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Verifying Chip Capacitors

  • Thread starter Darol Klawetter
  • Start date
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Darol Klawetter

Jan 1, 1970
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Say you've just received a board from the PCB assembly shop that has sea ofdecoupling 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.

Darol Klawetter
 
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Spehro Pefhany

Jan 1, 1970
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Say you've just received a board from the PCB assembly shop that has sea 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.

Darol Klawetter


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).
 
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Darol Klawetter

Jan 1, 1970
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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).

Spehro,

Thanks for responding. If I understand you, you're saying to remove a 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.
 
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Darol Klawetter

Jan 1, 1970
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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

--

Dr Philip C D Hobbs

Principal Consultant

ElectroOptical Innovations LLC

Optics, Electro-optics, Photonics, Analog Electronics



160 North State Road #203

Briarcliff Manor NY 10510 USA

+1 845 480 2058



hobbs at electrooptical dot net

http://electrooptical.net

Yes, I thought of that, though that wouldn't check for the correct distribution of various decoupling cap values.
 
Say you've just received a board from the PCB assembly shop that has sea of decoupling chip capacitors, many of which are too small (e.g., 0402) to have markings that would reveal the capacitance value. Other than inferringthat 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 theright capacitors have been used.



Darol Klawetter

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


Thanks for responding. If I understand you, you're saying to remove a decoupling
capacitor of each value and measure them. If they are correct, then we canhave 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 becorrect.
This would be costly for high volume production but could be worthwhile during
prototype checkout.

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
 
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Spehro Pefhany

Jan 1, 1970
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Spehro,

Thanks for responding. If I understand you, you're saying to remove a 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.
 
If there are multiple caps on a plane or pour, it would be very difficultto

characterize a single cap, even with a picosecond-range TDR.





--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

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

Jan 1, 1970
0
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.

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
 
J

Jasen Betts

Jan 1, 1970
0
On Fri, 4 Jan 2013 09:11:53 -0800 (PST), the renowned


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

LOL! In the wong context that's hilarious!
 
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UltimatePatriot

Jan 1, 1970
0
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.


You are such an idiot.
 
M

MrTallyman

Jan 1, 1970
0
I was too polite to point that out.

absolute bullshit.

Were it I who had written that, you would have been "all over it".

You as polite as a freshly laid turd in the town square!
 
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MrTallyman

Jan 1, 1970
0
No, I only point out your scat fetish,

A claim which is untrue, you RETARDED ****!
which we can see is still in overdrive.

You have made more references in the last year than I... Oooops!
Besides, Speff is a nice person, and you're not.


You made no reply to the spreadsheet I spent my personal time on
preparing for you. You were too busy latching onto the open source
solution you were given, and decided to simply ignore me and my
contribution.

You are a real prize. and you cannot figure out why you get referred
to as the most filthy media the earth produces. and then it comes out of
the orifice you claim is civil as well.

How quaint... No... how utterly vile of you.
 
C

Chairman Meow

Jan 1, 1970
0
I couldn't see that it did anything useful. But thanks for trying.

The gnuplot thing works great. I double-click on a batch file, and the pdf
appears in a couple of seconds. The x-axis scaling is adaptive to the number of
points in the file, something that is apparently difficult to do in Excel.

https://dl.dropbox.com/u/53724080/Truckee/Tplot.pdf

Purty, huh?


That is not very many data points at all. Not at all what you asked
for in your post.

Mine seems to be more fuzzy as the number of points gets larger.

It would seem that what you need is a "selection Window' where you pick
a specific week, and all entries for that week show up in the plot, based
on which week you selected.

Of Course, data for that week (or whatever time segment) would have to
be a full set of readings. Or the plot will screw up on looking up the
data.

I think I can do it. I think I already have a blood pressure log that
does a similar function which I can look back on ad use pieces of.

You just paste in new data sets occasionally, which can also be
automated. But you have to accept a macro rich VB spreadsheet at that
point.
 
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Darol Klawetter

Jan 1, 1970
0
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



--

Dr Philip C D Hobbs

Principal Consultant

ElectroOptical Innovations LLC

Optics, Electro-optics, Photonics, Analog Electronics



160 North State Road #203

Briarcliff Manor NY 10510



hobbs at electrooptical dot net

http://electrooptical.net

Thanks for your input, Phil. Though I agree that in many cases the low impedance of the plane reduces dependence on a careful distribution of high andlow freq caps, my concern was simply how one can verify that the decoupling caps on the board match the schematic.

Darol Klawetter
 
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Darol Klawetter

Jan 1, 1970
0
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.

Ahh, yes. One could have board-resident representatives each capacitor type, and it wouldn't, at least in my case, require much board area. Assuming P&P assembly, and a board functioning at full speed, one could have high confidence that the decoupling scheme matches the design.
 
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