Under bump metallizations for lead free solders

Several under bump metallization (UBM) schemes using CuNi alloys as the solderable layer were investigated. The nickel slows down the dissolution of the UBM into the solder and the formation of intermetallics during reflow. Ni containing UBMs were fabricated and reflowed with eutectic SnAg solder balls. The solder/UBM interfaces were analysed with SEM to find out how the Ni concentration affects the reaction, and how much Ni is needed to obtain a sufficiently slow reaction rate. Reflows were also made on top of bulk substrates to study the reaction when there is an unlimited amount of CuNi available. To determine the rate of dissolution of the substrate material into solder, CuNi foils of different concentrations were immersed in pure Sn and eutectic PbSn solder baths for soldering times ranging from 30 seconds to 30 minutes. Since nickel metallizations often have high stresses, stress in the UBMs was measured by the wafer curvature method. Stress vs. Ni content plots show that while stresses increase somewhat with the Ni content, the adhesion layer under the CuNi layer has a much larger effect on the stress.