The solidus of a solder alloy is the temperature above which the alloy begins to melt, and, below which is completely solid. At this point, part of the alloy becomes liquid, while the other part remains solid. The solidus temperature depends on the composition of the alloy. The reason for the solidus is the various microstructures/phases within the alloy. Changing the composition can change the solidus temperature. If you have more questions feel free to reach out to me or email Askus@indium.com.