How do I choose a stackup for PCB?

What is a PCB stackup?

A PCB stackup refers to the arrangement of copper layers and insulating materials that make up a printed circuit board (PCB). It is a critical aspect of PCB design, as it determines the board’s electrical properties, manufacturability, and overall performance. The stackup defines the number of layers, their order, and the thickness of each layer and insulating material.

Key components of a PCB stackup

  1. Copper layers: These are the conductive layers that carry electrical signals and power throughout the PCB. The number of layers can vary from one to many, depending on the complexity of the design.

  2. Insulating material: Also known as the substrate or dielectric, this material separates the copper layers and provides electrical insulation. The most common materials used are FR-4, Rogers, and Polyimide.

  3. Solder mask: A protective layer applied to the outer layers of the PCB to prevent oxidation and provide electrical insulation.

  4. Silkscreen: A layer of text and symbols printed on the solder mask to aid in component placement and identification.

Factors to consider when choosing a PCB stackup

1. Signal integrity

Signal integrity is a crucial factor in determining the performance of a PCB. It refers to the ability of the PCB to maintain the quality of the electrical signals as they travel through the board. Factors that can affect signal integrity include:

  • Impedance: The resistance to the flow of electrical signals. Mismatched impedance can lead to signal reflections and distortions.
  • Crosstalk: The unwanted coupling of signals between adjacent traces, which can cause interference and noise.
  • EMI/EMC: Electromagnetic interference and compatibility, which can affect the performance of the PCB and nearby electronic devices.

To ensure good signal integrity, consider the following when choosing a PCB stackup:

  • Use a sufficient number of ground and power planes to provide a low-impedance return path for signals.
  • Maintain consistent trace widths and spacing to control impedance.
  • Use differential pairs for high-speed signals to minimize crosstalk.
  • Incorporate shielding techniques, such as grounded copper pours, to reduce EMI/EMC.

2. Power delivery

Efficient power delivery is essential for the proper functioning of a PCB. Poor power delivery can lead to voltage drops, noise, and other issues that can affect the performance of the board. When choosing a PCB stackup, consider the following:

  • Use dedicated power and ground planes to minimize impedance and provide a low-resistance path for current flow.
  • Place power and ground planes close to the components they serve to minimize voltage drops.
  • Use multiple vias to connect power and ground planes to component pins to reduce inductance.
  • Incorporate capacitors and other filtering components to reduce noise and ripple.

3. Manufacturing constraints

The manufacturing process can impose constraints on the choice of PCB stackup. Some factors to consider include:

  • Layer count: More layers generally increase the cost and complexity of manufacturing.
  • Material availability: Some materials may be more readily available or cost-effective than others.
  • Minimum trace width and spacing: The manufacturing process may have limitations on the minimum trace width and spacing that can be achieved.
  • Drill sizes: The size of the vias and through-holes may be limited by the manufacturing process.

Work with your PCB manufacturer to understand their capabilities and constraints when choosing a stackup.

4. Cost

The cost of a PCB is directly related to its complexity, including the number of layers, materials used, and manufacturing process. When choosing a PCB stackup, consider the following cost factors:

  • Layer count: More layers generally increase the cost of the PCB.
  • Material choice: Some materials, such as Rogers or Polyimide, may be more expensive than standard FR-4.
  • Manufacturing process: Advanced manufacturing techniques, such as HDI (high-density interconnect) or microvias, can increase costs.

Balance the cost of the PCB against the performance requirements of your design.

Common PCB stackups

2-layer stackup

A 2-layer PCB is the simplest and most cost-effective option. It consists of a single core with copper on both sides and a solder mask and silkscreen on the outer layers. This stackup is suitable for simple, low-density designs with low power requirements.

Layer Material Thickness (mm)
Top Copper Copper 0.035
Core FR-4 1.6
Bottom Copper Copper 0.035

4-layer stackup

A 4-layer PCB is a common choice for more complex designs that require better signal integrity and power delivery. It consists of two cores, each with copper on both sides, and a prepreg layer between them. The outer layers are used for signals, while the inner layers are typically used for power and ground planes.

