9 Jul
2007
9 Jul
'07
6:22 a.m.
William Donzelli wrote:
No. The differences in expansion just are not that big. If you do the
math, you will see that the dimensional changes of the the two
disimilar substances just are not that big - way too small to cause
all sorts of wrinkles. At worst case, maybe *a* wrinkle would show up
on a long trace.
The differences are enough for many thermal cycles over a long period of time.
It seems that the greater the temperature delta and the more frequently the temp
is cycled the sooner it occurs.
I've seen boards cycled between 20 and 300 degrees C five times a day that have
wrinkles in about a year and trace lift by five years. I've seen similar
wrinkling in boards cycled between 7 and 98 degrees C hourly that have been in
service for as long as 20 years. Interestingly, the boards that are at a (more
or less) constant 200 to 300 degrees C show no wrinkling or trace lift.
The expansion coefficients may be have a relatively small delta but it is enough
given enough time, enough temp cycling and enough of a temp range.
[snip]
Well, when someone involved in getting mil-spec and space grade acceptance for
PCB's mentions it, one tends to believe him.
==
jd
Real programmers don't draw flowcharts. Flowcharts are, after all, the
illiterate's form of documentation. Cavemen drew flowcharts; look how
much good it did them.
From what I've seen, this is caused by the copper
and the base material
expanding and shrinking at different rates. It is seen quite
a bit where
temperature ranges between extremes, especially with cheap and older
stuff. I'm told
this is one big reason why PCB's were not accepted for
military and
space-grade stuff for quite some time before they managed to more
closely match
materials' thermal expansion coefficients.
No. It is almost all due to bureaucratic inertia. The military is
funny this way - sometimes a new technology will be scooped up as soon
as served, yet others they drag their feet with.
And of course, in the 1950s and 60 all the contractors were making a
mint, charging the government for all that hand wired work. They were
not about to change to a cost saving new idea.