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From: EdV on 14 Jun 2010 09:48
I have "inherited" a PCB design from a former contractor at a start up
company. In the process of adding some more circuits to an already
crowded board (to me at any rate using Eagle for the first time) I am
thinking adding one more route layer might help me make schedule.
Currently it is a top and bottom signal layer with a power plane and
two separate ground planes.

In the the past I have been encouraged not to have route layers that
are inaccessible by exacto knife. ;-)

Any comments, suggestions or horror stories are welcome.

Thanks,
Ed V.

From: Archimedes' Lever on 14 Jun 2010 10:17
On Mon, 14 Jun 2010 06:48:00 -0700 (PDT), EdV <edvogel56(a)gmail.com>
wrote:

>I have "inherited" a PCB design from a former contractor at a start up
>company. In the process of adding some more circuits to an already
>crowded board (to me at any rate using Eagle for the first time) I am
>thinking adding one more route layer might help me make schedule.
>Currently it is a top and bottom signal layer with a power plane and
>two separate ground planes.
>
>In the the past I have been encouraged not to have route layers that
>are inaccessible by exacto knife. ;-)
>
>Any comments, suggestions or horror stories are welcome.
>
>Thanks,
> Ed V.

Profile available? Make a daughter card and pin hole row to attach it
with. Or use a std SIMM socket or the like as your connection mode. Or
make a parallel attached daughtercard that stands over the board on pegs.

Then, all you have to adjust are node locations and add vias, etc to
accept the interconnections. Shove everything over and route to an edge
and use a ribbon flat cable.
From: John Larkin on 14 Jun 2010 10:27
On Mon, 14 Jun 2010 06:48:00 -0700 (PDT), EdV <edvogel56(a)gmail.com>
wrote:

>I have "inherited" a PCB design from a former contractor at a start up
>company. In the process of adding some more circuits to an already
>crowded board (to me at any rate using Eagle for the first time) I am
>thinking adding one more route layer might help me make schedule.
>Currently it is a top and bottom signal layer with a power plane and
>two separate ground planes.
>
>In the the past I have been encouraged not to have route layers that
>are inaccessible by exacto knife. ;-)
>
>Any comments, suggestions or horror stories are welcome.
>
>Thanks,
> Ed V.


1. Layers are expensive.

2. I can't say for sure, but two ground planes are usually
unnecessary, and often worse.

3. Inner-layer traces are a fact of life nowadays. Check your design
and layout carefully before you order boards. Have somebody else check
it, too.

4. Lately we use copper pours for ground and power, not planes. The
difference is that you can route traces inside or around or between
pours, but you can't on planes. That can be useful, used carefully.

John

From: D Yuniskis on 14 Jun 2010 12:47
Hi Ed,

EdV wrote:
> I have "inherited" a PCB design from a former contractor at a start up
> company. In the process of adding some more circuits to an already
> crowded board (to me at any rate using Eagle for the first time) I am
> thinking adding one more route layer might help me make schedule.
> Currently it is a top and bottom signal layer with a power plane and
> two separate ground planes.

(!!) Why two ground planes? If you need separate grounds,
can't you *split* the ground plane (use a pour)?

What sort of design is it (analog, digital, power)? How
"fast" (or "clean")? etc. What packaging technology(ies)?

> In the the past I have been encouraged not to have route layers that
> are inaccessible by exacto knife. ;-)

I much prefer cutting down layers at the expense of moving
signal layers to internal layers -- just don't make any mistakes! :>
(you can always turn the crank one more time to fix those screwups)

> Any comments, suggestions or horror stories are welcome.

If you can squeeze back *down* to 4 layers, go for it.
Above *6*? <frown> Costs go up really quick. Consider
enlarging the board (if possible) instead of making it
"thicker".

Good Luck!
From: Bill Sloman on 14 Jun 2010 13:24
On Jun 14, 3:48 pm, EdV <edvoge...(a)gmail.com> wrote:
> I have "inherited" a PCB design from a former contractor at a start up
> company.  In the process of adding some more circuits to an already
> crowded board (to me at any rate using Eagle for the first time) I am
> thinking adding one more route layer might help me make schedule.
> Currently it is a top and bottom signal layer with a power plane and
> two separate ground planes.
>
> In the the past I have been encouraged not to have route layers that
> are inaccessible by exacto knife.  ;-)
>
> Any comments, suggestions or horror stories are welcome.

Unless you have worked out how to embed chips inside the board, even
buried traces are accessible to the "exacto knife" at both ends. You
may have to bend up the relevant lead on the package tp break the
connection to the buried trace, but it's always do-able, though often
not that easy.

And I've certainly worked with circuits where it was a very good idea
to have a ground plane - or a least a ground or power pour - directly
under traces carrying fast signals, and on both sides of buried
traces. Extra layers do cost money, but so do the logic chips that are
fast enough to need them, and providing a sub-optimal electromagnetic
environemnt can throw away the money you spent on the faster chips in
the first place.

--
Bill Sloman, Nijmegen
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