Influence of Initial Morphology and Thickness of Cu_6Sn_5 and Cu_3Sn Intermetallics on Growth and Evolution during Thermal Aging of Sn-Ag Solder/Cu Joints

摘要

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 soldwer joints.In this study,the influence of initial intermetallic morphology on growth rate,and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu.The initial morphology of the intermetallic was tailered by cooling in water,air,or furenace conditions.Solder aging was conducted at 100 deg C,140 deg C,and 175 deg C and aged for 0-1,000 h.Cooling rate,aging temperature,and aging time played an important role on microstructure evolution and growth kinetics of Cu_6Sn_5 (eta) and Cu_3Sn (epsilon) intermetallic layers.Prior to aging,faster cooling rates resulted in a relatively planar Cu_6Sn_5 layer,while a nodular Cu_6Sn_5 mor9phology was present for slower cooling.Intermetallic-growth rate measurements after aging at various times,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 Cu_6Sn_5 and Cu_3Sn growth.