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Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder
MR-12188; No of Pages 10 Microelectronics Reliability xxx (2016) xxx–xxx
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Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder M.I.I. Ramli ⁎, N. Saud, M.A.A. Mohd Salleh, M.N. Derman, R. Mohd Said Center of Excellence Geopolymer & Green Technology (CeGeoGTech), School of Materials Engineering, Universiti Malaysia Perlis (UniMAP), Taman Muhibbah, 02600 Arau, Perlis, Malaysia
a r t i c l e
i n f o
Article history: Received 30 March 2016 Received in revised form 31 July 2016 Accepted 19 August 2016 Available online xxxx Keywords: Soldering alloy Intermetallic Alloys Powder metallurgy Fracture
a b s t r a c t The effect of TiO2 addition on the microstructure, melting behavior, microhardness and interfacial reaction between Sn-0.7Cu-0.05Ni and a Cu-substrate were explored. Samples with various TiO2 percentages (0, 0.25, 0.5, 0.75 and 1.0 wt.%) were prepared using a microwave-assisted sintering powder metallurgy method. Microstructural analysis reveals that TiO2 was uniformly distributed along the grain boundary of the bulk solder. Differential scanning calorimetry (DSC) results showed a decrease in the undercooling while melting temperature of the solder slightly increase. The thickness of the interfacial intermetallic compounds of the solder joint was reduced with the addition of TiO2 particles. This thickness reduction indicates that the presence of a small amount of TiO2 particles is effective in suppressing the growth of the intermetallic compound layer. Small dimples on the fracture surface have revealed that the Sn-Cu-Ni composite solder exhibits typical ductile failure. Overall, the addition of TiO2 to Sn-0.7Cu-0.05Ni solder dramatically increases its shear strength and hardness and improves its wetting properties and fracture surface. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Conventional Tin-lead (Sn-Pb) eutectic alloy systems have long been widely used in soldering applications in advanced electronic components. However, due to health and environmental concerns regarding the high toxicity of lead, European Union (EU) Waste of Electrical and Electronic Equipment (WEEE) and RoHS (Restriction of Hazardous Substances) legislation strictly prohibit the use of Pb in most electronic components [1,2]. Traditional lead solder alloy has been replaced with many lead-free solder alloys, including Sn-Bi [3], Sn-Bi-Ag [4], Bi-In-Sn [5], Sn-Zn [6], Sn-Zn-Bi [6], Sn-Ag-Cu [7,8], Sn-Cu [1,9], and In-48Sn [10]. Sn-Ag and Sn-Cu specifically well known for wave soldering [11] and commonly used as a replacement for the conventional Sn-Pb. However, due to its high silver content, Sn-Ag solder is more expensive than Sn-Cu solder alloys. Currently, Sn-Cu based solder is considered the cheapest leadfree solder candidate, it has received much attention as the lead-free solder of choice [12–14]. However, Sn-Cu has been reported to exhibit somewhat inferior mechanical properties and wettability compared to Ag-containing lead-free solder alloys [15]. Therefore, further improvements in Sn-Cu solders are needed. Several researchers [16] have reported that the addition of nickel has resulted in an improvement in wetting and solder joint reliability. This formulation (Sn-0.7Cu-0.05Ni) has been discovered by Nihon Superior Co Ltd. and has been commercialized as SN100C. SN100C would appear to be a real challenger, with a good compromise between cost and quality. However, due to high ⁎ Corresponding author. E-mail address: [email protected] (M.I.I. Ramli).
demands for better solder joint performance of lead-free solder, a viable way to enhance the performance of a solder is by introducing ceramic particles into the solder matrix [17]. Numerous studies have revealed that the addition of foreign particles provides a marked improvement in microstructural and mechanical properties [7,18–21]. Tsao et al. [22,23] has investigated the influence of reinforcing TiO2 and Al2O3 nanoparticles on the microstructural development and hardness of Sn-Ag-Cu systems. The microhardness measurements revealed that the addition of TiO2 and Al2O3 nanoparticles is helpful in enhancing the overall strength of the eutectic solder. Nai et al. [17] successfully synthesized Sn-3.5Ag-0.7Cu using titanium diboride (TiB2), multi-walled carbon nanotubes (MWCNTs) and ZrO2 nanoparticles via the powder metallurgical technique and found that the mechanical properties of the composite solders showed an overall improvement. However, the limitation of ceramic particles into solder is (i) homogeneous distribution of ceramic particles in a solder matrix is hard to be achieved, (ii) a high amount of ceramic particles may increase the brittleness of solder and (iii) ceramic additions do not normally stay in the solder during reflow, and are almost completely expelled with flux [11]. Powder metallurgy is the most common method used in developing composite solders [17,24–27]. The material in form of a powder is compacted as a green body and sintered to a net shape at elevated temperature. However, there are challenging demands from the powder metallurgy industry for new and improved sintering processes. This is where microwave technology is found to be advantageous. Microwave-assisted sintering provides finer microstructures and thus enhanced physical and mechanical properties [27,28]. Recently, it has been found that microwave sintering can also be applied efficiently
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Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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and effectively to powdered metals [29]. Y. Tang et al. [30] studied the effect of TiO2 addition on the microstructure and microhardness of SnAg-Cu and revealed that the microhardness of the composite solder is higher than that of the monolithic solder. Salleh et al. [2] added Si3N4 nanoparticles to Sn0.7Cu solder as a reinforcement, yielding improvements in microhardness and shear strength using microwave sintering. During the soldering process, a metallurgical reaction between the liquid solder and the copper substrate causes an IMC layer to form at the joint interface [32]. The formation of a thicker IMC layer affects the mechanical reliability of the solder joint, which weakens it due to the brittle nature of the IMCs. Decreasing the thickness of the IMC layer formed at the interface can increase the strength of the solder joint [7]. Leong et al. [8] reported that the addition of a small amount of TiO2 to the Sn-Ag-Cu solder can suppress the growth of the intermetallic compound layer. For the above reason, this present study concentrated to reveal the effect of TiO2 addition on Sn-0.7Cu-0.05Ni solder alloy by using powder metallurgy technique. The effect on microstructure evolution, melting property and wettability were investigated comprehensively. The interfacial reaction behavior and interfacial fracture surface on post mortem samples were analyzed using scanning electron microscope (SEM). The hardness and shear strength of composite solder were tested and compared with the Sn-0.7Cu-0.05Ni solder alloy.
2. Experimental procedures The base material, Sn-0.7Cu-0.05Ni solder powder with an average size 23.3–43.2 μm, obtained from Nihon Superior Co Ltd., Japan, was used. The solder alloy were mixed with TiO2 particle with average particle size 1.7– 19.8 μm by using powder metallurgy technique. These composite materials were mixed with five different percentages (0, 0.25, 0.5, 0.75 and 1.0 wt.%) of micron-size TiO2 in an airtight container using a roller blender. The blended mixtures were divided into samples of approximately 1 g using a weighing balance for each compact pellet. Each of the solder mixtures was pressed using a Specac 15-ton manual hydraulic press and uniaxially compacted in a 12-mm diameter mold. The compacted pellets were then sintered by microwave sintering at 185 °C under ambient conditions in an 800 W, 50 Hz Panasonic microwave oven. Silicon carbide (SiC) was used as a microwave susceptor material [28]. The samples were then reflowed onto Cu-substrates in a F4N reflow oven following the reflow profile for lead free solder Sn-0.7Cu as shown in Fig. 1. No clean mildly activated rosin flux from ASAHI Flux Medium
θ Cu-Pad
Fig. 2. Contact angle (θ) of Sn-0.7Cu-0.05Ni on copper substrate.
was applied prior to reflow soldering to act as a cleaning agent to remove and prevent the formation of oxides. The reflowed sample were mounted in epoxy resin, grinded, polished and finally etched with a solution of 93% methanol + 5%HNO3 + 2% HCl for clear grain structure observation. During solder reflow, there are probability of small amount of TiO2 particles were expelled in the flux as reported by Kotadia et al. [11, 33,34]. The microstructure was investigated using a scanning electron microscope (model JEOL JSM-6460LA SEM). A high-resolution scanning electron microscope equipped with an energy dispersive X-ray spectroscopy analysis system was used to inspect and analyze the microstructures of the five different compositions of composite solder and to perform the semi-quantitative analysis of the detected elements. The morphology of the intermetallic compound and the thickness of the IMC were observed by scanning electron microscopy (SEM) and evaluated using J-image analysis software. The IMC thickness measurement was carried out by dividing the IMC area (A) by its length (L) as illustrated in Fig. 3. The average thickness of IMC layer at various positions of the interface was calculated in 10 images. X-ray diffraction (XRD) was also used to inspect and analyze the phase formation of the composite solder. The phase analysis study was conducted using an X-ray diffractometer (XRD, D2 Phaser Bruker).
