Many years ago I got my private pilot’s license. It took a lot of study and a lot of time training in a small airplane in order to be ready for the final exam. I had already passed the written test for my student’s license, and now I was ready for the practical examination to demonstrate that I had the hands-on skills to be an official pilot. But when I arrived at the airport for the test, the testing instructor first took me into his office and had me pull up a chair while he started to verbally quiz me. Apparently he wanted to verify that I really knew the regulations and rules of flying before he would get into an airplane with me.
In hindsight it makes a lot of sense. He wanted to verify that I had accomplished the goals that were required of a pilot first so that our time in the airplane wouldn’t be wasted. You can’t get a pilot’s license if you don’t know the basic rules of flight. In the same way we also need to verify that the trace routing on our PCB designs has been done according to the specifications that are required of it. You can’t have a functioning board built if it isn’t routed correctly for signal integrity and manufacturability. Here are some ways that we can run some routing verification procedures in PCB design with our CAD tools.
Design Rules and Constraints for Routing Verification Procedures in PCB Design
The first thing to do in order to run some routing verification procedures is to fully set up your design rules and constraints in your PCB design tools. Don’t settle for just the basics here, spend some time in setting up all of the rules that are available to you. It may seem like setting up design rule checks isn’t an actual verification process, but I would disagree. Since the rules you set up are going to give you online guidance you while you are routing, they are going to be doing the verification for you while you work.
There are a number of different rules and constraints that you can set up for your routing. The basic setups include the following:
Trace Width and Spacing: PCB design CAD tools have always been able to assign a common trace width value that is used for routing, but the tools usually give you the ability to assign unique values to either a specific net or to groups of nets as well. This also goes for the spacing of the traces, and it isn’t limited to just trace to trace spacing. There will be options to set up spacing values to different design objects such as pads and vias.
Net Classes: In order to control the settings of a group of traces, PCB design tools usually give you the ability to set up groups or classes of nets. This way you can more easily control the trace width and spacing parameters of those nets through a single setting instead of having to work with each individual net.
Area Rules: In some instances it may be desirable to have a different set of trace parameters for a specific area then what is being used for the majority of the board. An example of this is in a BGA which often requires smaller trace widths and spacing in order to route out of the pads. An area rule allows the user to specify areas where the normal rules are temporarily replaced by a different set of rules.
Trace Lengths: Some nets must not exceed a specified length, or must be held within a tight tolerance of a specified length. For these nets you can usually assign a trace length rule to them. Most PCB design CAD systems will alert you when those rules are being violated while you are manually routing them.
Differential Pairs: Some high speed nets are routed as a pair so that their return path is configured exactly the same as the outgoing signal. CAD tools will have a special utility connected to a differential rule that will allow the user to route the two nets of the pair as one entity.
In addition to these rules, there are also rules and constraints specifically for high speed routing. These would include trace length matching of multiple nets, and the ability to add serpentine routing segments to traces in order to tune them to a specific length. Another specialized high speed constraint that can be added to a group of traces is a signal path rule. Since high speed signals often go through components like a resistor that is between the signal’s source and destination, a high speed signal path rule can be made up of multiple nets. This rule allows the user to control the length and other parameters of all the assigned nets as one single net.
Setting up the values for differential pair routing in a constraint manager
Design Rule Checking of Your PCB Trace Routing
As can be expected, once your PCB design rules and constraints are all set up, you can use them to fully check your design while in progress and after completion. Running the design rule checks manually like this will turn up problems that happened during routing that you might have introduced intentionally with the expectation that they would be cleaned up later. You may also have situations where your rules and constraints changed during the course of the design, and some previous routing may now need to be altered to make it compliant.
No matter what your circumstances are, it is always a good idea to routinely check your design for errors. It is absolutely critical that you check the rules after the design is completed and before you send it out for manufacturing. Yes, the PCB design tools have online checking built into them, but it is always possible that there may be routing errors due to the reasons mentioned above that need to be cleaned up.
Design rules and constraints will allow you to verify tuned routing like this
How PCB Design CAD Tools are Equipped to Help You
The best PCB design tools use a design rules and constraint system that is simple for the user to interface with, similar to a spreadsheet. These rules will also work in tandem with other features in the PCB design tools as well. Impedance calculators will input their values into the rules and constraints system, and differential pair routers will work off the width and spacing rules that have been set up for them.
There are also other features in advanced PCB design tools that will help you to verify your routing as well. Signal integrity analysis tools will give you information on how well your routing will work for the best board performance levels, and circuit simulation tools will allow you to perfect your design before you even take it into routing.
To arm yourself with all of the advanced routing features that we’ve been talking about here, you need a PCB design system created with routing verification procedures in mind. Allegro PCB Designer, from Cadence has this advanced functionality ready to go to work for you so that you can be sure that your routing will be exactly the way you intended it to be on your final design.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.