The fact that the cracks are on the prepreg not core
may be a red herring.

A couple things I would check out...
  1. Do the cracks appear near the outer layers.
(Layer 1 and n)?  You will tend to get more movement
towards the outside of the board.
  2. What do the Tensile Strength/ Elongation numbers
look like?  I find that when the Elongation goes below
10% we see things that we don't like in our sections.
We try to maintain our baths at 15% or greater.
  3. Have your laminate supplier run Tg on the board
to verify cure.  Be careful here, the Tg numbers will
include the core material and the prepreg. The core
material will almost certainly be cured but since your
testing both core and prepreg together the cured core
will affect the prepreg numbers. (I hope this made
sense) You might try pressing some prepreg together
(no core) and having this tested.  Use the exact same
press parameters used for the failed panel.  (Slip a
few sheets of prepreg in a book, don't just press the
prepreg by itself)
  4. This also might be an anomaly.  It's expensive
but I would send more product to the lab (from the
same lot) and have it themal cycled.  Have these
samples sectioned and evaluate.  You might also do
some testing to failure (thermal stress in a solder
pot) to assure it was an anomaly and not systematic.

Good luck:-)
pete


--- "Stoppel, Robert" <[log in to unmask]> wrote:
>         Hello fello technetters,  I am looking for
> help on a problem that we
> have on polyimide multilayer boards that we build.
> The issue is copper
> cracking in the plated hole after 100 thermal
> cycles.  The lab says the
> cracks are only near the prepreg and not by the core
> laminate in the
> stackup.  I have verified all press parameters to be
> within tolerance.
> We have seen this issue on parts that have been
> cured in the press and
> on parts that have a post bake cure in an oven.
>         Could this be a material cure issue or even
> a copper plating issue?
>
>                                         Thanks!
>
> Robert A. Stoppel
> General Technology Corp.
> 505-345-5591  X3068
>
>
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