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Verifying Chip Capacitors

Discussion in 'Electronic Design' started by Darol Klawetter, Jan 3, 2013.

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  1. 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
     

  2. 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).
     
  3. 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.
     
  4. Yes, I thought of that, though that wouldn't check for the correct distribution of various decoupling cap values.
     
  5. Guest

    Suck off them off? Are you on the right forum?
     
  6. Guest

    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.
     
  7. Guest

    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
     
  8. 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.
     
  9. Guest

    Doesn't make any difference, the caps are there to decouple switched capacitive loads in the first place.
     
  10. On Fri, 4 Jan 2013 09:11:53 -0800 (PST), the renowned
    My parts are so small I have to use tweezers:-

    http://stellartechnical.com/wmrttweezerset.aspx

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


    Best regards,
    Spehro Pefhany
     
  11. rickman

    rickman Guest

    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
     
  12. Jasen Betts

    Jasen Betts Guest

    LOL! In the wong context that's hilarious!
     
  13. German-made, what do you expect?


    Best regards,
    Spehro Pefhany
     

  14. You are such an idiot.
     
  15. MrTallyman

    MrTallyman Guest

    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!
     
  16. MrTallyman

    MrTallyman Guest

    A claim which is untrue, you RETARDED ****!
    You have made more references in the last year than I... Oooops!

    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.
     

  17. It did what you asked for.

    You said your data points were limited.

    It plotted your entire set.

    I updated it too.

    http://www.mediafire.com/view/?1ewf6pcseqfgft5

    You failed to put a header in naming what the data fields were so I
    guessed at them.
     

  18. 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.
     
  19. 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
     
  20. 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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