PCB Layout Guide: Circuit Trace Angle
If you open SOC PCB Layout Guide,you can see it mentions high speed signal line’s angle question: high speed signal line don’t layout in right-angle, it should layout in 45 degree angle,and arc is better then 45 degree. How’s the truth? How to design the circuit line? 45 degree or 90 degree? let’s discuss in this article. PCB line corner can be acute angle?
Rules of thumb PCB line corner can’t be acute angle.Follow PCB DFM,If lines corner are acute angle,it will easy appears acid traps problem.then lead to line over-etched when etching process.Despite PCB manufacture engineer will check and fill the acid traps, but it risks signal integrity question.thus we should avoid this problem at PCB layout process. Using the Enhanced Pad Entry tool can avoid acute angle between trace and pad at layout,avoid acid traps questions. Using toggle option can avoid acid traps of acute angle at Cadence Allegro layout. PCB line corner can be 90 degree
High-speed signal trace should avoid 90 degree,because impedance trace required width is accordance from beginning to end. If trace is 90 degree will lead to signal trace transmission delay. Follow article”Right angle corners on printed circuit board traces,time and frequency domain analysis” and”Who’s Afraid of the Big Bad Bend?”, we can get below conclusion: 90 degree can effect signal trace for high speed signal PCB. Trace 4-5mil, 90 degree capacity with 10ft,after tested,it delay 0.25ps as capacity. So 5mil trace and 90 degree is not very big influence. PCB line corner can be 45 degree
We use 45 degree in general except frequency signal and other special signals. The corner line length should be 1.5 time of trace width when line walk long. Space between trace to should be 4 times of width. PCB line corner can be arc
We think lines are not necessary to be arc if not microwave transmission or other special requirement. Arc trace occupy much space and it don’t adapt to repair at later period. Art is good for RF microwave signal transmission trace.