
What's the deal with square traces on PCBs? - IndrekR
https://hackaday.com/2019/06/26/whats-the-deal-with-square-traces-on-pcbs/
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the-dude
Meanwhile, in Soviet Russia, traces square you :
[https://en.wikipedia.org/wiki/TopoR](https://en.wikipedia.org/wiki/TopoR)

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bcaa7f3a8bbc
How do you ensure the proper requirements for routing analog circuits, high-
speed digital circuits, or RF circuits are satisfied in a freestyle routing?

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omilu
Don't use a router for a critical circuit paths.

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cnvogel
Here's a paper that shows reflections (Fig. 3, "S11") and insertion loss (Fig.
4, "S12") on coplanar waveguides, when using a sharp 90° bend, a 90° bend with
a champfer and the case where the bend is replaced by two successive 45° turns
("final design").

The frequency scale spans 10..90 GHz.

[http://tentzeris.ece.gatech.edu/ECTC09_Rida.pdf](http://tentzeris.ece.gatech.edu/ECTC09_Rida.pdf)

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klyrs
I'm a chip designer and this article kinda cracked me up. For microwave
signals on submicron lines, losses get very significant. It seems that PCB
design inherits our practices in places these issues aren't so critical.

But then I wonder about applications. If you ignore this rule of thumb, and
gang a few hundred boards together sharing a clock... will your signal
survive?

Sometimes a 'superstition' is just common sense regarding edge cases - walking
under a ladder won't have cosmic effects, but a dropped bucket of paint can
leave a mark

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akiselev
_> If you ignore this rule of thumb, and gang a few hundred boards together
sharing a clock... will your signal survive?_

No. But then if you need a few hundred boards off a single clock I assume your
phased array radar budget can handle the extra engineering.

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markus92
What I've been taught during my EE studies, is that the main reason to avoid
square traces is for EMC reasons, mainly due to reflections due to the hard
boundaries. Interesting that the article doesn't touch on that.

PCB design/layout is one of the more important factors influencing EMC. Taking
it into account early in the PCB design phase saves you a lot of trouble
during certification.

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krapht
The article obliquely touches on it by mentioning how square bends affect the
trace resistance and capacitance.

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crististm
Yeah, it dances around the corners. Pun aside, signal reflections are the main
reasons not to do it in high bandwidth circuits. Other circuits may have
different reasons.

I'm surprised about the hand-wavy way of ignoring a very well understood
phenomenon.

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femto
I avoid right angle turns on the theory that the sharp exterior corner
increases the risk of the copper foil peeling away from the substrate.

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russdill
Nearly every single pcb designer uses square pads with 90° corners.

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coryrc
Pads are much longer than the width across the smallest corner, so if it gets
rounded a little, there's no problem. But a 90° corner for the thinnest trace
is vulnerable to being etched from all sides and getting too thin.

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taneq
There are a lot of good technical arguments raised here but the real reason we
don't use right-angle bends on the 90% of PCBs which aren't carrying serious
current or running at GHz is that right angles look ugly.

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roland35
You can't fix ugly! There is nothing better looking than a beautifully laid
out PCB

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roland35
Another important consideration is ease of routing signals during design.
Using 45 degree corners in general produces pretty space-efficient routing
that makes it easy to cluster signals together as well as move them if
necessary during layout.

One point the article left out is the risk of traces lifting off of the board.
Having less sharp corners seems to reduce the risk of this happening with
components that have a mechanical risk of coming off (like USB connectors, or
large capacitors).

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gorio
Square corners seems likely to be less space efficient, which is one
consideration of pcb design. Why we don't use round corners or fluid traces
might be a better question.

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roland35
Round corners are nice but the radius is constantly increasing so they do not
stack as well as 45 degree corners. This is especially difficult when you have
to move a group of signals together during design!

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philpem
Acid trap was the one I kept hearing, and proved true on my home-etched PCBs.
The 90-degree traces would get undercut but the 45-degree ones were fine.

To be fair it's less likely to be an issue with more advanced etching
processes, but there are still limits. The thicker the copper layer, the wider
your minimum track and gap widths become, otherwise the copper traces are
excessively undercut.

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SJSque
I don't know about PCB level, but on smaller scales you should avoid making
right-angle turns in metal lines because of electromigration [1]. Energetic
electrons flowing along the line will crash into the 'wall' at the turn, and
will knock the atoms out of place a little bit. Over time, this will lead to
'erosion' of the corner.

[1]
[https://en.wikipedia.org/wiki/Electromigration](https://en.wikipedia.org/wiki/Electromigration)

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ajross
> on smaller scales you should avoid making right-angle turns in metal lines
> because of electromigration

Someone needs to tell the semiconductor industry that, then, because all
traces inside a modern IC are at 90 degrees. The whole chip is just a big
array of parallel lines in two orientation that get cut by photolithography to
form circuits. The vias between levels are 90 degrees too, of course.

