Effect of material properties on the fatigue life of dual solder (DS) ceramic ball grid array (CBGA) solder joints

The interconnection structure of industry-standard ball grid array (BGA) electronic packages, both plastic and ceramic, consists of a single alloy, typically eutectic Pb-Sn solder. However, IBM ceramic BGA packages utilize a unique dual-solder (DS) interconnection system, which consists of a low-melting alloy (eutectic Pb-Sn) to attach a higher melting alloy ball to the ceramic substrate at one end and card on the opposite end of the joint. Unlike industry-standard BGA joints which collapse upon reflow, DS joints do not since their high MP ball does not become molten during processing, providing a reproducible standoff and significant fatigue life advantage. Finite element modeling indicates that the highest stress occurs within the low-MP fillet area, verified by crack patterns generated during accelerated thermal fatigue testing. The purpose of this study was to determine which combination of material properties provide an optimum joint fatigue life. The intent is to more uniformly distribute thermally induced strains within the solder connection by transferring some of the stress-relief from within the fillet areas to the ball. Accordingly, several ball and fillet materials were investigated in various combinations ranging in yield, strength and creep resistance properties. Both thermal fatigue data and supporting metallography are presented for the various solder joint combinations investigated.