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Cu interconnects
Journal Pre-proof The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects Jian Guo, Xiuchen Zhao, Yingxia Liu, Chengwen Tan, Lijun Liu, Xianjin Ning, Zhihua Nie, Xiaodong Yu
PII:
S2352-4928(19)31378-9
DOI:
https://doi.org/10.1016/j.mtcomm.2020.100960
Reference:
MTCOMM 100960
To appear in:
Materials Today Communications
Received Date:
14 November 2019
Revised Date:
24 January 2020
Accepted Date:
24 January 2020
Please cite this article as: Guo J, Zhao X, Liu Y, Tan C, Liu L, Ning X, Nie Z, Yu X, The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.100960
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects
Jian Guo 1, Xiuchen Zhao *1, Yingxia Liu 1, Chengwen Tan 1, Lijun Liu 1, Xianjin Ning 1, Zhihua Nie 1, Xiaodong Yu 1
School of Materials Science and Engineering, Beijing Institute of Technology,
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1
Beijing, 100081, China
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*Correspondence should be addressed to Xiuchen Zhao; [email protected]
* E-mail: [email protected]
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ORCID
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Corresponding Author
Highlights
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Xiuchen Zhao: 0000-0002-0297-4015
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Ag doping reduces the thickness of intermetallic compounds at the
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Sn5Zn/Cu interface
Ag can reduce the growth rate of interfacial intermetallic compounds during aging
Doping Ag atoms increases the stability of Cu5Zn8-based unit cells Ag doping can increase the diffusion activation energy in Cu5Zn8based unit cells
Abstract. The reliability of electronic packages is primarily affected by the growth of the intermetallic compound (IMC) between the solder and the copper interconnection. It is therefore necessary to study the effect of element addition on the growth of the IMC interface with respect to an improvement in the package reliability. In this study, the influence of Ag on the growth of intermetallic compounds in the Sn5Zn
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solder/copper interconnection interface was investigated. The results revealed that in
comparison with Sn5Zn/Cu, the interfacial IMC thickness decreased initially and then
increased in accordance with an increase in the doping amount of alloying elements in
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the solder. Further, it was observed that the doping of the alloying element (Ag) can
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significantly reduce the IMC growth rate at the interface. Compared with the Cu5Zn8 unit cell formation energy (-0.632eV), one dopant atom and two dopant atoms of Ag
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can reduce the formation energy of a Cu5Zn8-based unit cell to the minimum values of
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-0.636 eV and –0.640 eV, respectively; consequently, the stability is increased. The maximum diffusion activation energies of Cu atoms and Zn atoms in a Cu5Zn8-based
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unit cell are 3.419 eV and 1.692 eV, which increase to 3.512 eV and 1.971 eV, respectively, after doping with Ag.
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Keywords. Solder; Intermetallic compound; Heat aging; Formation energy; Diffusion activation energy
1. Introduction With the rapid development of the electronic information industry, microchips have been more readily integrated in electronic packages, and the feature sizes have
decreased significantly, which further accelerates the development of electronic packaging technology. [1] This has led to an abrupt increase in the packaging density of electronic devices, and an abrupt decrease in the size of the interconnection pads. Solder plays a major role in electronics manufacturing, and solders of different systems play major roles in all areas of electronics manufacturing. The solder interconnection reliability is directly related to the service life of electronic products.
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With an increase in the density of solder joints, the requirements for the solder
interconnection reliability have become more stringent. Therefore, issues with respect to the solder interconnection reliability have a significant impact on the entire
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electronic device. [2] For the realization of highly reliable interconnections between
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the solder and the substrate, the formation of an intermetallic compound (IMC) due to the interaction between the solder and the metal plating on the substrate surface is
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critical. The IMC interface is formed by a reaction between the solder and metal
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atoms in the substrate at the interface [3], which is based on atomic diffusion. [4-13] At present, the method mainly employed to improve the reliability of solder
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interconnections is alloying. The alloying of different systems has a significant influence on the interconnection reliability, especially for the formation and growth of
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IMCs.
In many studies, the effects of alloying on the growth thickness, mechanical
properties, and electromigration resistance of IMC layers based on experiments have been reported. In addition, the influence of the addition of different Ni elements on the growth thickness of the IMC layer and Kirkendall voids in Sn-based solder/copper
joints was investigated. [14-16] The results of these studies revealed that the addition of trace amounts of Zn can effectively inhibit the growth of IMCs at the Sn/Cu interface, especially the growth of Cu3Sn, thereby inhibiting the formation of Kirkendall voids. [17-19]
However, the abovementioned studies clarified the results, and provided
explanations based on inference by conducting several tests. However, the results were not analyzed with respect to atomic diffusion. With the development of
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computer simulation technology, computational material science has been applied to the investigation of the IMC growth of the solder joint interface.
The first-principles method refers to the determination of the physical properties of
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atoms, molecules, and condensed matter based on five fundamental physical
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constants, in addition to the electronic structure of each element based on the theory of quantum mechanics. [20] Hence, it provides a new approach to the study of solder
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joint interface reliability. In recent years, calculations based on first principles have
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been proposed in studies on the atomic diffusion of Sn-based solders in the field of electronic packaging. Zhou et al. [21] studied the diffusion properties of Sn atoms and
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Ag atoms based on first principles. The calculation results revealed that the diffusion of Ag atoms at the solder joints is more difficult than that of Sn atoms. The results
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were consistent with the experimental results. Most recent researches were focused on the improvement of the Sn-Zn solder
performance. However, there were several studies conducted on the Cu5Zn8 generated at the Sn-Zn solder interconnection interface; although atomic diffusion was not discussed in detail. In this study, the effect of Ag addition on the growth of the Sn5Zn
solder/copper interface IMC was investigated. The first principle of the density functional theory was used to determine the Cu5Zn8 phase structure, phase stability, and atomic diffusion activation energy before and after alloying. The simulation results were then used to explain the experimental phenomenon.
2. Methods 2.1 Experimental
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A Sn5Zn-xAg ternary solder with a total weight of 20 g was prepared. The Zn content was 5 wt.%, and the Ag contents were 1 wt.%, 3 wt.%, and 4 wt.%. The
solder was prepared by molten salt-covered alloy smelting. After the holding time was
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reached, the prepared alloy sample was cooled to room temperature.
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For the accurate analysis of the phase, the solder was subjected to x-ray diffraction (XRD) tests using a D8-Advance model X-ray diffraction analyzer at a scan speed of
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0.2 s/step, scan angle (2θ) of 10–90°, and scan step size of 0.0205285°.
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The preparation of the interconnected joints and the isothermal heat aging performance test was as follows:
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(1) Welding. First, a brass substrate with dimensions of 1 cm × 1 cm was prepared. A solder with a suitable size was then prepared on the surface of a copper sheet. An
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appropriate amount of NC-559-ASMUV flux was then applied, and the sample was placed in a TYD-KW600 reflow oven for soldering, wherein reflow was carried out in air for 20 min. (2) Aging. For the isothermal heat aging performance test, the reflowed sample was placed in an electric blast drying oven. The aging temperature was set as 150 ℃, and
the aging times were 120 h, 240 h, and 360 h. When the sample reached a predetermined aging time, it was removed and air cooled to room temperature. (3) Interconnection interface sample preparation. The reflowed and aged samples were cut along the largest cross section. After mounting, grinding, polishing, and etching, back-scattered electron microscopy (BSEM; Phenom G2 Pro) was employed to observe the solder and the copper. The microscopic morphology of the intermetallic
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compound at the interface was then imaged.
(4) Interconnection interface IMC layer thickness calculation. The interface of the corroded sample was observed and imaged using BSEM. The area and width of the
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interface IMC layer in the BSEM image were measured, and the interface IMC
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thickness was calculated using Formula (1). The average value was then calculated multiple times to obtain the average thickness of the interface IMC layer.
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𝑑 = 𝑆/𝐿
(1)
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where d is the IMC interface thickness, and S and L are the measured interface IMC layer area and width, respectively.
2.2 Computational simulation
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All the calculations in this study were carried out using the Cambridge sequential
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total energy package (CASTEP) module in the Materials Studio software. The generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE) functional was used to determine the Brillouin zone based on the Monkhorst–Pack k-point mesh, and the k-points of all the Cu5Zn8-based cells were divided into cells with dimensions of 4 × 4 × 4. The calculated mid-plane wave truncation energy was 351 eV, the energy
variation convergence value was less than 1.0×10-5 eV/atom, and the stress variation convergence value was less than 0.03 eV/Å. The intermetallic compound investigated in this study was a Cu5Zn8-based IMC with a cubic structure and an I/43m space group. The Cu5Zn8 original unit cell structure used in the calculation is shown in Fig. 1. The three-dimensional unit cell structure, as shown in schematic in Fig. 1(a), contained 52 atoms, which comprised 20
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Cu atoms and 32 Zn atoms. The initial unit cell lattice constants were a = b = c =
8.878 Å, and α = β = γ = 90°. Based on the symmetrical structure, two Cu1 position
atoms, two Cu2 position atoms, two Zn1 position atoms, and three Zn2 position atoms
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were selected for substitution and diffusion correlations, as shown in Fig. 2. The
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different colors in each atom in Fig. 2 represent different atomic positions, which correspond to Cu1(1), Cu1(2), Cu2(1), Cu2(2), Zn1(1), Zn1(2), Zn2(1), Zn2(2), and
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Zn2(3).
