Fluxes Rosin base

EH 65/31 Rosin-based solder flux fume criteria document for an OEL... 

The use of VMS fluids to remove semi-polar soils requires the use of additives to tailor the solubility. We chose a rosin-based solder flux as a model for such soils. Rosin is a natural product which is a complex mixture rich in tricyclic, partially unsaturated carboxylic acids that contain twenty carbon atoms, and related derivatives. Figure 2 illustrates the impact that addition of a polar solvent (designated a polar solvency enhancer) has on the ability of the VMS fluids to remove solder flux. Although none of the VMS fluids were able to remove much solder flux unassisted, addition of 18 wt% l-methoxy-2-propanol greatly accelerated removal rate and resulted in thorough removal in 5-10 minutes at RT. Fortunately, the VMS fluids are miscible with many polar solvents, so it is easy to tailor solvency to particular soils. 

The flux can be any one of the various cxjmpositions—rosin-based, no-clean, low solids, and water-soluble. The flux also provides the tack that keeps the components attached to the board prior to the reflow step. Other ingredients in the paste are the thixotropic agents. It is the thixotropic agents, together with the metal content and flux, that determine the viscosity of the paste. 

Up to the mid-1970s, the predominant flux chemistry was rosin-based. Today, although rosin fluxes are still in use, great advances have been made in flux chemistries and a large variety of formulations with very different chemistries are being used. 

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. 

Solder fluxes and pastes have gone throngh significant evolution since the early 1980s. As of 2006, in North America about 70 percent of the fluxes were not cleaned, 25 percent were water-soluble, and 5 percent were rosin-based for military applications. However, in the lead-free soldering world, more cleaning is reqnired. 

MIL-F-14256, Flux, Soldering, Liqnid (Rosin Base). June 15,1995. 

The second is rosin-based, with additives to make it chemically active to varying degrees. Removal requires solvent, but it can be formulated so that the residue is inert and can remain on the board. This is termed no-clean flux. 

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. 

No-clean flux residue can vary in color from clear-transparent to amber. The color depends on the resin systems used in the flux formulation. Modified resin systems tend to give pale, transparent residues, while rosin-based no-cleans tend to give amber-colored residues. The residue also will be affected by the thermal profile it sees (hotter thermal profiles will darken the flux residue). Therefore, residues from lead-free no-clean solder pastes typically are darker than those of the Sn/Pb systems. The chemistry of the no-clean flux system also will determine if the flux residue remains on top of the soldered joint, or flows out to the perimeter of the soldered joint. 

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. 

A flux classification system for rosin fluxes that has been in use for many years defines the use of additives to a rosin base to increase the flux activity. Some applications require stronger fluxing action than is provided by rosin alone. Therefore, compositions with an increased activity have been formulated. The system consists of the designations R, RA, and RMA for rosin, activated... 

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. 

In solder wave applications, there are several ways to apply a flux to a PCB, the method selected depends on the flux system utihzed. A foam fluxer (Figure 43) can be used for rosin-based or noclean alcohol fluxes that meet the lead-free process temperature requirements. A foam fluxer is... 

Controlling health nsks from rosin (colophony) based solder fluxes How HSE assesses offshore safety cases WASP - Quality assurance for chemical analysis... 

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