Solder

Solder is a metal that melts at a fairly low temperature, used like a conductive glue. Melted between a part and a circuit board, it wets both, then freezes into a joint that holds the part in place and carries current.

Most electronics now use a lead-free alloy called SAC305: about 96.5% tin, 3% silver and 0.5% copper. Tin does the main work, reacting with copper pads to form a thin layer of tin-copper compound that anchors the joint. Silver makes the joint stronger and more resistant to fatigue. Copper lowers the melting point slightly and slows the solder from dissolving the copper pads. This mix melts at about 217 to 220 °C. For circuit boards, the alloy is turned into a fine powder and mixed with sticky flux to make solder paste.

The paste is printed onto a printed circuit board through a stencil, parts are placed on top, and the board passes through a reflow oven that melts it. Every part on the main board, timing controller board and power supply board is held this way, and chips like the system-on-chip carry solder balls underneath. Older solder was mostly tin and lead, until EU rules banned lead in most electronics from 2006.