Cup Alloys - Brazing Best Practice -


UK Silver Price
28th Mar 2017


  Brazing Best Practice

Capillary Flow

The brazing process, of which silver soldering is a part, has been used successfully for thousands of years to produce strong leaktight, ductile joints. The earliest examples of brazed joints were found in the tomb of King Tutankhamen and were made about 3500BC. Pictures on the walls showed the craftsmen at work over charcoal fires and using a blowpipe to create the heat. Silver soldering is an old and simple process capable of producing strong joints in a wide variety of material combinations. Although simple, it is a skillful process only made difficult by not adhering to the basic principles of the process.

Stick to the principles and you will be successful

Deviating from them leads to problems.

If you experience difficulties producing sound strong joints re-examine your techniques to adhere to the basic principles of the process. You will also find that you use less material!

One definition in British Standards defines brazing as :

"a process of joining generally applied to metals in which, during or after heating, molten filler is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined, by capillary attraction"

International Convention declares that brazed joints are made above the melting point of aluminium 610 degC. Below that temperature you are soldering. A brazed joint is identified by the temperature of the filer metal, not by the composition of the rod in the hand.

The key words are by capillary attraction. Everything that you do, joint design, fluxing, heating is aimed at promoting capillary flow. If this is not done you are not brazing. You are not getting all the inherent benefits and advantages of the process. You are simply using a very expensive filler rod to block a hole!


Capillary action is enhanced for brazing by considering five factors:

 Joint Design


  Heating Technique

  Alloy Application

  Parent Materials

Joint Design

"Closely adjacent surfaces" implies a degree of overlap between the components. This creates a joint in which all stresses are carried in shear or torsion. A brazed or soldered joint is at its' strongest when operating in shear or torsion. It is at its' weakest when operating in tension ie a butt joint because when the silver solder solidifies there is a local contraction causing it to "neck". That acts as a "stress raiser"  and joint failure.

Typical silver solder joint strengths in shear can be as high as 135,000 psi dependent on the joint gap and irrespective of the parent materials. This is way above the strength of annealed copper and brass and that required for the safe operation of locomotive boilers, hydraulic lines and most joints found in general industry.

For capillary attraction there must be a gap into which the filler metal will flow. All soft and silver solders have the ability to bridge,penetrate gaps. Narrow melting range, free flowing alloys eg 455 and 445 are better suited to small gaps. Joints with large gaps are better made with wider melting range alloys eg 430.



Typical joint designs for brazing/soldering

As in all cases, maximum joint strength is achieved in a brazed joint when thesilver solder fully penetrates the joint gap. To obtain that penetration, the joint gap must be controlled within certain tolerances dependent on the parent materials and silver solder. The alloy will not penetrate a gap that is too small or too large. The joint gap when using 455 to silver solder copper components should be 0.002 - 0.006'' (0.05 - 0.15mm). If a wider melting point solder is being used eg 440 the gap should be 0.004 - 0.008'' (0.1 - 0.2mm). Tight fits are to be avoided. In all tube joints the gap is radial

The strength of a joint simply depends on the load being transferred from one component to the other across the filler metal and relies on the tremendously strong inter-atomic strength between the silver solder and the parent materials. It does not rely on the bulk strength of the alloy.

Fillets contribute nothing to the strength of a joint. They may offer the user a sense of confidence of a job well done but that is all but in reality a fillet may be hiding a weak joint due to lack of penetration of the alloy into the joint! See "Alloy Application"

The joint strength is affected by the joint gap. Joints made with smaller gaps are more dependent on that inter-atomic strength. But there must be a gap into which the silver solder can flow.


There has to be however sufficient area over which to transfer the forces. BS EN 13133 Brazing Practice suggests that the joint length should be 3 - 4x the thickness of the thinner component. Longer joints run the risk of flux entrapment or incomplete penetration and potential weakness. They are also needlessly expensive and contribute nothing towards the joint strength.

Most joints are self-jigging but occasionally components may have to be clamped into position particularly with sheet/sheet joints. Clamping effectively removes the joint gap. Maintain it by centre punching dimples into one of the sheets or place a piece of foil in between. Hold components together with soft wire that will expand with the components preventing distortion. Any clamping arrangemment should be as light as possible to avoid it becoming a heat sink and slowing the heating process.

