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I have an undetermined short somewhere within this capacitor network which is causing the power rail to short and pull a large amount of current.

The MC_VDD_0P9V is connected through a ferrite bead to an ARRIA10 FPGA. I determined the short is in the capacitor network by successfully checking continuity after removing the ferrite bead from the power supply rail.

I have no access to traces as the board is a an 18 layer PCB.

I have tried providing power and looking at the board through a thermal camera, but the resolution is not powerful enough to determine which capacitor is shorted. The capacitors are 0402 and are lumped very close to each other so the FLIR thermal camera cannot differentiate between the capacitors.

Is there any other way the short can be determined?

Thank you

Aaron
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  • Thats a lot of capacitors... Maybe if you have a milliohm meter, you will be able to measure the caps one by one and figure out which is shorted. ( The one showing least resistance) – Linkyyy Mar 22 '22 at 16:21
  • Or possibly using a FLIR camera to see which capacitor is getting hot. – Linkyyy Mar 22 '22 at 16:24
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    Presumably each of those caps is placed near to a pin on your FPGA - in which case how do you know it's a cap at fault and not the FPGA itself? – brhans Mar 22 '22 at 16:36
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    If the FPGA is not currently fitted, then I'd suggest removing the ferrite bead and applying a higher voltage to that net in an effort to get the suspect cap to dissipate enough heat that you can see it on the thermal cam. – brhans Mar 22 '22 at 16:38
  • My favorite method for such cases is to feed current limited voltage into the rail and check with a thermal camera which one heats up. There’s your problem. – winny Mar 22 '22 at 16:57
  • @Linkyyy measuring the resistance of the capacitors will have to be done individually right? I assume once they are on board, the resistance of the short will take precedence, so they are in parallel. My FLIR camera gives a general area but the caps are pretty small (0402) and are lumped too close to each other. – Aaron Mar 24 '22 at 15:55
  • Tbh i have not tried it before, but i assumed that when probing each capacitor on board, you would measure slightly lower resistance when probing just the one that is shorted. But if they are 0402 and very closely placed, it might not be possible then. (Trace resistance between caps being very very small then) – Linkyyy Mar 24 '22 at 16:03

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I have tried providing power and looking at the board through a thermal camera, but the resolution is not powerful enough to determine which capacitor is shorted.

I'm assuming you see no thermal difference at all - it will be visible if enough current is injected.

Seriously, place a voltage-limited supply on it and crank up the current slowly while observing in visual and infrared. If the short is a very low resistance (milliohms), current may have to reach an amp or more before anything becomes visible. Do this very slowly; the board has significant thermal mass and a hot trace in layer 9 could burn out before even being seen if rushed. Try 0.25A for 15 min, 0.5A, 0.75A, etc.

Is there any other way the short can be determined? I originally thought to check continuity.

Continuity is not precise enough; a standard resistance measurement may not be either. Some handheld (and most bench) multimeters have a "high resolution" mode, usually giving an extra digit or two. In either case you'll need the ability to discern differences of 0.001Ω at least. If yours can do that, then you can probe (unpowered) across each cap for resistance - the lowest is suspect; remove that one and try the board again.

Isopropyl alcohol is another trick; put some in a spray-bottle and lightly mist the suspect area - the warmer areas evaporate first. Of course remember this is flammable.

And another device is the Polar ToneOhm which has been around a long time and can physically direct you the the lowest-impedance area. Of course, this is a lot of money for a specialist device which could get very little use.

While it could be a cap, it may also be the FPGA, or even "no-clean" flux issues. If it were mine, I'd soak it overnight with mild agitation and check in the morning, just to rule that out.

If all this is still inconclusive, remove the FPGA and check again. At least this way you'll know for sure where the problem lies.

rdtsc
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  • The thermal camera gives us a general area. Unfortunately most of the capacitors shown in the schematic fall under that area. So I just tested all the new boards we received and they all give the similar problem. I gave a look at the layout and haven't seen anything out of the ordinary. That alcohol trick seems like it might work, so Ill give it a shot. – Aaron Mar 24 '22 at 15:50
  • In my experience, FPGAs tend to be highly sensitive to over-voltage conditions and cascade into a jumble of shorts. I've swapped a shorted FPGA only to have the new one fail instantly due to an input being out-of-spec (this was my fault for not ensuring *every detail* was perfect before applying power.) I'd just feel more comfortable if one was removed to prove that it was still good and that all of these caps were really to blame. I'm also wondering if it could be an internal PCB issue, or damage during initial programming, etc. since all of them are exhibiting it. – rdtsc Mar 24 '22 at 16:48