Copper Brazing: The Basics You Should Know

Copper and copper alloys

Copper and its alloys have been widely used because of their excellent electrical conductivity, thermal conductivity, corrosion resistance and good formability.

Copper and its alloys can generally be divided into four categories: red copper, brass, bronze and white copper.

Copper Brazing: The Basics You Should Know 1

Material properties of copper and copper alloys

Red copper

Red copper is pure copper with copper content no less than 99.5%.

According to different oxygen content, it can be divided into pure copper and oxygen free copper.

Cu2O and CuO oxides may be formed on the surface of red copper.

The copper surface is covered with Cu2O at room temperature;

The oxide scale under high temperature is divided into two layers, the outer layer is CuO, and the inner layer is Cu2O.

Pure copper cannot be brazed in hydrogen containing reducing atmosphere.


Brass refers to copper zinc alloy, which has much higher strength, hardness and corrosion resistance than red copper, and maintains certain toughness and high corrosion resistance.

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Metallographic diagram of brass

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Special brass

Tin brass:

Tin brass contains about 1% of w (Sn), and the presence of tin does not affect the composition of surface oxides.

The solderability of tin brass is similar to that of brass, which is easy to solder.

Lead brass:

The lead in lead brass will form sticky slag when heated, which will damage the wetting effect and fluidity of solder.

Proper flux must be selected to ensure fluidity.

Manganese brass:

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

Manganese oxide is relatively stable and difficult to remove, so active brazing flux should be used to ensure the wettability of brazing filler metal.


There are many kinds of bronze, and different alloy elements are added, so the brazability is different.

If the added alloy element is tin, or some small amount of chromium or cadmium, it has little effect on the solderability, and is generally easier to braze.

If the added alloy element is aluminum, especially when the aluminum content is high (up to 10%), the aluminum oxide on the surface is difficult to remove, and the solderability is deteriorated.

Special flux must be used for brazing.

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For example, when silicon element is added to form silicon bronze, silicon bronze is quite sensitive to hot brittleness and stress cracking under the action of molten solder.

For example, the alloy element added is beryllium.

Although a relatively stable BeO oxide is formed, the conventional brazing flux can also meet the requirements of removing the oxide film.

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Copper Brazing: The Basics You Should Know 6

White copper

White copper is an alloy of copper and nickel, which has good comprehensive mechanical properties.

The white copper contains nickel.

When selecting the filler metal, avoid selecting the filler metal containing phosphorus, such as copper phosphorus filler metal and copper phosphorus silver filler metal.

White copper is very sensitive to hot cracking and stress cracking under the action of molten solder.

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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 used because of its moderate melting point, good processability, good strength, toughness, 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, because the addition of copper can reduce the melting temperature of silver, and does not form a brittle phase.

The addition of zinc can further reduce its melting temperature.

The addition of tin can reduce the melting temperature of silver copper tin alloy very low, but the alloy with low melting temperature is extremely brittle and has no practical use value.

In order to avoid brittleness, the tin content of silver copper tin solder is generally not higher than 10%.

In order 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 used in brazing copper and copper alloys due to its good technological performance and low price.

Phosphorus plays two roles in copper:

First, phosphorus can significantly reduce the melting point of copper;

Second, it acts as a self soldering flux when brazing copper in air.

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

Phosphorus and copper rattan silver filler metals can only be used to braze copper and copper alloys, and cannot be used to braze steel, nickel alloys and copper nickel alloys with nickel content greater than 10%.

This kind of filler metal has segregation effect when heating slowly, so the method of fast heating brazing should be adopted as far as possible.

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 using Sn based solder to braze copper, intermetallic compound Cu6Sn5 is easily formed at the interface between solder and base metal, so attention must be paid to brazing temperature and holding time.

In general, when soldering with soldering iron, the compound layer is very thin, which has no great influence on the joint performance.

The strength of brass joint brazed with tin lead filler metal is higher than that of copper joint brazed with the same filler metal.

This is because the dissolution of brass in liquid filler metal is slower than that of copper, so less brittle intermetallic compounds are generated.

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 existing brazing fluxes mainly use borax, boric acid and their mixtures as the matrix, and add some alkali metals or alkaline earth metals’ fluoride, fluoroborate, etc. to obtain appropriate activation temperature and enhance the ability to remove oxides.

Boric acid H3B03 decomposes to form boric anhydride B203 when heated.

The reaction formula is as follows:


The melting point of boric anhydride is 580 ℃.

It can form soluble borate with copper, zinc, nickel and iron oxides and float on the brazed joint in the form of slag, which can not only remove the film, but also play a role in mechanical protection.


