Soldering

The familiar solders contain mainly lead and tin. Molten lead will not wet the surface of copper however clean it is. If a little tin is added, the resulting alloy will readily flow over the copper.

The temperature at which the solder melts depends upon the composition of the alloy used.

So-called "hard" solders may be melted with a gas torch at about 850⁰ C, (850 degrees Celsius). Hard solder is a form of brass (zinc and copper) hence this process is called "brazing". A brazed joint is considerably stronger than a "soft" solder joint.

Soft solder melts at 200 - 250⁰ C and consists of tin and lead in varying proportions, with sometimes a trace of antimony. For electronic circuit work an alloy of 60 % tin: 40 % lead is common. (Plummer's solder contains 35 % tin: 64% lead: 1% antimony.)

It is not possible to amalgamate soft solder with all metals but copper, brass, zinc, most steels and precious metals may be soft soldered.

Metals that are to be soldered must be thoroughly cleaned before starting work. To further clean the surface and remove oxides, which form rapidly, a flux is applied. This seals the surface preventing tarnishing by the atmosphere.

Fluxes may be classified as corrosive, (e.g. Zinc chloride, Baker's fluid) or non-corrosive, (eg. resin, Fluxite).

Only non corrosive flux should be used for electrical work.

Cored solder is convenient because it contains flux within the solder wire.

Soldering: some general points

1. Surfaces to be soldered must be thoroughly cleaned.
2. Surfaces must be "tinned",(i.e. pre-coated with a thin film of solder). This may be accomplished with a soldering iron and resin cored solder.
3. The parts to be joined must be properly mated; the clearance between them being not too small to prevent the solder flowing nor yet so large as to cause a weak joint. It may help to clamp the parts to hold them in position.
4. Sufficient heat must be applied with a soldering iron to amalgamate the two tinned surfaces. For this to happen, the surface heat of the metals must be sufficient to melt the solder.

   If the temperature is insufficient, there will be a layer of solid solder beneath the melted solder and amalgamation will not take place. Such poorly made joints are called "dry joints" and are a common cause of electrical faults.

5. Remove any flux and inspect the joint which should appear fresh and bright.

Two long established makers of temperature controlled soldering irons are Antex and Weller.

Multicore makes solder wire with cores of "Ersin" non corrosive flux.

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Example:
Antex C Miniature Soldering Iron

• 15 watt iron
• a range of 11 bits available
• either 24V, 115V or 250V input
• constant tip temperature
• weight 21 gms
• price guide: £14.00 Stirling

Soldering: first steps for beginners.

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