It's certainly true that electromigration is going to concentrate more at
tight bends. But in practice if it isn't killing your 7nm interconnect
you're... probably going to be fine with that 1 mil trace that is a million
times wider.

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jgeada
The reason it isn't killing that 7nm interconnect is that those semiconductor
physical implementation designers run lots of EM simulations to determine the
reliability and lifetime of all traces, taking into account the trace
dimensions, electrical properties and the expected current flows and the
design is only signed off when those metrics meet or exceed the design
requirements. Note also that specialized metals (tungsten, ruthenium) are used
on really fine lower level traces of advanced nodes, due both to their
resistance to EM as well their better compatibility with silicon (smaller
passivation thickness).

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ajross
I think you missed the sarcasm. Electromigration is simply not an issue for
PCB design, period. The relative scales of the devices are separated by _many_
orders of magnitude. The effect wasn't even _discovered_ until
photolithography pushed scales to values much smaller than we see on even the
tiniest printed circuit.

Maybe, just maybe, there's a high current, high voltage board out there with
tiny traces that was designed with some kind of trivial electromigration
mitigation. But if there is, I've never heard of it. And electromigration is
absolutely not a reason to avoid 90 degree turns on your digital logic board,
don't be ridiculous.

~~~
madengr
Electrochemical migration is an issue in high-rel PCBs, though I don’t think
that is true electro migration from charge carriers. Silver and tin dendrites
have failed satellites.

~~~
ajross
Yeah, different things. Electrochemistry involves motion of charge carriers
through some kind of medium, and can happen over very large distances even if
the medium is just the polymer surface of the board. And needless to say it
doesn't care about junction angle at all.

"Electromigration" is the motion of individual conductor nuclei out of the
wire due to scattering with electron momentum. Effectively, when electrons are
asked (by the electric field in the conductor) to make ultra-sharp turns at
nanometer scales, they sometimes collide with the
aluminum/copper/tungsten/whatever nucleus instead, knocking that nucleus away
from where it should be in the crystal. Over time, this erodes the junction,
increasing electrical resistance, and eventually the chip starts to fail at
voltages and clock rates it used to handle well.

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amelius
Conclusion:

> The reason we don’t see square traces in most PCB designs is simply due to
> aesthetic convention. If it looks wrong, it is wrong. There is no specific
> reason why every circuit board design should shy away from 90-degree angles
> on traces, it’s just something that you shouldn’t do because it looks wrong.

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jiveturkey
It does matter for RF, as TFA says. So the tools do it for you. Since it's
automatic and easily done with the tools, everyone just does it.

Sure, you don't have to shy away from 90° but you also don't have to avoid it.
I strongly disagree with "looks wrong". It's simply an aesthetic choice, there
is no right or wrong about it.

I wonder if they did this kind of optimization back in the day when traces
were hand-taped. The sensitivity is to rise time, not actually frequency, so
even for the slower designs of the day, maybe 45° or arced corners were still
useful?

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krapht
Huh, article says not to worry about traces unless dealing with 100 MHz+
signal lines.

Isn't that very common? Hell even the MSP430 is running at 25 MHz these days.

And for all the other reasons listed in the article, you should avoid sharp
corners. It's lazy to do otherwise when mitred 45 degree bends or curves are
so easy to generate in a CAD tool. It's the hardware version of a code smell -
the board house (compiler) might not care, but it should be fixed - don't give
up freely obtained manufacturing slack because you couldn't set up a trace
bend rule.

I kind of feel like PCB layout ought to just be done by the antenna engineers
for any serious product with wireless comms or high speed I/O.

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IshKebab
100 MHz in traces is very uncommon for typical DIY PCBs. The only place I can
think you'd get that is in a microcontroller clock, but most people use
modules or internal clocks.

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nitrogen
Or USB 2.0 if you are using a USB-capable microcontroller.

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IshKebab
USB 2 High Speed support is still rare in microcontrollers. Most still only
support Full Speed which is only 12 Mb/s.

(If anyone is looking for this btw the best option I found is Atmel's SAM ARM
microcontrollers, e.g. as used in the Arduino Due.)

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Hendrikto
Why does the author keep saying “square”, when what he actually means is
“right angle”?

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alanbernstein

      adjective
    
      having or forming one or more right angles or being at right angles to something
    

According to
[https://www.dictionary.com/browse/square](https://www.dictionary.com/browse/square),
this is British usage, but I've often heard and used it in the US.

