Brazing vs soldering: how they work and when to use each

Brazing and soldering join two metal parts with a filler metal that melts at a lower temperature than the parts. The parts are heated but never melted. The liquid filler is pulled into the narrow gap between them by capillary action, the same effect that draws water up a thin tube, and it bonds to both surfaces as it cools. This makes both processes gentler than welding and lets them join metals that cannot be welded together.

The 450 °C dividing line

The only formal difference is the filler’s melting temperature. If the filler melts below 450 °C, the process is soldering. If it melts above 450 °C but still below the melting point of the parts, it is brazing. In practice that difference in temperature leads to very different strength, equipment and uses.

Soldering

Soldering uses tin-based fillers that melt between roughly 180 and 230 °C. Lead-free solders such as tin–silver–copper (around 217 °C) have replaced tin–lead in electronics and drinking-water systems. A soldering iron, hot-air tool or small flame provides the heat.

The process, step by step

1. Clean the surfaces back to bright metal; oxide stops the solder wetting the joint.
2. Apply flux. It removes the last oxide and stops new oxide forming while the joint heats.
3. Fit the parts with a small, even gap.
4. Heat the joint, not the solder. When the parts are hot enough, touch the solder to the joint and it flows in.
5. Let it cool undisturbed, then clean off flux residue, which can corrode the joint over time.

Advantages

• Low temperature: no distortion and no damage to nearby components or coatings.
• Quick, cheap and easy to learn; easy to undo for repair.
• Good electrical conductivity, which is why every circuit board is soldered.

Limitations

• Low strength (typically a few tens of MPa in shear), and creep under constant load.
• Not for temperatures much above 100 °C in service.
• Joints should be designed to carry little or no mechanical load.

Where soldering is used

Electronics and circuit boards, wiring, copper water tubes in buildings, radiators, sheet-metal seams and small tin-plate items.

Brazing

Brazing fillers melt above 450 °C. Common ones are silver alloys (about 600–800 °C), copper–phosphorus alloys for copper (about 700–800 °C), aluminium–silicon alloys for aluminium (about 580–600 °C) and pure copper for furnace-brazing steel (about 1100 °C). Heat comes from a gas torch, an induction coil or a furnace.

A well-designed brazed joint can be as strong as the parts it joins. The trick is a lap joint with enough overlap, typically about three times the thickness of the thinner part, so that the large bonded area makes up for the filler being weaker than the base metal.

The process, step by step

1. Design the gap. Capillary flow works best with a clearance of about 0.025–0.125 mm. Too wide and the filler won’t fill it; too tight and it can’t get in.
2. Clean and flux the parts (some fillers, such as copper–phosphorus on copper, flux themselves; furnace brazing uses a protective atmosphere or vacuum instead).
3. Fix the parts in a jig or with a press fit so the gap stays even.
4. Heat the whole joint evenly until it reaches the brazing temperature.
5. Feed the filler at one edge. It runs through the gap and forms a small fillet on the far side when the joint is full.
6. Cool, then wash off flux and inspect the fillets.

Advantages

• Joins dissimilar metals: copper to steel, brass to stainless steel, carbide to steel.
• Strong, leak-tight joints with neat fillets and little distortion.
• Many joints can be brazed at once in a furnace, which suits production.
• The base metals keep most of their properties because they never melt.

Limitations

• Joints need close fit-up, clean surfaces and a lap design; butt joints are weak.
• Flux residues must be removed.
• Service temperature is limited by the filler’s melting range.
• The filler may differ in colour and corrosion behaviour from the parts.

Where brazing is used

Carbide tips on cutting tools and saw blades, heat exchangers and aluminium car radiators, refrigeration and air-conditioning lines, bicycle frames with lugs, hydraulic fittings, electrical contacts and jewellery.

Soldering vs brazing vs welding

How to choose

• Electrical connection or a light, sealed joint at low temperature: solder.
• A strong, leak-tight joint between thin or dissimilar parts, or many joints at once: braze.
• Structural joints in thick sections of the same metal, or joints that must work at high temperature: weld. Our MIG vs TIG guide compares the two common arc processes.

When we design a brazed assembly in CAD, we model the joint as a lap or sleeve with the clearance set by the fit tolerance, and we add the filler and the joint method as notes on the drawing so the shop knows the gap is intentional.