In PCB design, electromagnetic compatibility (EMC) and the associated electromagnetic interference (EMI) have traditionally been two major headaches for engineers, especially in today’s circuit board designs and component packages continue to shrink, OEMs require higher speed systems. In this article, I will share how to avoid electromagnetic problems in PCB design.
1. Crosstalk and alignment is the focus
Alignment is particularly important to ensure the proper flow of current. If the current comes from an oscillator or other similar device, it is particularly important to keep the current separate from the ground layer, or to keep the current from running in parallel with another alignment. Two high-speed signals in parallel can generate EMC and EMI, especially crosstalk. It is important to keep the resistor paths as short as possible and the return current paths as short as possible. The length of the return path should be the same as the length of the transmit path.
For EMI, one path is called the “violation path” and the other is the “victim path”. Inductive and capacitive coupling affects the “victim” path due to the presence of electromagnetic fields, thus generating forward and reverse currents on the “victim path”. In this way, ripple is generated in a stable environment where the transmit and receive lengths of the signal are almost equal.
In a well-balanced environment with stable alignments, the induced currents should cancel each other out, thus eliminating crosstalk. However, we are in an imperfect world where such a thing does not happen. Therefore, our goal is that crosstalk must be kept to a minimum for all alignments. The effect of crosstalk can be minimised if the width between parallel lines is twice the width of the lines. For example, if the line width is 5 mils, the minimum distance between two parallel lines should be 10 mils or greater.
As new materials and components continue to appear, PCB designers must also continue to deal with EMC and interference issues.
2. Decoupling capacitors
Decoupling capacitors reduce the undesirable effects of crosstalk. They should be located between the power and ground pins of the device, which ensures a low AC impedance and reduces noise and crosstalk. To achieve low impedance over a wide frequency range, multiple decoupling capacitors should be used.
An important principle for placing decoupling capacitors is that the capacitor with the lowest capacitance value is placed as close to the device as possible to reduce inductive effects on the alignments. This particular capacitor should be placed as close as possible to the device’s power supply pins or the power supply raceway and the pads of the capacitor should be connected directly to the vias or ground level. If the alignment is long, use multiple vias to minimise ground impedance.
3. Grounding the PCB
An important way to reduce EMI is to design the PCB grounding layer. The first step is to make the grounding area as large as possible within the total area of the PCB board so that emissions, crosstalk and noise can be reduced. Particular care must be taken when connecting each component to a ground point or grounding layer, without which the neutralising effect of a reliable grounding layer cannot be fully utilised.
A particularly complex PCB design has several stable voltages. Ideally, each reference voltage has its own corresponding grounding layer. However, too many grounding layers would increase the manufacturing costs of the PCB and make it too expensive. A compromise is to use grounding layers in three to five different locations, each of which can contain several grounding sections. This not only controls the manufacturing cost of the board, but also reduces EMI and EMC.
A low impedance grounding system is important if EMC is to be minimised. In a multilayer PCB it is preferable to have a reliable grounding layer rather than a copper balance block (copper thieving) or a scattered grounding layer as it has low impedance, provides a current path and is the best source of reverse signals.
The length of time the signal takes to return to ground is also very important. The time taken for the signal to travel to and from the source must be comparable, otherwise an antenna-like phenomenon will occur, allowing the radiated energy to become part of the EMI. Similarly, the alignment of the current to/from the signal source should be as short as possible, if the source and return paths are not of equal length, ground bounce will occur and this will also generate EMI.
4. Avoid 90° angles
To reduce EMI, the alignment, vias and other components should be avoided to form a 90° angle, because a right angle will generate radiation. To avoid 90 ° angle, the alignment should be at least two 45 ° angle wiring to the corner.
5. The use of over-hole need to be careful
In almost all PCB layouts, vias must be used to provide a conductive connection between the different layers. In some cases, they also produce reflections, as the characteristic impedance changes when the vias are created in the alignment.
It is also important to remember that vias increase the length of the alignment and need to be matched. In the case of differential alignments, vias should be avoided where possible. If this cannot be avoided, vias should be used in both alignments to compensate for delays in the signal and return paths.
6. Cables and physical shielding
Cables carrying digital circuits and analogue currents can generate parasitic capacitance and inductance, causing many EMC related problems. If twisted pair cables are used, a low level of coupling is maintained and the magnetic fields generated are eliminated. For high frequency signals, shielded cables must be used, with both their front and back grounded, to eliminate EMI interference.
Physical shielding is the encasing of the whole or part of the system in a metal package to prevent EMI from entering the PCB circuitry. This shielding acts like a closed, ground-conducting capacitor, reducing the size of the antenna loop and absorbing EMI.
Post time: Nov-23-2022