Two 4-Layer PCB Stackups With 50 Ohms Impedance

In the world of high-speed digital design, controlling impedance is crucial for maintaining signal integrity and ensuring proper functioning of electronic devices. One of the most common impedance targets is 50 ohms, which has become a standard in many applications due to its balance between power handling and signal loss. Achieving this target impedance in a printed circuit board (PCB) design requires careful consideration of the board's stackup – the arrangement of copper layers and dielectric materials that make up the PCB.

This article will explore two different 4-layer PCB stackups that can achieve 50 ohms impedance. We'll delve into the details of each stackup, discussing their advantages, limitations, and best use cases. By understanding these stackups, PCB designers can make informed decisions to optimize their designs for performance, cost, and manufacturability.

Understanding PCB Stackups

Before we dive into the specific 4-layer stackups, let's briefly review what a PCB stackup is and why it's important.

What is a PCB Stackup?

A PCB stackup refers to the arrangement of copper layers and dielectric materials in a multi-layer printed circuit board. It defines the number of layers, their order, thickness, and the materials used for both the conductive layers and the insulating substrates.

Importance of Stackup Design

The stackup design plays a crucial role in several aspects of PCB performance:

1. Impedance Control: The stackup directly affects the impedance of signal traces.
2. Signal Integrity: Proper stackup design can minimize crosstalk and electromagnetic interference.
3. Power Distribution: The arrangement of power and ground planes impacts power delivery efficiency.
4. Thermal Management: The stackup can influence heat dissipation in the PCB.
5. Manufacturability: Some stackups are easier and more cost-effective to manufacture than others.

Key Factors in Stackup Design

When designing a PCB stackup, several factors need to be considered:

1. Layer Count: The number of copper layers in the PCB.
2. Layer Thickness: The thickness of each copper layer.
3. Dielectric Material: The type and thickness of insulating material between copper layers.
4. Layer Order: The arrangement of signal, power, and ground layers.
5. Prepreg and Core: The use of prepreg (pre-impregnated) layers and core materials.

Stackup 1: Signal-Ground-Power-Signal

Our first 4-layer stackup with 50 ohms impedance follows a Signal-Ground-Power-Signal configuration. This is a common arrangement that provides a good balance of signal integrity and power distribution.

Layer Configuration

Impedance Calculation

To achieve 50 ohms impedance for microstrip lines on the outer layers, we need to calculate the appropriate trace width. The impedance of a microstrip line is influenced by several factors:

1. Trace width
2. Trace thickness
3. Distance to the reference plane
4. Dielectric constant of the substrate

Using standard impedance calculators or electromagnetic field solvers, we can determine that for this stackup, a trace width of approximately 0.30 mm (11.8 mils) on the outer layers will yield 50 ohms impedance.

Advantages

1. Good Signal Integrity: With ground and power planes sandwiched between signal layers, this stackup provides excellent shielding and return path for signals.
2. Flexible Routing: Both outer layers are available for signal routing, providing flexibility in design.
3. Efficient Power Distribution: The inner power and ground planes allow for efficient power distribution across the board.

Limitations

1. Limited to Two Signal Layers: Only the outer layers are available for signal routing, which may be constraining for complex designs.
2. Potential for EMI: Signals on the outer layers are more susceptible to electromagnetic interference.

Best Use Cases

This stackup is well-suited for:

1. Designs with moderate routing density
2. Applications requiring good power distribution
3. Boards with a mix of high-speed and low-speed signals

Stackup 2: Signal-Power-Ground-Signal

Our second 4-layer stackup also achieves 50 ohms impedance but uses a Signal-Power-Ground-Signal configuration. This arrangement offers some unique advantages over the first stackup.

Layer Configuration

Impedance Calculation

For this stackup, to achieve 50 ohms impedance for microstrip lines on the outer layers, we need a slightly different trace width due to the change in dielectric thickness. Using impedance calculators, we find that a trace width of approximately 0.34 mm (13.4 mils) will yield 50 ohms impedance.

Advantages

1. Improved Power Integrity: The power plane is closer to the top layer, which can be beneficial for components on the top side of the board.
2. Good Signal Integrity: The ground plane provides a solid reference for signals on both outer layers.
3. Flexible Routing: Like the first stackup, both outer layers are available for signal routing.

Limitations

1. Slightly Wider Traces: The 50 ohm traces are slightly wider in this configuration, which may impact routing density.
2. Power Distribution: The power plane is further from the bottom layer, which may affect power delivery to bottom-side components.

Best Use Cases

This stackup is particularly suitable for:

1. Designs with many power-hungry components on the top layer
2. Applications where power integrity is a primary concern
3. Boards where most critical components are on the top layer

Comparison of the Two Stackups

Let's compare these two stackups across several key factors:

Considerations for Implementing These Stackups

When implementing either of these stackups in your PCB design, consider the following factors:

1. Manufacturer Capabilities

Ensure your PCB manufacturer can consistently produce the stackup you've chosen. Some manufacturers may have limitations on minimum dielectric thickness or specific material availability.

2. Impedance Control

If tight impedance control is crucial for your design, consider specifying controlled impedance to your manufacturer. This usually involves an additional cost but ensures more precise impedance on your critical traces.

