Vectronics VEC-1500K - Manual (Page 13)

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oxidation is more prevalent. The residue from RMA flux is relatively non-
conductive and non-corrosive.
RA Flux: "Rosin--fully activated" flux (RA) is the most widely-used flux, and
clearly the best choice for bench and field work. RA flux delivers more
aggressive cleaning action, and it activates a wider range of solderable metals
than RMA. Although RA residue is more conductive and corrosive than RMA,
it is also "self-encapsulating". This characteristic isolates corrosive agents from
air and moisture to prevent long-term contamination. On the down side,
encapsulation may interfere with probe contact during testing procedures, and its
protection may fail to hold up when exposed to extreme humidity and moisture.
Other rosin flux formulations are available, but rarely used for hand soldering.
For example, "R" flux (non-activated rosin) is too mild for most practical
applications, and "highly-activated RA" leaves a highly corrosive residue that
must be removed immediately after use. In addition to the popular RMA and
RA rosin fluxes, other formulations such as water-soluble and no-clean flux are
now widely available in wire solders.
No-Clean Flux: "No-clean" flux contains few solids (2-5%), and leaves only a
small trace of non-corrosive and non-conductive residue behind. This, in turn,
eliminates the need for cleaning after the job is completed. No-clean flux is less
aggressive than RA, which reduces its usefulness for general field and bench
work where oxidation my be poorly controlled. However, "no-clean" works very
well for hand-assembling new circuit boards, and is especially popular with
technicians and engineers for surface-mount prototype work.
Water-Soluble Flux: This organic water-soluble flux consists of citric,
glutamic, or lactic acids dissolved in a water or alcohol base. On the plus side,
water-soluble flux is more aggressive that rosin fluxes, and it successfully
activates some metals that RA flux cannot. On the negative side, water-soluble
fluxes leave organic acids and salts behind which are potentially corrosive and
conductive--and these must be removed immediately after use. Of course,
removal is relatively easy, since only water and mild non-toxic cleaning agents
are needed to do the job.
Supplemental Flux
When hand soldering, flux is delivered primarily through the core of solder wire.
In some cases, "core flux" may not provide sufficient chemical action to get the
job done, and additional flux is needed to fully prepare the area. Supplemental
fluxes are available from distributors in most popular types (RMA, RA, no-
clean, etc.), and may be dispensed as a paste or liquid--depending upon
packaging.