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Design007-Nov2020

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10 DESIGN007 MAGAZINE I NOVEMBER 2020 When we started planning this issue on via design, I was reminded of the Dunkin' Donuts commercials from the 1980s. The commer- cials starred a sleepy character named Fred the Baker dragging himself out of bed before dawn, muttering his mantra, "Time to make the donuts." Dunkin' Donuts retired Fred's character in 1997 and gave away six million donuts in his honor. With its annular ring, a finished via has always reminded me of a donut, and it's not much more compli- cated. A via is basically just a fancy hole, yet the industry contin- ues to have issues with vias. You've heard sto- ries of the OEMs who send out a dozen test boards, chock full of vias, to a dozen fabrica- tors to get a feel for how they manufacture their vias. And they get back a dozen boards with vias built in a dozen different ways. Vias are usually in the top five when we sur- vey readers about their biggest challenges, and there has been quite a bit of activity in the world of vias, especially microvias. Military contractors have been experiencing microvia failures that occur during reflow but remain undetectable at room temperature. A failure that you can't detect is never a good thing, especially if you're build- ing PCBs for missiles or fighter planes. In response, IPC recently came out with new thermal test meth- ods that require the coupon to undergo a solder paste reflow profile to reach 230˚C or 260˚C, where the latent fail- ures can be observed. IPC also formed a group of experts, led by Michael Carano, to investigate this prob- lem. The group, now known as the V-TSL- MVIA Weak Interface Microvia Failure Tech- nology Solutions Subcommittee, also includes Happy Holden as a founding member. The results of their research have been mixed so far. The problem seems to be restricted to complex stacked microvias, so this group rec- ommends using staggered microvias for the time being. But there doesn't seem to be a smoking gun. (If anyone has figured out the root cause, they're not sharing it.) Hopefully, the subcommittee will have some encourag- ing news to present at the IPC High-Reliability Forum in Baltimore, Maryland, next May. For this issue, we lined up a group of expert contributors to discuss the best methods for The Shaughnessy Report by Andy Shaughnessy, I-CONNECT007 Time to Make the Vias

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