Effect of Au/Pd bilayer metallization on solder/Cu interfacial reaction and growth kinetics of CuSn intermetallic compounds during solid-state aging

The present study investigated the effect of Au/Pd bilayer metallization, 0.46 μm Au and 0.04 μm Pd, on the solder/Cu interfacial reaction and growth kinetics of the CuSn intermetallic compounds during solid-state aging. The as-reflowed Ni/Sn1.8Ag + Sn3Ag0.5Cu/Au/Pd/Cu solder joint was aged at 150 °C for 500 and 1000 h. The solder joint incorporated with bare Cu pad was also investigated for comparison. The interfacial reaction between solder and Au/Pd metallized Cu pad formed the (Cu,Ni,Pd,Au)6Sn5 and Cu3Sn compounds during reflow. The AuSn4 compound was distributed throughout the solder joint. The AuSn4 compound gradually dissolved during thermal aging, which rendered Au incorporate with Cu6Sn5 for up to 6.58-8.61 at.%. The substantial inclusion of Au in the Cu6Sn5 stabilized the compound and accelerated the compound growth during solid-state aging. The fast growing Cu6Sn5 strongly suppressed the formation of the neighboring Cu3Sn compound and thus effectively suppressed the potential micro-void formation within the Cu3Sn layer. Additionally, the formation of the undesirable AuSn4 compound was inhibited in the solder joint incorporated with Au/Pd metallized Cu pad. These observations are expected to enhance the functional performance of the solder joint with the incorporation of Au/Pd metallization at the solder/Cu interface.