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For power pins of IC we add a decoupling capacitor, suppose a design misses the decoupling cap. Is there anything that can be done on layout to counter the missing decoupling capacitor? My first guess is to increase the width of the power traces.

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If you're modifying the layout, why don't you just add the missing capacitors? –  Dave Tweed Aug 31 at 18:30
    
If you insist on not modifying the circuit (with Dave and Olin I can't see why) you could add a large power/ground plane. But it would have to be VERY large! Or just add a few traces and stop-mask free copper islands that can accommodate an SMD capacitor... look boss, asolutely no circuit diagram changes! (but I'll have to talk to the boys at assembly about ... never mind, you don't want to know) –  Wouter van Ooijen Aug 31 at 19:07
    
Not much can be done, I guess. But in addition to lowering the resitance of the power traces, lowering the supply's output impedance via additional/lower ESR capacitors may also help a little. - Still, you'll probably have "less than optimum" EMI characteristics from the so-created antenna loop. –  Hanno Binder Sep 1 at 15:14

1 Answer 1

If you're still in layout, then you can go back to the schematic and fix the circuit. You may have to then move parts around to fit the new cap, depending on how tight the congestion is in that area. Still, fixing it now is far better than applying a kludge later.

Besides, this is one of the things that should be looked for in any design review, and checking this is also something that should fall out of normal layout procedures. When you go to lay out a part, the first thing you do is look at the power and ground connections. If you see pins connected to power that don't have a bypass cap, you go back to the schematic and check whether these are power pins or just signal pins permanently tied high.

It's hard to imagine how a missing decoupling cap would go undetected all the way thru layout. This therefore a non-question. If you're thinking about decoupling caps in layout, then you'll fix problems there. Conciously putting in a kludge around missing decoupling caps in layout makes no sense whatsoever.

If you do manage to screw up and miss a decoupling cap, the question then becomes how to do the rework. That depends a lot on the geometry of the part. Most of the time soldering a small leaded cap directly between the power and ground pins of the package will be the simplest fix, but this will vary case by case.

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"It's hard to imagine how a missing decoupling cap would go undetected all the way thru layout." Plenty of people have PCBs made as part of solo efforts, as non-commercial or hobbyist projects. –  whatsisname Aug 31 at 18:36
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@whats: That doesn't change anything. If you think about this issue in layout, then you'll go back to the schematic to fix any missing caps you find. It makes no sense to think about this in layout, but install a kludge because the cap might be missing than to simply check it and make sure. Besides, adding decoupling caps is the first thing you do when connecting a IC in the schematic. If you find yourself leaving off decoupling caps, then you need to seriously fix your schematic drawing process. There is really very little excuse. This is a really dumb problem to have. –  Olin Lathrop Aug 31 at 18:42
    
Having said all that, you can quite often get away with missing decoupling caps from the board without any obvious problems. The problems tend to arise when you are manufacturing batches of boards and/or when they are subject to extreme conditions. In which case you will start to see "mysterious" random problems in some of the boards. –  user32885 Sep 1 at 8:26

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