New PCB Materials: How to Use Them

New PCB Materials: How to Use Them

Over the past few years, we have witnessed major advancements in several electronic applications, resulting in the introduction of more and more innovative technologies. The sectors that saw the greatest growth included mobile communications (smartphones and tablets), wearables (including virtual and augmented reality devices), and electronic medical devices. Other important developments have also been made in the automotive and aerospace industries. The push from innovation, coupled with the availability of new manufacturing techniques, has allowed the introduction of new materials to produce thinner, lighter and, if necessary, flexible printed circuits capable of transmitting electrical signals at ever-increasing speeds and frequencies.

Demand for new materials

Traditional materials and substrates include fiberglass fabrics, plastics (resins) and copper. Different types of resins and glasses are used in the manufacture of printed circuit boards, and the way they are combined affects the electrical and mechanical properties of the material. The two main electrical properties that define a material are the dielectric constant (Dk) and the loss tangent (also known as the dissipation factor (Df)), both of which are highly dependent on the temperature and frequency to which the material or substrate is subjected . The dielectric constant specifies the amount of charge two conductors can hold when a certain voltage is applied to them. The constant Dk also determines the rate at which a given current flows in the conductor. Conversely, the loss tangent provides a measure of the electromagnetic energy absorbed by the dielectric material.

The most modern electronic applications require materials with properties that differ from those offered by the materials and substrates traditionally used in PCB manufacturing. Even though the reasons for deciding to choose are many and strictly depend on the specific application, a possible list includes:

l Need to manage electrical signals with higher frequencies;

l Increase the integration density of electronic components;

l Availability of new software packages for many components, affecting routing technology;

l Need to minimize power loss, especially in low power or battery powered applications;

l Need to provide adequate thermal management for the PCB to minimize the heat dissipated;

l Need to manage device connectivity (often wireless), which is a critical aspect of PCB design.

The increase in the frequency of signals traveling across the PCB seems unstoppable. This feature, coupled with increasingly lower supply voltages (especially for highly integrated digital components such as MCUs, SoCs and FPGAs), is creating serious signal integrity issues. Applications of this type include fiber optic transport cards and devices, computers, and most embedded systems equipped with processing units.

New Materials and Substrates

Based on the factors considered in the previous paragraph, we can identify two key factors that determine the choice of materials and substrates best suited for a particular application: the maximum power and heat that the PCB can withstand. While this rule is general and applies to all types of materials, there are greater benefits to adopting innovative materials, such as:

l Fluoropolymer: PCBs fabricated with substrates of this material have high resistance to corrosion, mechanical stress and high temperatures. Furthermore, at the mechanical level, fluoropolymers have excellent abrasion resistance, low adhesion and long life characteristics. Considering the non-negligible cost, this type of material is suitable for the manufacture of PCBs used in the medical, pharmaceutical and food industries.

l Polyimide: Due to the growing popularity of flexible and rigid-flex printed circuit boards, this material (also known as PI) has recently enjoyed great success. These PCBs are revolutionizing some electronic applications by solving electrical connection issues once considered critical in an efficient and simple manner, especially in terms of reliability. This task is possible due to their ability to bend and wrap themselves in narrow or irregularly shaped spaces. Unlike traditional rigid PCBs, flexible PCBs can be bent without changing the transmission of the electrical signals they carry. Composed of polyimide films deposited on conductive trace substrates, it is widely used in smartphones, wearables, electronic medical devices, and anywhere a flexible wiring solution for tight spaces is required. In addition to mechanical flexibility, the material thus obtained has excellent thermal and atmospheric resistance. By combining rigid parts and flexible parts together, a rigid-flex PCB as shown in Figure 1 can be obtained. The solution currently costs more than traditional PCBs and can be used in the automotive and motorcycle industries, military and aerospace.

l Acrylic Adhesives: These materials are highly appreciated for their ductility even after polymerization and are an excellent solution for all dynamic applications. Acrylic adhesives have a higher coefficient of expansion than other materials used as PCB substrates. Additionally, at temperatures approaching 180°C, the acrylic adhesive begins to soften, and the PCB layers in contact with the conductive traces may delaminate. If high flame retardancy is required, chemical flame retardants must be added to the substrate to reduce the dynamic properties of the material;

l Epoxy Adhesives: Unlike previous adhesives, epoxy adhesives polymerize to form rigid materials, making them unsuitable for many dynamic applications. However, due to their lower coefficient of expansion and higher bond strength, they are an excellent solution for building multilayer PCBs that can withstand higher operating temperatures. Epoxy adhesives have high chemical resistance and ability to absorb moisture and are therefore widely used as substrates for PCBs where sensors may come into contact with moisture, such as in medical and healthcare applications and many fitness and wearables in the device;

l Liquid crystal polymer: Also known as LCP, liquid crystal polymer is usually used in the manufacture of multilayer PCBs, where reducing thickness is a basic requirement. LCP is made of extremely inert, non-reactive material with high flame retardancy. They are lightweight and flexible, and have extraordinary electrical properties that make them an ideal solution for high frequency applications, especially where the weight and thickness of the PCB needs to be contained. Liquid crystal polymers also have good dielectric properties with very low loss and hygroscopicity.

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

New materials that offer superior performance compared to traditional materials require continuous development due to their ability to improve various aspects related to signal integrity. Lower Dk values improve impedance control, crosstalk, jitter and signal skew. On the other hand, lower Df values help improve rise and fall times and overall decay.