Circuits Assembly - August 2008 - (Page 18)

Screen Printing It’s Not All about Miniaturization Processing large boards means thinking big. I Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears bimonthly. t seems everything electronics-related I read these days has to do with miniaturization. I’ve even written several papers, columns and articles on the subject and, of course, it is the main driver in our industry. But, many other electronics devices being manufactured are at the opposite end of the spectrum: really large printed circuit boards. No doubt you’ve seen them – these mammoth 24" x 24" boards with 40 layers that weigh as much as 16 lbs. – which is why in some circles they are called surfboards. They can cost a pretty penny, too. Some of the larger server boards can set you back a good $20,000 or more. With a price tag like that, optimized manufacturing processes become even more critical. From a printing point of view, large board processing has been a subject of discussion for some time. Most major screen-printer equipment manufacturers have some type of large board system that accommodates these behemoths, and some are more flexible than others. There are, for example, systems dedicated to large-board processing. Others, however, are a bit more versatile, offering the ability to manufacture products of a variety of sizes, while still maintaining a standard platform footprint. But, as our company has learned, it’s not just about machine capability. As I always preach in this space – and large board processing is no different – the inputs have to be correctly specified for a precise output. For bigger boards, this includes not only the machine accuracy, but also the stencil quality and type, and the squeegee capability. Although the technology challenges don’t stem from geometry and feature size issues, as with highly miniaturized products, certainly repeatability and print quality hurdles must be overcome to ensure high yields for large boards. Let’s start with the stencil. Because of their construction dynamics, traditional mesh stencils require a fabrication area of several inches on each side of the quadrant, so a 29" x 29" stencil will yield a reduced printable area. For PCBs outside this print area, this solution is obviously not workable. Frameless stencil technologies, however, may lend the solution. Interchangeable frame technolCircuits Assembly AUGUST 2008 "Large-board processing is not just about machine capability; it's about correct inputs." ogy, whereby different stencil foils can be changed in and out of a stencil frame, offer the print area required for many large assemblies. Because there is no mesh element and tension is controlled through a frame/foil extrusion locking mechanism, the print area can be extended almost all the way to the edge of the foil. Therefore, with a 29" x 29" frameless stencil system, accommodating the print area on extra large boards is easily done, as you can take the print right to the frame edge. The other issue that is often problematic when dealing with large board printing is maintaining consistent transfer pressure when using larger squeegee lengths. With the 510 mm or 560 mm squeegee blades required for oversized boards, often the transfer forces vary across the stencil, as well as across the board, thus delivering inconsistent paste volumes. In fact, this is a fairly common problem for firms that process oversized boards, and one that we see often. With one customer in particular, the problem was even more prevalent. This company was placing a highpower, non-eutectic column grid array – an application that requires very tight deposition control. But, with a standard squeegee, this wasn’t possible. There was large volumetric spread that this particular package could not withstand. Because the board contained several of these devices, this customer was finding they were not within the required tolerance and, consequently, yields were poor. This, of course, added expense to an already costly assembly. Ultimately, the customer concluded that the standard squeegee just couldn’t deliver a repeatable process on a product this large. By switching to a 500 mm enclosed head printing system, the customer was able to maintain consistent pressure across the entire print head, which sustained and controlled consistent deposit volumes, and led to improved yields. Like any process, executing it perfectly once can be simple, but maintaining a level of repeatability and quality at high volume is the challenge. With large boards, as with any assembly, the end-product is only as good as the inputs. It’s not any one element, but all combined, that ensure a robust result. Think big. n circuitsassembly.com 18 http://dek.com http://circuitsassembly.com

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Circuits Assembly - August 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Auditing a Fabricatior Cutting Machine Programming Time Simultaneous Acoustic Imaging and Surface Mapping Tech Tips Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Component Advances Product Spotlight Ad Index Assembly Insider Techincal Abstracts

Circuits Assembly - August 2008

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