With the continuous emergence of high-speed circuits, the complexity of PCB boards is increasing. In order to avoid the interference of electrical factors, the signal layer and the power supply layer must be separated, so it involves the design of multilayer PCBs, which is also known as the design of stacked structures. A good stack design not only effectively improves power quality, reduces crosstalk and EMI, and improves signaling performance, but also saves costs and facilitates cabling, which is the first thing any high-speed PCB designer must consider.
10 Tips for PCB Stacking Design
In a multilayer pcb fabrication, there is usually a signal plane, a power (p) plane and a ground (GND) plane. The power plane and ground plane are usually indivisible solid planes that provide good low impedance current loops for currents in adjacent signal wiring. Most signal layers lie between these power or ground reference plane layers, forming symmetrical or asymmetrical striplines. The top and bottom layers of a multi-layer PCB are typically used for components and a small amount of wiring. These signal wires are required to be not too long to minimize direct radiation caused by the wiring.
2. Determine a single power supply to provide a reference data plane (power plane)
The use of decoupling capacitors is an important measure to address the integrity of the power supply. Decoupling capacitors should only be placed at the top and bottom of the PCB. The wiring, pads and vias of the decoupling capacitors can seriously affect the effectiveness of the decoupling capacitors. Therefore, it is required that the wiring connecting the decoupling capacitors should be as short and wide as possible, and that the wiring connecting the vias should also be as short as possible. For example, in a high-speed digital circuit, a decoupling capacitor may be placed at the top of the printed circuit board, and layer 2 may be assigned to the high-speed digital circuit (e.g., the processor) as the power supply layer, layer 3 to the signal layer, and layer 4 to the high-speed digital circuit ground.
In addition, try to ensure that signal alignments driven by the same high-speed digital device use the same power supply layer as the reference plane, and this power supply layer is the power supply layer for the high-speed digital device.
3. Determine the multi-power supply to provide reference data plane
The multi-supply reference plane will be divided into several physical regions with different voltages. If the signal layer is close to the multi-supply layer, the signal current on the signal layer close to it will encounter an undesirable return path, resulting in gaps in the return path. For high-speed digital signals, this illogical loop design can cause serious problems, thus requiring that high-speed digital signal cabling be routed away from the multi-supply reference plane.
4. Determine the need for multiple grounding can be reference data plane (ground plane)
Multiple ground reference planes (grounding layers) can provide a good low impedance current return path that can reduce common mode EMl. The ground plane and power plane should be tightly coupled, and the signal layer should also be tightly coupled with neighboring reference planes. This can be accomplished by reducing the dielectric thickness between layers.
5. The high-speed signaling layer is located in the middle signaling layer.
The high-speed signal transmission layer in the circuit should be sandwiched between the two copper layers of the signal intermediate layer. The two layers of copper film can provide electromagnetic shielding for high-speed signal transmission, and can effectively limit the high-speed signal radiation between the two layers of copper film, without causing external interference.
6. Setting the wiring direction
On the same signal layer, most of the wiring direction should be consistent and orthogonal to the wiring direction of adjacent signal layers. For example, the wiring direction of one signal layer can be set to "Y-axis", and the wiring direction of another neighboring signal layer can be set to "X-axis".
7. Multiple power supply layer away from the economic development of high-speed signal layer multiple power supply layer should pay attention to their own away from the social high-speed digital control signals for wiring. Because the multiple power supply layer will be divided into several operating voltage and different entity enterprise area, if the immediate vicinity of the multiple power supply layer is the signal layer, then its nearby signal layer on the signal output current will be encountered in the undesirable return path, so that the return path to appear some gaps.
8. The classic even-numbered layer structure of PCB stack design is almost always even-numbered layers rather than odd-numbered layers. Even-numbered printed circuit boards offer cost advantages, and even-numbered layers are superior to odd-numbered layers in preventing board warpage.
9. In order to accomplish layer-to-layer conversion of complex wiring, wiring combinations are inevitably laid out in neighboring layers. Two layers crossed by a signal path are called a "wiring harness". The best design of wiring combinations is to avoid loop currents flowing from one reference plane to another, but rather from one point (face) of a reference plane to another. Therefore, PCB factory it is desirable to arrange the wiring combinations in neighboring layers, since paths through multiple layers are not smooth for return currents. Although this effect can be minimized by placing decoupling capacitors near the vias or reducing the dielectric thickness between the reference layers, this is not a good design.
10. Cost Considerations
In terms of manufacturing cost, multilayer boards cost more than single and double layer boards for the same PCB area, and the more layers, the higher the cost. However, when considering the implementation of circuit functionality and miniaturization of the board to ensure performance in terms of signal integrity, EMI, EMC and other factors, we should try to use multilayer boards. Overall, the cost difference between a multilayer board and a single or double layer board is not much higher than expected.