Over the past few years, we have witnessed significant advancements in various electronic application fields, leading to the introduction of increasingly innovative technologies. Rapidly developing areas include mobile communications (smartphones and tablets), wearable devices (including virtual and augmented reality devices), and electronic medical devices. Significant progress has also been made in the automotive and aerospace industries. The driving force of innovation, coupled with the emergence of new manufacturing techniques, has enabled the introduction of new materials for producing thinner, lighter, and when necessary, more flexible printed circuits capable of transmitting electrical signals at faster speeds and frequencies.

　　 Demand for New Materials

　　Traditional materials and substrates consist of fiberglass fabrics, plastics (resins), and copper. Different types of resins and glass are used in the manufacturing of printed circuit boards, and the way they are combined affects the electrical and mechanical properties of the material. Two major electrical properties defining the material are the dielectric constant (Dk) and the dissipation factor (also known as the loss tangent or Df), both of which are significantly dependent on the temperature and frequency at which the material or substrate is operating. The dielectric constant specifies the amount of charge that can be held on two conductors when a certain voltage is applied across them. The constant Dk also determines the rate at which a given current flows in the conductors. The loss tangent, on the other hand, provides a measure of the amount of electromagnetic energy absorbed by the dielectric material.

　　Modern electronic applications require materials with different characteristics than those traditionally used for manufacturing PCBs. While the reasons behind the choice are numerous and strictly depend on the specific application, a possible list includes:

　　The need to manage higher frequency electrical signals;

　　Increased integration density of electronic components;

　　New packages are available for many components, impacting wiring techniques;

　　The need to minimize power losses, especially in low-power or battery-powered applications;

　　The need to provide adequate thermal management for the PCB to minimize the heat that needs to be dissipated;

　　The need to manage device connectivity (usually wireless), a critical aspect of PCB design.

　　The frequency of signals transmitted on PCBs seems to be relentlessly increasing. This characteristic, coupled with increasingly lower supply voltages (especially for highly integrated digital components such as MCUs, SoCs, and FPGAs), is causing serious signal integrity issues. Such applications include fiber optic transmission cards and devices, computers, and most embedded systems equipped with processing units.

　 　New Materials and Substrates

　　Based on the considerations expressed in the previous paragraph, we can identify two key factors that determine the choice of materials and substrates suitable for a particular application: the power and heat that the PCB can withstand. While this rule is general and applies to all types of materials, greater benefits can be achieved by adopting the following innovative materials:

　　Fluoropolymers: PCBs manufactured with substrates made of this material have high resistance to corrosion, mechanical stress, and high temperatures. Furthermore, at a mechanical level, fluoropolymers exhibit excellent wear resistance, low adhesion, and long life. Considering the non-negligible cost, this type of material is suitable for manufacturing PCBs used in the medical, pharmaceutical, and food industries;

　　Polyimides: Also known as PI, this material has recently gained enormous success due to the increasing popularity of flexible and rigid-flex printed circuit boards. These PCBs are revolutionizing multiple electronic applications, solving what were once considered critical electrical connection problems in an efficient and simple way, especially in terms of reliability. This achievement is due to their ability to bend and self-wrap in confined or irregularly shaped spaces. Unlike traditional rigid PCBs, flexible PCBs can bend without altering the transmission of the electrical signals they carry. Composed of thin polyimide films deposited on a substrate with conductive traces, they are widely used in smartphones, wearable devices, electronic medical devices, and anywhere a flexible wiring solution suitable for confined spaces is needed. In addition to mechanical flexibility, the resulting material also exhibits excellent thermal and atmospheric resistance. Rigid-flex PCBs (an example is shown in Figure 1) are obtained by combining rigid and flexible sections. This solution, currently more expensive than traditional PCBs, is used in the automotive and motorcycle industries, military, and aerospace sectors;

　　Acrylic Adhesives: Highly appreciated for remaining malleable even after polymerization, these materials are a good solution for all dynamic applications. Acrylic adhesives have a higher coefficient of expansion than other materials used as PCB substrates. Furthermore, at temperatures approaching 180°C, acrylic adhesives begin to soften, and delamination of the PCB layers in contact with the conductive traces may occur. If high flame retardancy is required, chemical flame retardants must be added to the substrate, but this may reduce the dynamic capabilities of the material;

　　Epoxy Adhesives: Unlike the previous adhesives, epoxy adhesives form a rigid material after polymerization and are therefore unsuitable for many dynamic applications. However, due to their relatively low coefficient of expansion and high adhesive strength, they are a good solution for the construction of multilayer PCBs capable of withstanding high operating temperatures. Epoxy adhesives have high chemical resistance and low moisture absorption and are widely used as PCB substrates where sensors may come into contact with moisture, such as in healthcare applications and many fitness and wearable devices;

　　Liquid Crystal Polymers: Liquid Crystal Polymers, also known as LCPs, are often used to manufacture multilayer PCBs because reducing thickness is a fundamental requirement. LCPs are made of a very inert, non-reactive, and highly flame-retardant material. They are lightweight and flexible, with exceptional electrical properties, making them ideal solutions for high-frequency applications, especially where controlling PCB weight and thickness is necessary. Liquid crystal polymers also have good dielectric properties with low losses and moisture absorption;

　　Aluminum: Aluminum printed circuits, also known as metal-clad PCBs or IMS (Insulated Metal Substrate), consist of a thin layer of thermally conductive but electrically insulating dielectric material laminated between a metal substrate and copper foil. The copper foil is etched with the desired PCB layout, while the metal substrate has the function of absorbing the heat generated by the circuit through the thin dielectric layer. The main advantage of aluminum PCBs is their better heat dissipation compared to common PCBs based on FR-4 material. Initially designed for high-power electronic applications, metal-clad PCBs are now becoming the ideal solution for supporting high-brightness LED lighting systems, both in the consumer and automotive sectors. Figure 2 shows an aluminum PCB used in applications in the field of ultra-bright LED lighting.

　　New materials are able to offer superior performance compared to traditional ones, experiencing continuous and ongoing development due to the ability to improve various aspects related to signal integrity. Lower Dk values improve impedance control, crosstalk, jitter, and signal skew. Lower Df values, on the other hand, help improve rise and fall times and overall attenuation.