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 um × 10 um, 20 um × 20 um, and 30 um × 30 um to Cu pads capped with Sn-Ag solder at 245oC-330oC 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 Cu 6 Sn 5 in the Cu interfaces, Cu 6 Sn 5 in the interior with Cu 3 Sn in the Cu interfaces, and one single Cu 3 Sn phase. The joint with only the Cu 3 Sn phase had the highest shear strength. Specimens were aged at 150oC for up to 1000 h. During aging, the microstructures of all the joints became Cu 3 Sn phase only. The shear strength of the joints was very sensitive to the formation of Cu 3 Sn and microvoids.