Interfacial Reaction And Electromigration Of SnZnBi-χNi Lead-free Solders And Cu Substrate

Under the current micro-electronic condition, the lead-free solder alloys whichare used as the conjunct solder junction in the electronic packaging industry wouldreact with the commonly-used Cu substrate to form the intermetallic compound （IMC）with or without current stressing. Interfacial reaction between SnZnBi-xNi and Cusubstrate and electromigration of one-dimensional Cu/SnZnBi-xNi/Cu under differentcurrent stressing time are systematically investigated in this paper.A diffusion-controlled mechanism of the intermetallic compound growth at thesolder/Cu interface was found in the interfacial reaction of SnZnBi-based lead-freesolder and Cu substrate: the thickness of the IMC show an exponential increase withthe aging time. The Ni addition can consume part of the Zn atoms in the solder matrix,so the fast growth of the Cu₅Zn₈IMC was effectively retarded and the reliabilityproblem of the solder junction caused by the characteristic brittle feature of the IMCwas modified.We performed an experiment of the conjunct Cu/SnZnBi/Cu structure withdifferent time duration under current stressing. The IMC at both the anode and thecathode side show a stress relaxation behavior after a long stressing time because ofthe mass transfer which was caused by the fast atomic diffusion. Due to the molarvolume expansion accompanied by the IMC growth, part of the IMC was extruded outof the free surface to form the hillock. This "bulged" phenomenon in theelectromigration of Ni-added Cu/SnZnBi-1Ni/Cu structure was greatly inhibited.Additionally, the IMC thickness show a "reversion" in the periodic current-stressedCu/SnZnBi/Cu sample because of the back stress gradient built up by the Sn diffusioninside the solder matrix. We hope that the results can provide a reference for the realelectronic packaging industry.