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Why does a typical PCB always has rounded tracks? What harm can a sharp edged PCB track can cause? Please explain!

Anindo Ghosh
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Vinit Shandilya
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2 Answers2

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This is a great question, because the default answer is usually wrong for 99% of applications. The default answer is: to avoid reflections and other problems with high frequency signals.

The default answer assumes that you are dealing with very high frequency signals - signals with a wavelength that is small enough to fit a couple of times in your trace. When you regard such a signal as a wave, when it hits the end of a trace or a 90 degree corner, it gets reflected back and causes destructive interference with itself, attenuating the signal.

However, almost all signals you will ever route through a PCB are either DC or - in terms of these kinds of problems - very low frequency. Even 1MHz is a very low frequency and you will not run into these kinds of problems. It's 100+ MHz that starts entering into routing problems. A great example of signals that benefit from clean layout in this respect are serial buses: PCIe, USB 2.0+, etc.

This does not mean that it's good practice to make sharp corners all over the place. There are a couple of reasons why you even want DC signals and basically all your routing to have nice 45 degree angles or rounded corners:

  • First of all, board area use. 90 degree angles or worse still: more than 90 degree sharp corners will always cause longer traces (higher impedance, more copper use) than traces that try to snake around obstacles. And often, your board size is limited, so you want to use as much area for the actual components, not the traces in between.
  • Cleanliness. A clean, nice-looking board layout is easier to optimize, transfer and troubleshoot.
  • Manufacturability. This is much less of a concern than in the past, but still something to consider if you plan on prototyping this on a hand-etched or milled PCB. Sharp corners tend to get loose when milled or get under-etched when using crude manual etching methods. Fluent lines are easier to produce.

However, if you know what you are doing, don't hesitate to use sharp corners when you need them. As always: strict rules are for beginners and dumb people, once you know what you are doing you know when it's alright to deviate from the rules.

JYelton
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user36129
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    And if an electron tries to take a sharp corner too fast, it could end up flying off. – John Jul 19 '13 at 16:00
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    @John This is why I always upgrade my boards with banked traces. However they are not rated for microgravity. – JYelton Jul 19 '13 at 18:01
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    Don't forget "that's what most software does by default, and most people have no reason to do it differently" as a reason :) – Phil Frost Jul 19 '13 at 20:08
  • I am just adding few points. One way whenever there is sharp corner, charge will accumulate(electrons), they can radiate efficiently. 2nd in terms of SI issues, at the 90degC bend you will have trace width changes ==> gives impedance mismatch >>>> gives rise reflections . – user19579 Jul 20 '13 at 15:25
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    Signal Integrity expert Howard Johnson put it this way (paraphrased): "Do 90 degree turns matter? Sure, they do at high frequency. But you have bigger problems if they do, because every via you put on a board is two 90 degree turns as your trace dives down the via and onto the next segment of trace." Source: [Interview on The Amp Hour](http://www.theamphour.com/the-amp-hour-77-winsome-waveform-wizardry/) – Chris Gammell Jul 23 '13 at 20:08
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    electronons don't fly off, Dr Howard Johnson explained the answer to this questions and the surrounding misunderstandings quite clearly in his course on high speed digital design I must say. – quantum231 Oct 09 '13 at 09:15
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Electrons like any other masses have inertia and when they reach to corner conductor it is likev a obstacle for them and stack for a moment behind the corner then pass through and that corner act like a capacitor. In 90 degree (and actually in any angle other than straight path)we impose an extra capacitance to the circuit. The higher frequency the more significant capacitance.

saeid amoozgar
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    Good joke, but no, electrons don't behave like that. – fraxinus Apr 03 '20 at 15:06
  • @fraxinus While it may not really be relevant at electronics scale, electrons do indeed have inertia, which plays a large part in many physics mechanisms. – MadHatter Apr 04 '20 at 03:44
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    I am pretty sure electrons have mass and inertia, there are even a simple experiment showing it. I meant that angles in conducting path don't add capacitance (in most cases, they add inductance instead) and it is not because electrons are reluctant to take turns. – fraxinus Apr 04 '20 at 08:56