Hi Kenny,

When testing capacitors you need to make sure you are testing at an
appropriate frequency. Check the manufacturer's data sheets. Typically caps
in the range in which you are interested are tested at 1kHz. 

We do most of our capacitor measurements using a GenRad digibridge. Handheld
meters with capacitance measurements built in can also be used but I
typically trust the digibridge readings a bit more. I don't have any
particular reason for this, just a personal preference. 

Blair


On Fri, 27 Jun 2008 14:13:32 -0700, Ken Bloomquist
<[log in to unmask]> wrote:

>Happy Friday,
>
>I need some help understanding how you test for the value of ceramic
>capacitors. We build a board that has some 0603, .470 uF ceramic caps, +/-
>10%. The assembly that these parts are on has been having a higher failure
>rate at our ICT then we've seen in the past. We tracked the failure to this
>cap which had a low reading. We replaced the cap and the unit passed ICT
>just fine.
>
>We measured the cap using our HP 4262A LCR Meter and sure enough it was
>reading low, around .398 uF.
>
>We pulled some new parts off the reel and they read low also ranging
>anywhere from .398 to .425. We suspected that we had a bad reel of parts
>even thought they were from a respected cap manufacturer.
>
>Being curious we tested some more reels from different lot codes and found
>the same thing. Being even more curious we tested some other parts from the
>same manufacturer and found them to be low by around 5% - 7% and in some
>cases slightly below 10%. We then tested some other parts from a different
>manufacturer and they tested low also.
>
>Now we are suspecting our test equipment so we went to an independent lab
>and had them tested on an Andeen Hagerling 2500A Bridge. Guess what, we got
>the same low readings.
>
>After all that my question is, what are we doing wrong to get such low
>readings on these ceramic caps? It just doesn't seem likely that four
>different parts from three different major suppliers tested on two different
>pieces of equipment would all be that far down on the low end.
>
>Any education in capacitor testing and reading would surely be appreciated.
>
>Best regards,
>
>KennyB
>
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