Whenever I go to buy a new big-ticket appliance, I always weigh my options. Sometimes I let this paralyze me, and I end up making a spending decision that I later regret. But it doesn’t have to be this way when deciding how to route signals in your next PCB.
When you have many components and interconnects to place on your PCB, you need to weigh your options. Dense routing is as much an art as it is science, and having as much information as possible on your options will help ensure that you can implement the best routing strategy for your next circuit board.
Trace and Component Density
Routing traces around your board becomes progressively more complex as trace density and component density increase. Your board can likely be routed entirely on a single layer as long as you are not routing between high pin count devices and you have plenty of room between components. Even when working with moderate to high-speed circuits, it is still possible to route everything on one layer.
As trace density and component density increase, routing on a single layer becomes more difficult or even mathematically impossible, and you must move on to multilayer design. Signals must be routed on the inner layers, and the inner layers can be accessed using vias. In certain devices, particularly in HDI design, this is the only way to continue packing more interconnects and components into a single PCB without increasing its size.
Since using vias is the only way to access the inner layers in a multilayer board, this begs the question: when should you use vias to make a shorter connection instead of routing a longer trace in a signal layer?
PCB with length-matched differential pairs
Be Mindful of Your Frequency Spectrum
Vias are useful as they can greatly simplify routing around other components, or when there is a high density of interconnects on a board. Components like FPGAs with BGA packages routinely use vias for escape routing. All vias have some capacitance and inductance, just like a PCB trace. This is usually ignored over a broad range of frequencies and switching speeds as the length of the vias are typically very small compared to the rest of the trace.
All digital signals have power concentrated at a frequency higher than the switching frequency. Compared to high-frequency analog signals, even moderate speed digital signaling can create signal integrity issues when routed through vias due to the power spectrum at higher frequencies. The small geometry of vias causes them to have a high fundamental resonance frequency (100’s of MHz to GHz). Resonance in a via causes EMI issues with high frequency (>100 MHz) analog or moderate to high-speed digital signals.
Obviously, the easiest way to avoid signal integrity problems from vias when routing high speed/ high-frequency signals is to avoid using vias on critical interconnects. At the very least, routing through vias should be minimized in these devices when possible. Opting for longer traces may be a better choice, but pay attention to a transition to transmission line behavior as the trace length is increased.
In many modern PCBs, the use of vias will be unavoidable. In lower speed or lower frequency devices, using vias to make routing easier is more acceptable. The frequency spectrum in digital signals and the frequency of analog signals in these devices is simply too low to cause resonance and subsequent EMI problems. Extending the trace lengths in these devices is also appropriate as the longer length is unlikely to cause a transition to transmission line behavior.
Working With Differential Pairs
Differential pairs require precise length matching in order to maintain signal timing. They also require precise spacing and symmetry in order to fully suppress crosstalk. Differential pairs can be extended rather than being routed through vias, but you will need to be mindful of skew that can accumulate if the pairs do not have precisely matched lengths.
Make sure to keep your differential signals timed properly
If you’re trying to decide whether to extend the length of differential pairs or use vias as a shortcut, you should apply your design choice to the entire signal net. The two traces in a differential pair must be length matched in order to maximize crosstalk suppression and eliminate skew. If the entire net is not length matched, skew will accumulate in the extended differential pairs.
Therefore, if you must route through vias, you might consider routing every differential pair in the net through vias as this can help ensure length matching across the signal net. Both ends of a differential pair should be routed through vias to ensure symmetry, thus suppressing EMI. You could also route some interconnects through additional vias and some on the surface layer, but you should always make sure that you maintain length matching in order to reduce skew.
If you must increase the length of your differential pairs in order to place your required interconnects in an effort to avoid signal integrity problems posed by vias, watch out for a transition to transmission line behavior, especially with high speed/high frequency signals. Once the length of a trace exceeds a critical length, the trace starts to behave as a transmission line and must be terminated in order to avoid further signal integrity problems.
With so many routing options and issues to consider during design, routing can be made simpler and faster with a great PCB design package like Altium Designer 18.1. You’ll have access to the best automated routing tools, CAD features, and signal integrity tools. The design rules engine in Altium Designer can help ensure that your traces remain length-matched and terminated.
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