The advent of multilayer PCBs has contributed largely to the electronics industry. Due to the increasing need for more compact electronic devices, there is more demand for multilayer PCBs. A 4 layer PCB is a type of multilayer PCB.
This type of multilayer board requires serious attention during its fabrication. Therefore, it is important one understands everything about this PCB type. In this article, we will discuss 4 layer PCB stackup, its fabrication, design, and more.
What is a 4 Layer PCB Stackup?
A 4 layer PCB is a type of multilayer circuit board that features two blind vias and internal layers. This PCB type features 4 layers that rout electrical signals. The layers include a top layer, 2 inner layers, and a bottom layer. The top and bottom layers serve as the outer layers. The two inner layers are in the core. These layers serve as power planes or can be used for signal routing.
Therefore, a 4 layer PCB stackup can include 3 signal layers and a ground plane. It could also feature 2 signal layers, a GND layer, and a VCC layer. The arrangement of these layers plays a significant role in the performance of a 4 layer PCB. For instance, a good PCB stackup will minimize crosstalk and radiation.
The ground and power planes are the inner layers of a 4 layer PCB stackup. The traces are usually on the outer layers. The ground and solid planes improve the quality of traces and minimize EMI emissions. Also, it is much easier to connect components to the planes. A 4 layer PCB stackup featuring ground planes on outer layers is the best option.
The top layer is the uppermost layer that includes the solder masks and surface finishes. The second and third layers include the ground and power plane. The bottom layer is the last layer of a 4 layer PCB stackup which carries the weight of the three layers.
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Types of Arrangement on a 4 Layer PCB Stackup
There are two major types of arrangements on a 4 layer PCB stackup. This type of PCB has only four layers. However, there are two different ways to arrange these layers.
Type 1
Signal
GND
VCC
Signal
GND is the ground plane while VCC is the power plane. In this type of arrangement, the ground and power planes serve as the inner layers of the PCB stackup. Manufacturers can switch the ground plane and the power plane. This depends on the layer having more signals. Here, the signal layers are close to the plane layers. This means that the return current of a signal can move on an adjacent plane.
This reduces the loop area caused by the current flow; hence reducing the current return path inductance. When the inductance return path is low, there is a reduction in board radiation. It is important to keep the signal layers very close to the plane.
Type 2
GND
Signal
Signals
GND
This stackup is ideal when all ground pins will not be connected through vias. For a 4 layer PCB stackup, this might be a better option. This is because the signal layers are adjacent to the GND. The two signal layers are also coupled to the adjacent planes in this type of stackup.
The ground planes can also protect the signal layers. Manufacturers will achieve better results when traces are kept to plane height. The two ground planes in this stackup help to minimize common-mode radiation. The ground planes serve as a shield for the signal layer. Since they serve as a shield, they can withstand the radiation from high-speed signal traces.
This type of stackup doesn’t include a power plane. The manufacturer can either use a gridded power structure or power pours.
4 Layer PCB Fabrication
When a PCB has multiple layers, it helps to ensure the proper distribution of energy in a circuit. A 4 layer PCB reduces EMI and cross-interference. However, the fabrication of these boards requires certain procedures. If these procedures are carefully carried out, 4 layer PCB fabrication can be a simple task. A 4 layer PCB stackup should be fabricated with quality in mind.
The fabrication process of a 4 layer PCB begins with preparing the necessary materials for the stackup. You will need materials like substrate and copper layers. Ensure that all the materials are in accurate sizes and measures. The substrate materials will function as insulation materials. These materials prevent any heat flow or electric current through the stackups.
The next stage in a 4 layer PCB fabrication involves printing the layout. For this purpose, a plotter printer is required. The printer makes the film of the 4 layer PCB stackup. The film will guide you through the fabrication process. Once you print the layout, you will align the films.
The next step is the inner layer etching. This involves using a chemical substance to remove unwanted parts on the board. Ensure you thoroughly clean resists left on the copper. You will need to inspect the board to check for any defects.
There are different inspection methods. However, manufacturers can employ automatic optical inspection. With this, you will be able to trace and detect incomplete patterns. Then, you can arrange all the layers of the stackup using a machine. This machine will heat up and join all the layers of the stackup.
4 Layer PCB Design
A 4 layer PCB design is much easier using design software. There are certain procedures manufacturers carry out to achieve a good PCB design.
Create the schematic
To start a 4 layer PCB design, you need to create a schematic. The schematic serves as the blueprint or the foundation for your design. You will need it throughout your design process. While creating the schematic, ensure the details below are included:
- Components for the layer stackup design
- Relationship between the several groups of components
- Connection between the various components
You can make a PCB layout for the design. This layout will include the file where you will save the design. Ensure you include the dimensions on your stackup. You will have to verify the design and get all the documents needed for the project.
