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Re: Experiments and considerations for more layer


On Mon, Sep 02, 2013 at 03:45:23AM -0700, Cirilo Bernardo wrote:
> Using IDF also gives us another method of specifying the board outline, but to be honest the specification has severe defects; for example, arcs are not correctly constrained so for any arc specified in IDF there are 4 different ways to draw the arc if we assume a circular arc; for elliptical arcs there are an infinite number of ways to draw the arc and every single one will conform to the standard as written.

AFAIK IDF is generally used only from ECAD to MCAD, even if it's
designed to work both ways. The IPC standard will explicitly add
a 'mechanical outline' layer for IDF generation and most probably
provisions for attaching a STEP model. However it isn't out yet, but the
pattern calculator already generates it (on screen, I don't have
licenses for the CAD exporters).

I see two problems generating IDF3: 1) correctly finding polygon loops
(more or less like for the SPECCTRA board border) and 2) height values;
AFAIK the eagle people hack the line width to represent height;
a similar special arrangement could be done for the 'body model outline'
layer, used for IDF generation.

As for the arcs, well, we should fix before the 90 degrees limitation
(and do anyone actually use elliptic arcs?)

> Nice work Lorenzo. As for how many layers we can ultimately use, I imagine at least twice as many as the copper layers in a pathological case thanks to things like 'no route' areas underneath ICs. But with 64 layers total I think we'll be safe for a long time.

We can't raise the copper layer number without breaking things (i.e.
renumber layers); IMHO whoever needs to use more than 16 coppers
probably should use something more advanced than pcbnew at the moment :P
We need *at least* a more complete padstack model to correctly handle
modern HDI assemblies... at least different antipads (call them
clearances, if you want) for internal layers and even more control for
thermals. And of course a script for fanning out BGAs :P

The layer increase project (started more than a year ago, IIRC) is more
to enable more auxiliary/technical layers (courtyard, assembly,
mechanical and electrical clearance to name few), and maybe more
user comment layers.

Lorenzo Marcantonio
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