Soldering terminology

During solidification of a metal, atomic groups of identical spatial orientation will form equal crystal grains. Sub -grains are smaller sub- crystals in these grains. By deep etching of a solder, tiny lines in the Sn and Pb crystals can be made visible, as shown in the illustration. These are sub-grains, each having its own alignment. The boundaries of these units are referred to as sub-grain boundaries.