Solder
Fig. 1. Reflow profile used in the F4N reflow oven.
Fig. 3. Area of IMC layer at the solder joint interface for average thickness measurements.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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The pellet samples were exposed to Cu Kα radiation with a 20°–80° diffraction scan range with a step size of 0.02°. The melting temperature characteristics of the solder were analyzed using differential scanning calorimetry (DSC). A total 10 mg of solder was placed into an aluminum pan and scanned to 250 °C at a rate of 10 °C/min under a nitrogen atmosphere. The micro hardness test was conducted using a Vickers micro hardness test machine operated according to the ASTM B933-09 standard test method with a 1 kgf indenting load for a 10 s dwell time. Five points for each composition will be used to calculate the accuracy of the average results. Coefficients of thermal expansion (CTE) of the solder material were determined using a dilatometer with 5 °C/min heating rate. The displacement of the sample (each 5 mm) as a function of temperature (50–150 °C) was measured using an alumina probe under an argon atmosphere. The contact angle was measured by using J-image analysis software, as shown in Fig. 2. A shear test was conducted using an Instron Machine, and the specification of the substrate is based on ASTM D1002, which is a 101.6 mm × 25.4 mm × 1.5 mm Cu-substrate. The fractography of the solder joint after the shear test was also observed using SEM. 3. Result and discussion 3.1. Microstructure and phase analysis To observe the homogeneity of the TiO2 inside the solder matrix, the etched Sn-0.7Cu-0.05Ni-xTiO2 composite solder was examined by SEM. Fig. 4(a–d), has shown that the TiO2 particles were distributed on the surface of the solder matrix along the grain boundary of the bulk solder.
a)
3
Results indicated that particles may agglomerate with the increase of reinforcement percentage. As expected, the appearance of TiO2 particles on the surface of the solder matrix increased with the weight percentage of TiO2, as seen in Fig. 4. The grain boundary can be observed clearly in Fig. 4, and the microstructure consists of eutectic phases, reinforcement particles and primary phases. As indicated by the scanning elemental maps of the solder with 0.75 wt.% TiO2 (Fig. 5), it can be seen that the Ti element is segregated on the surface along the grain boundaries. Fig. 6 shows the solidified microstructure of the Sn-0.7Cu-0.05Ni and composite solder alloys. As seen in Fig. 4a, Sn-0.7Cu-0.05Ni is the neareutectic alloy, and it contains a large number of primary β-Sn dendrites and a small intermetallic compound Cu6Sn5. Interestingly, with a minor addition of TiO2, the grain size of β-Sn was refined compared to Sn0.7Cu-0.05Ni solder. On the other hand, the eutectic area has increased and distributed uniformly. This phenomenon was attributed to the unique properties of the active element, TiO2. According to the adsorption theory of the surface active material, the increase in the adsorption of active elements could decrease the surface energy and thereby decrease the growth and size of the grains. The presence of TiO2 particles can act as obstacles to diffusion, such as of Cu and Sn atoms, which are responsible for the growth of the intermetallic [35]. It also can be seen that with the addition of reinforcement particles, the amount of the Cu6Sn5 IMC slightly increased. Salleh et al. [36,37] had found with Synchrotron imaging that TiO2 may act as nucleation sites and increase the numbers of Cu6Sn5. Apart from that, TiO2 particles can also act as heterogeneous nucleation sites [2]. The TiO2 particles are dispersed in the molten solder and then precipitate on top of the Cu substrate during
b) Grain boundaries
Ti
Sn-rich
c)
Ti
d) Ti Ti
Ti
Sn-rich
Sn-rich
Fig. 4. SEM micrographs of the Sn-0.7Cu-0.05Ni lead-free solder with a) 0.25 wt.%, b) 0.50 wt.%, c) 0.75 wt.%, and d) 1.0 wt.% TiO2 reinforcement additions.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Eutectic phase
Reinforcement phase Primary phase
Fig. 5. Scanning electron microscope micrographs and elemental maps of the Sn-0.7Cu-0.05Ni solder with 0.75 wt.% TiO2 reinforcement addition.
a)
b) β -Sn
Cu6Sn5 Cu6Sn5
Fig. 6. Fully solidified microstructure of (a) Sn-0.7Cu-0.05Ni and (b) Sn-0.7Cu-0.05Ni-1.0% TiO2.
4.0 3.80 3.48
Thickness (μm)
soldering. The Cu6Sn5 IMCs will nucleate on the reinforcement surface to reduce the thermodynamic barrier. The reduction of the thermodynamic barrier then results in a decreased surface energy and growth velocity of the Cu6Sn5 grains [38]. The microstructure of the Sn-0.7Cu-0.05Ni-1.0 wt.% TiO2 composite solder (Fig. 6) becomes finer due to the presence of reinforcement particles hindering grain growth. Previous research by Tang et al. [30] reported that the addition of a trace amount of TiO2 nanoparticles can influence the microstructure of Sn-3.0Ag-0.5Cu solder, and the size and spacing of the Ag3Sn decreases significantly. The same phenomenon was also demonstrated by Chuang et al. [39] when Ti is added into Sn3.5Ag0.5Cu solder. They stated that the grain size of the β-Sn phase and the size of the IMCs could be refined to obtain a more stable microstructure and better mechanical properties. Therefore, the refinement of the microstructure due to TiO2 addition could improve the mechanical properties of Sn-0.7Cu-0.05Ni solder. Fig. 7 present the thickness of the IMC layer of Sn-0.7Cu-0.05Ni composite solder versus the wt.% of TiO2 particles. The IMC layer thickness of the composite solder joint decreased with the increased TiO2 particle addition. The thinnest IMC layer was achieved with the addition of
3.5 3.19
3.0
2.83 2.68
2.5
0
0.25% 0.50% 0.75% 1.0%
TiO2 (wt.%) Fig. 7. IMC layer thickness of Sn-0.7Cu-0.05Ni composite solder joint.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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1.0 wt.% of TiO2 Fig. 8. Additionally, the IMC layer thickness for the composite solder was smaller compared to that of the Sn-0.7Cu-0.05Ni solder. This phenomenon suggests that the addition of TiO2 particles could suppress the IMC layer growth. During the reflowing process of the composite solder, the TiO2 particles do not melt due to the high melting temperature of TiO2 (1843 °C), causing them to be absorbed in the liquid. The presence of TiO2 particles can act as an obstacle to the diffusion of atoms such as Cu atoms from the substrate. The formation of the IMC layer is mainly due to the diffusion of atoms from the substrate into the solder matrix. The TiO2 particles are believed to suppress the diffusion process, as they are distributed in the diffusion path of the Cu atoms. The SEM-EDX mapping results in Fig. 9 and Table 1 prove the presence of TiO2 particles distributed in the diffusion path that leads to the growth suppression of the IMC layer. Similar to previous findings, TiO2 may also remain attached on the IMC gloves and subsequently hinders the dissolution and diffusion of Cu from the substrate during soldering [37]. In addition, TiO2 particles may act as a heterogeneous nucleating agent that can affect the formation of the IMC layer [40]. The changes in the IMC layer morphology are considered as preferred changes because the elongated scallop shape induces brittle fracturing at the interface leading to cracking and the eventually decrease in the solder joint reliability. A similar result was reported by Leong [8], who found that the addition of TiO2 particles reduced the Sn3.5Ag0.5Cu IMC layer thickness. However, a contrasting phenomenon occurred when Leong [8] increased the addition of TiO2 up to 1.25 wt.%. Nai et al. [41] also reported the suppressor effect of CNTs on the growth of the IMC layer between the SAC composite solder and the Cu-substrate after aging. The X-ray diffraction results corresponding to the Sn-0.7Cu-0.05NixTiO2 composite samples are shown in Fig. 10. The results reveal that the main peaks in Fig. 10 are indexed by β-Sn and Cu6Sn5 in the composite solder. It was found that with the increasing amount of TiO2, there was some alteration and modification of Cu6Sn5 peak height when. However, the TiO2 phase was not detected in the XRD pattern of the Sn-0.7Cu-0.05Ni alloys (Fig. 10b–d). It is believed that the 1.0 wt.% TiO2 addition in the composite solder is too small to be detected. A similar phenomenon was encountered by Billah et al. [42], who could not detect a small amount of 1% Ni rich phase additions in Sn8Zn-3Bi with XRD. 3.2. Thermal analysis The melting temperature is a critical solder characteristic because it determines the soldering temperature. A higher melting temperature
5
Fig. 9. EDX analysis of IMC layer of Sn-0.7Cu-0.05Ni-TiO2.
