Solder
A metal that melts low enough to join others without melting them — and the material every electronic device is held together with.
Solder exists to melt. Every other property is subordinate to the requirement that it become liquid at a temperature the things being joined will survive, wet their surfaces, and freeze into a joint that conducts electricity and holds mechanically.
For most of industrial history that meant tin and lead in roughly equal parts, an alloy with a genuinely remarkable property: at 63 per cent tin it is eutectic, melting and freezing at a single temperature rather than passing through a pasty range. A joint that sets instantly is what made automated assembly possible.
Regulation ended that. Lead-free solders are tin-based with small additions of silver and copper, melt some thirty-five degrees hotter, and behave differently enough that the entire electronics industry had to re-qualify its components and its ovens.
Processing
Soldering is not welding and not brazing. Nothing being joined melts — the solder alone does — and the joint's strength comes from a thin intermetallic layer that forms where the molten alloy meets the copper beneath it. Too little of that layer and the joint is mechanically weak; too much and it becomes brittle, which is what a joint that has been overheated or has aged badly looks like under a microscope.
Flux does the invisible work. Copper oxidises within minutes in air and molten solder will not wet an oxide, so the flux dissolves what is there and keeps more from forming until the joint has set.
The convention that solder is drawn into a gap rather than laid across one is what makes plumbing joints possible: capillary action pulls the alloy along the whole length of a fitting from a bead applied at one end.
Economic significance
Solder is where a large share of the world's tin goes, which makes an ancient metal's demand a function of consumer electronics — and makes tin one of the few metals whose price is discussed alongside semiconductor cycles.
The lead-free transition, mandated in the European Union from 2006 and followed widely, is one of the clearest cases of a material being changed by regulation rather than by engineering. It also raised silver demand measurably, because the replacement alloys need a few per cent of it.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Tin — element · 60–99% · the base of every solder in current use, leaded or not
- Lead — element · 0–40% · in the traditional alloys, at 37% for the eutectic that melts and freezes at a single temperature — withdrawn from electronics by regulation
- Silver — element · 0–4% · a few per cent in the lead-free alloys, which measurably raised world silver demand when the transition happened
- Copper — element · 0–1% · under a per cent, to slow the rate at which the joint dissolves the copper it is sitting on
is used as
- Electrical conduction — application · every joint in every electronic device, where the requirement is to melt low rather than to conduct well
is produced by
- Alloying and melting — process · melted and cast into wire or paste; the alloy is simple and the composition control is not
is used in
- Electronics manufacture — industry · and the move away from tin-lead under RoHS from 2006 is the most visible material change the industry has made
- Printed circuit board — object · every joint on it, and tin-silver-copper since RoHS pushed the lead out from 2006
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing