In PCB design, electromagnetic compatibility (EMC) and related electromagnetic interference (EMI) have always been two major problems that have caused engineers to headache, especially in today’s circuit board design and component packaging are shrinking, and OEMs require higher-speed systems Situation.
1. Crosstalk and wiring are the key points
The wiring is particularly important to ensure the normal flow of current. If the current comes from an oscillator or other similar device, it is especially important to keep the current separate from the ground plane, or not to let the current run parallel to another trace. Two parallel high-speed signals will generate EMC and EMI, especially crosstalk. The resistance path must be the shortest, and the return current path must be as short as possible. The length of the return path trace should be the same as the length of the send trace.
For EMI, one is called “infringed wiring” and the other is “victimized wiring”. The coupling of inductance and capacitance will affect the “victim” trace due to the presence of electromagnetic fields, thereby generating forward and reverse currents on the “victim trace”. In this case, ripples will be generated in a stable environment where the transmission length and reception length of the signal are almost equal.
In a well-balanced and stable wiring environment, the induced currents should cancel each other out to eliminate crosstalk. However, we are in an imperfect world, and such things will not happen. Therefore, our goal is to keep the crosstalk of all traces to a minimum. If the width between parallel lines is twice the width of the lines, the effect of crosstalk can be minimized. For example, if the trace width is 5 mils, the minimum distance between two parallel running traces should be 10 mils or more.
As new materials and new components continue to appear, PCB designers must continue to deal with electromagnetic compatibility and interference issues.
2. Decoupling capacitor
Decoupling capacitors can reduce the adverse effects of crosstalk. They should be located between the power supply pin and the ground pin of the device to ensure low AC impedance and reduce 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 smallest capacitance value should be as close as possible to the device to reduce the inductance effect on the trace. This particular capacitor is as close as possible to the power pin or power trace of the device, and connect the pad of the capacitor directly to the via or ground plane. If the trace is long, use multiple vias to minimize the ground impedance.
3. Ground the PCB
An important way to reduce EMI is to design the PCB ground plane. The first step is to make the grounding area as large as possible within the total area of the PCB circuit board, which can reduce emission, crosstalk and noise. Special care must be taken when connecting each component to the ground point or ground plane. If this is not done, the neutralizing effect of a reliable ground plane will not be fully utilized.
A particularly complex PCB design has several stable voltages. Ideally, each reference voltage has its own corresponding ground plane. However, if the ground layer is too much, it will increase the manufacturing cost of the PCB and make the price too high. The compromise is to use ground planes in three to five different positions, and each ground plane can contain multiple ground parts. This not only controls the manufacturing cost of the circuit board, but also reduces EMI and EMC.
If you want to minimize EMC, a low impedance grounding system is very important. In a multi-layer PCB, it is best to have a reliable ground plane, rather than a copper thieving or scattered ground plane, because it has low impedance, can provide a current path, is the best reverse signal source .
The length of time the signal returns to the ground is also very important. The time between the signal and the signal source must be equal, otherwise it will produce an antenna-like phenomenon, making the radiated energy a part of EMI. Similarly, the traces that transmit current to/from the signal source should be as short as possible. If the length of the source path and the return path are not equal, ground bounce will occur, which will also generate EMI.
4. Avoid 90° angle
In order to reduce EMI, avoid wiring, vias and other components forming a 90° angle, because right angles will generate radiation. At this corner, the capacitance will increase, and the characteristic impedance will also change, leading to reflections and then EMI. To avoid 90° angles, traces should be routed to the corners at least at two 45° angles.
5. Use vias with caution
In almost all PCB layouts, vias must be used to provide conductive connections between different layers. PCB layout engineers need to be especially careful because vias will generate inductance and capacitance. In some cases, they will also produce reflections, because the characteristic impedance will change when a via is made in the trace.
Also remember that vias will increase the length of the trace and need to be matched. If it is a differential trace, vias should be avoided as much as possible. If it cannot be avoided, use vias in both traces to compensate for delays in the signal and return path.
6. Cable and physical shielding
Cables carrying digital circuits and analog currents will generate parasitic capacitance and inductance, causing many EMC-related problems. If a twisted-pair cable is used, the coupling level will be kept low and the generated magnetic field will be eliminated. For high-frequency signals, a shielded cable must be used, and the front and back of the cable must be grounded to eliminate EMI interference.
Physical shielding is to wrap the whole or part of the system with a metal package to prevent EMI from entering the PCB circuit. This kind of shielding is like a closed grounded conductive container, which reduces the antenna loop size and absorbs EMI.