barcode scanner integration in asp.net PRINTED CIRCUIT BOARD SURFACE FINISHES in Software

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with OSP or HASL finishes. For gold wirebonding, only electrolytic Ni/Au is widely used. Good bond strengths have been achieved on ENIG, Ni/Pd/Au, electroless gold, direct gold, and silver, but most production uses soft electrolytic Ni/Au to meet gold wirebonding demands.
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Compliant Pin, Press-Fit Connectors Compliant pin, or press-fit, connectors rely on contact with the surface finish in plated through holes. The main concern is the dimension of the finished hole size. When using HASL, the PCB designer must allow for the extra thickness of solder that will reduce the diameter of the PTH. HASL alternative finishes are flat, so will have negligible effect on finished hole size, since the thickness of the finishes is less than the tolerance of the drilling and plating operations. The material properties of the finish have some affect on compliant pin capability. Soft materials such as HASL and immersion tin perform better by providing low insertion force. Hard materials, such as copper and nickel, yield higher insertion force. OSP and silver act like copper due to the thinness of the coating.
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See Table 32.11 for descriptions of failure modes of specific surface finishes
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TABLE 32.11 Failure Modes Specific to PCB Surface Finishes OSP PTH Hole Fill Exposed Copper after Soldering Post-Assembly Corrosion of Exposed Surfaces Handling Defects HASL Solder Bridging Plugged Holes Equipment Safety Paste misprints ENIG Black-Pad, Black-Line Nickel Brittle Fracture Solder Mask Attack RF Signal Loss Post-Assembly Corrosion of Exposed Surfaces Immersion Tin Tin Whiskers Ionic cleaning Failures Solder Mask Attack Post-Assembly Corrosion of Exposed Surfaces Immersion Silver Post-Assembly Corrosion of Exposed Surfaces Tarnish Solderjoint Microvoids Electrochemical Migration
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Whiskers Whiskers form as a single crystal growth from the surfaces of mainly pure tin coatings (see Fig. 32.6). The whiskers are tiny, but can form lengths exceeding the spacing between electrical features on the circuit board or components. As pure tin is electrically conductive, reliability engineers fear that tin whiskers will form short circuits. In fact, there have been several high-profile failures attributed to tin whisker growth. Whiskers form more quickly from pure deposits of tin over copper and brass. Although they have been reported in literature from other elements, the vast majority of whiskers in electronics have resulted from tin plating.
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FIGURE 32.6
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Tin whiskers on fine-pitch contacts.
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The use of lead in tin plating and solder has been used to inhibit tin whisker growth. Rare reports of tin whiskers have been reported from tin-lead coatings. Whiskers evolve from tin surfaces as a stress-relief mechanism between tin crystals. Most stress in tin deposits arises from the growth of copper-tin intermetallics at the interface with underlying copper. With a much slower rate of nickel-tin intermetallic formation, tin whiskers form at a much lower rate than tin coatings on nickel underplate. Several tin-whisker mitigation techniques have been proposed:
Inclusion of lead, silver, and other metals in the tin deposit Use of a nickel tin coating as a barrier between copper and tin Annealing of tin as a stress relief Conformal coating as a barrier to whisker growth
In practice, the only effective whisker mitigation strategies involve nickel barriers and contamination of the tin deposit with a sufficient amount of metal, such as 2 percent Pb.
Interfacial Fracture Interfacial fracture is a failure mode usually associated with ENIG, and is also known as black-pad, brittle nickel, and black-line nickel (See Fig. 32.7). Technically, interfacial fracture simply refers to the location of physical separation when a solderjoint is tested to failure. Black-pad (black-line nickel) is a phenomenon of ENIG that, when sufficiently severe, results in interfacial fracture. Brittle nickel refers to the inherent weakness of solderjoints formed from nickel-tin intermetallics, regardless of the mechanism of failure. Black-pad is a name given to the surface of a circuitry feature after interfacial fracture. The separation occurs between the solder and an underlying surface of irregular, high-phosphorous, rough nickel. Aggressive gold displacement corrodes the rough nickel selectively along the nodule boundaries of the electroless nickel. The nickel is easily corroded due to the low phosphorous content of the as-deposited electroless nickel coating. Low phosphorous deposits (less than 9 percent) are far more easily corroded than higher phosphorous (10 to 12 percent by weight) EN coatings. The simple remedy in prevention of black-pad is the use of high phosphorous coatings, based on the operating parameters and chemical formulation of the nickel bath. However, any nickel bath can be destabilized with contamination such as divalent sulfur from undercured solder mask. Less stable EN baths plate at a higher rate and result in low phosphorus deposits.
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