Rosin-based fluxes

The activity of a rosin-based flux will be determined by the adivators and surfactants, which are part of the formulation. Some activators help to remove metal oxide but also leave residues that are essentially noncorrosive. 

Military specifications in the past have required the use of rosin-based fluxes. These were defined as pure rosin (R), rosin mildly activated (RMA), rosin activated (RA), and rosin super-activated (RSA), based upon the level of halide activators they contained. Typically only R or RMA fluxes were approved for high-reliability military applications. 

For processes utilizing potentially corrosive rosin based fluxes, simple test options to determine cleanliness such as resistance of solvent extract (ROSE) tests can be used. Other more significant tests may be required to determine the residue properties of organic residue fluxes, such as those commonly used during hot air solder leveling (HASL) of boards.The most common of these are ion chromatography tests that can characterize the residuals and their potential hazards to the product. 

In the case of carbon steels and stainless steels, and many of the non-ferrous alloys, the fluxes are based on acidic inorganic salts, e.g. chlorides, which are highly corrosive to the metal, unless they are removed subsequently by washing in hot water. For soldering tinplate, clean copper, and brass, it is possible to formulate rosin-based fluxes having non-corrosive residues, and these are essential for all electrical and electronic work. 

Soldering processes associated with Pb-Sn solders have been performed with fluxes from each of three categories, the (natural) rosin-based fluxes, resin-based fluxes, and the organic acid (water-soluble) fluxes. ... 

A detailed study on the wetting characteristics of eutectic Bi-Sn solder has been reported in terms of impurity effects, fluxes, base metals, and soldering temperature [18]. It was determined that eutectic Bi n solder is far less tolerant of impurities than eutectic Plr n [19]. In particular, the presence of impurity elements which form intermetallic compounds with Bi-Sn solder, such as Cu, Ni, Fe, and Pd, is especially critical, while Sb and Pb appear to be beneficial in terms of promoting wetting characteristics. Table 1 compares the solderabihty of several low-melting-point solders on various surface metallizations [20]. Only the Au/Ni-plated metallization is considered acceptable for Bi n solder, while the Cu, Ni, and Au/Ni metallizations are all acceptable for eutectic Sn Pb solder when a rosin-base flux is used. Another study confirmed that Bi Sn solder does not wet Cu-base metallizations as well as eutectic Sn-Pb solder does when a rosin-base flux is used [21]. However, if the Cu surface is pre-tirmed, then Bi-Sn wetting is acceptable even with a rosin-base flux [22]. Based on the wettabihty studies, eutectic Bi-Sn solder can only be considered a viable candidate if a suitable flux system is developed which allows it to be utilized for metallizations other than a Au/Ni overplate. 

Modern solder and flux teclinology began in the 20th century with the demands of the electronics industry. For most of the century, the most extensive utilization encompassed tin-lead solders and rosin-based fluxes. 

As was noted with rosin-based fluxes, the activators added typically contain halides such as chlorides and bromides, which are fairly corrosive. There are, however, several categories of milder, halide-free activators such as organic acids and amine compounds. Because these fluxes are less aggressive, they are slower-acting and do not have the ability to remove thicker tarnish layers. For these fluxes to be effective, it is necessary to provide pristine solderable surfaces. 

As already noted, one way to increase the activity of rosin-based fluxes is to increase the activity of the activator material, hence the progression of R to RMA to RA fluxes. Another less drastic approach is to increase the rosin content, which is typically 35-45%. Because rosin is mostly abietic acid as noted earlier, increasing the rosin content increases activity. Doing so, of course, increases the potential for corrosion. If a rosin flux was not removed postsoldering for an application, it may be necessary to remove it if the rosin content is increased. 

It was noted in Sec. VI. L. 3 that one method to increase the activity of a rosin-based flux is to increase the rosin concentration in the flux. It should be noted that high-solids-content fluxes typically do not readily form foam. Wave fluxing is often used to dispense flux for high-solids-content fluxes. A tradeolf analysis is necessary if raising the solids level to increase activity results in the need to change the method of flux application. 

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