Microwave Tech - How does smart audio HDI determine the stratification?

Smart audio HDI can be divided into single-panel, double-panel and multi-layer boards in structure. Different boards have different design points. In this article, we mainly come to understand the PCB layering strategy and the design principles of PCB multilayer boards.

PCB layering strategy

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 is as follows:

1. 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 .

2. Try to avoid setting up adjacent wiring layers. Because parallel signal traces on adjacent wiring layers can cause signal crosstalk, if it is impossible to avoid the adjacent wiring layers, the layer spacing between the two wiring layers should be increased appropriately, and the layer spacing between the wiring layer and its signal circuit should be reduced.

3. 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.

Design Principles of HDI Multilayer Board for Smart Audio

When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to control the signal loop area well, it is generally necessary to use a multi-layer board design (high-speed PCBs are generally designed with multi-layer boards). When designing multilayer boards, we should pay attention to the following principles:
1. The key wiring layer (the layer where the clock line, bus, interface signal line, radio frequency line, reset signal line, chip select signal line and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two ground planes, such as Shown in Figure 1. The key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the area of the signal loop, reduce the radiation intensity or improve the anti-interference ability.

Figure 1 The key wiring layer is between the two ground planes
2. 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 Figure 2.

Figure 2 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 Figure 3.

Figure 3 The power plane should be close to its ground plane
3. 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.

Knowledge expansion: The number of layers of multi-layer smart audio HDI depends on the complexity of the circuit board. The number of layers and stacking scheme of a PCB design depends on the hardware cost, the wiring of high-density components, signal quality control, schematic signal definition, Factors such as the baseline of PCB manufacturers’ processing capabilities.

PCB single-layer board and double-layer board design

For the design of single-layer and double-layer boards, the design of key signal lines and power lines should be paid attention to. 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 board should have a large area of ground, or the same method as the single-layer board, design "Guide Ground Line", as shown in Figure 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.

Figure 4 "Guide Ground Line" is placed on both sides of the key signal line of the single-layer board

Figure 5 Large-area paving on the projection plane of the key signal line of the double-layer board