Application of the Taguchi method to chip scale package (CSP) design

A three-dimensional (3-D) nonlinear finite element model of an overmolded chip scale package (CSP) on flex-tape carrier has been developed by using ANSYS^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 Joint Electron Device Engineering Council (JEDEC) limits. An L₁₈ Taguchi matrix has been developed to investigate the effect of die thickness and die size, mold compound material and thickness, flex-tape thickness, die attach epoxy and copper trace thicknesses, and solder bail collapsed stand-off height on the reliability of the package during temperature cycling. For package failures, simulations performed represent temperature cycling 125°C to -40°C. This condition is approximated by cooling the package which is mounted on a multilayer printed circuit board (PCB) from 125°C to -40°C. For solder ball coplanarity analysis, simulations have been performed without the PCB and the lowest temperature of the cycle is changed to 25°C. Predicted results indicate that for an optimum design, that is low stress in the package and low package warpage, the package should have smaller die with thicker overmold. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of solder ball with the highest strain level. The results indicate that the highest strain levels are attained in solder balls located at the edge of the die. The strain levels could then be used to predict the fatigue life of individual solder balls