Assembly - February 2009 - (Page 48) ASSEMBLY Electronics Assembly Building Big Boards Large, thick circuit boards pose considerable challenges for a lead-free assembly process. W bread box. ith electronic devices getting smaller and smaller, it’s somehow comforting to know there are still printed circuit board assemblies bigger than the proverbial engineering manager for Fuji America Corp. (Vernon Hills, IL). “Generally, anything bigger than 18 inches square is considered a large board, but there’s a new level of large board out there now. These boards can have 25 to 40 layers; they can be as thick as 3/8 inch; and they can weigh 10 to 15 pounds without components. I’ve handled a board that was 25 by 36 inches, and they can get bigger than that.” “There’s a new level of large board out there.” —Scott Wischoffer, Fuji America Such large PCBs are typically backplanes and motherboards for telecommunications, Internet, aerospace, defense and high-end server applications. These are high-mix, low-volume assemblies, and they are very expensive. Printing and Placement When applying solder paste to large PCBs, maintaining a level surface on which to print is a major challenge. Unusually wide boards tend to sag under their own weight. They can also exhibit warpage, particularly if the board is double-sided and has already gone through reflow once. Bottom-side board support is critical. Another concern is ensuring even coverage and squeegee pressure. The standard size for a framed stencil is 29 Large boards should be conveyed in and out of placement equipment slowly and carefully. If they’re moved too quickly, the machine can overshoot the target or disturb previously mounted components. Photo courtesy Juki Automation Systems Inc. By John Sprovieri Editor It’s comforting, that is, unless you’re responsible for assembling a PCB more than 18 inches wide and 0.125 inch thick. That’s because large, thick circuit boards are challenging for every aspect of the assembly process, from printing to reflow. Even the simple task of transferring these boards from one machine to another requires specialized equipment. Indeed, most electronics assembly machines are designed for boards less than 14 inches wide. “A tremendous amount of people are asking for machines that can handle boards 24 inches square or bigger,” says Scott Wischoffer, applications 48 ASSEMBLY / February 2009 www.assemblymag.com http://www.assemblymag.com
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