PCB design constrain

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Hi all,

My question is simple. Why shouldn't we use 90 degree turns in layout ? What is the exact reason behind it ?. I have tried googling it but i cant find a satisfying answer.

-Krishna Balan S

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"Heroes are ordinary people with extraordinary commitment"

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1. because it looks plain ugly,
2. with less sophisticated manufacturing technologies they could form acid traps and the strength of the trace is reduced,
3. they cause current crowding,
4. they seem to be bad for EMC and EMI.

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I believe the 4th issue raised is the key for your PCB to be working properly. At lower frequencies a 90 degree isn't going to matter much, but 90 degree turns in high speed designs will result in issues.

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It is usually only a problem at frequencies above 100MHz but is good practice to always follow. When a signal hits a corner, part of it is reflected backwards and causes serious integrity issues. The same applies for any kind of stub trace that you introduce.

I went to a seminar on USB put on by TI. The speaker said he just added a small test point to the USB lines on one of his boards and it failed to work at high speed because of the signal reflections in the trace stubs.

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So, instead of a right angle corner, you use a 45 degree segment? I remember in the days of black tape on mylar the layout guys preferred curved lines to straight angles, but they also thought 20MHz was an extreme frequency.

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Curved lines were the easiest solution in the tape days.... Not so easy now. Look at any PC motherboard and you will see how they produse(draw) clock equalisation traces...For you, just try and make 45 degrees instead of right angles where you can.

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Curved lines also have the shortest effective length for the same topology. It matters mainly for impedance controlled or high-current high frequency traces, everything else under 100MHz it really doesn't matter that much.

The problem with squared 90' angles is the bend shows a difference in impedance, as the copper is wider at the bend and it presents a higher parasitic capacitance. You can do your 90' bends, just round the corners to keep the trace width constant. Worse EMC/EMI would be a result of the change in impedance, on a badly designed board, but the bend itself has no direct effect on them.

PS.: Yeah 90' looks ugly... Rounded looks awesome, but from doing one board I can tell you they are much harder to "read" with your eyes, and harder to rework. Just not needed. They also are not more expensive, but if you have a lot of them or slimmer traces, you might not want to have the board produced by the cheapest Chinese manufacturing house. The chances for manufacturing problems increase with fancyness.

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Hi all,

Thank you all for your replies.

-Krishna Balan S

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"Heroes are ordinary people with extraordinary commitment"