Solder ball failure mechanisms in plastic ball grid array packages

Plastic ball grid array packages were aged for up to 2000 hours. Various solder ball pad metallurgies were studied and solder ball shear tests were conducted at a range of ageing times. The solder ball shear strength was found to decrease after an initial hardening stage. The deterioration of solder ball shear strength was found to be mainly caused by the formation of intermetallic compound layers, together with microstructural coarsening and diffusion related porosity at the interface. For the ball pad metallurgy, two distinct intermetallic compound layer structures were observed to have formed after ageing. Once two continuous intermetallic compound layers formed fracture tended to occur at their interface. For the ball pad metallurgies which do not form two continuous intermetallic compound layers, the shear strength still decreased, due to the coarsening of the microstructure, intermetallic particle formation and diffusion related porosity at the surface of the Ni₃Sn₄.