Development of Mechanical Stress Simulator for Surface-Mount Devices

Abstract

A mechanical stress simulator for surface-mount devices was developed, which employed the finite-element method. The moisture distribution stage can calculate the moisture distribution of the LSI package exposed to temperature and humidity conditions. The heat conduction stage determines the time-dependent temperature distribution of the package immersed in a solder bath. Using the moisture and temperature distributions thus calculated, the mechanical stress stage gives the mechanical stress distribution in the package. Two groups of samples were prepared for solder dipping experiments. In one group, after dry baking for initialization, samples absorbed moisture by exposure at 30°C 85% (absorption process). In the other group, samples were exposed at 85°C 85% for 168 h and then dry-baked at 70°C (desorption process). These two groups have different moisture distributions in their packages. The results of solder dipping experiments are analyzed from the calculated mechanical stress values. In the absorption process, calculated maximum von Mises equivalent stress over the sectional molding compound σ; max increases monotonically with absorption time. Then, σ; max approaches its saturated value as absorption time tends to infinity. In the desorption process, σ; max decreases with desorption time and approaches the thermal stress value when desorption time tends to infinity. These calculations explain the effect of moisture distribution on package cracking.