Electrically driven matter transport effects in PoP interconnections

The migration issue of PackPackage-on-Packageage-on-package (PoP) is investigated in this article. The migration phenomenon can be amplified as the density of solder bumps increases, while the paths are very finer as well in PoP. These migration effects were first evaluated by three dimensional finite element simulations in ANSYS in a previous article, in which the bottom of the bump was found out to be the weakest part of the structure in the electrical, thermal and stress fields driven by current loads in the PoP assemblies. To verify this conclusion, the experiments were performed in the assembly cards for analyzing migration effects in PoP. "Top", "bottom" and PoP components were assembled on test cards. Daisy chained bumps were chosen to investigate the migration phenomenon by comparing the behaviors between the single package ("top" and "bottom") and PoP. The test assembly cards were placed into a temperature-controlled chamber with one ampere current stressing the components. This experiment was performed at different temperatures. In conclusion, the migration problem of PoP was studied by experiments, and new simulations were also carried out. The IMCs were analyzed for understanding the migration phenomenon. The comparison of the simulation and experimental results indicates high electromigration mass flux divergences at the position of the open circuit.