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because of the condensates from these cooling volatiles. Exhaust stack monitors should help pinpoint such problems. The smoke from a piece of smoldering cotton twine can be used to sense whether there is a net flow into or out of the oven. When venting is correct, there should be a lazy net flow into the oven at both the entrance and exit. Of course, the oven should be hot and fully operational for proper exhaust setup. Similarly, it would be instructive to perform this test using hottest and coolest reflow profiles to ensure that heater module settings do not affect the exhaust conditions for minimal net flow into the oven at entrance and exit. Once the exhaust properties are set, repeat the measurements with dummy boards in the oven and observe whether the properties presence has an influence on net flow. Fine-tune the exhaust flows and repeat testing as needed. Proper exhaust setting will help achieve consistency in oven reflow soldering and protect plant personnel from exposure to noxious chemical or metal particles generated from the reflow soldering process.

Reflow Oven Requirements 47331 Multiple Heating Zones Reflow ovens are designed with multiple heating zones at the top and bottom along the tunnel length Each heater or zone is independently settable to a different temperature and with sensor circuitry for stable temperature control Note that a zone here is the top and opposing bottom heater pair, as shown in Fig 476 Multiple heater zones allow the required adjustment and shaping of the time-temperature soldering profile necessary for successful heating, activating of flux, reflow of solder paste, and cooling of the soldered assembly The number of heating zones required depends on the thermal mass of the product being assembled; the more heating zones, the more controllable the reflow process 47332 Heater Independence Zone separation the influence of one thermal zone upon another is also essential for controllable and repeatable reflow soldering.

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One heating zone set to 250 C should have minimal affect on an adjacent zone set to 200 CThere are no set rules for zone separation requirements; however, the greater the zone separation, the more controllable the processTesting this parameter prior to buying or deploying a reflow oven is necessary for proper process setup To test, set one heater zone high and adjacent zones lower Allow the oven to stabilize Determine conditions that cause the higher temperature zone to influence the lower set-point zonesThe lower set zones may rise in temperature above their set point; an indication that the hotter zone is causing uncontrollable temperature rise in adjacent zone(s) In setting the oven for circuit board assembly, avoid the large differential zone-to-zone thermal conditions and stay within the range of safe zone separation as previously characterized 47333 Temperature Distribution.

Fig 82: Moore s law showing the increase of component counts per unit area [4]

Temperature distribution across the width of the tunnel determines the quality and uniformity of heating Top-quality ovens have rail-to-rail thermal uniformity of about 2 C, even for large board widths Such tight thermal distributions are necessary for controllable and consistent soldering 47334 Heater Responsiveness Another important characteristic of an oven is responsiveness of the oven s heaters and accompanying thermal controller circuitry As a board or multiple boards pass through the oven, the oven s heat is absorbed by those cooler boards on their way to reflow The oven has to respond to that net heat loss absorbed by the train of PWBs with their solder paste and components The thermocouples built into the oven should sense the thermal impact on the tunnel s ambient and the net loss in temperature from the flow of product in the tunnel.

0.5 0.45 0.4 0.35 0.3 Voltage (Normalized) 0.25 0.2 0.15 0.1 0.05 0 0 0.2 0.4 Time (Normalized) 0.6 0.8 1 (1 - exp(-x)) .72*x

It is important not to space the boards too closely together, as doing so will fully segregate the top from the bottom heaters and alter oven performance The spacing between the boards in a forced-air convection oven must be sufficient to obviate this top-to-bottom divisional isolation of the heater zones The spacing is dependent on line.

throughput requirements and thermal mass of the product design Board separation may be as small as a few inches, but the effects of board loading and spacing need to be determined empirically and checked to detect any impact on the process time-temperature profile Any assessment of new equipment ought to include load testing to ensure that the oven can handle anticipated process throughput and capacity The oven s profile needs to be examined before, during, and after loading of the board train and compared to the performance with the same oven settings for a single board 47335 Ovens and RoHS The onset of the Restriction of Hazardous Substances (RoHS) legislation and the move to Pb-free reflow has implications for reflow equipment.

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