Re: gEDA-user: six-layer stack

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

Friends -

On Wed, Feb 01, 2006 at 01:18:32PM -0500, Stuart Brorson wrote:
> 
> The web page you linked to is not really a stackup.  Rather, it is
> just a drawing showing  where they use prepreg vs. core material.  

OK.  But it does define the dielectric layer thicknesses.

> The usual  six layer stackup looks like this:
> 
> ----  sig ----
> 
> ====   gnd   ====
> 
> ----  sig ----
> 
> ----  sig ----
> 
> ====  pwr/gnd  ====
> 
> ----  sig ----
> The above stackup also gives each sig layer a
> reference plane next to it.  The laminate thickness between the two
> inner sig layers should be large. 

Which is pretty much what I had in mind when I started.
5+5+30+5+5, right?  And I consider my collection of eight
power supplies as signals, which need to have a high-capacitance
low-inductance coupling to the nearby ground.

> The layers need to be symmetrical in the vertical direction to
> prevent board warpage.

I haven't ever seen this effect first-hand.  I'll certainly
ask the fab house before placing the order.  I can always add
filler (carefully) on the under-used planes to improve symmetry.

> If you Google around, you can find lots of discussion about good
> stackups.  Here's one: http://www.ce-mag.com/ARG/Grasso.html

Good picture, kind of minimal on-topic discussion.

> Also, the Howard Johnson book ("High speed digital design, a
> handbook of black magic") has a chapter about good stackups.

I've heard that book recommended before.  Maybe I should get
a copy for myself.  <clicky-clicky>  $78 on Amazon, $40 used
at half.com.

      - Larry