Effect of (Au, Ni)Sn 4 Evolution on Sn-37Pb/ENIG Solder Joint Reliability Under Isothermal and Temperature-Cycled Conditions

The ternary intermetallic compound Au_0.5Ni_0.5Sn₄ forms at the Sn-37Pb/ENIG solder interface during aging and temperature cycling, leading to increased interfacial cracking and a corresponding decrease in solder joint reliability for 15 mm ball grid array (BGA) structures. (Au,Ni)Sn ₄ forms at both the board finish (bottom) and component side (top) of the solder joint for isothermally aged, temperature-cycled, and (aged + cycled) joints. For control specimens (reflow only), no cracks or interfacial Au are observed. For isothermally aged joints (170 and 340 h at 125degC), a broken, discontinuous layer of (Au,Ni)Sn₄ is present, but no cracking. For temperature-cycled joints, lowered reliability and interfacial cracking occurs along a continuous (Au,Ni)Sn₄ intermetallic layer on the solder side of the interface after ~450 h of cycling. Aging + cycling did little to inhibit cracking or formation of (Au,Ni)Sn₄. Development of a continuous (Au,Ni)Sn₄ film at the interface is the key failure mechanism. At low cycle numbers where high joint reliability is observed, the (Au,Ni)Sn₄ layer is discontinuous and not fully developed. At higher cycle numbers and longer aging times, the (Au,Ni)Sn ₄ layer becomes continuous and encourages crack growth along the intermetallic interface and consequent lower reliability. The correlation of interfacial smoothness with lowered reliability is consistent with recent work showing that, when intermetallic compounds form smoothly at the solder interface, the mechanical properties are degraded (compared to a rough intermetallic) due to the decreased resistance to shear along the interface