soldering

Soldering is brazing's low-temperature cousin: the same idea of joining parts with a molten filler that wets the surfaces and freezes into a bond, but with a filler that melts below 450 degrees C. Classic solder was a tin-lead alloy that melts near 180 to 190 degrees C, soft and easy to flow with a small iron. It is the everyday metal glue of electronics and plumbing, holding every component onto a circuit board and sealing copper pipe joints.

You clean the surfaces, apply flux to strip oxide and help wetting, heat the joint, and feed in solder. The molten solder wets the metal, is drawn into the gap, and solidifies to make a joint that is electrically conductive, sealed, and modestly strong. Modern electronics use lead-free solders, typically tin-silver-copper alloys that melt a bit higher, around 217 to 220 degrees C. The reliability of a solder joint depends heavily on good wetting and on the thin intermetallic compound layer that forms where solder meets the base metal.

The honest limits follow from the low melting point. Solder joints are weak and creep readily even at modest temperatures, so soldering is for electrical connection and sealing, not for carrying structural loads. Overheating or a bad flux gives cold, non-wetted joints that fail intermittently, a classic source of flaky electronics. Lead-free solders solved a toxicity problem but melt hotter and can grow tin whiskers, tiny conductive filaments that cause shorts. Soldering is indispensable but should never be asked to be strong.