Rules For PCB Layout

In addition to component selection and circuit design, good printed circuit board (PCB) layout is also a very important factor in electromagnetic compatibility. Since PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB wiring will not bring additional costs to the final product.


Anyone should keep in mind that a poor PCB layout can cause more EMC problems than eliminate them. In many cases, even adding filters and components will not solve these problems. In the end, the entire board had to be rewired. Therefore, it is the most cost-effective way to develop good PCB wiring habits at the beginning. The following will introduce some general rules of PCB wiring and the design strategies of power lines, ground lines and signal lines. Finally, according to these rules, the typical printed circuit board circuits of air conditioners are proposed to improve measures.


1. Separation of wiring

The role of wiring separation is to minimize crosstalk and noise coupling between adjacent lines on the same layer of the PCB. The 3W specification indicates that all signals (clock, video, audio, reset, etc.) are isolated from line to line, edge to edge. In order to further reduce the magnetic coupling, the reference ground is placed near the critical signal to isolate the coupling noise generated on other signal lines.


2. Protection and shunt circuit

Setting up shunt and protection circuits is a very effective way to isolate and protect critical signals, such as system clock signals in a noisy environment. The protection circuit not only isolates the coupling magnetic flux generated by other signal lines, but also isolates the key signals from the coupling with other signal lines. The difference between the shunt line and the protection line is that the shunt line does not have to be terminated (connected to ground), but both ends of the protection line must be connected to ground. In order to further reduce coupling, the protection circuit in the multilayer PCB can be added to the ground every other section.


3. Power cord design   

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the ability to resist noise. In single-sided or double-sided boards, if the power line is very long, decoupling capacitors should be added to the ground every 3000 mils. The value of the capacitor is 10uF + 1000pF.


4. Ground design   


The principle of ground design is:   

(1) Separate digital ground and analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should use single-point parallel grounding as much as possible. When the actual wiring is difficult, it can be partially connected in series and then connected in parallel. High-frequency circuits should be grounded with multiple points in series, the ground wire should be short and leased, and grid-like large-area ground foil should be used around the high-frequency components.  


(2) The ground wire should be as thick as possible. If the grounding wire uses a very strong line, the grounding potential changes with the change of current, which reduces the anti-noise performance. Therefore, the grounding wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2~3mm.  


(3) The ground wire constitutes a closed loop. The printed circuit board composed only of digital circuits, the ground circuit is arranged in a group of loops, most of which can improve the ability to resist noise.



5. Signal line design

For key signal lines, if the board has an internal signal routing layer, key signal lines such as clocks are laid on the inner layer, and the wiring layer is preferred. In addition, the key signal lines must not be routed across the partition, including the reference plane gap caused by vias and pads, otherwise it will cause an increase in the signal loop area. And the key signal line should be ≥ 3H away from the edge of the reference plane (H is the height of the line from the reference plane) to suppress the edge radiation effect.


For sensitive signal lines such as clock lines, bus lines, radio frequency lines and other strong radiation signal lines, reset signal lines, chip selection signal lines, system control signals, etc., they should be far away from the outgoing signal lines of the interface. Therefore, the interference of the strong radiation signal line is prevented from being coupled to the outgoing signal line and radiating outward; and the external interference brought in by the outgoing signal line of the interface is prevented from being coupled to the sensitive signal line, causing the system to malfunction.


For differential signal lines, they should be of the same layer, equal length, and parallel running lines, keeping the impedance consistent, and no other routing between the differential lines. Because the common-mode impedance of the differential pair is equal, its anti-interference ability can be improved. According to the above wiring rules, the typical printed circuit board circuit of the air conditioner is improved and optimized


In general, the improvement of PCB design on EMC is: before wiring, first study the design scheme of the return path, there is the best chance of success, and the goal of reducing EMI radiation can be achieved. And before the actual wiring is done, changing the wiring layer does not cost any money, which is the cheapest way to improve EMC.