Brazing and soldering

Filler only: a lower-melting filler flows into a tight gap

Brazing and soldering join parts with a melted filler metal that flows into the joint and freezes, a bit like hot glue made of metal. The parts themselves never melt, so delicate, thin or dissimilar metals can be joined. Every circuit board is soldered, and most air-conditioning pipes are brazed.

Photo: U.S. Navy, Public domain, via Wikimedia Commons

Brazing · cross-section of a tube in a fitting, with a which-metal-melts ladderCircled numbers match the steps below. The ladder on the right shows what separates soldering, brazing and welding.

How it works

Clean and flux. The surfaces are cleaned and coated with flux (a paste that dissolves oxide films and keeps air off). Filler only sticks to clean metal.
Heat the joint. A torch, furnace, induction coil or soldering iron heats both parts evenly. They must get hotter than the filler's melting point but stay well below their own.
Feed the filler. The filler rod or wire touches the hot joint and melts on contact. The heat should come from the parts, not straight from the flame, so the filler flows toward the hottest metal.
Let capillary action work. The liquid filler wets (spreads over) the clean surfaces and is sucked into the narrow gap, even uphill. This only works if the gap is tight, roughly 0.05–0.15 mm for brazing.
Cool and clean. The filler freezes into a thin layer bonded to both parts, with a small fillet (a curved bead) at the edge. Leftover flux is washed off because it can cause corrosion.

Brazing or soldering? The official difference is only temperature. Soldering uses fillers that melt below 450 °C, such as tin alloys at about 180–230 °C. Brazing uses fillers that melt above 450 °C, often 600–900 °C for silver or copper-phosphorus alloys, and makes much stronger joints.

Key facts

Materials: Almost any metal pair, even dissimilar ones (copper to steel, carbide to steel); ceramics with special fillers
Typical parts: Plumbing and air-conditioning tubes, heat exchangers, carbide-tipped tools, circuit boards, lugged bike frames
Part size: From chip leads under 1 mm to heat exchangers a meter or more across, brazed in furnaces
Precision: Good: the parts do not melt, so assemblies typically stay within a few tenths of a millimeter, better in furnaces
Surface finish: A smooth, thin fillet at the joint edge; flux residue must be cleaned off
Batch size it suits: One-offs by hand up to millions of joints a day in furnaces and reflow ovens
Tooling and machine cost: Tens of dollars for a torch or soldering iron; tens to hundreds of thousands for furnaces and reflow lines
Speed: Seconds to a minute per joint by hand; a furnace or reflow oven makes thousands of joints in one pass of several minutes

Strengths and limits

Strengths

Joins dissimilar metals and thin parts that welding would burn through
The parts do not melt, so there is little distortion or damage
Many joints at once in a furnace or oven
Neat, sealed joints that conduct heat and electricity well

Limits

Joints are weaker than the parts, especially soldered ones
Needs tight, clean gaps and careful fit-up
Flux residue can corrode and must be removed
Weakens at high temperature, so not for hot service

Where it's used

ElectronicsEvery component on a circuit board (soldering)
Plumbing and HVACCopper water pipes (soldered), air-conditioning and refrigerant lines (brazed)
AutomotiveAluminum radiators and condensers (furnace brazed)
ToolsCarbide tips on saw blades and drill bits (brazed)
Jewelry and instrumentsSilver and gold pieces, brass wind instruments
BicyclesClassic lugged steel frames (brazed)

How to recognize it

A thin, smooth fillet of a different color at the joint edge: silvery solder, or a gold, silver or copper-colored braze line.
Parts that overlap or slot into each other (tube in socket, lugs) rather than meeting edge to edge.
No weld ripples and no melted-looking edges on the parts.
On circuit boards: shiny little cones of solder around each component lead.

Compare with

Arc welding7.1melts the parts themselves, so joints are stronger and handle heat better, but distortion is greater.Ultrasonic welding7.7also joins metals without melting them, using vibration; it often replaces soldering for wires and battery tabs.Explosion welding7.8another way to bond dissimilar metals, but for large plates rather than small joints.

Watch it

James Sword Engineering · How the three joining methods differ, for beginners

Lucas-Milhaupt · Why a tight gap pulls the filler through the joint

Lucas-Milhaupt · A torch-brazed copper tube joint, start to finish

Eurocircuits · Circuit boards soldered in a reflow oven

See real photos

Wikipedia: Brazing — photos of torch brazing and brazed joints
Wikipedia: Soldering — hand soldering, solder joints on circuit boards and copper pipes
Wikimedia Commons photo search

Remember

The filler melts; the parts never do. That is what separates brazing and soldering from welding.
A tight, clean, fluxed gap lets capillary action pull the filler all the way through the joint.
Below 450 °C it is soldering; above 450 °C it is brazing, which is much stronger.