Finite element analysis of hygrothermally induced stresses in plastic IC packages

In the present study, the moisture and temperature distributions and residual stresses inside plastic encapsulated IC packages are evaluated in order to asses product reliability. Numerical procedures based on finite element analyses are presented to calculate the hygro-thermally induced deformations and stresses in plastic IC packages during the surface mounting process preceded by the moisture soaking test. For example residual stresses in thin LOC (Lead-On-Chip) TSOP packages during the reflow soldering process preceded by the 168 hours 85°C/85% RH moisture soaking test have been studied. Numerical results show that when TSOP packages undergo the reflow soldering process substantially high tensile σ₂ stresses arose in the silicon chip while high compressive stresses were in the encapsulant below the chip. Such high compressive and tensile stress development in the silicon chip and encapsulant below the chip is responsible for the delamination and popcorn crack. The results also show that the magnitudes of residual stresses in IC packages may depend not only on the magnitude of loading but also on the loading history due to the hygro-thermo-viscoelastic behavior of plastic mold compound materials