scan barcode asp.net mobile PRESS-FIT INTERCONNECTION in Software

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than go into their respective through-holes. Sometimes solder from the HASL surface finish can redistribute during SMT reflow cycles, causing a bulging meniscus within the PTH. This condition is depicted in Fig. 49.6a. Compare this illustration to that in Fig. 49.6b, which shows a normally finished PTH with even wall FIGURE 49.6 Barrel profile as a result of surface coating, a relatively uniform barrel diameter, finish coating: (a) A PTH barrel with excess surface and an evenly coated primary-side annular finish from HASL, which has formed a meniscus ring. With the advent of RoHS, HASL finishes upon solder reflow. Other plating defects can result in such a meniscus, although the problem is uncommon. are less common, but press-fit is commonly (b) A normal PTH barrel with evenly coated walls used in high-end servers and telecommunica- and annular rings. (c) Excessive plating on PTH bartions equipment where the use of Sn-Pb solder rel walls and annular pads. (d) Excess plating on is currently exempt from RoHS Pb-free annular ring causing pin interference and premature EON pin collapse. (e) An overly compressed pin requirements. excess PTH Excessive plating on PTH barrel walls and resulting from poses a plating onrisk. annular ring. This condition reliability annular pads (see Fig. 49.6c, d) can cause premature collapse of the compliant section of the press-fit pin. This condition may result in excessive pressing force to get the pin into the hole as well as insufficient normal contact force from the collapsed press-fit pin section to the barrel once fully seated in the hole. This condition poses an electrical contact reliability risk, as shown in Fig. 49.6e, where the pin is not actually in contact with the PTH barrel walls. Figure 49.7a shows a precollapsed press-fit pin that is making only intermittent contact due to the FIGURE 49.7 (a) Excess nickel plate on a PWB condition as depicted in Fig. 49.7b. PTH annular ring has caused two problems. High-aspect-ratio holes (PWB thickness: PTH First, it has shaved material from the press-fit diameter) may have irregular plating where the thick- pin (arrows labeled x ).Additionally, it caused ness closest to the board s outer layers is within compression of the compliant section of the acceptable thickness limits, whereas plating thickness pin. When fully seated in the PTH, the pin was in the middle of the PTH may have unusually thin too loose for reliable contact. The y labels deposits. These thin deposits are caused by plating demark a gap between the press-fit pin and the PTH wall. Compare this to (b), where pin shavbath stagnation and exhaustion a zone of poor elec- ing is evident but the pin is in intimate contact trolyte circulation within the high-aspect-ratio barrel. with the PTH barrel. (Scanning electron microThis condition may also lead to poor contact of the scope (SEM) micrographs courtesy of A. press-fit pin against the PTH barrel wall and contact Alexander, Jabil Circuits, Penang, Malaysia.) reliability issues. The opposite can be detrimental also, where the PWB vendor strives to attain the proper plating thickness within the PTH barrel, but plates too much material at the barrel openings and on annular rings. 49.6.3 Slivers The Pb of Sn-Pb solder-coated barrels is known to reduce the force required for pressfit connector insertion and seating. Although Sn-Pb coating is one of the best surface finishes for pressing, if it is thick, then the pressfit connector pin may skive a thin sliver of solder and push it out the secondary side of the PWB (see Fig. 49.8). Very often these slivers
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FIGURE 49.8 A solder sliver (dashed outline) pushed out of a HASL-coated PTH.
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remain attached to the pin or the PWB, or they become jammed between the pin and barrel. The sliver may break free of its source and cause an electrical short locally or elsewhere in the system.
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