Screaming Circuits: Does Angle Matter


Does Angle Matter

It's standard practice to avoid joining small PCB traces at 90 degrees, but instead to join them at an angle. But, does it matter for thick traces?

Right angle traces Here's some 20 or 24 mil traces. Is it really going to matter, with a pad-size trace, what angle the joining trace hits the other one?

Convention would have you do something like the alternative layout on the right. Either like "A" or like "B". But, is it really necessary and worth the extra timeNot right angle traces required to do that?

Part of me really wonders and another part of me says, it shows attention to detail and implies that the entire design was produced with the same care. It's that elegance in design thing.

The other question I have relates to "B", in the image on the right. Does it matter which direction the 45 degree trace intersects? Does it matter based on the direction of current flow or does it matter at all?

Which of the three illustrated techniques do you prefer and use?

Duane Benson
Winslow Leach says hi.

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Bob - My understanding is that the convention illustrated in "A" came from the days when PCB manufacturing was still a much more manual process and not so well understood or optimized as is today.

Right angle bends used to be called "acid traps" and allegedly could lead to the risk of under or over etching. My bet is that we really don't have to worry about it anymore.

In really high speed and RF designs, things like reflection come into play, but designs in that world are a whole different animal anyway.

Personally I think the original photo is the best.
90deg angle bad? 'A' has 2x45deg angles, AND a 90deg angle where they meet.

I agree that acute angles (like the one in B) look bad, probably b/c there used to be a problem with acid traps. Acid trap folklore shapes my perception.

I do not accept concerns about high speed signal integrity issues. The feature in question is 50 mils long, very small compared to even high-edge-rate digital signals. We're talking about 10ps travel time on the trace.

For microwave RF, subtle changes in trace geometry can make a different, but I can't tell which will work just by looking.

Personally I prefer neither A nor B but instead A with the ^ notch filled. I would like to see an angular transistion area feeding the pin. "A" disrupts the flow along the trace. "B" would seem worse than the original due to lack of symmetry. Just do an inside 45 degree fill from each side on the original.

Something about the "B" just feels wrong. I've never had a problem with acid traps, but then I'd never route something like "B." I typically do something like "A" but I agree with the comments, it probably does not matter much... there was a good paper where the use of 90 vs. 45 degree angles is compared and there is not much difference until you get really high frequency.

I would think it would all be a matter of frequency. If the tracks were slow changing digital signals (if changing at all) then I think it really doesn't matter.

If you were looking at transporting extremely high speed digital, or had a board which dealt with RF stuff, I would avoid using sharp corners. Sharp corners would increase noise sensitivity and increase the chance of radiated emissions. 90 degree turns pretty much make every connected track look like a little antenna! :)

But I do agree that in pursuing "best practice" principles, the engineer should be consistent in his design choices.

D.

P.S.

On a side note, have you ever seen a board specifically using rounded corners in an effort to increase noise/emissions immunity?

Hmm, isn't "B" *worse* than having a 90 degree bend? "A" is OK, though I personally can't see what difference it would make, and yes, I've heard the various arguments.
To me, "B" is just plain wrong -- if there's still such a thing as an acid trap, the the right-hand side of the "B" junction is one for sure!

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