Electromigration Mechanisms of Solder Joints with Limited Sn Volume in Advanced Electronic Packaging

Electromigration (EM) of Cu/Solder/Ni/Cu and Cu/Solder/Cu joints are studied. Both solder systems have limited Sn volumes that they can all form full IMC joints under EM before UBMs are fully consumed. EM induced cathode side IMC voiding can be found in Cu/Solder/Ni/Cu system for both electron wind directions, but no such voiding in Cu/Solder/Cu system. The voiding mechanism is believed to be Sn flux divergence induced Sn/IMC interface voiding before full IMC joints are formed. No such mechanism happens in Cu/Solder/Cu system because it has less solder volume, which makes top and bottom IMC merging earlier than EM induced Sn/IMC voiding. EM also cause fast cathode side UBM dissolution. The dissolved cathode side UBMs, including Cu and Ni, diffuse toward anode side to react with Sn forming IMCs. When Ni/Cu UBM is on cathode side, the full IMC joints are mixture of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4; when Cu UBM is on cathode side, the full IMC joint is mainly Cu6Sn5 IMC for both systems. Cu3Sn IMC can always be seen between Cu and Cu6Sn5 based IMC. Abnormal Cu3Sn/Cu6Sn5 separation during EM can also be found when contamination is present at the interface.