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FIGURE 47.26 A wave-solder pallet for a PWB. The light areas are cutouts that allow the component solder-tails to come into contact with the solder wave. The dark areas shield the board, component vias, and critical components from the wave. (Courtesy of Hewlett-Packard).
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The typical wave solder pallet is made of a nonwetting, electrostatic discharge (ESD) safe, thermally insulating material such as Delmat or another high-temperature epoxy-glass composite. 47.4.11.1
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Pallet Uses.
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A pallet may serve several purposes, including the following:
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Bottomside SMD solder-joint protection Via and via field shielding from solder10 Edge card finger shielding from solder Press-fit connector site shielding PWB distortion prevention Test-point protection Plated chassis hole protection
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47.4.11.2 Wave-Solder Pallet Design. Although the wave solder pallet can prevent damage to a board, if not designed properly it can also interfere with soldering. There are only a few rules governing wave pallet design:
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Limited pallet thickness The pallet must not be too thick. The design must not hold the board so high as to preclude the molten solder wave from contacting areas to be soldered. Largest possible pallet openings Pallet openings should be as large as possible for good PCA preheating and to permit free access of the molten solder to the targeted areas of solderjoint formation. Clearance between cutout walls and component pins Clearance must be provided between the edge of the pallet cutout and the component pins to be soldered. If the pins are too close to the pallet opening wall, the wave may not contact the pins. Further, the pallet is somewhat of a thermal sink and inhibits the thermal rise necessary for soldering. Board locks and overclamps Board locks to clamp the board into the pallet are useful. If none are included, the PWB will float up when contacted by the wave. Overclamps may be
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needed to keep the board tight against the pallet during the wave-solder process. Overclamps are also useful to hold connectors and other nonclinched parts from buoying up when contacted by the wave. Care must be taken that the overclamps and board locks are not so massive or otherwise interfere with preheating of the board. Pallet support The pallet design should include enough areas of support under the board so that a thin, large PWB will not sag as it traverses the wave. Bevel cutouts Cutout openings should be beveled to aid in the hydrodynamics of the wave, ensuring that the wave smoothly enters and exits each cutout.
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No-Clean Flux Residue and In-Circuit Test During wave soldering, most of the flux from that process is consumed or washed off the bottom side of the board when in contact with the molten solder wave. However, if no-clean flux residue seeps between the pallet and the board, it will not be removed by the wave process. Any remaining residue, if covering in-circuit test pads, may be an impediment to proper incircuit test probe contact. Care should be taken to minimize flux deposition for wave soldering. In terms of board design, test pads should be moved as far from intended wave-solder pallet openings as possible.
Designing PWBs for Wave Soldering Complex PCAs with BGAs and fine-pitch SMT components on boards thicker than about 2.36 mm (0.093 in.) may be difficult to wave-solder if care in layout is not observed.
Use appropriate heat relief It is good design practice to use appropriate heat reliefs (e.g., spoked pads) to provide electrical conduction but limit thermal conduction from the PTHs to ground or power planes internal to the board (see Fig. 47.27). This will enhance heatup of the PTH and limit thermal sinking to the ground or power planes and thus also enhance PTH barrel fill if process parameters are correct.
FIGURE 47.27 Schematic of a thermal relief feature within a PWB. The spokes connect to the PTH to the conduction plane and limit thermal transfer. The limited thermal path enhances heating of the PTH for soldering.
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