PCB layers can be one or multiple layers of dielectric and conductive materials that are laminated together to make circuit boards used in manufacturing a wide range of electronic devices and machinery across different industries, such as consumer, automotive, telecommunications, aerospace, military, and medical industry.
A printed circuit board is typically made up of copper, substrate, and resin, and is used to connect and support electrical components in electronic devices. The number of layers and dimensions of a particular board determines the PCB's power and capacity. As layers are added, there is increased functionality, offering vast benefits for PCB fabrication and electronics manufacturing. 
The single layer PCB, also known as Single Sided PCB is a circuit board type that is produced with only one layer of conducting material (usually copper) on one side of the board, while the other side is used for attaching different electronic components to the board.
A single-layer PCB, therefore, embodies a substrate layer, a conductive metal layer, a protective solder mask, and silk-screen, which is used to make a complete board consisting of pads, vias, mounting holes, wires, components, connectors, filling, and electrical boundaries.
Single layer PCBs are the foundation of Printed circuit board technology and still play a significant role in the electronics industry.
Low manufacturing cost
Easier to design with fewer chances of errors
One layer makes the drilling, soldering, and mounting of components uncomplicated.
Ideal for low-density designs
Cost-effective for the production of consumer devices
Ideal for quick, high-volume production.
The simplistic and easily manufactured design of a single-layer PCB may be helpful for users but it also has some impediments.
Not enough space and connection points for circuit boards that require many components.
Slower speed and lower operating capacity.
Added functionality results in a larger size and higher weight, sacrificing compactness.
Single layer PCBs find application in simple home appliances like coffee machines, radios, calculators, as well as camera systems and mobile phones. 
Multi-layer PCBs are made with double-sided boards that are stacked on top of each other. These stack-ups usually contain three double-sided boards, but they can contain as many boards as needed for a project. However, an odd number of layers could result in twisting or warping after soldering, so it is advisable to use an even number of layers.
Multilayer PCBs are produced by laminating alternating layers of prepreg and core materials under high temperatures. This process ensures that there is no trapped between layers, since conductors are completely covered by resin, and the adhesive that holds the layers together is melted appropriately. Material combinations for multilayer PCB fabrication range from basic epoxy glass to sophisticated ceramic or Teflon materials, among others.
Applications of multilayer PCBs include computers, file servers, GPS technology, satellite systems, handheld devices, medical equipment like X-ray machines, heart monitors and cat scan technology, nuclear detection systems, space probe equipment, signal transmission, and a host of other high-tech devices.
Multi-layer boards can be expanded by integrating more layers, providing room for additional circuits with extra connections. This makes the board suitable for complex devices that require additional circuits and components.
The more layers are added, the thicker the board will be, making it durable. This way, the board can withstand indelicate handling and last longer.
Several components would usually need more than one connection point, but the multi-layer PCB only needs a single connection point. This advantage is beneficial to producing devices with simple designs and lightweight features.
The extra density of multi-layered PCBs makes them useful for power-intensive devices. So there is increased capacity for powerful devices to operate efficiently.
Multi-layered PCBs require additional materials, expertise, and time to formulate. Therefore, they cost a lot more than the basic single-layer board. The manufacturer must ensure that the project is well worth the cost of the multilayer board.
The intricate demands of the layering process make multi-layer boards take longer to develop.
The overall completion time is extended due to the number of layers involved in the manufacturing process.
There may be difficulties involved in repairing a problematic multi-layered PCB. This is because some internal layers that are not visible from the outside would make it difficult to locate the source of the issue. The more layers contained on the board, the more complicated it would be to repair. 
Due to the difficulty in producing flexible multilayer PCBs, multilayer PCBs are often classified as rigid PCBs. Most of the standard multilayer PCBs have between 4 and 8 layers. Depending on how complicated the application is, smartphones can have up to 12 layers. Also, since laminating an odd number of layers can result in a circuit that is excessively complex and has problems, manufacturers prefer even layers over odd ones. High cost is another consideration to take into account.
Recommended reading: Printed Circuit Boards: Differences Between Rigid, Flex, and Rigid-flex PCBs
However, the following layer stackings are the layers of a typical multilayer printed circuit board:
The 2-layer PCB is copper coated on both sides with an insulating layer in the middle. It has components on both sides of the board, which is why it is also called a double-sided PCB. They are fabricated by joining two layers of copper together, with a dielectric material in between. The copper on each side can transmit different electrical signals, which makes them suitable for applications where high-speed and compact packaging is essential.
The electrical signals are routed between the two layers of copper, and the dielectric material between them helps to keep these signals from impeding on each other. 2 layer PCBs are the most common and also the most economical boards to manufacture. 
4-layer PCBs are printed circuit boards that have four conductive layers: the top layer, two inner layers, and the bottom layer. Both inner layers are the core, usually used as power or ground planes, while the top and bottom outer layers are for placing components and routing signals.
The outer layers are typically covered in solder masks with exposed pads to provide placement spots to attach surface mount devices and through-hole components. Via holes are normally used to provide connections between the four layers, which make a single board when they are laminated together.
Here's a quick breakdown of these layers:
Layer one: This is the bottom layer, normally made of copper. It functions as a base for the entire board, providing support for the other layers.
Layer two: This is the power plane. It is so called because it provides a clean and stable power source for all components on the board.
Layer three: This is the ground plane layer, which acts as the ground source for all components on the board.
Layer four: The top layer serves to route signals and provides connection points for components.
This is the arrangement for a standard 4-layer PCB stack-up, but it can be switched depending on the design specifications and the layer with the most signals.
4-layer PCBs are much more versatile than traditional 2-layer PCBs, affording a wide range of applications. Also, the additional layers ensure better signal integrity and improved thermal management. For these reasons and more, they are considered more advantageous than 2-layer PCBs. 
A 6-layer PCB is essentially a 4-layer board with 2 extra signal layers added between the planes. The 6-layer PCB standard stack-up includes 4 routing layers ( two outer layers and two internal layers) and 2 internal planes (one for ground and the other for power). This amplifies the EMI (Electromagnetic interference) considerably by offering 2 internal layers for high-speed signals and 2 external layers to route low-speed signals. The EMI is the energy that disrupts the signaling in an electronic device through radiation or induction.
There are several arrangements for a 6-layer PCB stack up, but the number of power, signal, and ground layers used is determined by the application requirements.
The standard 6-layer PCB stack-up includes top layer - prepreg - internal ground plane - core - internal routing layer - prepreg - internal routing layer - core - internal power plane - prepreg - bottom layer.
Although standard, this arrangement will not be suitable for every PCB design, so there might be a need for reshuffling layer positions or having more of a particular layer. Nevertheless, placements must be done with consideration of routing efficiency and crosstalk minimization. 
An 8-layer PCB features four signal layers and four planes that are stacked together. These layers include the ground plane, power plane, and signal layers. The ground and power planes separate and minimize crosstalk between the signal layers. The stack-up of layers on an 8-layer PCB offers quality routing, enhances signal traces, and increases efficiency for complex compact devices.
8-layer PCBs are also beneficial for their high current traces, which are usually very thick and have a low impedance.
Additionally, they provide better power and ground plane separation, increased EMC (Electromagnetic compatibility) performance, and high-speed signal routing. 
Decades before now, single-layer PCBs were used in most electrical devices, but the introduction of multilayer PCBs which offer optimized benefits for complex needs took over the industry since the world is getting more technologically advanced. Nonetheless, single-layer PCBs still have relevance for simpler devices, while multilayer PCBs populate high-tech devices.
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