Re: gEDA-user: six-layer stack

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

Matt -

On Wed, Feb 01, 2006 at 10:02:43AM -0800, Matt Ettus wrote:
> > I'm "nearly done" with a six-layer design using pcb.  I have assumed
> > all along that the layers come in triplets, so with two ground planes
> > every trace is close to ground.  Nominal dielectric thicknesses would
> > be 5+5+30+5+5 mils.
> 
> That's a very odd stackup.

Maybe odd from a construction point of view, but it makes
perfect sense electrically.  Hence my question.

> Normally, they build a six layer board
> from 2 laminates (2 sides each) plus the 2 outer layers.  The
> laminates are usually thicker than 5 mils.  The outer layers are built
> differently from the laminates, which is why you can get the 5-8 mil
> thicknesses there.

5 mils is the (approximate) lower limit for laminates.
The 5+5+30+5+5 stack would require 5-mil laminates, a 30 mil
prepreg in the center, and 5 mil prepreg for the outer layers.
It sounds possible to me, although apparently not commonplace.

My conclusion is that I should expect most board fab shops to
prefer the 4+14+4+14+4 style construction, and therefore adapt
my electrical design.

> What you are describing is more like a 4 layer board (one laminate
> plus outer layers) with 2 additional outer layers on top.

OK.  Are there shops that will do this in prototype quantities?

        - Larry