Brittle fracture of intermetallic compounds in SAC solder joints under high speed ball pull/pin pull and Charpy impact tests

A comprehensive experimental study on the brittle fracture of intermetallic compounds (IMCs) of a Pb-free solder was performed and presented in this paper. The solder joints were subject to thermal aging for accelerating the growth of IMCs. The brittle fracture of IMCs was investigated using high speed ball pull/pin pull and Charpy impact tests. In addition to qualitative analysis of failure mechanism, quantitative characterization of ball pull force and impact toughness was also performed. It was found that the decrease in the ball pull force and the impact toughness due to thermal aging could not be simply attributed to the growth of IMC thickness. In order to understand the whole scenario of failure mechanism, the variation in IMC composition and the shift of fracture location must be considered as well. From direct test data, it seems that Cu₃Sn should be the weakest link in the solder joint. But in cases subject to limited thermal aging, the brittle fracture propagated along the interface between Cu₃Sn and Cu₆Sn₅, instead of inside Cu₃Sn. With detailed fractographic analyses, the number of stacks of Cu₃Sn grains was considered the key factor for determining the fracture location.