Joint Strength and Microstructure for Sn-Ag-(Cu) Soldering on an Electroless Ni-Au Surface Finish by Using a Flux Containing a Cu Compound

The effect of a flux containing Cu(II) stearate (barrier flux) on the strength of soldered joints between an electroless Ni-Au surface finish and two Pb-free soldering systems, Sn-3Ag-0.5Cu and Sn-3.5Ag (wt.%), was examined. Pull and shock tests showed that barrier flux gave a higher joint strength for both solder compositions than did a flux containing no Cu compounds. Interface analysis revealed that a thin P-rich layer and refined Cu-Ni-Sn intermetallic compounds were formed at the joint interface when barrier flux was used. It is assumed that the supply of Cu from barrier flux suppresses diffusion of Ni into the solder.