> >"R. Steve Walz" wrote:
> >>
> >> BM wrote:
> >> >
> >> > I was told the other day that there are no right angle traces on PCBs
(the
>> > traces go to something like 45 degrees between perpendicular traces)
because
>> > of something to do with something or other. Could something please
explain
>> > this or point me to some resources so I can book up.
> >> >
> >> > Thanks
> >> >
> >> > -BM
> >> ------------
> >> Because the electrons slide off the copper if they take the corners
> >> too fast! ;-> No, it's because chemical etching rounds off sharp
> >> corners quite destructively, as it presents more side of the trace
> >> to the etchant per lineal than a rounded one does.
> >
> >
> >Nearly right! The ones on the outside of any corner have to travel
> >faster than the ones on the inside. If the corner is 45 degrees or less
> >this is OK, since the outside ones don't have to travel faster than C
> >unless the track is wider than 100mils. But the transition is so sharp
> >on a right angle corner that the outside ones have to break the C
> >barrier on any track wider than 6 mils. The problem is that this is no
> >longer legal, having been banned by Albert E. Thus the outside electrons
> >are forced to crowd the inner lanes. Since they have already been
> >accelerated more than the inner electrons they carry more inertia, and
> >it's actually the inside electrons that get bumped. They then scramble
> >around to the other side of the track and try to jump back on, hence the
> >illusion that they have slipped from the outer edge.
> >
> >Al
>
> Not quite ;-) But current crowding does occur in corners. There was
> a paper by Jim Dunkley on this subject, rigorously deriving the
> current density in corners, clear back in the mid 1960's.
>
> ...Jim Thompson
Indeed, that is to be expected. The electrons are charged, so they repel their neighbours. They can only change direction or move if there is an inbalence in forces, so they will tend to bunch up at the corner and push adjacent electrons around it. There is therefore an increased density of electrons at the corner, and a high electric field as a result. Notwithstanding this the direction is being changed as they turn 90 degrees which is an acceleration, and accelerating electrons radiate. Anyway, that's how I look at it!
Mark.