The assembly of plastic electronic package requires a sequence of process steps. Every process step induces thermal residual deformation and stresses on the assembled components, which cause mechanical effects on the subsequent process step. Processing model with considering the effect of chemical shrinkage on molding compound is built to simulate the package warpage and stresses in assembly. The processing model, a nonlinear model with element birth and death used to activate and deactivate the processing materials, can more realistically simulate a series of assembly processes in a plastic package. The stresses in package components are primary factor for the damage of package. The induced package stresses can cause the delamination between die/die attach interface or die attach/die pad interface. In this study, effects of material properties as well thickness of die attach on warpage and stresses of TSOP (Thin Small Outline Package) during assembly are discussed.