Characterize the microstructure and reliability of ultra fine pitch BGA joints

The solder ball pitch of BGA packages in mass production now is normally above 0.5 mm. To fulfill the future demand of package miniaturization, the 0.4 mm ultra fine pitch BGA solder joint was researched in this paper. The solder volume shrinkage along with the ball pitch decrease was found to affect the joint microstructure, which could be characterized in two aspects: the Sn dendrites in bulk solder became finer, and the so called ldquocross-interactionrdquo in interfacial layer occurred. The joint reliability variation due to volume shrinkage was paid much attention to. Four different alloy content solder balls were evaluated through thermal cycling and drop reliability test, to find out the most proper soldering materials on both NiAu and OSP pad finishes. Our research result indicated that reliable ultra fine pitch joint could be gotten if proper materials and processes adopted. Sn1.0Ag0.5Cu0.02Ni was recommended as the solder alloy composition for the ultra fine pitch BGA usage.