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Care should be taken to ensure that the attachment of stiffeners does not interfere either mechanically or thermally with components that may be positioned close to the board edge Design rules should preclude the use of reflow-challenging components or component fields too close to the circuit board edges Additionally, some ovens are sold with an accessory steel cable strung the oven length that is meant to serve as an antisag support An additional adjustable supporting chain is sometimes installed for the same reason and meant to be in contact with the bottom side of the board The cable or support chain must not contact components on the bottom side of the board, as the chain could dislodge components; thus, clearance must be designed into the board or panel for the support cable or chain The chain and its supporting rail can interfere with the reflow profile.

The board will be cooler in the shadow of this additional rail as it blocks air flow from the bottom blowers In a properly designed oven, the pin-chain inflicts little thermal influence on the conveyor However, some edge-hold systems have been known to impart a dramatic effect on the thermal transfer at board edges, resulting in overheating or heat sinking, depending on the oven s performance and rail position Some vendors offer rail heaters to counteract this effect It is best to avoid this type of system because it adds further complexity to the oven and makes the job of process control all the more difficult This should not be a problem with the conventional pin-chain conveyor because in actuality the pin-chain is only in point-contact with the circuit board Thermal transfer through these contact points is very poor.

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Further, most of the newest reflow oven manufacturers have replaced carbon steel chains with stainless steel chains Stainless steel has better wear characteristics and is a poorer thermal conductor than other commonly available chain materials 473122 Mesh Belt Conveyor Mesh belts used in reflow ovens are generally fabricated of stainless steel links Some are wide open with large spaces, many centimeters from link to link Others are more like chain mail armor (Fig 473) The widely spaced links allow more air flow to the bottom side of the board..

FIGURE 47.3 mesh (b).

MICROELECTROMECHANICAL SYSTEMS (MEMS)

The mesh belt is close to the full width of the tunnel and traverses its entire length Mesh belts offer exceptional versatility, as there is no need to adjust for various board widths and never a concern for board sagging or dropping The best situation, however, is a combination of conveyors with the pin-chain running above a mesh belt In this case, redundancy can save boards, reduce maintenance time, and enhance personal safety The mesh belt here serves as either an optional means of conveyance for boards populated on a single side or as a safety net to catch a board that may fall off the pin-chain conveyor Dropped boards are a rare occurrence if the pin-chain conveyor is set properly and the equipment, process, and materials are properly specified and controlled.

Then the PB0 is pin programmed as an input with no pull-up resistors, and either a software counter or Timer0 (or Timer1) is triggered to start counting The capacitor starts charging to 5 V through the resistor R1 When the voltage on the capacitor becomes more than input voltage on the Ain1 pin, the comparator output switches 1 When this is detected by the program, which is polling in a loop to detect the change of state of the comparator to 1 , the software counter (or the Timer0) is stopped and the accumulated count is proportional to the input voltage on Ain-pin The larger the voltage, on Ain-, the capacitor will have to charge to a voltage higher than that, and that will take more time, which means the internal counter will be clocked for more time, accumulating a larger count.

If the mesh belt were not used with the pin-chain, any dropped boards would cook on the heater assemblies, resulting in the release of noxious, irritating fumes Additionally, the heater or heaters affected could become encrusted with the decomposed board laminate, which may impact their thermal performance If a thermocouple, the sensing instrument for oven temperature control, is damaged or insulated by the laminate decomposition products, overheating of the zone could occur, causing oven damage and manufacturing downtime The biggest problem with mesh belt conveyors is that the bottom side of the board is in direct contact with the conveyor surface Any components mounted on the bottom side of the board could be damaged or dislodged before or after reflow While the board is at reflow temperatures, components could be forced into misalignment or nudged off bonding pads by contact with the conveyor mesh.

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