Morphology of wetting reaction of eutectic SnPb solder on Au foils

We have studied the wetting behaviors of eutectic SnPb solder caps on pure Au foils at 200°C. Surface morphology, wetting angle, and wetting tip stability were examined by scanning electron microscopy and energy dispersion x-ray analysis. In addition, interfacial reaction in the bulk diffusion couple of the solder and Au was studied. Intermetallic compound (AuSn₄) was observed on the surface of the solder cap, often called the cold joint, as early as 2 s of reflow at 200°C. The wetting angle decreased with reflow time but remained constant after 180 s of reflow. However, the side-band width and the diameter of the solder cap continuously increase with the reflow time to 5 min when the entire eutectic SnPb solder cap is fully consumed by intermetallic compound formation. In the bulk diffusion couple of eutectic SnPb and Au, the growth and morphology of the intermetallic compound (AuSn₄) was influenced by Au dissolution. Since Au, showing excellent wetting behavior, forms the platelet-type intermetallic compound, we postulate that the wetting rate may be improved with a system containing platelet-type intermetallic compound formation for the eutectic SnPb solder.