Hi Steve,
From a PCB layout perspective.......
We do similar looking stackups, and the fab vendors we use have no
issues with specifying different dielectric thicknesses, as long as the
stack up is balanced. In fact to accommodate the range of impedances
designers are required to route with, it would be impossible not to have
varying dielectric thicknesses.
It is imperative to ask the sorts of questions at the beginning of the
PCB layout, so your client has done the right thing. This will ensure
there will be no delay due to technical queries regarding the stack from
the fab vendor after you have shipped the Gerbers to them. Stackup
design and stackup confirmation from the fab vendor should be distinct
parts of the layout procedure. Don't gloss over it, it can cost you a
lot of time, usually when you can least afford it.
I would suggest that your client talk directly with the board shop, and
be prepared to spend some time on it, it will be an iterative process,
there are a few variables to manage.
If this is the first time your client has done this sort of thing, I
would suggest they obtain an copy of the stackup as the vendor is going
to fabricate it, getting them to call out the prepregs and cores, copper
weights, impedance calculations and trace widths for each layer, and
file it with the job. The reason for this? None of our vendors match our
Hyperlynx calculations exactly, and each of our vendors wants to do
build the board slightly differently, so for traceability and as an aid
to understand where the fab vendors are coming from, this archived
material is invaluable. I would also recommend that they get a tour of a
fab shop to see how PCBs are built. I was unaware of a lot of what goes
on, (even though I had read lots of books) until my first visit to a MLB
shop.
Two comments:
The board is staring to get quite thick, which will place a limitation
on the minimum through hole size. Your client should check this out with
who ever is going to fabricate it. This may impact on the range of
vendors that can fabricate it reliably, and the cost.
And as David has pointed out, a typically fab vendors have a core in
the middle, and alternate prepregs and cores out from there. This will
mean the core between 4/5 and 8/9 will have 1oz foil on one side and
0.5oz on the other, which will add cost to the fab if the fabricator has
to deviate from his normal process or source non standard material. A
board vendor who is prepared to work with you and suggest alternative
ways of doing things (hopefully saving you money) is worth their weight
in gold.
Regards
Steve.
>>> [log in to unmask] 26/02/05 07:53:51 >>>
Happy Friday All!
I've posted a drawing of a PCB layer stack-up at:
http://www.stevezeva.homestead.com/files/Stackup.jpg
I'd like to verify its manufacturability. The reason for the
verification
is that there
are dielectric thicknesses that vary quite a bit from layer to layer.
Can anybody comment on this?
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
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