Multilayer PCB Board introduction and Prototype Fabrication Service

Here you can learn more about the definition, laminate structure, manufacturing process, advantages, and applications of multilayer Printed circuit boards.

Compared with single-layer PCB and double-sided PCB, a multi-layer PCB is a printed circuit board with at least three conductive layers. The electrical connection between multilayer printed circuit board layers is realized through plated through holes on the circuit board. The number of layers of Multilayer circuit board can be 4, 6, 8, 10 Up to 100 layers.

More and more PCB manufacturers are involved in the multilayer PCB manufacturing industry, so the manufacturing process and technology have also developed rapidly. Fast turnaround multilayer PCB prototyping and assembly has become a reality.

Electronic devices, equipment, appliances, and other products are evolving with the passage of time. A few decades ago, mobile phones had a fraction of features compared to the mobiles we use today, and they were heavy and bulky. Nowadays, mobile phones are slim and compact and offer tons of features that we couldn’t imagine. Likewise, users and consumers wish for a compact, reliable, and lightweight alternative to every product. Now, we have a compact version of everything, and with the passage of time, the size, weight, and cost are being reduced.

All this was possible because of advancements in technology, especially the introduction of multilayer PCBs. In the current era, multilayer PCBs play an important role in every sector and industry. It has completely transformed the appearance, structure, and performance of electronic products.

What is multilayer PCB

What is Multilayer PCB Board?

Definition of the Multilayer PCB Board

The term multilayer represents the number of copper/conductive layers in a PCB. A single-layer PCB has only one conductive layer; however, it has other layers, such as the base layer, lamination, and insulating layer. A PCB is known or defined by the number of conductive/copper layers it inherits.

A multilayer PCB contains three or more copper layers. Thus, there can be 3, 4, 5…, 50…, 100, or even more layers. Keep in mind that it is not possible to place the components on all layers of a multilayer PCB, as they can only be placed over the surface layers. The other layers of PCB are used to distribute the connection and wiring between. The connections are distributed among the layers, which is why complex operations are possible, and connections can be made without wasting the space on the board.

multilayer printed circuit board

Why Is There Any Need for Multilayer PCBs?

It is quite difficult, and most of the time impossible, to implement a complex design and obtain the product requirements from a single-layer or double-layer PCB. Single or double-layer PCBs are suitable for simple products, equipment, and devices, but when it comes to implementing complex designs, such as computer motherboards and smartphone PCBs, there are many constraints.

The first crystal-clear constraint is the space on the board. For complex designs, the single-layer and double-layer PCBs will take too much space, and consequently, the product will be bulky. We cannot make a compact product without using a multilayer PCB.

The most disturbing constraint in simple PCBs is the connection of the components. For connecting different components, there will be more space required, and it is important to use jumpers and wires. End-to-end wiring of complex designs is almost impossible in simple PCBs.

In a multilayer PCB, the presence of different layers offers a conductive route for the connection of components and passage of signals, which is why it has the potential to implement complicated designs.

Advantages of Multilayer PCBs

Why use multilayer PCB?

The first advantage of multilayer PCBs is their compact size. These PCBs have a very small size, even if the design is complex. The reason behind this is the presence of multilayers. Thus, there is less circuit on the board, and there is more space for the components. A multiplayer PCB is the perfect choice if you are looking for a small PCB and PCBA. The assembly will take less space and make your final product compact. Due to this advantage and benefit, these PCBs are preferred over other types and used widely in many applications.

The final PCB is quite lightweight compared to its single-layer and double-layer alternative. Due to the presence of multiple layers, there is no need for bulky jumpers and wires. All these are eliminated, and the complex design is implemented without any wires. The interconnection of the layers also makes the circuit compact; thus, it reduces the weight.

Usually, there are SMDs on the PCB, so they save a lot of space and make the PCB lightweight.

Multilayer PCBs are more reliable. There are no external wires and other connectors that can create issues in the future. Everything is tidy and clean. The internal connection of the layers is more reliable than the external connector, and it offers the best performance even in extreme conditions. The durability of such boards is also commendable. They can withstand more pressure and temperature stress. The board is thick due to the presence of core, copper layers, and prepreg, so it is more durable than a single-layer PCB or double-layer PCB.

