Re: gEDA-user: next PCB release - 1.99za vs 4.0

[DJ Delorie <dj@xxxxxxxxxxx>]

> ...but your number of "sub-composites" grows exponentially. In
> theory, whatever the lowest-level composite is, your sub-composite
> set is the power set of that!

No, because fabs can't drill that way.  You can't have a copper layer
that's part of two separate sub-assemblies before those sub-assemblies
are assembled to each other.

Let's say we're doing the top half of an 8-layer board...

copper  ----- 1 top
   fr4  ===== 2
copper  ----- 3
   fr4  ===== 4
copper  ----- 5
   fr4  ===== 6
copper  ----- 7
   fr4  ===== 8 center

Now, depending on how the fab makes it, determines what combinations
of buried vias are possible.  For example, if the fab makes
two two-sided boards and glues them to a core, you get this:

copper  ----- 1  --
   fr4  ===== 2  ][
copper  ----- 3  --

   fr4  ===== 4

copper  ----- 5  --
   fr4  ===== 6  ][
copper  ----- 7  --

   fr4  ===== 8

Thus, you can via from 1-3 or from 5-7, but not 3-5 because you never
have 3/4/5 as a separate drillable unit.  Thus, the composite is:

((123) 4 (567) 8 ...)

Blind and micro vias are just special cases of this.  You'd sub-composite
the top copper/fr4 so you could list drills there:

(((12) 3) 4 (567) 8 ...)


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