To Autoroute or Not To Autoroute.. That is the question.

[MyElectronsFellOut]

When laying out your PCBs, how many start with the Autorouter?  I personally dislike it and think it just makes a mess of the board. (the auto populate is even worse) but I've only used Designspark and Eagle.
I prefer to work though components and circuit clusters systematically and place and route as I go. Having said that, I've only ever done 4 layer small to medium sized boards.

What are the advantages / disadvantages of the auto router ( and/or auto place) ?

[BradC]

I always start with the autorouter, have a cup of coffee & a good laugh before I un-route everything and get started. Sometimes during a session I'll make a backup and run the autorouter to complete my work, then restore the backup. It's not uncommon for me to get some ideas from the auto routes, but I always go back to my manual process.

Sometimes (very occasionally) it does inspirational work, but it's more often amusing.

[T3sl4co1l]

I only use it for complicated boards where routing a great many nets is a chore (typically digital logic, buses and interfaces).  I usually route everything around the area of interest first, and exclude those nets from the autorouter.  (Examples: support components, power converters, supply rails, etc.)

Significant cleanup is always necessary, and it feels like 10-20% savings in space and average trace length can be had by inspecting the results and shorting across "dumb" decisions.

Starting with a routed layout, even poorly done, can be valuable.  It speeds up subsequent changes or optimization, because you can see at a glance where everything needs to go, and how the existing layout is wrong (electrically, or to your aesthetics) and how to fix it.

Tim

[Moriniman]

Manual routing every time.

If you take the time to set up all the rules, you can get the autorouter to do a mediocre job.

I used to use Altium and we produced our own boards, so for double sided you'd often utilise component legs as vias. The AR can't do that.

I'd spend quite a bit of time on placement simplifying the rats nest, pin/part swapping as needed, then put on a set of headphones load up a CD and go into the 'layout zone'.

[georges80]

Anything analog needs manual routing.
Anything with power supply circuitry needs manual routing.
Anything with high speed signals/clocks needs manual routing.
I've used contractors for many years to do board layouts and they've always manually routed whether 4/6/8 etc layers.

Modern chips typically use high speed serial interfaces so that needs manual routing (impedance matching/trace length matching etc). Anything that uses DDR3/DDR2 requires trace length matching.

To ensure minimal EMI, impedance matching is needed as well as how traces do or don't cross layers and how ground stitching is performed. These are all manual layout necessities.

For placement just turning on the rats nest allows pretty obvious decisions to be made. No need to create an autorouted mess.

Anyhow, that's my experience.

Maybe try to auto route if you are doing a z80 board that runs at 4MHz

cheers,
george.

[MyElectronsFellOut]

I'd spend quite a bit of time on placement simplifying the rats nest, pin/part swapping as needed, then put on a set of headphones load up a CD and go into the 'layout zone'.

Ahh the layout zone... Good suggestion with the headphones. Might try with music next time.

I always try to challenge myself to route as much as I can on the same layer. For instance, I've just put together a small analog circuit to generate a 4 - 20mA and 0- 10 V output using two 8 pin op amps plus passives, mosfet etc.. I managed to route that entire part of the circuit on the same layer with strategic component placement.  The inputs to the op-amps are PWM generated (passed through a crude ADC consisting of a resistor and capacitor) and that meant I could bring the PWM inputs out on the other side of the board as close to the ADC resistor / capacitor as possible before passing them through a via.

[mazurov]

Auto is good for tasks like adding some last-minute gizmo to almost finished multi-layer PC motherboard.  Another good application for autorouter is when you only ever lay out PC motherboards, say, or some other specific type of digital, like an oscilloscope back end. You won't learn much from autorouter either, don't use it if you are a beginner.

[naxxfish]

The only time I'd ever trust an autorouter would be if I had already manually routed all the important sections and signals and was left with a couple of relatively long distance, low speed signals that I don't really care much about.  Other than that, more trouble than it's worth.

I have tried to use the autorouter in OrCAD, Eagle and KiCAD, but every time ended up spending more time ripping up stupid tracks than I would have doing it manually in the first place. 

