By | September 12, 2015

SOLDERING AND BRAZING                                

Soldering and brazing provide permanent joint to bond metal pieces. Soldering and brazing process lies somewhere in between fusion welding and solid state welding. These processes have some advantages over welding process. These can join the metal having poor weldability, dissimilar metals; very less amount of heating is needed. The major disadvantage is joint made by soldering and brazing has low strength as compared to welded joint.


In case of brazing joining of metal pieces is done with the help of filler metal. Filler metal is melted and distributed by capillary action between the faying surfaces of the metallic parts being joined. In this case only filler metal melts. There is no melting of workpiece metal. The filler metal (brazing metal) should have the melting point more than 450oC. Its melting point should be lesser than the melting point of workpiece metal. The metallurgical bonding between work and filler metal and geometric constrictions imposed on the joint by the workpiece metal make the joint stronger than the filler metal out of which the joint has been formed.


Soldering is very much similar to brazing and its principle is same as that of brazing. The major difference lies with the filler metal, the filler metal used in case of soldering should have the melting temperature lower than 450oC. The surfaces to be soldered must be pre-cleaned so that these are faces of oxides, oils, etc. An appropriate flux must be applied to the faying surfaces and then surfaces are heated. Filler metal called solder is added to the joint, which distributes between the closely fitted surfaces. Strength of soldered joint is much lesser than welded joint and less than a brazed joint.

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Most of the solder metals are the alloy of tin and lead. These alloys exhibit a wide range of melting point so different type of soldering metal can be used for variety of applications. Percentage of lead is kept least due to its toxic properties. Tin becomes chemically active at soldering temperature and promotes the wetting action required for making the joint. Copper, silver and antimony are also used in soldering metal as per the strength requirements of the joint. Different solder their melting point and applications are given in the Table “Common Soldering Alloys and their Applications”.

Common Soldering Alloys and their Applications

Filler Metal Compositions  

Melting Point



Tin Lead Silver Zinc Antimony
96 04 305oC Joint making at elevated temperature
60 40 188oC Electronic circuits
50 50 199oC Wire joining
40 60 208oC Automobile radiators
91 09 200oC Joining of aluminium wires
95 05 238oC Plumbing, etc.

A solder is selected on the basis of its melting point. If metals to be joined have higher melting point solder of higher melting point is generally selected. Solder of high melting point provides better strength of the joint.


Soldering fluxes can be classified as:

  • Organic, and
  • Inorganic

Organic Fluxes

Organic fluxes are either rosin or water soluble materials. Rosin used for fluxes are wood gum, and other rosin which are not water soluble. Organic fluxes are mostly used for electrical and electronic circuit making. These are chemically unstable at elevated temperature but non-corrosive at room temperature.

Inorganic Fluxes

Inorganic fluxes are consists of inorganic acids; mixture of metal chlorides (zinc and ammonium chlorides). These are used to achieve rapid and active fluxing where formations of oxide films are problems.

Fluxes should be removed after soldering either by washing with water or by chemical solvents. The main functions performed by fluxes are :

  • remove oxide films and tarnish from base part surfaces,
  • prevent oxidation during heating, and
  • promote wetting of the faying

The fluxes should

  • be molten at soldering temperature,
  • be readily displaced by the molten solder during the process, and leave a residue that is non-corrosive and non-conductive.
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