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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.
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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.
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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.
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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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Component Compatibility Not all components are compatible with the water-wash process. Circuit board mounted switches are notable in this regard. Even some switches sold as environmentally sealed or water-wash compatible have been found to leak during the high-pressure wash spray or submersion steps of aqueous cleaning. This can result in failure of the switch to work properly (the water causes an electrical short within the switch) or, if there is enough ionic content in the wash water or in the switch itself, switch contact corrosion may occur in the long term. Many circuit assemblies are very dense with parts.The higher the part density, the less effective aqueous cleaners are. Like stones in a stream, the water swirls around them and eddy currents of relatively stagnant water form behind the stones. Components on a circuit board act in the same way, as they block the free flow of water and impede proper rinsing even in the most turbulent water spray systems. To counteract this, aqueous wash machines have multiple spray nozzles directed at board surfaces in several directions and angles. Many components have low headroom spaces beneath them, inhibiting rinsing effectiveness. In the case of PGAs, ball grid assemblies (BGAs), ceramic column grid arrays (CCGAs), ceramic ball grid arrays (CBGAs), etc., the water s path is further frustrated by the presence of solder joints beneath the component. Increasing I/O count, increased package size, and decreasing space between the PWB and component underside make water washing all the more difficult. The same issues hold true for chip-scale packages (CSPs) where the headroom is little more than a capillary and good washwater exchange is unlikely. High-density, multirow connectors are also difficult to rinse. All of the preceding are equally difficult to dry. Water beneath components or within components may interfere with in-circuit test or functional test results immediately following aqueous cleaning. Water from aqueous wash has been known to enter integrated circuit (IC) encapsulant vent holes and cause shorts internal to the IC (see Fig. 46.4).
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