Re: gEDA-user: Preliminary report on gerbers with buried, hidden and micro vias

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

Every question generates another.

I think it is a little more complicated then I let on.

I am under the impression (and I will try to get this confirmed) that
for boards of more then 2 layers. For example 4 layers, 6 layers, 8,
layers etc (there are always even number of layers once you have more
then 1 layer)

So from a plastics stack up a 4 layer board would look like

1   first plastic

2   
-   second plastic
3

4   third plastic

and a 6 layer board would look like 

1   first plastic

2   
-   second plastic
3

4   
-   third plastic
5

6   forth plastic

So (for a 4 layer board) you can have a buried (or micro) via from layer
2 to layer 3  but not from layer 3 to layer 4. Though you could have a
via which includes layers 2,3 and 4.

Don't take this for gospel I will try to confirm.


Now to try to respond to your pondering....

I think assuming micro vias for layer pairs (2/3, 4/5, 6/7 .... etc are
okay. The outer layers may never be paired for micro vias.

The other layers maybe should be grouped to check for mixing layers.
Maybe clicking on a via brings up a dialog which tells you what is legal
for that via based upon what you have done already?

Your idea of once you have committed layers 2/3 and 4/5 to have a buried
via you can't also commit a via to being between 4/5 and 5/6 without
including 2/3.

Also, again i presume, every new pair of layers added to a stackup means
passing the board through the lamination process, drilling process and
plating process. The laminating process alone can take an hour so the
cost of the board is likely to go up up up.

Steve Meier  




On Thu, 2005-01-13 at 15:01, DJ Delorie wrote:
> > You can't mix layers. If you have buried via connecting layers 2, 3, and
> > 4 you then can not have buried vias connecting layers 3, 4 and 5.
> > This is because they will build up the layers 2, 3, and 4 as if it was
> > its own board. Drill and plate the holes for them. Next they add the
> > other layers to make the complete board. Then they drill and plate all
> > the normal vias.
> 
> That sounds like what I'd expect based on how boards are made.  You
> always drill, plate, etch.  So you can drill buried vias, plate them,
> and etch the outer layers of the sandwich.  Then you glue more stuff
> on the outsides of that to make a thicker sandwich, which you drill,
> plate, etch.
> 
> Hmmm... should we assume that any given FR4 layer has a maximum of one
> set of buried vias?  I.e. any given layer would be drilled at most
> twice, once for its buried vias and once for the through holes?  If
> so, we can probably add "via groups" into pcb easier than if we
> allowed arbitrary heirarchies of drill/plate/etch (like drill 3-4,
> drill 5-6, combine, drill 3-6, combine, drill 1-8)