Patty’s Perspective: Everything You Want to Know about Vias but Were Afraid to Ask


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“Via” is one of those words appropriated by our industry and completely redefined—one of hundreds, I believe (think of mouse bites, tombstones, etc.). In my mom’s old Funk and Wagnall’s Dictionary (1968), “via” is defined as a preposition: “By way of; by a route passing through.” Merriam-Webster online today has the same basic definition: “By going through… By way of (a particular place).” Not a noun as we so blithely use it and not a thing about very small holes and electrical connections. But in our industry, IPC defines no less than seven types of vias (a noun!). (See Keith Sellers’ column for those.)

Regarding our survey on vias, almost 85% of our respondents said they used blind/buried vias. And since 15% of those answering were either suppliers or consultants, it seems that approximately 100% use the B/Bs. OK then, on to a tougher question. We then asked how often, and it turns out that “use” covers everything from rarely to pretty much all the time, with a definite undercurrent of “because we have to.”

When asked about thermal vias, over 82% of respondents said they use them and nearly as many expect to see increasing numbers of them, as one person noted, “Absolutely. ICs are getting smaller and hotter.” We asked several other open-ended questions, including, “What are the greatest challenges you face when it comes to vias?” The number of different responses was widely varied and seemed to cover everything: impedance match, routing density, aspect ratio, annular ring, DFM, diminishing pad size, cost efficiency, tenting, filling, via-in-pad, CAF concerns, signal integrity, plating, equipment, reliability concerns… And of course, density, density, density.

We also asked, “What are the greatest challenges in back-drilling your vias?” We received many different answers here, too: not enough internal copper clearance, thickness variations, alignment of primary drill to backdrill, stub length tolerance, machine precision, spacing, via integrity, processing time, and finally, “the solution is to avoid back drilling.” Ouch, I think back-drill is a pain point.

 

Editor's Note: To read this entire article, which appeared in the November 2016 issue, click here.

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