Fig. 1 Schematic diagram of Cu5Zn8 unit cell structure: (a) three-dimensional structure and (b) central symmetric atomic occupancy
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Fig. 2 Schematic of the selected atomic position calculation
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When only one atom in the Cu5Zn8 unit cell is substituted, the type of unit cell
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constructed can be expressed as (Cu4Ag)Zn8; and when two atoms in the unit cell are substituted, the type of unit cell constructed can be expressed as (Cu3Ag2)Zn8.
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The calculation formula for the formation energy of the Ag-doped Cu5Zn8-based IMC unit cell can be expressed as follows:
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𝑡𝑜𝑡𝑎𝑙 𝐸𝑓 = [𝐸𝐶𝑢 − (𝑚𝜇𝐶𝑢 + 𝑛𝜇𝐴𝑔 + 𝑘𝜇𝑍𝑛 )] ÷ (𝑚 + 𝑛 + 𝑘) 𝑚 𝐴𝑔𝑛 𝑍𝑛𝑘
（2）
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𝑡𝑜𝑡𝑎𝑙 where 𝐸𝐶𝑢 is the total energy of the Ag-doped Cu5Zn8-based IMC; m, n, and 𝑚 𝐴𝑔𝑛 𝑍𝑛𝑘
k are the number of Cu, Ag, and Zn atoms, respectively, in the Cu5Zn8-based cell; and 𝜇𝐶𝑢 , 𝜇𝐴𝑔 , 𝑎𝑛𝑑 𝜇𝑍𝑛 are the chemical potentials of the Cu, Ag, and Zn atoms, respectively. The calculation method of the formation energy of atomic vacancies in the crystal is as follows. An artificial atomic vacancy is created in the obtained stable crystal
structure, and then the structure that contains the missing atom is then optimized to obtain the total energy of the structure. Thereafter, the vacancy formation energy is calculated based on the formula. The formula for calculating the vacancy formation energy can be expressed as follows: （3）
𝑡𝑜𝑡𝑎𝑙 𝐸𝑣 = 𝐸𝐴𝑡𝑜𝑡𝑎𝑙 + 𝜇𝐴 − 𝐸𝐼𝑀𝐶
where 𝐸𝑣 is the vacancy formation energy, 𝐸𝐴𝑡𝑜𝑡𝑎𝑙 is the total energy of the structure
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that contains a missing A atom, 𝜇𝐴 is the chemical potential of the A atom, and 𝑡𝑜𝑡𝑎𝑙 𝐸𝐼𝑀𝐶 is the total energy of the crystal structure without the vacancy.
The atom is in a lower energy state when it is in a stable position; thus, it is
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required to overcome an energy barrier during its migration to the adjacent vacancy,
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and there is a path with the lowest barrier in the process. In this study, the TS Search function in the CASTEP module was used to search for the transition state, determine
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the lowest energy barrier, and then calculate the diffusion activation energy of the atom based on Formula (4).
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𝑚 𝑄𝑎 = 𝐸𝑣 + 𝐸𝐴−𝑉
（4）
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where 𝑄𝑎 is the diffusion activation energy of the atom, 𝐸𝑣 is the vacancy 𝑚 formation energy, and 𝐸𝐴−𝑉 is the vacancy-atomic exchange barrier.
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3. Results
3.1 Experimental 3.1.1 Effect of Ag on solder phase Fig. 3 presents an XRD analysis graph of the Sn5Zn-xAg solder. As can be seen from the figure, there were only Sn and Zn phases in the Sn5Zn solder and only Sn
and Ag5Zn8 phases in the Sn5Zn-xAg (x = 1,3,4) solder. In particular, when Ag was present, the formation energy of Ag5Zn8(–0.658 eV) was less than that of Ag3Sn(– 0.636eV), and the Zn element tended to form the compound Ag5Zn8 instead of Ag3Sn with Ag. In the absence of the Ag element, the Zn element was in a simple substance
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form in the solder matrix.
Fig. 3 XRD analysis of Sn5Zn-xAg solder.
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3.1.2 Study on the interface between solder and copper after reflow
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Fig. 4 presents a cross-sectional BSEM photograph of the IMC at the Sn5Zn/Cu and Sn5Zn-xAg/Cu interface after reflow.
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Fig. 4 BSEM image of the IMC cross-section after reflow: (a) Sn5Zn/Cu; (b) Sn5Zn-1Ag/Cu; (c) Sn5Zn-3Ag/Cu; and (d) Sn5Zn-4Ag/Cu.
As can be seen from Fig. 4, the IMC at the Sn5Zn/Cu interface was relatively flat
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and uniform in thickness after reflow, whereas the portion of the Sn5Zn-xAg/Cu
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interface IMC near the solder exhibited distinct irregularities.
To further analyze the effect of the doping of an alloying element Ag on the
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interfacial IMC thickness after reflow, the IMC thickness variation curve of the
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Sn5Zn/Cu and Sn5Zn-xAg/Cu interface after reflow was plotted, as shown in Fig. 5. As can be seen from the figure, the thickness of IMC at the Sn5Zn-xAg/Cu interface
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after reflow was less than that at the Sn5Zn/Cu interface. The thickness of the IMC interface was slightly less than that of the Sn5Zn/Cu interface when the Ag element
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was doped with a small amount of Ag. Moreover, the thickness of the IMC at the Sn5Zn-xAg/Cu interface was found to increase in accordance with an increase in the Ag content. In general, the thickness of the IMC interface decreased in accordance with the addition of Ag, when compared with the case wherein Sn5Zn/Cu was not added. Thus, Ag can inhibit the growth of IMC during reflow.
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Fig. 5 Variation in IMC thickness at the interface of Sn5Zn/Cu and Sn5Zn-xAg/Cu after reflow.
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Fig. 6 presents the electron probe micro analyzer (EPMA) test results for the Sn5Zn-4Ag/Cu interface after reflow. As can be seen from the figure, most of the
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solder matrix consisted of Sn, and there were almost no Zn or Ag elements in the
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region where the Sn element was distributed. In addition, Zn and Ag were found mainly in the precipitate phase; the interfacial IMC was divided into two layers, the
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main element on the side closest to the solder was Zn/Ag; and the side closest to Cu
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contained a small amount of Ag, in addition to the main elements Zn and Cu.
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Fig. 6 (a) SEM image of Sn5Zn-4Ag/Cu interface; EPMA diagrams of the distribution of (b) Sn elements; (c) Zn elements; (d) Cu elements; (e) Ag elements.
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To more accurately determine the composition of the precipitated phase in the
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solder shown in Fig. 6, an energy spectrum analysis was conducted, and the results are shown in Fig. 7. As can be seen from the figure, only the Zn and Ag elements
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were present in the precipitated phase, and the phases in which the needles were
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uniformly distributed (Part 1 of Fig. 7) exhibited Ag/Zn atomic ratios close to 5:8. Based on the XRD test results, the acicular phase may be Ag5Zn8. In the bulk phase
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(Fig. 7, Part 2) the Ag/Zn atomic ratio was close to 1:2, which was presumed to be a
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mixture of Ag5Zn8 and AgZn3.
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Fig. 7 Precipitated phase energy spectrum in the Sn5Zn-4Ag solder matrix. To compare and analyze the composition of the IMC at the interface of Sn5Zn/Cu
and Sn5Zn-xAg/Cu after reflow, the energy spectrum of the IMC layer at the interface
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of Sn5Zn/Cu and Sn5Zn-4Ag/Cu after reflow was tested. The test results are shown in
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Fig. 8. Fig. 8(a) presents an SEM image of the IMC layer at the Sn5Zn/Cu interface after reflow. As can be seen from the figure, the thickness of the IMC interface layer
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was even and low, and no distinct stratification was observed. The energy spectrum
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analysis results for a randomly selected region revealed that there was only Cu and Zn in the IMC layer, and the Cu/Zn atomic ratio was close to 5:8. Based on the binary
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phase diagram of Cu-Zn, Cu-Zn can form various IMCs such as CuZn, CuZn3, Cu5Zn8, and CuZn5. Therefore, the IMC of the Sn5Zn/Cu interface after reflow was
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Cu5Zn8. Fig. 8(b) presents the IMC layer at the Sn5Zn-4Ag/Cu interface after reflow. As can be seen from the SEM image, there were two phases at the interface of Sn5Zn4Ag/Cu, and the energy spectra of the two phases were tested. In the upper layer of the interface (on the side near the solder), there were only Ag and Zn elements, and the atomic ratio of Ag/Zn was close to 5:8. According to the Ag-Zn binary phase
diagram, the phase was Ag5Zn8. Moreover, in the lower layer of the interface (near the side of the Cu plate), Cu atoms were present, in addition to Ag and Zn atoms, and the ratio of the sum of Ag and Cu atoms to Zn was close to 5:8. It was therefore inferred
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that this IMC layer was (Cu, Ag)5Zn8.
