Copper Brazing: The Basics You Should Know

Copper and copper alloys

Copper and its alloys are widely used due to their outstanding electrical conductivity, thermal conductivity, corrosion resistance, and formability. These alloys can generally be divided into four categories: red copper, brass, bronze, and white copper.

Material properties of copper and copper alloys

Red copper

Red copper is a pure form of copper with a copper content of at least 99.5%.

It can be further divided into pure copper and oxygen-free copper based on its oxygen content.

Cu2O and CuO oxides can form on the surface of red copper.

At room temperature, the copper surface is covered with Cu2O.

Under high temperatures, the oxide scale is composed of two layers: the outer layer is CuO and the inner layer is Cu2O.

It is important to note that pure copper cannot be brazed in a hydrogen-containing reducing atmosphere.


Brass refers to a copper-zinc alloy that has greater strength, hardness, and corrosion resistance compared to red copper while still retaining toughness and high corrosion resistance.

Metallographic diagram of brass

Special brass

Tin brass:

Tin brass contains approximately 1% tin (Sn) and the presence of tin does not alter the composition of surface oxides.

The solderability of tin brass is comparable to that of brass, making it easy to solder.

Lead brass:

Lead brass contains lead, which when heated forms a sticky slag that impairs the wetting effect and fluidity of solder.

It is important to select the proper flux to ensure proper fluidity.

Manganese brass:

The surface of manganese brass is comprised of zinc oxide and manganese oxide.

Manganese oxide is relatively stable and difficult to remove, so it is necessary to use an active brazing flux to ensure the wettability of the brazing filler metal.


There are various types of bronze, each with different alloy elements, which affects their brazability.

When the alloy element added is tin, or a small amount of chromium or cadmium, it has minimal impact on solderability and is generally easier to braze.

However, if the added element is aluminum, particularly when the aluminum content is high (up to 10%), the aluminum oxide on the surface is difficult to remove, causing a deterioration in solderability.

In such cases, it is necessary to use a special flux for brazing.

For instance, when silicon is added to form silicon bronze, it becomes highly sensitive to hot brittleness and stress cracking when exposed to molten solder.

Another example is when the added alloy element is beryllium.

Although a relatively stable BeO oxide forms, conventional brazing flux is sufficient for removing the oxide film.

White copper

White copper is an alloy of copper and nickel that boasts excellent comprehensive mechanical properties.

It contains nickel.

When selecting filler metal, it is important to avoid those containing phosphorus, such as copper-phosphorus filler metal and copper-phosphorus-silver filler metal.

White copper is highly sensitive to hot cracking and stress cracking when subjected to molten solder.

Brazability of common copper and copper alloys

alloy Copper T1 Oxygen free copper TU1 Brass Tin-bronze
Manganese brass
Tin-bronze Lead Brass
H96 H68 H62 QSn58-2 QSn4-3
Brazeability Excellent Excellent Excellent Excellent Excellent Excellent Good Excellent Excellent Good
Alloy aluminium bronze
beryllium bronze
silicon bronze
chromium bronze
cadmium bronze
Zinc-Copper-nickel alloy
Mn copper nickel alloy
QAl9-2 QAl10-4-4 QBe2 QBe1.7
Brazeability Bad Bad Good Good Good Good Excellent Good Difficult

Brazing filler metal – silver based brazing filler metal

Silver-based solder is widely utilized due to its moderate melting point, good processability, strong and tough qualities, conductivity, thermal conductivity, and corrosion resistance.

The main alloy elements of silver-based solders are copper, zinc, cadmium, and tin. Copper is the most important alloy element, as it reduces the melting temperature of silver without forming a brittle phase.

The addition of zinc further lowers the melting temperature.

While the addition of tin can significantly lower the melting temperature of silver-copper-tin alloys, this low melting temperature results in extreme brittleness and lack of practical use.

To avoid brittleness, the tin content in silver-copper-tin solder is typically not higher than 10%.

