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PCB design? Answered

Why do modern PCBs always use straight tracks and 45-degree diagonal tracks? I know it is good practice and design all my boards to this standard, but is there a reason behind it other than it looks visually appealing. It appears only relatively modern boards took this into account, I was looking at some 1970's electronics and quite a lot of them have rounded tracks of no definite thickness.


Here's one interesting fact about this. I was at an EMC seminar once, and the presenter brought up a commonly stated reason for using the 45 degree corners. The reason was that supposedly sharp 90 degree bends would reprsent an abrupt discontinuity to the characteristic impedance and cause reflections on the line.

The presenter had done considerable reasearch and experimentation with EMC issues of high speed circuit PCB layout over his career, and he had found that there was in fact no appreciable difference. He had found the text that was the origninal source of this common wisdom, and he contacted the author. The author acknowledged that the EMC impact was in fact neglibible, yet the notion of EMC improvement persists due to so much other EMC best practices literature repeating it over the years.

I was always taught the EM theory was crap, but that it helps track adhesion.

I've heard that as well. The fellow who gave the presentation did acknowlege that there were other possible, sound reasons for using 45 degree corners, most likely related to manufacturability. It was just the supposed EMC justifications that he was trying to dispell.

True, although once you get up into the gigahertz range a track isn't just a track - It's a resistor, a capacitor and an inductor as well.
If you look inside something like a satellite LNB you'll see tracks which go nowhere and all sorts of weird squiggles.  High frequency RF design is a black art to the normal logic designer.

In the good old times, electronic parts where large, circuits had (relatively) few connections and boards were routed and painted manually - freestyle!

Then came the digital parts, ICs still big as DIP but with lots of connections and circuits with a lot of traces. Luckily the first computers could help with routing PCBs. But those machines didn't have a lot of memory, so designing in a grid style was the easiest way. One side of the PCB for up-down, one side for left-right traces. Used a lot of interconnections (vias) but worked. And - the devices to produce the PCBs were state-of-the-art, but compared to todays plotters pretty simple. A set of given shapes for pads and some fixed width stencils that could be moved to produce tracks. Moving only one direction at a time (straight tracks) was easier than moving in two axis simultaneously.

Flash forward to today - We have super fine-pitch devices with some hundreds of connections and thousands of parts. But we also have grid-free autorouters and enough processing power to design the PCBs. And there is still some processing power left to do some nice stuff like 'avoid 90 degree bends', 'fill unused areas with GND' or 'make sure the vias and copper area under ICx allow the transport of y W dissipated heat'. Not to speak of the modern PCB machines that can generate any shape.

So coming from hand made stuff through a time where straight lines were a necessity we now are at a point where straight lines are still a good idea but not a must.


7 years ago

A round PCB. A

I think you misunderstood the question. I am asking about the actual traces on the board being rounded, I have seen (and designed) many circular boards before. orksecurity and seandogue pretty much answered that.

You are exactly right, it was the fellow from Missouri. I think his name is Lee Ritchy, or something similar. He said he had worked on radios that went to the moon, so he must have had some relationship with NASA at some point.

The seminar I went to was just a few years ago, and the guy looked to be about in his 70's. But he was sharp and put on a very informative seminar.

Most modern PCBs, other than the most trivial ones, have their masks generated by CAD/CAM software, which is generally going to go for the simplest straight-line solutions.

When folks were designing PCBs by hand, they sometimes optimized for minimum etching -- which is why you get the irregular lands rather than traces.

In multi-layer boards, having wires cross at 90 degrees minimizes the risk of crosstalk between them. Which may or may not matter, depending on the circuit.

Ahh I see, It makes more sense now. My CAD software actually enables you to add non-straight paths, but I have always stuck to straight, even when designing on paper.