>The customer says they can tell when there's a bad cap on the board, by
>putting it on a test fixture and putting finger pressure on the cap and
>watching for the value to change for the capacitor. If it changes, it's bad,
>if it doesn't, it's good.
>
>Does that sound like a good test? The reason I ask is because I've read on
>one of KEMET's web page "Knowledge Edge" articles that dielectrics such as
>X7R are considered ferroelectric dielectrics, and have a common
>characteristic of piezoelectricity, so by putting pressure on the cap it is
>going to change the values, won't it?

There was a paper published last year in the journal that I edit showing that
resistors can also significantly (up to ~1%) change their value as a direct
result of mechanical strain. Publication details below if anyone interested
in following up.

David Whalley



Kühl R.W. "Mechanical stress and deformation of SMT components during
temperature cycling and PCB bending"

Soldering & Surface Mount Technology, 22 April 1999, vol. 11, no. 2, pp.
35-41(7)


    Abstract: A very common method to predict the reliability of components
soldered on printed circuit board (PCB) or substrates is by bending
    tests and temperature cycle tests, for instance between -55°C and 125°C
(up to 2,000 cycles at 1h cycle period). Sensitive SMD
    constructions such as chips with ball grid array mounting or multilayer
chip capacitors (MLCC) are often a major issue due to their “flex
    cracking” problems. This paper describes the real behaviour of
deformation at temperature cycling and PCB bending of chip components
    (body size 0603). By using the piezoresistive effect in thick film
resistors the effects of stress on the alumina body can be determined and
    described for the whole temperature range of interest. The complete
system of component, PCB/substrate and solder joint will be discussed
    and different influences will be isolated. It will be shown that
CTE-matching of the component and substrate does not lead to an optimum
    situation. The influence of the solder joint plays an important part.
Optimization potentials and design rules for the whole system will be given.
    The basis of this paper is a quite unusual “measurement tool” the
effect of piezoresistivity. The investigation into that phenomenon will be
    described very thoroughly first.

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