Reliability study of package-on-package stacking assembly under vibration loading

The vibration reliability of lead-free solder joints of Package-on-Package (PoP) is investigated by experimental tests and finite element method (FEM) simulations in this paper. A 14 × 14 mm two-tier PoP module was selected for this study. The natural frequencies and modes were determined by FEM and verified by experimental tests. The printed circuit board (PCB) assemblies are tested under harmonic vibration. Vibration test results show that the vibration reliability of top package is better than the bottom package, and the outermost corner solder joints of the bottom package are the critical solder joints for the PoP under vibration loading. The stress characteristics of solder joints obtained by FEM are well correlated with the experimental results. Failure mechanism analysis indicates that the bottom solder joints become the most vulnerable part of the PoP under vibration due to the bigger relative displacements between the PCB and the bottom package. The micro-structural analysis indicates that cracks usually originate in the bottleneck position of the solder balls, extend within bulk solder and then propagate along the interface between the IMC layer and the bulk solder. The influence of bottom solder joints standoff for vibration reliability was analyzed by FEM as well. Results show that the higher the bottom solder joints' standoff, the more difficult the failure for the PoP assembly.