Essential knowledge for PCB engineers: Understand PCB stack-up design in one article

The number of layers of PCB depends on the complexity of the circuit board. From the perspective of PCB processing, multi-layer PCB is manufactured by stacking and laminating multiple “double-sided PCBs”. However, the number of layers of multi-layer PCB, the order of stacking between layers and the selection of board materials are determined by the circuit board designer. This is the so-called “PCB stack-up design”.
Factors to consider in PCB stack-up design

The number of layers and stacking scheme of a PCB design depends on the following factors:

1. Hardware cost: The number of PCB layers is directly related to the final hardware cost. The more layers, the higher the hardware cost. Hardware PCBs represented by consumer products generally have a maximum limit on the number of layers, such as notebook computers. The number of motherboard PCB layers is usually 4 to 6 layers, and rarely exceeds 8 layers;

2. Outlet of high-density components: High-density components represented by BGA package devices. The number of outgoing layers of such components basically determines the number of wiring layers on the PCB board;

3. Signal quality control: For PCB designs where high-speed signals are concentrated, if the focus is on signal quality, it is required to reduce adjacent layer wiring to reduce crosstalk between signals. At this time, the number of wiring layers is different from the number of reference layers (Ground layer or The ratio of Power layer) is preferably 1:1, which will increase the number of PCB design layers; conversely, if signal quality control is not mandatory, adjacent wiring layer solutions can be used to reduce the number of PCB layers;

4. Schematic signal definition: The schematic signal definition will determine whether the PCB wiring is “smooth”. Poor schematic signal definition will lead to uneven PCB wiring and an increase in the number of wiring layers;

5. PCB manufacturer’s processing capability baseline: The lamination design plan (laminated method, lamination thickness, etc.) given by the PCB designer must fully consider the PCB manufacturer’s processing capability baseline, such as: processing flow, processing equipment capabilities, commonly used PCBs Plate model, etc.

PCB stack-up design requires priority and balance among all the above design influencing factors.

General rules for PCB stack-up design

1. The ground layer and the signal layer should be tightly coupled, which means that the distance between the ground layer and the power layer should be as small as possible, and the thickness of the medium should be as small as possible to increase the capacitance between the power layer and the ground layer (if you don’t understand here) , you can think of a flat capacitor, the size of the capacitor is inversely proportional to the spacing).

2. Try not to directly adjacent the two signal layers. This will easily cause signal crosstalk and affect the performance of the circuit.

3. For multi-layer circuit boards, such as 4-layer boards and 6-layer boards, it is generally required that the signal layer be adjacent to an internal electrical layer (ground layer or power layer) as much as possible, so that the large area of copper on the internal electrical layer can be used to achieve the desired effect. It shields the signal layer, thereby effectively avoiding crosstalk between signal layers.

4. The high-speed signal layer is generally located between two internal electrical layers. The purpose of this is to provide an effective shielding layer for high-speed signals on the one hand, and to limit the high-speed signals to the two internal electrical layers on the other. between layers to reduce interference to other signal layers.

5. Consider the symmetry of the stacked structure.

6. Multiple grounded internal electrical layers can effectively reduce grounding impedance.

Recommended Layer Structure

1. Place high-frequency traces on the top layer to avoid using vias and introducing inductive inductance during high-frequency traces. The data lines of the top-layer isolator and the transmitting and receiving circuits are directly connected with high-frequency traces.

2. Place a ground plane under the high-frequency signal line to control the impedance of the transmission connection line and provide a very low-inductance path for return current to flow.

3. Place the power layer below the ground layer. These two reference layers form an additional high-frequency bypass capacitor of approximately 100pF/inch2.

4. Arrange low-speed control signals in the bottom wiring. These signal lines have greater headroom to withstand impedance discontinuities caused by vias, allowing for greater flexibility.

If a power supply layer (Vcc) or signal layer needs to be added, the added second set of power layers/ground layers must be stacked symmetrically. This way the laminated structure is stable and the board will not warp. The power supply layers and ground layers of different voltages should be closer together to increase high-frequency bypass capacitance and suppress noise.

Reminder: There is another layer here which means to use even numbered layers of PCB and avoid using odd numbered layers. Because odd-numbered circuit boards are prone to bending.

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