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Relationship between morphologies and orientations of Cu6Sn5 grains in Sn3.0Ag0.5Cu solder joints on different Cu pads
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
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Relationship between morphologies and orientations of Cu6Sn5 grains in Sn3.0Ag0.5Cu solder joints on different Cu pads Yanhong Tian⁎, Rui Zhang, Chunjin Hang, Lina Niu, Chunqing Wang State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
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Article history:
The morphologies and orientations of Cu6Sn5 intermetallic compounds in the Sn3.0Ag0.5Cu
Received 15 September 2013
solder joints both on polycrystalline and single crystal Cu pads under different peak reflow
Received in revised form 1 December
temperatures and times above liquids were investigated. The relationship between Cu6Sn5
2013
grain orientations and morphologies was clarified. At the interface of Sn3.0Ag0.5Cu/
Accepted 6 December 2013
polycrystalline Cu pad, scalloped Cu6Sn5 intermetallic compounds formed at 250 °C and roof shape Cu6Sn5 formed at 300 °C. Both scalloped Cu6Sn5 and roof shape Cu6Sn5 had a preferred orientation of {0001} plane being parallel to polycrystalline Cu pad surface.
Keywords:
Besides, the percentage of large angle grain boundaries increased as the peak reflow
Lead-free solder
temperature rose. At the interface of Sn3.0Ag0.5Cu/(111) single crystal Cu pad, the Cu6Sn5
Intermetallic compounds
intermetallic compounds were mainly scallop-type at 250 °C and were prism type at 300 °C.
Hollowed structure
The prismatic Cu6Sn5 grains grew along the three preferred directions with the inter-angles
Grain orientation
of 60° on (111) single crystal Cu pad while along two perpendicular directions on (100) single
Electron backscattered diffraction
crystal Cu pad. The orientation relationship between Cu6Sn5 grains and the single crystal Cu pads was investigated by electron backscatter diffraction technology. In addition, two types of hollowed Cu6Sn5 intermetallic compounds were found inside the joints of polycrystalline Cu pads. The long hexagonal Cu6Sn5 strips were observed in the joints reflowing at 250 °C while the hollowed Cu6Sn5 strips with the ‘ ’ shape cross-sections appeared at 300 °C, which was attributed to the different grain growth rates of different Cu6Sn5 crystal faces. © 2013 Elsevier Inc. All rights reserved.
1.
Introduction
The trend toward the miniaturization of electronic products leads to the shrinkage of joint size, resulting in a high volume fraction and a limited number of Cu6Sn5 intermetallic compounds (IMCs) generated at the Sn-based solders/Cu interface. The morphology and anisotropic properties of interfacial Cu6Sn5 grains can largely influence the mechanical characteristics and reliability of solder joints [1–5]. Therefore, an understanding of the relationship between morphology and orientation of Cu6Sn5 IMCs is necessary. Up to now, most of the reported Cu6Sn5 grains ⁎ Corresponding author. Tel.: +86 451 86418359; fax: +86 451 86416186. E-mail address: [email protected] (Y. Tian).
1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.12.006
at the interfaces of Sn-based lead-free solders/polycrystalline Cu pads are scallop, roof or prism shape [6–9]. Kumar et al. [10] reported that the Cu6Sn5 grains had no preferred orientation relationship with the polycrystalline Cu pads, while Gong et al. [8] and Li et al. [11] came to the opposite conclusion that the [0001] orientation of Cu6Sn5 tended to be parallel to Cu pad surface. Zou et al. [12,13] found that for a short time reflowing process, the Cu6Sn5 grains generated on (100) and (111) single crystal Cu pads appeared to be regular prism or roof shape, while on (123) single crystal Cu pad, the Cu6Sn5 grains were mainly scallop-type. With a long reflowing or aging process, the prismatic Cu6Sn5 at the joint interface would transform to be scallop-type. In addition, some studies showed that the roof-shape Cu6Sn5 grains on single crystal Cu pads arranged
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
with a preferred orientation in the early stage of reflowing, identified by transmission electron microscopy and electron backscatter diffraction (EBSD) methods [14–16]. Besides, previous research has revealed that the morphologies of Cu6Sn5 inside the solder joint were different from those at the joint interface. The tubular-shape Cu6Sn5 grains with the hollowed structure usually formed inside the solder joints [17,18]. Even though the formations of tubular-shape Cu6Sn5 grains were reported massively, the formation mechanism of the hollowed structure grains is not quite clear up to now. In this study, the morphologies and grain orientations of Cu6Sn5 generated at the interfaces of Sn3.0Ag0.5Cu/polycrystalline Cu pads and Sn3.0Ag0.5Cu/(111), (100) single crystal Cu pads during reflowing process were investigated respectively. Two types of long hollowed Cu6Sn5 grains with different cross-sectional shapes were found inside the solder joint. The relationship between the morphologies and grain orientations of interfacial Cu6Sn5 grains was also discussed. The formation mechanisms of the preferred oriented Cu6Sn5 at the joint interface and the hollowed Cu6Sn5 grains inside the solder joint were illustrated.
2.
Experimental Procedures
Polycrystalline and (111), (100) single crystal Cu sheets with the size of 2 × 3 × 1 mm3 were used as the joint pads and Sn3.0Ag0.5Cu solder balls with the diameter of 400 μm were applied as solder alloy. The used flux was the commercial NC-559-ASM type flux (AMTECH, USA). The peak reflow temperatures (PRTs) were 250 °C, 275 °C and 300 °C, and the times above liquids (TALs) were 10 s, 60 s and 300 s, respectively. After soldering, the samples were water-cooled to room temperature. Scanning electron microscopy samples were deep-etched by solution (10 vol.% HNO3 + 90 vol.% H2O) for 15 min to remove residual Sn and reveal three dimensional (3D) morphologies of Cu6Sn5 grains, which were observed on a Quanta 200FEG field emission electron microscope at 20 kV. To prepare the EBSD samples, the joints were mounted in epoxy resin, and then the top faces of the joints were carefully ground and polished. After that, a conductive layer of carbon with the thickness of 2 nm was sputtered onto the specimens to eliminate charging effect during the process of EBSD analysis. During the EBSD characterization, the specimens were tilted to 70° with respect to the horizontal and an EDAX/TSL OIM EBSD system attached to the field emission electron microscope, operated at 30 kV with a 14 mm working distance, was used. The diffraction pattern was imaged on a phosphor screen and captured by the CCD camera (Hikari EBSD Detector). The magnification of each mapping was 10,000, and the EBSD detector camera was at a 4 × 4 (160 × 120 pixels) binning setting, with an exposure time of 30 ms. Besides, standard background subtract function was adopted for EBSD pattern enhancement and a step size of 0.5 μm was chosen during the EBSD measurement. Afterwards, the TSL OIM analysis software version 5.31 was used to analyze the EBSD data. In order to remove isolated points that were not indexed correctly or at all due to dust particles or pits on the surface or at grain boundaries [19], single iteration grain dilation, a type of clean-up methods,
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with the parameters of grain tolerance being 5° misorientation and a minimum grain size of 2 pixels was used before further EBSD data analysis.
