Verifying Chip Capacitors

decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

Assuming they're all the same value, measure the capacitance from plane to plane and divide by the number of caps. If there are some bigger reservoir caps there, suck those ones off first.

Cheers

Phil Hobbs

decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

There's probably only a couple types of bypass caps, so maybe add a couple more and connect one of each between ground and a test point. That should give a pretty high degree of confidence, especially assuming machine assembly, and catch errors such as loading the wrong reel onto the machine. Often you can tell one value from another by slight color differences in the ceramic for visual inspection. I guess it depends on what your goal is in the testing. You're not easily going to be able to catch parts of the wrong voltage rating being installed (though it might be possible for non-NP0 types by looking at the voltage-capacitance curve).

of decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferrin g that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that th e right capacitors have been used.

Spehro,

Thanks for responding. If I understand you, you're saying to remove a decou pling capacitor of each value and measure them. If they are correct, then w e can have high confidence that all caps of these values are correctly popu lated because the pick and place machine must have been loaded correctly fo r our small sampling to be correct. This would be costly for high volume pr oduction but could be worthwhile during prototype checkout.

a of decoupling chip capacitors, many of which are too small (e.g., 0402) t o have markings that would reveal the capacitance value. Other than inferri ng that they are mostly correct because the board works or removing them an d measuring their value, do any of you have some procedure to verify that t he right capacitors have been used.

Yes, I thought of that, though that wouldn't check for the correct distribu tion of various decoupling cap values.

decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

distribution of various decoupling cap values.

If it's a power plane, you don't care very much about how the capacitance is distributed, because the effective impedance of the plane is so low. If the power is routed in traces, it's more of a problem.

OTOH if the board was stuffed using automatic P&P, you should be okay just desoldering and checking one cap of each part number, to make sure they loaded the right reels in the right slots.

Cheers

Phil Hobbs

Suck off them off? Are you on the right forum?

of decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

You could rig a pin tester in parallel with the capacitor and high-speed sw itch into a capacitive load, say at 1/10 C, observing the voltage transient developed across it. If the circuit is mostly digital, that's the main rea son the decoupling is there in the first place.

ea of decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferr ing that they are mostly correct because the board works or removing them a nd measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

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I think you'd be ok to only check for one board per panel per production batch, (or when a new reel is added)

I guess you could even put one of each size on a footprint in the panel frame

-Lasse

decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

of various decoupling cap values.

If they are scattered around planes or copper pours, the distribution most likely doesn't matter. I use all the same value bypass caps, 330 nF. The "Black Magic"/datasheet stuff about mixing bypass values is mostly silly.

I wouldn't bother checking at all.

decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferring that they are mostly correct because the board works or removing them and measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

into a capacitive load, say at 1/10 C, observing the voltage transient developed across it. If the circuit is mostly digital, that's the main reason the decoupling is there in the first place.

If there are multiple caps on a plane or pour, it would be very difficult to characterize a single cap, even with a picosecond-range TDR.

decoupling capacitor of each value and measure them. If they are correct, then we can have high confidence that all caps of these values are correctly populated because the pick and place machine must have been loaded correctly for our small sampling to be correct. This would be costly for high volume production but could be worthwhile during prototype checkout.

I'm saying add a couple extra caps to the circuit (schematic and layout), of the same types as the decoupling caps. Connect them to test points. Nothing needs to be removed. You can always choose not to populate them at some point in the future.

ea of decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferr ing that they are mostly correct because the board works or removing them a nd measuring their value, do any of you have some procedure to verify that the right capacitors have been used.

switch into a capacitive load, say at 1/10 C, observing the voltage transi ent developed across it. If the circuit is mostly digital, that's the main reason the decoupling is there in the first place.

to

Doesn't make any difference, the caps are there to decouple switched capaci tive loads in the first place.

My parts are so small I have to use tweezers:-

They tend to do a number on stacked film though..

Best regards, Spehro Pefhany

distribution of various decoupling cap values.

"Black

I have to say I don't get you. Sometimes you seem like you know your stuff and other times you say stuff like this, the sort of stuff that hacks would say.

If you have a design that needs a properly designed power distribution system (PDS) then mixing values of caps can do a lot of good in reducing the impedance over a wide frequency range. But if your designs just don't need the PDS to provide a low impedance over a wide frequency range, then of course this is not a useful technique.

There are tons of bad info for designing the PDS of high speed digital systems. One piece of bad info is that all systems need to be treated as high speed. But assuming that the info on how to design the PDS for a high speed digital system is "silly" is just more bad info.

Rick

LOL! In the wong context that's hilarious!

On a sunny day (Fri, 04 Jan 2013 19:05:23 -0500) it happened Spehro Pefhany wrote in :

I think their prices are insane.

German-made, what do you expect?

Best regards, Spehro Pefhany

On a sunny day (Sat, 05 Jan 2013 11:44:16 -0500) it happened Spehro Pefhany wrote in :

Not as bad as Microsoft asks for a 10 cent DVD copy of their OS. And better quality too:-) Actually lots of German made stuff is cheaper and better than US made.

I was too polite to point that out.