Soldering and Brazing
Soldering and brazing join metals with a molten filler that wets the joint by capillary action while the parent metal stays solid.
- Part
- Joining
- Variants
- 3
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- Joint clearance 0.001–0.005 in (0.025–0.13 mm) for brazing, 0.003–0.005 in (0.075–0.13 mm) for soldering, specified at brazing temperature; assembly position set by fixture or self-locating features
- Typical volumes
- 1 to 1,000,000+ — torch for one-offs and repair, continuous or vacuum furnace at production volume
- Lead time
- Seconds to minutes for torch and induction work; 20–90 minute furnace cycles including ramp and controlled cool
- Materials
- Metal
What it is
Soldering and brazing join metals with a molten filler that wets both faces and is drawn into a close-fitting joint by capillary action, while the parent metal never melts. The dividing line is a temperature: filler that melts below 840 °F (450 °C) is solder, above it is braze.
Because the base metal stays solid, there is no fusion zone, no dilution and almost no distortion — the joint can be made on finish-machined parts, on thin-wall tube, on carbide inserts, and on assemblies with dozens of joints made simultaneously in one furnace pass.
The number to design around: a lap joint with an overlap of 3x the thinner member's thickness develops the full strength of that member, so brazed assemblies are designed by overlap area rather than by filler strength. Silver brazing filler melts at 1,125–1,145 °F (607–618 °C); eutectic Sn63/Pb37 solder at 361 °F (183 °C); lead-free SAC305 at 423–428 °F (217–220 °C).
How it works
- Clean and fit. Oxide, oil and oxide-forming residue must be gone — capillary filler will not wet a dirty surface. Parts are then assembled to a controlled clearance, typically 0.001–0.005 in (0.025–0.13 mm) for brazing and 0.003–0.005 in (0.075–0.13 mm) for soldering. Wider than that and capillary action fails; tighter and the filler cannot enter.
- Flux or atmosphere. Flux dissolves the oxide film and protects the joint while it heats. Furnace brazing in vacuum or dissociated ammonia / hydrogen does the same job chemically and leaves nothing to clean off.
- Heat the joint, not the filler. The assembly is brought above the filler's liquidus so the parent metal itself melts the filler. Preplaced rings, shims, paste or clad layers are common; hand-fed wire works for torch brazing.
- Capillary fill. Molten filler is drawn through the clearance, flowing toward the hottest region. A complete fillet visible all the way around the joint is the signal that the gap filled.
- Cool and clean. The joint solidifies as the filler drops below solidus. Residual halide-containing flux is corrosive and must be washed off. Furnace-brazed assemblies cool under controlled atmosphere and need no cleaning.
The three heating methods
Conduction. A soldering iron, resistance-heated tip or hot plate conducts heat into the joint. Local, cheap and fully manual — electronics rework, sheet metal seams, jewelry.
Torch. Oxy-fuel or air-fuel flame heats the parts directly. The most flexible route, and the standard for one-off and repair work: HVAC copper tube, bicycle frames, tool tipping. Skill-dependent, because the operator controls the whole thermal cycle by eye.
Furnace. The whole assembly goes through a controlled thermal cycle — continuous belt with a reducing atmosphere, or a vacuum furnace for stainless and superalloys. Every joint in an assembly is made at once with identical, repeatable heat, which is why heat exchangers, honeycomb structures and multi-joint manifolds are furnace brazed. Cycles run 20–90 minutes including ramp and cool.
Induction and dip brazing sit between torch and furnace: fast, localized, and repeatable enough for production without a full furnace cycle.
Design guidelines
Joint clearance is the controlling dimension
Capillary flow needs a gap, and the optimum is narrow: 0.001–0.005 in (0.025–0.13 mm) for silver brazing with flux, essentially 0.000–0.002 in (0–0.05 mm) for nickel filler in a vacuum furnace, and 0.003–0.005 in (0.075–0.13 mm) for soldering. Design that clearance at brazing temperature, not at room temperature: a steel bushing in an aluminum housing closes up on heating, while a copper sleeve in a steel bore opens. Cylindrical joints are dimensioned as fits — see the ISO 286 fits and tolerances chart.
Design a lap, never a butt
A butt joint has only the cross-section as bond area and no capillary path, so it is weak by definition. Use a lap, a sleeve or a scarf. The standard rule is an overlap of 3x the thickness of the thinner member, which makes the joint stronger than the member it joins — the part yields before the braze shears. Longer overlaps add little.
Position the joint, and respect the thermal cycle
Capillary action can pull filler uphill, but do not rely on it: preplace filler where gravity assists flow and design a visible fillet exit so an operator or camera can confirm the joint filled through. Remember too that brazing temperature is a heat treatment — brazing 6061 aluminum at 1,070–1,080 °F (577–582 °C) puts it near solution temperature and it must be re-aged, brazing hardened steel above its tempering temperature softens it, and work-hardened copper anneals.
