Brazing and Soldering Theory Reference

Why this matters

Soldering and brazing both join metals with a filler that melts below the base metal's melting point. The distinction matters: soldering uses filler below 840 °F (water lines, low-temperature joints), brazing uses filler above 840 °F (refrigerant lines, gas lines, high-pressure joints). Knowing which technique applies where - and which filler / flux / heat source goes with each - is fundamental to plumbing and HVAC refrigerant work.

The temperature threshold

By industry definition (AWS standard):

• Soldering: filler melts below 840 °F (450 °C)
• Brazing: filler melts above 840 °F
• Welding: base metal melts and fuses (not used for copper plumbing or refrigerant work)

The threshold matters because the higher temperature of brazing changes everything - different fillers, different fluxes, different torches, different joint preparation, different safety considerations.

When to use which

Soldering (lead-free solder, below 840 °F):

• Potable water supply pipes (copper)
• Low-pressure systems
• Joints accessible for service
• DWV (waste) connections when codes allow
• Most residential plumbing copper-to-copper

Brazing (silver braze or brass-alloy braze, above 840 °F):

• Refrigeration / HVAC refrigerant lines (always brazed)
• Gas piping requiring high strength (sometimes; flares and threaded common)
• High-pressure plumbing
• Joints that will be permanently buried or inaccessible
• Joints subject to vibration or thermal stress
• Stainless steel to copper transitions

Soldering fillers

95/5 tin-antimony (Oatey Safe-Flo, Harris Stay-Clean):

• The residential standard
• NSF 61 certified for potable water
• Melts 450-464 °F
• Strength: ~14,000 psi tensile

Tin-silver (Sn96.5/Ag3/Cu0.5):

• Higher strength than 95/5
• Used in HVAC refrigeration low-temp work, food-service, semiconductor
• Slightly higher melt range

50/50 tin-lead:

• BANNED for potable water since 1986 (SDWA amendment)
• Still legal for DWV
• Don't carry on your truck if you don't do non-potable work; risk of grabbing wrong solder

Brazing fillers

Silver braze (BAg series):

• BAg-1 (45% Ag): melts 1125-1145 °F, good general purpose
• BAg-5 (45% Ag low-temp): melts 1225-1370 °F, used where higher temp is acceptable
• BAg-7 (56% Ag): melts 1145-1205 °F, cadmium-free, food-safe
• Standard HVAC refrigerant brazing material

Phosphorus-copper (BCuP series):

• BCuP-2 (no silver): melts 1310-1450 °F, copper-to-copper only
• BCuP-3 (~5% Ag): melts 1190-1495 °F, more flexible joint
• BCuP-5 (~15% Ag): melts 1190-1465 °F
• Used in HVAC, sometimes refrigerant brazing - self-fluxing on copper-to-copper
• NOT for use on steel or copper-to-iron joints

Brass / bronze braze rods:

• Higher melting point (~1650 °F)
• Used for steel-to-steel, sometimes brass plumbing
• Rare in HVAC

Fluxes

Soldering flux:

• Water-soluble paste flux (Oatey No. 5, Harris Stay-Clean): standard for plumbing soldering
• Tinning flux (Oatey No. 95): contains powdered solder; flux turns silver when joint reaches solder melt temp (visual indicator)
• Apply to BOTH pipe and fitting before assembly
• Excess flux residue is corrosive long-term; wipe joint clean

Brazing flux:

• White paste, higher temperature tolerance
• Harris Stay-Silv white flux (for silver braze)
• Stay-Silv black flux (for stainless or where higher heat is involved)
• Apply to fitting only on copper-to-copper with BCuP filler (self-fluxing) - silver braze needs flux on both sides

No flux for BCuP on copper-to-copper:

• Phosphorus in the rod acts as flux
• One of BCuP's main advantages - faster work, less cleanup

Heat sources

Propane (1-lb cylinder or MC tank):

• Burns ~3600 °F
• Adequate for soldering 1/2"-3/4" copper
• Marginal for soldering 1"+ copper (heat loss to surroundings)
• NOT hot enough for brazing

MAPP gas (Propylene-MAPP equivalent, Map-Pro):

• Burns ~3700 °F
• Faster than propane; less ambient heat loss
• Used for soldering 1"+ copper or large fittings
• Marginal for brazing (some brazing alloys, with effort)

Oxy-acetylene:

• Burns up to ~6000 °F
• Standard for brazing refrigerant lines
• Faster heat input; precise control with mixing valve
• Required for thicker steel brazing or hard-to-heat geometries

Air-acetylene (Prest-O-Lite, B-tank acetylene with air mix):

• Burns ~4200 °F
• Acceptable for most HVAC silver brazing
• Less expensive setup than oxy-acetylene
• Common in residential HVAC service trucks

Induction (specialty):

• No open flame
• Used in factory settings; rare in field
• Heats joint via electromagnetic induction

Joint geometry

For both soldering and brazing, capillary action draws molten filler into the gap between pipe OD and fitting socket ID. The gap dimensions matter:

Optimal gap: 0.002-0.008" (0.05-0.20 mm) - standard fitting tolerance

Too tight: filler can't flow in fully; bonds at the edges only

Too loose: capillary action weak; filler sags or fails to fill

Pipe must be cut SQUARE and deburred for proper fit. Out-of-round pipe or burred ends prevent proper seating.

Brazing procedure for refrigerant lines

Refrigerant brazing has additional requirements beyond water-supply soldering.

Preparation:

1. Cut pipe square with tubing cutter
2. Deburr inside and outside
3. Clean both surfaces with appropriate abrasive - emery cloth (#150-180) or fitting brush
4. Wipe with clean cloth (no oil, no fingerprints)
5. Apply flux to fitting ID (if using flux-required alloy)
6. Assemble; the fitting should bottom against the pipe shoulder

Critical: nitrogen purge:

• Pure copper inside refrigerant systems oxidizes when heated
• Copper oxide flakes off later, can damage compressor or block metering devices
• Industry standard: low-flow nitrogen (1-3 SCFH) flowing through the joint area during brazing prevents oxide formation
• Nitrogen comes from refrigerant-grade dry nitrogen cylinder, regulated to 1-3 SCFH (NOT high flow)
• This is non-optional for refrigerant brazing per ASHRAE Standard 15 and best practice

Brazing:

1. Apply heat to the FITTING (not the pipe) - fitting's mass needs to reach brazing temperature
2. As fitting reaches temp, flux turns clear and active (silver braze) or just liquid (BCuP)
3. Touch filler rod to the joint
4. Capillary action draws filler around the joint
5. Move quickly; don't overheat
6. Heat the joint just enough to flow filler; not until pipe turns purple (overheat)

Cooling:

• Allow natural cool
• Don't quench with water (thermal shock can crack the joint)

References

• AWS A5.8 (filler metals for brazing)
• AWS A5.31 (fluxes)
• ASHRAE Standard 15 (refrigerant system safety, including brazing requirements)
• ASME B31.5 (refrigeration piping)
• IAPMO / IPC for plumbing solder requirements
• NSF 61 (potable water components, including solder)
• SDWA 1986 amendment (lead solder ban for potable water)
• Manufacturer technical data (Harris, Oatey, Lucas-Milhaupt, J.W. Harris)