Fracture mechanism map for the fracture of microelectronic Pb-free solder joints under dynamic loading conditions

Solder joints, which serve as mechanical and electrical interconnects in a package, are particularly prone to failure during a drop. Hence, the fracture behavior of solders at high strain rates and in mixed mode is a critically important design parameter. This study reports the effects of (a) loading conditions (strain rate and loading angle), (b) reflow parameters (dwell time and cooling rate), and (c) post-reflow aging on the mixed mode fracture toughness of a lead-free solder (Sn-3.8%Ag-0.7%Cu)/Cu joint. A methodology based on the calculation of critical energy release rate, G_C, which is equal to the fracture toughness of a material under limited plasticity condition, was employed. An increase in the strain rate results in limited plasticity ahead of the crack tip leading to a reduction in the fracture toughness of the solder joints. Fracture toughness also decreases with increasing mode-mixity (up to a loading angle of 75°). A slower cooling rate increases the fracture toughness whereas a longer dwell time adversely affects it. Also, aged samples show higher G_C value. A fracture mechanism map is developed to describe the correlation between the yield strength of the solder, which depends on the solder microstructure and the loading rate, the IMC morphology, which depends on the reflow conditions and aging, and the fracture toughness of the solder joint.