8 rules for designing PCB stack up

Introduction to PCB Stackup

Printed Circuit Board (PCB) stackup refers to the arrangement of copper and insulating layers that make up a PCB. It is a critical aspect of PCB design as it directly impacts the performance, reliability, and manufacturability of the final product. A well-designed PCB stackup ensures proper signal integrity, reduces electromagnetic interference (EMI), and minimizes manufacturing issues.

In this article, we will discuss eight essential rules for designing an effective PCB stackup. By following these guidelines, you can create a robust and reliable PCB that meets your specific requirements.

Request PCB Manufacturing & Assembly Quote Now

Rule 1: Determine the Number of Layers

The first step in designing a PCB stackup is to determine the number of layers required for your design. The number of layers depends on various factors such as the complexity of the circuit, the number of components, signal integrity requirements, and cost considerations.

Here is a general guideline for choosing the number of layers based on the complexity of the design:

Design Complexity	Number of Layers
Simple	1-2
Moderate	4-6
Complex	8-12
High-density	14+

It is important to strike a balance between the number of layers and the overall cost of the PCB. Adding more layers increases the manufacturing cost, but it also provides more routing space and helps in maintaining signal integrity.

Rule 2: Choose the Right Layer Arrangement

Once you have determined the number of layers, the next step is to choose the appropriate layer arrangement. The layer arrangement depends on the type of signals being routed and their sensitivity to noise and interference.

A typical 4-layer PCB stackup consists of the following layers:

Top Layer (Signal)
Ground Plane
Power Plane
Bottom Layer (Signal)

This arrangement provides a good balance between signal integrity and EMI reduction. The ground and power planes act as shields between the signal layers, reducing crosstalk and interference.

For more complex designs, additional layers can be added as required. It is important to maintain symmetry in the layer arrangement to minimize warpage and ensure proper manufacturability.

Rule 3: Use Proper Layer Spacing

The spacing between layers in a PCB stackup is critical for maintaining signal integrity and reducing crosstalk. The spacing depends on the dielectric material used and the frequency of the signals being routed.

Here are some general guidelines for layer spacing:

For high-speed signals (>1 GHz), use a dielectric thickness of 3-4 mils between signal layers and ground/power planes.
For medium-speed signals (100 MHz – 1 GHz), use a dielectric thickness of 5-8 mils.
For low-speed signals (<100 MHz), use a dielectric thickness of 10-15 mils.

It is also important to maintain consistent spacing throughout the stackup to avoid impedance mismatches and signal reflections.

Rule 4: Choose the Right Dielectric Material

The choice of dielectric material is crucial for maintaining signal integrity and controlling the impedance of the transmission lines. The Dielectric constant (Dk) and Dissipation Factor (Df) are the two main properties that determine the performance of the dielectric material.

Here are some commonly used dielectric materials and their properties:

Dielectric Material	Dk	Df
FR-4	4.2	0.02
Rogers 4350b	3.48	0.0037
Isola 370HR	3.7	0.01
Nelco N4000-13	3.6	0.0055

FR-4 is the most widely used dielectric material due to its low cost and good mechanical properties. However, for high-speed and high-frequency applications, low-loss materials like Rogers or Isola are preferred.

Rule 5: Control Impedance

Controlling the impedance of the transmission lines is essential for maintaining signal integrity and reducing reflections. The characteristic impedance of a transmission line depends on the trace width, dielectric thickness, and dielectric constant.

The most common target impedances are 50 ohms for single-ended signals and 100 ohms for differential pairs. To achieve the target impedance, you need to calculate the required trace width and spacing using impedance calculators or PCB design software.

Here is an example of trace width and spacing for a 50-ohm single-ended trace on a 4-layer PCB with FR-4 dielectric:

Dielectric Thickness	Trace Width	Trace Spacing
5 mils	12 mils	8 mils
8 mils	18 mils	12 mils
10 mils	24 mils	16 mils

It is important to maintain consistent impedance throughout the signal path to avoid reflections and signal distortion.

Rule 6: Route High-Speed Signals Carefully

Routing high-speed signals requires special considerations to maintain signal integrity and minimize crosstalk. Here are some guidelines for routing high-speed signals:

Keep the signal traces as short as possible to minimize signal propagation delay and attenuation.
Avoid sharp bends and corners in the signal traces. Use 45-degree angles or curved traces instead.
Maintain proper spacing between signal traces to reduce crosstalk. Use the 3W rule (trace spacing = 3 times the trace width) as a starting point.
Use ground planes or ground pours to provide a low-impedance return path for the signals.
Avoid routing high-speed signals near the edges of the PCB or near slots and cutouts.