In many cases, particularly those involving tubes, it is better if the joint is not clamped because as the  alloy melts and flows, the capillary forces will "centre" the tubes.

When joining dis-similar metals consideration has to be given to their relative coefficients of expansion. Brass and copper expand more than steels. It is important that the gap is suitable at brazing temperature. Joints involving tubes or bosses should be that the material with the lowest coefficient is the male. On cooling the joint is put under compression removing any risk of the silver solder cracking.

The rate of flow of the silver solder into the joint is proportional to the gap. The gap should be consistent within the capability of the silver solder. The alloy will first fill that part of the joint where the gap is largest. It will not fill the gap if it is too large. What may appear to be a sound joint is not but hides a void in the joint Failure could occur at any time due to thermal stresses during cooling, reheating or in service.

Joint Cleanliness

It is imperative that all joints are perfectly clean if an alloy is to flow properly and produce sound joints. It is, however, immaterial as to the cleanliness at room temperature, it is the state at soldering or brazing temperature that is all important.

Excessive attention to joint cleanliness during assembly is un-necessary. No matter how much you clean the components, you are going to create more oxide from the heat source than you have so painstakingly removed. 
Oxide removal is the function of the flux!

Certainly all parts should be free of oil and grease. If appropriate de-grease using warm soapy water, solvent, wire wool or a stiff wire brush. This is probably more relevant to soft soldering because the lower temperatures involved may not drive off or burn off grease from handling the components.

Do not use emery cloth or grit based products. These can leave deposits behind that the flux cannot remove leading to porous joints.


"the silver solder melts - but it doesn't flow - it just goes into a ball and drops off"

there is a flux problem!

In order for the molten alloy to flow, it must "wet" onto the surfaces to be joined. It takes up a flat not a spherical profile.

For wetting to occur, the surfaces should be free of grease and any oxides present before or during the heating cycle. Before soft soldering, the parts should be degreased or scoured with wire wool. Care should be taken not to handle the components in the joint area. When precleaning do not use a grit or emery cloth product or scotchbrite. They can leave behind deposits that the flux cannot remove. The result can be pinholes and leaks.

Oxide removal is the function of the flux. It prepares the surfaces of the parent material to bond with the silver solder.

Soft solder fluxes are normally liquid readily applied with a nylon brush.

Silver solder fluxes are normally supplied in powder form but are best used as a paste. It is more convenient, easier to use and get good coverage of the joint area. Paste is readily available commercially but can be made in the workshop.

Hint  When making up a paste, first add a couple of drops of detergent to the powder and mix to a yoghurt consistency with water. Keep it in an airtight container. The detergent aids mixing, helps the flux to stick to the components and allows remixing with a little extra water if it dries out. It will. Without the detergent the flux dries like a brick and needs to be discarded.

Ensure that there is a good coverage of the joint area. A good volume of flux helps to prevent it becoming spent and not working.  When heated the flux paste undergoes changes.

  The water is driven off as the water boils to leave a white deposit

  The flux starts to melt and appears to "sweat"

  The flux bexcomes a clear liquid and is now active.

  The flux flows by capillary attraction into and through the joint cleaning the joint.

Regrettably there is not a universal flux. The flux used will depend on various circumstances but must meet three criteria. If not all criteria are met the flux will not work, the alloy will melt and go into a ball. This is the flux problem. Which flux to use will depend on

  The oxides present on the surface of the parent materials

  Melting temperature of the silver solder

  Duration of the heating cycle.

Sound joints made with relatively short heating times, using a low melting or medium temperature silver solder eg 455, 445 and 438 can normally be achieved with EF flux. Joints made on stainless steel, or with longer heating times or using a high temperature alloy eg 430, 424, 418 are best done with HT5.

Avoid "hot-rodding" ie warming the rod in the flame and immersing the rod in the flkux powder to create a flux-coated rod. This restricts the amount of flux being placed on the joint. It limits the time that the flux has to clean the joint  but most detrimentally it promotes a welding type procedure of heating the rod and possibly leading to poor joints (see Heating Techniques) Use the practice only as a means of applying additional flux to a joint

Generally most soft solder joints can be made with Comsol/Bakers Fluid. Use Staybrite Flux or Stainbrite Aluminium when soldering stainless steel or aluminium respectively.

