What is a PCB?
A printed circuit board (PCB) is the base of almost all electronic devices and equipment. It is used to mechanically support and electrically connect electronic components through conductive copper pathways etched from copper sheets laminated onto a non-conductive substrate. PCBs provide the necessary interconnections between components and act as the foundation for designing and building electronic devices.

PCBs can be single-sided with copper traces on one side, double-sided with traces on both sides, or multi-layer with traces on multiple layers. As electronic devices become more complex and compact, multilayer PCBs are commonly used to accommodate more interconnections in a smaller space.

Benefits of using 8 layer PCBs

Compared to PCBs with fewer layers, advanced 8 layer PCBs provide several advantages:

More routing channels

With 8 layers, there are more layers available for routing signals which enables more complex routing without congestion. This increased routing capacity allows for higher component density.

Better signal integrity

Additional layers help segregate different signals to minimize noise and cross-talk. Sensitive signals like clocks can be isolated between ground planes while high speed signals can have multiple reference planes on inner layers. This improves overall signal integrity.

Better power distribution

Entire layers can be dedicated as ground or power planes. Multiple power planes help evenly distribute current, reducing voltage drops across the board. Noise from voltage fluctuations is also reduced.

Higher frequencies

Ground planes between signals act as a return path and control impedance. This helps maintain signal integrity at higher frequencies compared to 2 layer boards. Many RF and high speed digital circuits need >4 layers.

Fewer jumpers

With more layers, fewer jumpers wires are needed to route connections. This improves manufacturability and reliability.

Miniaturization

Higher component density from increased routing capacity allows for smaller and more compact PCBs. This is critical for fitting into smaller enclosures.

Lower EMI

Additional ground planes help block and absorb EMI emitted from components. This improves EMI performance.

When should 8 layers be used?

Here are some common applications where 8 layer PCBs provide advantages:

• High density interconnect (HDI) PCBs – When there is not enough space for routing with fewer layers
• RF/microwave boards – Isolating signals from ground planes reduces losses
• Mixed signal boards – Segregating analog and digital domains reduces noise
• High speed digital – Maintaining signal integrity at higher speeds
• Power electronics – Distributing high currents evenly to reduce voltage drops
• Complex multi-function boards – Combining circuits that need >4 layers
• Size constrained boards – Enabling higher component density

In general, 4-6 layers should be considered for medium complexity boards while 8+ layers are good for maximum complexity. Specifically, 8 layers provides advantages over 6 layers by adding 2 more routing layers and allowing 3 complete ground plane layers. The specific layer count depends on board complexity and layout constraints.

Stackup design

The stackup defines the layer structure of the PCB. For 8 layer boards, common stackups are:

Signal-Power-Ground-Signal-Signal-Ground-Power-Signal

Top and bottom layers are used for component placement and high density routing. Complete ground plane layers isolate internal power and signal layers.

Signal-Ground-Signal-Power-Signal-Power-Ground-Signal

Two internal ground planes isolate signal layers from the split power planes. This provides good isolation for signals.

Signal-Ground-Power-Ground-Signal-Signal-Ground-Signal

This offers two isolated ground planes around the central power plane. Good isolation with two reference planes for signals.

The exact sequence and use of each layer depends on the specific board requirements. Some considerations for 8 layer stackups are:

• Place ground planes between important signals
• Ensure power and ground planes are continuous
• Alternate signal and plane layers to increase capacitance
• Use a complete ground plane layer if isolating sensitive analog signals
• Assign power and ground as internal layers if distributing high currents

A well designed stackup is critical for maximizing performance in multilayer PCBs.

Layout considerations

Here are some layout techniques to utilize 8 layers effectively:

Partitioning

Group related circuits and partition across the layers. This localized routing improves manufacturability.

High density interconnection

Use microvias and laser drilled vias to route connections between layers. This enables higher component density.

Power distribution

Use an entire layer or polygons on multiple layers for power. Power islands can also be created.

Impedance control

Reference planes adjacent to signals control impedance. Vary spacing as needed for single-ended or differential pairs.

EMI control

Provide compartmentalized shielding on unused board areas. Ground vias can also help.

Thermal management

Distribute heat generating components across layers. Add thermal vias connecting to ground or power planes.

Sensitive circuits

Isolate analog, high speed digital, clock and RF circuits using ground planes. Adds shielding.

Testability

Include test points, vias and planes as needed to probe specific nets and layers during testing.

Manufacturing and assembly

There are some special considerations when manufacturing and assembling multilayer PCBs:

Registration

Each layer must be precisely aligned to avoid exposing copper during drilling. Tight layer registration is critical.

Blind and buried vias

These are used to route between layers without exposing vias. This expands routing options.

Fiducials

Additional fiducials may be needed on inner layers for alignment during assembly. Fiducial holes require extra drilling steps.

Layer thickness

Prepregs must be thicker to obtain the desired finished thickness with more layers. Thinner layers are more prone to issues.

Aspect ratio

Keep drilling aspect ratio in mind. Too high can lead to unreliable plated holes. May require smaller vias.

Test access

Include test points, vias or planes to access inner layers. Specialized probes or fixtures may be needed.

Warpage

The stacking process can induce warpage due to material stresses. Panel designs and layup must minimize warpage.

Impedance

Consistency in dielectric materials is needed to achieve desired impedance across layers. Tighter process controls are needed.

Cost considerations

Some cost factors when using 8 layer PCBs:

• Base material cost is higher – More layers require more raw materials
• More complex process – Additional steps like alignment and lamination
• Lower Yields – Defects spread across more area due to larger size
• Specialized equipment – Needed for microvias, blind vias, precise registration
• Testing overhead – Accessing inner layers requires special probing
• Qualification – Additional verification required to ensure reliability

However, the increased design flexibility, performance and miniaturization provided by 8 layers can justify the additional costs in many applications.

Frequent questions about 8 layer PCBs

What are the typical applications for 8 layer PCBs?

8 layers are commonly used for boards with high complexity and density like RF, high-speed digital, FPGAs, processors, mixed-signal, and miniaturized designs. The increased routing and isolation enables these applications.

How are layers numbered in an 8 layer board?

Layers are numbered 1-8 starting from the top. The top is layer 1, then the layers count up going into the board so the bottom is layer 8. This holds the layers in the correct order.

Can 8 layers replace the need for blind/buried vias?

Blind and buried vias are still useful on 8 layers for connections between non-adjacent layers. But 8 layers does reduce the need compared to fewer layer boards.

Does adding more layers infinitely improve performance?

There are diminishing returns as you go past 8 layers. Issues like aspect ratio, warpage, and yield may emerge. Target the minimum number of layers needed for performance and density.

What are the main disadvantages of 8 layer boards?

Cost is higher, yields lower, assembly is trickier, and testing access harder compared to fewer layers. There is also more qualification needed to ensure reliability.

Conclusion

Advanced PCBs with 8 copper layers enable highly complex and dense electronics design while maintaining critical signal integrity. The additional routing channels, isolation, signal layers, and improved power distribution provide significant advantages over simpler 2 or 4 layer boards. When designed properly, an 8 layer stackup can maximize performance. To determine if the benefits justify the extra cost and manufacturing considerations, evaluate layout complexity and circuit requirements early when targeting 8 layer PCB technology.