John Larkin wrote:
On Thu, 04 Oct 2007 00:00:26 GMT, Joerg
wrote:
Eeyore wrote:
Joerg wrote:
I'll second John's and Rich's comments. Besides rounding you might want
to consider flaring the traces into the pin header vias. I think
layouters call that "drop". That way there could be less stress fractures.
Teardrop. It's astonishing how many good practices of old have been lost as a
result of CAD layout.
There was once a Marconi (Instruments) IIRC guide to pcb layout from the early
days of tape-up. It covered all these subtleties. I've seen excerpts but never
the actual publication.
I HAVE seen foil fractures where a thin trace enters a pad resulting from rough
handling, rework or whatever. Tear drops reduce such stresses hugely. It's basic
engineering.
PADS IIRC is the only package I've seen that has a teardrop function built in as
standard.
Eagle can do that as well AFAIK. But I never do layouts myself.
A lot of layout is common-sense and it gets violated a lot. Why on earth
everyone thinks right angle looks more cool that round traces I will
never understand. And then the stuff breaks.
Do you think a right-angle trace is more likely to break than a curve
or bevel? I can't see why.
It offers a distinct starting point for a hair crack and as Arie pointed
out is not allowed in some hi-rel designs. This effect is most
pronounced on flex where I never allow right angles. Or any angle for
that matter, it all has to be curved. I have seen too many flex failures
and nearly all boiled down to hair cracks at trace bends or where
tear-drop hadn't been used.
Except maybe for high voltages, or 5+ GHz stuff, I don't think it
matters.
Agree, except for analog stuff where the cutoff can be much lower. In
the digital world a reflection 40dB down makes no difference but in
Radar apps it can matter.
My take on this is, why not do properly rounded traces if there is zero
cost difference and any CAD SW worth its salt can do that automatically?
Strangely, in the olden days folks shunned round traces. Not because CAD
program couldn't do them but because they did not want their design to
look hand-made.
--
Regards, Joerg
http://www.analogconsultants.com