Cracking and phase stability in reaction layers between Sn-Cu-Ni solders and Cu substrates

A method of limiting cracking in the Cu6Sn5 intermetallic compounds (IMCs) at the interface between lead-free solders and copper substrates has been developed. To explore the mechanism of crack inhibition in the nickel-containing IMC reaction layers, detailed synchrotron x-ray powder diffraction with Rietveld analysis and differential scanning calorimetry have been used. The results show that nickel stabilizes the high-temperature hexagonal allotrope of Cu6Sn5, avoiding stresses induced by a volumetric change that would otherwise occur on transformation to the monoclinic phase