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48 SMT Magazine • November 2017 by Stephen Las Marias I-CONNECT007 Screaming Circuits, a division of EMS firm Milwaukee Electronics, specializes in short- run, one-off, and prototype PCB assembly. For instance, if you need two or three prototypes and you need them in just a couple of days— that's what the company does. Or if you need 500 or 5,000 production boards, for example, if you had a Kickstarter and you sold 500 and you don't know if you'll sell any more—most man- ufacturing companies don't want that business, but that's where Screaming Circuits comes in. It's all about small volumes, unforecastable, uncertain volumes, according to Duane Ben- son, chief technology officer and marketing manager at Screaming Circuits. "That gives us a very large set of experience because we see so many different jobs; we see virtually any kind of component tree applications that you might possibly imagine." In another interview, Mike Creeden, vice president of layout services at sister compa- ny San Diego PCB Design LLC, said via-in-pad, even if it's a plated through-hole via, is a form of HDI. With that in mind, I interviewed Benson about the challenges when dealing with micro- vias and vias-in-pads from an assembly stand- point, and how in-circuit test (ICT) issues, such as access to test points, can be addressed. Generally, Benson said there's not that much issue for the assemblers when it comes to micro- vias or HDI. However, he notes that no matter how small the vias are, especially in via-in-pad, they have to be plugged and filled at the fabri- cator side, must be plated over, and has to have a very planar surface. "That's the biggest challenge with the high- density vias—making sure that they're proper- ly plugged and plated at the board house, and then that they leave a planar surface. If it's nice and flat like that, for assembly purposes, it doesn't matter—we don't really know that it's there really. Some people like to leave the microvias partially open—there would be a via open going from one layer to the next. With the tiny micro BGA, if you do that, you are go- ing to end up with an air bubble inside the BGA ball, which might crack under stress, and it might not totally connect," said Benson. "Basi- cally, the only thing for a super fine pitch BGAs and those types of vias, the only proper way to FEATURE

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