PCB Blog - Composition and design of a 4-layer flex pcb stackup

Basic Structure of 4 Layer Flex pcb stackup

The 4-layer flex pcb stackup consists of four layers, namely the top layer, bottom layer, inner layer 1, and inner layer 2. The top and bottom layers are signal layers, while inner layer 1 and inner layer 2 are power layers. Its characteristic is to add a power plane or signal layer between inner layer 1 and inner layer 2, thereby forming an internal shielding protection layer. This approach has the following advantages:

1. Reduce electromagnetic interference: When there is an internal layer between the signal layer and the power layer, the impact of electromagnetic interference can be reduced, ensuring the stability and reliability of the circuit board.

2. Reduce signal transmission delay: The presence of internal layer planes can enable faster signal transmission, reduce signal transmission delay, and improve the efficiency of circuit boards.

3. Enhance the noise suppression ability of the signal layer: The presence of the internal layer can effectively absorb the noise of the signal layer, thereby improving the noise suppression ability of the signal layer.

4. Improve the anti-interference ability of the circuit board: The presence of internal layers can enhance the anti-interference ability of the circuit board, effectively preventing the influence of external signals on the circuit board.

4 Layer Flex pcb

The 4-layer flex pcb stackup structure is mainly used in the design of mid to high-end circuit boards, especially in scenarios that require high-speed digital signal processing and high-frequency analog signal transmission. Some common application scenarios are as follows:

1. High-speed signal transmission: The 4-layer flex pcb stackuping structure can reduce signal transmission delay, thereby improving the efficiency of the circuit board. Widely used in scenarios that require high-speed signal transmission.

2. High power circuit design: The inner layer plane can effectively eliminate the inductance between the signal layer and the power layer, thereby improving the noise level of the circuit board, making it suitable for the design of high-power power circuits.

3. High frequency transmission: The 4-layer flex pcb stackuping structure can reduce signal transmission noise, improve the anti-interference ability of the circuit board, and is suitable for high-frequency transmission scenarios.

4. Multiple circuit board stacking: In some designs, multiple circuit boards need to be stacked. By using a four layer board structure, better shielding effects and signal processing capabilities can be achieved.

Design of 4-layer flex pcb stackup

1. SIG-GND (PWR) - PWR (GND) - SIG;

2. GND SIG (PWR) - SIG (PWR) - GND;

The potential issue with the above two stacked designs is the traditional 1.6mm (62mil) plate thickness. The interlayer spacing will become very large, which is not only unfavorable for controlling impedance, interlayer coupling, and shielding; Especially with the large spacing between power layers, the board capacitance is reduced, which is not conducive to filtering out noise.

For the first solution, it is usually applied in situations where there are many on-board chips. This solution can achieve good SI performance, but it is not very good for EMI performance. It mainly needs to be controlled through wiring and other details.

Main attention: Placing the strata in the connecting layer of the signal layer with the densest signal density is beneficial for absorbing and suppressing radiation; Increase the board area to reflect the 20H rule.

For the second solution, it is usually applied in situations where the chip density on the board is low enough and there is enough area around the chip (to place the required power supply copper layer). The outer layer of the PCB in this scheme is the ground layer, and the middle two layers are both the signal/power layer.

The power supply on the signal layer is wired with wide wires, which can reduce the path impedance of the power supply current and the impedance of the signal microstrip path. It can also shield the inner layer signal radiation through the outer layer. From the perspective of EMI control, this is the optimal 4-layer PCB structure currently available.

Main attention: The distance between the signal and power mixing layers in the middle two layers should be widened, and the wiring direction should be perpendicular to avoid crosstalk; Properly control the board area and reflect the 20H rule; If you want to control the impedance of the wiring, the above plan needs to be very careful in arranging the wiring under the copper island of the power supply and grounding.

In addition, the copper laid on the power supply or ground should be interconnected as much as possible to ensure the connectivity between DC and low-frequency.

The above is the composition and design of the 4 layer flex pcb stackup shared by iPCB.