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PCB traces - 90 degrees or not?

T

techfuzz

Diamond Member
I have a question that I hope some EE people can explain to me to reasons why.

Using a PC's motherboard as an example, I have always thought and read that the traces on motherboards cannot "turn" at 90 degrees (or less) otherwise bleeding or other data corruption could occur between lines. How is it that on these motherboards that traces are able to move from 1 layer of the board to the next? One would assume by looking at the top of a motherboard and the bottom that traces moving from the top layer to the bottom are turning at 90 degrees (perpendicular to the layers)?
 
What do you mean by "cannot turn at 90 degrees"?
I work at a company that makes pcb's and i have yet to hear of this.
 
Generaly, 90 degree bends are frowned upon because the trace can get thin and/or break if there is a problem or you are using a cheap PCB manufacturer.. Try to do 2 45 degree angles if possable.

<edit>
90 degrees verticle is totaly different. The holes are plated through. turning between layers is not a problem.
 
90 degree bends aren't used as much because of current crowding in the inner corner of the bend. It's okay to use them for low current/signal traces, but for high current, designers avoid them for that reason. Because of this current crowding, very little current is passed through the outer edge. And besides, why make a 90 degree bend when you can make two small 45 degree bends, or even three 30 degree bends?
 
Cheap tools.

Miters and nice rounded traces are very nice. Sharp transitions, or even minor, can also result in impedance mis-matches which can cause reflections, ringing, and all sorts of neat effects.
 
To add to this, electromagnetic compatiblilty and compliance is also a general issue with signals being routed at 90deg angles. I'd have to go back and look it up, but it has something to do with too much EM emissions i think.
 
Don't you get weird magnetic interference from 90º bends? I think I remember that from Physics E&M last year.
 
So how is it that these traces are able to bend at 90 degrees when moving from 1 layer to the next? How come the same rules do not apply?
 
I think the annular ring around the via has something to do with this.
I made this up in cam software, the yellow section is the annular ring and the green circle is the hole/via.
If a signal from the bottom of the annular ring were to reflect/bleed when going across the 90 degree bend it would hit the other side of the annular ring or the inside of the barrel, so it wouldnt really interfere with anything else.
I might be pulling this from hot air since i havent heard of this "problem" but you really are nitpicking here.
 
it doesn't apply to through holes/vias because in a 90 degree bend, the cross sectional area available for current to pass depends on the thickness of the copper, while in a through hole/via, the cross sectional area is the width of the via/throughhole component lead. That's my explanation 🙂
 
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