3.
Results and Discussion
3.1. Morphologies of Cu6Sn5 Forming on Polycrystalline Cu Pads Fig. 1 shows the interfacial Cu6Sn5 grains at the joints of polycrystalline Cu pads reflowing at 250 °C and 300 °C for 10 s, 60 s, and 300 s, respectively. When the PRT was 250 °C and TAL was 10 s, scalloped Cu6Sn5 grains formed on polycrystalline Cu pads (Fig. 1a). Then the grain coarsening phenomenon occurred when the TAL increased gradually (Fig. 1b, c). In contrast, the Cu6Sn5 grains forming at the PRT of 300 °C appeared to be roof shape, of which the inter-angle between two top neighboring crystal planes was 120° (Fig. 1f). The sizes of Cu6Sn5 grains forming at 300 °C also increased with longer TAL (Fig. 1d–f). The morphology transformation of Cu6Sn5 grains can be explained by the following formula: ΔG = VΔGV + Aγ, where ΔGV is free energy change per unit volume and γ stands for surface free energy of a specific area. The surface area (A) of scalloped Cu6Sn5 grain is smaller than that of roof shape Cu6Sn5 grain when same volume grain (V) formed, which means that the formation of scalloped Cu6Sn5 requires comparatively smaller nucleation energy (ΔG). Thus, scalloped Cu6Sn5 grains preferred to form at the nucleation stage with less input energy. With more input energy, roof shape Cu6Sn5 grains tended to form. Apart from the scalloped Cu6Sn5 grains forming at the joint interface, the hollowed hexagonal Cu6Sn5 grains were found inside the joint reflowing at the PRT of 250 °C for 300 s (Fig. 2a– c), which was consistent with the findings in Ref. [17]. In addition, another type of hollowed Cu6Sn5 grains formed inside the solder joint when the PRT increased to 300 °C (Fig. 2d–f). The cross-sectional shape of this type Cu6Sn5 grain was defined as ‘ ’ in this study. It is worth noting that the ‘ ’ shape of Cu6Sn5 grains were only observed inside the joint reflowing at 300 °C. 3D morphologies of these two types of hollowed Cu6Sn5 grains are shown in Fig. 3. It is known that η-Cu6Sn5 with hexagonal structure is normally stable above 186 °C but also can be retained at room temperature when the available time was insufficient for the solid-state transformation during soldering and subsequent cooling process [17,20–22]. According to the structure of η-Cu6Sn5, the side faces of the hexagonal prism shape Cu6Sn5 in Fig. 3b were determined to be 1010 crystal planes, and the axial direction was < 0001> crystal orientation.
3.2. Morphologies of Cu6Sn5 Forming on (111), (100) Single Crystal Cu Pads Figs. 4 and 5 present the interfacial Cu6Sn5 grains at the joints of (111) and (100) single crystal Cu pads, generated under different reflowing conditions. At the PRT of 250 °C for 60 s, Cu6Sn5 grains forming on a (111) single crystal Cu pad were mainly scallop-type and only a few Cu6Sn5 grains were prism-type (Fig. 4a). As the PRT increased to 275 °C, most of
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Fig. 1 – Interfacial Cu6Sn5 grains forming on polycrystalline Cu pads under different conditions: (a) 250 °C, 10 s, (b) 250 °C, 60 s, (c) 250 °C, 300 s, (d) 300 °C, 10 s, (e) 300 °C, 60 s and (f) 300 °C, 300 s.
Cu6Sn5 grains changed from scallop into prism shape (Fig. 4b). All Cu6Sn5 grains transformed into prism type when the PRT reached 300 °C and they extended along three directions with the angle of 60°, as shown in Fig. 4c. However, the prismatic
Cu6Sn5 grains forming on (100) single crystal Cu pads at 300 °C for the same time extended along two perpendicular directions (Fig. 5b). Meanwhile, Cu6Sn5 grain size increased over TAL, as shown in Figs. 4d–f and 5a–c. Based on the above
Fig. 2 – The joints of polycrystalline Cu pads reflowing at different temperatures for 300 s. (a) reflowed at 250 °C, (b) Cu6Sn5 inside the joint in (a), (c) local amplification of Cu6Sn5 in (b), (d) reflowed at 300 °C, (e) and (f) Cu6Sn5 inside the joint in (d).
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Fig. 3 – (a) Cu6Sn5 grains inside the joints reflowing at 250 °C for 300 s, (b) a hollowed hexagonal Cu6Sn5 in (a). Cu6Sn5 grains inside the joints reflowing at 300 °C for 300 s: (c) two grooves on the crystal surfaces, (d) back side of prismatic Cu6Sn5, (e) ‘∃’ shape on one side, (f) ‘ ’ shape. observations, it can be concluded that the influence of reflowing temperature on morphology of Cu6Sn5 was obviously more significant than that of TAL. Besides, the hollowed Cu6Sn5 grains were not observed inside the solder joints of (111) or (100) single crystal Cu pads.
3.3.