Layer Material Thickness (mm)
Top Copper Copper 0.035
Prepreg FR-4 0.2
Inner Layer 1 (Ground) Copper 0.035
Core FR-4 1.2
Inner Layer 2 (Power) Copper 0.035
Prepreg FR-4 0.2
Bottom Copper Copper 0.035

6-layer stackup

A 6-layer PCB is used for even more complex designs that require additional routing space and better signal integrity. It consists of three cores, each with copper on both sides, and two prepreg layers between them. The outer layers are used for signals, while the inner layers are used for power, ground, and additional signal routing.

Layer Material Thickness (mm)
Top Copper Copper 0.035
Prepreg FR-4 0.2
Inner Layer 1 (Signal) Copper 0.035
Core FR-4 1.2
Inner Layer 2 (Ground) Copper 0.035
Prepreg FR-4 0.2
Inner Layer 3 (Power) Copper 0.035
Core FR-4 1.2
Inner Layer 4 (Signal) Copper 0.035
Prepreg FR-4 0.2
Bottom Copper Copper 0.035

FAQ

1. How do I know how many layers my PCB needs?

The number of layers needed for a PCB depends on the complexity of the design, the number of components, and the routing requirements. A simple design with few components may only require a 2-layer board, while a more complex design with many components and high-speed signals may require a 6-layer or even an 8-layer board. Work with your PCB designer to determine the appropriate number of layers for your project.

2. What is the difference between FR-4 and Rogers materials?

FR-4 is a standard, cost-effective material used for PCB substrates. It is a composite material made of woven fiberglass and epoxy resin. FR-4 is suitable for most general-purpose applications.

Rogers materials, such as RO4000 and RT/duroid, are high-performance substrates designed for high-frequency and high-speed applications. They have better electrical properties than FR-4, including lower dielectric loss and more stable dielectric constant over temperature. However, Rogers materials are more expensive than FR-4.

3. Can I mix different materials in my PCB stackup?

Yes, it is possible to mix different materials in a PCB stackup. This is often done to optimize the performance of specific layers or to reduce costs. For example, you might use Rogers material for a high-speed signal layer and FR-4 for the other layers. However, mixing materials can increase the complexity of the manufacturing process and may require additional design considerations, such as accounting for differences in thermal expansion.

4. What is the minimum trace width and spacing for a PCB?

The minimum trace width and spacing for a PCB depend on the manufacturing process and the capabilities of the PCB manufacturer. For a standard PCB using an FR-4 substrate, a typical minimum trace width and spacing is around 0.15mm (6 mil). However, advanced manufacturing techniques, such as HDI, can achieve trace widths and spacings as small as 0.075mm (3 mil) or less. Consult with your PCB manufacturer to determine their capabilities and design rules.

5. How do I choose the right thickness for my PCB?

The thickness of a PCB is determined by the stackup, including the number of layers, the thickness of each layer, and the thickness of the insulating materials. The choice of thickness depends on several factors, including:

  • Mechanical requirements: Thicker boards are generally more rigid and durable, while thinner boards are more flexible and lightweight.
  • Electrical requirements: Thicker boards can provide better power delivery and heat dissipation, while thinner boards may have better high-speed performance.
  • Component requirements: The thickness of the board must accommodate the height of the components, including any through-hole components.
  • Manufacturing constraints: The PCB manufacturer may have limitations on the minimum and maximum thickness they can produce.

Work with your PCB designer and manufacturer to determine the appropriate thickness for your project based on these factors.

Conclusion

Choosing the right PCB stackup is a critical step in the pcb design process. It involves balancing the requirements of signal integrity, power delivery, manufacturability, and cost. By understanding the key factors to consider and working closely with your PCB designer and manufacturer, you can select a stackup that meets the needs of your project and ensures optimal performance and reliability.