Designing High-Frequency PCBs: Mitigating Electromagnetic Interference (EMI)
A factor that engineers need to consider when designing PCBs is Electromagnetic Interference (EMI). This occurs when there is a disturbance generated by an external source which affects the electrical circuits. This interference may be through electromagnetic induction, electrostatic coupling, or conduction.
When high frequencies are involved, the level of EMI occurring within a PCB trace and in components increases. It is important to remember that every device, trace and via can act as a source of EMI. Boards are considered to be high frequency when the signal is greater than 3 MHz, and is rapidly changing.
It is important when designing any PCB to follow basic rules of component placement, but even more so when considering high frequency boards. The order should be:
- Power circuits;
- Sensitive and precision circuits;
- Critical circuit components;
- Everything else.
A DRC will only be able to identify errors that it is programmed to monitor. Many of the issues associated with high frequency boards will pass through a DRC check, as they are not functionally ‘errors’. Things to watch in DRC checking include package-to-package spacing, shorted or unconnected nets, and air-gap violations. When considering the PCB layout, there are many factors to pay close attention to:
It is also important to consider trace widths when planning high frequency boards. Trace widths of 10 to 20 mils are used for traces carrying 10 to 20 mA; while trace widths of 5 to 8 mils should be used for traces carrying less than 10mA.
The routing also needs to be carefully considered, as does the need for high-impedance nodes.
It is important to consider the placement of sensitive nodes and circuits. They will need to be shielded from sources of noise. A solder mask should be in place between pins and vias. It is also essential that the silkscreen is clear and concise.
It is recommended that the first inner layer below the component sides is used as ground. Other layers should then be assigned power planes. Stack-ups need to be planned in a way that balances the board around the midpoint of the Z axis.
There are two common structures – microstrip and stripline circuits. Both have different properties, and the preferred layout should be tested to ensure it meets your specific needs. Microstrip designs may be subject to crosstalk. To minimize crosstalk on adjacent traces, the traces should be separated by at least twice the trace width.
If the circuit is differential, the traces for both signals need to have the same length to ensure that propagation delay times are equal, ensuring that the electromagnetic fields effectively cancel each other out.
As with all PCB design, it is best to have the design reviewed on an ongoing basis through the design stages, with adjustments made iteratively to ensure the EMI impacts are minimized in the high frequency board.