kicad-developers team mailing list archive

[Andrew Zonenberg <azonenberg@xxxxxxxxxxxxxxx>]

The current code in the repo would silently convert all vias to standard
through-hole vias, my code makes it behave properly for blind/buried
vias. With some additional modifications it could support microvias too
but I haven't implemented that.

Are you saying you don't think this patch should be accepted until it
also supports microvias?

On Tue, 2014-07-22 at 20:06 +0200, jp charras wrote:
> Le 22/07/2014 18:47, Andrew Zonenberg a écrit :
> > With the attached patch, the router fully supports both routing 
> > over/under blind/buried vias and shoving them, but cannot yet add 
> > new blind vias.
> > 
> > The current code cannot distinguish between blind vias and 
> > microvias. It uses a simple heuristic: if a via goes from the top 
> > to the bottom it must be a through-board via, otherwise 
> > blind/buried. This means that when a microvia is shoved, it's 
> > silently converted to a regular blind via.
> 
> "it's silently converted to a regular blind via."
> This is not a good idea. Why to convert them?
> I do not understand the reason.
> 
> > 
> > This may be the time for a discussion of why microvias are 
> > considered "special" compared to normal blind vias. I'm working on 
> > a board now that uses layer 1-2 and 1-3 microvias. These are 
> > implemented at the fab by drilling from layer 2-3 anywhere that a 
> > layer 1-3 via is needed, then laminating layer 1 and drilling from 
> > 1-2 anywhere that either 1-2 or 1-3 vias are used.
> > 
> > In other words, real microvias (when using a stacked process) can 
> > go to layers other than 2 and N-1. If this is the only distinction 
> > between microvias and blind vias in KiCAD, I propose eliminating 
> > the microvia entity entirely. There could perhaps be a DRC mode to 
> > only allow blind vias between specific layer pairs if you're 
> > targeting a process with a limited number of microvia layers.
> 
> Eliminating the micro vias is perhaps not a good idea.
> 
> I agree the fact a micro via is a blind/buried via, just with
> constraints on layers count.
> 
> Micro vias are laser drilled and the hole has a limited depth.
> (this is not the case for blind/buried vias which are not laser drilled)
> 
> Most of time they are used to connect a footprint like a bga therefore
> an external copper layer to the next internal layer.
> 
> Having a special type for this kind of via is an easy way to handle
> layers count constraints and make placing very small vias more easy.
> 
> It also allows to easy place 3 different vias: through, blind and very
> small (said microvias, with constraints) vias.
> 
> If you are thinking eliminating the micro vias is a good idea, I
> suggest you first to write an user documentation about usage of vias,
> blind vias with no constraints and vias with constraints.
> 
> In Kicad, micro vias (only allowed from an external layer to the first
> internal layer) is just a convenient way to place very small vias,
> which are expected laser drilled, i.e. which have a limited depth.
> 
> However you easily can add a DRC test on vias which have a limited
> depth, depending on the drill hole, and use only blind/buried vias.
> 

-- 
Andrew Zonenberg
PhD student, security group
Computer Science Department
Rensselaer Polytechnic Institute
http://colossus.cs.rpi.edu/~azonenberg/

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