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Brazing and Soldering

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Comparing Brazing and Soldering

Both use heat, filler metal and usually flux.
Primary difference is melting temperature of filler metal.

Brazing filler metals melt above 840°F (liquidus).

Filler metal in both cases melts below melting temperature of base metal.
Sometimes (especially with brazing) the difference in melting temperatures is small (small process window).
Process window for soldering usually dictated by electronic or other heat-sensitive components

This is what I found in response to your question about why AWS chose 840F for the liquidus point of filler metal as the threshold between soldering and brazing.

From http://www.copper.org/applications/plumbing/techcorner/soldering_brazing_explained.html

" by definition the temperature that defines the difference between soldering and brazing of copper is approximately 840°F/449°C. This temperature is much more important than just an arbitrary definitional threshold. It is important because 700°F/371°C is the temperature at which copper begins to anneal, or be changed from hard temper (rigid) to annealed temper (soft). With this change in temper comes an inherent loss in strength – hard temper copper is stronger than annealed temper copper. The overall amount of annealing that occurs, and thus strength that is lost, is determined by the temperature and the time the material spends at that temperature. The higher the temperature, the less time it takes to change from hard temper to soft temper.

Since brazing temperatures must exceed the melting point of the brazing alloys, between 1,150°F/621°C and 1,550°F/843°C, the process of making a brazed joint causes the base metals to anneal or soften, resulting in a reduction in the overall strength of the assembly. While a brazed joint is demonstrably stronger than a solder joint, the Rated Internal Working Pressure, that is the 24/7 allowable working pressure of the system, is lower for annealed tube (see Copper Tube Handbook, Tables 3a through 3e). "

Also, see a chart of liquidus/solidus temperatures for various filler metals here.

http://www.harrisproductsgroup.com/~/media/Files/PDF/Requested%20Resources/FillerMetalSelectionChart.ashx

Note that all of the common solder compositions used on copper are well below 840F and all brazing compositions used on copper are well above that point. The only composition that comes near the threshold is Alcor which, according to http://www.harrisproductsgroup.com/~/media/Files/PDF/Requested%20Resources/GuidetoBrazingandSoldering.ashx is a Zn/Al composition not intended for use with copper.

(Thanks, Dustin!)

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Comparing Brazing and Soldering

Both rely on capillary action to create a strong joint.

Joint clearance is critical
Typical range is .001 to .005 for brazing, .003 to .006 for soldering (inches)
Joint design may need to compensate for thermal expansion

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Flux

Prevents, dissolves and/or facilitates removal of oxides and other surface contaminants. They also facilitate wetting and encourage intermetallic bonds to form.

Fluxes are not designed to be cleaners. Joint should be clean before brazing begins.

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Brazing Fluxes

Six Categories

Type 1, 2, 3A, 3B, 4, 5
Selection is based on base metals.
Contain combinations of fluorides, chlorides, borates (including borax) and wetting agents
Each type has a different working temperature
Available as liquid, paste, powder. Sometimes compounded with filler metal. Braze rod with flux covering or core is available.

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Brazing Filler Metals

Filler metals are available as

wire

foil

paste

powder

preforms

braze sheet

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Brazing - Advantages

The joining of dissimilar metals, and materials
Very thin material can be brazed which would otherwise be damaged by welding
Inaccessible joints can more easily be brazed
Brazing is easily and more economically automated than many welding processes
Leak-tight joints are easily attained
Multiple joints can be made simultaneously
Less-skilled operators are required
Braze joints are ductile
Braze process is readily automated

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Brazable Materials

Most ferrous and non-ferrous metals.

Many carbides and cermets

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Brazing Limitations

Size limitation, Extremely large parts may be more easily welded.
Parts must be made to close tolerances to ensure proper joint clearance.
"Process window" can be small. Temperatures must be controlled accurately to avoid melting base metal.

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Torch Brazing

Uses an oxyfuel gas on fluxed joints. Can be manual or automated.

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Automated Torch Brazing

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Dip Brazing

Uses a molten chemical or metal bath.

Assembled parts are typically dipped in a heated chemical bath which serve as both fluxing agent and heat source to melt pre-applied filler material.

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Induction Brazing

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Furnace Brazing

Batch furnace brazing
Continuous furnace brazing

Can be controlled atmosphere/vacuum

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Brazing vs. Braze Welding

Brazing must use capillary action to draw the molten filler metal into the joint.

Braze welding is identical in terms of definition except the filler metal is not drawn into the joint with capillary action.

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Soldering

Soldering is one of the oldest methods of joining metals. Because filler metals melt at low temperatures there is minimum part distortion and heat damage to sensitive parts.

Note: “soft solder,” “hard solder,” & “silver solder” are slang terms that are used generally to distinguish solder from brazing.

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Solderable Materials

Depends on wetting

Many combinations of metal to metal or ceramic to metal may be joined.

Soldering is used extensively in the electronics industry where it’s limited mechanical strength is not a major factor.

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Soldering Fluxes

Organic

Contain carbon, corrosive at elevated temperature, non corrosive at room temp.�Water soluble

Inorganic

Very Corrosive, provide better cleaning, do not char or burn easily. Parts must be cleaned after brazing.

Rosin Flux

Non Corrosive, used for electronics least effective cleaning ability

A rosin flux

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Soldering Filler Metals

Filler materials include combinations of tin-lead, tin-silver-lead, tin-zinc, silver-copper-zinc and zinc-aluminum alloys.

As with brazing filler metals, solders are supplied as wires, foil, sheets, pastes, preforms, or as bars and ingots.

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Iron Soldering

Iron soldering is the oldest and simplest soldering method and is still widely used today. Soldering irons have copper tips which easily store and transfer heat to the joint

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Through-Hole Devices

Printed Circuit Board

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Wave Soldering

Wave soldering is a specific method used in the fabrication of electronic components and printed circuit boards (PCB). In this method, continuously circulating fountains or waves of solder are lifted into contact with the joints.

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Surface Mounted Devices

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SMT Reflow Soldering

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Reflow Soldering

Usually used with SMT (to mount SMD's)

Such as ball grid array (BGD) components. Done by Reflow oven or Infrared Lamps (or even heat guns).

Start at 8:30

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SMT Defects

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Laser Soldering

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Torch Soldering ("Sweating")

Used for pipe soldering (plumbing applications)

For more information see:

Copper Tube Handbook

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Soldering Sheet Metal

Lap joint, lock seam joint, using soldering coppers

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