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FIGURE 40.30 Hand-soldering techniques using point, tweezers, and hot-bar tips to perform repair or rework procedures. (Courtesy of OK International and Electronic Products and Technologies.)
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In the case of rework procedures, the most difficult step is often de-soldering the damaged component for the latter s removal from the circuit board. In the case of axial leaded devices, it may be possible to cut the leads of the damaged component. The component can then be removed from the circuit board, thereby increasing access to the joint area for the de-soldering operation. Lead-cutting is more difficult for radial leaded and high-pin-count components such as DIPs, PGAs, and connectors due to their shorter lead lengths and limited clearances between the package and circuit board surface. Typically, the operator must remove the solder from the joint, usually one joint at a time, to release the component. A light coating of flux is applied to the Cu braid; the braid is then placed between the fillet and the soldering iron, with preference given to resting the tip near the top of the fillet, against the lead. This technique limits possible damage to the Cu pad, solder mask, or laminate. Sufficient solder must be removed from each joint so that the component can be extracted with minimum force to prevent damage to the Cu barrel or the Cu pad. The dressing of the through-hole and Cu pad is the next step. An excess of solder remaining in the hole will hinder lead insertion for the new component. It is difficult to extract physically all solder residue form inside a hole, even at modest aspect ratios (board thickness-to-hole diameter).This difficulty is further compounded with thicker, multilayer circuit boards because the soldering iron tip cannot compensate for the large thermal mass of the laminate or the heat-sink effects caused by the internal Cu layers. Finally, it is often necessary to contact the Cu pads of the circuit board briefly, with the braid between the pad and the tip, to remove any excessive solder residue. Soldering of the replacement component on to the circuit board has fewer complications than does part removal. Using a hand-soldering process, after the device has been inserted into the circuit board, each joint is made one at a time. The operator provides real-time inspection of the joint for any defects. Finally, if necessary, flux residues can be removed from the circuit board using the appropriate cleaning procedures. Thicker, multilayer boards are more difficult to solder due to their greater thermal mass and the heat-sink effects of the internal layers. Preheating of the circuit board can be used to augment the soldering iron heat source. Typically, the circuit board is heated to a temperature in the range of 100 to 125 C. However, because the preheating step is applied to a full-up
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assembly (minus the defective part), consideration must be given to the heat sensitivity of other components on the circuit board. Preheating with a hot plate may be acceptable for some applications. However, when temperature-sensitive components are nearby, other heating methods such as hot-air (gas) sources may be required to localize the temperature rise away from other components or structures. Several techniques can be used to assist with the hand-soldering process (de-soldering or re-soldering), or even substitute for it altogether. For example, there is equipment that can heat multiple joints at a time.The so-called hot-bar technique is a soldering iron with a long bar for the tip. The bar simultaneously heats up a row of leads for the de-soldering or re-soldering step. Hot air or hot gas guns can provide more diffuse heat sources that can melt all of a component s solder joints at the same time, thereby facilitating the removal and replacement of larger components that have a greater number of I/Os. Selective soldering, or solder fountains that perform like miniature wave-soldering machines, are an effective approach for the de-soldering and re-soldering of through-hole components. In fact, this technique is particularly well suited for connectors. The thermal energy provided by the molten solder fountain can overcome the heat-sinking effects of the connector structure and thick, multilayer circuit boards. However, it may be necessary to preheat particularly thick circuit boards (>3.0 mm) having multiple internal layers in order to melt the solder joints of a defective product successfully, as well as to achieve hole fill and adequate fillets on the replacement part. Again, care must be exercised to prevent thermal damage to neighboring components or the undesirable remelting of their solder joints. It was noted previously that a Pb-free solder option raises two concerns: the higher soldering temperatures required by the new alloys, and the mixing of Sn-Pb and Pb-free solders, particularly when performing rework procedures on legacy hardware. Both concerns are generally of lesser magnitude for through-hole solder joints. The physical separation of axial or radial leads lessens the likelihood of thermal damage to the circuit board or replacement component. Potential processing and reliability effects resulting for the mixing of Sn-Pb residues with Pb-free solder are less significant due to the greater quantity of solder required in through-hole interconnections. In particular, the solder composition within the fillets, which provide most of the strength and service life of the joints, would be least impacted by Sn-Pb contamination.
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Surface-Mount Technology Solder-joint repair and the rework of surface-mount components pose a greater challenge due in large part to the reduced size of the devices and associated I/Os. (In fact, the difficulty of repairing or reworking surface-mount circuit boards was one of the leading drivers for the electronics industry, as a whole, to re-embrace more stringent process control techniques, including SPC, to minimize assembly defects.) Contributing hurdles included the higher board densities that accompany the small part sizes; an increased sensitivity of component materials to potential thermal damage including the Si die resulting from flip-chip attachment and thinner molded packages; as well as solder joints that are out of direct view from the operator, as in the case of area-array packages. Another important consideration is the overall fragility of surface-mount Cu pads on the circuit board compared to through-hole structures. In the latter case, damage to the bond between the Cu pad and the laminate was less likely due to a greater robustness of the feature. When such damage in the form of lift-off occurred, it was of a lesser consequence because the hole barrel provided an additional mechanical attachment to the laminate. On the other hand, surface-mount pads are smaller in order to accommodate finer package I/O pitches and overall higher board densities, giving them less overall bond area with which to adhere to the board. Also, the pads are attached to the laminate without a hole or, at best, with only a small via, which also increases the sensitivity of Cu pads to thermal damage in the form of lift-off.
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