Effect of Al and Zn alloying elements on Sn-3.8Ag-0.7Cu and Sn-3.6Ag solder reaction with Cu and Ni(P) substrate

Sn-Ag-Cu (SAC) and Sn-Ag alloys are considered to be the most promising Pb-free solders for electronics applications. The properties of these solders can be improved by the addition of minor alloying elements to control the intermetallic compounds (IMCs) that are formed during soldering and high temperature storage/cycling. Additional alloying elements into the solder alloys change the solidification path and reaction products. In this study, the effects of Al addition in the range 0.5 to 2 wt.% and Zn in the range 0.5 to 1.5 wt.% were studied, together with the effect of varying solder volume. The interfacial reaction studies were carried out on Cu and Ni(P) substrates. The resultant solder joint microstructure after reflow and isothermal aging at 150°C up to 500 h were investigated under Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray analysis (EDX) for phase identification and Optical microscopy (OM) for qualitative and quantitative analysis. Our experimental results have confirmed that addition of Al and Zn alloys forms Al₂Cu and Cu₅Zn₈ on Cu substrate and Al₃Ni and Ni₅Zn₂₁ on Ni(P) substrate respectively. These IMC layers have previously been found to act as barrier layers preventing excessive IMC growth. Additional effects of the additives on the eutectic structure of the solder and its properties are also reported. It should be noted that Al and Zn are currently avoided in reflow soldering processes due to their adverse effects on solder wetting. Given the optimum concentrations of Al and Zn found in the present study for barrier layer formation, future work will concentrate on avoiding the wetting problems by injecting the reactive elements (Al, Zn) into the solder in the form of nanoparticles, which release the active element after the initial wetting processes, are complete.