Designer’s Notebook

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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Don’t be afraid of the ground.

Doing special things with a circuit pattern is a hallmark of analog design. All the important signals on the board added together are equal in importance to one net; that net is the ground net. Every active component will have at least one of its pins tied to ground. An RF device could use any number of voltages and will likely want a dedicated power supply for each voltage required. Characteristic impedance relies on a ground plane or two.

Faster digital circuits start to behave like their analog counterparts. The typical routing rules involve fanning out the surface mount pins with short segments and doing the main course of the routing on an innerlayer. An elegant placement could make it possible for the bus of related traces to run entirely on the outer layers.

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