barcode scanner programming asp.net FIGURE 47.2 Cross-sectional view of circuit board carried on a pin-chain conveyor. in Software

Painting Denso QR Bar Code in Software FIGURE 47.2 Cross-sectional view of circuit board carried on a pin-chain conveyor.

FIGURE 47.2 Cross-sectional view of circuit board carried on a pin-chain conveyor.
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There is one such chain on each side of the reflow oven.The circuit board rests on these pins on the inner aspect of the chains and is transported through the oven during the soldering process. The two chains are driven from a common motor, appropriately geared to ensure that the circuit board is conveyed through the oven evenly and to preclude angling and jamming of the board in the oven. Conveyor speed is adjustable and is one of the major factors of reflow soldering in the thermal (time-versus-temperature) profile development. Pin-chain conveyors are best for circuit boards with components on both sides since the circuit board is conveyed by its edges. This edge-hold method prevents misregistration of parts to be reflowed and also eliminates movement of or mechanical interference with previously reflowed components on the bottom side of the board during transport through the oven.
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Of course, an edge keep-out area must be designed into the board such that there will be no pin-chain or component interference. When reflowing thin, large circuit boards, they may sag when held by the edges on a pinchain conveyor. The sagging is due to thermal excursion above the glass transition temperature (Tg) of the circuit board epoxy during the reflow process. Tg is the temperature at which a partially crystalline polymer will change from a hard structure to a rubbery, viscous state, as in the case of FR-4 and other such laminate materials. The Tg of most circuit board laminates is in the range of about 135 C to about 200 C, well below the peak reflow process temperatures of most solders. Sagging will be pronounced due to the fact that the board is unsupported at its middle as it surpassed its Tg during transport on the pin-chain conveyor. Copper ground and power planes within the board provide significant support, but may not impart enough stiffness to counter this problem. Sagging alone or together with rail-to-rail nonparallelisms at process temperature can even lead to boards falling off the conveyor. Most of the major vendors in the reflow oven market have conquered the rail twist problem, but hot testing an oven with precisely toleranced test vehicles or even aluminum plates should be performed prior to final acceptance of the equipment or reliance on it. Board sagging can be mitigated, to some extent, by means of mechanical stiffeners affixed permanently or temporarily to the circuit board or by reflowing the board on a pallet. It should be noted that stiffeners and pallets may affect the thermal mass of the board, making reflow more of a challenge. 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. 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. 47.3.1.2.2 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. 47.3). The widely spaced links allow more air flow to the bottom side of the board.
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FIGURE 47.3 mesh (b).
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