PCB Layering Strategy-One of The Key Factors of PCB EMC
Good PCB design is a very important factor in electromagnetic compatibility. The key to PCB EMC design is to minimize the reflow area and let the reflow path flow in the direction of the design. The most common return current problems come from cracks in the reference plane, changing the reference plane layer, and the signal flowing through the connector. Jumper capacitors or decoupling capacitors may solve some problems, but the overall impedance of capacitors, vias, pads, and wiring must be considered.
Table of Contents
1.PCB layering strategy
2.Multilayer board design
3. Single-layer board and double-layer board design
1.PCB layering strategy
The thickness, via process and the number of layers in the circuit board design are not the key to solving the problem. Good layered stacking is to ensure the bypass and decoupling of the power bus and minimize the transient voltage on the power layer or ground layer. The key to shielding the electromagnetic field of the signal and power supply. From the perspective of signal traces, a good layering strategy should be to put all signal traces on one or several layers, and these layers are next to the power layer or ground layer. For the power supply, a good layering strategy should be that the power layer is adjacent to the ground layer, and the distance between the power layer and the ground layer is as small as possible. This is what we call the “layering” strategy. A good PCB layering strategy has the following advantages.
A.The projection plane of the wiring layer should be in its reflow plane layer area. If the wiring layer is not in the projection area of the reflow plane layer, there will be signal lines outside the projection area during wiring, which will cause “edge radiation” problems, and will also increase the area of the signal loop, resulting in increased differential mode radiation .
B.Try to avoid setting up adjacent wiring layers. Because parallel signal traces on adjacent wiring layers can cause signal crosstalk, if adjacent wiring layers cannot be avoided, the layer spacing between the two wiring layers should be appropriately increased, and the layer spacing between the wiring layer and its signal circuit should be reduced.
C.Adjacent plane layers should avoid overlapping of their projection planes. Because when the projections overlap, the coupling capacitance between the layers will cause the noise between the layers to couple with each other.
2. Multilayer board design
When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to control the signal loop area well, a multi-layer board design is generally required. Pay attention to the following points when Multilayer board design:
A.The key wiring layer should be adjacent to the complete ground plane, preferably between the two ground planes, as shown in PIC 1.Key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the signal loop area, reduce its radiation intensity or improve anti-interference ability.
PIC 1 The key wiring layer is between the two ground planes
B.The power plane should be retracted relative to its adjacent ground plane (recommended value 5H～20H). The retraction of the power plane relative to its return ground plane can effectively suppress the “edge radiation” problem, as shown in PIC 2.
PIC2 The power plane should be retracted relative to its adjacent ground plane
In addition, the main working power plane of the board (the most widely used power plane) should be close to its ground plane to effectively reduce the loop area of the power supply current, as shown in PIC 3.
PIC3 The power plane should be close to its ground plane
C. Whether there is no signal line ≥50MHz on the TOP and BOTTOM layers of the board. If so,it is best to walk the high-frequency signal between the two plane layers to suppress its radiation to the space.
3. Single-layer board and double-layer board design
The design of single-layer and double-layer boards design should mainly pay attention to the design of key signal lines and power lines. There must be a ground wire next to and parallel to the power trace to reduce the area of the power current loop.
“Guide Ground Line” should be laid on both sides of the key signal line of the single-layer board, as shown in Figure 4. The key signal line projection plane of the double-layer boards design should have a large area of ground, or the same method as the single-layer board, design “Guide Ground Line”, as shown in PIC 5. The “guard ground wire” on both sides of the key signal line can reduce the signal loop area on the one hand, and also prevent crosstalk between the signal line and other signal lines.
PIC4 “Guide Ground Line” is placed on both sides of the key signal line of the single-layer board
PIC5 The key signal line of the double-layer board is laid on a large area on the projection plane