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48.4 AUTOMATED SOLDER FOUNTAIN
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This version of the solder fountain incorporates a programmable head and is used mostly for first-pass soldering, but can be an effective soldering repair tool once the PTH component is removed, the site is prepped, and a new component is placed. A programmable nozzle emitting a precise jet of molten solder is directed at the leads of the component to be soldered.The nozzles allow point soldering of very fine PTH pitches.
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48.5 LASER
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Laser is a recent innovation in rework and repair. The fundamentals of laser for initial soldering or for repair have already been discussed in Chap. 47. In the commercial incarnation of this technique, a laser beam is quickly scanned around component leads or, package surface, in the case of area-array devices such as BGAs and chip-scale packages (CSPs). It is most effective for plastic packaged components rather than the thermally massive CBGAs, CCGAs, and so on. In this technique, the component body is heated. Otherwise, there is little difference between this and alternative rework or repair techniques. Since the energy of the laser beam is more tightly confined to the area it irradiates, the heating is more localized. This can be advantageous when trying to remove or replace a component on a densely populated, double-sided PCA. Otherwise, the same caveats must be kept in mind regarding IMC formation and pulling components before they are fully reflowed. Care must be taken to assess carefully the assembly for laser damage threshold so as not to char components or PCAs during either removal or component resoldering.
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48.6 CONSIDERATIONS FOR REPAIR
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48.6.1 Mixing Aqueous With No-Clean Chemistries Care must be taken when using fluxes for the rework and repair processes. If an aqueousclean solder chemistry is used for building the PCA, then either no-clean or aqueous flux formulations can be used. If an aqueous chemistry is used for the repair, then the board must be subjected to another aqueous cleaning cycle. Be sure to check that all components at that stage are compatible with the aqueous cleaning process. No-clean solder flux formulations can be used for rework and repair even if the PCA was manufactured with an aqueous clean chemistry, although the reverse is not recommended. Sometimes when a no-clean board is subjected to an aqueous cleaning, the no-clean flux residue takes on a white, gummy characteristic that is conducive to dendritic corrosion, which can result in soft or hard electrical shorting. Saponified aqueous cleaning can be used, but must be tested for effectiveness in removing the polymerized flux residue from the no-clean process and for compatibility with the selected no-clean flux to avoid generation of corrosive byproducts.This is particularly important under connectors, area-array devices, and other components with low headroom between the underside of the body and the PWB surface.
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SOLDERING REPAIR AND REWORK
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Pooled Flux Another consideration with no-clean solder chemistries is that of pooled, unactivated flux residue. No-clean solder flux is generally harmless after it has gone through the thermal cycle associated with solder reflow. It is activated and denatured by the heat. If the flux is applied generously and the flux pool does not see the high temperatures associated with solder joint formation, acids remaining in the unheated flux residue can cause reliability problems. These acids are normally denatured, evaporated, or sublimed through the high-temperature excursions of the soldering process. If the flux acids remain, corrosion can occur in the long term. There are commercial flux solvents that can be used to dilute and wash away excess flux; otherwise, the assembly can be baked briefly to about 120 C to denature the flux. Check with the flux manufacturer for specific recommendations regarding flux residue management. The problem can be avoided by eliminating the use of liquid flux and using small amounts of paste flux instead. In the case of core wire, use the smallest diameter practical to limit flux residue.
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Component Bake-Out Components that have been sitting out may be subject to water absorption. When components are heated during rework, where the heating cycle is more localized and perhaps more extreme than in the slow heating experienced during mass oven reflow soldering, moistureinduced popcorning may be a more serious problem. Follow IPC or component manufacturer recommendations for presoldering moisture bake-out. Refer to Sec. 50.2.2.5 for further discussion on this topic.
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Gold Finger Protection There is ample opportunity to ruin gold edge connector fingers on a circuit board during the repair operation. Care should be taken to keep both solder and flux away from gold fingers during repair. Mechanical shields or acrylic-adhesive polyimide tape can be used to protect fingers from smeared solder paste, loose solder debris, or flux. If solder comes into contact with gold, the gold will rapidly dissolve into the solder mass and the solder will spread on the finger. The solder cannot be removed from the gold finger without scraping and replating, a costly process. Solder is not a reliable connector contact material, especially with today s highdensity, low normal-force electrical connectors. Keep solder away from gold fingers. Keep gold finger temperature as low as possible. Ensure that gold fingers are never handled, even with gloved hands. Avoid lint and fibers. After rework, clean gold fingers with a suitable solvent. Use a soft brush, lint-free cloth, or other nonabrasive material for this cleaning. Clean gold fingers on each side of the board separately. Avoid folding the lint-free cloth over both edges and wiping repeatedly; the cloth will wear against the glass-epoxy expunge and shred, leaving lint on the board. A better alternative is to use a soft, ESD-safe brush with solvent to clean the gold fingers and follow with a filtered-air blow-off to dry. Avoid any materials that will scratch the gold surface finish on the fingers. The underlying nickel, if exposed to the atmosphere will oxidize and, is known to be problematic for separable contact reliability.
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