Introduction
In a 4-layer PCB, there are 4 layers to detect and circulate the electrical signals, these layers are the Top Layer, Inner Layer 1, Inner Layer 2, and Bottom Layer. The TOP and bottom layers are the outer layers where the components are soldered and are used for routing signals. Inner layer 1 and inner layer 2 are inside the core and are used as power planes for signal routing. This article will discuss the 4 layers PCB in full detail.
What is a 4-Layer PCB?
Structure of a 4 layer PCB
A 4-layer PCB is a multilayer circuit board, having two blind vias and internal layers, designated for routing electrical signals. These layers consist of an upper layer, two inner layers, and a lowermost layer. The upper and lower layers act as the external layers, while the two inner layers are present inside the core, a 4-layer PCB stack-up has three signal layers accompanied by a ground plane. Alternatively, it consists of two signal layers, a ground (GND) layer, and a power (VCC) layer, playing a vital role in influencing the performance of a 4-layer PCB.
Advanced 4 Layer PCB Stackup
Cap lamination of a 4 layer PCB
In an advanced 4-layer PCB stack-up, the arrangement is dependent on the layer with the most signals, and the PCB stack-up follows the following configuration:
- Outer Layers: Outer layers consist of the top and bottom layers that are only designed for signal routing.
- Inner Layers: Inner layers consist of the two middle layers that serve as the power ground and signal router.
Let’s discuss each layer in more detail:
- Layer One:
Layer one is situated at the bottom of the PCB. This layer is typically made up of copper. It is the foundational base for the whole circuit board. It provides structural support to the other layers of the PCB.
- Layer Two:
Layer two works as the power plane of the PCB by providing a continuous and clean power source to all the components on the circuit board.
- Layer Three:
Layer three acts as the ground plane by ensuring a durable and clean ground source for all the components of the circuit board.
- Layer Four:
Layer four is situated at the top of the PCB. This layer is functional in routing the signals and offering the connection points for various PCB components.
Here are the functions performed by these layers:
- Signal Layers:
The signal layer is responsible for signal routing and most components are placed on this layer. The inner signal layers are utilized for routing high-speed and differential signals.
- Ground and Power Plane Layers:
These layers give clean and continuous power as well as ground sources for all components while reducing the noise on the board.
- Solder Mask Layers:
Solder mask layers protect the copper traces from soldering. They also provide a uniform and professional appearance to the circuit board.
- Wiring Layers:
Wiring layers are used for interconnecting the different parts of the circuit board by offering structural support to it.
- Mechanical Layer:
The mechanical layer helps in component mounting. It provides full support and stability to the circuit board.
- Ground Layer (GND Layer):
This layer works as a common ground source for all board components and helps minimize noise.
- Reference Layer:
The reference layer supplies the reference voltage for board components and helps to promote the stability of the components.
- Two Additional Routing Layers:
4-layer PCBs often have extra routing layers for signal routing between the different board sections and for improving the connections between them.
4-Layer PCB Stack-Up Types
The manufacturers of 4-layer PCBs provide two standard configurations for circuit boards, designing the VCC and GND layers to suit the specific requirements of the application.
Configuration 1: Signals – GND – VCC – Signals: In this layout for a 4-layer PCB, the uppermost layer is for critical signals. It is advisable to keep the core board from becoming excessively thick. This minimizes the distributed impedance in the ground plane and in the power supply.
Configuration 2: GND – Signals – Signals – GND: This stack-up arrangement is suitable for enhancing the electromagnetic compatibility (EMC) performance of the circuit board. It maintains a close proximity between the signal layers and the planes. Incorporating a sizable core layer between the ground and power planes helps in maintaining the overall thickness of the substrate.
Considerations to keep in mind for a 4-layer PCB stack-up
Designing a 4-layer printed circuit board can be challenging for designers interested in making PCBS of greater size. Instead of dedicating layers to routing, they prefer to share space with copper pours and broad traces for power distribution. Following are the design factors that should be considered while working with a 4-layer PCB stackup.
Grounding Is Essential
It is necessary to make a different return path for creating optimal signal integrity in the PCB to achieve high-speed signals. This principle is applied to all signal types including, digital, analog, and power-related signals. To achieve the best grounding, one of the internal layers in the 4-layer board is dedicated to the ground plane only. The outer layers are allocated for component placement and the internal layer is allocated for signal and power routing.
