Sn-RICH PHASE COARSENING DURING ISOTHERMAL ANNEALING ON Sn-Ag-Cu SOLDER. N. Saud and A. Jalar

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1 International Journal of Mechanical and Materials Engineering (IJMME), Vol. 4 (2009), No. 2, Sn-RICH PHASE COARSENING DURING ISOTHERMAL ANNEALING ON Sn-Ag-Cu SOLDER. N. Saud and A. Jalar School of applied Physic, Faculty of Science & Technology, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia intanniza@yahoo.com ABSTRACT The microstructure study of eutectic Sn-3.8Ag-0.7Cu (SAC) during isothermal aging has been investigated. Isothermal aging temperatures were 30 C, 60ºC, 90ºC, 120ºC, 150 C and 180ºC for 216 hours of aging time. Infinite Focus Microscope (IFM) is used for microscopy analysis. The influence of microstructure changes is established. Results showed that Sn-rich phases and eutectic phases appear immediately after reflow. However, the diffusion of eutectic component into Sn-rich phases has altered the microstructure of the SAC solder, as will be discussed further. The calculated activation energy value for Sn-rich phase coarsening is 76kJ/mol, which represent the volume diffusion mechanism. Keywords: Sn-Ag-Cu, Annealing, Sn-rich phase, Diffusion. INTRODUCTION Lead based solders have been used in microelectronics industry for a long time. But Pb in solder can cause not only toxicity but also as the radioactive elements which are harmful to the environment. Therefore the global trend in microelectronics packaging nowadays has switched to lead free solder; Sn-base solders. Most Sn-base lead free solders contain only minor amount of alloying additions. Usually, more than 95 wt% of the solder is Swenson (2007) as can be seen in the most promising candidates of lead free solder, Sn-Ag-Cu. Previous studies, (Chen et al., 2007; Fix et al., 2005; Fix et al., 2008; Kim et al., 2003; Lewis et al., 2002; Peng et al., 2007) have proved that this solder family system is one of the potential choices. It also was recommended by some organizations such as Consortium of National Physics Laboratory (NPL), International Research Institute (ITRI), National Electronics Manufacturing Initiative (NEMI) and Department of Trade & Industry (DTI) (Zhong et al., 2006). Although this solder has satisfied essential technological demands as melting point, wetting properties, and surface tension, still there are some issues to be resolved for these alloys. For instance, formation of different phases in the solders is still 147 under consideration. Phase changes especially due to thermal are still not fully understood. Knowledge of phase evolution in solder joint is important especially in the metallurgical view point. It is very useful to make prediction on the reliability of the solder joint. In this study, the changes of different phases of solder joint were investigated due to the effect of thermal interactions of the microstructure during heat treatment. Since most solder alloys have low melting points, diffusion at solder joints cannot be ignored even at room temperature. The annealed microstructure of the as-joint solder before and after annealing process is expected to be different due to the diffusion mechanism. As the temperature increase, volume, lattice and atomic diffusion would starts to dominate the diffusion mechanism (Lauria et al., 2005). This paper will also discuss on how the morphological changes due to thermal related to the growth energy. MATERIALS & METHODS As-joint solder materials are known to behave differently than the bulk solder materials. Therefore, the samples were hand soldered to form a solder joint. Eutectic SAC solder used was a 0.6mm-diameter solid wire from a commercial vendor, RedRing Solder (M) Sdn.Bhd. A clamping device (third hand) was used to facilitate the hand soldering of two (0.57mm diameter) Cu wires together with a joint gap of about 0.7 mm as shown in figure 1. When the melting point of the solder reached, each Cu end was hand soldered by melting solder along the top of the gap, using gravity and capillary forces to pull the solder into the gap. A small amount of activated rosin flux was added to break Temperature is one of the key factors in determining diffusion process and microstructure evolution. Therefore, aging test was carried out to understand these mechanisms on SAC solder. The test samples were loaded into an ambient atmosphere oven for isothermal aging at 30 C, 60ºC, 90ºC, 120ºC, 150 C and 180ºC for 216 hours. up the oxide layer on the Cu wire end, prior soldering process. For microstructural analysis, the samples were grinded and polished with standard metallographic technique and etched with a solution consisting of 5% HNO 3 + 2% HCl + 93% Methanol for 7 seconds to reveal the microstructure.

2 Figure 1: Clamping device for hand soldering 90 C (c) Figure 2: IFM photograph of microstructure of SAC solder (after reflow process). 60 C (b) 30 C (a) 120 C (d) 148

