Effect of Cu electroplating parameters on microvoid formation and high-speed shear strength in Sn-3.0Ag-0.5Cu/Cu joints

Abstract

The effect of Cu electroplating parameters, i.e., the bath composition and current density, on the impact strength of Sn-3.0Ag-0.5Cu (SAC)/Cu joints was investigated using a high-speed ball shear test. An SAC solder ball was attached to an electroplated Cu layer by a reflow at a peak temperature of 260 degrees C. Thermal aging at 180 degrees C formed a Cu3Sn intermetallic compound (IMC) between the Cu6Sn5 IMC and the electroplated Cu, and microvoids were formed within the Cu3Sn IMC layer. The total microvoid area increased with increased Cu electroplating current density. When Cl- ions alone were added to the electroplating bath, the total microvoid area decreased with thermal aging. In contrast, when both polyethylene glycol (PEG) and Cl- ions were added to the bath, the total microvoid area increased. However, the overall IMC thickness was similar for all samples, irrespective of the electroplating parameters. The shear strength of the SAC solder joint reflowed on the electroplated Cu decreased with increased aging time. For the same aging time, the shear strength decreased with increased current density. In addition, the shear strength decreased when the electroplating bath contained both PEG and Cl- ions. As the aging time increased, the fracture site shifted from the solder/Cu6Sn5 interface to the Cu3Sn/Cu interface. An increase in the total microvoid area in the Cu3Sn IMC as well as an increase in the IMC thickness decreased the shear strength of the SAC solder on the electroplated Cu.