Fig. 8 Interface energy spectra of (a) Sn5Zn/Cu and (b) Sn5Zn-4Ag/Cu after reflow.
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3.1.3 Evaluation of interface between solder and copper after isothermal aging
Fig. 9 presents a BSEM image of the IMC at the Sn5Zn/Cu and Sn5Zn-xAg/Cu interfaces after isothermal aging for different aging times. As can be seen from the figure, with an increase in the aging time, the thickness of the IMC layer at the
interface between the solder and the copper interconnection of each component
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gradually increased.
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Fig. 9 BSEM image of interfacial IMC cross-section after isothermal aging for different aging times: (a) Sn5Zn/Cu; (b) Sn5Zn-1Ag/Cu; (c) Sn5Zn-3Ag/Cu; and (d) Sn5Zn-4Ag/Cu.
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Fig. 10 presents the average thickness of the IMC at the Sn5Zn/Cu and Sn5ZnxAg/Cu interfaces with respect to the isothermal aging time. As can be seen from the
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figure, the thickness of the interfacial IMC between the solder and the copper of each
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component increased in accordance with an increase in increase in the isothermal aging time. However, at the same aging time, the thickness of the interconnected IMC interface after doping with Ag was less than that of the undoped IMC interface; which indicates that doping with the alloying element Ag suppressed the growth of the interfacial IMC during the isothermal aging process.
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Fig. 10 IMC thickness at Sn5Zn/Cu and Sn5Zn-xAg/Cu interfaces after isothermal aging at different times.
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To more accurately compare the growth trends of the solder and copper IMC
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interconnection interface of each component during the isothermal aging process, the following formula was used to calculate the growth rate constant k of each IMC
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interface layer:
𝑘 = (𝑑 − 𝑑0 )/𝑡1/2
（5）
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where d is the IMC interface thickness, d0 is the IMC interface thickness after
reflow, and t is the isothermal aging time. The IMC interface thickness d at different aging times was linearly fitted to the aging time t1 / 2, and the slope of the linear fitting curve was the growth rate constant k of the IMC interface. Fig. 11 presents the linear fitting curve of the IMC thickness d of the Sn5Zn-
xAg/Cu interface and the aging time t1 / 2. The slope of the linear curve in the figure represents the growth rate of the IMC interface. The black line in the image represents the linear fitting curve of the IMC thickness of the Sn5Zn solder and copper interconnection interface without alloying elements. As can be seen from the figure, the slope was significantly higher than that of the linear fitting curve of the solder and copper interconnection interface with Ag. In particular, the IMC growth rate at the
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interface can be significantly reduced after doping with Ag.
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Fig. 11 Linear fitting curve of IMC thickness d and aging time t1 / 2 at Sn5ZnxAg/Cu interface
3.2 Computational simulation 3.2.1 Effect of Ag element addition on the stability of IMC phase in Cu5Zn8-based interface After the geometric optimization of the Cu5Zn8 unit cell, the lattice constants were
a = b = c = 8.886 Å, α = β = γ = 90°. The formation energy was calculated as −0.632 eV based on the formula. To determine whether a stable Cu5Zn8-based structure and a stable substitution position can be formed after Ag doping, the formation energies of the (Cu4Ag)Zn8 unit cell and the (Cu3Ag2)Zn8 unit cell were calculated, and the statistical data are shown in Tables 1(a) and (b), respectively.
Formation Energy (eV) Doped Position
Position Co-ordinate (Cu4Ag) Zn8
Cu2
(0.500, 0.856, 0.500)
Zn1
(0.391, 0.609, 0.391)
Zn2
(0.535, 0.813, 0.813)
−0.633 −0.636
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(0.673, 0.673, 0.327)
−0.617
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Cu1
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Table 1 (a) Formation energy of (Cu4Ag)Zn8 unit cell
−0.619
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Table 1 (b) Formation energy of (Cu3Ag2)Zn8 unit cell Formation Energy (eV)
Doped Position
Position Co-ordinate
(Cu3Ag2)Zn8
−0.634 −0.640
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Cu2-two
(0.173, 0.173, 0.827) (0.673, 0.673, 0.327) (0.000, 1.000, 0.644) (0.500, 0.856, 0.500)
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Cu1-two
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As can be seen from the data in Table 1(a), the formation energy of the unit cell in which the Ag element was substituted for the Cu position was lower than that of the substituted Zn position, which indicates that the Ag atom tended to replace the Cu2 (12e). When Ag is substituted for the Cu position, a more stable structure than the Cu5Zn8 original unit cell can be obtained, and the obtained steady state unit cell is represented by (CuAg)5Zn8.
The structure diagram of the most stable unit cell obtained after doping Ag is shown in Fig. 12, and the Ag atom co-ordinates and the lattice constants of the unit
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cell are shown in Table 2.
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Fig. 12 Schematic of the unit cell structure of steady-state (Cu4Ag) Zn8 and (Cu3Ag2)Zn8.
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Table 2 Lattice constants of steady-state (Cu4Ag)Zn8 and (Cu3Ag2)Zn8 unit cells and Ag atomic occupancy Structure Doped Position Position Coordinate Lattice Constants a = 8.920 Å; b = 8.918 Å (0.499, 0.856, c = 8.932 Å; α = (Cu 4 Ag)Zn 8 Cu2 (12e) 0.497) 90.01° β = 90.01°; γ = 88.84° a = 8.946 Å; b = (0.000, 0.000, 8.955 Å 0.644) c = 8.958 Å; α = (Cu 3 Ag 2 )Zn 8 Cu2 (12e) (0.499, 0.856, 89.85° 0.497) β = 90.62°; γ = 89.01° The formation energies of the (CuAg)5Zn8 unit cell (minimum of −0.640 eV) and
Ag5Zn8 unit cell (−0.658 eV) were lower than that of the Cu5Zn8 unit cell (−0.632 eV). The formation energy of the unit cell can be lower than that of a Cu6Sn5 unit cell (−0.064 eV). In particular, under the same conditions, a (CuAg)5Zn8 unit cell and a
Ag5Zn8 unit cell were preferentially formed and more stable than a Cu5Zn8 unit cell. Moreover, the Cu5Zn8 unit cell was preferentially formed and more stable than the Cu6Sn5 unit cell. As can be seen from Fig. 8, when the alloying elements were not doped; only Cu5Zn8 was formed in the interface between the solder and the copper. When the alloying elements were doped, (CuAg)5Zn8 and Ag5Zn8 were formed in the solder and the copper interconnection interfaces. Therefore, the test results were
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consistent with the theoretical calculations.
3.2.2 Evaluation of atomic diffusion properties in Cu5Zn8 unit cell
Based on Formula (3), the vacancy formation energy at each position in the
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undoped Cu5Zn8 structure was calculated, and the results are shown in Fig. 13. As can
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be seen from the figure, the Cu site vacancy formation energy was lower than 1 eV, and the Zn site vacancy formation energy was approximately 1.5 eV. Comparing the
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different positions of Cu atoms and the vacancy formation energies of Zn atoms in
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different positions with respect to the symmetry, Cu2 was found to have two different positions, and the vacancy formation energies were 0.821 eV and 0.858 eV,
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respectively. The formation energies of Cu1 at two different positions were 0.877 eV and 0.902 eV, respectively, and the formation energy of Cu2 was slightly lower.
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Moreover, Zn1 occupied two different positions, and the vacancy formation energies were 1.505 eV and 1.504 eV, respectively. Furthermore, Zn2 occupied three different positions, and the formation energies were 1.491 eV, 1.498 eV, and 1.506 eV, respectively. The difference in the vacancy formation energy between Zn1 and Zn2 was in the order of 10–3 eV. The vacancy formation energies of each position of Zn
atoms were therefore similar.
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Fig. 13 Formation energies of vacancies at different positions in undoped Cu5Zn8 unit cell.
Based on the different position vacancies, neighboring atoms were selected for the vacancy-atomic exchange barrier calculation. In addition, three Cu atom diffusion
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paths and three Zn atom diffusion paths of Cu5Zn8 unit cells were respectively
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different paths are shown in Fig. 14.
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established. According to Formula (4), the atomic diffusion activation energy data of
Fig. 14 Atomic diffusion activation energies of different paths in undoped Cu5Zn8 unit cell. As can be seen from the data in Fig. 14, the Zn atom diffusion activation energy was small. The diffusion activation energy of the Cu1Cu2 path was higher than the that of the Cu2Cu2 path by a minimum of 0.5 eV, and the diffusion activation
energy of the Zn atoms diffused through each path was similar. In particular, upon diffusion, Cu atoms may be diffused along the Cu2Cu2 path, whereas Zn atoms are simultaneously diffused along multiple paths. In summary, the diffusion of Zn atoms in the Cu5Zn8 unit cell was easier than that of Cu atoms, and the diffusion rate was higher.
3.2.3 Effect of Ag on the diffusion properties of Cu atoms in Cu5Zn8-
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based unit cells
After completely relaxing the Cu5Zn8-based unit cell structure shown in Fig. 12, the Cu atom vacancy formation energy in the corresponding unit cell was calculated, and
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the results are shown in Fig. 15.
