§1Joining without melting
Soldering and brazing bond two parts by melting a third metal — the filler — between them, while the parts themselves never melt. The filler wets the surfaces and, on freezing, holds them together.
This is the crucial difference from welding (its own page): welding fuses the parts by melting them together, whereas soldering and brazing leave the parts solid and rely entirely on a filler that melts at a lower temperature than they do. Because the parts stay solid, there is little distortion and no melting of the base metal, dissimilar metals can be joined, and delicate or thin work survives — which is why electronics are soldered and pipework and tooling are brazed. The strength comes not from deep fusion but from a thin, well-wetted film of filler over a generous joint area (§4), drawn in by capillary action (§3). Understand wetting, the gap and the overlap, and both processes follow the same logic — they differ mainly in temperature (§2).
Contents§2The 450°C line
Soldering and brazing are the same process at different temperatures, divided by convention at 450°C — soldering below it, brazing above.
The distinction is simply the melting point of the filler. Soldering uses fillers melting below about 450°C — tin-based “soft” solders melting around 180–190°C — giving low-temperature, low-strength joints ideal for electrical connections, tinplate and sealing. Brazing uses fillers melting above 450°C — silver alloys and brass melting around 600–900°C — giving much stronger, more heat-resistant joints for structural and mechanical work. In both, the base metal stays well below its own melting point, so the only real difference is how hot you must get and how strong the result: soldering is cooler and weaker, brazing hotter and stronger. Everything else — capillary flow, close gaps, lap joints, flux — is common to both, which is why they are treated together.
Contents§3Capillary action and the gap
Molten filler is not spread into the joint — it is drawn in by capillary action, the same force that pulls liquid up a narrow tube. That force needs a close-fitting gap to work.
A narrow gap between two wetted surfaces draws molten filler in and holds it, exactly as a thin space pulls water up between two plates — and the narrower the gap, the stronger the capillary pull. But too tight a gap starves the flow and traps flux, while too wide a gap kills the capillary action and leaves a weak, filler-poor joint. The optimum is a small, controlled clearance — roughly 0.05 to 0.1 mm for most brazed and soldered joints — close enough for strong capillary draw yet open enough to fill fully. This is why brazed and soldered joints are designed as close, parallel fits, not loose ones: the gap is a working dimension, sized for capillarity. Heat the assembled joint, touch filler to one edge, and capillary action pulls it right through the gap of its own accord.
§4Why lap joints
The strength of a soldered or brazed joint comes from the area of overlap, not the thickness of filler — so the parts are lapped over one another, not butted end to end.
The thin filler film is far weaker than the base metal, so a butt joint — end to end, with only the cross-section’s worth of filler — is feeble. A lap joint, with the parts overlapping, spreads the load over a large bonded area, and its strength is that overlap area times the filler’s shear strength: a 10 mm overlap on 20 mm-wide parts gives 200 mm² of bond, which at a modest filler shear strength of 30 N/mm² carries 200 × 30 = 6000 N. Lengthen the overlap and the joint grows stronger in proportion, up to the point where the parts themselves would fail — so a brazed or soldered joint is made strong by generous overlap, not by a thick glue-line of filler. This is the governing design rule of both processes: lap, don’t butt, and size the overlap for the load.
§5The eutectic solder
The classic soft solder is a tin–lead alloy at its eutectic composition — the one mix that melts sharply at a single low temperature instead of through a pasty range.
As the materials pages’ phase diagrams show, most alloy compositions melt over a range — pasty between a lower and upper temperature — but the eutectic composition melts and freezes at one sharp point, the lowest for that alloy system. For tin–lead, that is 63% tin, 37% lead, melting cleanly at 183°C with no pasty stage. The sharp melting point makes it the traditional electronics solder: it flows the instant it reaches temperature and freezes solid the instant it cools, giving quick, sound joints and no weak mushy interval in which a disturbed joint would crack. Non-eutectic solders, with a pasty range, suit wiping and filling where a plastic stage is useful. Modern electronics use lead-free tin–silver–copper solders (melting a little higher) for health and environmental reasons, but the eutectic principle is the same: pick the composition that melts sharply at the temperature you want.
Contents§6Flux and wetting
Filler will only bond to clean metal, but hot metal oxidises instantly — so a flux is used to strip the oxide and let the filler wet the surface.
Wetting — the filler spreading into a thin, adherent film rather than balling up — is the whole basis of the joint, and it happens only on chemically clean metal. But heating metal in air grows an oxide skin that the filler cannot wet, so a flux is applied: a chemical that dissolves and removes the oxide at soldering temperature, shields the clean surface from re-oxidising, and helps the molten filler flow and wet. Rosin fluxes serve electronics (mild, non-corrosive residues), more active acid fluxes serve plumbing and general work (but must be cleaned off, being corrosive), and special fluxes serve brazing’s higher heat. Without flux — or without the clean, close, well-fitted joint it enables — the filler beads up and will not bond. So the recipe for a sound joint is constant: clean, close-fitting parts, the right flux, enough heat to melt the filler (not the parts), and capillary action to draw it home.
Contents§7Quick reference
The working core of the page on one card rack.
Principle
filler melts, parts don't
(welding fuses the parts)
The line
solder < 450°C · braze > 450°C
Capillary gap
~0.05–0.1 mm close fit
draws filler in
Lap joint
strength = overlap × shear
10 mm × 20 mm → 6000 N
Eutectic / flux
63Sn-37Pb melts sharp at 183°C
flux strips oxide → wetting