will lead to a higher soldering temperature, which could cause damage to the electronic components in the packaging. Fig. 11 shows the DSC curves of the composite solder reinforced with different wt.% of TiO2 particles. Lead-free Sn-0.7Cu-0.05Ni solder shows a near-eutectic alloy with a melting point of 226.5 °C. The DSC results for the Sn-0.7Cu0.05Ni-xTiO2 composite are summarized in Table 2. The slight increases in the melting point of the composite solder could be influenced by the addition of TiO2. However, the slight changes in the melting point will not affect the typical soldering process for Sn-0.7Cu-0.05Ni solder. The same phenomenon has been found in SAC-(Ni-CNTs) [43] and Sn3.7Ag-0.9Zn/SiC [44]. Those studies revealed that no adjustment to the soldering process is needed if the change in the melting temperature of the newly fabricated solder is less than 1.0 °C. The reinforcement addition could modify the surface instability and the variation in the physical properties and the grain boundary/interfacial characteristics [45]. Apart from that, the degree of undercooling is also important in the solidification process of the solder alloy. The undercooling is defined as the temperature difference between the melting temperature of the solder during heating and the solidification temperature during cooling. The degree of undercooling is the one of the factors that affects the growth of IMCs. Table 2 shows that the undercooling of Sn-0.7Cu-0.05Ni solder is significantly reduced from 22.85 °C to 18.38 °C with the addition of TiO2 particles.
b)
a)
Cu6Sn5 Cu3Sn
Cu
Fig. 8. Backscattered images of intermetallic compounds formed at the interfaces of the as-reflowed solder joints (a) Sn-0.7Cu-0.05Ni and (b) Sn-0.7Cu-0.05Ni-1.0% TiO2.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Table 1 EDX analysis results of Sn-0.7Cu-0.05Ni-1.0TiO2.
Line 001 (10 lines) Line 002 (10 lines) Area 003 (5 area)
Element
Average atomic (%)
Standard deviation
Sn Cu Ti Sn Cu Ti Sn Cu Ti
35.87 64.13 nd 47.06 52.94 nd 86.67 12.95 0.38
1.095 2.339 nd 1.356 1.305 nd 1.822 0.937 0.073
This is because the undercooling phenomenon is related to the difficulty of nucleating a solid phase in the liquid state. The larger the undercooling is, the larger the driving force for the growth of IMCs will be [45]. The large amount of undercooling in the solder can influence its microstructure by changing its solidification behavior. In general, a faster cooling rate would produce a finer microstructure and also a greater hardness [9]. This undercooling will also reduce the time for IMCs to grow. These phenomena are also proposed by Chen et al. [9], who stated that a minor Ti addition effectively reduces the amount of undercooling of Sn-rich solders. 3.3. Wettability Fig. 12 shows the contact angle as a function of the TiO2 addition to the Sn-0.7Cu-0.05Ni solder. A decreasing trend of the contact angle was observed as the wt.% of the TiO2 addition increased. A minimum contact angle of 22.38° was observed with a 1.0 wt.% TiO2 addition. The results
show that a small amount of reinforcement in the Sn-0.7Cu-0.05Ni composite solder decreases the contact angle and thereby improves the wettability. The lower the contact angle of the solder on the Cu substrate, the better wettability of the solder joint [46]. These improvements can be attributed to the effect of the TiO2 in the molten solder, probably acting as an agent to reduce the surface tension of the alloy and increases the wettability between Cu substrate [21]. This can be observed with the decrease in contact angle as indicated in Fig. 12. It can also be explained based on the theory of the absorption of surface-active materials (TiO2 particles) on wetting [21]. The change in wetting angle could also be due to the segregation of TiO2 particles near the wetting triple point (Fig. 13). Similar results were also attained by Mohd Salleh et al. [2], who found that an improvement in the wettability of the Sn-0.7Cu/Si3N4 composite lead-free solder to the optimal level was achieved by the addition of 1.0 wt.% Si3N4. 3.4. Mechanical properties 3.4.1. Microhardness measurements The influence of the TiO2 particle reinforcement on the microhardness is summarized in Table 3. Fig. 14 shows that the microhardness of the composite solder increases as the TiO2 reinforcement increases. Microstructural observations in Fig. 4 revealed that the distribution of TiO2 particles along the grain boundaries can be considered the main reason for the improvement in the microhardness values of the Sn0.7Cu-0.05Ni composite solder. It was observed that the microhardness of the composite solder increases from 16.6 Hv to 17.76 Hv with the increase in the weight percentage of TiO2. The presence of TiO2 particles in the solder matrix pins the grain boundaries, impeding their sliding [30] and apparently enhancing the microhardness of the composite solder according to the dispersion
Fig. 10. X-ray diffraction patterns of Sn-0.7Cu-0.05Ni solder with (a) 0 wt.%, (b) 0.25 wt.%, (c) 0.50 wt.%, (d) 0.75 wt.%, and (e) 1.0 wt.% TiO2 reinforcement addition.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Fig. 11. DSC curves of Sn-0.7Cu-0.05Ni solder and Sn-0.7Cu-0.05Ni composite solder containing TiO2 particles. (a) Sn-0.7Cu-0.05Ni, (b) Sn-0.7Cu-0.05Ni-0.25TiO2, (c) Sn-0.7Cu-0.05Ni0.5TiO2, (d) Sn-0.7Cu-0.05Ni-0.75TiO2 and (e) Sn-0.7Cu-0.05Ni-1TiO2.
