The effects of solder deformation on the wetting characteristics and interfacial reaction of Sn–3.5Ag solders on Cu substrates in fluxless soldering

The effects of solder deformation on the wetting characteristics during fluxless soldering were studied when deformed Sn–3.5Ag solder balls were reacted with Cu or oxidized Cu substrates. The Cu surfaces were oxidized at 100 °C for 2 or 4 h in air. After the 760 μm diameter solder balls were deformed on the substrates under 0–30 N, they were then reflowed at 300 °C for 30 s without flux. An optical microscope and a scanning electron microscope equipped with energy dispersive spectroscopy were used to measure the wetting angles and to characterize interfacial microstructures. As solder deformation increased, the wetting angle of solder bumps on the Cu or oxidized Cu substrates decreased and the spreading area increased. The oxide layer on the Cu surface decreased the wettability of the solders. Intermetallic compound (IMC) growth was suppressed in the solder interface when the solder reacted with oxidized Cu, while the IMC thickness increased with solder deformation. Solder deformation exposed a fresh Sn surface and improved contact between the solder and Cu substrate, thereby increasing the wettability of the solders.