Tips for PCB Stackup Design

The rapid development of the electronics industry is evident, and electronic products are becoming increasingly powerful. A notable example is the smartwatch, which, in addition to basic time display, can perform various functions such as making calls, video calls, and heart rate monitoring. To achieve multifunctionality, more components need to be integrated onto the circuit board. To meet this goal, there is a shift towards the development of multilayer PCB. Multilayer PCB have more inner layers and are formed into a cohesive whole through stackup and lamination. In this article, TechSparks will provide you with comprehensive knowledge about PCB layer stackup, covering not only basic concepts but also aspects of design and manufacturing.

pcb stack up

What is PCB Layer Stackup

Stackup is a technique in the PCB manufacturing process where multiple inner layers are stacked together. This technique is applicable to multilayer PCB and not to single or double-layer ones. Taking a double-layer PCB as an example, it consists of two layers of copper foil and one layer of dielectric material. Components and circuits are distributed on the top and bottom layers, and signal transmission and power distribution between these two layers are achieved through plated through-holes. For multilayer PCB, additional copper foil layers are used as circuit layers to increase circuit density, allowing for more components and conductors to fit into a smaller space. During the design phase, the functionality of each layer and interlayer connection methods need to be considered, while during the manufacturing phase, adhesive is used to bond the layers together, forming a cohesive whole.

PCB Stacking Process

Implementation of the PCB Stacking Process begins at the design stage, where designers meticulously plan the total number of layers in the PCB and the specific functions of each layer, covering aspects such as signal transmission, power distribution, and electromagnetic shielding. Each layer serves a specific purpose, laying the foundation for overall performance improvement.

Once the design is finalized, physical connections between different layers of the PCB are established through through-holes. To address the complexity of the design, the selection of different types of PCB vias becomes crucial. Blind vias, an effective strategy, are particularly suitable for higher density designs as they avoid complex routing through external layers, thereby reducing the number of signal pins.

In high-frequency and digital signal applications considering interlayer impedance consistency, designers need to adjust parameters such as dielectric material, interlayer distance, and width of intralayer traces. This ensures the stability and reliability of signal transmission.

After completing the design, the CAD software exports the design patterns to CAM software. Using the information exported by CAM software, PCB graphic negatives are created. Subsequently, the designed patterns are transferred to multiple copper foils through processes like exposure and etching. These copper foils, imprinted with circuit patterns, are systematically stacked together with substrate material and adhesive. Finally, through a high-temperature lamination process, they form an integrated multilayer PCB structure.

To enhance the structural strength and stability of the PCB, designers may choose to fill embedded materials between layers or within the overall structure. This step helps reduce the pressure exerted on the PCB during lamination, improving mechanical strength and minimizing the risk of deformation.

PCB Stackup Example

4-Layer PCB Stackup

When the number of layers in a PCB is ≥3, it is considered a multilayer PCB requiring stack-up design. In practical projects, 3-layer PCB are usually not used because the cost quoted by manufacturers is typically the same as for 4-layer PCB. However, 4-layer PCB offer superior performance.

For optimal performance in 4-layer PCB design, the following stack-up strategy is recommended:

  • Top Layer: Serves as the signal layer for connecting various circuit components. Since it contains the main functionality of the circuit, it is crucial for testing and debugging.
  • Second Layer: Functions as the ground (GND) plane, providing the ground connection for the circuit to ensure that the entire circuit is at the same potential. This significantly reduces signal loops and enhances electromagnetic compatibility.
  • Third Layer: Serves as the power signal layer, possibly including connections to power pins, ensuring a stable power supply.
  • Bottom Layer: Similar to the top layer, serving as another signal layer. Placing the signal layer on the bottom contributes to making the PCB more compact.

4-Layer PCB Stackup

6-Layer PCB Stackup​

For the design of a 6-layer PCB, the following stack-up strategy is recommended:

  • Signal Layers: These should always be positioned on the top and bottom layers, and their functionality has been explained previously.
  • GND Layers: To consider electromagnetic compatibility and signal integrity, it is advisable to maintain symmetry for two GND layers. Therefore, it’s best to arrange them on the second layer and the second-to-last layer.
  • High-Speed Signal Layers: It is recommended to place high-speed signals on the third and fourth layers. This arrangement helps reduce coupling effects between adjacent layers. Additionally, compared to the second and fifth layers, the third and fourth layers typically have fewer connectors and pins, significantly lowering the risk of interference.

6-Layer PCB Stackup

8-Layer PCB Stackup​​

The stack-up design for an 8-layer PCB is fundamentally similar to that of a 6-layer PCB, with the key difference being the presence of additional inner layers. This allows for a more refined distribution of power, enabling the distribution of power across multiple inner layers to reduce power noise. Below is an example illustration of the stack-up design structure:

8-Layer PCB Stackup​​​

PCB Stackup Design Rules

Stacking technology, while increasing the number of layers in a circuit, also introduces complexity in design and manufacturing. To ensure circuit performance and reliability, designers should adhere to a series of validated best practices in PCB stackup design:

  • Minimize the distance between the ground layer and power layer, reducing the dielectric thickness to increase capacitance between the power layer and ground layer.
  • Avoid placing two signal layers directly adjacent to each other to reduce signal crosstalk and maintain circuit performance.
  • Place a signal layer adjacent to an inner power layer, taking full advantage of the large copper coverage in the inner power layer for effective shielding of the signal layer.
  • Position high-speed signal layers between two inner power layers to achieve both shielding effects and reduced interference with other signal layers.
  • Always maintain symmetry in the stack-up structure to preserve circuit balance and stability.
  • Carefully design the thickness of each signal layer to ensure stable signal transmission characteristics.
  • In microwave circuit and hybrid dielectric circuit design, consider adding a blank signal layer between dielectric layers to minimize stack-up imbalances.
  • If the power layers are even, and signal layers are odd, consider adding an additional signal layer to improve symmetry.

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