Multilayer PCBs have a very high component density. They save space on the surface layer, which is why more components can be placed. Moreover, the SMDs are compact, so they require only a little space. The components can be placed on both sides of the PCB, and they ensure more components in a small space.

Complex and complicated designs can be implemented on multilayer PCBs. They have more layers, so no matter how complex your design is, it can be easily implemented. However, it requires expertise in designing.

Jumpers and wires take more time to transmit the signal because there is a considerable distance between the sender and receiver. On the other hand, multiplayer PCBs do not use jumpers and wires. The distance between the layers is minimal, and one layer is just beneath the other. Thus, the performance is commendable.

Advantages of Mulitilayer PCBs

What are the advantages of multilayer circuit boards?

Disadvantages of Multilayer PCBs

The cost of multilayer PCBs is much higher than other types. The reason behind this is the complex design that needs more expertise and exceptional requirements. The benefits come with an extra cost. However, people are willing to pay for a compact device and lightweight technology to get extra features. Therefore, irrespective of the high price, multilayer PCBs are used in various applications, especially handheld devices.

The procedure of creating a multilayer PCB is quite complex. There are various layers, and the interconnection between them requires extra precision. The process is carried out under experts that understand the nitty-gritty.

The latest machinery is required to create multilayer PCBs; these cannot be created using the normal PCB machinery used for a single-layer PCB. Therefore, not every manufacturer can handle such PCBs.

Due to complex design and procedure, multilayer PCBs require more time. The buyer needs to wait, and the batch production is slow compared to single-layer and double-layer PCB.

It is difficult to find the issues and errors in multilayer PCBs. Due to the presence of SMDs, an X-ray inspection is required. Moreover, it is difficult to repair such PCBs and rectify the errors.

Disadvantages of Multilayer PCBs

Disadvantages of Multilayer PCBs

Multilayer PCB Applications

Due to the surprising benefits of multilayer PCBs, they are widely used in different industries. Firstly, it is the top priority of designers to implement their complex circuits, so wherever complex operation is required, these PCBs are used.

Consumer electronics have a wide range of products. However, multilayer PCBs are used in complex functions and where compactness matters a lot. You might be using a device containing multilayer PCB while reading this article. It is commonly used and the only option for smartphones, wearable devices, computers, microwaves, smart LEDs, cameras, and hundreds of other devices.

Industrial equipment and machinery perform complex processes and functions. These require multilayer PCBs. Even the PCB manufacturing machine also contains multilayer PCBs to carry out the whole process. The advanced operations and tons of functionality can only be achieved using multilayer PCBs.

Almost all electronic medical devices require multilayer PCBs. Even a small blood pressure machine needs it. Multilayer PCBs are the basis of advanced medical equipment, such as ECG machines and X-ray machines. The machines have complex multilayer PCBs to perform the operations.

Rockets, satellites, and other similar commodities are quite advanced. Their complex operations can only be performed through multilayer PCBs. The controllers, actuators, sensors, and other devices consist of multilayer PCBs.

The transmission of signals requires a higher level of functionality. GPS systems, satellites, signal towers, and other telecommunication systems use multilayer PCBs.

Military equipment, devices, vehicles, and defense systems need more accuracy, precision, and high performance. They have multilayer PCBs for this purpose.  

Multilayer PCB Applications

What are multi-layer PCBs used for?

There are dozens and hundreds of applications of multilayer PCBs. We can say it has taken the whole market and improved the performance and functionality of PCBs. These are used almost in every industry and sector.

Development Trend of Multilayer PCB

With the development of electronic components to “light, thin, short and small,” multilayer PCB will become the most influential and vital category in the printed circuit board industry and become the leading product. The structure of Multilayer PCB will be a diversified, thin, and high multilayer. At the same time, multilayer PCB also requires high equipment and technical investment.

