Impact of PBGA-ball-coplanarity on formation of solder joints

Type 225L PBGAs with eutectic balls are specially processed and assembled to determine the effect of ball-coplanarity on solder joint formation. A mathematical model is developed to predict the formation and shape of the joints as a function of the initial ball volume, coplanarity, and other part and process variables. The model-predicted values are in very good agreement with the observed ones. A method is also developed for characterizing the effect of PCB warpage, and sag during assembly reflow, on joint formation. The overall methodology may be used to set package coplanarity specifications that ensure acceptable solder joints. Hewlett-Packard will continue purchasing PBGAs that meet the JEDEC current 6-mil coplanarity specification. This study suggests that industrial processes can tolerate larger than 6-mil ball-coplanarities for PBGA's with collapsible balls. Hopefully, this work starts momentum within the industry to relax the JEDEC specification. The potential benefit is a substantial package cost savings for package suppliers and end-users.