Since we are working close together with industry we are frequently asked
questions concerning the reliability of electronic equipment. Now I face a
problem where I have trouble with the total explanation of the effect:
- The PCB's send to us show short circuits within the board between inner
layers.
- I worked out that the shorts are caused by through plated holes,
connecting inner layers
- I also saw delamination between inner Cu-layers and the Epoxy

So far no problem. But:
- To localise the short circuit I watch the area where I found the short (
by measuring ) with a infrared microscope and apply a certain current in
order to detect local heat up. It is always the case that I can have the
current applied ( last time 10 amperes ) without observing any considerable
heat up. But after the current is applied for a while ( minutes up to hours
) a very rapid heat up occurs ( over 200 Celsius ). This heat up is so fast
that I have rarely the chance to cut off the current. Naturally if I am
analysing the short with microsections I see the totally carbonised PCB and
the track where the short circuit went.

Does anyone the reason for this behaviour ?

Thanks for your help.

Guenter Grossmann
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