How Do You Join Metal Without Melting It? A Third Metal in Between

The filler metal is heated until it melts and flows into the joint, drawn in by capillary action into narrow gaps, and it wets the surfaces of the parts, meaning it spreads across them rather than beading up. Where it wets properly, a thin layer of alloy forms at each interface as atoms from the filler and the parent metal interdiffuse, and that alloy layer is what actually holds the joint together rather than the filler simply sticking. The parent metals never melt, which is the whole point, since it allows joining components that heat would destroy and allows work on assembled structures without distortion.

The conditions are specific and failing any of them produces a weak joint:

• •Surfaces clean and free of oxide, which is what flux exists to achieve
• •Both parts heated, rather than the filler being melted onto a cold surface
• •The filler melted by the work rather than by the iron directly
• •A concave fillet where the filler meets both parts, indicating proper wetting
• •A bright smooth surface once cooled, since a dull grainy one indicates movement while solidifying
• •No movement of the parts until the joint has fully set

Metal surfaces oxidise in air immediately, and molten filler will not wet an oxide layer, so a joint made on unprepared metal simply beads up and falls off. Flux is a chemical applied before or during the operation that dissolves that oxide, keeps further oxidation from occurring while the metal is hot, and lowers the surface tension so the filler flows. Electronics work uses a resin-based flux built into the core of the wire, which is mild enough to leave in place. Plumbing uses an acid-based flux that is far more aggressive and must be cleaned off afterwards, since residue continues corroding the joint. Using plumbing flux on electronics destroys the circuit over months, which is a classic beginner's mistake.

Industrial assembly of circuit boards uses processes quite unlike hand work, and knowing them explains what the inside of a device looks like. Reflow soldering prints a paste of powdered alloy mixed with flux onto the board through a stencil, places components into the sticky paste, and passes the whole board through an oven whose temperature profile is controlled to the second, melting every joint simultaneously. Surface tension in the molten alloy actually pulls slightly misplaced components into alignment, which is why placement need not be perfect. Wave soldering passes a board over a standing wave of molten metal, which suits components with legs through holes. Inspection is done by machine vision and by X-ray, since joints beneath a chip cannot be seen at all.

The traditional filler for electronics was an alloy of tin and lead, which melts at a conveniently low temperature, flows beautifully and produces reliable joints, and it has been largely eliminated by regulation because lead is toxic and electronic waste enters the environment in enormous quantities. European legislation from 2006 restricted it in most consumer equipment, and other jurisdictions followed. The replacements are alloys of tin with silver and copper, which melt around thirty degrees higher, wet less readily, look duller when correctly made and are more prone to a failure mode in which thin whiskers of tin grow from the surface and short circuits result. Aerospace, medical and military equipment retain exemptions for exactly those reliability reasons.

A filler metal melts, flows into the gap and forms a thin alloy layer with each parent metal by interdiffusion, and that layer holds the joint while the parts themselves never melt. Clean oxide-free surfaces, both parts heated and no movement while setting are the requirements. Flux dissolves oxide and stops it reforming. Lead-based alloys were largely eliminated by regulation from 2006, and the replacements melt hotter and wet less readily.