The significance of printed circuit boards lies in their ability to replace complex wire connections. However, as human needs have evolved, much like the transition from land transportation to air travel, electronic technologies have been constantly advancing. Simple single-layer PCB and double-layer PCB are no longer sufficient to meet these evolving requirements.
In response, circuit boards with more intricate layer structures have emerged. Multilayer PCB incorporate additional conductive material layers between insulating substrate materials. This layered design greatly facilitates the creation of complex electronic circuits within a compact form factor. If you want to delve deeper into this topic, TechSparks is the place to explore it further!
What is Multilayer PCB
In the construction of a multi-layer PCB, additional layers are created through lamination, and each layer contains traces and interconnecting wires. In theory, the number of layers in a PCB can be virtually unlimited, as long as the manufacturing technology can support it. So, how do you determine the number of layers in a PCB?
For a 2-layer PCB, it consists of a top layer, a bottom layer, and a dielectric layer in between. In this context, we typically ignore the dielectric layer and consider the number of copper layers contained within the PCB as the PCB’s total number of layers. For example, in the illustration above, the circuit board has 6 copper layers, making it a standard 6-layer PCB.
In the provided image, you can observe that both the top and bottom layers each contain one copper layer, while the inner layers consist of two copper layers separated by a dielectric layer. Consequently, the number of layers in a multilayer printed circuit board is often an even number, which is an important distinction. While it’s possible to mention 3-layer PCB, consisting of a top layer, bottom layer, and a middle copper layer, they are rarely found in modern electronics. Therefore, a multilayer PCB is defined as a PCB board with 4 or more layers, and the number of layers is determined by the copper layers present.
Single vs. Double vs. Multilayer
|Single Layer PCB
|Double Layer PCB
|Only one side is covered with copper foil and the circuit connection pattern is stamped on it.
|Both the top and bottom layers of the PCB have copper layers.
|Apply more copper layers inside the PCB, except for the top and bottom.
|Components are placed on one side of the PCB only.
|Can be arranged on both sides of the PCB to increase component density.
|Inner layer routing is allowed. Under the same size, the higher the number of layers, the higher the routing and component density.
|Longer distance between the signal plane and the ground plane can result in higher signal return path impedance, affecting signal integrity.
|Lack of inner layers to route power and ground planes can cause signal integrity issues.
|Provide required power and ground planes that reduce signal interference and signal reflection reduction and ensure accurate transmission of data.
|It is difficult to achieve miniaturization, which is one of the main reasons why it has been phased out.
|Although electronic volume can be reduced using SMT technology or small components, there are still drawbacks.
|Best for the miniaturization of electronic devices. Their compact design, high component density, and effective space use help the production of small and portable projects.
|Although the design is simple, it is difficult to realize diversified designs of products.
|Limited impedance control and weaker power distribution make it less flexible than multilayer circuit boards due to routing space.
|Highly flexible to meet various requirements through custom layer configuration combined with advanced features such as blind and buried vias.
|Bare boards are inexpensive to manufacture, but multiple pieces may be required in a project to perform the desired function.
|Due to the impact of required materials and process difficulty, the higher the number of layers, the higher the price.
|Simple circuits, prototyping, education and training.
|Widely used in consumer electronics and cost saving projects.
|The application fields are unlimited, and the higher the number of layers, the more complex the application.
Multilayer PCB Board Manufacturing Process
- Design: The manufacturing of multilayer PCB begins with the design phase. However, as the number of layers increases, the layout complexity also rises. Various considerations come into play, such as routing signals through multiple layers, arranging the power supply sensibly, and configuring inter-layer interconnections. Therefore, adhering to PCB design guidelines throughout the design process is crucial for ensuring accuracy.
- Layer Stacking: To create multiple layers of conductive materials, the PCB is stacked according to the schematic design. This stacking process involves the use of prepreg layers, which are epoxy resin layers bonded onto the core layer.
- Copper Cladding: The substrate serves as an insulating layer that supports the components. To facilitate electrical conductivity on the components, a thin layer of copper is applied to the PCB substrate.
- Etching: Chemical materials are employed to remove excess copper from the board, defining the traces. Since it’s a multilayer board, both the outer and inner layers undergo this etching step.
- Drilling: CNC or laser drilling equipment is used to create holes on the PCB, which serve purposes such as component mounting and electrical connections. In addition to through holes, there are other types of vias that are manufactured based on project requirements.
- Plating: For holes that serve as electrical connections, an electroplating process is employed to deposit a layer of copper on them. This process is known as Plated Through-Hole (PTH) plating.
- Protective Layer: To safeguard the circuit, a solder mask or protective covering layer is applied to the surface of the PCB, depending on the PCB type.
- Testing: Comprehensive testing is conducted throughout the PCB manufacturing process. This includes AOI after both the inner and outer circuits are manufactured, as well as X-Ray inspection, functional testing, environmental testing, and more. Some of these tests are covered by the PCB manufacturer, while others may incur additional costs depending on project requirements.
Multilayer PCB and Via
As mentioned earlier, the purpose of multilayer PCB is to achieve a denser component layout through a multilayer structure, and the electrical connections between different layers are accomplished using vias. Here’s a brief introduction to the concept of vias.
Vias, also known as “vertical interconnection channels” on circuit boards, offer numerous advantages, including electrical connections, signal routing, power distribution, and heat dissipation. When designing, they can be created as through holes, partially through holes, or concealed within the board, depending on specific needs. Here are a few common types:
- Through Holes: These pass through the entire circuit board and are divided into N-PTH and PTH. They can serve for mechanical fixation or provide electrical connections for components.
- Blind Vias: These facilitate connections between the outer and inner circuits without traversing the entire circuit board.
- Buried Vias: These are concealed within multilayer PCB, enabling efficient handling of complex designs.
- Microvias: Commonly found in multilayer PCB, microvias are valuable for creating fine-pitch components and routing them within tight spaces. This is particularly beneficial for high-density circuit boards.
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