The spreading of Sn-3Ag-xBi solders on Fe-42Ni has been studied using a drop transfer setup. Initial spreading velocities as fast as ~0.5 m/s have been recorded. The results are consistent with a liquid front moving on a metastable, flat, unreacted substrate and can be described by using a modified molecular-kinetic model for which the rate controlling step is the movement of one atom from the liquid to the surface of the solid substrate. Although the phase diagram predicts the formation of two Fe-Sn intermetallics at the solder/substrate interface in samples heated at temperatures lower than 513oC, after spreading at 250oC only a thin FeSn reaction layer could be observed. Two interfacial layers (FeSn and FeSn2) were found after spreading at 450oC.

The wetting and interfacial integrity of lead-free Sn-Ag and Sn-Ag-Bi solders with 42 alloy (Fe-42wt% Ni) were investigated. A tin-iron intermetallic layer formed at the interface and, during cooling, Ni3Sn4 platelets precipitated in the solder. After bonding, the Sn-3Ag joints have a eutectic microstructure, with a fine network of Ag3Sn particles surrounding large primary -Sn grains. Bismuth addition to Sn-3Ag improved wettability, but decreased the joint strength, owing to Bi segregation to the interface.