kicad-developers team mailing list archive

[Bob Gustafson <bobgus@xxxxxxx>]

On 11/26/2014 01:16 PM, Nick Østergaard wrote:
> 2014-11-26 20:00 GMT+01:00 jp charras <jp.charras@xxxxxxxxxx>:
> > Le 26/11/2014 18:43, Nick Østergaard a écrit :
> > > This is indeed an annoyting bug, it has existed since 2009 at least.
> > > There is already a bug report about it. See:
> > >
> > > https://bugs.launchpad.net/kicad/+bug/1035621
> > Sorry, but I already said why a trapezoidal pad is designed.
> >
> > The fact an user is thinking something is not expected is not a proof of
> > a bug.
> I do follow you, but I still thought it was worth linking this thread
> with the bug report.
>
> > Trapezoidal pads have a very specific purpose.
> > I am pretty sure a copper zone around tracks in a microwave design
> > breaks the design.
> Indeed, IIRC this special behaivour of the trapez pad is not described
> in the manual, so one could only assume it was a bug.
>
> On top of my head I can not come up with a design that requires a
> trapez pad instead of an oval, rectangular or circular pad. Maybe
> wayse could fill in, since he has them in his design.

As the system frequency gets higher, board traces begin to act like
transmission lines.

Trapezoidal pads use transmission line theory.

> > So :
> > First, define the purpose of trapezoidal pads.
> > After, and depending on this purpose, decide if thermal reliefs are good
> > or not.
> >
> > > Nick
> > >
> > > 2014-11-26 18:03 GMT+01:00 jp charras <jp.charras@xxxxxxxxxx>:
> > > > Le 26/11/2014 17:18, Wayne Stambaugh a écrit :
> > > > > I just discovered something I never noticed before.  Trapezoidal SMD
> > > > > pads are not connected to zone files (see attached screen shot).  The
> > > > > zone fill also does not follow the contour of the pad outline.  Is this
> > > > > by design or should I file a bug report?
> > > > Currently, this is by design.
> > > >
> > > > The primary goal of these trapezoidal pads is microwave applications.
> > > >
> > > > For these microwave applications, they are used to connect a large track
> > > > to a narrow pin ( transistor or IC) ( or a narrow track to a large pin)
> > > > *without discontinuity* of the copper width.
> > > > (A discontinuity between a track and a copper pad area can create signal
> > > > integrity issues. In fact any discontinuity on the signal path creates
> > > > issues, at very high frequencies )
> > > > A trapezoidal shape with an edge having the same size as the track, and
> > > > the opposite edge having the size of the transistor or IC pin does not
> > > > create discontinuity.
> > > > One could use also rectangular pads, and trapezoidal track segments to
> > > > avoid discontinuity, but Pcbnew does not know trapezoidal track
> > > > segments, mainly because they are not easy to handle in DRC.
> > > >
> > > > When you are using trapezoidal shapes for pads, you are expected connect
> > > > a track to these pads using the right edge, and the right track width
> > > > (or the right pad size)
> > > > If this is not the case, a rectangular pad or an oval pad is better
> > > > (more easy to use).
> > > >
> > > > Adding thermal relief to a trapezoidal shape creates discontinuity, and
> > > > the shape is no more a trapezoid.
> > > > In fact, using a trapezoidal shape for a pad in a copper zone has no
> > > > interest.
> > > >
> > > > For these reasons, I did not spent time to code thermal reliefs for
> > > > trapezoidal shapes.
> > > >
> > > > Of course, trapezoidal shapes can have now applications outside the
> > > > microwave applications, but I don't know these applications.
> > > >
> > > > --
> > > > Jean-Pierre CHARRAS
> > > >
> > > > _______________________________________________
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> > --
> > Jean-Pierre CHARRAS
> >
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