Thermal Fatigue Crack Initiation and Related Microstructural Evolution in Sn/3.0Ag/0.5Cu Solder Joints

Abstract

Recently, Sn/3.0Ag/0.5Cu solder has been noted as one of the most practical lead-free solder. In order to investigate the relation between thermal fatigue crack initiation and microstructural evolution, a series of thermal cycle tests for Sn/3.0Ag/0.5Cu solder joints was carried out by using some PCBs on which resistance chips with different sizes were mounted. Additionally, microstructural observation of the joints was carried out by SEM, and average size of Ag_3Sn phase d was measured by image processing. Consequently, the phase grows as a number of thermal cycles N increases, and the phase growth is greatly influenced by both temperature and strain change. The relation between N and phase growth parameter S, which is defined by d^4,enable us to evaluate the thermal fatigue crack initiation of the solder joints.