Effect of excess temperature above liquidus of lead-free solders on wetting time in a wetting balance test

A wetting test a as conducted using a wetting balance tester to evaluate the wettability of Sn-Ag and Sn-Zn lead-free solders with the addition of bismuth or indium up to 25 mass%. The base metal and flux used were oxygen-free high-conductivity copper and rosin-based flux containing 0.2 mass%Cl and 35 mass% solid rosin diluted with isopropyl alcohol. The wetting time decreased with increases in the amount of additional elements at a fixed test temperature. The wetting force had only a slight dependence on the element content: Sn-Zn alloys exhibited less wetting force than Sn-Ag. The plots between the excess temperature above liquidus. (test temperature)-(liquidus), and wetting time correlated well. An increase in the excess temperature decreased the wetting time, indicating that the wetting time is closely controlled by the temperature-rising stage of the wetting test specimen. Therefore, a comparison of the contact angles is recommended for precisely evaluating the wettability on lead-free solders. The interfacial tension between the soldering flux and molten solder was measured by a simple method using a wetting balance tester to obtain the contact angle from the wetting balance test. The calculated contact angle in Sn-Ag system solders is smaller than that in Sn-Zn system solders, indicating that Sn-Ag system solders exhibit superior wettability.