How to Design PCB Stackup

What is PCB Stackup?

PCB stackup refers to the arrangement of layers in a printed circuit board (PCB). It involves the specific arrangement of copper layers, insulating layers (prepreg or core), and the outer layers (solder mask and silkscreen). The stackup design is crucial as it determines the electrical, mechanical, and thermal properties of the PCB.

Key Components of a PCB Stackup

1. Copper Layers:
2. Signal layers: Used for routing traces and interconnections
3. Power and Ground planes: Provide stable power supply and reference planes
4. Insulating Layers:
5. Core: Thicker, pre-cured laminate material
6. Prepreg: Thinner, uncured material that bonds layers together during lamination
7. Outer Layers:
8. Solder Mask: Protective coating that prevents solder bridging and corrosion
9. Silkscreen: Used for text and component markings

Factors to Consider When Designing PCB Stackup

1. Board Thickness

The overall thickness of the PCB is determined by the number of layers and the thickness of each layer. Standard PCB Thicknesses range from 0.4mm to 3.2mm. Consider the following factors when deciding on board thickness:

• Component requirements
• Mechanical strength
• Space constraints
• Manufacturing capabilities

2. Layer Count

The number of layers in a PCB stackup depends on the complexity of the circuit and the routing requirements. Common layer counts include:

• Single-sided (1 layer)
• Double-sided (2 layers)
• Four-layer
• Six-layer
• Eight-layer and above

As the layer count increases, so does the manufacturing complexity and cost. Therefore, it’s essential to choose the optimal number of layers based on the design requirements.

3. Material Selection

The choice of materials for the PCB stackup affects its electrical, thermal, and mechanical properties. The most common materials used in PCB fabrication are:

• FR-4: Glass-reinforced epoxy laminate, suitable for most applications
• High-Tg FR-4: Improved Thermal stability compared to standard FR-4
• Isola 370HR: High-performance material with lower dielectric loss and better thermal stability
• Rogers 4003C: High-frequency laminate with low Dielectric constant and low loss

Consider the operating frequency, temperature range, and dielectric properties when selecting materials for your PCB stackup.

4. Impedance Control

For high-speed designs, it’s crucial to maintain consistent impedance throughout the signal paths. The PCB stackup plays a significant role in achieving impedance control. Factors that affect impedance include:

• Dielectric constant of the insulating material
• Thickness of the dielectric layer
• Width and thickness of the copper traces
• Proximity to reference planes

Use PCB stackup calculators or consult with your PCB manufacturer to determine the appropriate trace widths and dielectric thicknesses for your desired impedance.

5. Via Types and Placement

Vias are used to interconnect traces on different layers of the PCB. The type and placement of vias can impact signal integrity and manufacturability. Common via types include:

• Through-hole vias: Drilled through the entire board
• Blind vias: Connect an outer layer to an inner layer
• Buried vias: Connect inner layers without reaching the outer layers

Consider the signal requirements, layer transitions, and manufacturing capabilities when deciding on via types and placement.

Designing a Four-Layer PCB Stackup

A four-layer PCB stackup is a common choice for many designs as it offers a good balance between complexity and performance. Here’s an example of a typical four-layer stackup:

Layer	Material	Thickness (mm)	Description
Top	Copper	0.035	Signal layer
Prepreg	FR-4	0.2	Insulating layer
Inner 1	Copper	0.035	Ground plane
Core	FR-4	0.4	Insulating layer
Inner 2	Copper	0.035	Power plane
Prepreg	FR-4	0.2	Insulating layer
Bottom	Copper	0.035	Signal layer

In this stackup:

1. The top and bottom layers are used for signal routing and component placement.
2. The inner layers are dedicated to ground and power planes, providing a stable reference for the signals.
3. The prepreg layers bond the copper layers together and provide insulation.
4. The core layer provides additional mechanical strength to the board.

