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The ideal flux would be one that is potent, requires only a small quantity, leaves little or no residue (and what residue does remain would be thin, transparent, nonreflective), easily penetrated by in-circuit test probes, electrically insulating, and chemically inert. If not inert, it should be easily and completely removable in an inexpensive solvent. However, this is far from actual materials and process reality. Flux must be applied in sufficient quantity for thorough oxide removal and to avoid both metal reoxidation and flux exhaustion prior to solder wetting. Some post-soldering flux residues or form by-products may have deleterious effects on the circuit assembly since they may be chemically reactive and result in corrosion of exposed metals over the long term (see Fig. 46.3). The corrosive agents can be electrically conductive, resulting in electrical leakage (soft shorts) between closely spaced, oppositely charged conductors on the circuit board surface exacerbated by the residue s hygroscopic moisture uptake from the atmosphere. Corrosion may be augmented either by natural electromagnetic field (EMF) couples between two different metals or electrochemically driven by a charge between two adjacent conductors. The metallic ions, electrochemically driven, can plate out as metal dendrites on the surface of the board and produce intermittent or even persistent hard short circuiting (see Fig. 46.3). Flux must be heated to a temperature to allow the best reactive conditions without drying out or denaturing. For the sake of economy and cleaning, a thin application is necessary and should be confined to the bonded site(s) of interest.

Up to 100 C no limitations; from 100 to 250 C stabilized Attention to oxidation resistance bearings and of lubricant is necessary special lubrication procedures are probably required For below 30 C special lubricants are required; consideration of starting torque necessary May impose limitation; consult manufacturer Lubricant may impose Lubricant may limitations; consideration impose limitations of starting torque necessary

FIGURE 46.3 Corrosion dendrites caused by corrosive, hygroscopic flux residues. (Courtesy of Hewlett-Packard.)

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The 1989 adoption of the Montreal Protocol2 imparted far-reaching changes to electronic manufacturing. In the circuit board assembly arena, it drove the elimination of chlorofluorocarbons and the reduction of volatile organic compounds for earth s ozone layer preservation. Many fluxes that used volatile organics such as alcohols were reformulated to use water as their solvent. Solvent-clean fluxes, which required non-aqueous cleaning chemicals (alcohols, chlorofluorocarbons [CFCs], hydochlorofluorocarbons [HCFCs], etc.), were practically eliminated from the marketplace. Today only two flux categories are in widespread use for PCB assembly: water-clean and no-clean. The former leaves a residue that must be removed from the PCA; otherwise, corrosion will set in. The latter, as the name implies, leaves a benign residue that is meant to be left on the circuit board. In fact, some no-clean residues perform as a conformal coating shielding the board from moisture and atmospheric contaminants. The choice between water-clean and no-clean has a profound effect on the process and may also affect subsequent process steps or long-term reliability. Both can be effective oxide strippers for soldering, but there are several considerations for choosing and using the proper flux.

Also referred to as aqueous-clean fluxes or organic acid (OA) fluxes (a misleading term, because most fluxes are organic acid-based), this category contains the most aggressive fluxing agents used for large-scale commercial board assembly. It can consist of strong organic acids, inorganic acids, amines, amino salts, or often a weak organic acid fortified with reactive materials such as halogen salts (Br , F , I , Cl ) that turn acidic during the flux decomposition process. For this reason, as the name implies, this type of flux requires an aqueous wash step to remove any reactive residue traces to avoid corrosion on the PCA.

Many of the water-soluble solder pastes are hygroscopic or leave hygroscopic, corrosive residues on the circuit board after soldering. This property can impact solder paste or flux shelf-life and the solder paste s physical attributes during printing and soldered assembly reliability. Most solder pastes are thixotropic; that is, they shear thin as they are being squeegeed onto the board, becoming thinner with use. Generally, pastes are formulated to limit the degree of thixotropy. In addition to thixotropic thinning, some solder pastes, when exposed to a humid atmosphere, pick up moisture, which further thins paste rheology. Paste stenciling becomes more difficult to control and tack, and slump characteristics can be adversely affected. Ironically, some of these same organic acid solder pastes are prone to dry out in lowhumidity conditions. Check paste manufacturer recommendations for optimal temperature and humidity regimes for storage and use. Localized environmental control of air temperature and humidity within the printer enclosure or factory-floor air conditioning can help to preserve solder paste rheological characteristics.

Attention to differential Low expansion and temperature starting torques is necessary Attention to possibility of fretting damage is necessary (except for hydrostatic bearings)

Cleaning Process The cleaning consists of more than a dip in a tank of water. Generally commercially available, conveyorized wash tunnels are used for effective removal of the reactive residue from the soldered PCA. Such equipment can be many meters long and may have numerous zones consisting of cold-water spray nozzles, hot-water spray nozzles, and hot-air drying. Some systems may incorporate sumps where the board is briefly conveyed underwater for a short time to enhance residue dissolution. High-purity (de-ionized) water is recommended to enhance washing to eliminate or minimize corrosion. Saponifiers are sometimes injected into the water stream to target organic residues and convert them to water-soluble entities. Following the saponification step, thorough rinsing with pure water is required to remove traces of the saponifying agent.

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