  • High density
  • High multilayer
  • Thinness
  • Structural diversification
  • High-performance thin copper foil and thin substrate
  • The high flatness of plate surface and surface coating technology
  • Flexible multilayer board and rigid-flexible multilayer board

How To Make Multilayer PCB?

The stack-up of a multilayer PCB is important. As there can be many layers, it is important to understand how these layers are stacked up. It is generally preferred to have an even number of layers in a multilayer PCB as it creates symmetry around the middle core layer. Having an odd number of layers is possible and still required in some applications, but even layers are preferred as they have more physical strength due to symmetry.

The stack up in a multilayer PCB is quite simple. The inner layers are just like double-layer PCBs, and the surface layers are like single-layer PCBs. It might be difficult to understand in the first place, but it is pretty simple.

Suppose there is a 4-layer PCB. The middle layer of the PCB will be the core layer. On both sides of the core, there will be conductive layers. This is just like a double-layer PCB. Now, there will be an insulation layer, prepreg, above these two layers. After that, there will be surface layers, one at the top and one at the bottom. These two layers are like a single-layer PCB. It means a 4-layer PCB has one double-layer PCB and two single-layer PCBs. As the number of layers increases, the number of double-layer PCBs increases, while the number of single layers remains constant; only two single layers.

Multilayer PCB Stack Up and Thickness

4 Layer PCB stack-up

4 Layer PCB  standard Stackup


4 Layer PCB 0.8 MM Standard Build


4 Layer PCB 1.6 MM Standard Build

6 Layer PCB Circuit Board Stackup

8 Layer PCB Stackup and Thickness


8 Layer 1.6 MM Standard Stackup And Thickness

A typical 8 Layer PCB Stackup

8 Layer PCB Stackup

8 layer PCB stackup

A Typical 8 Layer PCB Stackup and Thickness

The above image explains the stack up in an 8-layer PCB. There are three cores in the PCB, and each core has two conductive layers. After every conductive layer, there is a prepreg layer for insulation. Finally, there is a surface layer at the top and one surface layer at the bottom. The components are placed on the surface layers only.

The stack-up remains the same in all multilayer PCBs; only the number of double-layer cores increases. Moreover, it is up to the user which layer he chooses for power, ground, signal, or any other purpose.

10 Layer PCB Stackup and Thickness


10 Layer 1.6mm PCB Standard Stackup and Thickness


Example 10 Layer PCB Stackup

12 Layer PCB Stackup and Thickness

12 Layer PCB Stackup

1.8mm 12 Layer PCB Stackup

12 layer PCB stackup thickness

Stackup Thickness of 12-layer PCB

How are Layers Connected in Multilayer PCB?

The stack-up of the layers is clear, but the question that comes to mind is about the connection of these layers. These layers can be interconnected with all layers or a few selected ones. As the layers are stacked up one over the other, having insulation between them, the only way to connect them is vias.

Via can connect the layers of the PCB. It is actually a hole created to make a connection between the layers. The hole can be for the connection of two layers, three layers, or as many as you need. A hole is created in the PCB, and its walls are plated so that it can create a connection.

Primarily, there are three types of vias used in PCB.

1. Plated Through Hole (PTH)
2. Blind Via
3. Buried Via

PTH is the simple type of via that joins all layers together. It is just like drilling the PCB until the drill bit comes out from the other side. After drilling, the hole is plated so that connections can be made.

Blind via can be seen from one side because it creates the connection of the surface layer with any layer present under it. It can be used to create a connection with internal layers, and there can be one or more internal layers.

Buried vias are buried under the PCB. They are not visible from either side. These vias create a connection between the inner layers only. Thus, these are the most complicated vias.

According to the connections, the PCB layers are arranged. The vias are created as per the requirement. For the plating of via, the board is drilled and then cleaned to remove the dust and debris. A thin layer of copper is applied first. Then, the board is passed through an electroplating process. There are a series of chemical and electroplating baths. After this process, the vias create the connection between the layers.

