Why does the 62-percent-tin and 38-percent-lead solder melt at one temperature?

The 62-percent-tin and 38-percent-lead alloy melts at a single, specific temperature of 183 °C (361 °F) because it is a eutectic composition.12

In metallurgy, while most tin-lead alloys soften over a temperature range—creating a "pasty range" before they fully melt—the eutectic alloy is unique. It represents the specific proportion where the melting point of the mixture is at its absolute minimum, lower than that of either pure metal. Because it transitions directly from solid to liquid at this precise temperature, it does not exhibit the softening phase characteristic of other lead-tin ratios. This property makes eutectic solder highly valuable in the electronics industry, where precise, low-temperature melting is required to protect heat-sensitive components.213