To further reduce the melting temperature of silver-based solder, cadmium can be added to the silver-copper-zinc alloy.

Chemical composition and main properties of silver based brazing filler metal

Brazing filler metal Chemical composition(weight %) Melting temperature/℃ Tensile strength/MPa Electrical resistivity/μΩ·m Brazing temperature/℃

Ag Cu Zn Cd Sn other
BAg72Cu. 72±1 allowance 779~779 375 0.022 780~900
BAg50Cu. 50±1.1 allowance 779~850
BAg70Cu. 70±1 26±1 allowance 730~755 353 0.042
BAg65Cu. 65±1 20±1.1 allowance 685~720 384 0.086
BAg60Cu 60 ±1 allowance 10±0.5 602~718 720~840
BAg50Cu 50±1.1 34±1.1 allowance 10±0.5 677~775 343 0.076 775~870
BAg45Cu 45±1 30+1 allowance 677~743 386 0.097 745~845
BAg25CuZn. 25±1. 40±1 allowance 745~775 353 0.069 800~890
BAg10CuZn 10±1 53±1.1 allowance 815~850 451 0.065 850~950
BAg50CuZnCd 50±1 15.5±1 16.5±2 627~635 419 0.072 635~760
BAg45CuZnCd 45±1. 15±1 16±2. 607~618 620~760
BAg40CuZnCdNi 40±1 16±0.5 17.8±0.5 Ni0.2±0.1 595~605 392 0.069 605~705
BAg34CuZnCd 35±1 26±1 21±2 607~702 411 0.069 700~845
BAg50CuZnCdNi 50±1.1 15.5±1 15.5±2 Ni3±0.5 632~688 431 0.105 690~815
BAg56CuZnSn 56±1 22±1 17±2 50.5 50.5 618~652 650~760
BAg34CuZnSn 34±1 36±1.1 27+2 30.5 30.5 630~730 730~820
BAg50CuZnSnNi 50±1 21.5±1 27±1.1 10.3 10.3 Ni0.30~0.65。 650~670 670~770
BAg40CuZnSnNi 40±1 25±1 30.5±1 30.3 30.3 镍1.30~1.65 630~640. 640~740

Brazing filler metal -Copper phosphorus solder

Copper-phosphorus brazing filler metal is widely utilized in brazing copper and copper alloys due to its favorable technological performance and cost-effectiveness.

Phosphorus serves two functions in copper:

First, it significantly lowers the melting point of copper.

Second, it acts as a self-soldering flux during brazing in air.

To further reduce the melting temperature of the Cu-P alloy and improve its toughness, silver can also be added.

It is important to note that copper-phosphorus and copper-rattan-silver filler metals can only be used for brazing copper and copper alloys and cannot be used for brazing steel, nickel alloys, or copper-nickel alloys with a nickel content greater than 10%.

This type of filler metal may result in segregation when heated slowly, so it is best to adopt a fast heating brazing method.

Chemical composition and properties of copper phosphorus solder

Filler metal Chemical composition (mass fraction) (%) Melting temperature Tensile strength MPa Resistivity/μΩ·m
Cu P Ag Sn other
Bcu95P. allowance 5±0.3 710~924
Bcu93P allowance 6.8~7.5 710~800 470.4 0.28
Bcu92PSb allowance 6.3±0.4 Sb1.5~2.0 690~800 303.8 0.47
Bcu91Ag allowance 7±0.2 2±0.2 645~810
Bcu89Ag allowance 5.8~6.7 5±0.2 650~800 519.4 0.23
Bcu80Pag allowance 4.8~5.3 15±0.5 640~815 499.8 0.12
HLAgCu70-5 allowance 5±0.5 25±0.5 650~710
HLCuP6-3 allowance 5.7±0.3 3.5±0.5 640~680 0.35
Cu86SnP allowance 5.3±0.5 7.5±0.5 0.8±0.4 620~660
Bcu80PSnAg allowance 5.3±0.5 5±0.5 10±0.5 560~650
Cu77NiSnP. 77.6 7.0 9.7 Ni5.7 591~643

Soft solder-Tin based solder

When brazing copper with Sn-based solder, the formation of the intermetallic compound Cu6Sn5 at the interface between the solder and base metal is common. Therefore, it is important to carefully consider the brazing temperature and holding time.

