Effect of Ni on the kinetics of intermetallic compounds evolution on the Sn-0.7Cu-10Bi-xNi/Co interface during various reflow

In this paper, the interface reaction between the Sn-0.7Cu-10Bi-xNi (SCB-xNi, x = 0 and 0.20, in wt%) solders and Co substrate during multiple reflows was studied. The results indicated that the morphologies and thickness of intermetallic compounds (IMCs) layer were changed after addition of 0.20 wt% Ni. The thickness of IMCs increased with the increasing reflow cycles on the SCB-xNi/Co interface. It was found that the formation of Ni-Co-Sn IMCs layer effectively inhibited the diffusion of Co atoms from Co substrate to solder/IMCs interface, and the primary diffusion of Sn atoms in the interface reaction led to the growth of IMCs along with Co/IMCs interface. Moreover, the IMCs layer was stratified obviously and there was a significant difference in compositions of upper and lower IMCs layers on the SCB-0.20Ni/Co interface with the increasing reflow cycles. When the reflow reached to ten cycles, the IMCs layer was dissolved into solder matrix. The results further suggested that the growth and evolution kinetics of IMCs layer was changed at the SCB-xNi/Co interface during multiple reflow cycles. In addition, the mechanism of atomic diffusion and IMCs growth in the interface reaction between solder and Co substrate was improved. (paper)