90度走線(90 Degree Corners):Only a few topics generate the kind of enthusiastic
discussion that a right angle corner on a trace does. Just
the mention of 90o corners --- regardless of whether you
say that they shouldn’t be used of if you say that they are
harmless and can be used without concern--- guarantees
a response from people with the opposite view.
Arguments against 90o corners fall into two categories:
Impedance mismatch: A right angle corner is, necessarily,
wider than the rest of the trace. This results in a
decrease in Zo, the intrinsic impedance of the trace, and
therefore causes an impedance mismatch at the corner.
This, in turn, causes reflections, signal distortions, and
noise along the trace.
I’ve even heard one speaker at a conference use the
misguided analogy of electrons being like marbles; they
will (he alleged) reflect back from a sharp 90o corner but
“bounce around” a 45o one! (Honest, electrons don’t act like that.)