barcode scanner programming asp.net SOLDER ALLOYS AND CORROSION in Software

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Just as some metals are more corrosion resistant than others, the same is true of solder alloys. When a solder corrodes on a circuit board, it is more than an aesthetic issue. Contaminants on the surface of the PWB between adjacent, oppositely charged conductors can result in corrosion dendrites. These are tiny conductive crystals filament networks that may extend from one conductor to the other. They can have enough current capacity to cause electrical shorting or they may heat up to the point of fusion, melt, interrupt current flow, and return the assembly to normal operation. This can be a cyclical event with the dendrites growing, fusing, and regrowing, making diagnosis difficult. Finer-pitch surface-mount geometries are particularly susceptible to this phenomenon. M. Abtew et al. published a chart of the electromagnetic force (EMF) of various metal couples present in some lead-free solder alloys.1 As a rule, the lower the EMF, the more corrosion-resistant the alloy. Eutectic Sn-Pb, the basis of comparison, was by far the lowest value on the list at 0.010 volts. The next nearest, Sn-51In, was 0.201 volts or 20 times the voltage
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as that of Sn-Pb solder. Other values reported were Sn-57Bi at 0.323 volts, Sn-9Zn at 0.624 volts, Sn-3.5Ag at 0.937 volts, and Sn-80Au at 1.636 volts. So tin-lead solder is significantly more resistant to corrosion and dendritic growth than the other seven alloys studied. This will be an area to watch as the world moves forward with Pb-free soldering.
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45.4 PB-FREE SOLDERS: SEARCH FOR ALTERNATIVES AND IMPLICATIONS
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The European Union s RoHS legislation has spawned widespread changes in electronic assembly. The biggest among them is the required move to Pb-free solders for most electronic products. Although this topic is a hotly debated, the use of elemental lead (Pb) in the electronics industry and more specifically in solders is a small percentage of the total industrial use of Pb worldwide. Accurate and current numbers are difficult to come by, but estimates range from less than 1 percent to as much as 10 percent worldwide. There are some excellent reasons, though, to find suitable substitutes for Pb where possible. Pb is toxic and governments worldwide spend large sums on abatement programs, education, and health care assistance to those afflicted with toxic levels of lead, mostly from peeling Pb-based paint, which is already regulated. Pb salts are sweet to the taste but toxic. The Romans purportedly added Pb salts to sweeten their wine. Electronic assemblies are rarely linked to toxicity through oral ingestion, but may be the source of contamination through improper disposal of the assemblies, components, raw materials, or their by-products. Purity of the water table may be affected by inappropriate land-filling of large quantity of Pb-bearing circuit boards; solder dross; spent solder pot contents; trace concentrations released as part of aqueous cleaning processes; and Pb-bearing ion exchange filters from aqueous cleaning units, plating baths, etc. Replacement solders, while perhaps nontoxic themselves, may require deleterious raw stock or result in noxious by-products from processing or from ore refinement. It is wise to proceed cautiously and look at the impact of Pb removal from the electronics industry from all perspectives. There are many nonPb bearing materials associated with printed circuit board assembly, each with a set of material components and process chemicals that may be problematic in a public health sense. Products, by-products, raw materials, and their derivatives must be handled and disposed of in a conscientious and environmentally considerate manner. Attention to better reclamation and disposal methods of industrial and household waste streams, as well as adequate education concerning the same, probably holds more significance to public health than defining a quick substitution for Sn-Pb solder. However, it is the requirement and, therefore, a knowledge of alternatives is mandatory for design and process engineers. The database for the material properties of widely used Sn-Pb solders is still incomplete. Needless to say, for a new Pb-free solder system, it will be much less complete and take several years to characterize the qualities and performance of a new solder. Since so many new solders are being investigated, this will further slow down solder development and characterization since efforts will be spread out rather than concentrated on a common solder alloy. As of this writing, several corporations have had product recalls associated with the changeover to Pb-free solders where there were unknown interactions with soldering system materials or lower-than-expected field reliability.
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