Tin-Lead, many times called Solder Plate, is widely used for functional soldering applications. The ratio of Tin to Lead does affect the melting point of the deposit. Typical ratios of Tin to Lead would be 60/40, 90/10, and 93/7. 60/40 will have the lowest melting point at roughly 374F (190C) while 90/10 will be 421F (217C).
The main purposes for using Tin-lead is its excellent solderability and corrosion protection but also to prevent tin whisker formation that can typically be experienced in 100% pure Tin. The growth of tin whiskers over time can cause failure of electronic components by bridging conductor paths causing a short circuit. However, far reaching impacts have been seen by the Lead Free electronic manufacturing directives mandated by the European Union so alternatives to Tin-Lead deposits remain ongoing.
Mil-P-81728 and ASTM B579 are primary specifications used for Tin-Lead.