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Fig. 15 Cu atom vacancy formation energy after Ag doping with (a) one Ag atom and (b) two Ag atoms.
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Based on a comparison with the formation energy of Cu atoms in the undoped Cu5Zn8 unit cell, as shown in Figs. 15(a),15(b), and 13; the doping of Ag atoms can reduce the formation of Cu1 vacancies in the Cu5Zn8-based unit cells. With an increase in the number of doping atoms, the formation energy of the Cu1 vacancy decreased, in addition to the formation energy of the Cu1 (2) vacancy. The doping of one Ag atom can reduce the vacancy formation at the Cu1 (2) position by 3.5%, and
the doping of two Ag atoms can reduce the vacancy formation energy at the Cu1 (2) position by nearly 6%. Thus, a small amount of Ag doping can reduce the Cu2 position vacancy formation energy in a Cu5Zn8-based unit cell. Based on Formula (4), the statistics of the diffusion activation energies of Cu atoms in Cu5Zn8-based cells before and after doping are shown in Table 3. The atomic diffusion activation energy was positive, which represents the absorption of heat
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during the diffusion. With an increase in the value, the energy required for diffusion increased, and the diffusion was more difficult. According to Table 3, for the Cu1
(1)Cu2 path, the diffusion activation energies of Cu atoms in the (Cu4Ag)Zn8 and
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(Cu3Ag2)Zn8 unit cells were 0.065 eV and 0.044 eV, respectively, which were higher
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than that of the undoped Cu5Zn8 unit cell. These results can be mainly attributed to the
doping with Ag atoms.
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increase in the path vacancy-atomic exchange barrier, which was mainly due to
Table 3 Cu atom diffusion activation energy before and after doping
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Atom Diffusion Energy (eV)
Vacancy-atom Site
Cu5Zn8 (Cu4Ag)Zn8 (Cu3Ag2)Zn8 2.233
2.298
2.277
Cu1 (2)Cu2
3.419
3.512
3.185
Cu2Cu2
1.705
1.634
1.668
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Cu1 (1)Cu2
The addition of Ag can increase the diffusion activation energy of several Cu atoms
in the Cu5Zn8-based unit cells to an extent. The diffusion activation energy of Cu atoms in the unit cells doped with two atoms was slightly higher than that of the unit cells doped with one atom. In particular, Ag can suppress the diffusion of Cu atoms,
reduce the reaction rate, and inhibit the growth of the IMC interface.
3.2.4 Effect of Ag on the diffusion properties of Zn atoms in Cu5Zn8based unit cell After completely relaxing the structure shown in Fig. 12, the Zn vacancy formation energy in the Cu5Zn8-based unit cell was calculated, and the results are shown in Fig.
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16.
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Fig. 16 Zn vacancy formation energy after Ag doping with (a) one Ag atom and (b) two Ag atoms.
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Figs. 16(a) and 16(b) present a comparison of the Zn atom vacancy formation energy with Ag doping with that of the undoped Cu5Zn8 unit cell shown in Fig. 13. As
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can be seen from the figures, the doping of Ag can improve the vacancy formation
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energy of Zn atoms in the Cu5Zn8-based unit cell. The statistics of the diffusion activation energies of Zn atoms in the Cu5Zn8 unit
cell before and after doping were calculated using Formula (4), and the results are shown in Table 4. As can be seen from the table, the diffusion activation energy of Zn atoms in Cu5Zn8-based cells after doping with Ag atoms was higher than that of Zn atoms in undoped Cu5Zn8 cells.
Table 4 Diffusion activation energies of Zn atoms before and after doping Atom Diffusion Energy (eV) Vacancy-atom Site Cu5Zn8
(Cu4Ag)Zn8
(Cu3Ag2)Zn8
Zn1Zn2 (3)
1.692
1.745
1.883
Zn1Zn2 (1)
1.679
1.845
1.883
Zn2Zn2
1.674
1.963
1.971
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Based on the analysis of the variations of the diffusion activation energy of Zn atoms in each path according to Table 4, the diffusion activation energy of Zn atoms
in each unit cell was found to satisfy the following relationship from the lowest to the
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highest: Cu5Zn8 < (Cu4Ag)Zn8 < (Cu3Ag2)Zn8. In particular, doping with Ag can
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increase the diffusion activation energy of Zn atoms in Cu5Zn8-based unit cells; and
Zn atoms increased.
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with an increase in the number of doping atoms, the diffusion activation energy of the
From a comparison of Tables 3 and Table 4, the diffusion activation energy of the
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Zn atoms before and after doping was found to be generally lower than that of Cu
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atoms; i.e., the diffusion of Zn atoms in the unit cell occurred more easily than the diffusion of Cu atoms, and the diffusion rate was higher.
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Based on the results of the abovementioned data analysis, doping with Ag was found to inhibit the diffusion of Cu and Zn atoms, and the inhibition of Zn atom diffusion was more distinct. Moreover, Zn atoms were the main diffusion atoms; thus, it was predicted that the alloying element Ag in the solder can inhibit the growth of the IMC interface.
As shown in Fig. 10, the isothermal aging process after the solder and copper interconnection is a process of holding at a higher temperature for a long time. During this period of time, the growth mechanism of the IMC interface was the diffusion of atoms in the solder and the copper sheet to the interface, after which the interface reaction occurred. Therefore, the IMC interface growth rate is closely related to the atomic diffusion rate. Doping with Ag can increase the diffusion activation energy of
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Cu atoms in a partial path and increase the diffusion activation energy of Zn atoms. The diffusion activation energy of an atom when doping with two atoms is higher
than that when doping one atom. This indicates that doping with Ag can inhibit the
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diffusion of Cu atoms and Zn atoms, especially that of Zn atoms. The effect of doping
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with two atoms was found to be more distinct than doping with one atom. In particular, the addition of the alloying element Ag can suppress the growth of the IMC
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interface. The experimental results were found to be in good agreement with the
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simulation results. The inhibition of IMC growth at the interface of the alloying element Ag can be mainly attributed to the increase in the diffusion activation energy
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of Cu atoms and Zn atoms in Cu5Zn8 due to doping with Ag.
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4. Conclusions
A Sn5Zn-xAg solder was prepared in this study. The influence of the alloying
element Ag on the formation and growth of intermetallic compounds in the Sn5Zn solder/copper interconnection interface was investigated, and the effects of doping with the alloying element Ag on the phase structure, phase stability, and diffusion activation energy of Cu and Zn atoms in Cu5Zn8-based unit cells were determined
based on first principles. The main conclusions are as follows: (1) After reflow, compared with the IMC of the Sn5Zn/Cu interface, the interfacial IMC thickness initially decreased and then increased in accordance with an increase in the doping amount of alloying elements in the solder. When the amount of alloying element added was 1 wt.%, the interfacial IMC thickness was the minimum.
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(2) With an increase in the isothermal aging time, the thickness of the IMC layer at the interface of each component gradually increased. The doping of the alloying
element Ag can therefore significantly decrease the IMC growth rate at the interface.
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(3) The Cu5Zn8 unit cell was geometrically optimized, and a stable Cu5Zn8
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structure was obtained. The lattice constants were a = b = c = 8.886 Å, α = β = γ = 90°, and the formation energy was −0.632 eV.
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(4) Compared with the Cu5Zn8 unit cell, the doping of Ag atoms decreased the
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formation energy of the Cu5Zn8-based unit cells and increased the stability. The Ag atoms tended to replace the Cu2 (12e) atoms, and the stable structure of the Ag-doped
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Cu5Zn8-based unit cell was determined. (5) Doping with Ag can increase the diffusion activation energy of several Cu
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atoms in Cu5Zn8-based unit cells, and therefore inhibit the diffusion of Cu atoms. Moreover, doping with Ag can effectively improve the diffusion activation energy of Zn atoms in Cu5Zn8-based unit cells. With an increase in the number of doped atoms, the diffusion activation energy of the Zn atoms increased.
Prime Novelty Statement
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The Sn5Zn-xAg solder was prepared in this paper. The influence of Ag on the formation and growth of intermetallic compounds in Sn5Zn solder/copper interconnect interface was studied. At the same time, the effects of alloying element Ag doping on the phase structure, phase stability and diffusion activation energy of Cu and Zn atoms in Cu5Zn8-based unit cells were calculated by first-principle. The article explains the experimental phenomena with simulation calculations. In particular, based on the correlation calculation of diffusion activation energy, it is concluded that the increase of diffusion activation energy of Cu and Zn atoms in Cu5Zn8 by Ag doping is the essential reason for the inhibition of IMC growth at the interface of Ag. This method of interpreting the growth of intermetallic compounds by simulation calculations is relatively novel.
Author information
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Author Contributions
All authors have approved the final version of the manuscript.
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Notes
Acknowledgements
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The authors declare no competing financial interest.
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The authors would like to acknowledge the support received from China Aerospace
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Science and Technology Innovation Fund (2016).
