Investigation of interface delamination in a SO8 package under reflow

In the semiconductor industry, efficient and effective reliability qualification is very critical. In order to investigate the moisture effect for nonhermetic packages, reliability test such as preconditioning is designed and characterized. Interface delamination is a very common moisture-related failure mode. This is the comprehensive results from coefficient of thermal expansion (CTE) mismatch and hygroscopic swelling mismatch between different materials, degradation of materials and adhesion strength, and large vapor pressure buildup in high temperature. In this paper, the finite element analysis (FEA) modeling is conducted to study the interface delamination of the SO8 package during reflow with and without moisture. The moisture diffusion simulation is firstly conducted using the thermal-moisture analogy. Then the vapor pressure is calculated based on the moisture distribution and steam table. Finally, the equivalent CTE method is used to simulate the stress, which includes the CTE mismatch induced stress, hygroscopic stress and vapor pressure induced stress. The impact of 4 parameters are studied, die attach (DA) material, bond line thickness (BLT), DA epoxy coverage, and reflow with and without moisture. Totally 54 simulation legs are generated through the design of experiment (DoE) for the 4 variables with different levels. Simulation results show that the stresses in DA epoxy and mold compound increase when subjected to reflow with moisture compared with the pure reflow without moisture. Thin BLT might increase the risk of interface delamination between DA epoxy and die attach pad (DAP), mold compound and lead frame. Full DA coverage is not the best case to obtain the lowest interface stress and avoid interface delamination.