Sumerian swordsmiths were soldering metal five thousand years ago
The trick that holds your phone's circuit board together is older than writing in most of the world. Sumerian swords from around 3000 BC were assembled with hard solder, and metalworkers probably learned to join pieces this way before 4000 BC. Today a single machine can solder thousands of joints at once.
Soldering works by melting a filler metal into the gap between two surfaces. Unlike glue, the molten solder actually alloys with the metal it touches, a process called wetting, so the finished joint conducts electricity and seals against gas and liquid. Capillary action pulls solder into tight gaps and up between the strands of a wire. Historically the technique made jewellery, cookware and stained glass; today it joins copper water pipes, food cans, roof flashing, car radiators and the bodies of brass instruments.
Temperature defines the family. Soft soldering happens below about 400 °C and can be done with an electric iron; it is gentle on components but weak under mechanical load. Silver soldering runs hotter and is favoured by jewellers and refrigeration technicians. Above roughly 450 °C the process is called brazing, which uses a brass filler, needs a torch or furnace, and makes the strongest joint of the three. Instrument makers often solder brass bodies and braze the keywork.
For decades electronics relied on an alloy of 63 percent tin and 37 percent lead. It is eutectic, meaning it melts at a single temperature, 183 °C, rather than over a range, so it wets quickly, sets quickly and exposes delicate parts to the least heat. The nearly identical 60/40 mix melts between 183 and 190 °C. Environmental rules such as Europe's restriction on hazardous substances have pushed industry toward lead-free solders, partly to keep lead out of groundwater.
Some metals cooperate more than others. Copper, brass, silver and gold take solder easily; iron and nickel are harder; stainless steel and some aluminium alloys resist because of tough oxide films; titanium, ceramics and graphite must first be plated with a solderable metal. In factories, wave soldering and reflow ovens make every joint on a crowded circuit board in a single pass, which dramatically lowered the cost of electronic devices.
Source: Soldering