References
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[3] D.R. Frear, H.S. Morgan, S.N. Burchett, J. H. Lau, The Mechanics of Solder Alloy Interconnects, Springer Science & Business Media. (1994). [4] R. Mayappan, Z.A. Ahmad, Effect of Bi addition on the activation energy for the growth of Cu5Zn8 intermetallic in the Sn–Zn lead-free solder, Intermetallics. 18 (2010) 730-735. [5] F. Bartels, J.W. Morris, G. Dalke, W. Gust, Intermetallic phase formation in thin
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PII:
S2352-4928(19)31378-9
DOI:
https://doi.org/10.1016/j.mtcomm.2020.100960
Reference:
MTCOMM 100960
To appear in:
Materials Today Communications
Received Date:
14 November 2019
Revised Date:
24 January 2020
Accepted Date:
24 January 2020
Please cite this article as: Guo J, Zhao X, Liu Y, Tan C, Liu L, Ning X, Nie Z, Yu X, The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.100960
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The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects
Jian Guo 1, Xiuchen Zhao *1, Yingxia Liu 1, Chengwen Tan 1, Lijun Liu 1, Xianjin Ning 1, Zhihua Nie 1, Xiaodong Yu 1
School of Materials Science and Engineering, Beijing Institute of Technology,
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1
Beijing, 100081, China
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*Correspondence should be addressed to Xiuchen Zhao; [email protected]
* E-mail: [email protected]
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ORCID
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Corresponding Author
Highlights
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Xiuchen Zhao: 0000-0002-0297-4015
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Ag doping reduces the thickness of intermetallic compounds at the
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Sn5Zn/Cu interface
Ag can reduce the growth rate of interfacial intermetallic compounds during aging
Doping Ag atoms increases the stability of Cu5Zn8-based unit cells Ag doping can increase the diffusion activation energy in Cu5Zn8based unit cells
Abstract. The reliability of electronic packages is primarily affected by the growth of the intermetallic compound (IMC) between the solder and the copper interconnection. It is therefore necessary to study the effect of element addition on the growth of the IMC interface with respect to an improvement in the package reliability. In this study, the influence of Ag on the growth of intermetallic compounds in the Sn5Zn
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solder/copper interconnection interface was investigated. The results revealed that in
comparison with Sn5Zn/Cu, the interfacial IMC thickness decreased initially and then
increased in accordance with an increase in the doping amount of alloying elements in
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the solder. Further, it was observed that the doping of the alloying element (Ag) can
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significantly reduce the IMC growth rate at the interface. Compared with the Cu5Zn8 unit cell formation energy (-0.632eV), one dopant atom and two dopant atoms of Ag
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can reduce the formation energy of a Cu5Zn8-based unit cell to the minimum values of
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-0.636 eV and –0.640 eV, respectively; consequently, the stability is increased. The maximum diffusion activation energies of Cu atoms and Zn atoms in a Cu5Zn8-based
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unit cell are 3.419 eV and 1.692 eV, which increase to 3.512 eV and 1.971 eV, respectively, after doping with Ag.
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Keywords. Solder; Intermetallic compound; Heat aging; Formation energy; Diffusion activation energy
1. Introduction With the rapid development of the electronic information industry, microchips have been more readily integrated in electronic packages, and the feature sizes have
decreased significantly, which further accelerates the development of electronic packaging technology. [1] This has led to an abrupt increase in the packaging density of electronic devices, and an abrupt decrease in the size of the interconnection pads. Solder plays a major role in electronics manufacturing, and solders of different systems play major roles in all areas of electronics manufacturing. The solder interconnection reliability is directly related to the service life of electronic products.
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With an increase in the density of solder joints, the requirements for the solder
interconnection reliability have become more stringent. Therefore, issues with respect to the solder interconnection reliability have a significant impact on the entire
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electronic device. [2] For the realization of highly reliable interconnections between
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the solder and the substrate, the formation of an intermetallic compound (IMC) due to the interaction between the solder and the metal plating on the substrate surface is
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critical. The IMC interface is formed by a reaction between the solder and metal
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atoms in the substrate at the interface [3], which is based on atomic diffusion. [4-13] At present, the method mainly employed to improve the reliability of solder
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interconnections is alloying. The alloying of different systems has a significant influence on the interconnection reliability, especially for the formation and growth of
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IMCs.
In many studies, the effects of alloying on the growth thickness, mechanical
properties, and electromigration resistance of IMC layers based on experiments have been reported. In addition, the influence of the addition of different Ni elements on the growth thickness of the IMC layer and Kirkendall voids in Sn-based solder/copper
joints was investigated. [14-16] The results of these studies revealed that the addition of trace amounts of Zn can effectively inhibit the growth of IMCs at the Sn/Cu interface, especially the growth of Cu3Sn, thereby inhibiting the formation of Kirkendall voids. [17-19]
However, the abovementioned studies clarified the results, and provided
explanations based on inference by conducting several tests. However, the results were not analyzed with respect to atomic diffusion. With the development of
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computer simulation technology, computational material science has been applied to the investigation of the IMC growth of the solder joint interface.
The first-principles method refers to the determination of the physical properties of
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atoms, molecules, and condensed matter based on five fundamental physical
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constants, in addition to the electronic structure of each element based on the theory of quantum mechanics. [20] Hence, it provides a new approach to the study of solder
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joint interface reliability. In recent years, calculations based on first principles have
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been proposed in studies on the atomic diffusion of Sn-based solders in the field of electronic packaging. Zhou et al. [21] studied the diffusion properties of Sn atoms and
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Ag atoms based on first principles. The calculation results revealed that the diffusion of Ag atoms at the solder joints is more difficult than that of Sn atoms. The results
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were consistent with the experimental results. Most recent researches were focused on the improvement of the Sn-Zn solder
performance. However, there were several studies conducted on the Cu5Zn8 generated at the Sn-Zn solder interconnection interface; although atomic diffusion was not discussed in detail. In this study, the effect of Ag addition on the growth of the Sn5Zn
solder/copper interface IMC was investigated. The first principle of the density functional theory was used to determine the Cu5Zn8 phase structure, phase stability, and atomic diffusion activation energy before and after alloying. The simulation results were then used to explain the experimental phenomenon.
2. Methods 2.1 Experimental
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A Sn5Zn-xAg ternary solder with a total weight of 20 g was prepared. The Zn content was 5 wt.%, and the Ag contents were 1 wt.%, 3 wt.%, and 4 wt.%. The
solder was prepared by molten salt-covered alloy smelting. After the holding time was
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reached, the prepared alloy sample was cooled to room temperature.
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For the accurate analysis of the phase, the solder was subjected to x-ray diffraction (XRD) tests using a D8-Advance model X-ray diffraction analyzer at a scan speed of
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0.2 s/step, scan angle (2θ) of 10–90°, and scan step size of 0.0205285°.
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The preparation of the interconnected joints and the isothermal heat aging performance test was as follows:
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(1) Welding. First, a brass substrate with dimensions of 1 cm × 1 cm was prepared. A solder with a suitable size was then prepared on the surface of a copper sheet. An
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appropriate amount of NC-559-ASMUV flux was then applied, and the sample was placed in a TYD-KW600 reflow oven for soldering, wherein reflow was carried out in air for 20 min. (2) Aging. For the isothermal heat aging performance test, the reflowed sample was placed in an electric blast drying oven. The aging temperature was set as 150 ℃, and
the aging times were 120 h, 240 h, and 360 h. When the sample reached a predetermined aging time, it was removed and air cooled to room temperature. (3) Interconnection interface sample preparation. The reflowed and aged samples were cut along the largest cross section. After mounting, grinding, polishing, and etching, back-scattered electron microscopy (BSEM; Phenom G2 Pro) was employed to observe the solder and the copper. The microscopic morphology of the intermetallic
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compound at the interface was then imaged.
(4) Interconnection interface IMC layer thickness calculation. The interface of the corroded sample was observed and imaged using BSEM. The area and width of the
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interface IMC layer in the BSEM image were measured, and the interface IMC
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thickness was calculated using Formula (1). The average value was then calculated multiple times to obtain the average thickness of the interface IMC layer.
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𝑑 = 𝑆/𝐿
(1)
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where d is the IMC interface thickness, and S and L are the measured interface IMC layer area and width, respectively.
2.2 Computational simulation
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All the calculations in this study were carried out using the Cambridge sequential
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total energy package (CASTEP) module in the Materials Studio software. The generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE) functional was used to determine the Brillouin zone based on the Monkhorst–Pack k-point mesh, and the k-points of all the Cu5Zn8-based cells were divided into cells with dimensions of 4 × 4 × 4. The calculated mid-plane wave truncation energy was 351 eV, the energy
variation convergence value was less than 1.0×10-5 eV/atom, and the stress variation convergence value was less than 0.03 eV/Å. The intermetallic compound investigated in this study was a Cu5Zn8-based IMC with a cubic structure and an I/43m space group. The Cu5Zn8 original unit cell structure used in the calculation is shown in Fig. 1. The three-dimensional unit cell structure, as shown in schematic in Fig. 1(a), contained 52 atoms, which comprised 20
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Cu atoms and 32 Zn atoms. The initial unit cell lattice constants were a = b = c =
8.878 Å, and α = β = γ = 90°. Based on the symmetrical structure, two Cu1 position
atoms, two Cu2 position atoms, two Zn1 position atoms, and three Zn2 position atoms
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were selected for substitution and diffusion correlations, as shown in Fig. 2. The
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different colors in each atom in Fig. 2 represent different atomic positions, which correspond to Cu1(1), Cu1(2), Cu2(1), Cu2(2), Zn1(1), Zn1(2), Zn2(1), Zn2(2), and
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Zn2(3).
