>
> I'm a self-confessed newbie on multi-layer PCBS (aside from double-sided)
and I'm sure that I'm not the only one who would benefit from a brief
treatment of the subject from a respected, experienced designer such as
yourself, perhaps even with a few choice clues on the subject, with which
the inexperienced might search for more detailed information from which to
learn?
>
Briefly, it causes several problems.   If a combination of copper pours and
traces are used on internal layers it creates difficulty for your board
house to maintain even etching. If the etching is achieved, the lamination
becomes difficult to produce reliably because the uneven copper distribution
causes the epoxies to squish out at different rates weakening the strength
of the laminate .  Because a large amount of copper is used in conjunction
with signals, the epoxies will have different strengths over the copper than
over the trace areas.  The latter leads to delamination on internal layers
because the copper causes pressure voids during lamination.  Reference
www.hadco.com   "Design for Manufacturabity Guidelines"     But I have heard
this from several manufacturing engineers.     To overcome some of this we
have used cross hatching with escapes built in to improve etching, squish
and peal strength.   On large production runs, I have moved some internal
copper to external layers to achieve the same connectivity.    Some good
board houses have flatly send us our files back, refusing to make production
runs, if we use copper pours on internal layers.       I used to use PADS
when I was at NASA,  and PADS had a neat feature that allowed the use of
copper planes and traces on internal layer.  I always thought this was a
cool feature that Protel should have until I learned that it was simply a
bad way to design.  Alot of spacecraft boards have been designed with
compromised reliabity built in.   Scary. I keep ooking up for one to fall on
my house now.  I am really surpized that all the precauations NASA takes for
Quality Assurance, no one ever inspects Printed Circut designers for
competence.

Mike Reagan
EDSI Frederick Md



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