barcode scanner programming asp.net FLUX ACTIVITY AND ATTRIBUTES in Software

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FLUX ACTIVITY AND ATTRIBUTES
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A flux s ability to prepare a surface for soldering is dependent on its chemical strength or activity. Just like reagents on the chemistry lab bench, there are various concentrations available. A flux may be naturally strong or it may have additives to enhance its reactive power.
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These additives are referred to as activators.1Activators accelerate the fluxing action either by direct chemical contribution or by catalysis. The activator can be an inorganic or organic material and combines with other ingredients in the flux, or decomposes with temperature to form an acid that is effective for oxide removal. Halogens, amino-halides, organic acids, and other materials are common activators. As the flux is heated, the activator begins its work. Flux heating for the soldering process has to be tempered for the particular type of flux formulation and solder system. Some activators may decompose if heated too quickly. Others may not be effective for a long enough time. Weaker fluxes (flux + activator) may require a longer preheat time and higher temperature in order to have sufficient chemical activity to remove oxidation and tarnish from parts to be joined as well as from the solder. Flux selection has to be appropriate for the solder alloy melting temperature as well as for the type of soldering process. The solder chemistry chosen will greatly influence the process time-temperature profile. Like most chemical reagents, a flux s reactivity is in part a function of the exposure time and the temperature; many exhibiting Arrhenius-like behavior, where time-dependent chemical performance increases sharply with temperature. Some fluxes are weakly active at room temperature, but most require a significant thermal assist to enable them to be useful. Ultimately the only valid test of flux performance is whether the system works for the types and conditions of printed circuit boards and components encountered in the course of the required soldering process. A more active flux may be needed for low melting point solder, especially if manufacturing demands high throughput for this process step. In this case, the flux will not have the benefit of longer exposure times and higher process temperatures to assist in its job of oxidation removal. Higher-temperature alloys require fluxes that can withstand higher process temperature for reflow. It must not volatilize or decompose too rapidly nor polymerize or char at temperatures commensurate with soldering. Some metals are more prone to oxidation than others. Bismuth and zinc used in solder and copper on circuit boards are notable examples that may require a more active flux. A nitrogen-rich environment is helpful in augmenting flux performance. (See also Chap. 43, Fluxes and Cleaning. )
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FLUX: IDEAL VERSUS REALITY
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The ideal flux would be one that is potent, requires only a small quantity, leaves little or no residue (and what residue does remain would be thin, transparent, nonreflective), easily penetrated by in-circuit test probes, electrically insulating, and chemically inert. If not inert, it should be easily and completely removable in an inexpensive solvent. However, this is far from actual materials and process reality. Flux must be applied in sufficient quantity for thorough oxide removal and to avoid both metal reoxidation and flux exhaustion prior to solder wetting. Some post-soldering flux residues or form by-products may have deleterious effects on the circuit assembly since they may be chemically reactive and result in corrosion of exposed metals over the long term (see Fig. 46.3). The corrosive agents can be electrically conductive, resulting in electrical leakage (soft shorts) between closely spaced, oppositely charged conductors on the circuit board surface exacerbated by the residue s hygroscopic moisture uptake from the atmosphere. Corrosion may be augmented either by natural electromagnetic field (EMF) couples between two different metals or electrochemically driven by a charge between two adjacent conductors. The metallic ions, electrochemically driven, can plate out as metal dendrites on the surface of the board and produce intermittent or even persistent hard short circuiting (see Fig. 46.3). Flux must be heated to a temperature to allow the best reactive conditions without drying out or denaturing. For the sake of economy and cleaning, a thin application is necessary and should be confined to the bonded site(s) of interest.
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