How do You Stack 4 Layer PCB?

Introduction

A 4 layer PCB (printed circuit board) refers to a PCB with 4 copper layers that are used to route signals and distribute power. Stacking a 4 layer PCB properly is important to ensure good signal integrity, reduce noise and crosstalk, and effectively manage power distribution.

In a 4 layer board, the layers are typically stacked up in the following order from top to bottom:

Top Layer

This is the topmost layer and is used for routing signals, components and connectors. It is also sometimes referred to as the component layer.

Layer 2

The second layer down is commonly used as a ground plane, providing a low impedance return path for signals routing on the top layer. A solid ground plane helps reduce noise.

Layer 3

The third layer is often used as a power plane for distributing power to different sections of the board. Splitting power planes allows for better power distribution and decoupling.

Bottom Layer

The bottom layer mirrors the top layer and is used for additional signal routing, routing of traces exiting the board, and for components and connectors.

4 Layer Stackup Configurations

There are several common 4 layer PCB stackup configurations that can be used depending on the needs of the design.

Signal-Ground-Power-Signal

This is a very common stackup used for mixed-signal or digital boards:

LayerUseTopSignalsLayer 2Ground PlaneLayer 3Power PlaneBottomSignals

The ground plane provides shielding between the top and bottom signal layers, and the power plane delivers power efficiently.

Signal-Power-Ground-Signal

Another option is to swap the power and ground planes:

LayerUseTopSignalsLayer 2Power PlaneLayer 3Ground PlaneBottomSignals

This provides good power distribution while maintaining a ground plane in the middle for shielding.

Ground-Power-Ground-Signal

For boards with high speed or RF signals, a stackup with ground planes sandwiching the power plane may be used:

LayerUseTopGround PlaneLayer 2Power PlaneLayer 3Ground PlaneBottomSignals

The double ground planes prevent noise coupling and improve signal integrity at high frequencies.

Signal-Ground-Signal-Power

Another high frequency stackup option is:

LayerUseTopSignalsLayer 2Ground PlaneLayer 3SignalsBottomPower Plane

The ground plane provides shielding between the two signal layers.

Layer Stackup Design Considerations

Here are some key factors to consider when designing the layer stackup in a 4 layer PCB:

• Signal integrity - Ensure adequate isolation and shielding between critical or high speed signals to prevent crosstalk or EMI problems. Use ground planes between signal layers.
• Power integrity - The power plane layer should distribute power evenly to different sections of the board. Use multiple power planes or splits if needed. Decouple power planes with bypass capacitors.
• Heat dissipation - The layer stackup impacts heat spreading and cooling. Place low power layers next to high power layers to help dissipate heat.
• EMI - Adding ground planes helps block EMI emissions and prevent external EMI from disrupting sensitive circuits. Minimize slots or openings in ground planes.
• Component placement - Consider component placement and routing needs when deciding layer usage. Critical traces may need to be routed on inner layers.
• Manufacturability - Simpler layer configs are easier to manufacture. Also consider fabrication shop capabilities.
• Future expandability - If designing an evolutionary product, consider future layer stackup needs and high speed signals.
• Cost - More layers increase cost. Balance performance needs with budget.

4 Layer PCB Routing Tips

Here are some useful routing practices for a 4 layer board:

• Route critical signals on inner layers adjacent to a ground plane whenever possible. This provides shielding and isolation.
• Route traces orthogonally between layers to avoid overlapping traces and crosstalk.
• Flood unused areas on signal layers with ground to provide additional shielding and EMI reduction.
• Use vias to transition signals between layers. Minimize the number of vias used.
• Avoid routing signals through cutouts or splits in the ground/power planes. This can cause noise coupling.
• Use thicker traces for high current power nets, especially near connectors.
• Include stitching vias around the edges of board to connect ground and power planes. This improves bypassing.
• Route traces perpendicular to slots or openings in planes. Avoid routing signals parallel to slots.
• Use blind and buried vias when possible to optimize routing between inner layers.
• Assess RF designs for transmission line effects. Control impedance with trace width/spacing.

Following good layout practices for a 4 layer PCB will help ensure the board meets all of its signal and power integrity requirements.

Example 4 Layer PCB Stackups

Here are a few examples of typical 4 layer PCB designs and their layer stackup configurations:

Digital Electronics Board

This board has both digital and analog circuits and needs to manage moderate speed signals and power requirements. A signal-ground-power-signal stackup is used:

LayerUseTopDigital and Analog SignalsLayer 2Ground PlaneLayer 33.3V and 5V PowerBottomMixed Signal Routing

The ground plane isolates the top and bottom layers. The power plane distributes multiple voltages.

RF Transceiver Board

To maintain signal integrity at high RF frequencies, this wireless board uses a ground-power-ground-signal stackup:

LayerUseTopGround PlaneLayer 23.3V Power DistributionLayer 3Ground PlaneBottomRF Circuits and Traces

The double ground plane configuration shields the RF components and traces from EMI and crosstalk.

High Speed Data Acquisition Board

With multi-gigabit data channels, a signal-ground-signal-power configuration is used:

LayerUseTopAnalog SignalsLayer 2Ground PlaneLayer 3Digital SignalsBottom5V and 3.3V Power

This provides isolation between the analog and digital signals, minimizing interference.

Summary

• A 4 layer PCB is stacked starting with signals on the top, followed by ground, power, and bottom signal layers.
• Key stackups are signal-ground-power-signal, signal-power-ground-signal, ground-power-ground-signal, and signal-ground-signal-power.
• Carefully consider signal integrity, power delivery, thermal design, EMI, component placement, manufacturability, and cost when selecting the layer configuration.
• Use good routing practices on 4 layer boards like orthogonal traces, minimizing vias, flooding with ground, and proper via transitions.
• Choose a 4 layer stackup that is optimized for the design requirements like RF performance, high speed signals, power levels, etc.

FAQ

What is the most common 4 layer stackup?

The most common stackup is signal-ground-power-signal. This provides a good balance of shielding, signal routing, and power distribution for many mixed signal PCB designs.

How do you decide on 4 layer stackup?

Consider factors like signal types (digital, RF, analog), speed and isolation requirements, power levels, thermal design, EMI, manufacturability, and cost constraints. Choose a stackup that optimizes for the critical design parameters.

Is 4 layer better than 2 layer?

4 layer PCBs provide better performance than 2 layer for many designs. The additional ground and power planes improve shielding, power delivery, and heat conduction. 4 layers can support more complex, higher speed designs. However, 4 layers are also more expensive.

What is the best material for a 4 layer PCB?

FR-4 glass epoxy is the most common and cost effective material for 4 layer PCBs. For boards with very high speeds (> 5Gbps), RF signals, or special thermal/mechanical needs, materials like polyimide, ceramic filled epoxy, or Rogers laminates may be preferable.

How do you design a 4 layer board?

Start by planning the layer stackup and usage. Use CAD best practices for trace routing, plane design, via transitions, and component placement. Run signal and power integrity simulations. Accurately specify characteristics like trace widths, dielectric materials, finishes, and board thickness for fabrication. Review with your PCB manufacturer.