Hi All,
In a message dated 08/25/00 18:52:27, [log in to unmask] writes:
>The fact that the cracks are on the prepreg not core
>may be a red herring.
Not likely; we have simply seen too much of the barrel cracks being at the
prepreg layers starting with IPC-TR-579, Round Robin Reliability Evaluation
os Small Diameter Plated Through holes in Printed Wiring Boards, in 1987/88
>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.
Also not likely. Failures at layers 1, 2, n-1 and n are typically inner layer
separations (see results of Post Separation committee) because those are the
locations with the most land rotation; barrel cracking occurs near the PCB
center because that is where the loads are highest for the barrel.
>  2. What do the Tensile Strength/ Elongation numbers look like?  I find
that when the >Elongation goes below10% we see things that we don't like in
our sections. We try to >maintain our baths at 15% or greater.
Very good point.
>  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)
No need for this; it is clear that the load on the PTH barrel was too large
giving the construction details. One thing that should be looked at is
whether the product saw manual solder operations (typically the highest
thermal excursions by far) near the failed PTHs.
I hope this helps, Robert.
Werner Engelmaier

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