Microstructural evolution and cracking of Pb-free ball grid array assemblies under thermal cycling

Two kinds of CBGA (ceramic ball grid array) assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy (SEM) and finite element method (FEM). Before thermal cycling, intermetallic compounds (IMCs) Cu₆Sn₅ and Ag₃Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu₃Sn was observed near the Cu pad in both two assemblies and the layers of Cu₆Sn₅ and Ag₃Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.