Grain Orientation of Interfacial Cu6Sn5
EBSD technology was employed to investigate the orientation relationship between Cu6Sn5 and Cu pads in solder joints. The harmonic series expansion technique, with the parameters of series rank being 16 and Gaussian half-width of 2°, was used to calculate orientation distribution functions (ODFs). In this study, the plane parallel to the Cu pad surface was defined as TD–RD plane, and the direction perpendicular to the Cu pad was defined as ND direction. In the inverse pole figure, the RD, TD, and ND directions represented the [100], [010], and [001] directions of the sample coordinate
respectively. The hexagonal structure η-Cu6Sn5 (a = 1.1022(5) nm, b = 0.7282(4) nm and c = 0.9827(2) nm) [22] was adopted during EBSD characterization. Fig. 6 shows the EBSD results of scalloped Cu6Sn5 in the solder joint of polycrystalline Cu pad after reflowing at 250 °C for 60 s. Fig. 6a presents SEM image of the EBSD analysis area. In the plane inverse pole figure (Fig. 6b), the same grain would have a same color. The indices of crystal plane parallel to TD–RD plane (Cu pad surface) could be obtained according to the color-coded map on the right. In order to present the EBSD results of Cu6Sn5 grains, which were our primary concerns, more clearly, a process of Cu6Sn5 phase data partition from the original database was implemented, resulting in black regions, where the EBSD data of other phases were absent, in Fig. 6b. Since most of Cu6Sn5 grains in Fig. 6a were indexed by red, yellow, or purple, they demonstrated a preferable orientation of Cu6Sn5 {0001} plane being parallel to TD–RD plane. In the (0001) pole figure (Fig. 6c), the red regions mainly located in the center area, which also
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Fig. 4 – Interfacial Cu6Sn5 grains forming on (111) single crystal Cu pads under different conditions: (a) 250 °C, 60 s, (b) 275 °C, 60 s, (c) 300 °C, 60 s, (d) 300 °C, 10 s, (e) 300 °C, 60 s and (f) 300 °C, 300 s.
meant that the {0001} plane of Cu6Sn5 tended to be parallel to the TD–RD plane. From the [001] inverse pole figure (as shown in Fig. 6d), it was found that the [0001] direction of Cu6Sn5 was paralleled to the ND direction with some offset, which coincided with the results of plane inverse pole figure (Fig. 6b) and (0001) pole figure (Fig. 6c). However, no obvious orientation relationship could be found in [010] or [100] inverse pole figures (not shown here). Fig. 7 shows the EBSD results of prismatic Cu6Sn5 grains generated on a polycrystalline Cu pad after reflowing at 300 °C for 60 s. The result was basically the same as that in Fig. 6. Namely, both scalloped and prismatic Cu6Sn5 forming at the Sn3.0Ag0.5Cu/polycrystalline Cu interface at different PRTs had a preferred orientation of Cu6Sn5 (0001) plane being parallel with Cu pad surfaces.
The misorientation angles of neighboring Cu6Sn5 grains in Figs. 6 and. 7 were shown in Fig. 8a and b, respectively. In order to avoid the negative effect of noise on the conclusion, only misorientation angles ranging from 2° to 90° were included in the calculation. For Cu6Sn5 grains forming at 250 °C, 62% grain boundaries were low angle grain boundaries (LAGBs) with a boundary misorientation of 2° to 15°, and the misorientation angles of high angle grain boundaries (HAGBs) uniformly distributed from 15° to 90° (Fig. 8a). On the contrary, for Cu6Sn5 grains forming at a relatively higher temperature (300 °C), LAGBs accounted for about 40% of Cu6Sn5 grain boundaries, and the percentage of HAGBs increased to 60% (Fig. 8b). As the interfacial energy of LAGB was lower than that of HAGB, neighboring Cu6Sn5 grains prefer to form LAGBs to reduce the
Fig. 5 – Interfacial Cu6Sn5 grains forming on (100) single crystal Cu pads under different conditions: (a) 300 °C, 10 s, (b) 300 °C, 60 s and (c) 300 °C, 300 s.
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Fig. 6 – SEM and EBSD results of interfacial scalloped Cu6Sn5 forming on a polycrystalline Cu pad at 250 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) (0001) pole figure and (d) [001] inverse pole figure.
nucleation energy when input energy was relatively low. With more input energy, the percentage of HAGBs would increase. Figs. 9–10 present the EBSD results of prismatic Cu6Sn5 forming on (111), (100) single crystal Cu pads at the PRT of 300 °C. Cu6Sn5 phase data partition was also processed in
Figs. 9–10 as mentioned before. In Figs. 9b and 10b, all the prismatic Cu6Sn5 grains were indexed by green color, which 10 crystal plane of Cu6Sn5 was parallel to implied that the 21 the Cu pad. This result was consistent with the conclusion derived from the (0001) pole figure and [001] inverse pole
Fig. 7 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on a polycrystalline Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) (0001) pole figure and (d) [001] inverse pole figure.
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Fig. 8 – Histograms of the misorientation angles of Cu6Sn5 grains forming at Sn3.0Ag0.5Cu/polycrystalline Cu interface. (a) Scalloped Cu6Sn5 at 250 °C and (b) prismatic Cu6Sn5 at 300 °C.
figure, as shown in Figs. 9c, d and 10c, d. Through the [010], pffiffiffi pffiffiffi 310 and − 310 inverse pole figures in Fig. 9d, it was found that the [0001] direction of prismatic Cu6Sn5 forming on (111) single crystal Cu pad was aligned to be parallel to the pffiffiffi pffiffiffi [010], 310 or − 310 direction of sample coordinate. For prismatic Cu6Sn5 forming on (100) single crystal Cu pad, the
[0001] direction was parallel to the [110] or [− 110] direction of sample coordinate (as shown in Fig. 10d). Since for (111), (100) single crystal Cu pads, the (111), (001) crystal planes were parallel to the TD–RD plane and the [110], [010] orientations were parallel to the TD direction ([010] direction of sample coordinate) respectively, the orientation
Fig. 9 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on (111) single crystal Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) pole figure and (d) inverse pole figure.
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Fig. 10 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on (100) single crystal Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) pole figure and (d) inverse pole figure.
relationships between prismatic Cu6Sn5 grains and (111), (100) single crystal Cu pads could be obtained as below:
structure Cu6Sn5 was chosen for analysis in Refs. [12] and [15].
3.4.
10 ‖ð111Þð111Þ Cu ; < 0001> Cu6 Sn5 ‖½110 ð111ÞCu ; 21 Cu6 Sn5
< 0001> Cu6 Sn5 ‖½011 ð111ÞCu ; < 0001> Cu6 Sn5 ‖½101 ð111ÞCu ; 10 21 ‖ð100Þð100Þ Cu ; < 0001> Cu6 Sn5 ‖½110 ð100ÞCu ; Cu6 Sn5 < 0001> Cu6 Sn5 ‖½110 ð100ÞCu :
The result was different from that in Refs. [12] and [15], as the monoclinic structure Cu6Sn5 instead of the hexagonal
Formation Mechanism of Hollowed Cu6Sn5
Fig. 2 shows the hollowed hexagonal Cu6Sn5 grains forming inside the solder joint and the mechanism can be explained by Bravais Law. As concentration profiles in front of the Cu/liquid interface during reflowing process are not necessarily uniform due to local differences in dissolution rates along polycrystalline Cu pad surface, and therefore there some areas
Fig. 11 – Schematic drawing of the formation process of a ‘ ’ shape Cu6Sn5 grain.
