Printed Circuit Design & Fab Online Magazine - Current Issue

Designer’s Notebook

John Burkhert

Or why aspect ratio rules all.

The humble via comes in many flavors. By connecting one layer of a conductor pattern to another, vias have connected the world. My career has depended on them as part of the hardware I used to design for others to use. A foundational innovation in electronics brought plated through-holes to the masses. As a leap forward from wire-wrap technology, multilayer printed circuit boards put a “mainframe” in each of our pockets almost overnight.

From the first plated through-hole to the latest, the trend is to support higher-density interconnect. The key driver in plating holes is the aspect ratio, the hole’s width relative to its depth. For a through-hole, the depth is the thickness of the PCB. Most reputable fabricators can handle a 10:1 ratio, such that a common 0.062" board thickness will require a minimum finished hole size 0.006" (FIGURE 1).

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John Burkhert

Design reviews can easily go off on tangents. Make sure you’re the one sharing your screen.

Every job eventually gets to tape-out day. But before that day comes, a lot of moving parts are wrangled into place. Even the simplest layout will require deliverables for assembly, including custom paste stencil and a bill of materials to associate the correct component for each location on the board. Along the way, a set of physical and electrical properties will be used to gauge line width and length, among other parameters (FIGURE 1). Getting the responsible parties to give guidance on the many assumptions made during layout is the point of the design review.

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DfM means design for money. If it can’t be built, that’s a waste.

Great ideas come together with great timing; what’s left is great execution. Flipping the switch that sets the factory in motion causes a few pain points. These “opportunities for improvement” will dictate your agenda down to the minute with all the little things that go wrong. Let’s say there is a factory downstairs from you. Further, the factory is doing slow and laborious rework on old printed circuit boards, aka PCBs. There is a solution! New PCBs. We’re going from P0 (zero) to P1 (one). That’s where we, the designers, come in.

At new PCB time, the first order of business is improving the electronics in some way. The fix could be better performance, lower cost, higher reliability and, in some cases, all the above. Venturing into wireless technology and gaining FCC approval to play in its allocated spectrum is no slam dunk. Beefing up the power grid is a typical step. A good power distribution network has been known to cover for otherwise iffy routing. Every engineer will have some considerations carried forward.

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Why you should be mindful of long parallel routing situations.

It was a warm Saturday morning last August when we decided to visit my favorite Swedish flatpack furniture store. We, of course, means me tagging along with my spouse as she chose some stuff to seal the back-to-school deals. (One more year!) We weren’t the only ones with that plan, so it was fortunate I knew the parking lot well.

For the uninitiated, the store layout – including the garage – is a giant maze of hallways. You pass by little rooms of staged furniture on the top floor, then another habitrail running past an assortment of sundry household items on the floor between that one and the parking level. It is supposed to be a treasure hunt, with cheap, bland food at either end.

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