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This represents an extreme reduction in waste stream to the environment as compared to the use of water-soluble paste and flux. There are volatile organic compound (VOC) free no-clean formulations that make no-clean even more environmentally friendly and attractive. These are generally compounded with a water or water-soluble solvent instead of alcohol. 46.6.2 The Negatives of No-Clean Before talking about the negatives of no-clean, it should be understood that the no-clean approach to circuit board assembly is widely embraced and is the mainstay of the industry. Any negatives of this technology are counterbalanced by major benefits. No-clean is impractical for only a small number of niche applications. 46.6.2.1 Difficulty of Inspection. No-clean residues, especially from surface-mount technology (SMT) soldering, are generally hard, transparent polymerized deposits that interfere to some degree with solder-joint inspection. The deposit can crack and sometimes operators will mistake the cracked flux residue deposit for solder-joint cracking. The residue is also reflective, which can interfere with visual inspection or automatic optical inspection (AOI) methods. 46.6.2.2 Decrease in Barrel Fill at Wave Soldering. Since no-clean fluxes are weaker fluxing agents than water-soluble formulations, wave soldering is a somewhat less robust process. Poorer barrel fill is generally found with no-clean fluxes, although in most cases subtle changes to the wave solder machine configuration and wave solder time-temperature profile are sufficient to gain excellent or acceptable plated through-hole (PTH) barrel fill. 46.6.2.3 ICT Probe Noncontact. Much of the no-clean flux residue associated with wave soldering of the secondary side of the board is washed off by the molten solder during the wave solder operation. Generally, ICT is accomplished by bed-of-nails testing where pointed spring probes contact dedicated test pads located on the secondary side of the board (see Fig. 46.5a). Although these test points are masked off during wave soldering using selective wave soldering pallets (shields), sometimes the liquid flux, when dispensed to the secondary side for the wave solder operation, finds its way between the pallet and the board (see Fig. 46.5b). Flux-covered test pads may be difficult or impossible to probe, depending on the thickness of the solder flux deposit and type of flux used. Without cleaning, these residues will persist. Adjustments to probe tip style and spring force may be necessary to overcome the noncontact issue. There are different types of test probes and different spring forces available. Generally, single-point probes rather than crown probes will have the best chance to penetrate the hardened flux residue. With a single-point contact, point force is maximized rather than spread out over multiple points. Using probes with high spring rates may cause the board to flex and damage solder joints or pierce through pads. 46.6.2.4 Not Recommended for Conformal Coating or Under-Fills. No-clean residue may inhibit adhesion of under-fills or conformal coatings. In fact, the flux residue may wick under a component and completely block an under-fill from entering between the board and the IC package. There are some new developments in this area and there are under-fill/flux combinations that may be effective. 46.6.2.5 Corrosion from Repair. Very often, even water-clean boards are repaired with no-clean formulations to avoid another aqueous cleaning step. Sometimes after a board is totally assembled, components of the board may not be suited for exposure to water. Although no-clean residues after soldering are generally benign, care must be taken to minimize the flux dispensed during board repair. There is a chance that the pooled flux will not be
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FIGURE 46.5 ICT probe noncontact: (a) If there is flux on a test pad, it will harden after soldering and may prevent the test probe from making electrical contact to the test pad. The test result may be interpreted as an open circuit. (b) As the board is fluxed in preparation for the wave solder process, the liquid flux may be drawn by capillary action between the wave solder pallet and the PWB. When this occurs, the in-circuit test pad (the target for bed-of-nails testing) may become fouled with solder flux. The flux residue inhibits probe contact.
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fully heated and divested of its organic acids (activators). If this happens, the flux will remain acidic and promote corrosion. Therefore, it is important to ensure that:
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The smallest amount of flux is used for a repair Flux is confined to the area, that is, the lead(s) to be repaired The use of liquid fluxes is avoided, as they are prone to spreading The smallest diameter flux-core solder wire is used for repairs Ultra-fine, electrostatic discharge (ESD) safe, felt-tip flux pens or paste flux is applied with an ultra-fine artist s brush, fine pick, or pin All the flux dispensed has reached a near soldering temperature
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