The effect of substrate surface roughness on the wettability of Sn-Bi solders

The effect of substrate surface roughness on the wettability of Sn-Bi solde rs is investigated by the eutectic Sn-Bi alloy on Cu/Al2O3 substrates at 19 0 degrees C. To engineer the surface with different roughnesses, the Cu-sid e of the substrates is polished with sandpaper with abrasive number 100, 24 0, 400, 600, 800, 1200, and 1 mu m alumina powder, respectively. Both dynam ic and static contact angles of the solder drops are studied by the real-ti me image in a dynamic contact angle analyzer system (FTA200). During dynami c wetting, the wetting velocity of the solder drop decreases for the roughe r surface. However, the time to reach the static contact angle seems to be identical with different substrate surface roughness. The wetting tip of th e solder cap exhibits a waveform on the rough surface, indicating that the liquid drop tends to flow along the valley. As the solder drops reach a sta tic state, the static contact angle increases with the substrate surface ro ughness. This demonstrates that the wettability of solders degrades as the substrates become rough.