Ingredients in a soldering flux

Written by carlos mano
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Ingredients in a soldering flux
Soldering is a method for joining together two metal objects. (soldering image by Bube from

Soldering is a method for joining together two metal objects by using a filler (or solder) that melts at a relatively low temperature. If differs from welding in that the joined metals are not melted. A soldered join is much weaker than a welded join, but it is strong enough, waterproof enough and electrically conductive enough for many purposes. Soldering is commonly used in plumbing, electronics and in various arts and crafts.

Flux for Soldering Electronics

The traditional solder for electronic and electrical equipment is a mixture of lead and tin. The traditional flux for this type of solder is rosin based with a small amount of hydrochloric acid or a chloride of either ammonium or zinc. This flux cleans off the area and also helps the solder flow into the joint. Rosin flux is also known as water soluble flux--it is the residue that is water soluble, not the flux. In other words, clean up is easy after the soldering is done. There are increasingly expensive grades of rosin flux depending on if it is for consumer, industrial or military use.

Flux for Soldering Plumbing

It is illegal to use lead on any plumbing, so the tin-lead solder is not used on plumbing--especially on pipes that carry drinking water. The fluxes that contain chlorides and strong acids are inappropriate for use with plumbing. The plumbing fluxes usually contain organic acids like stearic acid or acetic acid. Most plumbing flux comes in the form of a paste that is spread on the joint before it is soldered. There are different grades of flux formulated for small pipes, large pipes and hot weather pipes.

Flux for Soldering Aluminum

Aluminium is difficult to solder, and requires fluxes with exotic materials such as fluoroboric acid, cadmium fluoroborate, triethanolamine, sodium fluoride and chlorides of ammonium, tin and zinc. Aluminium must be extremely clean before the solder will work. The flux should be applied and left to set for an hour and then wiped off and reapplied before soldering.

Flux for Soldering Jewelry

Jewellery is in constant contact with the human body, so the solders and fluxes used in jewellery are even more benign than those used in plumbing. Fluxes used in jewellery tend to be used more for cleaning the area to be soldered than to assist in filler flow. Typical flux has borax and salt as the only active ingredients. Potash is also used. It is important that the flux be non-toxic and that it does not cause pitting in metals. Fluxes used for soldering jewellery never contain strong acids.

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