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FIGURE 47.22 Schematic of a board going over a solder wave.The board is conveyed over the pumped standing wave of molten solder. Only the secondary side of the board is exposed to the wave. The wave height is set to contact the bottom of the board very slightly.
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When exposed to the wave, the adhesive-attached surface-mount devices pick up solder on their metal contacts and the solder bridges from the contact to its corresponding bonding pad on the bottom side of the PWB. In the case of solder-tail components, the molten solder is drawn by capillary action between the lead and the PTH barrel. If the barrel and lead are hot enough and well fluxed, the solder fills up the barrel and wick to form fillets from pin to barrel. As the board continues past the wave, it cools, solder solidifies, and joint formation is complete. One attribute of this process is the speed at which solder joints are formed. Much quicker than oven reflow soldering, the wave-solder process allows little time for preheating, fluxing, and solder-joint formation, which also explains the variability of this process.
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Wave-Solder Machine Subsystems There are five basic subsystems of a wave soldering machine, including conveyors, fluxers, preheaters, solder pots with pumps and heaters, and ventilation. If alcohol-based fluxes are being used, it is wise to install a fire suppression system also. Conveyor. As for the reflow oven, the wave solder machine is generally equipped with a mechanized, speed-controlled conveyor to transport the board through the soldering process. In the case of wave soldering, the board is always held by a width adjustable edge-finger conveyor, which grabs the board by its edges and neither occludes solder exposure nor interferes with wave dynamics. The stainless-steel edge fingers need to be cleaned routinely to avoid flux build-up. Fluxer. The fluxer is responsible for delivering a uniform application of soldering flux in a sufficient quantity for solder-joint formation. The short preheat and liquidus times characteristic of the wave-soldering process dictate the use of slightly stronger fluxing agents
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than are used in reflow soldering. Although this is the case, the vast majority of wave solder applications have migrated to the use of no-clean fluxes. For the flux to do its job, it must be heated to an ample temperature to permit best reactive conditions without drying or denaturing it. For the sake of economy, the thinnest flux application is deposited. The flux quantity has to be sufficient to remove oxides from the PTH barrels, solder, and component leads. On the other hand, since post-wave solder no-clean flux residues can confound in-circuit test probe contact, the thinnest application that still permits adequate fluxing is recommended. Flux quantity may also be of concern in the wave-soldering process for yet another reason: fire hazard. Flux-laden boards are preheated going into the wave. If the flux application is too heavy, the flux may drip onto preheater elements. This may cause the flux to volatilize rapidly, combine with oxygen in the atmosphere, and provide the right conditions for flame initiation. Even if there is not direct exposure of the liquid flux to preheaters, if the quantity of volatile, flammable components is high enough to be an ignition source, then an explosive condition may develop.With the advent of more eco-friendly, water-based fluxes, fire hazard is less of a concern. Foam and wave fluxing have given way to spray fluxing as the predominant flux application method. All three techniques are discussed here. Foam Fluxer. Foam fluxing is accomplished by pumping and aerating a stream of liquid flux through a porous metal nozzle, a fritted glass, or porous stone. The nozzle, also called a chimney, shapes the flow of the aerated flux. The board to be soldered is run over the foaming flux and then heated to activate the flux before reaching the solder wave. As it moves past the solder wave, the solder wets to solderable metals and solidifies to complete the soldering process. Foam fluxing is particularly effective for soldering of PTH assemblies. Surface tension draws the foam into the PTH barrels, just as is the case with solder.The resulting flux deposit on the pin and barrel is thin and uniform. Wave Fluxer. The wave fluxer works much as the solder wave itself. The printed circuit board is moved over a standing wave of solder flux. The height of the wave and depth of the board penetration into the wave are adjusted to allow for proper flux application thickness. Capillary action, as in all the fluxing methods for through-hole components, draws the flux into interstice between component lead and the barrel. As with foam-fluxing, it is difficult to control the amount of flux delivered. For the most part, flux applied for wave soldering is largely removed by the turbulence of the solder wave; however, it is possible to bake on the flux if the preheater temperatures are set to too high. This may impede fluxing, and the residues can even preclude otherwise solderable contacts from the solder during initial contact with the wave. Spray Fluxing. Because of its economy and accuracy, spray fluxing has grown to be the predominant technology. Precise amounts of a low-solids flux can be applied generally to a board or can be selectively delivered to small areas of the board to be soldered. There are two principal methods of spray fluxing: air-spray and ultrasonic. Both are effective and well tested. Ultrasonic methods minimize flux volumes consumed in manufacturing. If not set properly, spray fluxing may result in a flux-laden airborne mist that deposit wherever the air or process gases carry it. Flux contamination of edge-card gold fingers and connectors is most notable. This can disrupt proper electrical contact when a daughter-card is inserted into a connector. It is wise to cover any exposed gold fingers with an acrylic adhesive polyimide tape3 or other shielding to prevent contact fouling. Fluxer Maintenance. As fluxes are exposed to the atmosphere, they are vulnerable to evaporation of the volatile constituents. This is the case even with water-based fluxes. Evolution of volatiles is significantly enhanced in systems such as wave-fluxer or foamfluxer, where the flux is open to the atmosphere or processing environment, has significant exposed flux surface area, and is constantly being recirculated. Therefore, the flux needs to be monitored and maintained. Although some automatic systems are now available, most require routine measurement and adjustment of the flux s specific gravity with a hydrometer. Flux thinner must be added to restore the specific gravity to compensate for evaporative losses. In addition, the volume of the flux in the system must be adjusted to the proper level. The fluxer must be maintained to prevent impact on process yield. The flux manufacturer can
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