Indiana Jones was not the biggest fan of snakes. If you remember his signature line from “Raiders of the Lost Ark” he bemoaned; “Snakes, why did it have to be snakes!” I have to agree with him, I would rather not have any close encounters with those particular reptiles either. But snakes can serve some useful purposes. They keep all kinds of pests in check out in my garden, and I’m certainly glad that my plumber has a snake when he comes to unclog the drains. The back and forth serpentine motion of a snake is also used to describe a specific routing pattern that we use on our printed circuit boards.
Sometimes in PCB design a trace has to be routed to a specific length that may be longer than the distance between the point-to-point connections. If you’re working on a large design this may not be a problem as you’ve got room for your trace to wander around. On a smaller design however this can be a challenge to find a way to get the extra length routed in. The best method to resolve this issue is to use a serpentine routing pattern to add the necessary length to your trace. Let’s take a closer look at serpentine routing and how it can be helpful to you in your next PCB design.
Tuned Traces and the Need for Serpentine Routing
Certain high speed nets in a PCB design need to be routed to a specific trace length in order to satisfy the performance requirements of the signal they are carrying. This is generally referred to as tuning the trace. Sometimes it is just a single net that needs to be tuned to a certain length, and other times there may be a whole group of nets that have to be tuned. Often the nets in these groups have to all be tuned to the same length, which is referred to as matched net lengths, as well as being tuned to a specific length.
There are also nets that have to be routed exactly together, as in the case of a differential pair. These nets often have to be routed to a specific length as well as exactly matching the other net in the pair. In some cases there are groups of these pairs that have to be matched together in length as well.
A high speed design can contain many different nets and groups of nets that require different levels of trace length tuning. To accomplish all of this, PCB designers use a serpentine pattern when routing the traces. This will allow the designer to increase the length of a trace without having to change the placement of the components.
Tuned traces in a PCB layout
Serpentine Routing Techniques
At one time trace tuning was completely dependant on the skill of the designer. Without the advanced routing features of modern PCB design tools, the designer was required to manually route in the trace tuning and guess at the actual lengths. On those older tools there were times that we would hold a ruler up to the screen in order to get an idea of how long our traces actually were. Fortunately that has all changed now, and there are different features in the tools for reporting trace lengths and to automate their tuning.
Trace tuning still requires a lot of skill from the designer however, both in understanding what needs to happen and in being able to work with the tools the most effectively. To start off with, high speed design experts usually recommend routing measured lines in large trombone patterns in order to leave room for tuning later. Then when it is time to tune the trace, convert those trombone patterns into the tighter serpentine patterns that you need in order to hit your target lengths. You should use 45-degree corners in the serpentine routing, and space the traces out at a minimum distance of 3 times the trace width.
Tuning a trace with serpentine routing in OrCAD PCB Designer
Put Your Design Tools to Work
As we stated above, today’s PCB design tools usually come loaded with different advanced features to help you with your serpentine routing. You can set up rules and constraints that will allow you to specify trace lengths, matched lengths, differential pairs, and other high speed routing properties.
There are also trace length notifications that will pop up while routing to let you know how your routing is aligning with the rules that you have set up, and there will be different trace reporting features available too. To route your tuned traces there will be different manual and automated routers that will obey the trace length rules and constraints that you have set up. You will also have automated features to add serpentine routing patterns to your traces as well.
Today’s PCB design tools can do the hard work of tuning your traces for you. Adding serpentine routing can be as simple as selecting an area of trace routing to edit, and commanding the system to add the tuning as shown in the picture above. The key is to make sure that you are using the best tools possible that will give you these advanced features to help you perfectly tune your traces.
To help you create the most precise serpentine routing in your PCB designs, the PCB design systems from Cadence have the features and functionality that you are looking for. With its advanced trace routing capabilities, OrCAD PCB Designer is the PCB design tool that you need to use to successfully tune your traces with serpentine routing every time.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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