Thanks Paul.  I believe you have hit the nail on the head.  This may seem
like an elementary question, but is it better to spec more pre-preg, like
your standard second sketch, or go with your first sketch?

I guess I don't have a good concept of what prepreg is or when it should be
used.  What I think I know about it is that it is not fully cured FR4 and
that the pressing of the layers together is what finishes the cure process.
I think I believed that it was sort of the 'glue' that held a multilayer
board together and thought it should be placed in the middle.
We have only just started to try to be more specific in what materials we
spec, and before, we trusted the manufacturer to figure out what to use.  I
believe we probably used to get a construction based exactly on your second
sketch.  But this board is built like your first sketch, which, amazingly
enough, was what we spec'd in our brand new revised readme file.
It is comforting to know they read my readme file...even if I don't know
what I am talking about.
Thanks.  Any good sources of info on what pre-preg is and where it should be
used would be much appreciated...

-----Original Message-----
From: Paul Brionez [mailto:[log in to unmask]]
Sent: February 26, 2002 11:00 AM
To: TechNet E-Mail Forum.; Genny Gibbard
Subject: Re: [TN] The board looks funny...


Genny

It sounds like your PCB was built with two B stages with FR4 between the
copper layers and the prepreg
between layers 2-3.


Similar to this               Most 4-layers

1- cu                            1- cu
     FR-4                            pre-preg
2- cu                            2- cu
    pre-preg                        Filler FR-4
3- cu                            3 - cu
    FR-4                              pre-preg
4- cu                            4 - cu





Genny Gibbard wrote:

> Hi,
> We just received a new rev of a PWB in house.  It has not been populated
> yet.  It looks like it has been built correctly - the right layer order,
> connections, etc - but it looks different than the rest of our 4 layer
> boards.  Normally the traces on the inner layer that is closest to the
> surface appear dark in colour and the deeper layer appears lighter.  This
> board is the opposite.  The layer closest to the surface is light in
colour
> and the deeper layer is dark.  Also, the FR4 seems much easier to see
> through than usual.
>
> Any thoughts or explanations for this?
> Thanks for your time.
>
> Genny Gibbard (mailto:[log in to unmask])
>
>
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