barcode scanner asp.net mvc SOLDERING FUNDAMENTALS in Software

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If too much solder wicks up the lead, it may cause solder shorting, particularly near the package body, where the lead spacing may be finest. Therefore, fillet examination alone is not sufficient to accurately assess soldering process performance. But, for a fixed set of process parameters, fillet appearance may provide a good means of first-pass comparative assessment of solderability and soldering results. 44.5.6 Intermetallic Compound Formation Intermetallic compound (IMC) formation is the key to soldering. IMC is the local alloying created at the boundary of the solder liquidus and the surfaces of the metal with which it is in contact. It is the essence of soldering. There is much misinformation about IMCs. One thing is certain: Without intermetallic formation, there is no solder joint. This is stressed because some erroneously claim lack of IMC in such processes as laser soldering. In the case of that technology, the IMC can be exceptionally thin immediately after soldering due to the short time of liquidus, but it is and must be present for soldering to occur. High temperature, time, and intimate contact of the liquid solder enhance the rate and volume of intermetallic compound formation. The IMC is a crystalline intermediate alloy phase composed of some or all of the contacting metal constituents. This high-alloy concentration region generally has properties that are vastly different from those of either the solder or the contacted metals. It is a brittle material that is electrically conducting or even semiconducting. Solder is thought of as a soft, forgiving material of low melting-point and good electrical conductivity. It is ironic that the essence of soldering the actual bond, or more correctly, the bond s anchor (IMC) is in fact, a brittle, high melting-point composition of generally poor electrical conductivity. 44.5.7 Cooling and Phase Change to Solidus As the solder approaches its solidus temperature, the rate of intermetallic compound formation decreases significantly and the solder joint is formed. It is critical that the solder freezes prior to circuit board handling for two important reasons. First, accidental movement of the PWB may cause the components to move in the molten solder, which could result in defects such as solder joint opens (disbonds) or solder shorts (unwanted bridging of solder from one bond pad to the next). Second, a solder joint disturbance at the critical transition from liquid to solid may result in a bond of degraded reliability.This type of bond has an inherently grainy look and can even have microfractures within caused by motion-induced interruption of orderly crystal growth during solidification. The cooling rate at the tail end of the process is as important as at the onset of heating. Extreme cooling rates may result in component cracking or electrical performance degradation induced by differential contraction of materials.
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SOLDERING MATERIALS AND METALLURGY
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45.1 INTRODUCTION
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Numerous considerations are crucial for successful circuit board assembly and useful product life. Assembly processes and materials that result in solder-joint formation can also have a profound effect on long-term solder-joint reliability as well as a product s environmental exposure and handling during its lifetime. Design and assembly engineers need to take into consideration temperature extremes and thermal excursion ramps that the product may endure as well as anticipated mechanical shock and vibration, the number of power (on-off) cycles expected over its lifetime, exposure to airborne particulates, gaseous pollutants, humidity, and handling. Such exposures may impact the printed circuit assembly (PCA) during soldering, when being handled on the factory floor, or while in the hands of the product s end user. These are but a sampling of variables that influence product reliability and should be considered in defining design and assembly attributes. PCA designs are most often driven by electronic device availability, electrical performance requirements, affordability, and the product s end-use environment and operating conditions. This chapter will strictly avoid offering up generalized rules for all soldering process applications. The materials and requisite solder-joint reliability properties for a desktop computer are markedly different from those for the ignition system printed circuitry associated with the engine of a motor vehicle. Similarly, mobile electronics (mobile phones, laptop computers, PDAs, etc.) have the added requirements of mechanical shock-resistance as well as high-volume manufacturing needs, including low cost, ease of assembly, and high yields. In some cases, the cost to repair a PCA or an entire product may exceed the value of the combined material and labor content of the first-pass assembled product. It is also well understood that the reliability of a repaired assembly is not as good as a properly soldered first-pass printed circuit assembly due to additional board handling and solder-joint degradation as a result of re-soldering. The solder flux chemistry and how it is applied and heated may also impact product reliability. These points reinforce the need to understand PCA processes thoroughly and to solder properly the first time. There are fundamental rules that will become evident in discussions of metallurgies, fluxes, and processes. If these rules are heeded, the results are optimal soldering yield and the best reliability. However, the only way to know whether an assembly will endure its intended use and environment is to develop appropriate accelerated testing methods that can accurately gauge product field reliability.
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