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kijin
The image at the top of the article shows a number of traces that look like
they're trying to climb a steep hill. I don't know much about PCB design, so
my best guess is that those squiggles are trying to match a certain latency.
But if 90-degree turns are so meticulously avoided in the industry, why is it
okay to do multiple 180-degree turns in quick succession?

~~~
DanBC
The reason those traces are shaped like that is to keep them the same length
as some other trace.

This question has some answers:
[https://electronics.stackexchange.com/questions/74789/purpos...](https://electronics.stackexchange.com/questions/74789/purpose-
of-wave-shaped-pcb-traces)

Designers would say that none of those corners are right angled, and that they
each have a bit of bevelling.

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childintime
Related: looking at the main image, there are lots of places where traces
routinely deviate to go around a pad, with an extremely low clearance,
needlessly so. The designer probably took a defensive approach: the traces
have been routed to leave as much space as possible to easily allow additional
traces in the same gap.

So to me this pcb is a rough draft, an unoptimized initial version. The
optimization obviously wasn't worth any time (and I agree with that), but I
think an EDA tool worth its price should take such a design and optimize it.
Such details really aren't the job of a designer, but still have the potential
to significantly improve a layout.

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IndrekR
On this board it looks like just a functionality/beauty thing and does not
matter much. But depends on the target. While for the pads I agree, spacing
can be optimised, then for the densely routed areas _space_ is your friend. At
higher frequencies (shallow skin depth) most energy is guided by the trace,
but transferred in the space. Traces become bounds of transmission lines along
what transverse electromagnetic waves are propagating. This is the reason why
dielectric constant and dielectric loss matter a lot at higher frequencies;
and why good impedance matching is needed for efficient energy transfer.
EMC/EMI/RFI is mostly energy management issue.

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EmpirePhoenix
Um wait, the main reasons for I know about is, that square corners are a huge
issue with higher currents, as they tend to burn from the inside out, due to
the lower resistance in the corner and non equal flowing of electrons.

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bookofjoe
When I (non-techie) first read the title, I thought this post would be about
strange shapes on chromatography results measuring pesticide residues.

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dillonmckay
This made me laugh.

PCBs can come from PCBs, though.

It is a valid interpretation.

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madengr
I take issue with this article.

Electrons do bunch up on sharp corners, as far as high voltage design is
concerned.

Sharp corners cause inductance when the current has to “turn the corner”. An
infinitely sharp edge would have infinite inductance. Think of a charge having
momentum, and the H field the result of that momentum. You can’t just change
it.

This is more troublesome at RF due to the currents crowding at the edges of
the line, where the path length is now different for inside and outside
radius, irrespective of lumped discontinuities.

In the RF world we mitre the bends, and sometimes used swept bends. The swept
bends supposedly radiate more than mitered, but have not proven that to
myself.

The case where you do want sharp, 90 degree bends are on electrically small
monopole type antennas. The inductance will compensate for the high capacitive
reactance of the antenna.

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aswanson
Have you ever done a design specifying molex connectors?

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madengr
Most have a Molex brand connector, though nothing > 50 VDC. I use a lot of the
Molex Pico-Lock connectors, and similar sizes; all board level stuff.

What I was revering to in the above post is the susceptibility to breakdown
and corona discharge from sharp edges (example at link below). I have done
some static E field modeling in CST for HV breakdown in potted modules.

[https://www.comsol.se/paper/modeling-of-avalanche-
breakdown-...](https://www.comsol.se/paper/modeling-of-avalanche-breakdown-in-
silicon-and-gallium-nitride-high-voltage-diod-66642)

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srfilipek
For simple and slow communication busses on hobbyist boards, sure it probably
doesn't matter.

But I really can't trust this very superficial attempt to prove a negative
with regards to a very complex phenomenon that I do not think he understands,
especially since he mentions nothing about inductance, reflections, or
impedance changes.

Hell, RF experts with decades of experience will still find new corner-cases
and issues with PCB layouts when it comes to analog signals.

In terms of "actual wisdom" regarding 90 degree traces: Some time ago at my
job we had to actually scrap a PCB because of right-angle traces causing too
much interference. The end effect was a comb filter on the signal and a ton of
emitted RF interference.

TLDR: Yes, the phenomenon is real. Do not trust these hand-wavy attempts to
dismiss it.

~~~
everybodyknows
First sign of the author's confusion:

>a square of copper two millimeters on a side has the same resistance as a
square of copper ten millimeters on a side. It’s counter-intuitive.

The phenomenon is perfectly intuitive for anyone who understands DC resistance
of metals, and the geometric assumptions of the "sheet resistance" thought
experiment.

Resistance is proportional to conducter length divided by cross-sectional
area. For a sheet conductor, cross-sectional area is thickness times width. A
necessary assumption, unstated in the article, is that the hypothetical test
circuit somehow makes perfect, uniform electrical contact over the entirety of
both of the sheet's chosen "width" edges.

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DanBC
You can fit more in if you avoid 90 degree corners. You have shorter traces
too.

Also, auto-routing exists so most people aren't handcrafting these corners.

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abetusk
TLDR "conventional" wisdom about square traces having adverse effects on
introducing noise for high frequency lines turns out to be negligible. Instead
square traces were/are frowned on because of acid pooling from the etching
process for making PCBs.

