Effect of hygro-thermo-mechanical stress on reliability of stacked die package

Mechanical reliability of epoxy molding compounds in plastic packages of integrated circuits (IC) is greatly affected by the compound ability to absorb moisture. In this paper, the hygro-thermal effect of a 2-layer stacked die package was investigated which emphasized on the hygroscopic stress and thermal mismatch stress. By finite element analysis (FEA), the distribution of moisture diffusion, thermo-mechanical stress, hygro-mechanical stress and hygro-thermo-mechanical stress under hygro-thermal environment were simulated and calculated. The simulation results showed that the bottom die-attach endured higher thermal stress after the moisture preconditioning under 85°C /85% RH. By simulation of hygroscopic swelling stress during reflow process, it was indicated that the critical position for the package reliability located at the corner of the bottom die and the interface between the bottom die-attach and die. Therefore, the reliability of the bottom layers is relatively low under hygro-thermal environment.