By Qian Jiang
Heterogeneous integration is leading to unprecedented miniaturization of solder joints, often with thousands of joints within a single package. The thermo-mechanical behavior of such SAC solder joints is critically important to the assembly performance and reliability. However, this response can be extremely challenging to predict due to the tremendous joint-to-joint variability observed in solder joints, as each joint consists of a unique arrangement of a few large highly-anisotropic grains. Similar variability has been observed in recent experiments where thermal cycling of BGA joints under axial compression (caused by the clamping force of heat sinks) produced large lateral creep deformations that were difficult to predict with simple homogeneous isotropic viscoplastic models.