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FIGURE 46.4 If water enters an improperly sealed encapsulant vent/fill hole on an IC package, shorting or corrosion can occur. (Courtesy of Hewlett-Packard.)
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Manufacturing Impact of Water-Clean Process There are significant issues to consider when using water-clean fluxes, and the decisions are dependent on the specific parts and final product:
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Expense Aqueous washers are expensive and require a large footprint on the manufacturing floor. Generally, high-purity, de-ionized water is used instead of tap water to enhance rinsing effectiveness and to minimize corrosion or shorts caused by tap water particulates or ionic species such as chlorine or fluorine as found in treated public water supplies. Additional process steps Washing represents several additional handling steps, and with each handling step comes risk to the board. Due to the force of the water jets in the cleaning machine, boards, especially smaller ones, are put in baskets or carriers to go through the water-wash process. This involves additional materials (baskets/carriers) and handling. There is ample opportunity to knock parts off the board during loading or unloading of these baskets. Occasionally tall parts (such as surface-mount capacitors) are blown off the board by the force of the water blast. Boards that somehow escape the water-wash process will develop corrosion in short order. Additional labor Besides the need for additional operators to load and unload the conveyorized wash machine, increased maintenance is required for the aqueous cleaning machine, de-ionized water system, and waste-water treatment. Environmental impact The de-ionized water system requires frequent monitoring and filter replacement. Solder fines washed off the PCA must be filtered from the waste-water stream, and dissolved metals and other species harmful to the environment must be monitored and removed if of a high enough concentration before releasing spent water to the drain. This process is not suited to geographies with water shortages, water restrictions, or poor-quality water, or where the cost of water is high.
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Benefits of Water Washing There are significant benefits to using water-clean fluxes, but the decision involves trade-offs and must be made based on the specific needs of the parts and the product:
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Unimpeded inspection PCAs that have gone through a proper aqueous cleaning process are easy to inspect because there is no flux residue on the surface to interfere with solderjoint inspection by eye or automated optical inspection devices.
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Best adhesion for conformal coating Some boards that are intended for harsh environments rely upon a conformal coating process to enhance their resistance to moisture and airborne contaminants. The coating is silicone, epoxy, or other material. For good adhesion of these coatings, aqueous cleaning is usually prescribed for the boards. Recommended for use with under-fill compounds For added solder-joint reliability, epoxies or other adhesives are sometimes dispensed under certain critical components to anchor them to the PWB surface. Most under-fill compounds require a clean substrate for best bonding, so the underside of the component and board to be bonded in this method should be free of flux residues. Possible electrical implications for RF circuitry Flux residues can interfere with some high-speed circuit conduction by changing top surface dielectric properties and increasing parasitic capacitance. It is for this reason that radio-frequency (RF) circuitry is often specified to be assembled using water-soluble fluxes.
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NO-CLEAN FLUX
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Boosted largely by cost, floor space, and environmental concerns, no-clean fluxes now dominate printed circuit assembly. They are generally composed of rosins (e.g., natural acidic plant residues such as pine pitch) and/or resins. Often, but not exclusively, the term resin is used to denote a manmade rosin analog. Historically, pine sap colophony, as it is referred to in older publications is the most notable natural product used as a flux. Once extracted from the tree, the pine rosin is purified and neutralized to remove excess acidity imparted by the extraction process and dissolved in alcohol. Termed water-white rosin, the key fluxing agents are a mixture of abietic and pimaric acids. Water-white used to be the basis for many commercial flux offerings. Rosin is a solid at room temperature and can be dissolved or partially dissolved in alcohols. During the soldering process, the alcohol or other volatiles in the flux begin to evaporate and the rosin-based solids precipitate. The solids soften at low temperatures (50 to 70 C) and begin to flow, covering the surfaces to be fluxed. With higher temperatures come higher reactivity and oxides and other tarnishes are stripped, readying surfaces for solder wetting. The most widely used fluxes today are resins but act in the same fashion. Generally weak organic acid activators such as succinic, malonic, and adipic acids are added to increase the reactivity of the flux. These, too, volatilize or decompose during the reflow cycle and only a clear polymerized, non-corrosive residue is left on the board. The need for cleaning this residue is absent, hence the term no-clean. The best additives are ones that will not evaporate or decompose too early in the heating cycle but will completely volatilize or decompose at or near the peak temperature of the soldering process. Halogenated activators should be avoided with no-clean chemistries. Some high-temperature organic acids may not completely volatilize or decompose, so reliability testing of the flux residues should be conducted to ensure that corrosion will not impact product reliability. Besides adding activators, one can boost a flux s chemical reactivity by increasing the rosin or resin content, but the higher the rosin/resin content, the heavier the post-soldering flux residue. Pure rosinic residues, especially from high-rosin-content formulations, are benign, hydrophobic, and electrically insulating, but they tend to be sticky.
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Benefits of No-Clean No-clean benefits include lower manufacturing costs through fewer process steps, less board handling, less maintenance, and reduced floor space than is required for board assembly via water-soluble fluxing. There are also environmental benefits of no-clean, as there is almost no waste effluent other than from the solvent within the paste or flux that evaporates during the process and from solvents used to clean the stencil, squeegee blades, and any misprinted boards.
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