The PCB substrate is the physical base on which the circuit components and traces are placed and connected. Choosing the right substrate for your PCB ensures the best functionality and durability. The various thermal and electrical characteristic features of a PCB substrate are the determining factors to select a material that is best suited for your product requirement. We will go through some of the important material properties, understand different types of substrates available in the market with their advantages, and also list a few best practices to choose the right substrate based on different applications.
Some of the thermal, electrical, mechanical, and chemical properties of a substrate will define its requirement in diverse applications.
Glass Transition temperature (Tg):
The temperature range in which the substrate softens and loses its rigid structure is known as the glass transition temperature. It is measured in degrees Celsius. Once the temperature reverts, the PCB material hardens and restores to its original form. A standard PCB substrate will have a Tg = 130°C. High-performance circuit boards require a Tg value >= 170°C.
Coefficient of Thermal Expansion (CTE):
The rate at which the PCB material expands when exposed to a high temperature is known as CTE. It is measured in parts per million. If the temperature is more than the Tg, then the CTE value also increases.
Thermal conductivity (K):
The rate at which heat is transferred from a thermal source towards cooler PCB areas is the thermal conductivity value. Watts per meter per degree Celsius is its unit of measurement. Most dielectric materials will have a value between 0.3 and 0.6 w/m -°C. Copper layers have higher thermal conductivity (386 w/m-°C) and hence can dissipate heat quicker than the dielectric material.
Dielectric constant (Dk):
The capacity of a dielectric substrate to store electrical energy and impede signal transmission is known as its dielectric constant. A PCB will have a Dk value of 3.5 to 5.5 in general. It varies inversely with the operating temperature. So, a high-frequency circuit should use a low Dk substrate material.
Peel strength:
The ability of the substrate to resist a peeling force to separate dielectric and copper layers in a PCB is its Peel strength.
Moisture absorption:
The capacity to resist water penetration in the PCB substrate is an important property. It will be in the range of 0.01–0.02% for most of the substrate materials. Lesser absorption value will improve the PCB performance, as the moisture can affect the thermal and electrical properties of the substrate.
The basic substrate materials are classified into rigid and flexible types. Rigid PCBs are made of a hard substrate (ceramic-based) that prevents the PCB from bending. The advantages of rigid PCBs are high thermal conductivity (170 w/m — °C), operating temperature > 350°C, strong dielectric, and minimum outgassing issues.
Some of the popular ceramic-based substrates are aluminum, aluminum nitride, beryllium oxide, etc. Rigid PCBs are suitable for applications that focus on strength, easy maintenance, and lower cost. Typical usage can be in sensor modules, LED boards, medical circuits, etc.
Flexible PCBs are made of soft substrates that can be easily folded or packed in small enclosures. The latest Wearable gadgets are available in the market because of these flexible PCBs. Substrates suitable for flexible PCBs are polyimide and polytetrafluorethylene (PTFE). To improve flexibility, plastic is commonly used in flex PCB substrates.
The advantages of Flexible PCBs are their compact design and lightweight nature. They are suitable for high-density trace routing. The flexible PCB substrates are resistant to chemicals, radiations, and extreme temperatures that make them suitable for applications like aerospace and satellite communications.
A combination of Rigid-Flex PCBs is also used in various applications as they are lightweight and provide low dielectric loss at high frequencies. Applications like aerospace, medical and military sectors are choosing Rigid-flex PCBs as the best option due to their high reliability and high-density features.
FR4 is the frequently used PCB substrate material with advantages like versatility, dimensional stability, and excellent electrical properties. But, the FR4 is not precisely a type of substrate material. It indicates the grade of the substrate material. FR means ‘Flame Retardant’ as per the National Electrical Manufacturers Association (NEMA) standards. It indicates that the material is compliant with the UL94 standard that is required for the manufacturing product’s safety and operability.
FR4 grade materials are classified as standard, high Tg, and halogen-free types. The standard grade FR4 is made of fiberglass materials with higher bromine content. If the glass transition (Tg) value required is around 170°C and above, then a high Tg-FR4 grade can be selected. Bromine is toxic to humans and hence halogen-free FR4 grade materials are used in applications that include human interactions.
FR4 is the low-cost material available in the market. Hence it is widely used in many applications like DC boards, and digital and low-frequency applications. They are best suited for prototype designs and multilayer PCBs with complex designs. In high-speed applications, it is suggested to avoid FR4 materials. They have a high dielectric loss which is not a desired feature in high-speed applications.
Choosing a tightly woven substrate can provide an evenly distributed Dk value throughout the substrate. In a multilayer PCB, the CTE value of substrates in different layers should match, else there may be an uneven expansion of the layers during the PCB fabrication process leading to degraded performance. High peel-strength substrates are required in PCBs that operate in harsh environments. Applications with high power circuits can use substrates having high thermal conductivity to facilitate optimal thermal dissipation.
In most recent electronic applications, flex and rigid-flex PCBs are widely used. The polyimide material used in these PCBs has numerous advantages like flexibility, resistance to chemicals, excellent thermal conductivity, and operating temperatures. The tensile strength of polyimide substrates makes them durable and their flexibility enables seamless integration into a system.
A rightly chosen PCB substrate ensures your product quality. It is the first step toward building a robust and high-performance PCB product. Compromising quality to reduce the production cost is not a correct approach. With the knowledge of different materials and associated parameters, you can now choose the best substrate for your PCB design.
