PCB Routing Guidelines - Part 7

December 20, 2018 Indian

Let us continue our discussion on the good PCB Layout techniques which help the engineers to develop a noise free PCBs thus helping in compliance.

1. Never try to place the power circuitry near to oscillators. Especially, the passive components like the conductors must be kept away from clock circuits. The frequency components of the crystal clock also called the harmonics can get coupled easily and propagate to parts of the circuit. Especially, unintentional noise on the power supply is a kill.
2. The above point applies to the high frequency traces and fast switching PWMs. Try to route them away from the power circuitry.
3. Follow the 20H rule. 20H rule helps to reduce the emission from the board. When we say, 20H, we are talking about the extent to which ground plane must extend beyond power plane. 'H' is the distance between the two layers. So, this indicates that power plane should never extend towards the edge of the board. The minimum it can be from the edge is 20H away from the ground plane.
4. Follow the 3W rule. If two traces, one aggressor and one victim are to be run parallel on a single layer, to avoid cross talk between them ensure that trace-to-trace center points get separated by 3W. Here, 'W' is the width of the trace on the PCB. While following the 3W rule, designers must ensure that signals are not run parallel for longer distances. For designer crippled with board space these days, achieving the 3W is always a huge task. so, most designers do not prefer parallelism.
5. Try to route the power and ground as radial as possible. This is to ensure that there are no sharp edges and hence resulting in reflections and radiation.
6. Always prefer to have planes for the power supply, even if current doesn't demand a plane, prefer to have it. This helps improve the inductance of the power supply rail and helps to reduce the radiation from the trace.
7. Please, follow the recommended footprint to create a footprint for the component. Ensure that enough tolerance is provided as indicated in the dimensions.
8. Ensure that you design your PCB Layout keeping the DFM (Design for Manufacturing) recommendations in mind.
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