Design007 Magazine

PCBD-Nov2016

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22 The PCB Design Magazine • November 2016 From their traditional use to more uncon- ventional uses, the via has gone through some changes over the years. In this month's column, I will examine issues such as: • Expressing tolerances for vias • Blind and buried vias • When, where, and why you should fill vias • Via-in-pad • Stacked vias • Vias for thermal applications Expressing Via Tolerances on a Drill Drawing I get this phone call at least once a week: "Hey, Mark, what is the smallest mechani- cal via that can be done by your company?" This is followed on my end by, "What will the tolerance for the vias in question be?" If they say, "Oh, your standard +/-.003" tol- erances," I am obligated to tell them the min via would be around .0078" with a signal pad of at least .014" and an anti-pad of at least .018". Usually, at this point I hear a lot of choking and coughing and they say, "But I am egressing from a .4mm pitch BGA, I don't have that kind of room!" This is where we talk tolerance. If they are true vias where the finished size is NOT of any consequence, we say, "Why not call them out as +.003" minus the entire hole size?" At that point I tell them that we can drill smaller and require less signal pad and anti-pad size, which opens the customer up for some routing for these fine-pitch parts. While we are at it, let's talk about pitch. Many times, we are approached by customers talking about a specific pitch between mounts or pads, such as .4mm or .5mm by Mark Thompson, C.I.D. PROTOTRON CIRCUITS Hey, They're Just Vias— or Are They? FEATURE COLUMN: THE BARE (BOARD) TRUTH

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