Fig. 1 Schematic diagram of Cu5Zn8 unit cell structure: (a) three-dimensional structure and (b) central symmetric atomic occupancy
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Fig. 2 Schematic of the selected atomic position calculation
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When only one atom in the Cu5Zn8 unit cell is substituted, the type of unit cell
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constructed can be expressed as (Cu4Ag)Zn8; and when two atoms in the unit cell are substituted, the type of unit cell constructed can be expressed as (Cu3Ag2)Zn8.
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The calculation formula for the formation energy of the Ag-doped Cu5Zn8-based IMC unit cell can be expressed as follows:
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𝑡𝑜𝑡𝑎𝑙 𝐸𝑓 = [𝐸𝐶𝑢 − (𝑚𝜇𝐶𝑢 + 𝑛𝜇𝐴𝑔 + 𝑘𝜇𝑍𝑛 )] ÷ (𝑚 + 𝑛 + 𝑘) 𝑚 𝐴𝑔𝑛 𝑍𝑛𝑘
（2）
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𝑡𝑜𝑡𝑎𝑙 where 𝐸𝐶𝑢 is the total energy of the Ag-doped Cu5Zn8-based IMC; m, n, and 𝑚 𝐴𝑔𝑛 𝑍𝑛𝑘
k are the number of Cu, Ag, and Zn atoms, respectively, in the Cu5Zn8-based cell; and 𝜇𝐶𝑢 , 𝜇𝐴𝑔 , 𝑎𝑛𝑑 𝜇𝑍𝑛 are the chemical potentials of the Cu, Ag, and Zn atoms, respectively. The calculation method of the formation energy of atomic vacancies in the crystal is as follows. An artificial atomic vacancy is created in the obtained stable crystal
structure, and then the structure that contains the missing atom is then optimized to obtain the total energy of the structure. Thereafter, the vacancy formation energy is calculated based on the formula. The formula for calculating the vacancy formation energy can be expressed as follows: （3）
𝑡𝑜𝑡𝑎𝑙 𝐸𝑣 = 𝐸𝐴𝑡𝑜𝑡𝑎𝑙 + 𝜇𝐴 − 𝐸𝐼𝑀𝐶
where 𝐸𝑣 is the vacancy formation energy, 𝐸𝐴𝑡𝑜𝑡𝑎𝑙 is the total energy of the structure
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that contains a missing A atom, 𝜇𝐴 is the chemical potential of the A atom, and 𝑡𝑜𝑡𝑎𝑙 𝐸𝐼𝑀𝐶 is the total energy of the crystal structure without the vacancy.
The atom is in a lower energy state when it is in a stable position; thus, it is
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required to overcome an energy barrier during its migration to the adjacent vacancy,
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and there is a path with the lowest barrier in the process. In this study, the TS Search function in the CASTEP module was used to search for the transition state, determine
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the lowest energy barrier, and then calculate the diffusion activation energy of the atom based on Formula (4).
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𝑚 𝑄𝑎 = 𝐸𝑣 + 𝐸𝐴−𝑉
（4）
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where 𝑄𝑎 is the diffusion activation energy of the atom, 𝐸𝑣 is the vacancy 𝑚 formation energy, and 𝐸𝐴−𝑉 is the vacancy-atomic exchange barrier.
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3. Results
3.1 Experimental 3.1.1 Effect of Ag on solder phase Fig. 3 presents an XRD analysis graph of the Sn5Zn-xAg solder. As can be seen from the figure, there were only Sn and Zn phases in the Sn5Zn solder and only Sn
and Ag5Zn8 phases in the Sn5Zn-xAg (x = 1,3,4) solder. In particular, when Ag was present, the formation energy of Ag5Zn8(–0.658 eV) was less than that of Ag3Sn(– 0.636eV), and the Zn element tended to form the compound Ag5Zn8 instead of Ag3Sn with Ag. In the absence of the Ag element, the Zn element was in a simple substance
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form in the solder matrix.
Fig. 3 XRD analysis of Sn5Zn-xAg solder.
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3.1.2 Study on the interface between solder and copper after reflow
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Fig. 4 presents a cross-sectional BSEM photograph of the IMC at the Sn5Zn/Cu and Sn5Zn-xAg/Cu interface after reflow.
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Fig. 4 BSEM image of the IMC cross-section after reflow: (a) Sn5Zn/Cu; (b) Sn5Zn-1Ag/Cu; (c) Sn5Zn-3Ag/Cu; and (d) Sn5Zn-4Ag/Cu.
As can be seen from Fig. 4, the IMC at the Sn5Zn/Cu interface was relatively flat
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and uniform in thickness after reflow, whereas the portion of the Sn5Zn-xAg/Cu
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interface IMC near the solder exhibited distinct irregularities.
To further analyze the effect of the doping of an alloying element Ag on the
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interfacial IMC thickness after reflow, the IMC thickness variation curve of the
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Sn5Zn/Cu and Sn5Zn-xAg/Cu interface after reflow was plotted, as shown in Fig. 5. As can be seen from the figure, the thickness of IMC at the Sn5Zn-xAg/Cu interface
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after reflow was less than that at the Sn5Zn/Cu interface. The thickness of the IMC interface was slightly less than that of the Sn5Zn/Cu interface when the Ag element
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was doped with a small amount of Ag. Moreover, the thickness of the IMC at the Sn5Zn-xAg/Cu interface was found to increase in accordance with an increase in the Ag content. In general, the thickness of the IMC interface decreased in accordance with the addition of Ag, when compared with the case wherein Sn5Zn/Cu was not added. Thus, Ag can inhibit the growth of IMC during reflow.
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Fig. 5 Variation in IMC thickness at the interface of Sn5Zn/Cu and Sn5Zn-xAg/Cu after reflow.
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Fig. 6 presents the electron probe micro analyzer (EPMA) test results for the Sn5Zn-4Ag/Cu interface after reflow. As can be seen from the figure, most of the
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solder matrix consisted of Sn, and there were almost no Zn or Ag elements in the
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region where the Sn element was distributed. In addition, Zn and Ag were found mainly in the precipitate phase; the interfacial IMC was divided into two layers, the
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main element on the side closest to the solder was Zn/Ag; and the side closest to Cu
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contained a small amount of Ag, in addition to the main elements Zn and Cu.
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Fig. 6 (a) SEM image of Sn5Zn-4Ag/Cu interface; EPMA diagrams of the distribution of (b) Sn elements; (c) Zn elements; (d) Cu elements; (e) Ag elements.
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To more accurately determine the composition of the precipitated phase in the
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solder shown in Fig. 6, an energy spectrum analysis was conducted, and the results are shown in Fig. 7. As can be seen from the figure, only the Zn and Ag elements
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were present in the precipitated phase, and the phases in which the needles were
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uniformly distributed (Part 1 of Fig. 7) exhibited Ag/Zn atomic ratios close to 5:8. Based on the XRD test results, the acicular phase may be Ag5Zn8. In the bulk phase
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(Fig. 7, Part 2) the Ag/Zn atomic ratio was close to 1:2, which was presumed to be a
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mixture of Ag5Zn8 and AgZn3.
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Fig. 7 Precipitated phase energy spectrum in the Sn5Zn-4Ag solder matrix. To compare and analyze the composition of the IMC at the interface of Sn5Zn/Cu
and Sn5Zn-xAg/Cu after reflow, the energy spectrum of the IMC layer at the interface
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of Sn5Zn/Cu and Sn5Zn-4Ag/Cu after reflow was tested. The test results are shown in
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Fig. 8. Fig. 8(a) presents an SEM image of the IMC layer at the Sn5Zn/Cu interface after reflow. As can be seen from the figure, the thickness of the IMC interface layer
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was even and low, and no distinct stratification was observed. The energy spectrum
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analysis results for a randomly selected region revealed that there was only Cu and Zn in the IMC layer, and the Cu/Zn atomic ratio was close to 5:8. Based on the binary
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phase diagram of Cu-Zn, Cu-Zn can form various IMCs such as CuZn, CuZn3, Cu5Zn8, and CuZn5. Therefore, the IMC of the Sn5Zn/Cu interface after reflow was
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Cu5Zn8. Fig. 8(b) presents the IMC layer at the Sn5Zn-4Ag/Cu interface after reflow. As can be seen from the SEM image, there were two phases at the interface of Sn5Zn4Ag/Cu, and the energy spectra of the two phases were tested. In the upper layer of the interface (on the side near the solder), there were only Ag and Zn elements, and the atomic ratio of Ag/Zn was close to 5:8. According to the Ag-Zn binary phase
diagram, the phase was Ag5Zn8. Moreover, in the lower layer of the interface (near the side of the Cu plate), Cu atoms were present, in addition to Ag and Zn atoms, and the ratio of the sum of Ag and Cu atoms to Zn was close to 5:8. It was therefore inferred
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that this IMC layer was (Cu, Ag)5Zn8.
