Any Angle Routing: When Should You Use It?
This is what any angle routing looks like...
EDA tools have come a long way since the advent of personal computing. Now advanced routing features like auto-routers, interactive routing, length tuning, and pin-swapping are helping designers stay productive, especially as device and trace densities increase. Routing is normally restricted to 45-degree or right angle turns with typical layout and routing tools, but more advanced PCB design software allows users to route at any angle they like.
So which routing style should you use, and what are the advantages of any angle routing? Like many engineering questions, switching from standard trace geometries to any angle routing brings a set of tradeoffs, and any angle routing is a better choice in some designs than in others. Hopefully, the advice we’ve compiled here can help you decide when to use any angle routing in your next PCB.
What is Any Angle Routing?
As its name suggests, any angle routing allows a designer to route a trace at any angle they like, even routing along a complicated curve. Interactive routing tools in high-quality PCB design software will include a feature that allows you to define an arc for routing a curve, or to simply drag the endpoint of a trace along the desired direction. This eliminates the conditional constraint where traces were routed at 45-degree angles and at right angles.
Although routing at 45-degree and 90-degree angles helps keep your traces organized and visually pleasing, it is not the only way to route your signals. The primary advantage of routing at any angle is the ability to decrease length mismatches between groups of single-ended traces, which helps save board space and reduces the level of trace meandering required for length matching.
In some cases, the ability to route along a relatively straight line can eliminate the need for two or more vias on a signal trace. It can also be used to overcome a problem that arises when using an unassisted auto-router, known as clinching. Creative use of vias along with any angle routing allows you to avoid routing around another group of traces and vias. Instead, you can route directly between two points on the surface layer or with a minimized number of layer transitions.
Routing on Curves: Placing an Arc in Any Angle Routing
One interesting routing scheme is to route traces with an arc, which brings some advantages in certain boards. In particular, in a circular board, routing along an arc allows you to save space as you route traces along the outside of the board. This is a better choice than octagonal routing as you can reduce the overall size of the board, fit more traces into a given space, or fit more components into a tight arrangement.
Any angle routing with circular arcs is not an either-or decision; you can use straight segments and arcs together on a single trace when you have access to the right routing tools. This can help you solve a few interesting routing problems beyond simply routing along a curved board. There are two examples of this that I’d like to present briefly.
One is the case where you have a square or rectangular board, and a large square or rectangular component is angled along the board diagonal (i.e., at 45-degrees compared to the board edge). In this case, using any angle routing to break off from the pads on the component allows you to route out to straight traces with less space and/or higher trace density compared to the use of standard 45-degree routing.
Any angle routing between ICs
There is another benefit to this, which may not be obvious when we look at the above image. Suppose that the 5 signal traces that break off from the central IC need to be length matched. If we were to use 45-degree turns in the above image, there would be a greater length mismatch between parallel traces, so a larger amount of meandering would be needed for the inside traces before reaching the downstream ICs. Routing along a circular arc between the 45-degree portion and the straight portion of the trace is shorter than routing along straight lines, so any length mismatch between these 5 traces is reduced. If the signal rise time is long enough, you may not need length matching at all.
BGA Breakouts and Differential Pairs
With a BGA, you can also use any angle routing as you break out from the pads beneath your component. This could make your routing scheme more flexible than using a simple dog bone fanout. When combined with the case of arc routing and a rotated component shown above, you can break out of a BGA in any way you like.
If you’re routing differential pairs under a specific signalling standard, you can still use any angle routing, although you will need to maintain consistent, symmetrical coupling along the length of the pair in order to meet differential impedance requirements. However, when taken alongside the arc routing benefits shown above, you may still be able to reduce length mismatch when routing groups of differential pairs around your board.
Any angle routing provides a nice alternative to dog bone fanouts
Just like your other routing tools, any angle routing may not be right in all situations, but it allows you to be more flexible with your routing scheme in different boards. The interactive routing features in Altium Designer® are ideal for implementing any routing style you can imagine. The routing features in Altium Designer automatically check your layout against your design rules as you create your layout.