The demand for multilayer printed circuit boards is growing. Small and fast device electronics have made multilayer PCB very popular. These types of printed circuit boards allow for unprecedented miniaturization, with a very high component density rate. Multilayer PCBs are printed circuits made up of more than two layers. They must therefore have at least three layers of conductive material within the insulating material. On the other hand, double-sided PCBs only have two layers of conductive material, the top and the bottom. Today, extremely complex devices such as mobile phones, smart TVs, computers and others could not exist without the support of excellent multilayer PCBs. In this article, we examine the areas of their uses and the occasions in which they must be adopted.
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Overview of multilayer PCBs
With new technologies and the high integration of components, electronic circuits are now extremely complex and the connections between the various parts have multiplied enormously. Single-sided and double-sided circuits are no longer sufficient and for high frequency applications it is essential to maintain extremely low mounting thicknesses to avoid unwanted capacitive increases. In other words, today, multilayer PCBs are the only solution to allow the creation of the electrical and electronic circuits we know today. Mobile phones, computers, motherboards, embedded systems and more all have a multilayer PCB and if, apparently, they are not visible, their presence is guaranteed thanks to the extreme compactness of the devices themselves. As the name suggests they are a combination of several layers. Single-sided and double-sided PCBs are combined together to create this complex final PCB. Having a greater number of layers, they have a greater area available for mounting the electronic components and for wiring. The various levels are separated from each other with insulating material. Each layer is dedicated to a specific function when the PCB is optimally designed. The arrangement of the layers should not create problems for the other layers.
The techniques for the production of multilayer PCBs are extremely complex and only seriously advanced companies can achieve them. Today it is easy to find PCBs composed of 32 or more layers and the techniques for their realization are extremely advanced. Theoretically, PCBs with 100 and more levels can also be created. Smartphones can have up to 12 levels, depending on the complexity of the applications provided. The various alternating layers (see figure 1) are compressed together at high temperature and pressure to eliminate any residual air that remains trapped. In this way, even the conductive layers are perfectly encapsulated by the resin for a safe and guaranteed final result. Recall that the conductive layers are the real tracks, on which the electric current passes. Multilayer PCBs find an extremely wide field of application today and practically all complex devices use this connection method. They can be found in computers, telephony circuits, handheld devices, industrial and medical systems.
With the universality of use, multilayer PCBs obviously have great advantages, they allow the creation of circuits with reduced dimensions, with a considerable saving of space and weight, the external wiring is reduced to a minimum, the electronic components can be mounted by adhering to a higher assembly density, impedances and parasitic capacitances are drastically reduced and there is efficient EMI shielding thanks to the accurate positioning of the power and ground layers. The use of multi-layered PCBs allows you to implement very complex electronic circuits on small PCBs.
Figure 1: the standard composition of a multilayer PCB
Advantages of multilayer PCBs
Multilayer PCBs offer numerous benefits and their use is increasing exponentially. Here are the main advantages of these cards:
- current technology allows the creation of rigid or flexible PCBs. It is obvious that the greater the number of layers, the less its flexibility is;
- with PCBs the dimensions of the devices are smaller. The stacking of the layers, in fact, allows saving space and reduced dimensions, and the circuits are also lighter due to the absence of connectors;
- increases the capabilities and complexities of the circuits;
- probably the only negative aspect is represented by the prices, which are much higher than traditional PCBs. However, this small problem is amply compensated thanks to the functionality and performance achieved.
Incorrect accumulation of layers
The design of a multilayer PCB is an extremely complex operation, it is almost an art. If the circuit that the PCB houses works at low frequency (or even in direct current) there are no substantial design problems. If, on the contrary, the system has to work at high frequencies, many criteria dictated by physics, electronics, electrostatics and magnetism must be respected, in order to preserve the optimal operation of the circuit, in all conditions. When the layers of a multilayer PCB are designed incorrectly, some anomalies can occur, often fatal, for the proper functioning of the system. Some of these problems are overshoot, undershoot, electromagnetic interference, incorrect coupling of signals, crosstalk and even their loss or attenuation. As we can well understand, these are insurmountable problems that a quality electronic circuit must absolutely not have. In fact, if it has anomalies and signal defects, it will probably not even receive the necessary insulation, compliance and safety certifications, to be able to be regularly marketed. A perfect PCB design, therefore, is an essential step for its subsequent production. An efficient design of the layers certainly saves subsequent wastes of time and avoids the occurrence of possible future problems that could arise due to improper design.
The order of the various PCB layers is extremely critical and the different stacking priorities must be respected, for which the materials and thicknesses of which the substrate and the conductive traces are made are also considered. As you can see from the previous figure, the different 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, relative to the project in object. Therefore the position of the ground planes and of the different types of signal levels plays an essential role in the success of the final project.
If the layers are planned incorrectly, unwanted electromagnetic interference and poor quality signal may occur. For a perfect multilayer PCB it is necessary to ensure a sufficient loop return of the signal. Furthermore, all the cases of a possible overlap or cross connection between the signals should be considered and, above all, carefully study the types of frequencies of the signals to be routed.