Filler and material selection
| Filler | Melting range | Use on | Notes |
|---|---|---|---|
| Sn63/Pb37 solder | 361 °F (183 °C), eutectic | Cu, brass, steel, PCBs | Shear strength only ~3,000–5,000 psi (20–35 MPa) — never structural |
| SAC305 (Sn/Ag/Cu) | 423–428 °F (217–220 °C) | Electronics, RoHS work | Higher process temperature than SnPb; wets less readily |
| BAg silver filler | 1,125–1,145 °F (607–618 °C) for BAg-1 | Steel, stainless, copper, carbide, nickel | The general-purpose braze; needs flux in air |
| BCuP (copper-phosphorus) | 1,310–1,460 °F (710–795 °C) | Copper to copper only | Self-fluxing on copper; never on ferrous — brittle iron phosphides |
| BCuZn brass/bronze | ~1,600–1,650 °F (870–900 °C) | Steel, cast iron | Also used as bronze fillet-braze without capillary action |
| BNi nickel filler | 1,780–2,100 °F (970–1,150 °C) | Stainless, superalloys | Vacuum furnace; high-temperature and corrosion service |
| BAlSi (Al-12Si) | 1,070–1,080 °F (577–582 °C) | Aluminum to aluminum | Uncomfortably close to the parent's solidus — tight furnace control |
Base-metal melting points, which set the process window against the filler ranges above, are on the metal melting points chart.
Inspection
Visual inspection of the fillet all the way around the joint is the primary method and is genuinely informative — a continuous fillet at the far side means the capillary gap filled. Beyond that: helium leak testing to 10⁻⁹ std cc/s for pressure and vacuum joints, radiography for internal voids in critical brazements, ultrasonic testing for bond coverage, and destructive peel or shear coupons run alongside a production lot. Be careful with dye penetrant — it wicks into any porosity and can be impossible to remove.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Braze clearance, flux brazing | 0.002 in (0.05 mm) | 0.001–0.005 in (0.025–0.13 mm) | Outside this band capillary flow stops |
| Vacuum braze clearance | 0.001 in (0.025 mm) | 0.000–0.002 in (0–0.05 mm) | Nickel fillers flow in near-zero gaps |
| Solder joint clearance | 0.004 in (0.1 mm) | 0.003–0.005 in (0.075–0.13 mm) | Solder is more viscous than braze |
| Lap overlap | 3x thinner member thickness | 2x minimum | Makes the joint stronger than the member |
| Joint type | Lap, sleeve or scarf | Never a plain butt | No capillary path and minimal bond area |
| Fillet visibility | Both ends of the joint | — | The only cheap confirmation the gap filled |
Cost drivers
The split is between manual and furnace routes. Torch brazing is labor and skill: no tooling, immediate start, and a cost per joint that never falls with volume. Furnace brazing inverts that — fixtures, filler preforms and a qualified cycle up front, then a cost per joint that collapses because a single cycle can make hundreds of joints across a full furnace load at once.
Filler metal is a real line item when it contains silver, so preform sizing matters: hand-fed wire routinely puts two or three times the necessary filler into a joint. Vacuum furnace time is the other significant cost, and it is charged by the cycle, so a partly loaded furnace is expensive per part.
- Use preplaced preforms — rings, washers, shims or clad layers — instead of hand-feeding. Filler use drops and joints become repeatable.
- Braze every joint in one cycle. Design the assembly so all joints are made in one furnace pass rather than sequenced with progressively lower-melting fillers.
- Hold the clearance with the parts, not the fixture. A machined shoulder, knurl or dimple that self-centers the joint deletes fixture cost.
- Check whether a cadmium-free or lower-silver filler works. Filler choice can move joint cost significantly with no design change.
- Do not over-specify the filler. A BAg alloy on a joint that a copper-phosphorus filler would handle is a pure cost adder on copper-to-copper work.
Variants
3 named
Conduction Method
Torch Method
Furnace Method
Questions
5 questionsWhat is the difference between soldering and brazing?
Filler melting temperature, and nothing else about the mechanism. Filler that melts below 840 °F (450 °C) is solder; above that it is braze. Both rely on capillary action to fill a close-fitting joint while the parent metal stays solid. The practical difference is strength: brazed joints can develop the strength of the base member, while tin-lead solder shears at only about 3,000–5,000 psi (20–35 MPa).
What joint clearance does brazing need?
0.001–0.005 in (0.025–0.13 mm) for silver filler with flux, and essentially 0.000–0.002 in (0–0.05 mm) for nickel filler in a vacuum furnace. Outside that band capillary action stops — too wide and the filler will not bridge, too tight and it cannot enter. Specify the clearance at brazing temperature, since differential expansion of dissimilar metals changes the gap on heating.
How strong is a brazed joint?
Strong enough that the joint is not the weak point, if it is designed as a lap with an overlap of 3x the thinner member's thickness. At that overlap the parent member yields before the braze shears, so brazed assemblies are sized by overlap area rather than by filler strength. A plain butt joint, by contrast, has no capillary path and minimal bond area.
Can you braze aluminum?
Yes, with an Al-12Si filler (BAlSi-4) melting at 1,070–1,080 °F (577–582 °C), typically in a controlled-atmosphere brazing furnace. The difficulty is that this is only a few degrees below the solidus of common aluminum alloys, so furnace temperature uniformity has to be tight. Brazing also puts 6061 near solution temperature, so the part needs re-aging afterward.
How are brazed joints inspected?
Visual inspection of the fillet all the way around is the primary and surprisingly informative method: a continuous fillet at the far end of the joint means the capillary gap filled through. Beyond that, helium leak testing to 10⁻⁹ std cc/s for pressure joints, radiography for internal voids, and destructive shear coupons run with the lot. Avoid dye penetrant, which wicks into porosity and cannot be removed.