High Speed Printed Circuit Board (PCB) Design Guidelines (2022)
pcb-hero.com
pcb-hero.com
Is it a feature size thing, the macroscopic size of the traces that are effectively transmission lines (that cant be, most pcbs are smaller than \lambda?). Or is it a skin effect of the conductor?
Signals travel through traces. It's just that the motion of charges in the conductor is mediated by electromagnetic forces, and electric and magnetic fields are not constrained to the conductor. Indeed, electrochemical batteries aside, most of the energy storage in an electrical circuit is mediated by fields interacting with the surrounding space.
At low switching speeds, these effects are inconsequential outside discrete components specifically designed to exploit them (capacitors, inductors). At high speeds, these small capacitances and inductances begin to have perceptible effects, for example turning the PCB a non-negligible capacitor that induces voltages and currents on other layers.
A related problem is that when trace length is no longer negligible in function of wavelength, you can no longer treat signals as changing the potential across the entire conductor in an instant. You get different voltages in different places, and all of sudden, you have to worry about wave propagation - reflections, etc.
Some of the advice in this article isn't entirely sound; for example, you certainly don't need to worry about sharp trace angles or vias at 50 MHz. Similarly to the fetish of (often discontinuous!) ground planes, there's a lot of PCB design advice that matters in specific use cases (and even there, needs to be done well) that over time morphed into a bit of a "you absolutely need to do this in your Arduino project or else".
If you click on the "Tech Tips" link on the left side of that page, you will get to his discussion of multi-layer PCBs. ("PCB Stackup").
Read This Section, and Become Enlightened! ;-)
90 degree vs 45 degree bends do not matter.
Ground planes are not a necessity for high speed digital, you just need either a power OR a ground plane referenced to the digital. They're both identical when talking about high speed.
Generally speaking (although almost no one seems to do this), you would prefer to have differential pairs NOT be tightly coupled. Tightly coupled pairs means impedance goes down, which drives very small traces on high speed signals (which, screws you if you have single-ended signals... because the traces need to be fat in comparison).
Overall not a horrible start, but it's pretty surface-level for high speed design.
The book "Right The First Time" devotes a whole chapter to debunking 90-degree bends as problematic, except under extraordinary circumstances like transmission lines > 50GHz. But ask yourself: have you ever seen something like a DDR4 length-matching snake that used 90-degree bends? Why are beveled corners absolutely universal? Is there some manufacturability problem with right angles in traces? Or are there weird problems caused by dozens of equally-spaced right angles on the same trace?
* Institutional misinformation or cargo cult behavior
* Ease of layout based on convention. If you need to match differential trace lengths, 90 degree angles are going to give you pretty big difference length wise, so you need more squiggles to compensate.
My point on 90 vs 45 is that some people seem to take the opinion that the 'ideal' trace is some sort of sweeping trace with absolutely no angles, and a 45 degree is a compromise. It's really not. It does not matter. Electrons don't 'bunch up' in corners like some people claim.