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It's been 30 years since I was working closely with PCB designers, but back in the day, the professional CAD folks would absolutely run the autorouter first, an

It's been 30 years since I was working closely with PCB designers, but back in the day, the professional CAD folks would absolutely run the autorouter first, and then use that as a starting point to tweak the layout. I would have thought that autorouting would have improved in all that time, but from this conversation, it sounds like it has become worse. What gives?

syntheticgate 3y ago
I'd suggest design complexity has scaled at a faster rate than the autorouting tech. In a closer-to-DC world years ago, optimal routing was nice but often not a requirement. In today's high-speed designs, proper routing can be the difference between working and not working, and there is a constant battle of constraints and trade-offs that humans can deal with more effectively. Attempting to constrain a complex design such that an auto-router can be successful can often result in an over-constrained problem which then makes the auto-router unsuccessful. More focus on hybrid routing tools where there is some automation going on while a user routes things is typically were you get much more useful help in modern ECAD.
dvdkon 3y ago
Back then, many boards used to be routed with vertical traces on one layer and horizontal traces on another, "Manhattan routing". Somehow, the circuits worked. Nowadays you can't be so "careless" to hit EMI targets or even to get a working board. Switching PSUs, data buses, clock signals, all those have spacing, impedance control, ground coupling and other requirements. Maybe you could encode all of them in the schematic and then let a modern algorithm suggest a board layout, but in practice nobody seems to do it that way (note that I am just a hobbyist with limited experience).
- RF_Savage 3y ago
  And the eighties computers with low cost two layer boards and complex designs like that on them are one of the reasons why all products have to pass FCC Part 15 radiated emissions EMC tests, not just the intentional radiators. The old computers were rather bad on that front.

syntheticgate 3y ago

I'd suggest design complexity has scaled at a faster rate than the autorouting tech. In a closer-to-DC world years ago, optimal routing was nice but often not a requirement. In today's high-speed designs, proper routing can be the difference between working and not working, and there is a constant battle of constraints and trade-offs that humans can deal with more effectively. Attempting to constrain a complex design such that an auto-router can be successful can often result in an over-constrained problem which then makes the auto-router unsuccessful. More focus on hybrid routing tools where there is some automation going on while a user routes things is typically were you get much more useful help in modern ECAD.

dvdkon 3y ago

Back then, many boards used to be routed with vertical traces on one layer and horizontal traces on another, "Manhattan routing". Somehow, the circuits worked. Nowadays you can't be so "careless" to hit EMI targets or even to get a working board. Switching PSUs, data buses, clock signals, all those have spacing, impedance control, ground coupling and other requirements. Maybe you could encode all of them in the schematic and then let a modern algorithm suggest a board layout, but in practice nobody seems to do it that way (note that I am just a hobbyist with limited experience).

RF_Savage 3y ago
And the eighties computers with low cost two layer boards and complex designs like that on them are one of the reasons why all products have to pass FCC Part 15 radiated emissions EMC tests, not just the intentional radiators. The old computers were rather bad on that front.