strengthening mechanism. The refined grain boundary and homogeneous dispersion of Cu6Sn5 at the grain boundary of the β-Sn dendrites prevent the slipping of dislocation, in addition to playing a strengthening role that increases the microhardness. This phenomenon is also explained by the Hall-Petch relationship [47], by which the strength increases as the grain size decreases. 3.4.2. Shear strength Fig. 15 shows the values of the shear strength of the Sn-0.7Cu0.05Ni-xTiO 2 composite solder. Monolithic Sn-0.7Cu-0.05Ni has an ultimate shear of 8.167 MPa. The addition of TiO 2 particles had a significant effect on the ultimate shear stress of the composite solder. Shear strength shows an increasing trend as the wt.% of TiO 2 increases. As the wt.% of TiO 2 approaches 1.0 wt.%, the shear strength reaches a maximum value of 11.09 MPa. The increasing trend of the shear strength is similar to that of the microhardness. This indicates that when the amount of TiO2 increases, the microstructure of the composite solder is improved because the TiO2 pins the grain boundaries, preventing grain dislocation and retarding grain growth. Therefore, it should be noted that the ductility increases in the composite solder. This could be connected to the reinforcement of particles along the grain boundaries. The shear
strength is usually governed by the IMC that forms between the solder and the Cu-substrate. The type of IMC is also important, in addition to its morphology and distribution. Through the addition of TiO2 into Sn-0.7Cu-0.05Ni solder alloy, the thickness and morphology of the IMC composite solder changes. The TiO2 addition is believed to suppress the formation of Cu6Sn5 and thereby decrease the thickness of the IMC. This indicates that the composite solder has a thicker IMC, and the scallops at the interface are more smoothly shaped. 3.5. Fracture surface analysis To verify the variation in the shear strength, the fracture surface was examined using SEM, as presented in Fig. 16. Based on the images, the monolithic solder sample in Fig. 16(a) exhibits a brittle fracture mode, indicated by a rectangular shape with a relatively smooth surface. It can be speculated that cracks initiate and propagate at the brittle Cu6Sn5 in an early stage of the shearing. However, the addition of TiO 2 particles dramatically improved the fracture mode of the Sn-Cu-Ni composite solder. Fig. 16(b–e) presents a mixture of ductile and brittle fracture modes with much rougher surfaces and dimpled surfaces due to the homogeneous distribution of the TiO2 particles.
Table 2 Solidus, liquidus, and melting range of lead-free Sn-0.7Cu-0.05Ni-xTiO2 composite solder. Specimens
TiO2 (wt.%)
Ts (°C)
TL (°C)
ΔT (°C)
Undercooling
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni-0.25% TiO2 Sn-0.7Cu-0.05Ni-0.50% TiO2 Sn-0.7Cu-0.05Ni-0.75% TiO2 Sn-0.7Cu-0.05Ni-1.0% TiO2
0 0.25% 0.50% 0.75% 1.0%
226.5 226.3 226.2 226.3 226.4
230.6 230.0 230.0 230.2 230.0
4.1 3.7 3.8 3.9 3.6
22.85 21.92 18.71 18.53 18.38
TS: solidus; TL: liquidus; ΔT: melting range.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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18.5
35.60° Microhardness (Hv)
Contact Angle (Degree)
38 36 34 32 30
28.66°
28
27.64° 24.99°
26
22.38°
24
17.76
18 17.5
17.18
17.32
16.84
17
16.6
16.5 16 15.5 0%
22
0.25%
0.50%
0.75%
1.00%
TiO2 (wt%)
20 0
0.25%
0.50%
0.75%
1.0% Fig. 14. Microhardness values of the Sn-0.7Cu-0.05Ni-xTiO2 solders.
Amount of TiO2 (wt%) Fig. 12. Influence of TiO2 addition on the contact angle in Sn-0.7Cu-0.05Ni composite solder.
thermal-fatigue reliability, so it is preferable to have a lower CTE value in the composite solder [49]. 4. Conclusion
A typical ductile fracture mode with the characteristic of dimples has been observed for samples with and without TiO2 addition. The dimplelike fracture surface is usually equiaxed in shape and normally occurs in ductile material. Generally, the larger and deeper the dimple is, the better the plastic property is [48]. The fracture surface of the composite solder appeared to be less ductile with a very fine dimple compared with the non- reinforced solder.
3.6. Coefficient of thermal expansion The CTE results obtained from various compacted pellets of composite solder samples are listed in Table 4. The decreases in the CTE value with the increasing TiO2`wt.% reveals that the dimensional stability of the Sn-0.7Cu-0.05Ni composite solder is better than that of the non-reinforced solder. The CTE of the Sn-0.7Cu-0.05Ni alloy is 16.2 × 10 − 6 /k, and the addition of TiO2 reduces the CTE value of the composite solder to 10.6·10− 6/k when 1 wt.% TiO2 was added. This result can be attributed to the lower coefficient of thermal expansion of titanium (9.1 × 10− 6/C), the uniform distribution of reinforcement particles, and good interfacial integrity between the reinforcement and matrix. A good solder joint should provide better
Table 3 Microhardness measurements of the Sn-0.7Cu-0.05Ni-xTiO2 solders. Sample
Addition of TiO2 (wt.%)
Average microhardness indention results (Hv) (5 indentions for each sample)
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni
0 0.25 0.50 0.75 1.0
16.6 16.84 17.18 17.32 17.76
(a) The microstructural observations revealed that the TiO2 particles were uniformly distributed on the surface of the solder matrix along the grain boundaries of the bulk solder. (b) The XRD analysis found that the growth rate of the Cu6Sn5 grains was markedly increased due to the addition of TiO2 nanoparticles. (c) The reinforcement of the TiO2 in the Sn-0.7Cu-0.05Ni resulted in a significant decrease in the undercooling temperature. However, because the change is less than 1.0 °C, there is no need for adjustments to the existing soldering process. (d) The wetting properties decrease with the addition of TiO2, and a minimum contact angle of 22.38° was observed with a 1.0 wt.% TiO2 addition. (e) The interfacial IMC thickness of the Sn-0.7Cu-0.05Ni composite solder joints is reduced compared to that of the Sn-0.7Cu0.05Ni solder. This indicates that the presence of TiO2
Shear Strength (MPa)
Fig. 13. Particle at triple point.
The effect of TiO2 addition into Sn-0.7Cu-0.05Ni solder alloy on the microstructure, mechanical and thermal properties has been investigated. The recommended content of TiO2 particles to be added in Sn-0.7Cu0.05Ni solder alloy is 1.0 wt.% since it showed significant increase of mechanical properties and wettability. The main conclusions are as follows:
14
11.09
12 10
8.167
8.399
0%
0.25%
9.095
9.796
0.50%
0.75%
8 6 4 2 1.00%
TiO2 (wt%) Fig. 15. Shear strength of Sn-0.7Cu-0.05Ni-xTiO2 composite solder joint.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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9
a) Brittle Fracture
b)
c)
d)
e)
Mixture of brittle and ductile fracture
Dimple
Dimple
Fig. 16. SEM micrographs of fracture surfaces of Sn-0.7Cu-0.05Ni composite solder (a) Sn-0.7Cu-0.05Ni, (b) Sn-0.7Cu-0.05Ni-0.25TiO2, (c) Sn-0.7Cu-0.05Ni-0.5TiO2, (d) Sn-0.7Cu-0.05Ni0.75TiO2 and (e) Sn-0.7Cu-0.05Ni-1TiO2.
particles is effective in suppressing the growth of the Cu6Sn5 IMC layer. (f) The shear strength of the Sn-0.7Cu-0.05Ni composite solder joints was improved linearly with the increasing weight percentage of TiO2.
(g) The increasing quantity of TiO2 particles decreased the CTE value from 16.2 × 10−6/k to 10.6 × 10−6/k with a 1.0 wt.% TiO2 addition. (h) The fracture surfaces of Sn-0.7Cu-0.05Ni show the brittle fracture mode, which is different from the Sn-0.7Cu-0.05Ni composite solder that exhibits a typical ductile failure with small dimples.
