Influence of Initial Morphology and Thickness of Cu6Sn5 and Cu3Sn Intermetallics on Growth and Evolution during Thermal Aging of Sn-Ag Solder/Cu Joints

Abstract

Intermetallic-layer formation and growth in Pb-free solder joints during solder reflow or subsequent aging has a significant effect on the thermal and mechanical behavior of solder joints. In this study, the intermetallic-growth rate, morphology, and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu. The joints were cooled in water, air, or furnace conditions. Solder aging was conducted at 100°C, 140°C, and 175°C and aged for 0–1,000 h. Cooling rate and aging temperature played an important role on microstruc-ture evolution and growth kinetics of Cu6Sn5 (η) and Cu3Sn (ε) intermetallic layers. Faster cooling rates resulted in a relatively planar Cu6Sn5 layer, while a nodular Cu6Sn5 morphology was present for slower cooling. Inter-metallic-growth rate measurements indicated a mixed growth mechanism of grain-boundary and bulk diffusion. These mechanisms are discussed in terms of the initial intermetallic thickness and morphology controlled by cooling rate, diffusion kinetics, and the competition between Cu6Sn5 and Cu3Sn growth