I often use a thin section of track to squeeze between pads on a component. 
These are usually controlled impedance traces (ECL logic, 50 ohms, 10-12 
mils, typically), and are terminated at the receiving end. The thin section 
changes the impedance, not much, perhaps from 50 ohms to 60-65 ohms. And 
it's short, only 100-150 mils long, about 20 picoseconds. If the rise time 
of the signal is much longer than this, the effect is almost imperceptible. 
High speed ECL has rise times of 200-300 picoseconds, so I don't hesitate. 
If you're doing analog at >2 GHz you may have to worry.

For normal signal lines, which are not controlled impedance and are 
un-terminated, the effect can be ignored completely.

For power traces, the thin section just changes the resistance of the line, 
and is easy to calculate. Unless you're dealing with tens of amps, fusing 
is not a practical problem.

Richard


At 10:51 AM 8/15/2002 -0500, you wrote:
>Danny,
>I have always been under the belief that if you make a part of a trace/track
>thin you might as well make the *entire* trace/track that width. (holding
>even more true in power & ground runs. in which case the thin run could
>almost act as a fuse.?) If anyone can share logic otherwise I would really
>be interested in hearing it. Or am I safe in what I think to be true?
>
>----- Original Message -----
>From: "Andrew Jenkins" <[EMAIL PROTECTED]>
>To: "'Protel EDA Forum'" <[EMAIL PROTECTED]>
>Sent: Thursday, August 15, 2002 10:02 AM
>Subject: Re: [PEDA] Matched Lenghth Constraint
>
>
> >
> >
> > > -----Original Message-----
> > > From: Danny Bishop [mailto:[EMAIL PROTECTED]]
> > >...
> > > I wonder what conclusion we can draw from any potential
> > > benefits of using a thick trace for a tricky trace, but
> > > thinning it down when required to get through tight
> > > spots?
> >
> > I suppose that might depend on how one thins the trace. Under certain
> > conditions, I think such a thinning might very well result in an
> > unanticipated reflective interface. That is, if one doesn't take
>UltraCad's
> > analysis as canon, IMO, a safer thinking process than simply bleating a
>path
> > to success ;^)
> >
> > aj
> >
> >
> >
> > ************************************************************************
> > * Tracking #: 74A48E38F8030A4DAB0A361931AAA163D71A3EB1
> > *
> > ************************************************************************
> >

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