Optimization of high pin count cavity-up enhanced plastic ball grid array (EPBGA) packages for robust design

Abstract

Three-dimensional (3-D) nonlinear finite element models of epoxy encapsulated enhanced plastic ball grid array (EPBGA) packages with and without an aluminum lid have been developed using ANSYS finite element simulation code. The model has been used to optimize the packages for robust design and to determine design rules to keep package warpage within acceptable limits. An L/sub 18/ Taguchi matrix has been developed to investigate the effect of die attach and encapsulant properties along with the substrate, encapsulant, die attach, and internal copper plane thicknesses on the reliability of the package during temperature cycling. For package failures, simulations performed represent temperature cycling from 165/spl deg/C to -65/spl deg/C. This condition is approximated by cooling the package mounted on a multilayer printed circuit board (PCB) from 165/spl deg/C to -65/spl deg/C. For coplanarity analysis, simulations have been performed without the PCB and the lowest temperature of the cycle is changed to 20/spl deg/C. Predicted results indicate that for an optimum design, that is low stress in the package and low package warpage, encapsulant as well as die attach material should have low Young's modulus and low coefficient of thermal expansion. Furthermore, it is found that the substrate and the die attach epoxy thicknesses should be increased beyond the current design. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of the possible first solder ball failure.