Power Delivery
Modern designs demand the components requiring diverse voltage level needs. Typically, designers address this problem by utilizing a combination of segmented planes and wide traces for power routing. Moreover, the bypass capacitors must be carefully positioned and routed. It will guarantee that they have the shortest and most direct paths to their designated supply pins.
Escape Routing
Designing a 4-layer circuit board is a highly complex procedure. If the number of pin rows on a large-scale BGA component are more, then more layers will be needed for escape routing. The external rows of the component can be directly routed and the subsequent rows will make the connections through vias. Ultimately, a 4-layer board can exhaust its routing for accommodating a large device.
Trace Routing
Mixed-signal designs are common in modern electronics and it is necessary to follow the routing guidelines while working with a compact 4-layer board. A continuous ground plane is necessary in mixed-signal PCB design. Avoid routing when there are interruptions in the reference ground plane. Try to isolate digital and analog circuitry from each other as much as possible.
Copper Balancing and Heat Dissipation
Thermal equilibrium is maintained by utilizing ground vias to dissipate the heat created by large components in the ground plane. It is necessary to manage all the prerequisites for designing a successful 4-layer circuit board. It is mandatory to utilize the capabilities provided by the PCB design software. Some of the tool features and functionalities that are beneficial in this process are given below.
4 Layer PCB Fabrication
The fabrication process for a 4-layer PCB begins by gathering the necessary things for the stack up including substrates and copper layers. The layout is printed, and the films are aligned. The process involves inner layer etching through the chemical; any remaining resists on the copper are cleaned thoroughly. Subsequently, a board inspection is conducted to identify and eliminate any defects.
How to design a 4 Layer PCB
PCB Layers Design
Creating a 4-layer PCB design is quite simple due to the use of design software. Manufacturers follow special procedures to achieve a well-executed PCB design.
Create the schematic
For starting a 4-layer PCB design, the first step is to create a schematic. This schematic serves as the fundamental blueprint for the PCB design. It is crucial to follow the following details while creating a schematic:
- Components of the layers stack up design.
- Relationship between different PCB components.
- Connection linking between various PCB components.
Capture the schematic
During the design phase, it is necessary to generate connections between various elements of the stack up. Manufacturers ensure that the PCB components, the specifications of layer stackup, and the Bill of Materials (BOM) are interconnected. The bill of materials is a detailed list of components and materials needed for the project. If modifications are required during the design phase then, the entire file can be revised accordingly.
Design the PCB layer stack up
Before the installation of electronic components, it is vital to follow the suitable layer stackup configuration. During this phase, the laminate materials are required to be specified in the 4 layer PCB stackup. It is necessary to effectively manage the impedance in high-speed designs. An impedance profiler can be utilized to solve the matters related to impedance.
Determine the arrangement of layers
A 4 layer PCB stack consists of four layers, it is necessary to establish the layer arrangement by following some special guidelines:
- Position the signal layers close to one another.
- The signal layer should be adjacent to a plane layer.
- Keep the signal layers and internal power layers close to each other.
- Use multiple ground planes to end radiation and ground impedance on the circuit board.
- The gap between the power layers and the ground layers should be minimized.
- Avoid placing signal layers adjacent to each other.
Determine material type
The thickness of the signal layer plays a significant role in a PCB stackup. Determine the material required for the layers of the PCB. Try to decide the thickness of the prepreg and the core. This will help regulate the thickness of the signal layer.
PCB Layer Configuration Strategies
There are various methods for designing and constructing a circuit board. Although FR-4 is a common material group that is used for PCB substrates. The designers make sure to devise the most suitable PCB layer configuration strategy for their specific design. Nevertheless, various aspects of the 4-layer circuit board are considered for understanding the requirements of the PCB.
