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In this case, the board traverses a region of the oven devoid of heaters cooling along the wayThis is generally adequate for thin, low-thermalmass boards But the reality of multilayer, densely packed circuit boards necessitates some means of cooling to ensure that solder joints are solid prior to exiting the oven and before handling the boardTo accomplish this, many ovens incorporate an area of active cooling after the last heating zone within the oven (see Fig 477) Active cooling comes in many forms Forced-air cooling by fans is the most common Fans can be deployed on top, on the bottom, or in combination for board cooling Among the most exotic but the most efficient is the water-to-gas heat exchanger found in some ovens.

Microelectro-mechanical Systems (MEMS) 1980s as micromachining became popular With this progress, simple actuators such as piezoresistive, capacitive, field emitters, electrostatic, bimorph and piezoelectric became common, and a large portion of the efforts was directed towards research on MEMS This resulted in fabrication methods employing ion etching, laser machining and deep ultraviolet systems with the use of better materials to produce advanced microelectromechanical systems

This provides a stream of temperature-controlled cool air at the exit end of the oven for board cooldown and has distinct advantages in nitrogen atmosphere reflow systems This method uses whatever process gas is in the oven, cooling it and redirecting it onto the circuit board Thus, this method does not require additional nitrogen for cooling, does not draw any additional air into the oven that might degrade the nitrogen atmosphere, and precludes the need.

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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.

Figure 6.10 An analog signal being sampled and encoded by an ADC. The number output of the ADC is on the Y axis and the time is on the X axis.

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).

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.

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