Learn more about: Plated Through Hole, Blind Via, and Buried Via in PCB Fabrication

Specialized Equipment for Pressing Multilayer PCB Circuit Boards

PCBs Manufacturing multilayer PCBs requires a specialized hydraulic press with heated platens. Initially, the books are squeezed with a “kiss” pressure of 50 psi before being heated to 350F at 350 psi for a minimum of one hour. The assembly is then allowed to cool slowly before removal for further processing. At Omni, the maximum size of a multilayer board is “12×16″ while the board thickness can range from 0.015″to 0.125”.

Multilayer PCBs Fabrication Process

The outer layers of the multilayer consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (prepreg) and a thin copper foil.

The PCB material brands available are Rogers, Shengyi, Dupont (for multilayer flexible PCB), KB (King board), Arlon, Isola, ITEQ, EMC, etc.

For more information about PCB materials, you can read the following articles.

The lay-up operator has already placed a copper foil and two prepreg sheets on the heavy steel baseplate. Now he sets the pre-treated core carefully over the alignment pins. Then he adds two more layers of prepreg, another copper foil, and an aluminum press plate.

He builds up to three panels on the baseplate in the same way. Then he rolls the massive stack under a press which lowers down the steel top plate. He pins the stack together and runs the finished stack out of the cleanroom into a rack.

The press operator collects three stacks on a loader and loads them into the bonding press. This press uses heated press plates and pressure to bond the PCB layers together. The heat melts and cures the epoxy resin in the prepreg while the pressure bonds the PCB together. The process is computer controlled to build up the heat and the pressure correctly, hold it, and cool the press down. In this way, we ensure a permanent bond that will last the lifetime of the PCB. Our board has four layers, but complex PCBs for defense, avionic, and telecommunications applications can have more than 50. These may include sub-assemblies of cores, prepregs, and foils drilled and plated before being assembled into the final PCB.

Once the cycle is completed, the press operator unloads the press and carefully rolls the massive stacks into the cleanroom. Here the lay-up operator de-pins the stack and removes the top plate. He unloads each panel from the stack, eliminating the aluminum press plates used to ensure a smooth copper finish. The copper foil is now bonded in place to form the outer layers of the PCB.

For more step-by-step introduction to the PCB manufacturing process, please click here.

Recommended Reading

JHYPCB is a top-speed and professional PCB printer in China, specializing in high quality and cheap PCB prototype & production(Multilayer Aluminum PCB, multilayer metal core PCB, multilayer flexible PCB, etc.), PCB assembly, and SMT PCB stencil services.

Multilayer PCB Manufacturing Videos

Can’t visit the PCB manufacturing plant in person? Want to understand more intuitively how multilayer PCBs are made? The video below should help you.

How to Make a Printed Circuit Board (PCB) – Step By Step Guide

How to make a multilayer PCB? – Multilayer PCB Bonding

Introduction of multi-layer PCB production process taking 4 layers PCB manufacturing as an example

Quality Standard and Control of Multilayer Printed Circuit Board

The manufacturing of multilayer PCB requires perfection in every step to ensure the quality of the final product. The following are the IPC standards for multilayer PCB manufacturing.

IPC DOC#ITEMSPublication date
IPC-L-108Specification for Thin Metal Clad Base Materials for Multilayer Printed BoardsJun-90
IPC-L-109Specification for Resin Impregnated Fabric (Prepreg) for Multilayer Printed BoardsJul-92
IPC-CC-110Guidelines for selcting core constructions for multilayer printed wiring board applicationsDec-97
IPC-TR-470Thermal Characteristics of Multilayer Interconnection BoardsJan-74
IPC-TA-721Technology Assessment Handbook on Multilayer Boards 
IPC-HM-860Specification for Multilayer Hybrid CircuitsJan-87
IPC-ML-960Qualification and Performance Specification for Mass Lamination Panels for Multilayer Printed BoardsJul-94
IPC-4101ESpecification for Base Materials for Rigid and Multilayer Printed BoardsMar-17

Click here to learn more about PCB IPC standards.

How to store multilayer PCBs?

