Multi-layer printed circuit boards are becoming more and more popular. Electronics that are quick and small have made multilayer PCB more common. These printed circuit boards have a very high component density rate, enabling unprecedented downsizing. Printed circuit boards with more than two layers are known as multilayer PCBs. In the insulating substance, they must consequently have at least three layers of conductive material. On the other hand, the top and bottom are the only two layers of conductive material on double-sided PCBs. Without the help of great multilayer PCBs, modern, very sophisticated products like mobile phones, smart TVs, PCs, and others would not be possible. In this article, we look at the situations in which they must be used as well as the places in which they can be used.
Overview of multilayer PCBs
Electronic circuits are now incredibly complicated and the connections between the various sections have expanded tremendously due to new technologies and the high level of component integration. Single-sided and double-sided circuits are no longer adequate, and extremely thin mounting thicknesses must be maintained for high frequency applications to prevent undesirable capacitance increases. In other words, the only option available today for enabling the development of the electrical and electronic circuits we are familiar with is multilayer PCBs. All multilayer PCBs, including those found in motherboards, embedded systems, mobile phones, and computers, are present even though they may not be readily apparent due to the incredibly small size of the devices. They are a combination of numerous layers, as their name implies. This intricate final PCB is made by combining single-sided and double-sided PCBs. They have more layers, which means there is more space for attaching the electronic parts and wiring. Insulating material separates the several layers from one another. When the PCB is constructed properly, each layer has a defined purpose. The other levels shouldn’t experience any issues as a result of the layer arrangement. Only technologically very advanced firms are able to accomplish the fabrication of multilayer PCBs due to the exceedingly complicated procedures involved. These days, it is simple to obtain PCBs with 32 or more layers, and the methods used to create them are highly sophisticated. The creation of PCBs with 100 levels or more is also theoretically possible. In accordance with the degree of sophistication of the applications offered, smartphones might have up to 12 tiers. High temperature and pressure are used to compress the numerous alternating layers (see figure 1) in order to release any trapped air. For a secure and certain end product, the resin is used to perfectly wrap even the electrical layers. Remember that the actual rails on which the electric current travels are the conductive layers. Today, multilayer PCBs have an incredibly broad range of applications, and almost all complex devices link using this technique. They can be found in industrial, medical, telecommunication, handheld, and computer systems. Multilayer PCBs have many benefits beyond just being widely applicable. They enable the construction of circuits with smaller dimensions, saving a significant amount of weight and space, and reducing the amount of external wiring. They also enable the mounting of electronic components by adhering to higher assembly densities, greatly reducing impedances and parasitic capacitances. You can create extremely complicated electronic circuits on little PCBs by using multi-layered PCBs.
Advantages of multilayer PCBs
The adoption of multilayer PCBs is skyrocketing due to their many advantages. The following are the key benefits of these cards:
- using current technology, rigid or flexible PCBs can be produced. It is evident that the flexibility decreases with the number of layers;
- the size of the gadgets is reduced using PCBs. In reality, the stacking of the layers enables space-saving and reduced dimensions, and the absence of connectors makes the circuits lighter;
- increases the capabilities and complexities of the circuits;
- price, which is significantly costlier than regular PCBs, is likely the only drawback. Nonetheless, the functionality and performance acquired more than make up for this minor issue.
Incorrect accumulation of layers
A multilayer PCB’s design is a very difficult process that is nearly an art. There are no significant design issues if the circuit the PCB holds operates at low frequency (or even in direct current). On the other hand, if the system must operate at high frequencies, several requirements set out by physics, electronics, electrostatics, and magnetism must be adhered to in order to maintain the circuit’s best performance under all circumstances. When the layers of a multilayer PCB are constructed improperly, several abnormalities can happen, which are frequently catastrophic for the system’s proper operation. Overshoot, undershoot, electromagnetic interference, improper coupling of signals, crosstalk, and even their loss or attenuation are a few of these issues. These are insurmountable issues that a high-quality electronic circuit must absolutely not have, as we can well see. In fact, if it has abnormalities and signal flaws, it probably won’t even get the certifications for insulation, compliance, and safety needed to be offered routinely. So, creating a flawless PCB design is crucial to moving forward with production. A layer’s efficient design prevents potential future issues and subsequent time waste that could result from a layer’s inefficient design. The materials and thicknesses used to make the substrate and the electrical traces are also taken into account when determining the order of the various PCB layers and stacking priorities. The several conductive layers, such as the “normal signal layer,” the “power layer,” the “ground layer,” and the “high speed signal layer,” are positioned in a workmanlike manner with respect to the project in object, as you can see from the previous figure. As a result, the placement of the ground planes and the various signal levels is crucial to the success of the overall project. Inappropriate layer planning may result in undesirable electromagnetic interference and signal degradation. It is essential to guarantee a sufficient loop return of the signal for a flawless multilayer PCB. Also, any scenarios in which there might be a signal overlap or cross connection should be taken into account, and the sorts of frequencies of the signals that need to be routed in particular should be thoroughly examined.
