Brazing and Soldering

Why brazing and soldering matter

Fusion welding is the loud, obvious way to join metal, but it is the wrong tool for a huge fraction of real assemblies. Melting the base metal distorts thin sections, coarsens grain structure in the heat-affected zone, and forms brittle intermetallics whenever two different alloys share a molten pool. Brazing and soldering sidestep all of that: only the filler melts, so the parent parts keep their temper, their dimensions, and their surface finish. A brazed bicycle frame lug, a soldered copper plumbing joint, a vacuum-brazed aluminum radiator, and every solder joint on the circuit board in front of you are all the same physics — a lower-melting alloy pulled by capillary action into a narrow gap and frozen in place.

• Dissimilar metals. Copper to steel, stainless to brass, aluminum to copper — joints a weld pool would ruin.
• Leak-tight. A continuous, void-free filler seam holds pressure and vacuum — refrigeration lines, heat exchangers, pressure vessels.
• Low distortion. Uniform, sub-melting heating keeps flatness and tolerances that welding would warp.
• Thin and fragile parts. Foil-thin fins, fine wires, carbide cutting tips, and ceramic-to-metal seals.
• Mass production. Furnace and dip brazing, and reflow/wave soldering, join hundreds of joints simultaneously with repeatable quality.
• Electronics. Every surface-mount and through-hole connection on a PCB is a soldered joint carrying both current and mechanical load.

How it works, step by step

Whether you are silver-brazing a carbide insert at 700 C or reflow-soldering a chip at 240 C, the sequence is the same:

1. Clean and fit. Remove oil, scale, and gross oxide, then assemble the parts with a controlled clearance — typically 0.025 to 0.125 mm — held at that gap during heating.
2. Apply flux. A flux coats the faying surfaces; on heating it melts first, dissolves the thin oxide film, and shields the clean metal from re-oxidizing.
3. Heat the assembly. Bring the whole joint (not just the filler) above the filler's liquidus but below the base metal's solidus — torch, induction coil, furnace, or a solder iron/reflow oven.
4. Wet and flow. The molten filler wets the fluxed surfaces; surface tension pulls it into the gap by capillary action, filling the joint completely, sometimes against gravity.
5. Solidify. On cooling the filler freezes, and a thin diffusion/alloying zone at each interface forms the metallurgical bond that carries the load.
6. Clean up. Remove flux residue — many brazing and acid-flux solder residues are corrosive and must be washed off; no-clean and vacuum-brazed joints skip this.

The governing physics is capillary rise. When a wetting liquid climbs a narrow slot, the equilibrium height balances surface tension against the weight of the raised column:

h = 2 σ cos θ / (ρ g c)

• h — capillary rise (height the filler climbs), in metres (m)
• σ — surface tension of the molten filler, in newtons per metre (N/m); ~0.9–1.0 N/m for molten silver-copper brazes, ~0.4–0.55 N/m for tin solders
• θ — wetting contact angle (degrees); good wetting means θ < 30°, and cos θ → 1; non-wetting means θ > 90° and h goes negative (the liquid is pushed out)
• ρ — density of the molten filler, in kilograms per cubic metre (kg/m³); ~7,000–9,000 for these alloys
• g — gravitational acceleration, 9.81 m/s²
• c — joint clearance (gap width), in metres (m)

Two design levers fall straight out of this equation. First, clearance dominates: rise is inversely proportional to gap, so halving the clearance doubles how far and how fast the filler is pulled — the reason brazed gaps are measured in thousandths of an inch, not filled like a weld. Second, wetting is a switch: if cos θ is positive the filler is sucked in; if the surface is oxidized and θ exceeds 90°, cos θ is negative and the filler is actively expelled. Flux exists to keep θ small.

Brazing vs. soldering vs. welding

The three processes form a spectrum by temperature and by whether the base metal melts. The 450 C line is arbitrary but universal (AWS definition): it separates the filler families and, with them, the achievable strength.

Property	Soldering	Brazing	Welding
Filler liquidus	< 450 C	> 450 C	— (base melts)
Base metal melts?	No	No	Yes
Typical fillers	Sn-Pb 63/37, SAC305, Sn-Cu	BAg (Ag-Cu-Zn), BCuP, BNi, Al-Si	Matches base (or none)
Joint mechanism	Capillary + wetting	Capillary + wetting	Fusion of parent metal
Typical shear strength	20–60 MPa	300–500 MPa (Ag)	~ base metal strength
Distortion / HAZ	Minimal	Low	High
Dissimilar metals	Good	Excellent	Often impossible
Typical use	Electronics, plumbing, tinware	HVAC, tooling, aerospace, bikes	Structural steel, pressure vessels

Filler families, temperatures, and flux

Choosing a filler means matching its melting range and chemistry to the base metals and the service temperature. A joint should not see service anywhere near its filler's solidus.