The packaging and storage of printed circuit boards have strict requirements. The storage and shelf life of multilayer PCBs are specified in the IPC standard. Foam cotton and vacuum packaging are required for packaging.

Related Reading: How to Store PCB and PCBA?

Design for Manufacturability (DFM) for Multilayer Circuit Boards

PCB design plays a vital role in determining the mechanical, electrical, and thermal performance of the complete electronic system. Many advanced electronic components use multilayer PCB because it allows a large number of elements to incorporate on a single board, hence allowing the higher component density. Some PCBs are highly complex that make use of electronic components that are embedded in the substrate material.

Signal integrity and power integrity are two essential features you must take into consideration before you intend to make PCB layout design. You must adopt the following rules to maintain complete power integrity and signal integrity of the PCB.

Conductive PCB trances are composed of copper that comes with finite resistance. Voltage drop in many digital systems can severely influence the quality and accuracy of the system at that resistance when small current flows through the system. Controlled impedance trances are required to maintain the high efficiency of the system.

If significant current flows through the board, it exceeds the temperature beyond standard value because of the resistance of trances. To control board temperature and increase PCB reliability, the width of the trances must be increased, which, if not possible, due to the overall predefined circuitry. Copper thickness must be increased to 2 to 3 based on your needs and requirements.

Stray inductance can cause voltage spikes on the board, which can be handled by using the decoupling capacitors near the load.

The magnetic field is generated when current passes through the traces. This magnetic field can harm or influence the components that come under the range of the magnetic field. To avoid this process, elements must be placed with larger space between them, which is near to impossible in case of a compact multilayer design. So, an alternative way to overcome the effect of the magnetic field is to add a return path or ground plane. A ground plane will behave like a shield and helps in providing a return path of any signal. Ground planes generate decoupling in multilayer PCBs.

Analog and digital portions of the circuit must be separated from the ground planes, and they must be connected at a single point.

It is recommended to avoid 90-degree trances because of EMI issues.

Better to avoid long tracks for the traces containing both analog and digital signals. Noise coupling can be avoided if both signals cross at one point.

The Difference Between Single-layer PCB and Multi-layer PCB

Single Layer PCB vs multilayer PCB

When it comes to Printed Circuit Boards, an immediate question before the design is whether to use single or multi-layered PCBs for your circuit. The benefit and use of each depend entirely on what you intend to do. First, we should define each type of circuit board.

Single-layer or single-sided PCB

These PCBs simply have components on one side of the board and the conductor pattern on the other side. This reason is why it`s known as a single-sided or single layer PCB. Often, these are used for simpler devices as no wires can cross if the circuit is to function correctly. These are usually slightly cheaper to manufacture than multi-layer PCBs.

Multilayer PCB Example

Multilayer Rigid PCB
Multilayer Rigid PCB

Multilayer rigid PCBs are made of FR4 copper-clad laminates. They can be 4-layer, 6-layer, or even more copper layers.

Flexible PCB

Flexible PCB is a printed circuit made of insulating substrates such as PI. Flex PCB can be single-layer, double-sided or multi-layer flexible circuit boards.

Multilayer Metal Core PCB

A metal core PCB is a special PCB having a metallic base layer. This base layer is usually made of aluminum or copper. It is named MCPCB.

multilayer aluminum PCB

Due to its excellent heat dissipation characteristics, aluminum has been widely used as a thermally conductive material for printed circuit boards.

Multilayer Ceramic PCB
Multilayer Ceramic PCB

Ceramic PCB is better than fr4 PCB due to its heat dissipation performance, current carrying capacity, insulation, thermal expansion coefficient, etc.

Multilayer PCB FAQs

Before designing a multilayer PCB, the designer needs first to determine the circuit board structure to be used according to the circuit scale, the circuit board size, and electromagnetic compatibility (EMC) requirements, that is, to decide whether to use 4 layers, 6 layers, or more layers of circuit boards. After determining the number of layers, determine where to place the internal electrical layers and distribute different signals on these layers. This is the choice of multilayer PCB stack structure. The laminated structure is an essential factor that affects the EMC performance of the PCB board, and it is also an essential means to suppress electromagnetic interference.

