Thursday, 2 October 2008

Soldering Fluxes

Fluxes fall into 2 categories: rosin based, and so called water-soluble

1. Rosin Fluxes
Rosin based fluxes are made from rosin which is extracted from pine sap. The purified product is known as "Water White Rosin". The active ingredient is an organic acid, abietic acid and may contain homologs such as dehydro abietic acid and leviopmaric acid.

In addition to rosin other activators may be present at different levels to increase the ability to clean and deoxidise. Activators are compounds that decompose at soldering temperatures yielding ammonia or hydrochloric acid in the process. Flux activity is categorised as R (rosin only), RMA (rosin mildly activated) and RA (rosin activated). A low boiling solvent such as isopropanol is used as the vehicle so they are flammable.

Type R containing only rosin is the least active and is recommended for surfaces very clean to start with. It leaves virtually no residue behind. Thus this is the best rosin based flux for copper foil and lead cames.

Type RMA contains a small amount of additional activator to enhance cleaning and deoxidisation leaving only a minimum amount of inert residue behind. A characteristic of RMA fluxes is that the remaining residue be non-corrosive, tack free, and exhibit a high degree of freedom from ionic contamination after cleaning. These fluxes are acceptable, but more difficult to clean. They are not acceptable for conservation work.

Type RA are most active of the rosin fluxes, and leave the most residue, however the residues can be removed with appropriate flux cleaners. The residues are really difficult to remove in decorative glass work circumstances and should not be used.

2. Water Soluble Fluxes
These are called water-soluble, as the residue left after soldering is water soluble, although the flux is not. The so-called water-soluble fluxes are divided into two categories, organic and inorganic, based on composition. 

Organic fluxes are more active than RA rosin, and inorganic are the most active of all. Both of these are the best of all fluxes to use in decorative glass work, as the residues are water soluble making clean-up easier, and they are more effective in wetting and keeping the copper and lead free from oxidisation at soldering temperatures.

See Also:
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes

Soldering fluxes