kicad-developers team mailing list archive

[jp charras <jp.charras@xxxxxxxxxx>]

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.

-- 
Jean-Pierre CHARRAS