There are many factors to consider when determining the laminated structure of the multilayer PCB board. From the perspective of wiring, the more layers, the better the wiring, but the cost and difficulty of board manufacturing will also increase. For manufacturers, whether the laminated structure is symmetrical or not is the focus that needs to be paid attention to when manufacturing PCB boards. Hence, the choice of the number of layers needs to consider the needs of all aspects to achieve the best balance.

Printed circuit boards are divided into single-sided, double-sided, and multilayer PCBs, among which the number of layers of the multilayer PCB is not limited. There are already more than 100-layer PCBs, and typical multilayer PCBs are 4-layer boards and 6-layer boards.

The PCBs we often see are even-numbered layers, so why are PCB multilayer boards all even-numbered layers?

Because PCBs with even-numbered layers are indeed more advantageous than PCBs with odd-numbered layers.

1. Cost factors

The cost of raw materials used in producing odd-numbered PCBs is slightly lower than that of even-numbered PCBs. This is because odd-numbered PCBs lack a layer of dielectric and copper-clad laminate. However, the processing cost of odd-layer PCBs is significantly higher than that of even-layer PCBs.

2. The even-numbered PCB has a well-balanced structure to avoid warpage

The best reason not to design PCBs with odd layers is that odd-numbered circuit boards tend to warp. The substrate used for multilayer PCB manufacturing is double-sided, with copper on both sides. The factor that needs to be considered when designing or producing a multilayer PCB is that when the substrate has copper on one side and no copper on the other side, the thermal expansion coefficients of the two sides of the board are different after being heated. Due to the principle of thermal expansion and contraction, the board will warp after processing. Therefore, even if an odd-numbered PCB is required to achieve the required functions, we will design it as an even-numbered layer, design the 5-layer into 6-layer, and the 7-layer into 8-layer board.

In the multi-layer PCB design, one of the essential issues that need to be considered is how many circuit layers, ground layers, and power layers are needed to realize the functions required by the circuit. The stack-up design of a multi-layer PCB is usually decided after considering various factors.

So, what principles do you need to follow when designing a multi-layer PCB stack-up?

1. The distribution of layers

A multilayer PCB usually contains signal layer (S), power layer (P), and ground layer (GND). The power plane and the ground plane are usually entities without separation, which provide an excellent low-impedance current return path for the current of adjacent signal traces. The signal layer is usually located between these power or ground reference plane layers; the top and bottom layers of a multilayer PCB are usually used to place components and a small number of traces.

2. Single power plane

Decoupling capacitors can only be placed on the top and bottom layers of the PCB. The traces, pads, and vias of the decoupling capacitor will seriously affect the effect of the decoupling capacitor. This requires the design to consider the traces connecting the decoupling capacitors as short and wide as possible; the wires connected to the vias should also be as short as possible.

3. Multiple power layers

The multi-power plane will be divided into several physical areas with different voltages. If the signal layer is close to the multi-power supply layer, the signal current on the signal layer nearby will encounter an undesirable return path. Therefore, it is required that the high-speed digital signal wiring should be far away from the multi-power supply plane.

4. Multiple ground layer

Multiple ground planes can provide an excellent low-impedance current return path and can also reduce common-mode EMI. The ground plane and the power plane should be tightly coupled, and the signal layer should also be tightly coupled with the adjacent plane.

5. Reasonably design the wiring combination

The two layers spanned by a signal path are called a “wiring combination.” The best wiring combination design avoids the return current flowing from one reference plane to another but from one point on one reference plane to another.

Single-point and multi-point grounding methods:

  1. Single-point grounding: The ground wires of all circuits are connected to the same point on the ground plane, divided into series single-point grounding and parallel single-point grounding.
  2. Multi-point grounding: The ground wires of all circuits are grounded nearby, and the ground wires are short and suitable for high-frequency grounding.
  3. Mixed grounding: mix single-point grounding and multi-point grounding.