3. Via Strategy

In 4-layer boards, vias play a crucial role in connecting the layers. Consider your via strategy carefully:

• Use through-hole vias for connecting to inner layers
• Consider back-drilling for high-frequency applications to reduce via stubs
• Use multiple vias for power connections to reduce inductance

4. Component Placement

The placement of components can impact the effectiveness of your stackup:

• Place high-speed components close to their required power sources
• Keep sensitive analog circuits away from high-speed digital sections
• Consider the return path for critical signals when placing components

5. Trace Routing

Proper trace routing is essential for maintaining the designed impedance:

• Keep high-speed traces on outer layers when possible for easier impedance control
• Use the 3W rule (space between traces should be at least 3 times the trace width) to minimize crosstalk
• Route traces in orthogonal directions on adjacent layers to reduce coupling

6. Power and Ground Planes

Effective use of power and ground planes is crucial:

• Minimize splits or cuts in the planes, especially under high-speed traces
• Use stitching vias to connect ground areas on different layers
• Consider using plane areas on signal layers for better return paths

Advanced Considerations

While the basic stackups we've discussed can work well for many designs, there are some advanced considerations for more demanding applications:

1. Differential Pairs

For designs using differential signaling (like USB, HDMI, or PCIe), you'll need to calculate the differential impedance, which is typically 100 ohms (2 x 50 ohms). This requires careful control of both the trace width and the spacing between the pair.

2. Embedded Microstrip

For very high-frequency designs, consider using embedded microstrip lines. These are traces on inner layers that are referenced to a plane on an adjacent layer. They offer better shielding than surface microstrips but are more challenging to implement.

3. Hybrid Stackups

Some advanced designs use hybrid stackups where different areas of the board have different layer stackups. This can be useful for optimizing both signal integrity and power delivery in complex systems.

4. High-Speed Materials

For very high-frequency applications (>1 GHz), consider using specialized low-loss materials like Rogers 4350B or Taconic RF-35 instead of standard FR-4. These materials offer better performance but at a higher cost.

5. Copper Roughness

The roughness of the copper foil can impact the actual impedance, especially at high frequencies. Some high-speed designs specify extra-smooth copper foils to minimize this effect.

Future Trends in PCB Stackup Design

As electronic devices continue to evolve, PCB stackup design is also advancing. Here are some trends to watch:

1. Thinner Dielectrics: The push for miniaturization is driving the use of thinner dielectric layers, allowing for more layers in the same board thickness.
2. Advanced Materials: New dielectric materials with better electrical and thermal properties are being developed, offering improved performance for high-speed and high-frequency applications.
3. Embedded Components: Some designs are moving towards embedding passive components within the PCB layers, which can impact stackup design.
4. 3D Printed Electronics: As 3D printing technology advances, it may allow for more complex and customized stackup designs.
5. AI-Assisted Design: Artificial intelligence and machine learning are being applied to PCB design, including stackup optimization for complex, multi-objective designs.

Conclusion

Designing a 4-layer PCB stackup with 50 ohms impedance is a crucial skill for any PCB designer working with high-speed circuits. The two stackups we've explored – Signal-Ground-Power-Signal and Signal-Power-Ground-Signal – both offer viable solutions for achieving this goal, each with its own set of advantages and trade-offs.

The choice between these stackups, or variations thereof, will depend on the specific requirements of your design, including factors like signal integrity needs, power distribution demands, component placement, and manufacturing considerations.

Remember that while these stackups provide a solid starting point, every design is unique. It's essential to validate your stackup choice through simulation or prototyping, especially for high-speed or high-frequency designs. Always consider the entire system, including the components, connectors, and the intended operating environment, when making your stackup decisions.

As PCB technology continues to advance, designers must stay informed about new materials, manufacturing techniques, and design tools. By combining a solid understanding of stackup fundamentals with awareness of emerging trends, PCB designers can create boards that meet the ever-increasing demands of modern electronic devices.

Frequently Asked Questions (FAQ)

1. Q: Can I use these stackups for high-speed designs above 1 GHz? A: While these stackups can work for frequencies above 1 GHz, you may need to make some modifications for optimal performance. Consider using lower-loss dielectric materials instead of standard FR-4, and pay extra attention to via design and transition. For very high frequencies (>10 GHz), you might need more layers or specialized high-frequency laminates. Always validate high-speed designs through simulation and careful prototyping.
2. Q: How do I handle multiple power rails with these 4-layer stackups? A: Managing multiple power rails in a 4-layer board can be challenging. One approach is to split the power plane into sections for different voltages. Another is to use larger copper pours on the signal layers for additional power distribution. For designs with many power rails, consider stepping up to a 6-layer or 8-layer board. In all cases, ensure you have a solid ground return path for all signals.
3. Q: What if I need a different impedance, like 75 ohms or 90 ohms? A: The same stackups can be used for different impedances by adjusting the trace width. Wider traces will increase impedance, while narrower traces will decrease it. Use an impedance calculator to determine the correct trace width for your desired impedance. Keep in mind that very wide or very narrow traces may present manufacturing or routing challenges.
4. Q: How critical is the dielectric thickness in these stackups? A: Dielectric thickness is crucial for achieving the target impedance. Even small variations can significantly affect the impedance. When sending your design for manufacture, specify controlled impedance and work with your PCB fabricator to ensure they can consistently produce the required dielectric thicknesses. Some adjustment of trace widths may be necessary based on the exact materials and thicknesses available from your manufacturer.
5. Q: Can I use these stackups for flex or rigid-flex PCBs? A: While the general principles are the same, flex and rigid-flex PCBs often require special considerations. The dielectric constants of flexible materials are usually different from rigid FR-4, and the copper layers may need to be thinner for flexibility. Additionally, flex materials often have higher manufacturing tolerances. If you're designing a flex or rigid-flex board, consult with your manufacturer about appropriate stackups and adjust your impedance calculations accordingly.