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where all the Cu atoms have not been used in the interfacial Cu6Sn5 reaction. These surplus Cu atoms can form relatively long Cu6Sn5 tubes [17]. However, the Cu dissolution rate along single crystal Cu pad surface was uniform, so there were not surplus Cu atoms to generate long Cu6Sn5 grains inside the joints of single crystal Cu pads. According to Bravais law, as the growth rate of a crystal face (thickness increase along the perpendicular direction of a crystal face in a specific time) has an inverse relationship with its surface density, the final face occurring on crystal tends to be parallel to the netplane which has the largest surface density. Surface density of a crystal can be defined by the formula: ε = N/A, where A is the area of a crystal face and N stands for the atomic quantity in this area. The surface atom densities of {0001} and 1010 crystal faces in a Cu6Sn5 grain were −2 −2 6.571 nm and 9.472 nm , respectively. The surface density of 1010 plane is 44% higher than that of {0001} plane. Thus, the growth rate of 1010 crystal face in longitudinal direction was larger than that in transversal directions. As a result, the long strips of hollowed Cu6Sn5 with the 1010 crystal faces as the outside faces were generated. The evolution process of the ‘ ’ shape Cu6Sn5 grains was illustrated in Fig. 11. The roof-shape Cu6Sn5 with the inter-angle of 120° firstly formed at the interface and then grew into the solder joint to be a prism type. Meantime, two grooves on both intersection surfaces occurred. With various Cu6Sn5 growth rates of different crystal faces, the grooves became deeper and longer along the longitudinal intersection. Then the ‘∃’ shape formed on one crystal face and the ‘ ’ shape appeared at the opposite side. Finally, the long hollowed Cu6Sn5 strip with a ‘ ’ shape cross section was generated inside the joint after longer reflowing process.
3.5. Relationship Between Cu6Sn5 Morphology and Orientation Fig. 12a–c presents the cross-sectional grain mappings of interfacial Cu6Sn5 forming on different kinds of Cu pads. The c axis of Cu6Sn5 grains forming on polycrystalline Cu pad tended to be perpendicular to Cu pad surface (Fig. 12a). While for Cu6Sn5 grains generated on (111), (100) single crystal Cu pads, the c axis was parallel to Cu pad surface. Fig. 13 shows the Cu atomic
spacing of Cu and Cu6Sn5 on different crystal planes. The misfit of Cu atoms between Cu and Cu6Sn5 grains can be expressed as: d¼
aCu Sn −aCu 5 6 aCu6 Sn5 þ aCu =2
ð1Þ
where aCu represents the atomic spacing of Cu and aCu6 Sn5 represents the Cu atomic spacing of Cu6Sn5. Based on Eq. (1), the misfit between Cu and Cu6Sn5 in different orientations can be obtained and the results are listed in Table 1. It was found that there was an extremely low misfit of Cu atoms between Cu <110> orientation and Cu6Sn5 <0001> orientation (only 1.46%). However, the misfits between Cu and Cu6Sn5 in other orientations were larger than 14%. As the interfacial energy will be reduced when the misfit between Cu6Sn5 and Cu pad decreases, to minimize the interfacial energy, Cu6Sn5 prefer to nucleate along the directions with small misfit. That is the reason why Cu6Sn5 forming on (111), (100) single crystal Cu pads extended along three preferred directions with a misorientation of 60° and two perpendicular directions respectively, as shown in Figs. 4 and 5.
4.
Conclusion
At the Sn3.0Ag0.5Cu/polycrystalline Cu joint interface, scalloped Cu6Sn5 grains formed at 250 °C, and roof shape Cu6Sn5 with the inter-angle of 120° formed at 300°C. EBSD results showed that these two types of Cu6Sn5 had a preferred orientation of Cu6Sn5 {0001} plane being parallel to polycrystalline Cu pad surface. In addition, the percentage of HAGBs increased with the PRT. At 250 °C, the Cu6Sn5 grains at the Sn3.0Ag0.5Cu/(111) single crystal Cu interface were mainly scallop-type. As the PRT increased, Cu6Sn5 changed from scallop into prism type. At 300 °C, the prismatic Cu6Sn5 forming on (111), (100) single crystal Cu pads extended along three preferred directions with a misorientation of 60° and two perpendicular directions respectively, due to the low misfit between <110> orientation of Cu and <0001> orientation of Cu6Sn5. Two different types of hollowed Cu6Sn5 grains were found inside the solder joints of polycrystalline Cu pads at different
Fig. 12 – Cross-sectional grain mappings of Cu6Sn5 forming on: (a) polycrystalline Cu pad, (b) (111) single crystal Cu pad and (c) (100) single crystal Cu pad.
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Fig. 13 – Cu atomic spacing on: (a) (001) plane of Cu, (b) (111) plane of Cu, (c) (0001) plane of Cu6Sn5, (d) 1010 plane of Cu6Sn5, (e) 10 plane of Cu6Sn5. 21
PRTs. Hollowed hexagonal Cu6Sn5 with the side face of plane formed at 250 °C. The formation of the 1010 hollowed Cu6Sn5 structure was attributed to the different Cu6Sn5 growth rates of different crystal planes. The long hollowed Cu6Sn5 strip with ‘ ’ shape cross section appeared inside the joint at 300 °C in this study. Compared with TAL, PRT has more significant influence on the morphology of Cu6Sn5.
Table 1 – The misfit of Cu atoms between Cu and Cu6Sn5 grains in different orientations. Cu6Sn5 〈0001〉 〈0001〉
1010
1010
2110
2110
Cu
Misfit
<100 > <110 > <100 > <110 > <100 > <110 >
35.75% 1.46% 67.05% 95.85% 14.78% 48.49%
Acknowledgments The authors are grateful for financial support from the National Science Foundation of China (Grant No. 51075103).