Acknowledgement Table 4 CTE results of monolithic and composite solders. Sample
CTE (×10−6/k) at 100 °C
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni-0.25TiO2 Sn-0.7Cu-0.05Ni-0.50TiO2 Sn-0.7Cu-0.05Ni-0.75TiO2 Sn-0.7Cu-0.05Ni-1.0TiO2
16.2 14.5 13.4 10.8 10.6
The authors gratefully acknowledge the financial support from Nihon Superior Co. Ltd., Japan under grant no. 9008-00003 and the School of Material Engineering, University Malaysia Perlis (UniMAP) for supporting this research effort through materials and facilities. The support of Associate Professor Kazuhiro Nogita from the University of Queensland for his valuable advice throughout this study is greatly appreciated.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder M.I.I. Ramli ⁎, N. Saud, M.A.A. Mohd Salleh, M.N. Derman, R. Mohd Said Center of Excellence Geopolymer & Green Technology (CeGeoGTech), School of Materials Engineering, Universiti Malaysia Perlis (UniMAP), Taman Muhibbah, 02600 Arau, Perlis, Malaysia
a r t i c l e
i n f o
Article history: Received 30 March 2016 Received in revised form 31 July 2016 Accepted 19 August 2016 Available online xxxx Keywords: Soldering alloy Intermetallic Alloys Powder metallurgy Fracture
a b s t r a c t The effect of TiO2 addition on the microstructure, melting behavior, microhardness and interfacial reaction between Sn-0.7Cu-0.05Ni and a Cu-substrate were explored. Samples with various TiO2 percentages (0, 0.25, 0.5, 0.75 and 1.0 wt.%) were prepared using a microwave-assisted sintering powder metallurgy method. Microstructural analysis reveals that TiO2 was uniformly distributed along the grain boundary of the bulk solder. Differential scanning calorimetry (DSC) results showed a decrease in the undercooling while melting temperature of the solder slightly increase. The thickness of the interfacial intermetallic compounds of the solder joint was reduced with the addition of TiO2 particles. This thickness reduction indicates that the presence of a small amount of TiO2 particles is effective in suppressing the growth of the intermetallic compound layer. Small dimples on the fracture surface have revealed that the Sn-Cu-Ni composite solder exhibits typical ductile failure. Overall, the addition of TiO2 to Sn-0.7Cu-0.05Ni solder dramatically increases its shear strength and hardness and improves its wetting properties and fracture surface. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Conventional Tin-lead (Sn-Pb) eutectic alloy systems have long been widely used in soldering applications in advanced electronic components. However, due to health and environmental concerns regarding the high toxicity of lead, European Union (EU) Waste of Electrical and Electronic Equipment (WEEE) and RoHS (Restriction of Hazardous Substances) legislation strictly prohibit the use of Pb in most electronic components [1,2]. Traditional lead solder alloy has been replaced with many lead-free solder alloys, including Sn-Bi [3], Sn-Bi-Ag [4], Bi-In-Sn [5], Sn-Zn [6], Sn-Zn-Bi [6], Sn-Ag-Cu [7,8], Sn-Cu [1,9], and In-48Sn [10]. Sn-Ag and Sn-Cu specifically well known for wave soldering [11] and commonly used as a replacement for the conventional Sn-Pb. However, due to its high silver content, Sn-Ag solder is more expensive than Sn-Cu solder alloys. Currently, Sn-Cu based solder is considered the cheapest leadfree solder candidate, it has received much attention as the lead-free solder of choice [12–14]. However, Sn-Cu has been reported to exhibit somewhat inferior mechanical properties and wettability compared to Ag-containing lead-free solder alloys [15]. Therefore, further improvements in Sn-Cu solders are needed. Several researchers [16] have reported that the addition of nickel has resulted in an improvement in wetting and solder joint reliability. This formulation (Sn-0.7Cu-0.05Ni) has been discovered by Nihon Superior Co Ltd. and has been commercialized as SN100C. SN100C would appear to be a real challenger, with a good compromise between cost and quality. However, due to high ⁎ Corresponding author. E-mail address: [email protected] (M.I.I. Ramli).
demands for better solder joint performance of lead-free solder, a viable way to enhance the performance of a solder is by introducing ceramic particles into the solder matrix [17]. Numerous studies have revealed that the addition of foreign particles provides a marked improvement in microstructural and mechanical properties [7,18–21]. Tsao et al. [22,23] has investigated the influence of reinforcing TiO2 and Al2O3 nanoparticles on the microstructural development and hardness of Sn-Ag-Cu systems. The microhardness measurements revealed that the addition of TiO2 and Al2O3 nanoparticles is helpful in enhancing the overall strength of the eutectic solder. Nai et al. [17] successfully synthesized Sn-3.5Ag-0.7Cu using titanium diboride (TiB2), multi-walled carbon nanotubes (MWCNTs) and ZrO2 nanoparticles via the powder metallurgical technique and found that the mechanical properties of the composite solders showed an overall improvement. However, the limitation of ceramic particles into solder is (i) homogeneous distribution of ceramic particles in a solder matrix is hard to be achieved, (ii) a high amount of ceramic particles may increase the brittleness of solder and (iii) ceramic additions do not normally stay in the solder during reflow, and are almost completely expelled with flux [11]. Powder metallurgy is the most common method used in developing composite solders [17,24–27]. The material in form of a powder is compacted as a green body and sintered to a net shape at elevated temperature. However, there are challenging demands from the powder metallurgy industry for new and improved sintering processes. This is where microwave technology is found to be advantageous. Microwave-assisted sintering provides finer microstructures and thus enhanced physical and mechanical properties [27,28]. Recently, it has been found that microwave sintering can also be applied efficiently
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Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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and effectively to powdered metals [29]. Y. Tang et al. [30] studied the effect of TiO2 addition on the microstructure and microhardness of SnAg-Cu and revealed that the microhardness of the composite solder is higher than that of the monolithic solder. Salleh et al. [2] added Si3N4 nanoparticles to Sn0.7Cu solder as a reinforcement, yielding improvements in microhardness and shear strength using microwave sintering. During the soldering process, a metallurgical reaction between the liquid solder and the copper substrate causes an IMC layer to form at the joint interface [32]. The formation of a thicker IMC layer affects the mechanical reliability of the solder joint, which weakens it due to the brittle nature of the IMCs. Decreasing the thickness of the IMC layer formed at the interface can increase the strength of the solder joint [7]. Leong et al. [8] reported that the addition of a small amount of TiO2 to the Sn-Ag-Cu solder can suppress the growth of the intermetallic compound layer. For the above reason, this present study concentrated to reveal the effect of TiO2 addition on Sn-0.7Cu-0.05Ni solder alloy by using powder metallurgy technique. The effect on microstructure evolution, melting property and wettability were investigated comprehensively. The interfacial reaction behavior and interfacial fracture surface on post mortem samples were analyzed using scanning electron microscope (SEM). The hardness and shear strength of composite solder were tested and compared with the Sn-0.7Cu-0.05Ni solder alloy.
2. Experimental procedures The base material, Sn-0.7Cu-0.05Ni solder powder with an average size 23.3–43.2 μm, obtained from Nihon Superior Co Ltd., Japan, was used. The solder alloy were mixed with TiO2 particle with average particle size 1.7– 19.8 μm by using powder metallurgy technique. These composite materials were mixed with five different percentages (0, 0.25, 0.5, 0.75 and 1.0 wt.%) of micron-size TiO2 in an airtight container using a roller blender. The blended mixtures were divided into samples of approximately 1 g using a weighing balance for each compact pellet. Each of the solder mixtures was pressed using a Specac 15-ton manual hydraulic press and uniaxially compacted in a 12-mm diameter mold. The compacted pellets were then sintered by microwave sintering at 185 °C under ambient conditions in an 800 W, 50 Hz Panasonic microwave oven. Silicon carbide (SiC) was used as a microwave susceptor material [28]. The samples were then reflowed onto Cu-substrates in a F4N reflow oven following the reflow profile for lead free solder Sn-0.7Cu as shown in Fig. 1. No clean mildly activated rosin flux from ASAHI Flux Medium
θ Cu-Pad
Fig. 2. Contact angle (θ) of Sn-0.7Cu-0.05Ni on copper substrate.
was applied prior to reflow soldering to act as a cleaning agent to remove and prevent the formation of oxides. The reflowed sample were mounted in epoxy resin, grinded, polished and finally etched with a solution of 93% methanol + 5%HNO3 + 2% HCl for clear grain structure observation. During solder reflow, there are probability of small amount of TiO2 particles were expelled in the flux as reported by Kotadia et al. [11, 33,34]. The microstructure was investigated using a scanning electron microscope (model JEOL JSM-6460LA SEM). A high-resolution scanning electron microscope equipped with an energy dispersive X-ray spectroscopy analysis system was used to inspect and analyze the microstructures of the five different compositions of composite solder and to perform the semi-quantitative analysis of the detected elements. The morphology of the intermetallic compound and the thickness of the IMC were observed by scanning electron microscopy (SEM) and evaluated using J-image analysis software. The IMC thickness measurement was carried out by dividing the IMC area (A) by its length (L) as illustrated in Fig. 3. The average thickness of IMC layer at various positions of the interface was calculated in 10 images. X-ray diffraction (XRD) was also used to inspect and analyze the phase formation of the composite solder. The phase analysis study was conducted using an X-ray diffractometer (XRD, D2 Phaser Bruker).