The more layers of PCB, the more difficult it is to manufacture, and the higher the manufacturing cost. The number of PCB layers is determined by the actual application requirements of the product. When designing a multi-layer PCB, the designer also needs to consider the feasibility of the process, production cost, function realization, and other factors to determine the number of layers of the PCB.

Multilayer flexible PCB can be divided into the following three types

1) A multilayer PCB is formed on a flexible insulating substrate, and the finished product is specified to be flexible. This structure is to bond the two side ends of many single-sided or double-sided microstrip flexible PCBs together. Still, the central parts are not bonded together, thus having a high degree of flexibility. To have the desired electrical characteristics, such as the characteristic impedance performance and the rigid PCB to which it is interconnected, each circuit layer of the multilayer flexible PCB component must be designed with signal lines on the ground plane. A thin, suitable coating, such as polyimide, can be used on the wiring layer instead of a thicker laminated cover layer to have a high degree of flexibility. This multilayer flexible PCB is most suitable for designs that require flexibility, high reliability, and high density.

2) A multilayer PCB is manufactured on a flexible insulating substrate, and the finished product can be flexed. This type of multilayer flexible PCB is made of flexible insulating materials, such as polyimide film, laminated to make a multilayer board, which loses its inherent flexibility after lamination. This type of flexible PCB is used when the design requires maximum use of the insulating properties of the film, such as the low dielectric constant, uniform thickness of the medium, lighter weight, and continuous processing. For example, a multilayer PCB made of polyimide film insulating material is about one-third lighter than a rigid PCB with epoxy glass cloth.

3) Multi-layer PCBs are made on a flexible insulating substrate, and the finished product must be shapeable, not continuously flexible. This type of multilayer flexible PCB is made of flexible insulating materials. Although it is made of flexible materials, due to electrical design restrictions, such as the required conductor resistance, a thick conductor is required, or for the required impedance or capacitance, a thick conductor is required between the signal layer and the ground layer. It is insulated and isolated, so it is already formed in the finished application. The term “formable” is defined as a multilayer flexible PCB component that can be shaped into the required shape and cannot be flexed in the application—used in the internal wiring of avionics units. At this time, it is required that the conductor resistance of the stripline or three-dimensional space design is low, the capacitive coupling or circuit noise is minimal, and the interconnection end can be smoothly bent to 90°. Multilayer flexible PCB made of polyimide film material achieves this wiring task. Because the polyimide film is resistant to high temperatures, flexible, and has good overall electrical and mechanical properties. To achieve all the interconnections of this component section, the wiring part can be further divided into a plurality of multilayer flexible circuit components combined with adhesive tape to form a printed circuit bundle.

1. Alignment between layers

Since there are more layers in a multilayer circuit board, the requirements for alignment between layers are also higher. Generally, the alignment tolerance between the layers is controlled at 75 microns. Due to the influence of factors such as size and temperature, the degree of difficulty in controlling the alignment between layers of a multilayer board will be huge.

2. Inner circuit production

Multilayer circuit boards use unique materials such as high TG, high speed, high frequency, thick copper, the thin dielectric layer, etc. This puts forward high requirements for inner circuit production and pattern size control. For example, the integrity of impedance signal transmission increases the difficulty of internal circuit manufacturing. The line width and line spacing are small, the probability of open circuit and short circuit may increase; the increase of short circuit will reduce the pass rate; the number of thin lines signal layers will increase the probability of missing AOI detection in the inner layer; the inner core board is light and easy to wrinkle, Poor exposure and easy curling during etching; the unit size of multilayer PCB is more significant, and its product scrap cost is higher.

3. Pressing process

Many inner core materials and semi-cured boards are superimposed, and defects such as slippage, delamination, resin voids, and bubble residues are prone to stamping production. In the design of the laminated structure, the heat resistance, pressure resistance, glue content, and dielectric thickness of the material should be fully considered, and a reasonable material pressing plan for the multilayer circuit board should be formulated. Due to many layers, the expansion and contraction control and the dimensional coefficient compensation cannot maintain consistency, and the thin interlayer insulation layer is likely to cause the interlayer reliability test to fail.