Ater brazing, the flux residues (some of which are corrosive) must be removed from the components. With EF flux this is readily done by a soft brush under running water or immersion in a bath of dilute sulphuric acid or the safer, more environmentally friendly citric acid. When the citric is exhausted, it can be easily disposed of by tipping down a convenient drain. Mix 20gm of salts in 1 litre of water.

HT5 residues are best removed mechanically with a stiff wire brush or immersion in a 10% solution of caustic soda.

Soft solder fluxes are best removed in running water.

Hint Citric acid is an organic compound that can, after a period of time, develop an algae. If this occurs simply add more citric acid to disperse it.


Heating Technique

The most common form of heating is a gas torch. There are many on the market and it can be difficult to assess their true value when it comes to brazing and soldering. The basic premise is to have a torch that produces a focused flame capable of generating sufficient heat, quickly enough to achieve metal flow into the joint. Ideally it will have a defined flame that will allow the user to control the heat and create the right heat pattern that will determine where the alloy flows.

Sievert propane torches are recommended as they are extremely versatile and will deliver exactly what the engineer needs, capillary flow.


The integrity of the joint and its strength is dependent  upon a high level of joint soundness. The manner in which a joint is heated greatly influences the way a soft or silver solder melts and flows. Capillary flow increases with increasing surface tension, decreasing viscosity and reduced contact angle of the filler metal with the parent material. Since all these features work beneficially with increased temperature, the alloy will be encouraged to flow towards the hottest part of a joint. To achieve maximum penetration into the joint, there is a need to develop a heat pattern in the joint that promotes the capillary flow. 

Maximum strength is dependent on achieving full penetration into the joint. Failure to do so, effectively builds an unseen crack into the back of the joint so weakening it.

When heating the first thing to achieve is to get the whole joint hot by general heating of the area and avoid cold spots where the alloy might freeze and stop flowing and not create that strong joint. Initially apply heat to the whole area using that part of the flame just outside the blue zone of the flame. That is the hottest part. Watch the flux. When it melts and goes into a clear liquid (at about 580 deg C) concentrate the heat on the area of the joint to which the alloy is required to flow in relation to where it is being applied. Apply the alloy. Do not heat the rod or preplaced material. Allow the alloy get its heat from the joint not the torch. Once the alloy has flowed remove the heat. Nothing is to be achieved by continued heating only potential problems caused by excessive heating.

The flux melting, becoming a clear liquid is a good indicator that the joint is approaching the brazing temperature. Do not attempt to apply alloy before this. The joint is not hot enough. Be patient.

See the video cliip below.

Improve the effectiveness of your heating by using lightweight heat reflective bricks or ceramic insulation blanket Use them to form a brazing hearth to get and keep the heat in the workpiece.



Do not use conventional firebricks eg from storage heaters. They absorb and retain heat. They slow the heating process and create cold spots.

Hint To insulate larger shapes or create a custom built hearth. Soak the insulation blanket in water and form it to the required shape. Let it dry and the blanket will keep its' shape. Bend it through 90 degrees and it will stand upright.

Propane is the preferred heating gas. It is safe, efficient and economical. It is a less concentrated heat source compared to oxy-acetylene. It encourages general heating to ensure the whole joint reaches brazing temperature yet still develops the heat pattern to promote capillsry flow. Sievert torches offer the best range of heating options to match most users needs.(see Sievert torches on home page). Because propane flames are not so intensive, they not only get the surface hot but also develop "through heat" necessary for the alloy penetrate the joint. 

Use a burner large enough to heat the joint quicky but small enough to give the necessary control to develop that heat pattern. With a flame temperature of 1980 deg C, propane burners can produce between 0.25 and 84 kw of heat. The heat output is dependent purely on the volume of gas being burnt.

Use a neck tube between the handle and burner that keeps the hands sufficiently far away from the flame (for comfort) without sacrificing positional control of the burner. Larger neck tubes tend to be fitted to the bigger burners.