Capture the schematic
During the design process, you will need to link several aspects of the stackup. The manufacturer needs to link the components, the aspect of the layer stackup, and the BOM in the right place. The BOM refers to the bill of materials. If you need to make any changes, you can update the entire file.
Design the PCB layer stackup
Before mounting the electronic components, ensure you determine the appropriate layer stackup. In this stage, you will need to determine the type of laminate materials for the 4 layer PCB stackup. You can regulate the impedance in high speed designs by using the impedance profiler.
Tips for 4 Layer PCB Design
Careful PCB design is very important as a poorly designed PCB can cause a lot of damage. Therefore, PCB manufacturers should pay attention to a 4 layer PCB design.
Determine the arrangement of layers
Since a 4 layer PCB stackup features four layers, you need to determine the arrangement of the layers. There are rules to follow when deciding the arrangement of a 4 layer PCB.
- Place the signal layers close to each other
- Ensure the signal layer is adjacent to a plane
- Place signal layers and internal power layers closer
- Use multiple ground planes. These planes reduce radiation and the ground impedance of a board.
- Minimize the spacing between the power layers and the ground layers
- Avoid placing signal layers in adjacent positions
Determine material type
The thickness of the signal layer plays a significant role in a PCB stackup. You need to determine the type of material you will use for your layers. Decide the thickness of the prepreg and the core. This will help you regulate the thickness of the signal layer.
It is important you evaluate the properties of the material for your PCB design. These properties determine the types of material used for the 4 layer PCB stackup.
- Consider the glass transition temperature. This will help you know how the material will react under heat exposure
- Evaluate the coefficient of thermal expansion. CTE measures how a material changes in size due to varying temperatures.
- Consider the dielectric constant of the material.
Decide the routes and vias
The manufacturer has to decide the routing of the traces on the stackup. The copper weights, types of vias and routes, and the vias location have to be determined.
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Advantages of a 4 Layer PCB
4 layer circuit boards offer a lot of advantages. Four layer circuit boards feature four distinct copper layers. These layers can be used for power and routing.
Improved signal routing
A 4 layer PCB stackup enables signal routing inside the PCB stackup. The circuit board features GND on the bottom and top layers. However, it is common for the inner layers to be ground plane and power plane. The arrangement of layers on a 4 layer PCB stackup can play a significant role in signal routing. A good arrangement enables easier signal routing.
EMI Shielding
This is another advantage of a 4 layer PCB. Since this circuit features four different layers, it offers better EMI shielding. The ground planes serve as a shield for the inner layers. These planes protect against electromagnetic impedance. They can also prevent any EMI discharges in the circuit.
Thermal control
A 4 layer PCB stackup offers better thermal control. The several segments on a PCB will overheat and will need cooling over time. The design of a 4 layer PCB can distribute warmth throughout the board layer stackup.
Higher assembly density
These PCBs incorporate four separate layers into one board; hence, they increase the assembly density. Circuit boards can connect better due to the interconnection between close quarters.
Smaller and compact designs
A 4 layer PCB is smaller in size. This makes it ideal for the production of today’s electronic devices. Tablets, laptops, smartphones, and smartwatches are examples of devices that feature this PCB.
Important Tips for a 4 Layer PCB Manufacturer
The manufacturing of a 4 layer PCB should be done by an expert. A 4 layer PCB manufacturer needs to understand certain rules for 4 layer PCB stackup. Below are useful tips for a 4 layer PCB manufacturer;
- Use ground planes in your stackup design. This is because they enable signal routing in a stripline or microstrip configuration. Ground planes also help to minimize the ground impedance; hence, the ground noise
- Ensure you route high speed signals on intermediate layers placed between the different levels. The ground planes will serve as a shield and absorb the radiation from the tracks.
- Couple the mass and power planes tightly.
- Use a cross section to prevent deformations
- It is possible to enhance noise and EMI performance. Make the insulation between the signal layer and the plane adjacent to it thin.
- Consider the thickness of each signal layer. For a 4 layer PCB stackup, there is a standard thickness.
- Evaluate the thermal, electrical, and mechanical properties of the material you are using.
- Make use of good software for your 4 layer PCB design.
A 4 layer PCB manufacturer must understand the importance of good layering in a stackup. Good layering can help to minimize board impedance. A stackup that is poorly designed can cause circuit radiation. As a 4 layer PCB manufacturer, you need to determine the spacing between layers. Proper spacing will help to improve the performance of the board. The tips highlighted above help manufacturers design a good 4 layer PCB stackup.
Conclusion
The 4 layer PCB stackup is one of the commonest type of multilayer boards. This circuit board has its unique features; hence, it is ideal for use in several applications. The arrangement of layers on a stackup determines the performance of a 4 layer PCB to a large extent. In this article, we discussed the essential facts about a 4 layer PCB stackup.