The layers of the multi-level PCB
Ground provides shielding support for signals, and in this type of PCB, they should not be avoided. Indeed, due to the high communication speeds to which a circuit could be subjected, it is convenient to place a ground layer in correspondence with each signal layer. Often, to save in terms of final costs and thickness of the entire system, it is preferable to limit the overlapping of layers, reducing them. Therefore the mass layers are placed after every two signal layers, as shown in the figure. Power floors should also be placed very close to the ground floors. Considering, therefore, only the conductive layers, a possible division of the layers could be the following:
- 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.
Obviously the distribution and positioning of the various layers depends from case to case, therefore different solutions can be found on the market. The sequence of the various levels also depends, among other things, on the expense that the customer is willing to spend on their project. Either way, board designers try to logically separate the layers based on their logical function. For example, in a complex system, such as that of a computer, you could have the power supply lines, earth lines, general management, devices, memory, processor and chipset lines. These lines, given their extreme communication speed, cannot be inserted in the same level but must be distributed on different layers, in order to optimize the quality of the system operation. In multilayer PCBs the return traces or ground level is used as a shielding solution. Furthermore, traces at 90 ° should be avoided, in order not to run into EMI problems.
Where should the electronic components be mounted?
Multilayer PCBs, as mentioned above, allow the creation of very complex, compact and miniaturized circuits. Due to these particular characteristics it is possible to mount the components (also equipped with many terminals) extremely close to each other. In this way, inductances and parasitic capacities decrease drastically. The normal procedure for assembling electronic components foresees their positioning on the external layers, then on the upper level and on the lower level. However, when the circuits are very complex, it may be necessary to place them in the internal layers of the PCB, especially in extremely critical sectors, such as the medical, military and space sectors, for which the reasons for electrical and magnetic interference must be completely cleared and eliminated. The various conductive layers are effectively separated by the insulating layers. The internal layers of the PCB are usually double-sided boards, while the outer layers are made up of single-sided boards. For the realization of a multilayer PCB it is necessary to keep an eye on several characteristics. Signal integrity is one of the most important. If they are poorly designed, the copper traces on the PCB may have an electrical resistance which, in some cases, can alter the quality of the current. This is why traces with controlled impedance are needed, while also increasing the thickness of the copper.
The connection between the various levels
The electrical connections between the various levels are made with metallized holes (see figure 2). They can be passersby or blind. One of the solutions to save space (but also money) is to build the normal "hidden" tracks within the layers. These tracks, in fact, could waste space and are intelligently "sunk" inside the layer itself, resulting completely buried or blind. Connection types can be “via”, “buried via”, “Blind Via”, “Stacked Vias”, “Staggered Vias”, etc. The different levels can refer to different planes, such as the ground plane, the power plane, the signal plane, etc.
Figure 2: The layers of a PCB are linked together with different techniques
Software for creating multilayer PCBs
Today the software for the design of a multilayer PCB are more and more numerous. Both freeware and commercial, there are more and more programs that help developers to create a PCB with many levels (see figure 3). There are currently excellent multilayer PCB design software, some of which are recommended by several hardware engineers. Among them we can include Altium design, Eagle, KiCad and OrCAD. These and other software allow a correct stacking procedure of the levels, through a sophisticated stackup manager that ensures excellent synchronization with the various elements of the circuit.
Figure 3: New software often allows the creation of multilayer PCBs
The planning of the layered PCB stackup strategy allows alternation between signals and power or ground plans. Each layer is separated by a dielectric or a prepreg. The main goal is to suppress EMI noise, crosstalk and crosstalk between levels, allowing for efficient routing of various signals between levels. Routing should use vias wisely, following some fairly critical rules.
Unfortunately, multilayer PCBs, although irreplaceable, are characterized by a fairly high cost. Multilayer PCBs also take longer to design and manufacture, so companies need to receive project files much earlier. Repairing multilayer PCBs is also more complex than normal PCB designs. Figure 4 shows a general graph of the average costs of multilayer PCBs from three sampled manufacturers. The cost criteria of the chart 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 shows the average price of PCBs from three different companies, but it should not be taken as an absolute value but to get an idea of how prices increase depending on the number of levels. Obviously the shipping costs are not included in the calculation. There are valid calculators online, produced by the manufacturers themselves, which help the customer to evaluate the costs of their printed circuits, choosing different parameters such as, for example, the type of conductor, the dimensions, the quantity, the number of levels, the material of the insulation, thickness and many more.
Figure 4: The average cost graph of PCBs with a different level number
PCB manufacturers have seen an increase in demand for multilayer boards due to the need for smaller and more complex devices for various industries. These are PCBs that can only be manufactured at an industrial level and the production costs force customers to purchase large quantities. Having multiple layers within the circuit board allows for more circuitry and wiring for more complex applications. Manufacturers today prefer an even number of layers instead of odd ones, as laminating an odd number of layers can make the circuit too complex and present problems. The multilayer panels, in fact, are made with two-layer laminated panels. The design of a multilayer PCB is certainly one of the most complex techniques of modern electronics.