Fig. 8 Interface energy spectra of (a) Sn5Zn/Cu and (b) Sn5Zn-4Ag/Cu after reflow.
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3.1.3 Evaluation of interface between solder and copper after isothermal aging
Fig. 9 presents a BSEM image of the IMC at the Sn5Zn/Cu and Sn5Zn-xAg/Cu interfaces after isothermal aging for different aging times. As can be seen from the figure, with an increase in the aging time, the thickness of the IMC layer at the
interface between the solder and the copper interconnection of each component
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gradually increased.
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Fig. 9 BSEM image of interfacial IMC cross-section after isothermal aging for different aging times: (a) Sn5Zn/Cu; (b) Sn5Zn-1Ag/Cu; (c) Sn5Zn-3Ag/Cu; and (d) Sn5Zn-4Ag/Cu.
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Fig. 10 presents the average thickness of the IMC at the Sn5Zn/Cu and Sn5ZnxAg/Cu interfaces with respect to the isothermal aging time. As can be seen from the
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figure, the thickness of the interfacial IMC between the solder and the copper of each
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component increased in accordance with an increase in increase in the isothermal aging time. However, at the same aging time, the thickness of the interconnected IMC interface after doping with Ag was less than that of the undoped IMC interface; which indicates that doping with the alloying element Ag suppressed the growth of the interfacial IMC during the isothermal aging process.
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Fig. 10 IMC thickness at Sn5Zn/Cu and Sn5Zn-xAg/Cu interfaces after isothermal aging at different times.
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To more accurately compare the growth trends of the solder and copper IMC
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interconnection interface of each component during the isothermal aging process, the following formula was used to calculate the growth rate constant k of each IMC
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interface layer:
𝑘 = (𝑑 − 𝑑0 )/𝑡1/2
（5）
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where d is the IMC interface thickness, d0 is the IMC interface thickness after
reflow, and t is the isothermal aging time. The IMC interface thickness d at different aging times was linearly fitted to the aging time t1 / 2, and the slope of the linear fitting curve was the growth rate constant k of the IMC interface. Fig. 11 presents the linear fitting curve of the IMC thickness d of the Sn5Zn-
xAg/Cu interface and the aging time t1 / 2. The slope of the linear curve in the figure represents the growth rate of the IMC interface. The black line in the image represents the linear fitting curve of the IMC thickness of the Sn5Zn solder and copper interconnection interface without alloying elements. As can be seen from the figure, the slope was significantly higher than that of the linear fitting curve of the solder and copper interconnection interface with Ag. In particular, the IMC growth rate at the
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interface can be significantly reduced after doping with Ag.
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Fig. 11 Linear fitting curve of IMC thickness d and aging time t1 / 2 at Sn5ZnxAg/Cu interface
3.2 Computational simulation 3.2.1 Effect of Ag element addition on the stability of IMC phase in Cu5Zn8-based interface After the geometric optimization of the Cu5Zn8 unit cell, the lattice constants were
a = b = c = 8.886 Å, α = β = γ = 90°. The formation energy was calculated as −0.632 eV based on the formula. To determine whether a stable Cu5Zn8-based structure and a stable substitution position can be formed after Ag doping, the formation energies of the (Cu4Ag)Zn8 unit cell and the (Cu3Ag2)Zn8 unit cell were calculated, and the statistical data are shown in Tables 1(a) and (b), respectively.
Formation Energy (eV) Doped Position
Position Co-ordinate (Cu4Ag) Zn8
Cu2
(0.500, 0.856, 0.500)
Zn1
(0.391, 0.609, 0.391)
Zn2
(0.535, 0.813, 0.813)
−0.633 −0.636
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(0.673, 0.673, 0.327)
−0.617
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Cu1
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Table 1 (a) Formation energy of (Cu4Ag)Zn8 unit cell
−0.619
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Table 1 (b) Formation energy of (Cu3Ag2)Zn8 unit cell Formation Energy (eV)
Doped Position
Position Co-ordinate
(Cu3Ag2)Zn8
−0.634 −0.640
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Cu2-two
(0.173, 0.173, 0.827) (0.673, 0.673, 0.327) (0.000, 1.000, 0.644) (0.500, 0.856, 0.500)
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Cu1-two
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As can be seen from the data in Table 1(a), the formation energy of the unit cell in which the Ag element was substituted for the Cu position was lower than that of the substituted Zn position, which indicates that the Ag atom tended to replace the Cu2 (12e). When Ag is substituted for the Cu position, a more stable structure than the Cu5Zn8 original unit cell can be obtained, and the obtained steady state unit cell is represented by (CuAg)5Zn8.
The structure diagram of the most stable unit cell obtained after doping Ag is shown in Fig. 12, and the Ag atom co-ordinates and the lattice constants of the unit
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cell are shown in Table 2.
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Fig. 12 Schematic of the unit cell structure of steady-state (Cu4Ag) Zn8 and (Cu3Ag2)Zn8.
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Table 2 Lattice constants of steady-state (Cu4Ag)Zn8 and (Cu3Ag2)Zn8 unit cells and Ag atomic occupancy Structure Doped Position Position Coordinate Lattice Constants a = 8.920 Å; b = 8.918 Å (0.499, 0.856, c = 8.932 Å; α = (Cu 4 Ag)Zn 8 Cu2 (12e) 0.497) 90.01° β = 90.01°; γ = 88.84° a = 8.946 Å; b = (0.000, 0.000, 8.955 Å 0.644) c = 8.958 Å; α = (Cu 3 Ag 2 )Zn 8 Cu2 (12e) (0.499, 0.856, 89.85° 0.497) β = 90.62°; γ = 89.01° The formation energies of the (CuAg)5Zn8 unit cell (minimum of −0.640 eV) and
Ag5Zn8 unit cell (−0.658 eV) were lower than that of the Cu5Zn8 unit cell (−0.632 eV). The formation energy of the unit cell can be lower than that of a Cu6Sn5 unit cell (−0.064 eV). In particular, under the same conditions, a (CuAg)5Zn8 unit cell and a
Ag5Zn8 unit cell were preferentially formed and more stable than a Cu5Zn8 unit cell. Moreover, the Cu5Zn8 unit cell was preferentially formed and more stable than the Cu6Sn5 unit cell. As can be seen from Fig. 8, when the alloying elements were not doped; only Cu5Zn8 was formed in the interface between the solder and the copper. When the alloying elements were doped, (CuAg)5Zn8 and Ag5Zn8 were formed in the solder and the copper interconnection interfaces. Therefore, the test results were
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consistent with the theoretical calculations.
3.2.2 Evaluation of atomic diffusion properties in Cu5Zn8 unit cell
Based on Formula (3), the vacancy formation energy at each position in the
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undoped Cu5Zn8 structure was calculated, and the results are shown in Fig. 13. As can
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be seen from the figure, the Cu site vacancy formation energy was lower than 1 eV, and the Zn site vacancy formation energy was approximately 1.5 eV. Comparing the
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different positions of Cu atoms and the vacancy formation energies of Zn atoms in
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different positions with respect to the symmetry, Cu2 was found to have two different positions, and the vacancy formation energies were 0.821 eV and 0.858 eV,
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respectively. The formation energies of Cu1 at two different positions were 0.877 eV and 0.902 eV, respectively, and the formation energy of Cu2 was slightly lower.
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Moreover, Zn1 occupied two different positions, and the vacancy formation energies were 1.505 eV and 1.504 eV, respectively. Furthermore, Zn2 occupied three different positions, and the formation energies were 1.491 eV, 1.498 eV, and 1.506 eV, respectively. The difference in the vacancy formation energy between Zn1 and Zn2 was in the order of 10–3 eV. The vacancy formation energies of each position of Zn
atoms were therefore similar.
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Fig. 13 Formation energies of vacancies at different positions in undoped Cu5Zn8 unit cell.
Based on the different position vacancies, neighboring atoms were selected for the vacancy-atomic exchange barrier calculation. In addition, three Cu atom diffusion
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paths and three Zn atom diffusion paths of Cu5Zn8 unit cells were respectively
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different paths are shown in Fig. 14.
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established. According to Formula (4), the atomic diffusion activation energy data of
Fig. 14 Atomic diffusion activation energies of different paths in undoped Cu5Zn8 unit cell. As can be seen from the data in Fig. 14, the Zn atom diffusion activation energy was small. The diffusion activation energy of the Cu1Cu2 path was higher than the that of the Cu2Cu2 path by a minimum of 0.5 eV, and the diffusion activation
energy of the Zn atoms diffused through each path was similar. In particular, upon diffusion, Cu atoms may be diffused along the Cu2Cu2 path, whereas Zn atoms are simultaneously diffused along multiple paths. In summary, the diffusion of Zn atoms in the Cu5Zn8 unit cell was easier than that of Cu atoms, and the diffusion rate was higher.
