Mechanism on solder-IMC crack formation in fine-pitch BGA packages

Solder-IMC (intermetallic compounds) cracking is a reliability failure mode of solder joints that occurs during secondary-side SMT (surface mount technology) assembly and often leads to serious short-circuit issues. In the present work, the formation mechanism of solder-IMC cracks is investigated through a set of designed comparative experiments. The results reveal that the primary cause of such cracks is the combination of a partially molten state of the solder ball and applied stress. During the heating process, thermal mismatch and non-uniform heat transfer generate not only thermal stress and warpage deformation, but also asynchronous melting among different solder balls and within individual solder balls. Under stress, the solder balls at the device edge melt first, leading to separation between the solder and the IMC on the higher-temperature chip side. Increasing the stress level and reducing the heating rate are found to promote the tendency for solder-IMC crack formation.