25 March 2018

Right angle is often a source of concern for SI and EMC concern  

If you ask an electronic engineer his opinion on having a right angle bend on a track,  the most likely answer will be that it is a bad idea  because 90 degree corner can create a local change in impedance and be the cause of reflections and therefore Signal Integrity issues.

But is this true?

Douglas Brooks, which  without doubt is an authority as PCB designer, wrote an interesting article  called "90 Degree Corners, The Final Turn"  In his article Brooks shows how he tested a 90 degree tracks on a PCB, he shows the test setup and the results of his test.  His result showns that  a 90 degree tracks will not create any harm to your signal. The same test has been then repeated from other authorities and all arrived to the same conclusion.

"A 90 degree bend will not cause any measurebale  Signal Integrity issue or EMI issue."

But if this is wrong... "Where this rule of thumbs come from?" The author of  this excellent book "Right the First Time"  believes that comes from an flawed measurement published on the Motorola ECL Systems Handbook. 

Vias are the second source of SI and EMC concern.

This is another incorrect rule of thumbs and I strongly suggest to read chapter 25 of this excellent  book: "Right the first time". Where the author explains that a vias add a capacitive effect on a track that is usually neglectable to the capacitance of the track itself and therefore it adds a little contribution to SI distortion. 

Therefore : Lest stop back-drilling vias on your PCB!