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Limit the number or plane attachments Where possible, limit the number of ground or power plane attachments to a PTH barrel. Each attachment point will result in thermal dissipation into the copper plane. This lost heat will detract from the soldering process making the job of pre-heating and solder wetting all the more difficult. Cluster components and provide clearance Group wave-soldered PTH components together. Avoid interspersing nonwave soldered SMD components within PTH fields. Conversely, avoid designs that place an isolated PTH component amid a field of SMD components. Such a placement requires a small wave pallet opening that inhibits solder-wave fluid flow, pre-heating, and wave contact. Place the components properly in relation to edges and use selvage edges When possible, avoid placing PTH components directly against the edge of the board. There must be clearance from the board edge to the pallet to allow the board to nest within the pallet.A selvage edge (a removable and disposable board edge extension that is part of the board itself) can be designed into the PWB (see Fig. 47.28). After wave soldering, the selvage edge can be removed by rotary cutter or routing. Note that post-solder routing is not preferred, especially for no-clean soldered assemblies. The abrasive router dust may remain within connectors and scratch connector contact surfaces or mating connector contacts. The vibration from routing may also affect the reliability of certain components.
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FIGURE 47.28 Board schematic showing a removable selvage edge, a disposible edge to enable wave soldering of the PTH connector near the true edge of the board.
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Avoid double-sided wave-soldered PTH components Double-sided wave soldering is generally fraught with defects. Determine whether SMT or press-fit technology can be used to replace a second wave-soldering operation. The industry-standard preference is to restrict all wave-solderable components to one side (components inserted through the primary side) of the PWB. Do not place components near gold fingers Clearance from gold fingers is required so as not to contact the gold fingers with solder. The gold finger should be away from the pallet nest edges. Covering the gold fingers with an acrylic-based high-temperature adhesive tape will help to keep solder and flux from the gold fingers. The tape is not an effective mask if exposed directly to the solder wave.
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WAVE SOLDER DEFECTS
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There are many types of wave-soldering systems, each with its unique advantage as claimed by the manufacturer.The soldering engineer has to assess these improvements as they relate to the type of assembly being soldered. The technology has matured significantly, but the degree of equipment complexity attests to the complexity of the process.There are many process variables associated with this operation. If these are not understood or properly controlled, wave-soldering defects such as skip soldering (electrical opens) and bridges (electrical shorts) will occur as in any other soldering operation. Another important defect is the influence of secondary-side wave soldering on primary-side surface-mounted components. If parameters are not controlled properly, it is possible to cause primary-side SMDs to reach reflow temperatures.This can induce
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solder opens, shorts, or solder-starved joints. This phenomenon is most commonly associated with thin, densely populated, double-sided PWBs with fine-pitch surface-mount devices such as quad flat packs and BGAs soldered to the primary side of the board. Thermal conduction from the wave through the board itself, through electrical vias and along electrical traces in and on the board, may provide enough heating to cause previously soldered parts to reflow again. Solder can be drained away through vias or wicked-up surface-mount component leads, resulting in opens or weakened soldered joints with an extreme hourglass-shaped attachment of solder from the bottom of the lead to the bonding pad. A common defect associated with no-clean wave soldering is the seepage of flux between the pallet and the board. The resulting flux residue may inhibit in-circuit test probe contact. Care should be taken to ensure that the board fits well into the pallet nest, is rigidly indexed within the nest of the pallet, and is adequately retained against the pallet surface. As a pallet ages, it may shrink, take on twist, or bow and even delaminate. All of these may interfere with proper board seating. If a component is seated tight against the PWB, it may block the flow of solder into the PTH. A gas pocket may form at the primary side of the hole. Gas pressure may force liquid solder out of the hole, preventing adequate PTH hole fill. This can be remedied by applying a small spacer under the component as shown in Fig. 47.29. The spacer must be made of a heat-resistant material such as silicone.
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FIGURE 47.29 A component body tight atop a PTH can seal the hole: (a) The resulting gas pocket in the PTH can exclude liquid solder; (b) this situation can be remedied with silicone or other heat-resistant spacers applied beneath the component body.
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Wave Soldering in the Pb-Free Context The advent of lead-free solders has brought with it several issues:
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Solder pot temperature In wave soldering with eutectic Sn-Pb, the solder pot temperature is typically in the range of 240 to 260 C. Surprisingly, the move to Pb-free soldering has not necessitated a big change in solder pot temperature. For SAC305 (Sn-3 percent Ag-0.5 percent Cu), solder pot temperature is typically maintained between 255 and 265 C. It is wise to use the lowest practical solder pot temperature to reduce solder oxidation (dross formation) and to limit dissolution of PWB or component materials. In many cases, major assembly houses are using the same solder pot temperature for Pb-free solder as was used for Pb-containing solder, and conveyor speed is slightly reduced for better preheating and to compensate for the lower wetting rate characteristic of most Pb-free solders. Oxidation Some of the Pb-free solder constituents such as copper, zinc, and bismuth are notorious for oxidizing. When using alloys in which such constituents are major components, nitrogen inerting is mandatory to ensure good soldering and to conserve solder composition by retarding oxidation and excessive dross formation.
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