Control of Solder Microstructure Stimulated by Interface Condition of the UBM/Solder and Enhancement of Electromigration Reliability

This paper presents two example cases of controlling microstructures of lead-free solder joints in a direction of improving electromigration (EM) reliability. Specifically, this paper reports the change in the grain structure by using the biased nucleation of Sn grains. The biased nucleation and development of highly textured Sn solder grains are seen when a solder ball is reflowed on the surface of (111) Cu single crystal. The texture development is found to be stronger in smaller balls, evidencing the substrate affecting the nucleation process. Also reported is the process of the grain refinement of solder when subjected to AC EM condition. Significant refinement of Sn grains is found in our study when SAC(Sn-Ag-Cu) solder joint are subjected to the pulsed EM condition. The refinement, probably induced by a dynamic recrystallization, results in substantial delay in the EM failure kinetics especially in low percentage failures, also known as early failures. Our investigation indicates that the early failures are related to the presence of EM shortcut, that is [001] Sn grain aligned to the EM direction, suggesting that the two cases of grain structure control demonstrated in this paper may offer an effective way of improving EM reliability.