Chip to Chip Bonding Using Cu Bumps Capped with Thin Sn Layers and the Effect of Microstructure on the Shear Strength of Joints

Abstract

Chip to chip bonding techniques using Cu bumps capped with thin solder layers have been frequently applied to 3D chip stacking technology. We studied the effect of joint microstructure on shear strength. Joints were formed by joining Sn/Cu bumps on a Si die and Sn/Cu layers on another Si die at 245-330A degrees C using a thermo-compression bonder. Three different types of microstructures were fabricated in the joints by controlling the bonding temperature and time, (1) a Sn-rich phase with a Cu6Sn5 phase at the Cu interfaces, (2) a Cu6Sn5 phase in the interior with a Cu3Sn phase at the Cu interfaces, and (3) one single Cu3Sn phase throughout the whole joint. The joint having a single Cu3Sn phase had the highest shear strength. Specimens were aged up to 2000 h at 150A degrees C and 180A degrees C. During aging, the microstructures of all joints were transformed in a single Cu3Sn phase. The shear strength of the joints was very sensitive to the formation of Cu3Sn and microvoids. Microvoids formed in the solder joints with a Cu6Sn5 phase with and without a Sn-rich phase during aging and decreased the shear strength of the joints. Conversely, aging did not induce the formation of microvoids in the joints which originally had only a Cu3Sn phase and the shear strength was not decreased.