When using a soldering iron, the compound layer is typically thin and has minimal impact on the joint’s performance.

Brass joints brazed with tin-lead filler metal are stronger than copper joints brazed with the same filler metal. This is because the dissolution of brass in the liquid filler metal is slower, resulting in the formation of fewer brittle intermetallic compounds.

Brazing filler metal Chemical composition Fusion temperature Tensile strength Elongation
Sn Ag Sb Cu
HL606 96.0 4.0 221 53.0
Sn95Sb 95.0 5.0 233 39.2 43
Sn92AgCuSb 92.0 5.0 1.0 2.0 250 49.0 2.3
Sn85AgSb 84.5 8.0 7.5 270 80.4 8.8
Brazing filler metal Chemical composition Fusion temperature
97.0 3.0 Sn
HLAgPb97 97.5 1.5 304-305
HLAgPb97.5-1.0 92 2.5 1.0 310-310
HLAgPb92-5.5 83.5 1.5 5.5 287-296
HLAgPb83.5-15-1.5 97.0 3.0 15.0 265-270

Soft solder – cadmium based solder

Chemical composition and properties of cadmium based solder

Filler metal Chemical composition (mass fraction) (%) Melting temperature/ Tensile strength/MPa
Cd Ag Zn
HL503 95 5 338~393 112.8
HLAgCd96-1 96 3 1 300~325 110.8
Cd79ZnAg 79 5 16 270~285 200
HL508 92 5 3 320~360

Soft solder – lead-free solder

Lead free solder for brazing copper tubes

BrandComposition (mass fraction)Solid phase line/℃Liquidus/℃

Strength of Copper and Brass Joints Brazed with Part of Soft Solder

Solder brand Shear strength/MPa Tensile strength/MPa
copper brass copper brass
S-Pb80Sn18Sb2 20.6 36.3 88.2 95.1
S-Pb68Sn30Sb2 26.5 2740 89.2 86.2
S-Pb58Sn40Sb2 36.3 45.1 76.4 78.40
S-Sn90Pb10 45.1 44.1 63.7 68.6
S-P697Ag3 29.4 49.0
S-Cd96Ag3Zn1 73.5 57.8
S-Sn95Sb5 37.2
S-sn85Ag8Sb7 82.3
S-Sn92AgSCu2Sb1 35.3
S-Sn96Ag4P 35.339.2~49.0 5.339.2~49.0

Brazing flux

The commonly used brazing fluxes consist of a matrix of borax, boric acid, or a mixture of both, and are supplemented with fluorides or fluoroborates of alkali or alkaline earth metals to achieve an appropriate activation temperature and improve oxide removal capabilities.

When heated, boric acid (H3BO3) breaks down to form boric anhydride (B2O3).

The reaction formula is as follows:


The melting point of boric anhydride is 580°C.

It can react with copper, zinc, nickel, and iron oxides to form a soluble borate, which floats on the brazed joint as a slag. This not only removes the oxide film but also provides mechanical protection.


Borax Na2B4O7 melts at 741 ℃:


Boric anhydride and metal oxides react to form soluble borates. Sodium metaborate and borates combine to form compounds with a lower melting temperature, making them easy to rise to the surface of solder joints.


The combination of borax and boric acid is a commonly utilized flux. The addition of boric acid can lower the surface tension of the borax flux and enhance its spread. Boric acid also enhances the ability of the flux residue to cleanly detach from the surface. However, when using borax-boric acid flux with silver filler metal, its melting temperature remains too high and its viscosity is still too high.

