If the housing of the PCB board is a metal housing, the entire metal housing is the ground. The ground of the 220V power supply is connected to the ground. All the interfaces need to be connected to the earth, and the screws must be connected to the earth. In this way, the interference entered in the EMC (Electromagnetic Compatibility) test is directly discharged from the earth to the earth, ensuring that it does not interfere with the internal system. In addition, EMC's protection devices must have every interface and be close to the interface.
If it is a plastic shell, it is better to have a metal plate embedded in it. If there is no way to achieve this, then we need to consider a lot of wiring layouts. Sensitive signal (clock, reset, crystal, etc.) lines need special processing to increase the filter network (chip, crystal, power supply).
It is best practice to connect the plated mounting holes to the chassis, but it is not the only best practice. To ensure that the equipment is protected, the chassis ground must be connected to the appropriate ground. For example, if you build an automatic parking payment machine that is not properly grounded, customers may complain about “electric shock” when making payments. This can happen when the customer touches the non-insulated metal part of the enclosure.
When the chassis of the computer power supply is not properly grounded, it may also be subject to a slight electrical shock. This may also happen when the grounding cable that connects the power outlet to the ground of the building is disconnected. This may cause floating ground on the corresponding machine.
The principle of EMI shielding relies on proper ground connections. Having a floating ground connection will not only give your customers a slight electric shock, but if the equipment is short-circuited, it may endanger the safety of the user. As shown in the figure below, proper grounding is important for safety and EMI shielding.