A wetting test was 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.