Re: gEDA-user: Blind and buried vias?

[Gabriel Paubert <paubert@xxxxxxx>]

On Mon, Sep 28, 2009 at 08:28:19PM +0000, Michael Sokolov wrote:
> How about we move this thread back to its original topic of blind and
> buried vias, not arguments regarding whether or not PCB is part of gEDA.
> 
> I have some questions out of plain curiosity: completely aside from the
> question of how they ought to be handled by GNU PCB or any other PCB
> design tool, I wonder how these blind/buried vias work at a more basic
> level:
> 
> 1. How are blind/buried vias made physically?  I thought they glued the
>    layers together first, then drilled the holes.

By drilling layers before gluing them, although for the external
layer, there are other processes like laser etched microvias.

> 
> 2. How are they represented in the Gerber+drill file set that passes
>    from the PCB design tool to the fab?

By several sets of drill files. I was even bitten by what can be
considered as a bug in OrCad 15 years ago: I had moved a through hole
component to the other side of the board and I had two drill files
for the two layer board (the first one was something like drl0-15.ncd
and the other one drl15-0.ncd). I sent only one to manufacturing and
the holed for the component on the back side was not drilled, I don't
know why OrCad thought that a hole going from top to bottom was so
different from one going from bottom to top to put them into two
different files, but that is what happened.

By the way, it was only a double-sided board (on some expensive 
high frequency laminate from Rogers, signals were up to ~5GHz). 
Anyway, I managed to screw-up the first batch.

> 
> I'm asking out of plain curiosity - I hope that I never have to make a
> board with such vias as I've heard that they add a bit of sadomasochistic
> flavor to board bringup/debug efforts - but then I guess some boards are
> so cramped for space that you can't avoid them...

Indeed. Although I have never needed them.

	Regards,
	Gabriel


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