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Relationship between morphologies and orientations of Cu6Sn5 grains in Sn3.0Ag0.5Cu solder joints on different Cu pads Yanhong Tian⁎, Rui Zhang, Chunjin Hang, Lina Niu, Chunqing Wang State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
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Article history:
The morphologies and orientations of Cu6Sn5 intermetallic compounds in the Sn3.0Ag0.5Cu
Received 15 September 2013
solder joints both on polycrystalline and single crystal Cu pads under different peak reflow
Received in revised form 1 December
temperatures and times above liquids were investigated. The relationship between Cu6Sn5
2013
grain orientations and morphologies was clarified. At the interface of Sn3.0Ag0.5Cu/
Accepted 6 December 2013
polycrystalline Cu pad, scalloped Cu6Sn5 intermetallic compounds formed at 250 °C and roof shape Cu6Sn5 formed at 300 °C. Both scalloped Cu6Sn5 and roof shape Cu6Sn5 had a preferred orientation of {0001} plane being parallel to polycrystalline Cu pad surface.
Keywords:
Besides, the percentage of large angle grain boundaries increased as the peak reflow
Lead-free solder
temperature rose. At the interface of Sn3.0Ag0.5Cu/(111) single crystal Cu pad, the Cu6Sn5
Intermetallic compounds
intermetallic compounds were mainly scallop-type at 250 °C and were prism type at 300 °C.
Hollowed structure
The prismatic Cu6Sn5 grains grew along the three preferred directions with the inter-angles
Grain orientation
of 60° on (111) single crystal Cu pad while along two perpendicular directions on (100) single
Electron backscattered diffraction
crystal Cu pad. The orientation relationship between Cu6Sn5 grains and the single crystal Cu pads was investigated by electron backscatter diffraction technology. In addition, two types of hollowed Cu6Sn5 intermetallic compounds were found inside the joints of polycrystalline Cu pads. The long hexagonal Cu6Sn5 strips were observed in the joints reflowing at 250 °C while the hollowed Cu6Sn5 strips with the ‘ ’ shape cross-sections appeared at 300 °C, which was attributed to the different grain growth rates of different Cu6Sn5 crystal faces. © 2013 Elsevier Inc. All rights reserved.
1.
Introduction
The trend toward the miniaturization of electronic products leads to the shrinkage of joint size, resulting in a high volume fraction and a limited number of Cu6Sn5 intermetallic compounds (IMCs) generated at the Sn-based solders/Cu interface. The morphology and anisotropic properties of interfacial Cu6Sn5 grains can largely influence the mechanical characteristics and reliability of solder joints [1–5]. Therefore, an understanding of the relationship between morphology and orientation of Cu6Sn5 IMCs is necessary. Up to now, most of the reported Cu6Sn5 grains ⁎ Corresponding author. Tel.: +86 451 86418359; fax: +86 451 86416186. E-mail address: [email protected] (Y. Tian).
1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.12.006
at the interfaces of Sn-based lead-free solders/polycrystalline Cu pads are scallop, roof or prism shape [6–9]. Kumar et al. [10] reported that the Cu6Sn5 grains had no preferred orientation relationship with the polycrystalline Cu pads, while Gong et al. [8] and Li et al. [11] came to the opposite conclusion that the [0001] orientation of Cu6Sn5 tended to be parallel to Cu pad surface. Zou et al. [12,13] found that for a short time reflowing process, the Cu6Sn5 grains generated on (100) and (111) single crystal Cu pads appeared to be regular prism or roof shape, while on (123) single crystal Cu pad, the Cu6Sn5 grains were mainly scallop-type. With a long reflowing or aging process, the prismatic Cu6Sn5 at the joint interface would transform to be scallop-type. In addition, some studies showed that the roof-shape Cu6Sn5 grains on single crystal Cu pads arranged
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
with a preferred orientation in the early stage of reflowing, identified by transmission electron microscopy and electron backscatter diffraction (EBSD) methods [14–16]. Besides, previous research has revealed that the morphologies of Cu6Sn5 inside the solder joint were different from those at the joint interface. The tubular-shape Cu6Sn5 grains with the hollowed structure usually formed inside the solder joints [17,18]. Even though the formations of tubular-shape Cu6Sn5 grains were reported massively, the formation mechanism of the hollowed structure grains is not quite clear up to now. In this study, the morphologies and grain orientations of Cu6Sn5 generated at the interfaces of Sn3.0Ag0.5Cu/polycrystalline Cu pads and Sn3.0Ag0.5Cu/(111), (100) single crystal Cu pads during reflowing process were investigated respectively. Two types of long hollowed Cu6Sn5 grains with different cross-sectional shapes were found inside the solder joint. The relationship between the morphologies and grain orientations of interfacial Cu6Sn5 grains was also discussed. The formation mechanisms of the preferred oriented Cu6Sn5 at the joint interface and the hollowed Cu6Sn5 grains inside the solder joint were illustrated.
2.
Experimental Procedures
Polycrystalline and (111), (100) single crystal Cu sheets with the size of 2 × 3 × 1 mm3 were used as the joint pads and Sn3.0Ag0.5Cu solder balls with the diameter of 400 μm were applied as solder alloy. The used flux was the commercial NC-559-ASM type flux (AMTECH, USA). The peak reflow temperatures (PRTs) were 250 °C, 275 °C and 300 °C, and the times above liquids (TALs) were 10 s, 60 s and 300 s, respectively. After soldering, the samples were water-cooled to room temperature. Scanning electron microscopy samples were deep-etched by solution (10 vol.% HNO3 + 90 vol.% H2O) for 15 min to remove residual Sn and reveal three dimensional (3D) morphologies of Cu6Sn5 grains, which were observed on a Quanta 200FEG field emission electron microscope at 20 kV. To prepare the EBSD samples, the joints were mounted in epoxy resin, and then the top faces of the joints were carefully ground and polished. After that, a conductive layer of carbon with the thickness of 2 nm was sputtered onto the specimens to eliminate charging effect during the process of EBSD analysis. During the EBSD characterization, the specimens were tilted to 70° with respect to the horizontal and an EDAX/TSL OIM EBSD system attached to the field emission electron microscope, operated at 30 kV with a 14 mm working distance, was used. The diffraction pattern was imaged on a phosphor screen and captured by the CCD camera (Hikari EBSD Detector). The magnification of each mapping was 10,000, and the EBSD detector camera was at a 4 × 4 (160 × 120 pixels) binning setting, with an exposure time of 30 ms. Besides, standard background subtract function was adopted for EBSD pattern enhancement and a step size of 0.5 μm was chosen during the EBSD measurement. Afterwards, the TSL OIM analysis software version 5.31 was used to analyze the EBSD data. In order to remove isolated points that were not indexed correctly or at all due to dust particles or pits on the surface or at grain boundaries [19], single iteration grain dilation, a type of clean-up methods,
59
with the parameters of grain tolerance being 5° misorientation and a minimum grain size of 2 pixels was used before further EBSD data analysis.
