Grain structure evolution and its impact on the fatigue reliability of lead-free solder joints in BGA packaging assembly

This paper reports the findings supporting that solder joint grain structure within an assembly is not fixed but varies with the joint location and cooling rate. Also reported is the fact that such change in grain structure can make the joint fatigue resistance to be increased. These conclusions are made by comparing the grain structure and fatigue kinetics of the joints in BGA assembly with variation in cooling rate after thermal aging. Our study finds that the grain structure changes with cooling rate because of the kinetic interplay between plastic deformation by thermal strain and its relaxation process other than dislocation glide. The variation in the spontaneous plastic deformation and its storage in the solder promote recrystallization in one extreme, while the process of dynamic relaxation releases the stored energy and make the joint maintain its original grain structure in another extreme. Mechanical fatigue testing of these samples reveals that failure prone joint, corner joint, becomes immune to fatigue failure when its grain structure is changed to polygranular structure. The degree of reliability gain is not possible to quantify at the present moment because the failed joint is shifted to non-recrystallized joint and the failure kinetics is also affected by the amount of residual stress which is also affected by the cooling rate. However, there are many of indications that the joint with polygranular structure is much less susceptible to fatigue failure. Some of highlighting evidences are presented in this paper.