Dear Georg Beckmann,

Thanks for telling us about your interesting variation of the "Pad Cap board"
idea: put GND and VCC on outer layers.

Pros:
  more routing area. (Large footprint pads now eat area out of the power planes,
rather than the signal planes).
Cons:
  Less interlayer capacitance.
  slightly more crosstalk interference.
  more difficult to access signal traces.

The first 2 cons are irrelevant unless you're at really high speeds. The 3rd con
-- well, already over half of the signal traces on a standard 6 layer board are
already inaccessible. So it doesn't make it much worse if *all* of the signal
traces are inaccessible. (Pad Cap boards have no signal traces on the outside
layers, only pads and vias and the short stub between them. These boards don't
even have a solder mask).

>From a cost standpoint, it doesn't matter how you re-stack the board. A 6 layer
board is a 6 layer board, no matter if the power planes are inside or outside.
And a 6 layer board is always going to be more expensive than a 4 layer board.
If you can really go from a 6 layer board (standard) to a 4 layer board (with
unusual stackup), I would go for it. I already know of one case where it was
cheaper to buy one 6 layer board and one 2 layer board (the same size, so they
would stack) than to make the 6 layer board about 50 percent larger and put
everything on that one board -- even after factoring in the cost of the
connectors and assembly time.

Hmm -- how about putting the ground plane on an outside layer, and the power
plane immediately adjacent on the next internal layer ? That would give you the
same interlayer capacitance as your original stackup. Although I suspect this
"interlayer capacitance" idea is a red herring -- what's really important is the
impedance of the power planes, which doesn't much depend on where they are in
the stackup.

I don't know much about blind vias. Tell us what you find out about the price of
6 layer through-hole vs. 4 layer blind vias, OK ?

Advanced Circuits
http://4pcb.com/
has nifty on-line quote price calculator

> -----Original Message-----
> From: TSListServer [mailto:[EMAIL PROTECTED]]On
> Behalf Of Georg Beckmann
> Sent: Tuesday, 20 February 2001 5:34 PM
> To: Multiple recipients of list proteledausers
> Subject: [PROTEL EDA USERS]: Best layer arrangement
>
>
> Hi to all,
>
> This question is more a general one,
>
> I have to do a redesign of a 6 layer board. The layers sequence
> from top to
> bottom is,
> Top routing, second routing, GND, VCC, third routing, fourth routing.
> Top layer has all IC's. The bottom layer has the resistors, cap aso.
>
> If I change the GND and VCC to the top and bottom layer, there is
> more space
> for routing.
> Even more space I could get with the use of blind vias.
>
> What does you mean is the better solution: six layer standard or 4 layer
> with planes at top
> and bottom and perhaps blind vias.
>
> Georg Beckmann
> [EMAIL PROTECTED]
......
> # INTERNET:             http://www.BuE.de #
.....





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