Rosin Paste Fluxes are intended to be used to reduce solder balling, and bridging. The proper flux allows for proper solder flow and increased wetting of desired areas.

Rosin Fluxes

Rosin or colophony is a natural product that is extracted from the stumps or bark of pine trees. The composition of rosin varies from batch to batch, but a general formula is C19H29COOH. It consists mainly of abietic acid (70 to 85 percent, depending on the source) with 10 to 15 percent pimaric acids. Rosins contain several percent of unsaponifiable hydrocarbons; for rosin flux removal, saponifiers (a form of alkaline chemical to make the water soapy) must be added.
Rosin flux is composed primarily of natural resin extracted from the oleoresin of pine trees and refined. Rosin fluxes are inactive at room temperatures but become active when heated to soldering temperatures. They are naturally acidic (165 to 170 mg KOH per g equivalent). They are soluble in a variety of solvents but not water. This is the reason for using solvents, semiaqueous solvents or water with saponifiers to remove them.

The melting point of rosin is 172°C to 175°C (342°Fto 347°F), or just below the melting point of solder (183°C). A desirable flux should melt and become active slightly below the soldering temperature. A flux is not effective if it decomposes at soldering temperatures, however. This means that synthetic fluxes can be used at higher temperatures than rosin fluxes, because the former decompose at higher temperatures. In general, rosin fluxes are weak, and to improve their activity (fluxing action), the use of halide activators is required.

The general formula for oxide removal by rosin is:
RCO2H + MX = RCO2M + HX
where RCO2H is rosin in the flux (C19H29COOH mentioned earlier)
M = Sn, Pb or Cu
X = oxide, hydroxide or carbonate

As mentioned earlier, rosin fluxes are also referred to as rosin (R), rosin mildly activated (RMA) and rosin activated (RA). The various categories of rosin fluxes differ in the concentration of the activators (halide, organic acids, amino acids, etc.). R and RMA types are generally noncorrosive, hence safe. R and RMA fluxes are not even cleaned in some applications even though they are not classified as no-clean. However, without cleaning, assembly reliability may be compromised because the sticky rosin can attract dust and harmful contaminants in the field during service.

The fluxes described here require cleaning. To get away from cleaning, many companies have shifted to no-clean flux, which will be the focus of my next column.