Application of the Taguchi method on the robust design of molded 225 plastic ball grid array packages

Abstract

A three-dimensional nonlinear finite element model of a molded plastic ball grid array (PBGA) has been developed using the ANSYS/sup TM/ finite element simulation code. The model has been used to optimize the package 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 mold compound properties along with the substrate, mold compound and die attach thicknesses on the potential defects that would possibly occur during temperature cycling. For package failures, simulations performed represent temperature cycling 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 (low stress in the package and low package warpage) mold compound as well as die attach material should have low Young's modulus and low coefficient of thermal expansion. Furthermore, it is found that the overmold should be kept as thin as possible. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of the probable first solder ball failure. The results indicate that higher strain levels are obtained in solder balls located one pitch inwards from the edge of the die. >