What is the Layer of a PCB?

Printed Circuit Board (PCB) is the foundation of electronics. It is used to mechanically support and electrically connect electronic components. The layout and layers in PCB form the backbone on which components like integrated circuits, resistors, capacitors are assembled.

The PCB acts like a pathway for signals and power distribution between components. Layers allow signals to pass from one part of the board to other without interfering other signal layers. Understanding layers is crucial in PCB designing to avoid signal loss, electrical noise and maintain signal integrity.

Types of PCB Layers

There are several types of layers that make up modern, multi-layer PCBs:

Signal Layers

Signal layers, as the name suggests, carry signals between different components and ICs on the PCB board. Signal layers will have a large amount of copper traces etched into them that transmit analog and digital signals across the board.

Power Plane Layers

Power plane layers distribute power from the power supply to different components on the board. These layers will consist of large copper fills instead of traces that allow power to be distributed over a wide area. Common power planes include:

• +5V plane
• +3.3V Plane
• GND plane

Ground Plane Layers

Ground plane layers provide a common 0V reference voltage plane over a large surface area of the board. This helps reduce electrical noise interference in signal layers. Like power planes, ground planes will have large areas of copper fill connected together.

Dielectric Layers

Dielectric layers provide insulation between copper layers. Common dielectrics include FR-4, Rogers, and polyimide. The dielectric constant of the material impacts signal properties.

PCB Stackup and Layer Order

The sequence of layers in a PCB is known as the stackup. A proper stackup is critical for routing high-speed signals and ensuring signal integrity. Some common rules of stackup design include:

• Placing power and ground layers close to signal layers
• Adjacent layers should not have signals running in the same direction to avoid crosstalk
• Important signals should be closer to reference planes

Here is an example 6-layer board stackup from top to bottom:

1. Top Signal Layer
2. Ground Plane
3. Power Plane
4. Signal Layer 2
5. Signal Layer 3
6. Bottom Signal Layer

This stackup allows signals to easily reference the power and ground planes, enhancing signal integrity.

PCB Layer Count

The layer count refers to the total number of conductive copper layers in the PCB. Some common layer counts include:

• 2-layer: Used for simple, low-complexity PCBs. Consists of a top and bottom signal layer.
• 4-layer: Common in consumer electronics. Allows a power and ground plane to be added.
• 6-layer: Provides additional routing channels compared to 4-layer. Used in more complex designs.
• 8-layer Used in advanced electronics and allows large amounts of functionality. High layer counts (>10) used in high-speed applications like networking and telecom equipment.

Here is a table summarizing some common PCB layer counts:

Layer CountApplication2Simple electronics, hobbiest4Consumer electronics6-8Advanced consumer and industrial10+High-speed, advanced applications

PCB Layer Functionality

Each layer in the PCB stackup serves a particular purpose:

Top Layer

The top layer hosts surface mount components and exposes copper pads for those components. High-speed traces are often routed on the top layer when length matching is required. Top layer serves as a cooling path to dissipate heat from components.

Inner Signal Layers

Signal layers host traces for routing signals between different components and ICs on the board. Sensitive analog signals should be routed closest to a ground reference.

Ground Layer

Ground layers provide a flooding of 0V potential across the surface area of PCB to serve as ground reference for signals. It reduces EMI and coupled noise. For mixed signal designs, analog and digital ground planes are separated.

Power Layer

Distributes power from the input supply to different components on the board. Important for steady voltage supply to sensitive ICs. Decoupling capacitors are placed to remove noise from power distribution.

Bottom Layer

The bottom layer also contains traces and component pads. High power components are placed on bottom layer to facilitate heat transfer to the board edge or chassis. Exposed copper can be used for shielding.

HDI PCB Technology

High-density interconnect (HDI) PCBs contain stacked microvias and more thinner layers compared to conventional PCBs. HDI allows routing traces between layers using microvias which have smaller pad sizes and spacing compared to through-hole vias.

HDI PCBs provide several advantages:

• More routing channels by increasing layer count
• Smaller track widths and trace spacing
• Accommodate higher component density
• Improved electrical performance

Here is an image showing a comparison of microvias in HDI PCB with plated through hole vias:

With HDI PCBs, designers can achieve >20 layers with trace spacing and widths under 4 mils. This densification helps miniaturize PCB footprint area in applications like smartphones and wearables.

Flexible and Rigid-Flex PCBs

In flex PCB, flexible dielectric like polyimide is used which allows the PCB to bend and twist. By combining standard FR-4 dielectric with flex dielectric, rigid-flex PCBs can be constructed which provide both flexibility and rigidity in different areas.

Rigid-flex PCBs provide various advantages for portable electronics and space constrained applications:

• Can fold and wrap inside small mechanical enclosures
• Dynamic flexing ability
• Reduce connectors between separate rigid boards

Here is an image showing a rigid-flex PCB with both rigid FR-4 and flexible polyimide dielectric:

Blind and Buried Vias

In higher layer count boards, blind vias and buried vias allow inner signal layers to be interconnected without using through-hole drilling.

Blind Vias

Blind vias connect an inner layer to an outer layer. It spans a portion of the PCB thickness. Allows higher routing density.

Buried Vias

Buried vias connect two or more inner signal layers without penetrating top or bottom layers. Allows signals to crossover without using valuable outer layer space.

Buried and blind vias are extensively used in HDI PCB fabrication to interconnect component pads and traces across a high number of layers.

FAQ

What are the most common PCB layer counts used?

The most common layer counts are 4-layer and 6-layer PCB configurations. 4-layer provides two signal layers and allows a full ground and power plane. 6-layer adds two extra routing layers for more complex designs.

Why are power and ground layers important in a PCB?

Power and ground layers help distribute steady voltage levels across the entire area of a PCB. This provides clean power to sensitive ICs and also serves as a low impedance return path from signal layers. It enhances signal integrity through controlled impedance environment.

Can you have multiple ground layers?

For mixed-signal applications, designers often use split ground planes - a noisy digital ground and quite analog ground plane to avoid coupling digital return current noise into analog sections. So for large boards, you can have multiple ground layers dedicated to different sections.

What are the key advantages of HDI PCBs?

HDI PCB provide four main advantages - increased routing channels through more number of thin layers, smaller trace dimensions and spacing, ability to integrate blind/buried vias and higher component density. This allows packing more functionality into the same footprint area.

How flexible are flex PCBs?

Flex PCB dielectric material like polyimide allows tight folding radiuses enabling wrapping flex PCBs around compact mechanical profiles. Some flex PCB variants have flexural endurance >1 million cycles making them suitable for dynamic bending applications.