3.
Results and Discussion
3.1. Morphologies of Cu6Sn5 Forming on Polycrystalline Cu Pads Fig. 1 shows the interfacial Cu6Sn5 grains at the joints of polycrystalline Cu pads reflowing at 250 °C and 300 °C for 10 s, 60 s, and 300 s, respectively. When the PRT was 250 °C and TAL was 10 s, scalloped Cu6Sn5 grains formed on polycrystalline Cu pads (Fig. 1a). Then the grain coarsening phenomenon occurred when the TAL increased gradually (Fig. 1b, c). In contrast, the Cu6Sn5 grains forming at the PRT of 300 °C appeared to be roof shape, of which the inter-angle between two top neighboring crystal planes was 120° (Fig. 1f). The sizes of Cu6Sn5 grains forming at 300 °C also increased with longer TAL (Fig. 1d–f). The morphology transformation of Cu6Sn5 grains can be explained by the following formula: ΔG = VΔGV + Aγ, where ΔGV is free energy change per unit volume and γ stands for surface free energy of a specific area. The surface area (A) of scalloped Cu6Sn5 grain is smaller than that of roof shape Cu6Sn5 grain when same volume grain (V) formed, which means that the formation of scalloped Cu6Sn5 requires comparatively smaller nucleation energy (ΔG). Thus, scalloped Cu6Sn5 grains preferred to form at the nucleation stage with less input energy. With more input energy, roof shape Cu6Sn5 grains tended to form. Apart from the scalloped Cu6Sn5 grains forming at the joint interface, the hollowed hexagonal Cu6Sn5 grains were found inside the joint reflowing at the PRT of 250 °C for 300 s (Fig. 2a– c), which was consistent with the findings in Ref. [17]. In addition, another type of hollowed Cu6Sn5 grains formed inside the solder joint when the PRT increased to 300 °C (Fig. 2d–f). The cross-sectional shape of this type Cu6Sn5 grain was defined as ‘ ’ in this study. It is worth noting that the ‘ ’ shape of Cu6Sn5 grains were only observed inside the joint reflowing at 300 °C. 3D morphologies of these two types of hollowed Cu6Sn5 grains are shown in Fig. 3. It is known that η-Cu6Sn5 with hexagonal structure is normally stable above 186 °C but also can be retained at room temperature when the available time was insufficient for the solid-state transformation during soldering and subsequent cooling process [17,20–22]. According to the structure of η-Cu6Sn5, the side faces of the hexagonal prism shape Cu6Sn5 in Fig. 3b were determined to be 1010 crystal planes, and the axial direction was < 0001> crystal orientation.
3.2. Morphologies of Cu6Sn5 Forming on (111), (100) Single Crystal Cu Pads Figs. 4 and 5 present the interfacial Cu6Sn5 grains at the joints of (111) and (100) single crystal Cu pads, generated under different reflowing conditions. At the PRT of 250 °C for 60 s, Cu6Sn5 grains forming on a (111) single crystal Cu pad were mainly scallop-type and only a few Cu6Sn5 grains were prism-type (Fig. 4a). As the PRT increased to 275 °C, most of
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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
Fig. 1 – Interfacial Cu6Sn5 grains forming on polycrystalline Cu pads under different conditions: (a) 250 °C, 10 s, (b) 250 °C, 60 s, (c) 250 °C, 300 s, (d) 300 °C, 10 s, (e) 300 °C, 60 s and (f) 300 °C, 300 s.
Cu6Sn5 grains changed from scallop into prism shape (Fig. 4b). All Cu6Sn5 grains transformed into prism type when the PRT reached 300 °C and they extended along three directions with the angle of 60°, as shown in Fig. 4c. However, the prismatic
Cu6Sn5 grains forming on (100) single crystal Cu pads at 300 °C for the same time extended along two perpendicular directions (Fig. 5b). Meanwhile, Cu6Sn5 grain size increased over TAL, as shown in Figs. 4d–f and 5a–c. Based on the above
Fig. 2 – The joints of polycrystalline Cu pads reflowing at different temperatures for 300 s. (a) reflowed at 250 °C, (b) Cu6Sn5 inside the joint in (a), (c) local amplification of Cu6Sn5 in (b), (d) reflowed at 300 °C, (e) and (f) Cu6Sn5 inside the joint in (d).
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
61
Fig. 3 – (a) Cu6Sn5 grains inside the joints reflowing at 250 °C for 300 s, (b) a hollowed hexagonal Cu6Sn5 in (a). Cu6Sn5 grains inside the joints reflowing at 300 °C for 300 s: (c) two grooves on the crystal surfaces, (d) back side of prismatic Cu6Sn5, (e) ‘∃’ shape on one side, (f) ‘ ’ shape. observations, it can be concluded that the influence of reflowing temperature on morphology of Cu6Sn5 was obviously more significant than that of TAL. Besides, the hollowed Cu6Sn5 grains were not observed inside the solder joints of (111) or (100) single crystal Cu pads.
3.3.
