Does PCB Autorouter utterly useless ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: March 08, 2012, 06:22:53 am
content: First of all, this thread is NOT about asking how to magically convert or lay down a pcb traces from schematic only by a single click of mouse button using the autorouting feature, and expecting it performs a magic to make a perfect pcb.I'm aware that no computer program will beat a good pcb lay out/traces made by an expert/guru, its just I want to hear from you experts or experienced users perspective, cause I've been reading and hearing that no body use auto router feature, yet almost pcb software provide it and even there are many independent routing programs out there.My question is , at which circumstances, conditions or examples in your real life cases especially in a mass produced commercial product that youreally needto use it ?Currently I'm thinking pc motherboard industry, those mobo makers are making new different type of mobos (not just small variant) so frequently like there is no tomorrow, just curious how did their pcb design dept manage that ? Again, I don't have any clue at all, but I just can't imagine it if they did it using the hard way by laying the traces manually.

author: IanB
author_rank: Super Contributor
timestamp: March 08, 2012, 07:02:07 am
content: Quote from: BravoV on March 08, 2012, 06:22:53 amIt might not be as hard as you think...many of the connections on a computer board are bus connections where 16, 32 or 64 traces all go in parallel. The number of logical connections between components is much less than the number of individual traces.

author: Psi
author_rank: Super Contributor
timestamp: March 08, 2012, 07:58:56 am
content: You get better results if you have a 6+ layer pcb.For example, if you have power and ground layers plus 4 signal layers the autorouter is much more free to lay tracks in a more direct fashion between components.Trying to use an autorouter on a 2 layer pcb just leads to a mess with tracks taking ridiculous paths.It can also take quite a while to correctly setup the autorouter and for a simple 2 layer board that may not be the best use of your time.Also autorouted boards don't have that neat/professional look that good hand routed boards do.However, when you're dealing with 64bit parallel bus lines and impedance matched traces the autorouter can save you time. For computer motherboards i imagine they route all the power and simple (non bus) connections by hand and then let the autorouter do most of the bus runs.

author: westfw
author_rank: Super Contributor
timestamp: March 08, 2012, 09:40:51 am
content: What Psi said. Modern autorouters seem to get most of their power from switching signals from one layer to another, rather than being clever about how to lay down a track on a single layer. I'm pretty sure that those 500mm*500mm 20layer boards with multiple 1000pin BGAs you see inside of SotA IT equipment were mostly autorouted (by high end packages, mind you...)On two layer boards with generally inadequate 2-layer autorouters (EAGLE), I still find them useful for evaluating chip positioning alternatives, and figuring out which signals are toughest route and might need prior special treatment, and stuff like that. I wind up with a mostly autorouted board that I've then pushed and pulled or redrawn most of the tracks, and added more by hand. The autorouter sucked, but it was still a helpful tool.

author: AntiProtonBoy
author_rank: Frequent Contributor
timestamp: March 08, 2012, 12:20:49 pm
content: Considering routing is pretty much in the NP-complete/hard territory, it's always going to be problematic for a very large number of components. Obviously component arrangement, orientation and relative proximity is going to be a huge factor.That said, I always thought that motherboard auto-routing were done in stages, or something. They would start with large and critical routes, such as buses, then big power rails, then all the remaining bits & pieces around them.

author: BravoV
author_rank: Super Contributor
timestamp: March 08, 2012, 03:46:41 pm
content: Thanks for the insight & comments, looks like the real power is at the auto routing is at setup procedures and rules and the skill of the operator who is using it.Quote from: westfw on March 08, 2012, 09:40:51 amWhat kind of "high end packages" are those ? Just curious

author: tesla500
author_rank: Regular Contributor
timestamp: March 08, 2012, 09:58:26 pm
content: Quote from: BravoV on March 08, 2012, 03:46:41 pmA couple of examples would be Cadence Allegro and Mentor Graphics Expedition. Altium Designer would likely not fall into this category, too many high end features are missing.

author: Psi
author_rank: Super Contributor
timestamp: March 08, 2012, 11:19:54 pm
content: Quote from: BravoV on March 08, 2012, 03:46:41 pmYeah that's pretty much it.

author: bfritz
author_rank: Regular Contributor
timestamp: March 10, 2012, 09:16:33 am
content: Circuits with large amounts of digital content are normally autorouted. Quite often power is also autorouted.Analog circuits tend to follow pretty logical paths that lead to a specific flow. This makes them good candidates for placement by humans. Even these I will often hand place the components, and then autoroute that small analog section. If I have placed the components well it makes pretty good decisions on how to route the tracks, so I might only need to rip up a couple and reroute them, as I wanted to give a different priority than what the autorouter guessed.For digital, differential pairs will want equal length routes. The better routing packages will recognize this, as you put some direction into the schematic to identify differential pair. Things like clocks will often have skew requirements, and a good package has you identify the clocks and their skew requirements, so the autorouter can optimize those.Most high digital content will almost be fully autorouted. I come from an IC manufacturer, so half the die area on a chip was commonly digital. We would usually constrain where the inputs to the digital would come from, and where the outputs went to. Then we would define a placement area for all the gates (we called it the "sea of gates"), having given constraints to the autorouter, and hit route. 10's of thousands of gates would then route. It was not uncommon for an autoroute to start and run for 6-8 hours. But trying to route that by hand... would probably be measured in years for a person. It was common for a package to fail to autoroute a couple signals. This may then require shoving some of the analog around to open a little more room, and then shove some of the digital away to open a path for the remaining couple signals. At the end of a layout, I've seen it take a week to figure out how to connect 3 more signals.So, autorouters are necessary for highly complex designs. I believe a license for the autorouter we were using for IC layout, designed to work with Cadence, was running about 50K a seat for a 1 year license.