Is it better to have more layers in a high-density multi-layer board? What issues should be considered during design and manufacturing?

In the field of PCB design, many people believe that the more layers a board has, the more advanced the technology and the stronger the performance. Especially in servers, communication equipment, automotive electronics and artificial intelligence hardware, cases of high multi-layer boards with 12 layers, 16 layers, 20 layers or even 30 layers or more are frequently seen. So, is it better for high multi-layer boards to have as many layers as possible? Does increasing the number of layers necessarily improve product performance?

In fact, although increasing the number of layers can enhance the wiring capacity and signal performance, it also brings about problems in terms of cost, process, reliability and production cycle. For electronic products, a reasonable design of the number of layers is far more important than blindly increasing the number of layers.

What does a higher number of layers in a high-density multilayer board mean?

The increase in PCB layers is essentially for obtaining more wiring space and better electrical performance.

For example:

The 4-layer board is suitable for ordinary control systems.

The 6-layer board is suitable for medium-complexity products.

The 8-layer board is suitable for high-speed interface products.

Above the 12th floor, it is mostly used for high-performance equipment.

After the number of layers increases, the following can be achieved:

More signal layers

More power layers

More grounding layers

More complex wiring structure

For high-pin-count chips and high-speed signal systems, this advantage is particularly significant.

But this does not mean that the higher the number of layers, the better.

Why can't the number of layers be increased blindly?

The cost will soar significantly.

Each time the number of layers increases, the manufacturing cost will rise noticeably.

The reasons for the increase in costs are:

The number of circuit boards has increased.

Increase in semi-cured sheets

The pressing process has been enhanced.

The difficulty of drilling has increased.

The number of testing items has been increased.

For example:

The manufacturing cost of a 16-layer board is usually much higher than that of an 8-layer board.

Compared with the 16-layer board, the cost of the 24-layer board tends to increase more significantly.

If the product requirements are not complex, an overly high level of hierarchy will lead to waste of resources.

The production cycle becomes longer.

The higher the number of multi-layer boards:

More processes need to be carried out.

For example:

Inner layer fabrication

Repeated pressing

Drilling

Electroplating

AOI inspection

Inter-layer alignment detection

Every additional floor will likely increase the production time.

For projects that require rapid delivery, an excessive number of layers can affect the progress of research and development as well as mass production.

良率下降

The increase in the number of layers means

The interlayer structure is more complex.

Possible problems may include:

Interlayer offset

Copper porosity defect

Curvature deformation

Pressing the cavity

Signal layer damage

For example:

The 8-layer board may have a relatively high yield rate for mass production.

Products above the 20th floor require extremely strict control over the manufacturing process.

Once the yield rate drops, the overall cost will increase further.

The heat dissipation design becomes more challenging.

There are more copper layers and dielectric layers inside the multi-layer PCB.

The heat conducts through a longer internal path.

If the design is not reasonable:

It is prone to form local hotspots.

Regarding:

CPU board card

GPU accelerator card

Power control board

Higher layers do not necessarily facilitate heat dissipation.

The thermal management design must be considered simultaneously.

Under what circumstances would it be necessary to increase the number of layers?

Not all products require high-density PCBs.

Usually, the number of layers will be increased only in the following situations.

High-pin BGA chip For example:

FPGA

CPU

GPU

AI chip

The number of pins can often reach several hundred.

It is difficult to complete the wiring with the standard number of layers.

Increasing the number of layers can enhance the routing space.

There are a lot of high-speed signals. For example:

PCIe

DDR5

USB4

800G optical module

These interfaces require:

Impedance control

Equal-length design

Complete reference plane

Increasing the number of layers is beneficial for the layout of high-speed signals.

Multi-power system

Complex equipment may exist simultaneously:

12V

5V

3.3 volts

1.8 volts

1.2V

Multiple power supply networks.

Adding a dedicated power supply layer can improve the quality of power supply.

Products with high EMC requirements For example:

Medical equipment

Automotive Electronics

Avionics

Adding a grounding layer is beneficial for:

Reduce radiation

Improve the ability to resist interference

What issues should be considered when designing high multi-layer boards?

Properly plan the stack-up structure.

The stack-up design determines the performance of the PCB.

The design principles are as follows:

The signal layer is close to the reference plane.

This ensures the integrity of the signal return path.

The power layer is paired with the ground layer.

This improves power integrity.

The layered structure maintains symmetry.

Reduces warping risk.

High-speed layer-independent planning

Avoid being interfered by other signals.

Pay attention to impedance control.

High multilayer boards are often used in high-speed products.

Impedance design must be planned in advance.

Key considerations:

Medium thickness

Copper thickness

Line width

Pitch

Avoid difficulties in making adjustments later on.

Control the number of vias

Many designers prefer to solve the routing problems by frequently changing layers.

In fact:

Too many vias can lead to:

Signal loss increases

Impedance discontinuity

The manufacturing process has become more difficult.

High-speed lines should minimize the number of through holes.

Back-drilling technique should be adopted when necessary.

Pay attention to power integrity design.

High-performance chips have extremely high requirements for power quality.

When designing, one should:

Properly lay out the power layer

Add decoupling capacitors

Shorten the power supply path

Reduce power supply noise.

What issues should be paid attention to during the manufacturing of high-density multi-layer boards?

Lamination control

The more layers there are:

The greater the degree of pressing and bonding.

Key Control:

Temperature curve

Pressure curve

Resin flow

Avoid interlayer separation.

Inter-layer alignment accuracy

The alignment error of the upper layer panels will accumulate and magnify.

Must be controlled:

Inner graphic positioning

Compression shrinkage compensation

Drilling accuracy

Ensure reliable interlayer connections.

Quality of deep hole processing

The depth of the holes in the upper layer plates has increased.

Drilling off target

The inner wall of the hole is rough.

Uneven electroplating

The drilling and electroplating processes need to be optimized.

Warpage control

Higher-pitched boards are more prone to warping.

Common control methods include:

Symmetrical stacking

Balanced copper surface design

Reasonable plate thickness planning

Ensure the quality of subsequent assembly.

How to choose the appropriate number of layers?

In the industry, a general principle is usually followed:

It is sufficient to meet the functional requirements; there is no need to pursue overly complex layers.

For example:

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