Solder
Fig. 1. Reflow profile used in the F4N reflow oven.
Fig. 3. Area of IMC layer at the solder joint interface for average thickness measurements.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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The pellet samples were exposed to Cu Kα radiation with a 20°–80° diffraction scan range with a step size of 0.02°. The melting temperature characteristics of the solder were analyzed using differential scanning calorimetry (DSC). A total 10 mg of solder was placed into an aluminum pan and scanned to 250 °C at a rate of 10 °C/min under a nitrogen atmosphere. The micro hardness test was conducted using a Vickers micro hardness test machine operated according to the ASTM B933-09 standard test method with a 1 kgf indenting load for a 10 s dwell time. Five points for each composition will be used to calculate the accuracy of the average results. Coefficients of thermal expansion (CTE) of the solder material were determined using a dilatometer with 5 °C/min heating rate. The displacement of the sample (each 5 mm) as a function of temperature (50–150 °C) was measured using an alumina probe under an argon atmosphere. The contact angle was measured by using J-image analysis software, as shown in Fig. 2. A shear test was conducted using an Instron Machine, and the specification of the substrate is based on ASTM D1002, which is a 101.6 mm × 25.4 mm × 1.5 mm Cu-substrate. The fractography of the solder joint after the shear test was also observed using SEM. 3. Result and discussion 3.1. Microstructure and phase analysis To observe the homogeneity of the TiO2 inside the solder matrix, the etched Sn-0.7Cu-0.05Ni-xTiO2 composite solder was examined by SEM. Fig. 4(a–d), has shown that the TiO2 particles were distributed on the surface of the solder matrix along the grain boundary of the bulk solder.
a)
3
Results indicated that particles may agglomerate with the increase of reinforcement percentage. As expected, the appearance of TiO2 particles on the surface of the solder matrix increased with the weight percentage of TiO2, as seen in Fig. 4. The grain boundary can be observed clearly in Fig. 4, and the microstructure consists of eutectic phases, reinforcement particles and primary phases. As indicated by the scanning elemental maps of the solder with 0.75 wt.% TiO2 (Fig. 5), it can be seen that the Ti element is segregated on the surface along the grain boundaries. Fig. 6 shows the solidified microstructure of the Sn-0.7Cu-0.05Ni and composite solder alloys. As seen in Fig. 4a, Sn-0.7Cu-0.05Ni is the neareutectic alloy, and it contains a large number of primary β-Sn dendrites and a small intermetallic compound Cu6Sn5. Interestingly, with a minor addition of TiO2, the grain size of β-Sn was refined compared to Sn0.7Cu-0.05Ni solder. On the other hand, the eutectic area has increased and distributed uniformly. This phenomenon was attributed to the unique properties of the active element, TiO2. According to the adsorption theory of the surface active material, the increase in the adsorption of active elements could decrease the surface energy and thereby decrease the growth and size of the grains. The presence of TiO2 particles can act as obstacles to diffusion, such as of Cu and Sn atoms, which are responsible for the growth of the intermetallic [35]. It also can be seen that with the addition of reinforcement particles, the amount of the Cu6Sn5 IMC slightly increased. Salleh et al. [36,37] had found with Synchrotron imaging that TiO2 may act as nucleation sites and increase the numbers of Cu6Sn5. Apart from that, TiO2 particles can also act as heterogeneous nucleation sites [2]. The TiO2 particles are dispersed in the molten solder and then precipitate on top of the Cu substrate during
b) Grain boundaries
Ti
Sn-rich
c)
Ti
d) Ti Ti
Ti
Sn-rich
Sn-rich
Fig. 4. SEM micrographs of the Sn-0.7Cu-0.05Ni lead-free solder with a) 0.25 wt.%, b) 0.50 wt.%, c) 0.75 wt.%, and d) 1.0 wt.% TiO2 reinforcement additions.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Eutectic phase
Reinforcement phase Primary phase
Fig. 5. Scanning electron microscope micrographs and elemental maps of the Sn-0.7Cu-0.05Ni solder with 0.75 wt.% TiO2 reinforcement addition.
a)
b) β -Sn
Cu6Sn5 Cu6Sn5
Fig. 6. Fully solidified microstructure of (a) Sn-0.7Cu-0.05Ni and (b) Sn-0.7Cu-0.05Ni-1.0% TiO2.
4.0 3.80 3.48
Thickness (μm)
soldering. The Cu6Sn5 IMCs will nucleate on the reinforcement surface to reduce the thermodynamic barrier. The reduction of the thermodynamic barrier then results in a decreased surface energy and growth velocity of the Cu6Sn5 grains [38]. The microstructure of the Sn-0.7Cu-0.05Ni-1.0 wt.% TiO2 composite solder (Fig. 6) becomes finer due to the presence of reinforcement particles hindering grain growth. Previous research by Tang et al. [30] reported that the addition of a trace amount of TiO2 nanoparticles can influence the microstructure of Sn-3.0Ag-0.5Cu solder, and the size and spacing of the Ag3Sn decreases significantly. The same phenomenon was also demonstrated by Chuang et al. [39] when Ti is added into Sn3.5Ag0.5Cu solder. They stated that the grain size of the β-Sn phase and the size of the IMCs could be refined to obtain a more stable microstructure and better mechanical properties. Therefore, the refinement of the microstructure due to TiO2 addition could improve the mechanical properties of Sn-0.7Cu-0.05Ni solder. Fig. 7 present the thickness of the IMC layer of Sn-0.7Cu-0.05Ni composite solder versus the wt.% of TiO2 particles. The IMC layer thickness of the composite solder joint decreased with the increased TiO2 particle addition. The thinnest IMC layer was achieved with the addition of
3.5 3.19
3.0
2.83 2.68
2.5
0
0.25% 0.50% 0.75% 1.0%
TiO2 (wt.%) Fig. 7. IMC layer thickness of Sn-0.7Cu-0.05Ni composite solder joint.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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1.0 wt.% of TiO2 Fig. 8. Additionally, the IMC layer thickness for the composite solder was smaller compared to that of the Sn-0.7Cu-0.05Ni solder. This phenomenon suggests that the addition of TiO2 particles could suppress the IMC layer growth. During the reflowing process of the composite solder, the TiO2 particles do not melt due to the high melting temperature of TiO2 (1843 °C), causing them to be absorbed in the liquid. The presence of TiO2 particles can act as an obstacle to the diffusion of atoms such as Cu atoms from the substrate. The formation of the IMC layer is mainly due to the diffusion of atoms from the substrate into the solder matrix. The TiO2 particles are believed to suppress the diffusion process, as they are distributed in the diffusion path of the Cu atoms. The SEM-EDX mapping results in Fig. 9 and Table 1 prove the presence of TiO2 particles distributed in the diffusion path that leads to the growth suppression of the IMC layer. Similar to previous findings, TiO2 may also remain attached on the IMC gloves and subsequently hinders the dissolution and diffusion of Cu from the substrate during soldering [37]. In addition, TiO2 particles may act as a heterogeneous nucleating agent that can affect the formation of the IMC layer [40]. The changes in the IMC layer morphology are considered as preferred changes because the elongated scallop shape induces brittle fracturing at the interface leading to cracking and the eventually decrease in the solder joint reliability. A similar result was reported by Leong [8], who found that the addition of TiO2 particles reduced the Sn3.5Ag0.5Cu IMC layer thickness. However, a contrasting phenomenon occurred when Leong [8] increased the addition of TiO2 up to 1.25 wt.%. Nai et al. [41] also reported the suppressor effect of CNTs on the growth of the IMC layer between the SAC composite solder and the Cu-substrate after aging. The X-ray diffraction results corresponding to the Sn-0.7Cu-0.05NixTiO2 composite samples are shown in Fig. 10. The results reveal that the main peaks in Fig. 10 are indexed by β-Sn and Cu6Sn5 in the composite solder. It was found that with the increasing amount of TiO2, there was some alteration and modification of Cu6Sn5 peak height when. However, the TiO2 phase was not detected in the XRD pattern of the Sn-0.7Cu-0.05Ni alloys (Fig. 10b–d). It is believed that the 1.0 wt.% TiO2 addition in the composite solder is too small to be detected. A similar phenomenon was encountered by Billah et al. [42], who could not detect a small amount of 1% Ni rich phase additions in Sn8Zn-3Bi with XRD. 3.2. Thermal analysis The melting temperature is a critical solder characteristic because it determines the soldering temperature. A higher melting temperature
5
Fig. 9. EDX analysis of IMC layer of Sn-0.7Cu-0.05Ni-TiO2.
