In addition to the circuit design and components selection, electromagnetic compatibility is also very important factor in a good printed circuit design and fab. The key is to minimize the reflux area, so that return path is in the direction of current flow. The most common problem is return current from the reference plane cracks as well as a signal flowing through the connector, which converts the reference plane layers. A capacitor or decoupling capacitor may solve some problems, but will need to take into account the overall impedance of the capacitor, vias, pads and wiring/routings. EMC PCB prototyping design techniques and three routing rules are introduced layering strategy and layout techniques.
PCB Hierarchical Policy:
PCB thickness, Vias, routing, fine layer stacks and PCB layers optimisation solely will not resolve the problem. Power supply should have bypass and decoupling capacitors. Power layer, ground layer and signal layer isolation and transient voltage is equally important for electromagnetic shielding. A good layering strategy for all the layers is very important. Signal traces on layer next to power layer should be well planned to avoid any crosstalk. A good layering strategy involves power plane layered well with signal layer. Distance between power plane and ground plane should be as small as possible. These are few best practices from excellent PCB layering strategy. Few key strategies are mentioned as below:
1. Projection related wirings should be routed mostly in single layer. If wiring layer is not in the plane layer of reflux projected area, signal wiring line will outside the projection area. It will cause “edge of radiation” problem and signal loop area increases, which leads to increase in differential mode radiation.
2. Parallel wiring/routing on adjacent layer should be avoided. It is because parallel signal layers will lead to cross talk. In case parallel wiring cannot be avoided, then proper spacing should be provided between two wiring on the different layers. In case of signal layers, narrow spacing can be used.
3. Adjacent plane layer should avoid overlapping projection plane. Because of the projection plane overlap coupling capacitance between layers results in noise coupling between the layers to each other.
Multilayer PCB layout service, electronic PCB design:
When clock frequencies are above 5MHz, or a signal rise time of less than 5ns, in order to make the signal loop area well controlled, generally multilayer PCB design is used. In case of multi-layer board design following principles should be used:
1. The layer with clock line, bus, interface signal line, RF cable, the reset signal line, chip select lines and various control signal lines, etc. should be the key to the wiring layer. Ground plane should be on adjacent. It is good if wiring layer is kept between two ground layers (as shown in FIG 1). The key signal lines are generally very sensitive to strong radiation or signal lines, and close to the ground plane so that it can signal circuit wiring area is reduced, reducing the radiation intensity or improves anti-jamming.
2. The power plane should be smaller relative to its neighbor (recommended 5H ~ 20H) in the ground plane. Power plane relative to the ground plane reflux condensation can effectively suppress the “edge radiation” problem (as shown in FIG 2).
In addition, the single moderators working power plane (the most widely used power plane), it should be close to its ground plane, in order to effectively reduce the supply current loop area (as shown in FIG 3).
3. In case of TOP & BOTTOM layer if frequency ≥50MHz for signal line then best high-frequency signal to go between two plane layers, to suppress the radiation of space.