Finite element analysis of sequential processes for ball grid array packages

What has been troubling the community of electronics packaging in terms of the residual stress modelling is the so-called stress-free temperature. Although many researchers have investigated the process-induced residual stresses for various plastic electronics packages, to the authors´ best knowledge, only one stress free temperature is considered in their models. If multiple processes are involved, it is impossible to use this alleged stress-free temperature. The topological change has seldom been considered, but it has proven to be important even when all the material properties are assumed to be linear (elastic). If all the temperature and time dependent material properties are considered, the topological change and the sequence of processing steps can have significant effects on the final residual stress calculations. In this study, the superposition method, element death and birth (EDB) method, and constraint equation (CE) method have been used to simulate two simple models with extreme dimensions and an actual TFBGA (thin-profile fine-pitch ball grid array) package model. The results show that superposition is a reliable method in predicting the warpage and residual stress of an electronics package. The EDB method does not accurately predict the warpage especially on the surface of a package, but it is able to reliably obtain the residual stress. The CE method only gives accurate displacement results if the dimensions of the model are small, but it is unable to obtain the residual stress accurately.