scan barcode asp.net mobile COMPLIANT PIN CONFIGURATIONS in Software

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COMPLIANT PIN CONFIGURATIONS
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There are many different press-fit pin configurations far too many for this chapter to review each one. Instead, three commonly used styles will be described as an introduction to press-fit technology. The first has a lead of square or rectangular cross-section that is sized to deform
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PRESS-FIT INTERCONNECTION
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FIGURE 49.2
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A square or rectangular pin forced into round hole. This is the most primitive of press-fit techniques.
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the board s plated through-hole slightly upon insertion. This is the proverbial square pin in a round hole. The lead is tapered at its end to facilitate initial PTH insertion (see Fig. 49.2). Second is the square-pin variant sometimes referred to as the H section, which consists of a sculpted rectangle, an H in cross-section, whose corners are meant to deform (see Fig. 49.3). The third and most prevalent configuration is the collapsible pin known as eye-of-theneedle (EON) (see Fig. 49.4). Upon insertion and pressing into the PTH, the EON pin folds upon itself axially in a controlled manner, as depicted in Fig. 49.5. The plated through-hole barrel is slightly deformed from the mechanical interference of the press-fit pin against it. The resultant forces from the collapsed press-fit lead against the PTH barrel wall result in stable, long-term mechanical and electrical contact.
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FIGURE 49.3 H -section press-fit pin. The sculpted cross-section of the pin allows the pin to deform slightly. This deformation along with some deformation of the PTH barrel upon insertion allows for reliable mechanical and electrical contact.
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FIGURE 4 9 . 4 Eye-of-the-needle configuration press-fit pin, shown before compression.
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FIGURE 49.5 As the EON pin is driven into the PTH barrel, its compliant section collapses inward upon itself. The resultant forces between the pin and the PTH barrel wall result in a stable, intimate mechanical and electrical contact.
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PRESS-FIT CONSIDERATIONS
Proper Loading The load necessary to apply a press-fit connector depends on a variety of factors, including pin design, pin material, pin size, pin surface finish (plating), PTH surface finish, plating thickness, pin and hole size, and the number of pins per connector. The connector manufacturer should provide the required loading per pin for each connector for proper insertion and connector seating.
Correct Design and Size Press-fit connector leads are designed to work within a tight range of PTH barrel diameters.The compliant pin normal force exerted against the PWB plated through-hole barrel wall must be maximized to anchor the connector into the PWB reliably, both for mechanical stability and electrical contact. The connector manufacturers will recommend proper PTH diameter and tolerances for their products. The resultant press-fit coupling is analogous to the solder joint. If the pin and barrel are sized properly and materials are appropriately finished, the anticipated joint life will be better than that of soldered assemblies. Also if the pin and barrel are sized correctly, the pin-to-barrel normal force will be high enough to form a gas-tight seal for long-term electrical and environmental contact reliability. The pin must be strong enough longitudinally to prevent buckling during the pressing operation. Pin normal forces must be optimized so as not to
PRESS-FIT INTERCONNECTION
damage the PTH barrel high enough for good contact, but light enough that the pin insertion will not damage the barrel or the PWB. 49.4.3 Reproducible Connector Press Since pressing force generally ranges from some 10s of grams to kilograms per lead, the mechanical advantage of a reproducible press machine is required for press-fit connector application.
PRESS-FIT PIN MATERIALS
Base Material For purposes of affordability, mechanical integrity, electrical properties, and materials conservation, all electrical contacts, regardless of assembly methodology, are made from inexpensive base metals and plated with a minimum thickness of a more precious or more practical material. Many press-fit connectors have been made of beryllium-copper alloy, but due to beryllium dust toxicity, a factor in press-fit lead preparation, the switch to safer phosphor-bronze or other copper alloys has occurred throughout the industry.
Surface Finishes: Pin and Barrel Since most materials oxidize readily upon exposure to the atmosphere, gold or other noble metals are often used to prevent or retard oxidation of SMT or PTH component leads or PWB solder lands and plated though-hole barrels. However, most circuit boards and component leads are covered with less noble materials such as tin (Sn) or tin-lead (Sn-Pb) and instead rely upon a fluxing agent to remove naturally occurring oxides prior to and during the soldering process. In a solder joint, the component lead and pad interface materials are wetted by and sealed in solder. The solder gives the interconnection mechanical rigidity and seals the contact surfaces from the environment. This is not the case with press-fit technology. The press-fit process is compatible with all surface finishes on the pin and the PTH barrel. The frictional properties of some materials are more favorable to the press-fit process than others. Sn-Pb is known to have favorable properties for the press-fit process. Pb is known for good tribologic properties and acts as a lubricant for the press-fit process. Copper is less favorable but is not a problem if the PWB hole is sized properly for the press-fit pin.
Materials in Light of RoHS With the onset of the European Union s (EU) RoHS legislation, there is additional consideration for the press-fit process. The original directive banned Pb for most PWB materials and assembly. This resulted in two problems. First, press-fit connector contacts converted to Sn instead of Sn-Pb would risk Sn-whisker formation. Unlike Sn-coated SMT component leads, there is no reflow associated with press-fit technology. Without the reflow step, Sn-whisker formation is much more likely to occur. Second, increased frictional forces from Pb-removal on currently designed products made pressing difficult and in some cases resulted in damage to the PWB. EU s RoHS technical advisory committee was petitioned and an exemption to the RoHS legislation was enacted permitting the use of Pb in surface finishes for compliant pin connector systems. 1
The Gas-Tight Seal In the case of the press-fit connector pin, there is neither a physical wetting of material nor metallic encapsulation to protect the interconnection.Additionally, no chemical fluxing agents are used.
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