[Alphatronique]

always manual route  ,that permit to fine tune the placement  as you route  and also catch potential design error

i use 2 monitor 1 for PCB and one for Schematic this way you have better whole picture that a dumb auto-router never have

[mikeselectricstuff]

A large proportion of the work is placement. An autorouter might make a half-useable job of routing a well placed design, sometimes, but even then by the time you've set up all the rules and fixed the dumb things it does, you might as well have done it by hand, and in the process improved your PCB skills.
 
..and show me an autorouter that can say "I know I can route any of these lines to any of these pins and sort it out in software/FPGA pin mapping, so I'll map them in the way that routes the cleanest"

PCB routing is one of problems that people are just better at doing than computers most of the time. 

And as for auto placement...

[metri]

I don't really like the autorouter. I always manually route everything i the end. I have run it to see if things are easily routable or I have a problem I need to solve, but I don't like the results. I use the unrouted signal wires in eagle to group components in chunks, then move the chunks around and route them all together.
I wouldn't go so far as to say the auto router isn't useful, but I think you get a nicer board if you do everything manually (for small boards anyway). 

[mikeselectricstuff]

The only real use I can think of for an autorouter is to get a very quick idea of how routable a board is, though with experience you can usually judge this by looking at it and juggling the positions of major parts around.

[Howardlong]

Back when I did stuff with wide parallel busses, yes, it did seem to make some sense at the time, but I was always somewhat disappointed with the result. Perhaps it was the algorithm, I don't know, but invariably most of it was ripped up and done manually.

Nowadays, with mixed signal, largely serial busses and much higher speeds, the value of an autorouter has diminished in its entirety for me, I haven't used the feature for about a decade.

[DanielS]

I do not trust autorouters beyond filling in what should be obvious routes once I am done routing what I consider critical routes or routes that I do not want subjected to auto-router weirdness.

Most of the times I let auto-route run though, it is mainly out of curiosity to see how silly the auto-place/auto-route decisions will be. Sometimes it gives me different ideas about how to place or route a board but most of the time, it just makes me /facepalm from how hopeless the layout looks.

[zapta]

I always autoroute (Eagle). I first route manually the critical nets (~25%) and then let the Eagle auto router do its thing. I don't care about aesthetic traces, just correctness and functionality.

[tonyarkles]

I'm relatively green as far as routing goes, and as others have mentioned, it's pretty clear that good placement is critical for routability. I'm still learning what makes good placement and what doesn't, so I'll use the autorouter as a gut check to see if I've done a decent job on placement (or have a part somewhere else on the board that I didn't consider)

If the autorouter hits 100%, great, rip it up and start trying to do it manually. If the autorouter hits 98% and has some long ratsnests left over, that's a pretty good indicator that there are parts that should be moved. If the autorouter can only hit 50%, well... Time to reconsider how things are placed

After I'm pretty comfortable, I'll rip the autoroute up and do it by hand. If there's some low-speed headers that have to go to the edge of the board, I may let the autorouter handle those at the end, but definitely not for anything critical.

[Landrew2390]

I use autoroute to check the layout of the board.  If it can't come up with a 100% routed board, I start checking the nets that Eagle can't route for placement problems.  99% of the time, it's because I set a width on power traces that was wider than the pitch between the pins on TQFP part.

Standard procedure for me is start autorouter, go eat dinner, take the dogs for a walk and then rip up half the stuff autorouter did.  On smaller boards (<40 parts), it's worth using.  One most of my current designs (+250 parts), it's a waste of time.  It saves maybe 5% on the total development time for me once I fix all the dumb routing mistakes.  Put on some good music and do it manually.

[IanJ]

Hi,

Use the ratsnest to give you ideas on placement and when you should think about pin-swapping.
If the ratsnest is a complete and utter mangle AFTER you have placed your parts then there is a fair chance the layout isn't optimized and you'll have trouble routing manually.
If placement is good, pin-swaps are optimized then manual routing should be much easier.

I never auto-route.........but I do think that somewhere down the line (years!) one of the software houses will make a step change and auto-routing will be much better.........but it will only happen with a step change in auto-placement as well. The software has to mimic man.

That's how I see it anyways.......

Ian.