Soldering is a joining process wherein coalescence is produced by heating below 800°F, using a non-ferrous filler metal with a melting point below that of the base metal. The metals to be joined dictate the flux, solder, and heating methods to be used. Base metals are selected for specific properties such as electrical conductivity, weight, and corrosion resistance.

To achieve a sound soldered joint, the following should be considered:

• Joint design: They should be designed with the requirements of solders and their limitations in mind.
• Pre-cleaning: The surfaces must be thoroughly cleaned to allow the solder to wet the base metal.
• Fluxing: A flux must be provided to remove traces of surface film or oxides and to prevent formation of oxides during the soldering operation.
• Proper fixtures or alignment of parts must be maintained to insure a sound soldered joint.
• Heating of the base metals should be uniform or even on base metals, to insure good penetration of the filler alloy into the joint. If a noncorrosive flux is used no further cleaning is necessary. The use of a corrosive flux makes flux residue removal imperative.

Basic Steps of Soldering

1. Joint fitting: A clearance of 0.005" is suitable for most soldering. When soldering precoated metals, a clearance of 0.001" is recommended for maximum mechanical strength. 

2.  Types of cleaning include:

• Mechanical - Scotch Brite pad, emery cloth
• Chemical - cleaning using acids to remove rust, scale or sulfides. Most commonly used acids are hydrochloric and sulphuric.

3.  Application of flux

• Flux should be capable of removing oxides and stop them from reforming.
• Flux should permit displacement by the solder.
• Flux should promote wetting of the surface by the solder.

4.  Application of heat: Heating the joints evenly or uniformly is of utmost importance to insure a sound joint.

• Types of soldering equipment:
1. Soldering irons - electric
2. Plumbers torch - propane low heat
3. Dip soldering - large tank with molten solder to solder multiple joints
4. Oven heating - only used in production where other heating methods are impractical

5.  Applying the Solder takes place in two steps:

1. Wetting the metal surfaces
2. Filling the gap between the wetted surfaces with solder
   
   Depending upon conditions dictated by the application, each step can be done separately.  This allows for more easily controlled conditions.

6.  Cooling the Joint:

• As soon as possible after soldering the joint may be cooled using a water spray or air blast. Slow cooling could cause excessive alloying, resulting in a brittle joint.

7.  Flux Residue Treatment:

• Non-corrosive fluxes are ones which are rosin base and do not require removal. Corrosive fluxes are fluxes containing zinc chloride. Removal is a must to prevent corrosion.