3 (e) 150 C (f) 180 C Figure 3: IFM photographs of effect of annealing temperature on the microstructure of SAC solder After thesamples were finished with metallographic process, infinite focus microscope (ifm) was used to observe the phase evolution that occurred in the samples. RESULTS AND DISCUSSION Figure 2 shows the initial microstructure of SAC solder joint immediately after reflow process. The typical microstructure consisted of Sn-rich dendrites (white area) surrounded by ternary eutectic structure (dark area) of Cu 6 Sn 5 and little fraction of Ag 3 Sn, according to the calculated phase diagram (Chen et al., 2007; Fix et al., 2005; Fix et al., 2008; Kim et al., 2003; Lewis et al., 2002; Peng et al., 2007). Figure 3 shows the microstructures of solder after annealing for 216 hours at temperature of 30, 60, 90, 120, 150 and 180 ºC. The changes of the microstructure can be seen starting at 60 C of annealing temperature ahead as shown in figure 3 (b) (f). Starting from figure 3 (b), the percentage of Sn-rich phase showed an increments. As the temperature increased, the percentage of Sn-rich phase keeps growing while the percentage of eutectic phase (dark area) depleted. At 180 C, the microstructure was totally altered, where the small and clear grain boundary as can be observed in figure 3 (b)- (g) has become larger and blur as they approached each other. This showed that grain boundary is getting larger as the annealing temperature increased. As we can observe, starting at 45%, the value has ended up to 87%. Diffusion mechanism has decreased the percentage of eutectic phase especially when annealing temperature above 50ºC. In most alloys, when the temperature above Tm, diffusion driven process area relatively rapid (Kou, 1987). That is why, from the observation, the coarsening of Sn-rich phase started to occur at 60ºC instead at 30ºC. As we can see, there is 149 no distinct change between the microstructure at 30ºC and the unannealed one. Sn-rich area (% ) R 2 = Annealing Temperature (celcius) Figure 4: Graph on the effect of annealing temperature on Sn-rich phase (expressed as percentage of the area). The diffusion process of Sn in eutectic phase into Snrich phase is the cause for the microstructural changes. Recent studies suggested Sn as a dominant diffusing species (Bae and Kim, 2001). Schaefer et al. (1998) also assumed Sn as the more rapidly diffusing species, based on the reasoning that the element with the lower melting temperature is the faster diffuser. This was also suggested by Martin et al. (1997) in his book. Jung et al. (2001) also suggested that Sn usually diffuse through Cu 3 Sn and Cu 6 Sn in the eutectic phase to react with other element/phases. Hence, Sn in the eutectic phase which has lower melting point compared to primary Sn phase has diffused into primary Sn phase and coarsened the area. Figure 5 illustrates the diffusion of Sn atom into Sn-rich phase.

4 make a comparison. There are differences of Sn phase diffusion rate in different alloys. Moreover, the diffusion and growth of the phases is sensitive to the type of solder. However, Fix et al. (2008) has reported that volume diffusion controlled mechanisms are expected for SAC as the amount of pure tin is <95%. (a) (b) Figure 5: The sketch of (a) SAC eutectic phase which consist of three types of fibrous; Sn (grey), Cu 6 Sn 5 (purple) and Ag 3 Sn (dark grey) and (b) Sn-rich phase area where Sn particle has diffused into Sn-rich phase area. ln D R 2 = Figure 6: Arrhenius plot on Sn-rich coarsening area The activation energy value needs to be calculated in order to enhance the diffusion analysis. The diffusion activation energy for the coarsening of Sn-rich area was calculated by using Arrhenius equation. D = Do e (-Q/kT) 1/T (1/K 10-3) Where in this expression, D = percentage of Sn-rich area, Q = activation energy, k = Boltzman constant and T = absolute temperature. A plot of ln D versus 1/T has a slope of (Q/k). The graph for this analysis is shown in figure 4. From the graph, the activation energy for the diffusion that coarsened Sn-rich phase is 76 kj / mol. However, no such data in the literature which calculate Sn-rich phase coarsening could be found for alloy SAC to 150 CONCLUSIONS Sn-rich phase coarsening is observed under different annealing temperature. As the temperature increase, the percentage of Sn-rich area also increases. The reason for the coarsening is the diffusion of Sn into Sn-rich phase and the diffusion process was rapid starting from 60ºC onward. The activation energy for Sn-rich area coarsening is 76kJ/mol, which was presumably representing the volume diffusion mechanism. ACKNOWLEDGEMENTS This work is supported under the IRPA grant No PR001. The authors gratefully acknowledge the support provided by UKM and MOSTI. REFERENCES Bae, K-S. and Kim, S-J Interdiffusion analysis of the soldering reactions in Sn-3.5Ag/Cu couples. Journal of Electronic Materials, Volume 30, No 11. Chen, S-W., Wang, C-H., Lin, S-K. and Chiu, C-N Phase diagram of Pb-free solders and their related materials systems. Journal Materials Science: Materials Electronic. Volume 18, pp Chen, T. and Dutta, I Effect of Ag and Cu concentrations on the creep behavior of Sn-based solders. Journal of electronic Materials Volume 37, No 3. Fix, A.R., Lopez, G.A., Brauer, I., Nutcher, W. and Mittermeijer, E.J Microstructural development of Sn-Ag-Cu solder joints. Journal of Electronic Materials. Volume 34, No 2. Fix, A.R., Nutcher, W. and Wilde, J Microstructural changes of lead- free solder joints during long term ageing, thermal cycling and vibration fatigue. Soldering & Surface Mount Technology, 20 issue 1, Jung, K and Conrad, H Microstructure coarsening during static annealing of 60Sn40Pb solder joints: Intermetallic compound growth kinetics. Journal of Electronic Materials, Volume 414, No10.

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