Multisim and Ultiboard
Autorouter oddity
david_c_partridge,
Yes thanks for your observation and besides those already given here a few tips for you and others using the Ultiboard autorouting capabilities.
Straight line routing - make the autorouter 'dumb'...
For something like this - where it appears that a straight line is obvious - what is basically happening is that based on your 'Cost Factor Settings', the autorouter is trying to over analyze the traces and optimize the trace positions. In this case, I would make the autorouter 'dumb' by setting all of the settings on the Cost Factor tab to their lowest setting '0' or '2' in some cases and you'll see better results for straight line routing. When you set the cost factors to their lowest setting you're basically saying to the router - 'do whatever you need to do, don't overthink it'. Then if you see something not 'behaving' you can increase the cost factors for that particular item and it will try to pay attention to that setting more (also see about piece-meal autorouting as well). You can always go back to 'Default' if you need to...
General Tips on Autorouting...
Grid / trace width & clearance issues: Generally, if the default copper grid for your design is too coarse (Options -> PCB Properties -> Grids & Units and then Grid step name: Copper grid), you will get poor results and the autorouter will not complete if you are using grid based routing. I generally use the autorouter with the 'gridless' setting which does make sense most of the time for SMT designs (I see that you've set this to gridless). By the way for designs with fine pitch parts, I usually set my copper grid to 1 mil to accomodate the precision required in routing those types of designs. Also make sure that your trace width and clearances are set (in the bottom spreadsheet view) so that the autorouter can accomodate traces running between pins (such as with a connector) if needed.
Route critical traces first: You'll probably want to hand route (manually) any critical traces first. These might include low level analog traces, high speed traces (differential pairs) or traces that need to be kept away from noisy areas of the board. Also it is a good idea to do all of your inner ground/power planes first before routing if possible, especially if you have specific locations or requirements for your planes (in a multilayer design).
Managing nets and planes: In designs with 4 or more layers, you can usually do yourself a favor (and the autorouter) by constraining nets to run on certain planes and where it makes sense (such as not through a ground or power plane). You can manage the routing layer on a per net basis in the spreadsheet view -> nets tab. You'll need to include the top or bottom plane, depending on the SMT part location, if the net is originating from a SMT pad.
Autoroute in a piece-meal fashion: You can select certain nets in a piecemeal fashion and this can sometimes be an important strategy for leveraging the autorouter to assist you in routing critical areas of your design first. For instance if you click on a part or multiple parts, you can automatically route all traces attached to those parts by selecting the Autoroute -> Autoroute Selected Parts. Similarly if you want to route specific traces, you can highlight the nets (in the spreadsheet -> nets tab) and then select Autoroute -> Autoroute Selected Nets.
Regards,
Pat Noonan
