Microstructure Aspects and Performance Implications of Sn/Ag/Cu/Sb Solder Joints in the Presence of Gold
Weiqun Peng, Steve Dunford, Viswanadham Puligandla, and Stephen Quander
Nokia, Dallas, TX 75039
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Influence of gold coating in Leadless Ceramic Chip Carrier component terminal finish on the performance of tin/silver/copper/antimony (Sn/Ag/Cu/Sb) solder joints was investigated by metallurgical analysis. Solder joint structure and morphology were examined as reflowed, after 200, and 500 thermal cycles, respectively, using scanning electron microscopy. Comparisons were made with tin/lead/silver (Sn/Pb/Ag) solder joints.
Increased gold content in solder joints resulted in an increase in Au/Sn intermetallic formation and irregular shaped voids. Coefficient of thermal expansion influenced crack development and propagation. The different failure mechanisms of Sn/Ag/Cu/Sb and Sn/Pb/Ag solder joints in the presence of gold were observed. Cracks in Sn/Ag/Cu/Sb joints propagated along the boundaries of Au/Sn and Ag/Sn intermetallics, on the other hand crack propagated through the Pb-rich layer at the solder/component interface in the case of Sn/Pb/Ag solder joints. The microstructure of Sn/Ag/Cu/Sb solder was more stable through thermal cycling than in the case of Sn/Pb/Ag solder. There was no unusual change in microstructure in Sn/Ag/Cu/Sb solder after thermal cycling. Some joints were found to have 12-17 wt.% Au because of the design of some component terminations. Primary AuSn4 and eutectic structures were observed in the high-gold-content solder joints. Performance differences observed with different components, and the gold content variations, using the two interconnection materials will be discussed.