Understanding the Requirements of a 4-layer PCB
To effectively design the board’s layout, it is essential to have an understanding of the functions that the PCB is intended to perform. The following are the prerequisites that are considered for a 4 layer PCB:
Purpose:
Designers try to understand the purpose of creating a durable 4 layer PCB. They check if the board is functional in the surroundings in which the board is anticipated to function efficiently. Materials are modified to ensure the flexibility of the board with harsh conditions like extreme temperatures or high humidity levels. Manufacturers check the board and the materials used for its construction to ensure that the purpose of the PCB is achieved.
Size and shape:
The size, shape, and number of board layers are adjusted according to the required size of the PCB. The accurate size and shape of the PCB is essential to accommodate the required trace routing. The size and shape can influence the manufacturing process of the board. It is necessary to consider the size and shape of the board during the manufacture of 4 layers PCB.
Technology:
Technology is an important factor when considering the layer stackup configuration of the board. It will be influenced by the type and the circuitry speed of the components. Components such as BGA footprints having large sizes and high-density pin configurations will make it necessary to have additional layers for routing all of their pins effectively. This requirement enables the best functioning of the PCB if fulfilled properly.
Features:
The required number of layers is dependent on the features of the circuit board. The presence of holes, brackets, cut-outs, and other distinctive features has a direct impact on the PCB. Designers study the features of the PCB to achieve a functional product. The consideration of these requirements eliminates the flaws in the product.
Signal integrity:
A specific design criterion is considered to achieve and ensure the signal integrity in the layered PCB. Requirements include controlled impedance routing and EMI shielding leading to increased ground plane and routing layers.
Power integrity:
The power needs are dependent on the layers of the PCB. The additional layers are necessary to establish an excellent power distribution network on the board.
Advantages of a 4-Layer PCB
Four-layer circuit boards have unlimited benefits. They are made up of four separate copper layers that are utilized for both power distribution and signal routing.
Improved signal routing
A 4 layer PCB stackup helps in signal routing in the PCB structure. The circuit board has ground (GND) layers on both the top and bottom. However, the inner layers often work as ground and power planes. The special arrangement of layers in a 4 layer PCB stackup can greatly influence the signal routing. A planned arrangement enhances the process of routing signals.
EMI Shielding
This is another advantage of a 4 layer PCB. It provides superior EMI shielding with its four distinct layers. The ground planes function as protective shields for the inner layers, protecting against electromagnetic interference and EMI emissions in the circuit board.
Thermal control
A 4 layer PCB stackup gives enhanced thermal management. Various components on a PCB can generate excess heat that requires cooling. The design of a 4 layer PCB ensures the even distribution of heat across all the layers present in the stackup.
Higher assembly density
These 4 layers PCBs are made up of four distinct layers in a single board. They enhance the feature of assembly density. The close packing of these layers allows improved interconnections between the components of the circuit board.
Smaller and compact designs
A 4-layer PCB is compact and small, making it well-suited for the manufacturing of various electronic gadgets. Devices such as tablets, laptops, smartphones, and smartwatches utilize this type of PCB.
Applications of a 4-Layer PCB
The notable benefits of the circuit boards result in their widespread adoption in various sectors. Here are several well-known applications in which circuit boards are extensively used:
- Medical heart monitors
- Personal computers
- GPS technology
- Operational file servers
- Mobile phones gadgets
- Supporting data storage
- Transmission of signals
- Conducting weather analysis
- Equipping space probe instruments
- X-ray machines
- Atomic accelerators
- CAT scan technology
- Fiber optic sensors
- Central fire alarm systems
- Amplified cell phone transmission and repeaters
Conclusion
Four-layer PCBs are widely used multilayer boards in electronic devices. They are distinguished by specific characteristics that make them suitable for various applications. The attachment of layers in a stack influences the performance of a four-layer PCB. In this article, various key insights about the four-layer PCB stack-up have been explored.
FAQs
What is the rule for 4 layer PCB design?
The rule for 4 layer PCB design is 4 layer PCB = 3 Signal layer + GND layer or 4 layer PCB = 2 signal layer + a VCC layer + a GND layer.
Why do we use 4-layer PCB?
We use 4 layers of PCB due to higher levels of signal integrity and reduced interference levels.
What is the minimum thickness of a 4 layer PCB?
The minimum thickness of a 4 layer PCB is 1.6mm.
What is a 4 layer PCB made of?
4 layer PCB is made up of 4 layer of glass fiber.
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