Why should I route traces by hand when I have my autorouter?

[iamericmin]

Hi everyone,

I'm a teenager and as my hobby I design PCBs. I always autoroute them since I'm already so busy with learning about new chips, programming, peripherals, etc. I mean, why painstakingly hand-route when my computer can do it for me?

However, as I quickly found out that many hobbyists and experts hand-route most, if not all the traces on their boards. This got me worried since I want to be an EE major in college(I'm a senior) and I still don't know how to hand-route. I also still don't get why I would route everything by hand when the autorouter does everything perfectly for me.

So, why should I learn hand routing and when should I get started? How do I learn it? To me it just seems like a really hard math puzzle that I will never be able to learn. It looks scary and complicated.

Thanks!

[wraper]

Because simply trowing task on autorouter generally will result in very shitty PCB. Getting decent result out of it is not trivial.

does everything perfectly for me.

Probably you just don't realize how shitty it is.

[iamericmin]

Hi wraper, thanks for the quick reply.

Can you please explain on the "shitty" aspect? Like, how does the autorouter screw up my board? I thought this only mattered for high-signal or high-power circuits.

[ataradov]

It also matters for analog performance. And if you look close enough, everything is analog.

It also depends on complexity, If you have something like Arduino board, I bet even auto-router will not screw it  up bad enough.

But you still need to do placement by hand. And once you do this, routing is not all that hard.

You learn by doing and also looking at other people's routing.

[wraper]

Hi wraper, thanks for the quick reply.

Can you please explain on the "shitty" aspect? Like, how does the autorouter screw up my board? I thought this only mattered for high-signal or high-power circuits.

Post example of your board to discuss what's screwed up there.

[pmercier]

Exemples of cases you need to route manually :
- prevent crosstalk between some traces
- artwork
- you want a specific layout or trace path around some component
- traces with controlled impedance
- signal synchronisation (some pro autorouter must be able to do that)
- HF circuits

You'll know when it's time to route your traces by hand. For low speed circuitry, low power why bother doing it by hand if the autorouter give you satisfaction for your use case ?

[ataradov]

Also, here is the example of an excellent review of a very complex board (Server Motherboard):


Chances are you will never need 90% of the things mentioned there. But it is still a worthwhile watch if you have any interest in the industry. Even if you won't use it, it will help you understand how to read other people's work. It is a very entertaining and enjoyable series.

[tautech]

Good placement is key to getting a board nicely routed.

[ataradov]

For low speed circuitry, low power why bother doing it by hand if the autorouter give you satisfaction for your use case ?

Because you will never learn to actually do it when the design becomes more complex. Of course if you never expect to run into complex designs, then it is fine.

[pmercier]

One month ago i did a breakot board for an oscillo expension port.
I was lazy and asked the router to do it for me. It worked fine when using 4 layers, but i wanted 2 and it couldn't find a way to do it. But it was obvious how to do it by hand, so falled back to hand routing.

[konzill]

Dave has a video about this:

Basically routing is in the class of complex problems that computers are still not that good at. Its just too open ended. Specifying all of the things that a human will consider when routing a board is somewhere between very hard and impossible.

It falls into the same class of problems as the https://en.wikipedia.org/wiki/Travelling_salesman_problem, except in this case you are trying to plan routes for many travelling salesman and the routes are not allowed to intersect.

[pmercier]

For low speed circuitry, low power why bother doing it by hand if the autorouter give you satisfaction for your use case ?

Because you will never learn to actually do it when the design becomes more complex. Of course if you never expect to run into complex designs, then it is fine.

I agree on this one, but everything come in time. No need to rush every competences on a teenager

[iamericmin]

Thanks. Attached below is a PCB of a wristwatch I made. It contains two 7-segment LEDs, a Nordic nRF52832, 5 more LEDs, a coin cell, and some passives. It is my most complex project to date. The PCB files are in a zip folder in the Autodesk Eagle .brd and .sch format. I also have screenshots of the schematic and PCB(without silkscreen).

I always place all my components by hand, being conscious of the autorouter and silkscreen artwork. If the autorouter struggles to find a way, I reorient some of my components to make life easier for the computer. But I never route any of the traces myself.

Here is a link to my PCB page from the fabhouse's website: http://dirtypcbs.com/store/designer/details/harambe/6335/time-machine-mk-iv

Thanks!

[ataradov]

The layout here is not pretty. But here is the thing. You are using a radio, even if it is just a module. And for radios "works" is different from getting the actual performance. It may or may not matter in your case, or course.

In this case you see traces and a via near the antenna region. Usually this is not great for performance. Typically module manufactures will specify to either keep the area clear of any traces or flood fill it with the ground plane.

Yes, section 2.3. "RF Layout Suggestion (aka Keep-Out Area)" of the module datasheet suggests a proper layout for this module. The requirements are completely disregarded here. Again, it will work, but the range is likely to be compromised.

[pmercier]

Seem to be a pretty complex layout. Perhaps choosing the pin assignation on the nrf module differently can simplify it.