The effect of a concentration gradient on interfacial reactions in microbumps of Ni/SnAg/Cu during liquid-state soldering

We report new findings in Ni/SnAg–solder/Cu microbumps having a reduced solder thickness in the range of 40–10 μm; the growth rate of the intermetallic compounds depends strongly on the solder thickness. In the Ni/40 μm solder/Cu samples, the compound which grew on the Ni side grew slightly faster than that on the Cu side. However, the growth reverses as the solder thickness decreases below 20 μm due to the change in Cu and Ni concentration gradients in the solder.