3.2.3 Effect of Ag on the diffusion properties of Cu atoms in Cu5Zn8-
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based unit cells
After completely relaxing the Cu5Zn8-based unit cell structure shown in Fig. 12, the Cu atom vacancy formation energy in the corresponding unit cell was calculated, and
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the results are shown in Fig. 15.
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Fig. 15 Cu atom vacancy formation energy after Ag doping with (a) one Ag atom and (b) two Ag atoms.
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Based on a comparison with the formation energy of Cu atoms in the undoped Cu5Zn8 unit cell, as shown in Figs. 15(a),15(b), and 13; the doping of Ag atoms can reduce the formation of Cu1 vacancies in the Cu5Zn8-based unit cells. With an increase in the number of doping atoms, the formation energy of the Cu1 vacancy decreased, in addition to the formation energy of the Cu1 (2) vacancy. The doping of one Ag atom can reduce the vacancy formation at the Cu1 (2) position by 3.5%, and
the doping of two Ag atoms can reduce the vacancy formation energy at the Cu1 (2) position by nearly 6%. Thus, a small amount of Ag doping can reduce the Cu2 position vacancy formation energy in a Cu5Zn8-based unit cell. Based on Formula (4), the statistics of the diffusion activation energies of Cu atoms in Cu5Zn8-based cells before and after doping are shown in Table 3. The atomic diffusion activation energy was positive, which represents the absorption of heat
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during the diffusion. With an increase in the value, the energy required for diffusion increased, and the diffusion was more difficult. According to Table 3, for the Cu1
(1)Cu2 path, the diffusion activation energies of Cu atoms in the (Cu4Ag)Zn8 and
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(Cu3Ag2)Zn8 unit cells were 0.065 eV and 0.044 eV, respectively, which were higher
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than that of the undoped Cu5Zn8 unit cell. These results can be mainly attributed to the
doping with Ag atoms.
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increase in the path vacancy-atomic exchange barrier, which was mainly due to
Table 3 Cu atom diffusion activation energy before and after doping
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Atom Diffusion Energy (eV)
Vacancy-atom Site
Cu5Zn8 (Cu4Ag)Zn8 (Cu3Ag2)Zn8 2.233
2.298
2.277
Cu1 (2)Cu2
3.419
3.512
3.185
Cu2Cu2
1.705
1.634
1.668
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Cu1 (1)Cu2
The addition of Ag can increase the diffusion activation energy of several Cu atoms
in the Cu5Zn8-based unit cells to an extent. The diffusion activation energy of Cu atoms in the unit cells doped with two atoms was slightly higher than that of the unit cells doped with one atom. In particular, Ag can suppress the diffusion of Cu atoms,
reduce the reaction rate, and inhibit the growth of the IMC interface.
3.2.4 Effect of Ag on the diffusion properties of Zn atoms in Cu5Zn8based unit cell After completely relaxing the structure shown in Fig. 12, the Zn vacancy formation energy in the Cu5Zn8-based unit cell was calculated, and the results are shown in Fig.
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Fig. 16 Zn vacancy formation energy after Ag doping with (a) one Ag atom and (b) two Ag atoms.
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Figs. 16(a) and 16(b) present a comparison of the Zn atom vacancy formation energy with Ag doping with that of the undoped Cu5Zn8 unit cell shown in Fig. 13. As
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can be seen from the figures, the doping of Ag can improve the vacancy formation
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energy of Zn atoms in the Cu5Zn8-based unit cell. The statistics of the diffusion activation energies of Zn atoms in the Cu5Zn8 unit
cell before and after doping were calculated using Formula (4), and the results are shown in Table 4. As can be seen from the table, the diffusion activation energy of Zn atoms in Cu5Zn8-based cells after doping with Ag atoms was higher than that of Zn atoms in undoped Cu5Zn8 cells.
Table 4 Diffusion activation energies of Zn atoms before and after doping Atom Diffusion Energy (eV) Vacancy-atom Site Cu5Zn8
(Cu4Ag)Zn8
(Cu3Ag2)Zn8
Zn1Zn2 (3)
1.692
1.745
1.883
Zn1Zn2 (1)
1.679
1.845
1.883
Zn2Zn2
1.674
1.963
1.971
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Based on the analysis of the variations of the diffusion activation energy of Zn atoms in each path according to Table 4, the diffusion activation energy of Zn atoms
in each unit cell was found to satisfy the following relationship from the lowest to the
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highest: Cu5Zn8 < (Cu4Ag)Zn8 < (Cu3Ag2)Zn8. In particular, doping with Ag can
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increase the diffusion activation energy of Zn atoms in Cu5Zn8-based unit cells; and
Zn atoms increased.
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with an increase in the number of doping atoms, the diffusion activation energy of the
From a comparison of Tables 3 and Table 4, the diffusion activation energy of the
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Zn atoms before and after doping was found to be generally lower than that of Cu
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atoms; i.e., the diffusion of Zn atoms in the unit cell occurred more easily than the diffusion of Cu atoms, and the diffusion rate was higher.
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Based on the results of the abovementioned data analysis, doping with Ag was found to inhibit the diffusion of Cu and Zn atoms, and the inhibition of Zn atom diffusion was more distinct. Moreover, Zn atoms were the main diffusion atoms; thus, it was predicted that the alloying element Ag in the solder can inhibit the growth of the IMC interface.
As shown in Fig. 10, the isothermal aging process after the solder and copper interconnection is a process of holding at a higher temperature for a long time. During this period of time, the growth mechanism of the IMC interface was the diffusion of atoms in the solder and the copper sheet to the interface, after which the interface reaction occurred. Therefore, the IMC interface growth rate is closely related to the atomic diffusion rate. Doping with Ag can increase the diffusion activation energy of
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Cu atoms in a partial path and increase the diffusion activation energy of Zn atoms. The diffusion activation energy of an atom when doping with two atoms is higher
than that when doping one atom. This indicates that doping with Ag can inhibit the
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diffusion of Cu atoms and Zn atoms, especially that of Zn atoms. The effect of doping
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with two atoms was found to be more distinct than doping with one atom. In particular, the addition of the alloying element Ag can suppress the growth of the IMC
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interface. The experimental results were found to be in good agreement with the
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simulation results. The inhibition of IMC growth at the interface of the alloying element Ag can be mainly attributed to the increase in the diffusion activation energy
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of Cu atoms and Zn atoms in Cu5Zn8 due to doping with Ag.
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4. Conclusions
A Sn5Zn-xAg solder was prepared in this study. The influence of the alloying
element Ag on the formation and growth of intermetallic compounds in the Sn5Zn solder/copper interconnection interface was investigated, and the effects of doping with the alloying element Ag on the phase structure, phase stability, and diffusion activation energy of Cu and Zn atoms in Cu5Zn8-based unit cells were determined
based on first principles. The main conclusions are as follows: (1) After reflow, compared with the IMC of the Sn5Zn/Cu interface, the interfacial IMC thickness initially decreased and then increased in accordance with an increase in the doping amount of alloying elements in the solder. When the amount of alloying element added was 1 wt.%, the interfacial IMC thickness was the minimum.
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(2) With an increase in the isothermal aging time, the thickness of the IMC layer at the interface of each component gradually increased. The doping of the alloying
element Ag can therefore significantly decrease the IMC growth rate at the interface.
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(3) The Cu5Zn8 unit cell was geometrically optimized, and a stable Cu5Zn8
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structure was obtained. The lattice constants were a = b = c = 8.886 Å, α = β = γ = 90°, and the formation energy was −0.632 eV.
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(4) Compared with the Cu5Zn8 unit cell, the doping of Ag atoms decreased the
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formation energy of the Cu5Zn8-based unit cells and increased the stability. The Ag atoms tended to replace the Cu2 (12e) atoms, and the stable structure of the Ag-doped
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Cu5Zn8-based unit cell was determined. (5) Doping with Ag can increase the diffusion activation energy of several Cu
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atoms in Cu5Zn8-based unit cells, and therefore inhibit the diffusion of Cu atoms. Moreover, doping with Ag can effectively improve the diffusion activation energy of Zn atoms in Cu5Zn8-based unit cells. With an increase in the number of doped atoms, the diffusion activation energy of the Zn atoms increased.
Prime Novelty Statement
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The Sn5Zn-xAg solder was prepared in this paper. The influence of Ag on the formation and growth of intermetallic compounds in Sn5Zn solder/copper interconnect interface was studied. At the same time, the effects of alloying element Ag doping on the phase structure, phase stability and diffusion activation energy of Cu and Zn atoms in Cu5Zn8-based unit cells were calculated by first-principle. The article explains the experimental phenomena with simulation calculations. In particular, based on the correlation calculation of diffusion activation energy, it is concluded that the increase of diffusion activation energy of Cu and Zn atoms in Cu5Zn8 by Ag doping is the essential reason for the inhibition of IMC growth at the interface of Ag. This method of interpreting the growth of intermetallic compounds by simulation calculations is relatively novel.
Author information
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Author Contributions
All authors have approved the final version of the manuscript.
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Notes
Acknowledgements
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The authors declare no competing financial interest.
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The authors would like to acknowledge the support received from China Aerospace
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Science and Technology Innovation Fund (2016).
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