Single-joint shear strength of micro Cu pillar solder bumps with different amounts of intermetallics

This study investigates the effects of intermetallics (IMCs) on the single-joint shear strength of 50 μm diameter Cu pillar bumps joined with Sn-4.0%Ag-0.5%Cu (SAC405) solder caps. The correlations between the shear strength of the Cu pillar bumps and the IMCs aged at 180°C for 150, 300, 500, and 750 h are studied. Microstructure characterization reveals that the Cu₆Sn₅ morphology transforms from scallop-type to planar-type after 150 h of aging. Void formations are observed at the Cu ₃Sn/Cu interface after 300 h of aging and the amount of void formations increases with aging time. The shear test result shows that the shear strength decreases with aging time. Although the void formations did contribute to the deterioration of the solder joint strength, the main contributing factor was the planarization of the Cu₆Sn₅ morphology through the aging process. The failure modes of the solder joint are also discussed in this study.