Isothermal Aging of Low-Ag SAC with Al Addition

Zinc sulfide (ZnS) thin films have been successfully deposited via spray pyrolysis using an aqueous solution of thiourea and zinc Amongst the candidates of lead-free solders, eutectic or near eutectic Sn-Ag-Cu or SAC has been found to be the most suitable candidate to replace the traditional tin-lead (Sn-Pb) solder. However, due to the higher cost of SAC (expensive Ag), research have now been directed to find a lower cost lead-free solder with comparable or better performance to the Sn-Pb or SAC, i.e low Ag SAC or Sn-Cu solders. The solder alloys developed in this work were SAC 0305 (0.3 wt% Ag, 0.5 wt% Cu and bal. Sn) and SAC0305 with 1 wt% Al and 2 wt% Al. The addition of Al is expected to refine the β-Sn grains and contribute to higher solder strength. Characterization of the solder alloys focused on the bulk solder microstructure, intermetallic compound (IMC) evaluation and wettability of solder alloys in reflowed and aged conditions. Reflow temperature was 270 °C while aging was done isothermally in normal atmosphere for 100, 200 and 500 hours at 100 °C and 150 °C. Microstructure of bulk solder and the IMC formed at interface between solder and Cu substrate were observed using SEM equipped with EDX. Wettability of solder was evaluated via wetting force and time, and the wetting angle between solder and the Cu substrate. A low Ag content resulted in fine Ag3Sn IMC as compared to much larger needle Ag3Sn observed in the commercial SAC 305 (Ag 3.0 wt%). With Al addition, SEM result showed finer β-Sn dendrites with fine Ag3Sn and Cu6Sn5 distributed within the eutectic colony of the bulk solder, and also thinner IMC layer in reflowed samples. Isothermally aged samples on the other hand, showed thicker IMC layer for Al added solder samples. This could be attributed to the finer β-Sn dendrites in the bulk solder providing more diffusion path and increased the thickness of IMC layer. The IMC formed at the interface between solder and the copper substrate was identified as Cu6Sn5 in reflowed samples, and both Cu6Sn5 and Cu3Sn when aged. Wettability of the solder alloys decreased with the addition of Al.