The Effect of Epoxy Molding Compound on Thermal/Residual Deformations and Stresses in IC Packages During Manufacturing Process

Abstract

Thermal/residual deformations and stresses in plastic integrated circuit (IC) packages caused by epoxy molding compound (EMC) during the manufacturing process are investigated experimentally (only for deformations), theoretically, and numerically. A real-time Twyman-Green interferometry is used for measuring the out-of-plane thermal and residual deformations of die/EMC bi-material specimens. Dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) are for characterizing thermomechanical properties of the EMC materials. A finite element model (FEM) and theory associated with experimental observations are employed for understanding the thermal/residual deformations and stresses of IC packages due to EMC encapsulation. It is shown that EMC materials must be fully cured so that the material properties are stable enough for applications. Experimental results show that the EMC material experiences stress relaxation due to its viscoelastic behavior during the post mold curing (PMC) process. As a result, the strains (stresses) resulted from the chemical shrinkage of the EMC curing could be relaxed during the PMC process, so that the chemical shrinkage has no effect on the residual strains (stresses) for the plastic packages being post cured. Compared with numerical and theoretical analyses, the experimental results have demonstrated that die/EMC bi-material structure at high temperature (above Tg) warps less than expected, as a result of viscoelastic stress relaxation of EMC at high temperature (during solder reflow process). Meanwhile, this stress relaxation can also cause shifting this zero-stress temperature to the higher one, so that the residual deformations (stresses) of die/EMC bi-material specimens were found to increase by about 40% after the solder reflow process. The residual and thermal stresses have been resolved by FEM and theoretical analyses. The results suggest that the pure bending stresses (without shear and peel stresses) of the bi-material specimens are only limited in the region from x= 0 (the center) to x= 0.75 L due to the free edge effects, but this region is shrunk down to x= 0.4L at 200degC. And the maximum warpage and bending stress per unit temperature change is occurred around 165degC (Tg of the EMC). This study has demonstrated that the Twyman-Green experiment with associated bi-material plate theory and FEM can provide a useful tool for studying the EMC-induce residual/thermal deformations and stresses during the IC packaging fabrication