Best PCB Routing Practices after Auto Routing Goes the Distance
My wife just got one of those new programmable electric pressure cookers, and she has been creating all kinds of wonderful meals with it. She instinctively knows how to get the most out of the tools and ingredients in our kitchen, while I, on the other hand, tend to create meals that feature either burnt or undercooked foods. What we’ve discovered is that the pressure cooker is not the one-button-magically-cooks-everything answer to your kitchen dreams.
Don’t get me wrong—the pressure cooker is a very valuable kitchen tool, but to be the most effective it needs to be used in the way that it was designed to be used. Innovation takes time to be flexible, I’ve found. Similarly, auto routers were considered for years to be a one-button solution to routing your printed circuit board. Someone would throw their design into the auto router and expect magic. Unfortunately, these folks were often left with a big disappointment.
Using an auto router requires that your design is prepared up front in order to get the best results. Then once the auto router is complete, you need to finish the job by checking that the routing results are what you expected and that any auto routing irregularities are cleaned up. By keeping in mind best practices for finishing up the routing of a PCB design, your PCB stew won’t become PCB slop.
Best PCB Routing Practices: Review Your Auto Routing Results
The first thing to do when you get your design out of the auto router is to confirm the routing results that you were expecting. You should not have unrealistic expectations for the router that you are using; even with the best, it is unlikely that you are going to get everything routed perfectly. Don’t set yourself up for disappointment by anticipating more than what you can reasonably expect to get from the router.
What you need to look for is how the router dealt with different areas of routing. You may have pre-routed some of the critical areas and if so is that pre-routing still preserved? If you forgot to lock down your pre-routes down before using the auto router, you may find that these traces have been re-routed and are no longer acceptable. You also may have set up rules for the auto router to handle critical nets. If so, then did the auto router do as you instructed it to?
Catching Errors: Antennas, Jogs, and Traps
Next, you will need to spend some time correcting the auto routing results. You can expect to find certain areas of routing that were mishandled by the router and now need to be cleaned up. One such area is traces that are left as antennas. Auto routers will often attempt to route in one direction, and then change their minds and go a different way. Sometimes they will leave little trace stubs, or antennas, in their wake that you should remove.
Two other similar problems that auto routers can create in your routing are jogs and acid traps. As I mentioned earlier, the auto router may switch directions while routing a trace, and leave a very ungraceful corner or jog in the trace. Not only are these unsightly, but they could cause signal integrity problems on critical nets. Auto router clean-up routines should detect and remove these problems, but it pays to keep an eye out for them nevertheless.
In the same way, an auto router may also fold a trace back on itself forming an acute angle in the routing. This could create an acid trap, a condition that could cause some of the manufacturing chemicals to be trapped in the corner resulting in a possible breakdown of the trace later on. Whether or not modern manufacturing processes still have problems with this is debatable, but since many manufacturers will flag this as an error it’s best to avoid the problem altogether.
Cleanup PCB Routing Techniques, or is There a Better Way?
The auto router may have also done some things that are not strictly bad but are undesirable. You may find meandering traces that could be cleaned up by putting them on another layer, or traces that loop around that could be shortened. Sometimes, a trace is plain ugly and you just want to clean it up. Whatever the reason, you will probably spend some time in manual routing cleanup to improve the routing that you got from the auto router.
There is another option though, and that is to use an auto-interactive router. These routers give you control over the direction of the routing by following the path that you specify for the traces to follow. These routers also give you the speed of auto routing with the precision of manual routing. They can give you better results and remove the need for a lot of the manual routing cleanup that you have with an auto router.
Your PCB design tools will shine within the right PCB design software environment. An auto-interactive router that has all the functionality that we’ve been talking about here, that allows you to create precise manual routing results at the speed of an auto router can be found in Altium ®’s ActiveRoute®.
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Would you like to find out more about how Altium can help you with its advanced routing capabilities? Talk to an expert at Altium.