Borax Na2B4O7 melts at 741 ℃:


Boron anhydride and metal oxides form soluble borates, while sodium metaborate and borates form compound compounds with lower melting temperature, which are easy to float to the surface of solder joints.


The mixture of borax and boric acid is a widely used flux.

Adding boric acid can reduce the surface tension of borax flux and promote the spread of flux.

Boric acid can also improve the deslagging property of flux residue.

However, when the borax boric acid flux is used with silver filler metal, the melting temperature is still too high and the viscosity is too high.

To further reduce its melting temperature, potassium fluoride can be added.

The more important function of KF is to reduce the viscosity of the flux and improve the ability to remove oxides.

In order to further reduce its melting temperature and improve 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 copper alloys can be brazed by many methods, such as iron brazing, immersion brazing, flame brazing, induction brazing, resistance brazing, furnace brazing, contact reaction brazing, etc.

However, high heating current is required due to low resistance of copper during high-frequency brazing.

Copper and copper alloy brazing technology


The coordination of filler metal and flux is as follows when brazing copper.

Rosin flux can be used when soldering especially clean surfaces with tin lead and tin silver solder, otherwise active rosin, weak corrosive flux or corrosive flux can be used.

Except for oxygen free copper, pure copper is not brazed in reducing atmosphere to avoid hydrogen embrittlement.


The filler metal and flux for brazing brass are basically the same as those for brazing copper.

However, there is zinc oxide on the surface of copper, so it cannot be brazed with inactive rosin.

FB102 flux must also be used when brazing with copper phosphorus and silver solder.

Manganese brass

Phosphoric acid solution flux shall be used for tin lead brazing;

Zinc oxide solution brazing flux is used for lead based brazing.

Q205 brazing flux is used for cadmium based brazing.

FB102 or FB103 flux is used for brazing BAg45CuCdNi and BAg45CuCd solders.

FB102 is used for brazing other silver based solders, copper phosphorus and copper phosphorus silver solders.

It is recommended to braze in the furnace in a protective atmosphere with FB104 flux.

Beryllium bronze

For the beryllium bronze in the soft soldering quenching aging state, the brazing filler metal with melting temperature lower than 300 °C shall be selected, and 63Sn-37Pb shall be preferably used in combination with weak corrosive flux or corrosive flux.

Brazing and solution treatment shall be carried out at the same time during brazing.

Chrome bronze

Soft soldering has little effect on its performance index, so soft solders and fluxes similar to beryllium bronze can be used for brazing.

Brazing of chromium bronze cannot be carried out in its solution aging state, but should be carried out in the solution treatment state and then aging.

The silver solder with the lowest melting temperature, such as BAgA0 CuZnCdNi, shall be used for brazing by rapid heating method.

Cadmium bronze and tin bronze

It is similar to copper and brass, but there is no hydrogen embrittlement and zinc volatilization when brazing in a protective atmosphere.

However, the tin bronze containing phosphorus has a tendency of stress cracking.

Silicon bronze

Strong corrosive flux with hydrochloric acid shall be used for soft soldering.

There is a tendency of stress cracking and intergranular penetration of filler metal during brazing.

The brazing temperature is below 760 ° C.

Silver solders with lower melting temperature can be used, such as BAg65CuZn, BAg50 CuZnCd, BAg40 CuZnCdNi, BAg56 CuZnSn, etc.

The lower the melting temperature is, the better.

FB102 and FB103 are the best fluxes.

Aluminum bronze

During soft soldering, in order to remove the oxide film on the surface, a strong corrosive flux with hydrochloric acid shall be used.

The commonly used solder is tin lead solder.

Silver filler metal is used for brazing.

In order to prevent aluminum from diffusing to silver solder, the brazing heating time must be as short as possible.

Copper or nickel plating on the surface of aluminum bronze can also prevent the diffusion of aluminum to solder.

Zinc white copper

The soldering of zinc white copper is similar to that of brass.

The following silver solders are used for brazing: BAg56CuZnSn, BAg50CuZnSnNi, BAg40CuZnNi, BAg56CuZnCd, etc.

The flux is FB102 and FB103.

Manganese white copper

Phosphoric acid solution flux is used for soldering, or copper is pre plated on the surface.

Brazing filler metals used include BAg60CuZn, BAg45CuZn, BAg40CuZnCdNi, BAg50 CuZnCd, etc.

Copper phosphorus silver solder is not recommended, because 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

In addition to the heat treatment of the age hardenable copper alloy (such as beryllium bronze), the only process after brazing is to remove the residual flux and clean the workpiece surface.

The main purpose of removing the residue is to prevent the corrosion of the residue on the workpiece, sometimes to obtain a good appearance or further process the workpiece after brazing.

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