To further decrease the melting temperature, potassium fluoride can be added. The primary role of potassium fluoride is to lower the viscosity of the flux and enhance its ability to remove oxides. To further reduce the melting temperature and increase its activity, KBF4 can be added.

The melting point of KBF4 is 540 ℃, and the melting decomposition is:


BrandComposition (mass fraction) (%)Action temperature ℃Purpose
FB101Boric acid 30, potassium fluoroborate 70550~850℃Silver solder flux
FB102Anhydrous potassium fluoride 42, potassium fluoroborate 25, boric anhydride 35600~850℃The most widely used silver solder flux
FB103Potassium fluoborate>95, potassium carbonate<5550~750℃For silver copper zinc cadmium solder
FB104Borax 50, boric acid 35, potassium fluoride 15650~850℃Brazing with silver based filler metal in furnace

Soft soldering flux

Corrosive flux

1ZnCl21130g,NH4Cl110g,H2O4LBrazing copper and copper alloys, steel
2ZnCl21020g,NaCI280g,NH4CI,HCI30g,H2O4LWelding copper and copper alloys, steel
3ZnCl2600g,NaCl170gDipped brazing covering agent
4ZnCl2710g, NH4Cl100g, Vaseline 1840g, H2O 180gBrazing copper and copper alloys, steel
5ZnCl21360g,NH4Cl140g,HC185g,H2O4LBrazing silicon bronze, aluminum bronze, stainless steel
6H3P04960g,H20455gBrazed manganese bronze, Stainless steel
QJ205ZnCl250g,NH4Cl15,CdCl230,NaF6Brazing of copper and copper alloys with cadmium based filler metals

Weakly corrosive flux

1Glutamic acid hydrochloride 540g, urea 310g, water 4LCopper, brass, bronze
2Hydrazine monobromide 280g, water 2550g, non-ionic wetting agent 1.5gCopper, brass, bronze
3Lactic acid (85%) 260g, water 1190g, wetting agent 3gWrinkled bronze

Non corrosive flux

The main component of non corrosive flux is rosin.

There are three commonly used rosin fluxes:

  • Inactivated rosin;
  • Weak activated rosin;
  • Active rosin.

Surface preparation

  • Solvent degreasing or alkaline solution is applicable to copper and copper alloys.
  • Mechanical methods, wire brushes and sandblasting can be used to remove oxides.
  • Silicon brass;
  • Chromium bronze and copper nickel alloy;
  • Aluminum bronze beryllium bronze;
  • Copper, brass, tin bronze.

Brazing process

Copper and its alloys can be brazed using various methods such as iron brazing, immersion brazing, flame brazing, induction brazing, resistance brazing, furnace brazing, contact reaction brazing, and others. However, during high-frequency brazing, a high heating current is necessary due to the low resistance of copper.

Copper and copper alloy brazing technology


When brazing copper, the coordination of filler metal and flux is as follows:

When soldering clean surfaces, especially with tin lead and tin silver solder, rosin flux can be used. For other surfaces, active rosin, weak corrosive flux, or corrosive flux can be utilized.

It is important to note that pure copper should not be brazed in a reducing atmosphere, except for oxygen-free copper, in order to avoid hydrogen embrittlement.


The filler metal and flux used for brazing brass are generally similar to those used for brazing copper. However, it should be noted that due to the presence of zinc oxide on the surface of brass, it cannot be brazed with inactive rosin. Additionally, when brazing with copper phosphorus and silver solder, FB102 flux must be used.

Manganese brass

For tin-lead brazing, a phosphoric acid solution flux should be utilized. Lead-based brazing requires the use of a zinc oxide solution brazing flux. Q205 brazing flux is used for cadmium-based brazing. BAg45CuCdNi and BAg45CuCd solders should be brazed with FB102 or FB103 flux. Other silver-based solders, as well as copper phosphorus and copper phosphorus silver solders, should be brazed with FB102 flux. It is recommended to braze using FB104 flux in a protective atmosphere within a furnace.