will lead to a higher soldering temperature, which could cause damage to the electronic components in the packaging. Fig. 11 shows the DSC curves of the composite solder reinforced with different wt.% of TiO2 particles. Lead-free Sn-0.7Cu-0.05Ni solder shows a near-eutectic alloy with a melting point of 226.5 °C. The DSC results for the Sn-0.7Cu0.05Ni-xTiO2 composite are summarized in Table 2. The slight increases in the melting point of the composite solder could be influenced by the addition of TiO2. However, the slight changes in the melting point will not affect the typical soldering process for Sn-0.7Cu-0.05Ni solder. The same phenomenon has been found in SAC-(Ni-CNTs) [43] and Sn3.7Ag-0.9Zn/SiC [44]. Those studies revealed that no adjustment to the soldering process is needed if the change in the melting temperature of the newly fabricated solder is less than 1.0 °C. The reinforcement addition could modify the surface instability and the variation in the physical properties and the grain boundary/interfacial characteristics [45]. Apart from that, the degree of undercooling is also important in the solidification process of the solder alloy. The undercooling is defined as the temperature difference between the melting temperature of the solder during heating and the solidification temperature during cooling. The degree of undercooling is the one of the factors that affects the growth of IMCs. Table 2 shows that the undercooling of Sn-0.7Cu-0.05Ni solder is significantly reduced from 22.85 °C to 18.38 °C with the addition of TiO2 particles.
b)
a)
Cu6Sn5 Cu3Sn
Cu
Fig. 8. Backscattered images of intermetallic compounds formed at the interfaces of the as-reflowed solder joints (a) Sn-0.7Cu-0.05Ni and (b) Sn-0.7Cu-0.05Ni-1.0% TiO2.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Table 1 EDX analysis results of Sn-0.7Cu-0.05Ni-1.0TiO2.
Line 001 (10 lines) Line 002 (10 lines) Area 003 (5 area)
Element
Average atomic (%)
Standard deviation
Sn Cu Ti Sn Cu Ti Sn Cu Ti
35.87 64.13 nd 47.06 52.94 nd 86.67 12.95 0.38
1.095 2.339 nd 1.356 1.305 nd 1.822 0.937 0.073
This is because the undercooling phenomenon is related to the difficulty of nucleating a solid phase in the liquid state. The larger the undercooling is, the larger the driving force for the growth of IMCs will be [45]. The large amount of undercooling in the solder can influence its microstructure by changing its solidification behavior. In general, a faster cooling rate would produce a finer microstructure and also a greater hardness [9]. This undercooling will also reduce the time for IMCs to grow. These phenomena are also proposed by Chen et al. [9], who stated that a minor Ti addition effectively reduces the amount of undercooling of Sn-rich solders. 3.3. Wettability Fig. 12 shows the contact angle as a function of the TiO2 addition to the Sn-0.7Cu-0.05Ni solder. A decreasing trend of the contact angle was observed as the wt.% of the TiO2 addition increased. A minimum contact angle of 22.38° was observed with a 1.0 wt.% TiO2 addition. The results
show that a small amount of reinforcement in the Sn-0.7Cu-0.05Ni composite solder decreases the contact angle and thereby improves the wettability. The lower the contact angle of the solder on the Cu substrate, the better wettability of the solder joint [46]. These improvements can be attributed to the effect of the TiO2 in the molten solder, probably acting as an agent to reduce the surface tension of the alloy and increases the wettability between Cu substrate [21]. This can be observed with the decrease in contact angle as indicated in Fig. 12. It can also be explained based on the theory of the absorption of surface-active materials (TiO2 particles) on wetting [21]. The change in wetting angle could also be due to the segregation of TiO2 particles near the wetting triple point (Fig. 13). Similar results were also attained by Mohd Salleh et al. [2], who found that an improvement in the wettability of the Sn-0.7Cu/Si3N4 composite lead-free solder to the optimal level was achieved by the addition of 1.0 wt.% Si3N4. 3.4. Mechanical properties 3.4.1. Microhardness measurements The influence of the TiO2 particle reinforcement on the microhardness is summarized in Table 3. Fig. 14 shows that the microhardness of the composite solder increases as the TiO2 reinforcement increases. Microstructural observations in Fig. 4 revealed that the distribution of TiO2 particles along the grain boundaries can be considered the main reason for the improvement in the microhardness values of the Sn0.7Cu-0.05Ni composite solder. It was observed that the microhardness of the composite solder increases from 16.6 Hv to 17.76 Hv with the increase in the weight percentage of TiO2. The presence of TiO2 particles in the solder matrix pins the grain boundaries, impeding their sliding [30] and apparently enhancing the microhardness of the composite solder according to the dispersion
Fig. 10. X-ray diffraction patterns of Sn-0.7Cu-0.05Ni solder with (a) 0 wt.%, (b) 0.25 wt.%, (c) 0.50 wt.%, (d) 0.75 wt.%, and (e) 1.0 wt.% TiO2 reinforcement addition.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Fig. 11. DSC curves of Sn-0.7Cu-0.05Ni solder and Sn-0.7Cu-0.05Ni composite solder containing TiO2 particles. (a) Sn-0.7Cu-0.05Ni, (b) Sn-0.7Cu-0.05Ni-0.25TiO2, (c) Sn-0.7Cu-0.05Ni0.5TiO2, (d) Sn-0.7Cu-0.05Ni-0.75TiO2 and (e) Sn-0.7Cu-0.05Ni-1TiO2.
