Chip to chip bonding using micro-cu bumps with sn capping layers

The chip to chip bonding technique using a Cu bump capped with thin Sn layers has been frequently applied to 3D chip stacking technology. We studied the effect of the microstructure on the joints. The joints were fabricated by joining micro-Cu bumps capped with Sn-Ag solder with sizes of 10 mum times 10 mum, 20 mumtimes 20 mum, and 30 mum times 30 mum to Cu pads capped with Sn-Ag solder at 245^degC-330^degC using a thermo compression bonder. Three different types of microstructure were formed in the joints depending on the bonding condition: an Sn-rich phase with Cu₆Sn₅ in the Cu interfaces, Cu₆Sn₅ in the interior with Cu₃Sn in the Cu interfaces, and one single Cu₃Sn phase. The joint with only the Cu₃Sn phase had the highest shear strength. Specimens were aged at 150^degC for up to 1000 h. During aging, the microstructures of all the joints became Cu₃Sn phase only. The shear strength of the joints was very sensitive to the formation of Cu₃Sn and microvoids.