When designing a four-layer PCB stackup, consider the following tips:

• Keep the signal layers symmetrical to minimize warping
• Use appropriate via types and sizes for layer transitions
• Maintain adequate clearance between traces and vias to prevent manufacturing issues
• Follow the manufacturer’s design guidelines for minimum trace widths, spacing, and drill sizes

Advanced PCB Stackup Techniques

For high-speed, high-frequency, or high-density designs, advanced PCB stackup techniques can help optimize signal integrity and EMI performance. Some of these techniques include:

1. High-Speed Stackup

In high-speed designs, it’s essential to minimize crosstalk and maintain signal integrity. A high-speed stackup typically involves:

• Thin dielectric layers to reduce signal propagation delay
• Tight coupling between signal and reference planes to minimize loop area
• Differential routing for high-speed signals
• Adequate shielding and grounding techniques

2. Asymmetric Stackup

An asymmetric stackup can be used to achieve specific impedance requirements or to accommodate different signal types on different layers. For example:

• Using a thicker dielectric on one side of the board for high-voltage signals
• Placing high-speed signals on inner layers to minimize interference
• Using different materials for different layers based on the signal requirements

3. Hybrid Stackup

A hybrid stackup combines different material types in the same PCB to optimize performance and cost. For example:

• Using a high-performance material (e.g., Rogers) for RF signals and standard FR-4 for digital signals
• Combining rigid and flexible layers in the same board for specific mechanical requirements

PCB Stackup Documentation

Proper documentation of the PCB stackup is crucial for manufacturing and troubleshooting purposes. The stackup document should include:

• Layer arrangement and numbering
• Material specifications for each layer
• Thickness and tolerance of each layer
• Copper weight and finish
• Via types and sizes
• Impedance requirements, if applicable

Collaborate with your PCB manufacturer to ensure that the stackup documentation meets their requirements and guidelines.

Frequently Asked Questions

1. What is the minimum number of layers required for a PCB?

A PCB can have as few as one layer (single-sided) or two layers (double-sided). However, for most modern designs, a minimum of four layers is recommended to achieve better signal integrity and power distribution.

2. How does the PCB stackup affect signal integrity?

The PCB stackup has a significant impact on signal integrity. Factors such as dielectric thickness, trace width and spacing, and reference plane placement can affect impedance, crosstalk, and EMI. Proper stackup design is crucial for maintaining signal quality and minimizing interference.

3. What is the difference between core and prepreg in a PCB stackup?

Core is a thicker, pre-cured laminate material that provides mechanical strength to the PCB. Prepreg is a thinner, uncured material that bonds the layers together during the lamination process. The combination of core and prepreg layers determines the overall thickness of the board.

4. How do I choose the right material for my PCB stackup?

The choice of material depends on several factors, including the operating frequency, temperature range, dielectric properties, and cost. Standard FR-4 is suitable for most applications, while high-performance materials like Rogers or Isola may be necessary for high-speed or RF designs. Consult with your PCB manufacturer or material supplier for guidance on material selection.

5. Can I mix different materials in the same PCB stackup?

Yes, it is possible to use different materials in the same PCB stackup, known as a hybrid stackup. This can be done to optimize performance and cost by using high-performance materials for specific layers and standard materials for others. However, mixing materials can increase the complexity of the design and manufacturing process, so it should be done carefully and in consultation with your PCB manufacturer.

Conclusion

Designing a PCB stackup is a critical step in the PCB design process. It involves considering factors such as board thickness, layer count, material selection, impedance control, and via placement. A well-designed stackup ensures optimal electrical, mechanical, and thermal performance of the PCB while meeting the specific requirements of the application.

When designing a PCB stackup, it’s essential to follow best practices and guidelines, such as maintaining symmetry, using appropriate via types and sizes, and selecting suitable materials for each layer. Advanced techniques like high-speed stackup, asymmetric stackup, and hybrid stackup can be employed for more demanding applications.

Proper documentation of the PCB stackup is crucial for manufacturing and troubleshooting purposes. Collaborating closely with your PCB manufacturer throughout the design process can help ensure a successful outcome.

By understanding the principles and techniques of PCB stackup design, you can create reliable, high-performance PCBs that meet the needs of your application while optimizing cost and manufacturability.