Why do tin-lead solders soften over a range of temperatures?

Tin-lead solders soften over a range of temperatures because, with the exception of the eutectic composition, they do not transition directly from solid to liquid at a single point.12

Most tin-lead solders consist of varying proportions of the two metals. Except for the specific 62-percent-tin and 38-percent-lead mixture—which melts instantly at 183 °C (361 °F)—these alloys pass through a "pasty range" during heating. In this state, the material is neither fully solid nor fully liquid, a property that is intentionally utilized in specific joining processes like wipe-soldering and certain molding applications.1