The layers of the multi-level PCB
In this kind of PCB, ground serves as a shielding support for signals and should not be avoided. In fact, it is practical to align a ground layer with each signal layer due to the potential for high transmission speeds in a circuit. It is frequently beneficial to restrict layer overlap, hence lowering them, in order to reduce overall expenses and system thickness. As depicted in the picture, the mass layers are consequently positioned every two signal layers. Moreover, power floors must to be positioned extremely close to the ground floors. Considering simply the conductive layers, the layers may be divided into the following groups:
- copper layer: normal signal;
- copper layer: power layer;
- ground layer;
- copper layer: high speed signal layer;
- copper layer: high speed signal layer;
- ground layer;
- copper layer: power layer;
- copper layer: normal signal.
Several solutions are available on the market because the distribution and location of the various layers obviously vary from case to case. The order of the various stages is also influenced by the cost that the client is prepared to incur for the project, among other factors. In either case, board designers make an effort to rationally divide the levels according to their logical purpose. Consider the power supply lines, earth lines, general management, devices, memory, CPU, and chipset lines in a sophisticated system like a computer. In order to improve the efficiency of the system’s functioning, these lines must be dispersed among several tiers due to their extremely fast transmission rates. The return traces or ground level are utilized as a shielding solution in multilayer Boards. To prevent EMI issues, it is also advisable to avoid traces at 90 degrees.
Where should the electronic components be mounted?
As was already established, multilayer PCBs enable the development of extremely intricate, small, and miniature circuits. These unique properties allow for the mounting of the components—which are also furnished with several terminals—quite closely together. Inductances and parasitic capabilities substantially drop in this method. Electronic components are typically positioned on the external layers, then on the upper level, and finally on the lower level during assembly. However, in very important industries like the medical, military, and space sectors, where the causes of electrical and magnetic interference must be totally cleared and eliminated, it may be required to install the circuits in the internal layers of the PCB when they are exceedingly complicated. The insulating layers serve as a functional barrier between the various conducting layers. The outside layers of the PCB typically consist of single-sided boards, whereas the inner levels typically consist of double-sided boards. A multilayer PCB must be realized, and numerous properties must be monitored. Perhaps one of the most crucial is signal integrity. The copper traces on the PCB may have an electrical resistance if they are badly designed, which in some situations might change the current’s quality. This calls for the use of regulated impedance traces and an increase in copper thickness.
The connection between the various levels
Metallic holes are used to create the electrical connections between the various levels (see figure 2). They might be onlookers or blind. Building the common “hidden” tracks within the layers is one way to conserve space (and money). These tracks may actually take up unnecessary room because they are deliberately “sunk” inside the layer itself, leaving them fully hidden or invisible. Via, buried via, blind via, stacked vias, staggered vias, etc. are all examples of connection types. Several planes, such as the ground plane, the power plane, the signal plane, etc., might be referred to as the various levels.
Software for creating multilayer PCBs
There are a growing number of software programs available now for designing multilayer PCBs. More and more tools, both freeware and paid, are available to assist developers in building a PCB with multiple levels (see figure 3). Several hardware engineers propose some of the top multilayer PCB design software that is currently available. Altium design, Eagle, KiCad, and OrCAD are a few of them. Using a sophisticated stackup manager that provides perfect synchronization with the various circuit components, this and other software enable a correct level stacking technique.
The multilayer PCB stackup strategy’s planning enables switching between signal, power, and ground schemes. A prepreg or a dielectric is used to separate each layer. The major objective is to reduce EMI noise, crosstalk, and crosstalk between levels to enable effective signal routing between levels. Vias should be used wisely by routing by abiding by a few very important regulations.
Multilayer PCBs are unfortunately somewhat expensive, despite being irreplaceable. Moreover, multilayer PCBs require more design and manufacturing time, necessitating earlier project file delivery to businesses. Multilayer PCB repairs are also more difficult than repairs on standard PCBs. A broad graph of the average prices of multilayer PCBs from three sampled manufacturers is shown in Figure 4. The chart’s cost criteria are as follows:
- ordered quantity of PCBs: 100;
- PCB dimensions: 400 mm x 200 mm;
- number of levels: 2, 4, 6, 8, 10.
The graph displays the average cost of PCBs from three different suppliers, although it shouldn’t be interpreted as an absolute cost but rather as a representation of how costs rise as the number of levels increases. The shipping expenses are obviously not factored into the calculation. Customers can evaluate the costs of their printed circuits online using valid calculators created by the manufacturers themselves by selecting various parameters like, for instance, the type of conductor, the dimensions, the quantity, the number of levels, the material of the insulation, thickness, and many more.
Due to the demand for smaller and more complicated devices for numerous industries, PCB makers have noticed a surge in the demand for multilayer boards. Because to the high production costs, PCBs like this can only be made on an industrial scale, which forces customers to buy them in bulk. The circuit board’s numerous layers enable more circuitry and wiring for more complicated applications. Today’s manufacturers prefer an even number of layers over an odd one since laminating an odd number of layers can result in a circuit that is too complicated and has issues. In actuality, two-layer laminated panels are used to create the multilayer panels. Unquestionably one of the most difficult procedures used in contemporary electronics is the construction of a multilayer PCB.