Interface between Sn–Sb–Cu solder and copper substrate

Influence of antimony and copper in Sn–Sb–Cu solder on the melting and solidification temperatures and on the microstructure of the interface between the solder and copper substrate after wetting the substrate at 623 K for 1800 s were studied. Microstructure of the interface between the solder and copper substrates in Cu–solder–Cu joints prepared at the same temperature for 1800 s was observed and shear strength of the joints was measured. Influence of the atmosphere – air with the flux and deoxidising N2 + 10H2 gas – was taken into account. Thermal stability and microstructure were studied by differential scanning calorimetry (DSC), light microscopy, scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD). Melting and solidification temperatures of the solders were determined. An interfacial transition zone was formed by diffusion reaction between solid copper and liquid solder. At the interface Cu3Sn and Cu6Sn5 phases arise. Cu3Sn is adjacent to the Cu substrate and its thickness decreases with increasing the amount of copper in solder. Scallop Cu6Sn5 phase is formed also inside the solder drop. The solid solution Sn(Sb) and SbSn phase compose the interior of the solder drop. Shear strength of the joints measured by push-off method decreases with increasing Sb concentration. Copper in the solder shows even bigger negative effect on the strength.

► New lead-free solder materials based on Sn–Sb–Cu were designed and prepared.
► Melting and solidification temperatures of the solders have been determined.
► Cu-substrate/solder interaction has been analyzed and quantified.
► Phases formed at the solder–substrate interface have been identified.
► Composition and soldering atmospheres were correlated with joint strength.