Beryllium bronze

When brazing beryllium bronze in its soft soldering quenching aging state, it is important to select a brazing filler metal with a melting temperature lower than 300°C. The preferred combination for this application is 63Sn-37Pb in combination with a weak corrosive flux or a corrosive flux. Additionally, brazing and solution treatment should be carried out simultaneously during the brazing process.

Chrome bronze

Soft soldering has minimal impact on the performance index of beryllium bronze, thus soft solders and fluxes similar to those used for beryllium bronze can be utilized for brazing.

It is important to note that chromium bronze should not be brazed in its solution aging state, but rather in the solution treatment state followed by aging.

When using a rapid heating method for brazing, it is recommended to use the silver solder with the lowest melting temperature, such as BAgA0 CuZnCdNi.

Cadmium bronze and tin bronze

Brazing tin bronze is similar to brazing copper and brass, but with the added benefit of avoiding hydrogen embrittlement and zinc volatilization when brazing in a protective atmosphere.

However, it should be noted that tin bronze containing phosphorus has a tendency towards stress cracking.

Silicon bronze

For soft soldering, it is recommended to use a strong corrosive flux containing hydrochloric acid.

During brazing, there is a tendency towards stress cracking and intergranular penetration of the filler metal. The brazing temperature should be below 760°C.

Silver solders with lower melting temperatures, such as BAg65CuZn, BAg50 CuZnCd, BAg40 CuZnCdNi, and BAg56 CuZnSn, can be used. The lower the melting temperature, the better.

For optimal results, FB102 and FB103 are the recommended fluxes to use.

Aluminum bronze

When performing soft soldering, it is important to use a strong corrosive flux containing hydrochloric acid to remove the oxide film on the surface. The commonly used solder for this process is tin-lead solder.

For brazing, silver filler metal is typically used. To prevent aluminum from diffusing into the silver solder, the brazing heating time should be kept as short as possible. Plating the surface of aluminum bronze with copper or nickel can also prevent the diffusion of aluminum into the solder.

Zinc white copper

The soldering process for zinc white copper is similar to that of brass. The following silver solders are commonly used for brazing: BAg56CuZnSn, BAg50CuZnSnNi, BAg40CuZnNi, and BAg56CuZnCd, among others. The recommended fluxes for use are FB102 and FB103.

Manganese white copper

For soldering zinc white copper, a phosphoric acid solution flux can be used or the surface can be pre-plated with copper.

The brazing filler metals that can be used include BAg60CuZn, BAg45CuZn, BAg40CuZnCdNi, and BAg50 CuZnCd, among others.

It is not recommended to use copper-phosphorus silver solder, as phosphorus and nickel will form a brittle compound phase.

Joint Strength of Copper and Brass Brazed with Silver Solder

Filler metal Shear strength/MPa Tensile strength/MPa
copper brass copper brass
BAg45CuZn 177 215 181 325
BAg50CuZn 171 208 174. 334
BAg65CuZn 171 208 177 334
BAg70CuZn 166 199 185 321
BAg40CuZnCdNi 167 194 179 339
BAg50CuZnCd 167 226 210 375
BAg35CuZnCd 164 190 167 328
BAg40CuZnSnNi 98 245 176 295
BAg50CuZnSn 220 240

Mechanical properties of copper joints brazed with copper phosphorus and copper phosphorus silver solders

Filler metalTensile strength
Shear strength
Bending angle
Impact toughness
/J · cm-2

Post weld heat treatment

For age-hardenable copper alloys, such as beryllium bronze, that have undergone heat treatment, the only step after brazing is to remove the residual flux and clean the workpiece surface.

The primary reason for removing the residue is to prevent corrosion on the workpiece and, in some cases, to achieve a good appearance or prepare the workpiece for further processing.

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