Designers that take a lead role in building their PCB stack-up will have to select materials. PCB material selection is a critical part of the PCB engineering process, and it determines some of the most important aspects of electrical, mechanical, and thermal performance. There are additional materials that go into building a PCB that are often an important part of engineering a new product, and they often don’t get the attention they require.

We prepared this PCB material selection guide to help designers navigate the complexity of all the materials used in PCBs and PCBAs. The end application and level of reliability are the ultimate factors driving PCB material selection. With that in mind, we will start by looking at different material classes for a range of application areas and end-use products.

PCB Materials for Various Applications

An application for a PCBA can range from simple consumer electronics to high power handling or wireless communication. The application also determines the frequency range at which the PCB will need to operate, or the power range, or both. Therefore, in our opinion, frequency range and power handling are both good metrics to use to start selecting PCB materials.

Within these material types, there are a few important material properties that determine which PCB material should be selected for a new design:

• Dielectric constant and loss
• CTI or PLC value
• TG and CTE values
• Breakdown or decomposition properties
• Material thickness and type (core or pre-preg)

These properties of the PCB dielectric materials will determine the electrical, thermal, and mechanical performance and reliability. Let's look at some example applications to see how these important properties drive a new design.

High Voltage PCBs

Operating at high voltage does not necessarily mean operating at high power, but the voltage itself is an important parameter in easy material selection. PCBs operating at high voltage can experience reduced reliability for several reasons. Among the many potential failure mechanisms, high voltage PCBs can experience quick failures or long-term reliability problems:

• Arcing in the presence of metal near high voltage components
• Electrochemically driven dendritic growth on surface layers due to solder flux residues
• Growth of conductive anodic filamentation over long-term operation
• Dielectric breakdown of the PCB materials

For high-reliability equipment, such as military and aerospace PCBs, CAF is the most insidious failure mechanism. In fact, it happens to be related to the curing agent used in the dielectric material in a high voltage PCB. From a reliability perspective, the highest reliability curing agent option is DICY, and many commercial material suppliers provide this curing agent in their material offerings. When selecting a material, check the comparative tracking index (CTI) for a comprehensive rating for high voltage PCB materials based on operating voltage.

 

 

RF PCB Materials

RF PCB designs operating above Wi-Fi frequencies benefit greatly from having specialized materials, specifically PTFE materials and Teflon-free epoxy resin systems. These two classes of materials are intended to provide the lowest possible dissipation factor, which will reduce insertion loss. These materials are also engineered to have a broad range of dielectric constants, which allows the size of an RF circuit to be tailored to the wavelength of the signal traveling in the circuit.

The broad cross-section of PCB materials that are compatible with RF devices includes simple FR4 materials, which are also acceptable for use in RF devices, provided the insertion loss budget is understood. A cross-section of RF materials vendors includes:

• Rogers Corporation
• AGC Multi-Material
• Arlon
• Isola
• Ventec

These companies provide the best-known RF materials, with Rogers being a significant supplier of many materials for military and aerospace systems. There are also many Asian vendors that provide comparable materials, but they are less well known than the companies listed above.

 

 

High-Speed Digital PCBs

Pretty much every PCB counts as a high-speed digital PCB as long as it uses a modern microcontroller with at least an SPI interface. The process technology and physical design of today's microcontrollers ensure that most interfaces have fast edge rates with moderate or small capacitive loads. However, just because many devices operate at fast edge rates does not mean premium materials are needed for every digital PCB.

Standard FR4 materials are acceptable for high-speed PCBs operating at low edge rates (or equivalently, low channel bandwidth). Once you look at materials specifically marketed for high-speed applications, these are lower-loss FR4 with a DK value of approximately 3.9. As signal edge rate increases, loss tangents for materials tend to go down, which then reduces insertion loss in long channels.

 

 

HDI PCB Materials

There are a small number of materials specifically designed for HDI designs, with the primary factor being that they are very thin. Aside from this, technically, any material with low enough thickness and the appropriate drilling compatibility can be used in an HDI design. HDI PCB materials are sometimes referred to as buildup films; however, some PCB laminate manufacturers produce their standard materials in small thicknesses, which can be used to fabricate blind and buried vias required in an HDI design.

For the HDI-specific materials, a short list includes:

• Ajinomoto buildup film (ABF)
• Polyimides
• Thermoset resin systems
• Resin-coated copper foils
• FR4 epoxy glass materials
• Reinforced and unreinforced thin PTFE films

 

 

Which Materials Are Best for Your PCB Projects?

Ensuring the board meets its operational and reliability targets hinges on selecting the right materials. A knowledgeable PCB design services company and collaboration with a manufacturer can help narrow down the best material options for a new product. This will help satisfy the basic electrical, mechanical, and thermal requirements, as well as the DFM requirements that will be imposed by the PCB fabrication process.

I would like to note that we did not cover more specialized designs, such as metal-core or metal-backed PCBs, and we did not cover some exotic designs like hybrid-HDI, hybrid RF, and ceramics. I’ll address these in an upcoming article focused on the use of exotic PCB materials.

 

Whether you're designing an ultra-rugged aerospace system or feature-rich embedded computing products, make sure your design firm understands how to coordinate with electronics manufacutring services and contract manufacturers to help you produce military embedded systems with maximum quality. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.

 

---

---

---

---