Grain Orientation of Interfacial Cu6Sn5
EBSD technology was employed to investigate the orientation relationship between Cu6Sn5 and Cu pads in solder joints. The harmonic series expansion technique, with the parameters of series rank being 16 and Gaussian half-width of 2°, was used to calculate orientation distribution functions (ODFs). In this study, the plane parallel to the Cu pad surface was defined as TD–RD plane, and the direction perpendicular to the Cu pad was defined as ND direction. In the inverse pole figure, the RD, TD, and ND directions represented the [100], [010], and [001] directions of the sample coordinate
respectively. The hexagonal structure η-Cu6Sn5 (a = 1.1022(5) nm, b = 0.7282(4) nm and c = 0.9827(2) nm) [22] was adopted during EBSD characterization. Fig. 6 shows the EBSD results of scalloped Cu6Sn5 in the solder joint of polycrystalline Cu pad after reflowing at 250 °C for 60 s. Fig. 6a presents SEM image of the EBSD analysis area. In the plane inverse pole figure (Fig. 6b), the same grain would have a same color. The indices of crystal plane parallel to TD–RD plane (Cu pad surface) could be obtained according to the color-coded map on the right. In order to present the EBSD results of Cu6Sn5 grains, which were our primary concerns, more clearly, a process of Cu6Sn5 phase data partition from the original database was implemented, resulting in black regions, where the EBSD data of other phases were absent, in Fig. 6b. Since most of Cu6Sn5 grains in Fig. 6a were indexed by red, yellow, or purple, they demonstrated a preferable orientation of Cu6Sn5 {0001} plane being parallel to TD–RD plane. In the (0001) pole figure (Fig. 6c), the red regions mainly located in the center area, which also
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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
Fig. 4 – Interfacial Cu6Sn5 grains forming on (111) single crystal Cu pads under different conditions: (a) 250 °C, 60 s, (b) 275 °C, 60 s, (c) 300 °C, 60 s, (d) 300 °C, 10 s, (e) 300 °C, 60 s and (f) 300 °C, 300 s.
meant that the {0001} plane of Cu6Sn5 tended to be parallel to the TD–RD plane. From the [001] inverse pole figure (as shown in Fig. 6d), it was found that the [0001] direction of Cu6Sn5 was paralleled to the ND direction with some offset, which coincided with the results of plane inverse pole figure (Fig. 6b) and (0001) pole figure (Fig. 6c). However, no obvious orientation relationship could be found in [010] or [100] inverse pole figures (not shown here). Fig. 7 shows the EBSD results of prismatic Cu6Sn5 grains generated on a polycrystalline Cu pad after reflowing at 300 °C for 60 s. The result was basically the same as that in Fig. 6. Namely, both scalloped and prismatic Cu6Sn5 forming at the Sn3.0Ag0.5Cu/polycrystalline Cu interface at different PRTs had a preferred orientation of Cu6Sn5 (0001) plane being parallel with Cu pad surfaces.
The misorientation angles of neighboring Cu6Sn5 grains in Figs. 6 and. 7 were shown in Fig. 8a and b, respectively. In order to avoid the negative effect of noise on the conclusion, only misorientation angles ranging from 2° to 90° were included in the calculation. For Cu6Sn5 grains forming at 250 °C, 62% grain boundaries were low angle grain boundaries (LAGBs) with a boundary misorientation of 2° to 15°, and the misorientation angles of high angle grain boundaries (HAGBs) uniformly distributed from 15° to 90° (Fig. 8a). On the contrary, for Cu6Sn5 grains forming at a relatively higher temperature (300 °C), LAGBs accounted for about 40% of Cu6Sn5 grain boundaries, and the percentage of HAGBs increased to 60% (Fig. 8b). As the interfacial energy of LAGB was lower than that of HAGB, neighboring Cu6Sn5 grains prefer to form LAGBs to reduce the
Fig. 5 – Interfacial Cu6Sn5 grains forming on (100) single crystal Cu pads under different conditions: (a) 300 °C, 10 s, (b) 300 °C, 60 s and (c) 300 °C, 300 s.
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
63
Fig. 6 – SEM and EBSD results of interfacial scalloped Cu6Sn5 forming on a polycrystalline Cu pad at 250 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) (0001) pole figure and (d) [001] inverse pole figure.
nucleation energy when input energy was relatively low. With more input energy, the percentage of HAGBs would increase. Figs. 9–10 present the EBSD results of prismatic Cu6Sn5 forming on (111), (100) single crystal Cu pads at the PRT of 300 °C. Cu6Sn5 phase data partition was also processed in
Figs. 9–10 as mentioned before. In Figs. 9b and 10b, all the prismatic Cu6Sn5 grains were indexed by green color, which 10 crystal plane of Cu6Sn5 was parallel to implied that the 21 the Cu pad. This result was consistent with the conclusion derived from the (0001) pole figure and [001] inverse pole
Fig. 7 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on a polycrystalline Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) (0001) pole figure and (d) [001] inverse pole figure.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
Fig. 8 – Histograms of the misorientation angles of Cu6Sn5 grains forming at Sn3.0Ag0.5Cu/polycrystalline Cu interface. (a) Scalloped Cu6Sn5 at 250 °C and (b) prismatic Cu6Sn5 at 300 °C.
figure, as shown in Figs. 9c, d and 10c, d. Through the [010], pffiffiffi pffiffiffi 310 and − 310 inverse pole figures in Fig. 9d, it was found that the [0001] direction of prismatic Cu6Sn5 forming on (111) single crystal Cu pad was aligned to be parallel to the pffiffiffi pffiffiffi [010], 310 or − 310 direction of sample coordinate. For prismatic Cu6Sn5 forming on (100) single crystal Cu pad, the
[0001] direction was parallel to the [110] or [− 110] direction of sample coordinate (as shown in Fig. 10d). Since for (111), (100) single crystal Cu pads, the (111), (001) crystal planes were parallel to the TD–RD plane and the [110], [010] orientations were parallel to the TD direction ([010] direction of sample coordinate) respectively, the orientation
Fig. 9 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on (111) single crystal Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) pole figure and (d) inverse pole figure.
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
65
Fig. 10 – SEM and EBSD results of interfacial prismatic Cu6Sn5 forming on (100) single crystal Cu pad at 300 °C for 60 s. (a) SEM, (b) plane inverse pole figure, (c) pole figure and (d) inverse pole figure.
relationships between prismatic Cu6Sn5 grains and (111), (100) single crystal Cu pads could be obtained as below:
structure Cu6Sn5 was chosen for analysis in Refs. [12] and [15].
3.4.
