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FIGURE 47.6 Forced-convection heating modules. Cooler air (a) is drawn across the resistance heater (b) by a fan (d).The heated air (c) is forced through a perforated plenum tuned for even heat distribution to the PWB (e), which is riding on the conveyor (f). A second set of forced-convection heating modules is directed at the underside of the PWB. Several such modules are arrayed the length of the oven for precise thermal profile control.
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FIGURE 47.7 Forced-convection oven with 12 top and 12 bottom heating modules (a). Each module is independently heated and controlled. The last two top and bottom modules on the right (b) are for active cooling of the soldered circuit board to ensure that solder has reached solidus before exiting the oven. In this machine, cooling zones are aided by fans blowing over water-cooled radiators. (Courtesy of Heller Industries).
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for cooling fans outside of the oven adjacent to the oven exit. This approach minimizes air turbulence at that tunnel exit and air entrainment into the oven s nitrogen environment. Water-to-gas cooling is efficient, but it must be included in the reflow profile to ensure that thermal ramp rate is correct. Some ovens use polymeric muffin fans. These must be kept on at all times to prevent overheating of the fan blades and the motors plastic fan support
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structures. Other ovens use air rakes or even air amplifiers to direct a flow of cooling compressed air or nitrogen at the board. In-oven cooling schemes often result in condensation of flux volatiles and flux decomposition products. As deposits build up on the coolest surfaces within the oven, they change the thermal transfer characteristics of the cooling system. This alters the thermal profile of the oven over a long period of time. Flux condensates may also drip onto circuit boards as they pass through the oven. Nitrogen atmosphere ovens running no-clean s face the biggest challenge in trying to control these condensates. No-clean pastes give off abundant volatiles during reflow. Ambient air entrainment is necessarily low for precise control of the nitrogen ambient within the oven, whereas nitrogen volume is minimized to keep board processing costs reasonable. The result is low gas exchange within the oven s tunnel and little dilution of condensable vapors. Condensates build up rapidly on the cooler surfaces and are baked on over time and difficult to remove. Oven manufacturers have devised many elaborate schemes for flux condensate management. Gas stream filtration in and around cooling zones, replaceable filters, cold traps, and cold fingers with self-cleaning burn-off cycles (kitchen oven style) have been incorporated into some of the newest ovens. Several ovens have quick-change finned radiators that are swapped out, cleaned, and readied for the next change-out. Many silicon integrated circuit (IC) packages or passive component devices have specifications for maximum thermal ramp rate. Exceeding this heating or cooling rate may damage die attach materials, crack packages, or otherwise alter electrical performance of the package. This is especially true of ceramic devices. Often the ramp is recommended in the range of 1 4 C per sec. Additionally, some solders like Sn-Ag-Cu (SAC) require cooling rates in this range to allow for quick solidification and to discourage solder alloy segregation. Cooling too slowly encourages redistribution of alloy constituents and silver platelets, or needles can form that may detract from solder-joint strength. Venting. An often overlooked reflow oven subsystem is exhaust venting. This is most important from an industrial hygiene point of view, but it can also have a profound effect on the soldering process itself. During the soldering process, minute quantities of metal oxides accumulate in the oven atmosphere or coat oven tunnel surfaces. The dangers of prolonged exposure to microquantities of lead (Pb) or lead compounds are well documented. With the change to Pb-free materials, the risks are likely lower, but there should still be concern for inhalation of any metal particulates. As previously discussed, during reflow, solder paste volatilizes and releases paste reaction products. Since most solder pastes and fluxes are highly guarded proprietary formulations, the paste and flux vendors may neither accurately disclose the composition reagents nor report the decomposition products in the mandated Material Safety Data Sheet (MSDS). Additionally, if a PCB should fall off the pin-chain conveyor or a component should fall through the mesh belt, it may land on a high-temperature surface either the heater itself ,or, in the case of forced-air convection reflow machines, the perforated baffles above the heater assembly. Either the board or a plastic component will overheat, decompose, and release unpleasant or even dangerous fumes. A properly designed and implemented exhaust system will mitigate these hazards. A high-velocity exhaust may influence oven performance.Too much exhaust will cause a significant net flow of atmosphere through the oven.This can result in unwanted turbulence within the oven, upsetting intentional forced-convection patterns and making temperature regulation more difficult, reducing zone-to-zone influence (zone separation), decreasing ease of profile establishment, and resulting in process variability. Some ovens are set with one central exhaust flue, whereas others may be designed to have ports at both the tunnel entrance and exit. A dedicated manometer or other vent sensor should be installed in each exhaust line to monitor setup and routine exhaust performance. Blast gates in the exhaust line permit flow adjustment. It is customary to mount vent sensors after the flow damping device. Follow oven manufacturer s guidelines for exhaust requirements. Since the exhaust gases are laden with volatile materials, mostly from the solder paste, active exhaust systems should be checked routinely for performance. Impeller blades can foul
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