Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads w as studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growt h of intermetallic (IMC) layers during soldering at 270 degreesC and aging at 150 degreesC. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insi gnificant effects during aging. Rapid Cu dissolution from the pad during re flow soldering resulted in a fine dispersion of Cu6Sn5 particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer gre w over a critical thickness about 1.2 mum for all solders. The bulk solder strength decreased rapidly as the fine Cu6Sn5 precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded si gnificantly after 1 day of aging at 150 degreesC and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear streng th of BGA solder joints, especially during system use at high temperatures.