The Capillary Solder System

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copper-tubing-plumbers-portal8The Capillary Solder System 

Since the introduction of thin-walled copper tubes in the early eighties, this method of joining copper tubes has become the most widely used. This has been due to several factors:

  •  It’s ease of fabrication
  •  Inexpensive and up to 70% saving on time and labour 
  •  Excellent mechanical properties
  •  Non-bulky fittings makes for a very compact system 

Soldered joints depend on capillary action drawing free-flowing molten solder into the gap between the fitting and the tube. Capillary fittings soldered correctly with 97/3 soft solder have a shear strength of 60 Mpa (600 Bar) and are able to withstand a high degree of vibration, expansion and contraction. 

A full range of capillary fittings is readily available in all sizes to fit SANS 460 Classes 0, 1, 2, 3 and CTA Blue Tube. 

Capillary fittings used with water-based self-cleaning flux with no ammonia and 97/3 solder, make a strong and lasting joint. (97% Tin/3% Copper). 

Care should be taken to not use too much flux. The cleaning agents in the flux can be harmful to the copper system if not cleaned off properly.

Caution should be taken when choosing fittings:

  •  Oversized: Tolerance between fitting and tube is too great and capillary action will not occur. 
  •  Undersized: Tolerance between fitting and tube is too little and solder will not be able to enter the joint. 

In both cases the joint will lead to failure. 

Soldering Flux
The functions of the soldering flux are:

  •  To remove residual traces of oxides
  •  To promote wetting
  •  To protect the surface to be soldered from oxidation during heating

Standard practice calls for cleaning the tube and fittings by mechanical means. Cleaning by mechanically is also required if residues of cement, paint or tape gum are found on the surface to be joined. The use of steel wool is not recommended for cleaning. Use a CTA cleaning pad or sandpaper. 

Soldering flux should only be applied to a clean surface. Lightly coat the surface to be joined. An oxide film may re-form quickly on the copper tube after it has been cleaned, therefore the flux should be applied as soon as possible after cleaning. Any excess flux should be wiped off before soldering. 

Once the joint has been soldered, the flux residue should be removed with a damp cloth. At no time should the soldered joint be shock-cooled with water or a wet rag as this may cause the joint to fracture. Wipe only with a damp cloth. 

The type of flux required is self-cleaning, water-based, contains NO ammonia and is able to withstand temperatures in excess of 240°C.

Note: The term “water-based” means that the flux is soluble in water and should rinse out of the system once flushed.

CTA recommends water-based self-cleaning flux for use with lead-free solder

Note: Joint must be soldered on the same day. If not, flux will harden and capillary action will not occur, causing joint failure.


It is extremely important to use only the recommended solder. Any solder containing lead or having either a resin or acid core should be avoided at all times. Not only is there the obvious health hazard, but there is also the risk of mechanical failure of the joint. 

CTA recommends 97/3 (97% tin; 3% copper) lead-free solder. 

Heat Source 

A soft non-localised flame such as that produced by LPG/Propane/ Butane mix is required. The use of oxy-acetylene is not recommended as the flame is too concentrated and severe. This causes overheating of the tube, leading to burning and drying out of the flux, annealing of the tube and melting of the tube and fitting. 

Note: The selection of gas torches is very important. 35mm-108mm require different size torches.

Brazed Joints

The brazing of capillary fittings to SABS 460 Class 0, copper tube on sizes below 54mm OD and SABS 460 Class 1 copper tube on sizes below 35mm OD, is not recommended.  

Brazed joints would normally be used only where operating service temperatures are above 150°C. 

Unlike solder with a melting point of ±240°C, brazing materials melt in the range of 600°C to 750°C. Consideration of this must be taken when designing a system as these temperatures will anneal the tube and fitting. Pressure ratings should therefore be calculated as for annealed tubing and care taken to avoid mechanical damage and consideration be given to adequate and correct tube supports. 

There are two types of filler materials in normal use. These are alloys containing silver and alloys containing phosphorus. Fluxes are not normally required when joining copper to copper only when joining copper to brass or cast bronze fittings. Check with recommendations of the filler material manufacturer for the correct flux and application. 

The various steps of joint preparation and application of the filler material are the same as for the large size soldered joints. 

It is recommended that a slightly reduced flame be used i.e. Excess fuel. 

As with soldering the brazing filler material MUST flow into the capillary joint and fill the capillary gap for the full depth of the socket. It is extremely BAD practice to apply only a “fillet” weld around the mouth of the fitting. It is advisable to allow the fitting to cool naturally after brazing. Brass or bronze cast fittings should never be cooled quickly by means of water. 

The heat source for brazing would normally be Oxy-acetylene to provide the required temperature. It is important that this flame does not remain on the same point and damage the joint but kept in motion over the section being brazed.

Compression fittings 

Brass compression fittings, to avoid corrosion problems, should be dezincification resistant. It is not recommended that they be used in conjunction with SABS 460 Class 0, hard drawn tube, but only on the half hard classes i.e. 460/1 - 460/2 and 460/3 tubing. These fittings rely on the compression of the brass olive onto the tube. It is BAD practice to either solder the olive or fitting into the tube or apply any PTFE tape or hemp to the fitting. DO NOT over tighten the cap nuts.