barcode scanner programming asp.net Through-Lead and Through-Pad Bonding in Software

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Through-Lead and Through-Pad Bonding
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Generally, the laser beam is directed at the component lead to accomplish soldering, but when the lead material is highly reflective, as would be the case with a gold-plated finish, heating is slow and irradiation times impractically long. An alternative has been demonstrated whereby the beam is directed at solder-coated circuit board bonding pad. If there is leeway in design, the bonding pad can be extended to aid in this method, dubbed throughpad bonding (see Fig. 47.33b). In contrast to the usual beam impingement on the component lead, or through-lead bonding (see Fig. 47.33a), the beam is directed at the more absorptive solder or on the circuit board land, increasing process efficiency and locally melting the solder in the vicinity of beam impingement. Since the molten solder is in intimate contact FIGURE 47.33 Laser soldering: (a) through-lead laser soldering; (b) through-pad laser soldering (B). with the lead and pad, the heat from the bonding process is efficiently transferred. The process can result in rapid soldering along the entire length of the component lead and circuit board pad with rich solder fillets evident if the process is conducted properly.
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Single-Point Laser Soldering This method requires a laser beam that is smaller in diameter than the length or width of the component lead or bonding pad. The beam is stepped to each lead-pad combination and delivered with sufficient energy to result in solder reflow. The beam can be CW, pulsed, or
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multiply pulsed to accomplish soldering as long as there is enough radiation delivered to cause the solder to undergo the phase change required for soldering and wetting. Due to the small beam diameter, as required by this technique for fine-pitch components, the energy density can be exceptionally high. In fact, it can be so high that if the lead is not in good contact with the bonding pad, then the laser s beam can damage the lead, perhaps cutting through it rather than soldering it. Overly intense irradiation can also cause a bonding pad to delaminate from the PWB. In any laser soldering technique where very small spot size is required, the beam diameter must be precisely controlled. A small variation in beam diameter, either through change in laser parameters or in working distance (focus), can dramatically affect the delivered energy density. In the cases of a 10 W Nd:YAG laser beam focused to a spot of 0.1 mm (0.004 in.) and a spot of 0.2 mm (0.008 in.), this seemingly small change in spot size will result in a large change in power density (see Eq. 47.2). P = p/d where P = power density p = average power d = focused beam diameter. (47.2)
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For this example, the power density would vary from 1,273 W/mm2 for the 0.1 mm diameter beam to 318 W/mm2 for the 0.2 mm beam diameter, a fourfold reduction emphasizing the criticality of strict maintenance and process control when working with a small spot size. In most cases, a Gaussian distribution of the beam s energy distribution across the focused diameter is assumed. It is customary to measure the beam at the 1/e2 point (13.5 percent of the peak height), so, in fact, the beam is impinging on a slightly larger area. However, the most intense portion of the laser s beam is confined to the region bounded by the 1/e2 points in two dimensions, as shown in Fig. 47.34.
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FIGURE 47.34 Laser energy density is measured from points on the distribution curve where the beam s intensity is 1/e2 (13.5 percent) of peak intensity or between points A1 and A2 in this plot.
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Single-point laser bonding has been applied to soldering tape automated bonding (TAB) lead frames to silicon die, TAB leads to PWB, or multichip modules as well as for soldering gull-wing leaded components to circuit boards. This technique is not suited for bonding areaarray devices such as BGAs or CBGAs to PWBs.14
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