Filler	Process	Approx. melt range	Typical use
63Sn/37Pb (eutectic)	Soldering	183 C (eutectic)	Legacy electronics, tinning
SAC305 (96.5Sn/3Ag/0.5Cu)	Soldering	217–220 C	Lead-free reflow (RoHS)
BCuP-5 (Cu-Ag-P)	Brazing	~643–800 C	Copper-to-copper (self-fluxing)
BAg-7 (Ag-Cu-Zn-Sn)	Brazing	~618–652 C	Steel, stainless, brass (cadmium-free)
Al-Si (BAlSi-4)	Brazing	~577–600 C	Aluminum heat exchangers
BNi (nickel-based)	Brazing	~900–1,200 C	Turbines, high-temperature stainless

Flux is chosen to match the temperature and metals: rosin (mildly activated) for electronics, zinc-ammonium-chloride acid flux for plumbing and sheet metal, and borax/potassium-fluoroborate pastes (AWS Type 3A, good to ~870 C) for silver brazing. BCuP fillers are self-fluxing on copper because the phosphorus reduces copper oxide — but not on iron or nickel, where phosphorus forms brittle phosphides, so BCuP must never braze steel. Where residue cannot be tolerated, furnace brazing under a hydrogen/dissociated-ammonia reducing atmosphere or in vacuum removes oxides without any flux at all.

Worked example: capillary rise in a silver-brazed joint

A vertical copper lap joint is silver-brazed with a BAg filler. Estimate how high the molten filler climbs for a 0.05 mm clearance versus a sloppy 0.25 mm clearance, assuming good wetting.

Take σ = 0.9 N/m, θ ≈ 10° so cos θ ≈ 0.985, ρ ≈ 9,300 kg/m³, g = 9.81 m/s².

For the tight gap, c = 0.05 mm = 5×10⁻⁵ m:

h = 2 × 0.9 × 0.985 / (9,300 × 9.81 × 5×10⁻⁵) ≈ 0.389 m ≈ 389 mm.

The filler can climb nearly 40 cm against gravity — vastly more than any real joint needs, so the gap fills completely and quickly. Now open the gap five times to c = 0.25 mm:

h ≈ 0.389 / 5 ≈ 0.078 m ≈ 78 mm.

Still positive, but the capillary pull has collapsed to a fifth, filling is slow, and any tall joint may starve. Worse, the thick 0.25 mm filler layer is no longer constrained by the base metal, so the joint's strength drops toward the weak bulk shear strength of the filler alloy rather than the enhanced, thin-film value. This is exactly why brazing standards call for a hot clearance near 0.04–0.08 mm: tight enough for strong capillary rise and a constrained, high-strength filler film, but not so tight that it chokes flow or traps flux.

Common misconceptions and failure modes

• "More filler makes it stronger." False — a fat fillet is weaker. Strength comes from the thin, base-constrained filler film; excess metal just adds bulk-strength material and stress risers.
• "Tighter is always better." No — below ~0.01 mm the gap blocks capillary flow and traps flux, leaving voids. There is an optimum, not a limit.
• "Melt the filler and it flows." Only if the surface is clean and wetted. On an oxidized surface the filler beads up and rolls off (contact angle > 90°); flux or a reducing atmosphere is mandatory.
• "Heat the filler." Heat the joint. Filler flows toward the hottest region and is drawn into the gap by the base metal's heat, not by direct flame on the rod.
• Base-metal erosion. Overheating dissolves parent metal into the filler, thinning the parts and coarsening grains — a classic brazing overheat failure.
• Trapped flux and voids. Flux that cannot escape a blind or over-tight joint leaves gas pockets that leak and concentrate stress.
• Thermal fatigue. Solder joints creep and crack under temperature cycling as CTE mismatch strains the joint; cracks propagate through the brittle intermetallic layer (a top field failure in electronics).
• Galvanic corrosion. A filler dissimilar to the base metal in a wet, conductive environment sets up a galvanic cell that slowly eats the anodic member.