Brad,
You failed to either comprehend what I  wrote or I failed to communicate
what I wrote.

to clarify  IPC states"
 Class C is a fully documented procurement package. Documentation is to the
extent that the information is self sufficient and be sent to multiple
vendors, with each producing the identical product. This documentation
package requires that all the full manufacturing allowances are disclosed
and documented.   This is an IPC spec and a very important one.   I write
specifications all the time, I did aprox 80 designs last year ranging in
various sizes and speeds to 2.7 Ghz. ( fast enough)  Any specification
should be written with the minimum requirements without tying your vendors
hands.   I am disappointed with you, that you would attack my credibility
for design.  My reputation stands fairly solid  and I am proud and confident
and outright cocky about my ability to understand high speed design issues,
impedance, propagation, wavelength, and any other topic you wish to take
offline.   I have never blasted anyone on this list, but I take issue with
your comments below

Mike Reagan


>
> Michael,
>       while I can generally agree with your advice you leave one aspect
> wide open. If you do not specify your layer thickness, you could
> get a board
> that works fine from manufacturer A, switch to manufacturer B and have a
> board that doesn't work fine. The problem, one manufacturer used
> a stack up
> with inter-layer thickness X, manufacturer B used inter-layer thickness Y
> and the board now has different impedance, inductance
> characteristics. In a
> lot of designs this would not cause a problem but then there could be that
> one slightly longer clock trace or other timing or very level transition
> sensitive signal which may not meet specs and cause a board
> failure. You ran
> thousands of boards from one manufacturer and they worked fine, purchasing
> changed manufacturers, ordered 10,000 and now none of them work
> reliably. Or
> the techs nightmare, 50% of them fail but very unreliably.
>       Specifying just the minimum prepreg thickness does not give you any
> control over the repeatability of that design and therefore does not meet
> the IPC condition that any manufacturer should be able to build a
> "working"
> version of your PCB. If you do not specify your total laminated prepreg
> thickness, you are rolling the dice. I know of some manufacturers who will
> by default use a 30 - 40 mil core, others will use a 20 mil core,
> if you do
> not think this is significant to your design, that 'may' just be
> because you
> don't know your design well enough. It is not always a matter of
> specifying
> only for controlled impedances, it is a matter of specifying such that you
> can get a reliable 'known' product from multiple manufacturers as the IPC
> spec suggests.
>
> Brad Velander,
> Lead PCB Designer,
> Norsat International Inc.,
> #300 - 4401 Still Creek Dr.,
> Burnaby, B.C., V5C 6G9.
> Tel. (604) 292-9089 direct
> Fax (604) 292-9010
> website www.norsat.com
>
>
> > -----Original Message-----
> > From: Michael Reagan [mailto:[EMAIL PROTECTED]]
> > Sent: Monday, July 09, 2001 8:28 AM
> > To: Protel EDA Forum
> > Subject: Re: [PEDA] Layer Stackup Info.
> >
> >
> > Jeff,
> >
> > Word of advice...and this is a rule and a fab note we use for
> > all designs.
> > We specify  a MIN  core and prepreg  thickness for all
> > layers, in one fab
> > note. We spec .0035 inch min.   This gives your fabricator
> > the latitude to
> > adjust for copper distribution, laminates he has in stock, epoxies,
> > pressing, over all thickness variations, etc.  The only
> > conditions in which
> > we specify a thickness and we only spec it for these layers,
> > are controlled
> > impedance, and where the min dielectric breakdown voltage is
> > required for
> > Bell-Core, FCC, and space applications.   My advice is to
> > leave the majority
> > of your stack up determined by your board house, unless you
> > have specific
> > reason to do otherwise.
> > The IPC spec in 6012 for level 3 is to specify a design so
> > that any board
> > house can build your design with the same result.   Remember you are
> > designing/ writing  a specification,  give the fabricator
> > some latitude
> >
> >
> > Mike Reagan
> > EDSI
> > Frederick  MD
>

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