10 ‖ð111Þð111Þ Cu ; < 0001> Cu6 Sn5 ‖½110 ð111ÞCu ; 21 Cu6 Sn5
< 0001> Cu6 Sn5 ‖½011 ð111ÞCu ; < 0001> Cu6 Sn5 ‖½101 ð111ÞCu ; 10 21 ‖ð100Þð100Þ Cu ; < 0001> Cu6 Sn5 ‖½110 ð100ÞCu ; Cu6 Sn5 < 0001> Cu6 Sn5 ‖½110 ð100ÞCu :
The result was different from that in Refs. [12] and [15], as the monoclinic structure Cu6Sn5 instead of the hexagonal
Formation Mechanism of Hollowed Cu6Sn5
Fig. 2 shows the hollowed hexagonal Cu6Sn5 grains forming inside the solder joint and the mechanism can be explained by Bravais Law. As concentration profiles in front of the Cu/liquid interface during reflowing process are not necessarily uniform due to local differences in dissolution rates along polycrystalline Cu pad surface, and therefore there some areas
Fig. 11 – Schematic drawing of the formation process of a ‘ ’ shape Cu6Sn5 grain.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
where all the Cu atoms have not been used in the interfacial Cu6Sn5 reaction. These surplus Cu atoms can form relatively long Cu6Sn5 tubes [17]. However, the Cu dissolution rate along single crystal Cu pad surface was uniform, so there were not surplus Cu atoms to generate long Cu6Sn5 grains inside the joints of single crystal Cu pads. According to Bravais law, as the growth rate of a crystal face (thickness increase along the perpendicular direction of a crystal face in a specific time) has an inverse relationship with its surface density, the final face occurring on crystal tends to be parallel to the netplane which has the largest surface density. Surface density of a crystal can be defined by the formula: ε = N/A, where A is the area of a crystal face and N stands for the atomic quantity in this area. The surface atom densities of {0001} and 1010 crystal faces in a Cu6Sn5 grain were −2 −2 6.571 nm and 9.472 nm , respectively. The surface density of 1010 plane is 44% higher than that of {0001} plane. Thus, the growth rate of 1010 crystal face in longitudinal direction was larger than that in transversal directions. As a result, the long strips of hollowed Cu6Sn5 with the 1010 crystal faces as the outside faces were generated. The evolution process of the ‘ ’ shape Cu6Sn5 grains was illustrated in Fig. 11. The roof-shape Cu6Sn5 with the inter-angle of 120° firstly formed at the interface and then grew into the solder joint to be a prism type. Meantime, two grooves on both intersection surfaces occurred. With various Cu6Sn5 growth rates of different crystal faces, the grooves became deeper and longer along the longitudinal intersection. Then the ‘∃’ shape formed on one crystal face and the ‘ ’ shape appeared at the opposite side. Finally, the long hollowed Cu6Sn5 strip with a ‘ ’ shape cross section was generated inside the joint after longer reflowing process.
3.5. Relationship Between Cu6Sn5 Morphology and Orientation Fig. 12a–c presents the cross-sectional grain mappings of interfacial Cu6Sn5 forming on different kinds of Cu pads. The c axis of Cu6Sn5 grains forming on polycrystalline Cu pad tended to be perpendicular to Cu pad surface (Fig. 12a). While for Cu6Sn5 grains generated on (111), (100) single crystal Cu pads, the c axis was parallel to Cu pad surface. Fig. 13 shows the Cu atomic
spacing of Cu and Cu6Sn5 on different crystal planes. The misfit of Cu atoms between Cu and Cu6Sn5 grains can be expressed as: d¼
aCu Sn −aCu 5 6 aCu6 Sn5 þ aCu =2
ð1Þ
where aCu represents the atomic spacing of Cu and aCu6 Sn5 represents the Cu atomic spacing of Cu6Sn5. Based on Eq. (1), the misfit between Cu and Cu6Sn5 in different orientations can be obtained and the results are listed in Table 1. It was found that there was an extremely low misfit of Cu atoms between Cu <110> orientation and Cu6Sn5 <0001> orientation (only 1.46%). However, the misfits between Cu and Cu6Sn5 in other orientations were larger than 14%. As the interfacial energy will be reduced when the misfit between Cu6Sn5 and Cu pad decreases, to minimize the interfacial energy, Cu6Sn5 prefer to nucleate along the directions with small misfit. That is the reason why Cu6Sn5 forming on (111), (100) single crystal Cu pads extended along three preferred directions with a misorientation of 60° and two perpendicular directions respectively, as shown in Figs. 4 and 5.
4.
Conclusion
At the Sn3.0Ag0.5Cu/polycrystalline Cu joint interface, scalloped Cu6Sn5 grains formed at 250 °C, and roof shape Cu6Sn5 with the inter-angle of 120° formed at 300°C. EBSD results showed that these two types of Cu6Sn5 had a preferred orientation of Cu6Sn5 {0001} plane being parallel to polycrystalline Cu pad surface. In addition, the percentage of HAGBs increased with the PRT. At 250 °C, the Cu6Sn5 grains at the Sn3.0Ag0.5Cu/(111) single crystal Cu interface were mainly scallop-type. As the PRT increased, Cu6Sn5 changed from scallop into prism type. At 300 °C, the prismatic Cu6Sn5 forming on (111), (100) single crystal Cu pads extended along three preferred directions with a misorientation of 60° and two perpendicular directions respectively, due to the low misfit between <110> orientation of Cu and <0001> orientation of Cu6Sn5. Two different types of hollowed Cu6Sn5 grains were found inside the solder joints of polycrystalline Cu pads at different
Fig. 12 – Cross-sectional grain mappings of Cu6Sn5 forming on: (a) polycrystalline Cu pad, (b) (111) single crystal Cu pad and (c) (100) single crystal Cu pad.
MA TE RI A L S CH A R A CT ER IZ A TI O N 8 8 (2 0 1 4) 5 8–6 8
67
Fig. 13 – Cu atomic spacing on: (a) (001) plane of Cu, (b) (111) plane of Cu, (c) (0001) plane of Cu6Sn5, (d) 1010 plane of Cu6Sn5, (e) 10 plane of Cu6Sn5. 21
PRTs. Hollowed hexagonal Cu6Sn5 with the side face of plane formed at 250 °C. The formation of the 1010 hollowed Cu6Sn5 structure was attributed to the different Cu6Sn5 growth rates of different crystal planes. The long hollowed Cu6Sn5 strip with ‘ ’ shape cross section appeared inside the joint at 300 °C in this study. Compared with TAL, PRT has more significant influence on the morphology of Cu6Sn5.
Table 1 – The misfit of Cu atoms between Cu and Cu6Sn5 grains in different orientations. Cu6Sn5 〈0001〉 〈0001〉
1010
1010
2110
2110
Cu
Misfit
<100 > <110 > <100 > <110 > <100 > <110 >
35.75% 1.46% 67.05% 95.85% 14.78% 48.49%
Acknowledgments The authors are grateful for financial support from the National Science Foundation of China (Grant No. 51075103).
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