4. Drilling

The use of high-TG, high-speed, high-frequency, and thick copper special plates increases the difficulty of multi-layer PCB drilling roughness, drilling burrs, and decontamination. Due to a large number of layers, the copper thickness and plate thickness have also increased, and tool breakage is likely to occur during drilling; if there are many BGAs, the narrow hole wall spacing can easily lead to CAF failure; the increase in plate thickness can also easily lead to the problem of diagonal drilling.

The meaning of the different layers of a multi-layer PCB

a. Signal Layers: Signal layers include Top Layer, Bottom Layer, Mid Layer 1……30. These layers are all layers with electrical connections, that is, the actual copper layer. The middle layer refers to the middle board layer used for wiring, and the wires are distributed in this layer.

b. Internal Plane: Internal Plane or inner layer 1……16. These layers are generally connected to the ground and the power source and become the power layer and the ground layer. They also have electrical connections and are also the actual copper layer. Generally, there is no wiring, and it is composed of a whole piece of the copper film.

c. Silkscreen Overlay: The top silkscreen layer (Top overlay) and the bottom silkscreen layer (Bottom overlay). The silkscreen characters that define the top and bottom layers are generally text symbols printed on the solder mask, such as component names, component symbols, component pins, copyrights, etc., to facilitate future circuit soldering and error checking.

d. Paste Mask: Including the top paste layer and the bottom paste layer (Bottom paste), which refers to the exposed surface-mount pads that we can see, that is, before soldering the part that needs to be coated with solder paste. Therefore, this layer is also helpful for hot air leveling of the pads and producing the welded stencil.

e. Solder Mask: It includes the top solder mask and the bottom solder mask. Its function is opposite to that of the solder paste layer. It refers to the layer to be covered with green oil. This layer is non-sticky solder, preventing the excess solder of adjacent soldering points from short-circuiting during soldering. The solder mask covers the copper film wires, and the copper film is too quickly oxidized in the air but leaves a place at the solder joints and does not cover the solder joints.

f. Mechanical Layers: Up to 16 mechanically processed layers can be selected. Designing a double-sided PCB only needs to use the default option Mechanical Layer 1.

g. Keep Out Layer: Define the boundary of the circuit layer. After defining the keep-out layer, the wiring with electrical characteristics must not exceed the boundary of the keep-out layer in the subsequent wiring process.

h.Drill Layer: including drill guide and Drill drawing.

I. Multi-layer: Refers to all layers of the multilayer PCB.

The thickness of the PCB board is mainly determined by factors such as copper thickness, material, number of layers, and application requirements. The standard board thickness of the PCB is about 62 mils (that is, what we usually call 1.60 mm PCB). With the needs of the application, the copper thickness and the number of layers of the PCB are constantly increasing. Therefore, the PCB is often thicker than before. According to customers’ designs and requirements, multilayer PCB manufacturers can provide customers with various options. (For example, 93 mil or 2.36 mm, and 125 mils or 3.17 mm; this is 150% and 200% of the old standard thickness).

How to order multilayer PCBs?

For a fast and accurate multi-layer PCB quote, when ordering a multi-layer PCB from a PCB manufacturer, be sure to provide accurate information such as material selection, lamination sequence, board thickness, tolerance requirements, processing techniques, etc. When exporting the PCB to Gerber, it is recommended to use the RS274X format. The RS274X format has the following advantages: the CAM system can automatically input data, and the whole process does not require manual participation, which can avoid many troubles, and can maintain good consistency and reduce the error rate.

Get Your Multilayer PCB Today!

JHYPCB is one of the leading PCB manufacturers in China, fully equipped with state-of-the-art machinery and the latest technology. We offer high-quality multilayer PCBs of up to 30 layers without any restriction on the number of pieces; you can order one PCB or PCBA. Our experts understand the complexity of multilayer PCB and ensure high-performing PCBs and PCBA.

JHYPCB is RoHS, UL, and ISO compliant and certified. We ensure optimum quality in the minimal delivery time. Customers can get turnkey and quick prototype and assembling services. For more information and details, feel free to contact us.

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