Trusting the Autorouter

In this chat, we welcome Ben Jordan from Altium to discuss routing technology and PCB design

Friday, February 23, 2018 12:00 pm PST - Friday, February 23, 2018 12:30 pm PST Local time zone:
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Ben Jordan will be co-hosting the Hack Chat this week. This Hack Chat is at noon PST, Friday, February 23rd.

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An autorouter is a magic tool that many think of as a way to allow an engineer to complete a PCB design with just one click. Sadly, this is a misconception. An autorouter is an EDA tool add-on intended to reduce the cost of producing a finished PCB layout, but an autorouter is just a tool, and it will do what the designer tells it do…. therefore the designer needs to understand how the autorouter works in order to direct it properly.

In this chat, we welcome Ben Jordan from Altium to talk about Altium Designer and CircuitMaker. To get the best performance out of an autorouter, it's good to understand how the software works behind the scenes. If you're interested in trying Altium Designer, you can download a free trial.

Ben Jordan is Director of Community Tools and Content at Altium. Ben is a Computer Systems engineer, with 25 years experience in board-level hardware and embedded systems design. Ben is also very much a maker, having picked up his first soldering iron at 8 years of age, and cutting his first lines of assembly language at 12. Ben spends any spare moment playing with audio electronics and playing guitar.

In this chat, we'll discuss:

  • Routing technology 
  • Multiboard design
  • Multilayer design
  • Mixed signal design

  • Trusting the Autorouter Hack Chat Transcript

    Lutetium02/23/2018 at 21:10 1 comment

    Sophi Kravitz says:

    for those who are here now, the reason I asked @Ben Jordan to join us was because of the last EDA chat where a bunch of you were talking about autorouting

    in a not so favorable way :)))

    I thought it would be excellent to discuss from the standpoint of understanding how it works

    @Ben Jordan welcome! please introduce yourself!

    Ben Jordan says:

    Hi all, thanks for coming. I've worked at Altium for 13.5+ years

    I *was* an FPGA designer, then R&D applications engineer, then FAE, then product manager

    I've seen a lot change in EDA, and worked with some cool people including @technolomaniac who's now with Autodesk, and Dave Jones from EEVBlog. Heard every opinion there is about auto routing tech

    This topic is contraversial

    Sophi Kravitz says:

    @Ben Jordan why do you think it's contraversial

    Ben Jordan says:

    Most professional PCB designers I have talked to would not use an autorouter. They wear the T-shirt that says "never trust the autorouter".... and usually for good reasons

    But realistic people, also know if you have computers, you should use them... right?

    First, let me 'splain some autorouter history from our side…

    Sophi Kravitz says:

    our first q comes from @Lutetium : Where can someone who has felt duped by the autorouter time and time start to begin seeing it the useful tool it was meant to be?

    Ben Jordan says:

    Excellent question

    hopefully knowing the how it works a bit will help with that

    First, let me 'splain some autorouter history from our side...

    First there are different types of autorouters: Grid, Rectilinear, and Topological are the main ones

    Grid routing is the oldest tech, and what you'd find in most older PCB tools. It works like this:

    12:13 PM

    So everything's on a grid

    The router is a dumb algorithm that leaves the net's starting pad horizontally or vertically (can be arbitrary or biased) then takes right-angles to get around where it has to go. If an object (like a component pad) is in the way, it will take a left or right turn based on the overall direction.

    Simple, fast, but low completion rates, and wasteful of space.

    Grid based routing worked really okay for 1970s and 1980s boards which were mainly through-hole, and introduced the notion of path costs, bias, decision making

    Dave Blundell says:

    this is my autorouter experience, in a nutshell.

    Ben Jordan says:

    you can see why most poeple said, yeah, I like the idea but this can't cut it

    Sophi Kravitz says:

    but that was 30 years or more ago

    Ben Jordan says:

    yeah. so Autorouters back then were $$$$$$ add-ons to CAD workstations. A few were sold and then a few years went by and they were abandoned in favor of human labor

    Grid based router was in old Protel, old P-CAD, and older versions of EAGLE as well. So then, Cooper and Chyan Technology  (now part of Cadence) came up with "Rectilinear" routing which did away with the grid.

    Rectilinear routers divide the board geometry into rectangles with no grid (hence the name)

    routes are still mainly done horizontal, vertical, etc. but more room was available.

    For each trace being routed, it would create a new rectangle array of the board map.

    Much better completion rates, but still suffered problems because the further the board was completed the more crap was in the way.

    And the results never "looked" like what a human would do. And in spite of all practical opinions, apparently it matters what the routing looks like, even though the board will not likely be seen by humans once it's in the final product ;-)

    So the next evolution came in the late 1990s (still, a long time ago!) with "topological" autorouters

    A topological router (like the ones currently in EAGLE, PADS, Altium Designer, CircuitMaker etc) maps the board space with polygons, usually triangles:

    darryln says:

    12:24 PM

    (CCT router in 1996 sucked mightily, trust me)

    Ben Jordan says:

    A lot of people abandoned automation for PCB at many times throughout history

    So that's the triangle-based topology...

    Read more »

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Paul Stoffregen wrote 02/23/2018 at 20:54 point

For those skeptical of trusting autorouters, especially based on old experiences, are there success stories to casually view?  (sort of the opposite of the T-shirt)  I suppose like computer benchmarks, there's also the matter of how trustworthy they are?

  Are you sure? yes | no

John wrote 02/23/2018 at 20:50 point

Do you have any tutorials or lessons so we can begin to use the new AI autorouter?

  Are you sure? yes | no

Frank Leon Rose wrote 02/23/2018 at 20:49 point

Another question: Given a choice, would you prefer a "like a human" layout or one that is objectively superior in terms of board area, minimal material use, etc.?

  Are you sure? yes | no

Frank Leon Rose wrote 02/23/2018 at 20:47 point

The problem is difficult enough when you are exploring just the space of physically&electrically *valid* layouts of a complete board. If your autorouter is creating invalid layouts that seems like a qualitatively different problem. Why is creating a valid layout so difficult?

  Are you sure? yes | no

John wrote 02/23/2018 at 20:40 point

Can you describe the decision making process of when it would make sense to start using the autorouter functionality? Does it depend upon the number of nodes that require routing? Does it depend upon the physical density of the nets on the board? Is there a time when the autorouter is the only method to complete the routing? Is the autorouter a crutch for the newbie pcb designer? Is the autorouter a black magic art to setup and to run?

  Are you sure? yes | no

toybuilder wrote 02/23/2018 at 20:04 point

Is there a good way to handle things like a GND class w/ fat traces for power distribution AND still have ground for (say) pin-strapping with a normal trace and handle them (semi-) automagically?

  Are you sure? yes | no

anfractuosity wrote 02/23/2018 at 20:01 point

Given enough user specified parameters, such as current requirements etc. will someday it be possible to get human-like autorouting ability, possibly by using machine learning?

  Are you sure? yes | no

Lutetium wrote 02/23/2018 at 19:52 point

Where can someone who has felt duped by the autorouter time and time start to begin seeing it the useful tool it was meant to be?

  Are you sure? yes | no

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