Assembly - February 2009 - (Page 49) This machine uses a technique called terrain mapping to compensate for small variations in board height. Photo courtesy Panasonic Factory Solutions Co. of America When populating large PCBs, maintaining a level surface is a major challenge. Because wide boards tend to sag under their own weight, bottom-side board support is critical. Photo courtesy Fuji America Corp. by 29 inches, though the actual print placement. The machine can simply area might measure 22 by 22 inches, populate the board in sections. If the after allowing for the frame and binding placement head is unable to cover the material. Printing paste across a long, entire board, a special machine is needwide area can produce “serious lapses ed. In that case, engineers should look in printing accuracy, due to the longer for a machine equipped with linear, travel distance along the arc line,” says rather than rotary, encoders. Ed Stone, sales manager at Manncorp “Linear encoders provide true, dead(Huntingdon Valley, PA). “There can on positioning, regardless of where the be a dead spot in the center of the print placement head is located,” says Stone. area, and another dead spot at the edges. “Rotary encoders lose accuracy as they These spots do not come into play when travel away from the point of origin and printing smaller boards.” generate more revolutions.” A frameless stencil system will Whether a standard or a specialized enable engineers to print to the edges of machine is used, the placement process a 29 by 29 inch stencil. To print across will take more time, since the head has an even larger area, special printers and more ground to cover. custom frames are necessary. Engineers can avoid some problems Maintaining a level surface is just as with placement accuracy during the important to component placement as it is to printing. Here, too, bottom-side board support is important. In addition, some placement machines use a technique called terrain mapping to compensate for small variations in board height. Prior to placing components, the machine measures the height of the board at multiple locations. This data is used to create a topographical map of the board. Based on the map and the size of the component, the placement head adjusts its motion to ensure accurate positioning. Large boards like this backplane typically have Depending on the size of the components that require a large amount of board, engineers may not need a localized heat. special machine for component design stage, says Thomas Baggio, engineering manager at Panasonic Factory Solutions Co. of America (Buffalo Grove, IL). For example, extra fiducials can overcome tolerance stack-up issues with the board and longer travel distances by the placement head. If one area of the board contains more metal than another, engineers can add metal to the light area, even if it’s not connected to anything, just to balance out the board and help prevent warpage. Conveying large boards throughout the assembly line must be done carefully. “You can’t convey large panels at the same speeds as small panels,” says Baggio. “The heavier the board, the more momentum it has. So when you need to stop it, you can overshoot the mark or [disturb previously mounted components]. You have to slow the conveyor down. Our equipment accomplishes that with variable-speed belt motors, positive stops and controllable acceleration and deceleration rates.” Reflow As with printing and placement, specialized equipment may be necessary to reflow large boards. Reflow ovens with 22-inch-wide conveyors and center supports are available. Because big boards can absorb large amounts of heat, they need a powerful www.assemblymag.com February 2009 / ASSE M B LY 49 http://www.assemblymag.com
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