strengthening mechanism. The refined grain boundary and homogeneous dispersion of Cu6Sn5 at the grain boundary of the β-Sn dendrites prevent the slipping of dislocation, in addition to playing a strengthening role that increases the microhardness. This phenomenon is also explained by the Hall-Petch relationship [47], by which the strength increases as the grain size decreases. 3.4.2. Shear strength Fig. 15 shows the values of the shear strength of the Sn-0.7Cu0.05Ni-xTiO 2 composite solder. Monolithic Sn-0.7Cu-0.05Ni has an ultimate shear of 8.167 MPa. The addition of TiO 2 particles had a significant effect on the ultimate shear stress of the composite solder. Shear strength shows an increasing trend as the wt.% of TiO 2 increases. As the wt.% of TiO 2 approaches 1.0 wt.%, the shear strength reaches a maximum value of 11.09 MPa. The increasing trend of the shear strength is similar to that of the microhardness. This indicates that when the amount of TiO2 increases, the microstructure of the composite solder is improved because the TiO2 pins the grain boundaries, preventing grain dislocation and retarding grain growth. Therefore, it should be noted that the ductility increases in the composite solder. This could be connected to the reinforcement of particles along the grain boundaries. The shear
strength is usually governed by the IMC that forms between the solder and the Cu-substrate. The type of IMC is also important, in addition to its morphology and distribution. Through the addition of TiO2 into Sn-0.7Cu-0.05Ni solder alloy, the thickness and morphology of the IMC composite solder changes. The TiO2 addition is believed to suppress the formation of Cu6Sn5 and thereby decrease the thickness of the IMC. This indicates that the composite solder has a thicker IMC, and the scallops at the interface are more smoothly shaped. 3.5. Fracture surface analysis To verify the variation in the shear strength, the fracture surface was examined using SEM, as presented in Fig. 16. Based on the images, the monolithic solder sample in Fig. 16(a) exhibits a brittle fracture mode, indicated by a rectangular shape with a relatively smooth surface. It can be speculated that cracks initiate and propagate at the brittle Cu6Sn5 in an early stage of the shearing. However, the addition of TiO 2 particles dramatically improved the fracture mode of the Sn-Cu-Ni composite solder. Fig. 16(b–e) presents a mixture of ductile and brittle fracture modes with much rougher surfaces and dimpled surfaces due to the homogeneous distribution of the TiO2 particles.
Table 2 Solidus, liquidus, and melting range of lead-free Sn-0.7Cu-0.05Ni-xTiO2 composite solder. Specimens
TiO2 (wt.%)
Ts (°C)
TL (°C)
ΔT (°C)
Undercooling
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni-0.25% TiO2 Sn-0.7Cu-0.05Ni-0.50% TiO2 Sn-0.7Cu-0.05Ni-0.75% TiO2 Sn-0.7Cu-0.05Ni-1.0% TiO2
0 0.25% 0.50% 0.75% 1.0%
226.5 226.3 226.2 226.3 226.4
230.6 230.0 230.0 230.2 230.0
4.1 3.7 3.8 3.9 3.6
22.85 21.92 18.71 18.53 18.38
TS: solidus; TL: liquidus; ΔT: melting range.
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18.5
35.60° Microhardness (Hv)
Contact Angle (Degree)
38 36 34 32 30
28.66°
28
27.64° 24.99°
26
22.38°
24
17.76
18 17.5
17.18
17.32
16.84
17
16.6
16.5 16 15.5 0%
22
0.25%
0.50%
0.75%
1.00%
TiO2 (wt%)
20 0
0.25%
0.50%
0.75%
1.0% Fig. 14. Microhardness values of the Sn-0.7Cu-0.05Ni-xTiO2 solders.
Amount of TiO2 (wt%) Fig. 12. Influence of TiO2 addition on the contact angle in Sn-0.7Cu-0.05Ni composite solder.
thermal-fatigue reliability, so it is preferable to have a lower CTE value in the composite solder [49]. 4. Conclusion
A typical ductile fracture mode with the characteristic of dimples has been observed for samples with and without TiO2 addition. The dimplelike fracture surface is usually equiaxed in shape and normally occurs in ductile material. Generally, the larger and deeper the dimple is, the better the plastic property is [48]. The fracture surface of the composite solder appeared to be less ductile with a very fine dimple compared with the non- reinforced solder.
3.6. Coefficient of thermal expansion The CTE results obtained from various compacted pellets of composite solder samples are listed in Table 4. The decreases in the CTE value with the increasing TiO2`wt.% reveals that the dimensional stability of the Sn-0.7Cu-0.05Ni composite solder is better than that of the non-reinforced solder. The CTE of the Sn-0.7Cu-0.05Ni alloy is 16.2 × 10 − 6 /k, and the addition of TiO2 reduces the CTE value of the composite solder to 10.6·10− 6/k when 1 wt.% TiO2 was added. This result can be attributed to the lower coefficient of thermal expansion of titanium (9.1 × 10− 6/C), the uniform distribution of reinforcement particles, and good interfacial integrity between the reinforcement and matrix. A good solder joint should provide better
Table 3 Microhardness measurements of the Sn-0.7Cu-0.05Ni-xTiO2 solders. Sample
Addition of TiO2 (wt.%)
Average microhardness indention results (Hv) (5 indentions for each sample)
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni
0 0.25 0.50 0.75 1.0
16.6 16.84 17.18 17.32 17.76
(a) The microstructural observations revealed that the TiO2 particles were uniformly distributed on the surface of the solder matrix along the grain boundaries of the bulk solder. (b) The XRD analysis found that the growth rate of the Cu6Sn5 grains was markedly increased due to the addition of TiO2 nanoparticles. (c) The reinforcement of the TiO2 in the Sn-0.7Cu-0.05Ni resulted in a significant decrease in the undercooling temperature. However, because the change is less than 1.0 °C, there is no need for adjustments to the existing soldering process. (d) The wetting properties decrease with the addition of TiO2, and a minimum contact angle of 22.38° was observed with a 1.0 wt.% TiO2 addition. (e) The interfacial IMC thickness of the Sn-0.7Cu-0.05Ni composite solder joints is reduced compared to that of the Sn-0.7Cu0.05Ni solder. This indicates that the presence of TiO2
Shear Strength (MPa)
Fig. 13. Particle at triple point.
The effect of TiO2 addition into Sn-0.7Cu-0.05Ni solder alloy on the microstructure, mechanical and thermal properties has been investigated. The recommended content of TiO2 particles to be added in Sn-0.7Cu0.05Ni solder alloy is 1.0 wt.% since it showed significant increase of mechanical properties and wettability. The main conclusions are as follows:
14
11.09
12 10
8.167
8.399
0%
0.25%
9.095
9.796
0.50%
0.75%
8 6 4 2 1.00%
TiO2 (wt%) Fig. 15. Shear strength of Sn-0.7Cu-0.05Ni-xTiO2 composite solder joint.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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a) Brittle Fracture
b)
c)
d)
e)
Mixture of brittle and ductile fracture
Dimple
Dimple
Fig. 16. SEM micrographs of fracture surfaces of Sn-0.7Cu-0.05Ni composite solder (a) Sn-0.7Cu-0.05Ni, (b) Sn-0.7Cu-0.05Ni-0.25TiO2, (c) Sn-0.7Cu-0.05Ni-0.5TiO2, (d) Sn-0.7Cu-0.05Ni0.75TiO2 and (e) Sn-0.7Cu-0.05Ni-1TiO2.
particles is effective in suppressing the growth of the Cu6Sn5 IMC layer. (f) The shear strength of the Sn-0.7Cu-0.05Ni composite solder joints was improved linearly with the increasing weight percentage of TiO2.
(g) The increasing quantity of TiO2 particles decreased the CTE value from 16.2 × 10−6/k to 10.6 × 10−6/k with a 1.0 wt.% TiO2 addition. (h) The fracture surfaces of Sn-0.7Cu-0.05Ni show the brittle fracture mode, which is different from the Sn-0.7Cu-0.05Ni composite solder that exhibits a typical ductile failure with small dimples.
Acknowledgement Table 4 CTE results of monolithic and composite solders. Sample
CTE (×10−6/k) at 100 °C
Sn-0.7Cu-0.05Ni Sn-0.7Cu-0.05Ni-0.25TiO2 Sn-0.7Cu-0.05Ni-0.50TiO2 Sn-0.7Cu-0.05Ni-0.75TiO2 Sn-0.7Cu-0.05Ni-1.0TiO2
16.2 14.5 13.4 10.8 10.6
The authors gratefully acknowledge the financial support from Nihon Superior Co. Ltd., Japan under grant no. 9008-00003 and the School of Material Engineering, University Malaysia Perlis (UniMAP) for supporting this research effort through materials and facilities. The support of Associate Professor Kazuhiro Nogita from the University of Queensland for his valuable advice throughout this study is greatly appreciated.
Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011
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Please cite this article as: M.I.I. Ramli, et al., Effect of TiO2 additions on Sn-0.7Cu-0.05Ni lead-free composite solder, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.08.011