barcode scanner programming asp.net SOLDERING MATERIALS AND METALLURGY in Software

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The main defect is brittle failure of the solder joint a clean break beneath the intermetallic layer formed on the PWB bonding pad. Were the package completely pried loose from the board, a high surface area, dull finish on bonding pad would be evident the wellknown black-pad. Often the pad has an array of cracks visible on its surface reminiscent of mud cracks. Once the failed component is removed, the black-pad afflicted bonding pads is unsolderable or unreliable for further use and the board must be scrapped. Although the exact cause of ENIG failure is not well understood, it seems to be linked to low phosphorus concentration in the electroless nickel plating bath. ENIG s appeal is its modest cost, flat topography consistent with all plated surface finishes, excellent solderability, excellent testability, and reasonable shelf-life if plated well. Unfortunately, the risk associated with this surface finish must be weighed alongside its benefits. In general, the assembler will find good results with ENIG. Unfortunately, there are no quick screening methods to weed out failure-prone ENIG plating lots. 45.6.5 Immersion Silver (Imm-Ag) This electroless silver-plated surface finish is in common use for Pb d assembly and is also compatible with the most common Pb-free solders.Although it is highly solderable and known to be a good material for probe contact during in-circuit test measurement, it does bring with it some negative attributes. It has reasonable storage shelf-life if stored properly (air-tight bags), but is known to combine with sulfur contaminants in the atmosphere and tarnish or sulfidize. The silver sulfide is somewhat resistant to weak organic fluxes as used for no-clean soldering. A new failure mechanism associated with this surface finish has been reported.31,32,33 Referred to as planar microvoids or champagne voids, they are small voids in alignment atop the intermetallic layer of a solder-joint bond line (see Fig. 45.4).
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FIGURE 45.4 Cross-sectional micrograph of SAC BGA solder ball bonded to a silver surface finished PWB bond pad. Planar voids are clearly evident in the section and in the magnified inset. Voids in close adjacency above the intermetallic layer and the BGA ball can significantly detract from the solder-joint strength. (Courtesy of Hewlett-Packard).
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Planar voids result in diminished bond strength and solder-joint fracture. This defect has been responsible for some product failures and recalls.The root cause of the microvoids is generally believed to be the result of volatile organics incorporated during Ag plating, but there also seems to be a correlation between Ag thickness and Cu topology on the bond pad.
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FIGURE 45.5 Ionic contamination on a board surface can adsorb moisture from the atmosphere. If an electrical charge is present, silver from bond pads may corrode and electrochemically migrate, resulting in a short circuit between bond pads.
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Silver ions are known for their mobility in the presence of an electrical field (see Fig. 45.5). Ionic residues on a board with silver-coated surface finish together with adsorbed moisture from the atmosphere can result in electrochemical migration and corrosion dendrites originating from any exposed, unalloyed silver plating.These dendrites are most commonly seen on the board surface but can also occur under solder mask. If the dendrites are long enough, they may cause electrical shorting to one another or to adjacent traces. Since the corrosion dendrites are very thin, they are poor electrical conductors. If enough current is passed, the dendrites may fuse and the once-shorted circuit will become open again. If moisture and contaminant conditions are right, corrosion dendrites may form again and result in an electrical short. This cyclical growth-and-fuse phenomenon can complicate diagnosis of the source of the shorting. Silver is an expensive metal. With its increased demand for solders (e.g., Sn-Ag-Cu), it is expected to rise in price, making it economically less attractive. 45.6.6 Tin (Sn) Tin deposited directly over copper, whether by electrolytic or electroless methods, should be avoided except for low-end/low-cost, short-field-life products. Sn alloys with copper, even at room temperature, form a brittle intermetallic compound, albeit slowly. It has limited solderable shelf-life at room temperature and the Sn-Cu intermetallic can inhibit soldering and may result in incomplete solder wetting or solder joints of inferior strength. For tin to be used effectively, an appropriate non-porous barrier metal, such as Ni, should be deposited between the Cu and the Sn. Immersion Sn can be plated only as a very thin layer since its deposition is by a self-limiting replacement reaction. When there is no more exposed Cu on the surface of the PWB during the surface-plating operation, Sn deposition will cease. Therefore, it is impossible to plate enough Sn by this method to extend the solderable shelf-life or reduce the brittleness of the solder joint.
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