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June 2000

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From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
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Date:
Thu, 15 Jun 2000 13:44:39 +0200
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I missed that article, David, although I subscribe on SMART,
interesting article, really. We have a PWB of the kind with nervous caps also. After a few tempcycles, the nervous components ( just a few in stressed positions) are removed and superseded with new ones, most solder joints seem to have relaxed, no more problem. Clumsy method, but works. In fact, we can sometimes hear (!) what happens when running up and down in temperature. A tiny, crispy 'krzt' tells that a 'tension' release happened, same reason as for earthquakes, but at a smaller scale. Talking about this, one can wonder how some boards survive and work without these problems, I mean boards in movable equipments, toys, vehicles, weapon etc. jumping up and down mechanically, vibrating, slammering and running up and down in quite unpredictable manners, many of these just leave the line, are packed and sent to the market on the other side of the globe, are unwrapped and put immediately into work. A contradictory world, isn't it.
Thanks again

Regards // Ingemar



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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