Dynamics of moisture diffusion, hygrothermal stresses and delamination in plasticic packages

Abstract

Plastic-encapsulated IC packages absorb moisture during packaging, storage and transportation. This results in hygrostresses which exacerbates the thermal stresses induced during reflow soldering and cause "popcorn" cracking. Hence it is important to understand the dynamics of moisture diffusion and heat transfer in processes commonly encountered during the assembly and testing of IC packages. This paper describes a finite element study of heat transfer and moisture diffusion in a plastic IC package, and the concommitant development of thermal and hygro-stressess during moisture preconditioning, prebaking and solder reflow. The effects of hygro-swelling coefficient, package geometry and prebaking period were investigated. It was found that the residual hygrostress after 100 hours of prebaking was apparently significant, but, on going through solder reflow, it was shown that the moisture-induced stress intensity factor decreased rapidly and had no effect on package delamination. However, if no prebaking were done and a package was subjected to 168 hours of 85°C/85%RH moisture absorption preconditioning immediately prior to solder reflow, delamination would occur with the likelihood of popcorn cracking.