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

[Andrew Poelstra <asp11@xxxxxx>]

On Sun, Sep 12, 2010 at 11:49:33PM -0400, DJ Delorie wrote:
> 
> > ...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 ...)
>

Alright, you've convinced me we won't encounter computationally infeasible
problems ;) but we've still got a user-interface problem.

Specifically, I expect most users don't care -at all- about this stuff, and
if they do care, they don't know (yet) what the fab can/will do.

Maybe they'll just fab whatever's cheapest and don't care about the material
or thickness of the layers. Maybe they don't need blind or buried layers.
Maybe they're just organizing protoboards or drawing road maps or something.

So what happens for those users?


Andrew



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