Like most people these days, I find it convenient to use GPS when planning a trip. Just plug in your destination, and the gentle voice of your navigator will guide you all the way there. It’s hard to believe that at one time we relied on physical maps for our trips, using a neon-yellow pen to underscore the route from start to finish. With hundreds of roads and highways on it, the highlighter was essential for plotting your route through the jumbled mass of differently colored lines crisscrossing the page like tangled yarn.
When routing a trace on a printed circuit board, you can have some of the same difficulties. It can be challenging to figure out the best path for your routes through a multicolored maze of traces. Thankfully some of the tricks that we used to do with maps, like highlighting the route that we wanted to focus on, are also available to us when routing traces on a circuit board. Here we’re going to look at several PCB routing techniques that you can use to find the best course to your destination.
Start with a Solid Foundation in the Schematic
In the same way that a road trip needs the right map to follow, your PCB design needs an accurate and complete schematic for the layout to follow. Without it you won’t just be lost in your layout, you won’t even be able to get started. In addition to the circuitry, here are some of the things that your schematic should have to successfully drive the layout:
✓ Logical circuitry flow: To layout the components of a PCB design and route their nets correctly, the schematic should be organized with a logical flow of its circuitry.
✓ High-speed signal paths: These critical nets must be routed precisely in the layout for the best performance of the circuit board. The schematic must identify these nets (clocks, data, memory, etc.) so that the layout team can prioritize their routing.
✓ Power delivery network: Another critical aspect of PCB routing is to create a clean power delivery network (PDN), as well as adequate ground planes for signal return paths and noise control. The layout team will need potential areas of concern for both power and ground, as well as isolation between digital and analog PDNs identified.
✓ Design rules: Many design rules and constraints can be set up in the schematic and passed forward into the PCB layout. These can include high-speed constraints, groups or classes of nets, and specifying trace widths for controlled impedance lines or power and ground routing.
✓ Cross-probing: One of the helpful tools available to the layout designer is the ability to cross-probe with the schematic. Any setup required on the schematic side should be ready to go for the layout team to use this feature.
With a good schematic in place, the PCB layout designer is now ready to begin the first step of successful routing by carefully placing the components.
A Good Component Placement Will Lead to Successful Trace Routing
All too often the trace routing on a PCB is complicated by a component placement that isn’t optimized for the most direct routing paths. Imagine planning a trip to the west coast by first visiting Portland, then traveling south to Los Angeles, and then passing through Portland again on your way North to Seattle. You would most likely want to start from one end and visit each city in sequence, and the same goes for most PCB routing. To do this, the PCB layout designer needs to place their components as represented by the flow of logic in the schematic. Not only will this optimize the trace routing, but it will promote better signal integrity performance.
The first step towards good component placement is to work backward from connectors and other fixed components and place the processors and memory chips that connect to them. Next, place the parts that are directly associated with those already on the board, followed by the remaining non-critical components. Remember to reserve enough space between components for routing, which is especially important for dense parts like high pin-count BGA devices. Don’t forget that in addition to the room you’ll need for trace and PDN routing, your placement will also have to be optimized to pass design for manufacturability (DFM) rules.
Now with the component placement optimized for routing, it’s time to lay down some traces.
PCB Routing Techniques that Can Help
While a successful PCB trace routing strategy relies on a solid schematic and good component placement, like most things it also takes experience. To put it simply, the more you route, the better you are. There are however some features in your PCB design software that can provide a lot of help with trace routing including:
- Automatic routing features such as batch auto-routing and auto-interactive routers can help you to create your traces much faster than you can manually. The key is to understand how they work and learn the routing scenarios where they perform best.
- Editing features that control options (like how much of a trace to rip up when deleting them). These are often user-configurable and can be set to delete the entire line or just to a certain segment.
- Push and shove features that will move existing traces and vias out of the way when routing in new traces.
- Display features that control the color of different layers, or highlight specific nets.
In addition to the functionality of the CAD tools that can help, remember that trace routing is also an artistic task. Give yourself the freedom to be creative in how you route your boards. Don’t be afraid to rip up and re-route, or to move large portions of circuitry to create a better routing pattern. While you are at it, please use those design rules and constraints with your system’s online checking that you set up before the layout. Spending a little extra time beforehand to correctly set up the design constraints could end up saving you hours and hours of redesign to correct unexpected layout errors.
Many other PCB routing techniques can help you to increase your design productivity, and your local PCB contract manufacturer is a good resource for learning more about them. At VSE we have made a career of working with PCB design engineers who want to create the best possible layouts for their designs. Our engineering team is ready to work with you to help you maximize your board’s layout for its best performance and error-free manufacturing.