When I started running, I went with a friend who had finished several ultramarathons. She would run miles from her house, do a short loop with me, and then carry on without me. When I got faster, instead of getting to go home sooner, she made our loop longer. She was tricky about it, too. She always planned a route where it would be boring for me to turn around early, or on new trails so I’d get lost trying to take a “shortcut” back.
As an urban planner, she was very intentional in choosing what she wanted out of a running route. As an engineer, I wanted something very different, usually to run by the smoothie place, but I understand her deliberation. After all, the same thing is true when I design PCBs. I want my routing to accomplish something very specific in terms of cost and performance. Routing is especially important in ESD protection and helps you to keep your components safe from the induced EMI of ESD events.
Minimize circuit loops
Although our running routes were often circuitous loops, you want to do the opposite on your PCB. If you can minimize circuit loops, this will go a long way towards reducing damage from ESD events propagating across your PCB. This is because a loop that encloses a changing magnetic flux will experience induced current. If that flux is a response to an ESD event, the amount of induced current flowing unexpectedly into your components can cause some pretty catastrophic damage.
Sometimes there’s just no other option, and your layout is going to require a loop. In that case, minimize the area of the loop. The magnitude of the induced current will be proportional to the loop size.
Use ground planes
When you are designing a multilayer PCB, you should definitely use a ground plane. The most common loops formed on PCBs are power to ground traces. Those loops are so ubiquitous that they’re easy to overlook.
For designs that can’t implement a ground plane, you should use a grid pattern of vias to connect to power and ground. This basically emulates a ground plane with traces. You can think of it like a trellis, with power connections occurring at points along one line, and ground traces connecting along orthogonal lines. Semtech has a really nice illustration, and they recommend connections every 6 cm.
They also recommend keeping power and ground traces close together. However, this can cause etching to occur in your board, especially with AC power.
Using a grid pattern, similar to a trellis, to connect to power and ground can help minimize circuit loops if you’re unable to use a ground plane.
Optimize routing paths
Besides minimizing loops, you should try to eliminate traces that run parallel to each other. This is particularly true of parallel traces between interconnected devices. Parallel traces can easily couple to each other. If you have a line protected by a TVS parallel to an unprotected trace, EMI damage can easily propagate through the system despite the protection circuit.
A next level ESD suppression method is to use guard traces. If you simply cannot avoid using a long signal trace (Semtech considers this to be 30 cm or longer), then you can use a guard trace instead. A guard trace runs parallel to a high speed signal. I know I just told you to avoid parallel traces, but a guard trace is essentially sacrificial, providing a return path that cancels much of the radiating field pattern. Thus, it “guards” the nearby traces from crosstalk from your signal line.
Transpose long trace length
If you’re designing for high-speed applications, you probably already know you should keep your traces short. You want to avoid creating unintentional radiating antennas across your PCB, or long linear loops. However, once again the fates may have it in for you, and there’s no way around a long signal or power line. However, you can break those loops up by transposing the signal and ground line. Now, you have lots of smaller loops, like using twisted pair wiring, but inside your PCB layers.
Transposing PCB traces is like using twisted pair wiring to reduce EMI.
Whether it’s for a training run or for PCB traces, planning your routing in advance is worth the extra time. Improving your speed, performance, and protection doesn’t come easily, but the results stay with you for the long haul. Even if you don’t have a friend to do routing for you, there are excellent tools that can help. Software for PCB design, like Altium Designer, provides a unified design and schematic environment that can help you get your PCB design just right. Altium support can help you get started now!
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