[Robin] starts with a demonstration of just how far a little bismuth depresses the melting point of tin solder — to about 135°. That allows plenty of time to work, and freeing leads from pads ...

Early soft soldering used pure tin (Sn), yet gradually alloys were sought ... as did the addition of nickel (Ni) and bismuth (Bi), with the latter also decreasing the melting point of the alloy.