By following these guidelines, you can ensure reliable high-speed signal transmission in your PCB design.

Rule 7: Manage Power Distribution

Proper power distribution is crucial for maintaining signal integrity and reducing noise in the PCB. Here are some guidelines for managing power distribution in your PCB stackup:

Use dedicated power planes for each voltage level to provide a low-impedance power distribution network.
Use decoupling capacitors near the power pins of the ICs to reduce high-frequency noise and provide local power supply stabilization.
Use wide traces or copper pours for power distribution to minimize voltage drop and improve current-carrying capacity.
Avoid routing power traces near sensitive analog or high-speed digital signals to reduce noise coupling.
Use split power planes or isolated power regions for sensitive circuits like analog or RF sections.

By following these guidelines, you can ensure a clean and stable power supply for your PCB design.

Rule 8: Document and Communicate

Documenting and communicating your PCB stackup design is essential for ensuring manufacturability and avoiding problems during the fabrication process. Here are some guidelines for documenting and communicating your PCB stackup:

Create a clear and concise stackup diagram that shows the arrangement of layers, their thicknesses, and the materials used.
Specify the copper weights for each layer (e.g., 1 oz, 2 oz) and the finished copper thickness after plating.
Specify the dielectric material and its properties (Dk, Df) for each insulating layer.
Specify the solder mask color and material (e.g., green, red, black, LPI).
Specify the surface finish (e.g., HASL, ENIG, OSP) for the outer layers.
Communicate any special requirements or constraints to the PCB fabricator, such as impedance control or controlled depth drilling.

By providing clear and comprehensive documentation, you can ensure that your PCB stackup is manufactured according to your specifications and avoid costly mistakes.

Frequently Asked Questions (FAQ)

What is the difference between a power plane and a ground plane?
A power plane is a solid copper layer that is used to distribute a specific voltage level (e.g., +3.3V, +5V) to the components on the PCB. A ground plane, on the other hand, is a solid copper layer that is used as a low-impedance return path for the signals and provides a reference voltage (0V) for the circuit.
What is the purpose of using a dielectric material with a low dissipation factor (Df)?
A dielectric material with a low dissipation factor (Df) helps in reducing the signal loss and attenuation at high frequencies. This is particularly important for high-speed digital signals and RF circuits where signal integrity is critical. Low-loss dielectric materials like Rogers or Isola are commonly used in such applications.
What is the difference between a single-ended signal and a differential signal?
A single-ended signal uses a single trace to transmit the signal with respect to a common ground reference. A differential signal, on the other hand, uses two traces (positive and negative) to transmit the signal. The signal is the difference between the voltages on the two traces. Differential signaling is less susceptible to noise and interference compared to single-ended signaling.
What is the purpose of using a solder mask on the outer layers of the PCB?
A solder mask is a thin layer of polymer coating that is applied on the outer layers of the PCB. It serves two main purposes: (1) to protect the copper traces from oxidation and corrosion, and (2) to prevent solder bridging between adjacent pads during the soldering process. The solder mask also provides electrical insulation and improves the aesthetic appearance of the PCB.
What is the difference between HASL and ENIG surface finishes?
HASL (Hot Air Solder Leveling) and ENIG (Electroless Nickel Immersion Gold) are two common surface finishes used on PCBs. HASL involves dipping the PCB in molten solder and then using hot air to level the solder on the pads. ENIG involves plating a thin layer of nickel followed by a thin layer of gold on the copper pads. ENIG provides a flatter and more uniform surface finish compared to HASL and is suitable for fine-pitch components and high-density PCBs.

Conclusion

Designing a PCB stackup is a critical aspect of PCB design that requires careful consideration of various factors such as the number of layers, layer arrangement, dielectric material, impedance control, and signal routing. By following the eight rules discussed in this article, you can create a robust and reliable PCB stackup that meets your specific requirements and ensures optimal performance and manufacturability.

Remember to document and communicate your PCB stackup design clearly to avoid any misunderstandings or issues during the fabrication process. By working closely with your PCB fabricator and providing comprehensive documentation, you can ensure that your PCB stackup is manufactured according to your specifications and functions as intended.