Propane burners are basically a gas stream that draws in air through vents in the side of the burner to create a flammable mixture. When brazing in a confined space eg in a firebox the burner uses all the available air and the flame extinguishes. In these instances use a Sievert cyclone burner. These draw in air through vents in the neck tube so maintaining the flame.

Hint Always light the burner from underneath. Using a gas lighter position the flame on the edge of the burner and move the flame into the gas stream. The burner will ignite first time and not blow out the gas lighter.

Oxy-acetylene with a flame temperature of 3100 deg C is hotter than propane but that does not mean more heat. It is a far more concentrated flame than propane. Its intense localized heat makes more difficult and discourages creating the heat pattern for capillary flow. It encourages direct heating of the rod and can restrict alloy penetration. The intense flame makes it easier to overheat the alloy creating blowholes or excessive shrinkage when the alloy freezes. Both will lead to weaker joints.

Oxy-acetylene torches are best used in conjunction with propane torches. On their own, the intense flame can cause local thermal stresses. However when working on larger copper components eg boilers propane alone may be insufficient. In these cases use the propane as a background heat to get the joint area hot and expand, Use the oxy-acetylene to lift the temperature locally to brazing temperature.

Complex assemblies are best left to cool naturally or slowly. Quenching them can create thermal stresses that could cause cracking of joints, particularly in joints that are not sound.


Application of Alloy 

Silver solder should be applied in a manner suitable to meet the requirements of the joint design and heating technique. This may be rod, wire, foil or paste. Rod is best used bare and not flux coated. Using the heat pattern generated, allow the alloy to get its heat from the joint. The dsize of the filler metal should reflect the size of the component. Small joint = thin rod. Use the thinnest rod or wire possible. It reduces the chilling effect on the joint and is likely to be more economical.

Consider the use of preplaced alloy. It will control the volume and cost of the material used per joint. Placing a ring or cropped piece of rod internally and heating externally to draw the metal through is an ideal scenario.

  You have 100% visual inspection of a sound joint

  There is only a small witness of the alloy.

  There is a neat and tidy appearance

  There are no post-brazing operations

Hint Rings can be simply made.

  Light the propane torch.

  Wind the alloy onto a mandrel to the required size.

  Clamp or hold the spring in place.

  Warm the spring with the torch to anneal the spring and remove the tension.

  Allow to cool and slide the spring off the mandrel.

  Tin snips give a ring to size first time

For ring sizes see "FAQ"

On small joints use thin wire or paste. Care should be taken that they are not heated directly. 

Hint  Hold the wire in a hollow pin vice. It stops the wire moving about in the draught of the flame and keeps the fingers away from the heat.

For advice on alloy selection see "Help Me Choose"

Parent Materials

Brazing and soldering are known for their ability to produce strong joints in a wide range of material combinations. The alloys are all made to ISO 17672, the International Standard for brazing materials. They are made using virgin metals to control any impurity that may cause brittle joints by forming brittle intermetallic compounds. These are notably aluminium and phosphorus in steel and titanium with silver.

Lead is added to parent material to improve machining characteristics. It is present as free lead. At brazing temperatures, any lead in the surface is dissolved by the silver solder. Its' flow characteristics change dramatically and it becomes more sluggish. Penetration of the solder is reduced. Lead is found in some grades of plumbing copper and mild steel EN2A

Aluminium is added to bronze. If the amount is above 2%, the flux may have difficulty in removing the oxide. The alloy will not wet onto the surface and flow.

Copper is available in many grades. Tough pitch copper should be avoided. It contains dissolved oxygen. If the reducing (blue zone) of the flameheats the copper, the oxygen is converted into steam. This steam penetrates the grain boundaries of the copper causing cracks to form. This phenomenon is called "hydrogen embrittlement". Catastrophic failure can occur without significant deformation or obvious deterioration of the component.

Use oxygen free copper C101 and C106.



  Keep the work bench clean and free of dust

  Work in well ventilated areas.

  Wash hands thoroughly after soldering or brazing.

  Use protective gloves and goggles.

  Do not eat, smoke or drink while soldering or brazing.

  If you notice anything unusual - stop brazing and seek advice



Copyright CuP Alloys (Metal Joining) Ltd 2017