Abstract
Abstract The microstructure of ternary Sn-Ag-Bi solder alloys before and after thermal aging test is evaluated and the effect of substrate surface roughness and oxidation on the wettability of solders is investigated. The ternary Sn-Ag-Bi solders are fabricated by different processes to evaluate its microstructure and composition. The homogeneity of the solder alloys is improved by the increase of molten time. As solders are thermally aged at 150℃ for a long period of time, the Ag3Sn phase coarsens and the Bi-rich phase segregates at the grain boundaries. There also exist hillocks and voids around Bi-rich precipitates. The microstructure of the ternary Sn-Ag-Bi solders/Cu/Al2O3 assembly under thermal aging test is also investigated. The growth of the Cu6Sn5 layer is rather rapid and exhibits a parabolic time dependence. The roughness of the interfacial morphology at the solder/Cu6Sn5 side increases along with some cracks and voids present. The effect of surface roughness on wetting is first studied in the H2O/Cu system and then in the solder/Cu system. As the surface roughness of the substrate increases, the static contact angle in both systems increases. During dynamic wetting, the spreading velocity of water drop is faster as substrates become rough. However, a reverse result occurs in the solder/Cu system, i.e., the time to reach the static contact angle is shorter in the smooth surface. An oxide layer, identified as SnO2, forms rapidly and its growth follows parabolic behavior as the solder is melted to a liquid state. The surface tension of the liquid solder reduces with the temperature and the oxidation degree. The surface morphology of the oxide layer is smoother in the eutectic Sn-Bi solder rather than in the eutectic Sn-Ag and ternary Sn-Ag-Bi solders. Both the oxidation of solders and substrates degrades the wettability of the solder. In the fluxless soldering, although the solder drops are almost non-wetting on the Cu substrate for all oxidation, there also exist an intermetallic compound layer in the middle of the contact area. The compound layer does not form until a period time of reflowing, yet it grows thicker as the reflowing time is increased.