We put this webinar together to address the complex issue of autorouting. It is a ‘high level’ presentation with the purpose of looking at the history and algorithms that are commonly used. As you will see, the effort is not simple. It's one thing to route a particular line; however, planning and prioritizing the routes are difficult. It is much like the old ‘chicken and egg’ conundrum. The router must know the congestion points to prioritize routing; however, it does not know these congestion points until it starts to route.
We must also understand that the placement of components and the real estate also play a role in the ease or difficulty autorouting. These are factors that are controlled outside of the autorouter.
The biggest effort in autorouting is constraining it. There need to be rules in place to ensure that the routes are viable. There is a delicate balance. The more we constrain it, the more difficult it gets for the autorouter to complete the route.
More importantly, we must consider the time it takes to autoroute. It is a rare to get the routing correct on the first attempt. This is the process of reviewing the result, modifying the rules or constraints, tweaking the order of the strategy, and running it again. When the autorouter is done, the work must be reviewed. There will be odd routing formations that are ‘legal’ within the constraints but would be considered poor routing practices if routed manually. The question is whether one wants to visually pour over the layout and correct issues or simply hand route each line and review the work as they are progressing.
In the end, we must decide whether or not to use an autorouter. This question is completely dependent on the board real estate, the number of components and their pins, the clearances allowed, the strategies available, and the number of layers within the board to operate. Though autorouters are used, the vast majority of boards are still routed by hand. The human mind is still the better ‘autorouter.’
Several EDA companies like Altium have moved away from the all-encompassing autorouter. An all-purpose router becomes a no purpose router. There are too many scenarios and factors to make this viable. In recent times, they have introduced local routers which let the autorouter focus on a very specific location. Once the router has completed the task in the space provided, the user can immediately review and clean up before moving on.