6 Layer PCB Design Guidelines and Stack Up Options

Introduction

With the increasing complexity and density of electronic circuits, multilayer PCBs with 6 or more layers are becoming very common. Compared to 2 layer and 4 layer boards, 6 layer PCBs provide more flexibility for routing high-speed signals and splitting power and ground planes. However, designing 6 layer boards requires careful planning to utilize the layers effectively and avoid signal integrity issues.

This article provides a set of guidelines and recommendations for 6 layer PCB stack up design. It covers key considerations like layer count sequence, plane splits, material selection, trace routing and via design. Different 6 layer stack up configurations like symmetrical, asymmetrical, and hybrid are also explained with their pros and cons.

Key Design Guidelines for 6 Layer PCBs

Here are some best practices to follow when designing the layer stack up for a 6 layer PCB:

1. Layer Count Sequence

A typical 6 layer board will have the following sequence:

Top Layer (Component side)

Layer 2 (Plane)

Layer 3 (Plane)

Layer 4 (Plane)

Layer 5 (Plane)

Bottom Layer

The top and bottom layers are used for component placement and routing signals. Layer 2 and 5 are commonly used as power and ground reference planes. Layer 3 and 4 can be used as signal layers or additional power planes.

2. Plane Splits

The power and ground planes (layer 2/5) should be properly split to provide clean and stable power to different sections of the board. A common approach is to split the power plane into analog and digital sections to isolate noisy digital supplies.

3. Route Critical Traces on Inner Layers

Route high speed or noise sensitive traces on the inner layer 3/4 which are sandwiched between the reference planes. This provides shielding and prevents EMI/crosstalk issues. Avoid routing such signals on outer layers.

4. Reference Planes for High Speed Signals

Ensure there is a continuous ground plane below any high speed signal trace for controlled impedance. Disruptions in the ground plane can cause impedance discontinuities.

5. Minimum Trace Width/Spacing

Follow the fabricator's design rules for minimum trace width and spacing. For 6 layers, 5/5 mil trace/space is typical for external and 3/3 mil for internal layers.

6. Breakout Vias

Use breakout/stub vias when routing a trace from an inner layer to an outer layer. This confines any via stub effects to the breakout via region.

Material Selection Guidelines

The key considerations for PCB material selection in 6 layer boards are:

• Dielectric constant - Select low Dk glass reinforced epoxy laminates like FR408, N4000-13 to minimize signal loss and achieve matched impedance across layers
• Dissipation factor - Low loss materials with Df below 0.005 like Isola and Nelco products are preferred for high frequency boards
• Copper thickness - 1 oz. copper is typical for signal layers. Use 2 oz. or 3 oz. copper for power and ground layers to handle high current
• Dielectric thickness - 8 to 12 mil dielectric thickness recommended depending on glass content to control layer to layer capacitance
• Copper foil type - Very low profile or reverse treated foil for fine line patterning and HDI requirements
• Number of lamination cycles - Use 2 or 3 lamination cycles to obtain the 6 layer stack up

Via Design Guidelines

Vias play a critical role in routing traces between layers in multilayer PCBs. Here are some guidelines for via design on 6 layer boards:

• Minimize unnecessary vias as they disrupt reference planes and affect signal quality
• Limit via stubs by extending plane layers under the via
• Use backdrilling for high density or high frequency vias like BGA rows
• Follow minimum annular ring, diameter, drill size rules as per fabricator's capabilities
• Space vias adequately for signal integrity and to meet fabrication requirements
• Plan ahead for thermal relief connections for inner plane layers

6 Layer Stack Up Configuration Options

There are several ways a 6 layer PCB can be arranged in terms of the stack up sequence, plane splits and layer usage.

1. Symmetrical Stack Up

In a symmetrical 6 layer stack, the layer sequence mirrors about the mid plane:

Layer 1 (Top): Signal

Layer 2: Ground

Layer 3: Signal

Layer 4 (Mid layer): Power

Layer 5: Signal

Layer 6: Ground

Layer 7 (Bottom): Signal

• Provides identical reference planes above and below mid-layer
• Excellent SI and PI with continuous GND planes
• Widely used for digital, RF and high speed boards

2. Asymmetrical Stack Up

The asymmetrical arrangement has non-mirrored power and ground layers:

Layer 1 (Top): Signal

Layer 2: Ground

Layer 3: Signal

Layer 4: Power

Layer 5: Power

Layer 6: Ground

Layer 7 (Bottom): Signal

• Permits splitting power plane to serve analog and digital sections
• Ground planes are not symmetrical - can affect signal quality
• Used when symmetrical stack up does not meet power distribution needs

3. Hybrid Stack Up

This combines symmetrical and asymmetrical arrangements:

Layer 1 (Top): Signal

Layer 2: Ground

Layer 3: Signal

Layer 4: Ground

Layer 5: Power

Layer 6: Ground

Layer 7 (Bottom): Signal

• Provides good SI/PI with continuous top and bottom ground planes
• Allows splitting power plane if required
• Often preferred hybrid approach optimizing Signal Integrity and Power Integrity

The optimal stack up configuration depends on the board layout and the isolation and decoupling needs of critical signals and components. A continuous ground reference plane close to the routing layers is highly recommended for signal and EMI integrity.

Conclusion

Designing an effective layer stack up is critical to achieve signal and power integrity in complex multilaayer PCBs. For 6 layer boards, selection of appropriate dielectrics, smart layer planning, symmetrical reference planes, impedance control and minimizing discontinuities are key considerations. Utilizing all 6 layers judiciously by allocating signals and planes reduces the board area footprint. A well executed 6 layer PCB design provides flexibility to partition circuits, route dense interconnections and distribute power effectively across the board.

Frequently Asked Questions

Q1. What are the key advantages of using a 6 layer PCB?

Some of the main advantages of 6 layer PCBs compared to 2 or 4 layer boards are:

• More layers to route signals on inner layers separately from components
• Ability to split power and ground planes to reduce noise
• Additional shielding for sensitive signals by sandwiching between planes
• Handling higher component density and interconnect complexity
• Flexible power distribution to different sections of the board
• Controlled impedance environment for signals by close reference planes
• Better EMI and crosstalk performance

Q2. What dielectric materials are used in 6 layer PCBs?

The commonly used dielectric materials in 6 layer PCBs are FR-4, Nelco N4000-13, Isola FR408, Rogers 4000 series, Arlon 85N, Panasonic Megtron, Taconic TLY and Park Electrochemical Nelco. These glass reinforced, low loss laminates provide good performance for high density multilayer boards.