The selection of substrate materials shapes the effective performance and traits of printed circuit boards. Therefore, it is vital to select suitable materials when designing your circuit board.
Most PCB materials consist of the epoxy resin substrate, glass fiber, and epoxy resin substrate attached to the copper foil. Copper is commonly used in PCBs because it is a good conductor of electricity. This article will dive deeper into the various PCB materials and the design consideration you should consider when selecting PCB materials.
Types of PCB Materials
There are four main types of PCB substrate materials: Hard/rigid, soft/flexible, Flex-rigid, and FR-4 materials. Let’s have a closer look at each of them.
Soft/Flexible PCB Materials
Soft materials present multiple application options because of their ability to bend, wrap, and fold into many shapes. This is why they have gained more attention in modern consumer electronics than their rigid counterparts.
Soft materials are applied particularly for electronics that require a PCB that bends during operation without hindering the current flow. Various industries, such as healthcare, smartphone, and wearables, have embraced flexible PCBs widely.
Manufacturers integrate a polyimide film known as Kapton in flexible materials. The film displays good traits like heat resistance, dimensional consistency, and dielectric property of 3.6. Polyimide and polytetrafluoroethylene are the common flexible PCB materials.
The Benefits of Flexible PCB Materials
They are space-intensive and minimize the PCB weight.
With plastic dielectric materials offering little stability to the boards, flexible PCB materials such as epoxy guarantee good stability to the board by creating a coat near the filler.
They minimize the PCB sizes, making them suitable for high-signal trace density applications.
They are ideal for making boards for use in harsh environmental conditions as they are waterproof and corrosion-resistant.
When to Use Flexible PCB Materials
When designing for harsh environments.
When you need a miniaturized and lightweight board.
If the application requires a bending PCB.
You require rigid and ceramic-based materials when you want to maintain your circuit board shape throughout its lifespan. The solid feature of these materials prevents boards from forming other shapes. Rigid PCB materials are primarily applied in making computer motherboards.
The most common rigid materials are aluminum, aluminum nitride, and beryllium oxide.
Benefits of Rigid Materials
They display an excellent functioning temperature of above 3500C.
They are packaged in small sizes and integrate well with PCB components.
They have a high dielectric constant.
They do not experience outgassing problems.
They have the least water absorption rates, facilitating hermetic packaging.
When to Use Hard PCB Materials
When you want quick repairs and maintenance – hard materials are integrated with clearly labeled components.
When you want to minimize costs in high-volume production runs
When your application requires a stable board.
When you need a clear signal path.
RF-4 PCB Materials
This is a fiberglass-epoxy material that is cost-effective, an excellent electrical insulator, and more fire-retardant than fiberglass-only material. Besides, FR-4 is not just a PCB material but a graded material recommended by various international trade bodies, like NEMA.
FR is an abbreviation for ‘fire retardant’ and implies that it complies with the UL94VO standard. FR-4 materials exhibit high flame resistance properties because they contain bromine halogens. There are multiple sub-grades of FR-4 materials, such as TG130, high TG, and FR4-Rogers.
The Benefits of FR-4 PCB Materials
They are cost-effective and versatile.
They are made of prepreg sheets, which consist of fiberglass matting and epoxy resins.
They represent the electronic industry standard regarding easy boring and metallization.
When to Use FR-4 Materials
When your application requires low-speed circuits.
When you want to minimize the production costs.
When making a complex design that requires multiple layers.
Flex-rigid PCB Materials
Flex-rigid materials are a combination of soft and hard materials. They have multiple layers, mostly made of polyimide, and are widely applied in making circuit boards for the aerospace, medical, and military industries.
Benefits of Flex-Rigid Materials
You can machine them easily.
They are precision built; thus, suitable for applications that require high precision levels, like military and medical equipment.
They provide a 60% reduction in weight and space.
They display minimal dielectric losses at microwave frequencies.
When to Use Flex-Rigid PCB Materials
When you require lightweight boards.
When designing boards for medical and military use.
When handling relative frequency identification tags.
When building automotive radar and sensors.
When designing cellular receivers and power amplifiers.
Factors to Consider When Selecting PCB Materials
You need to consider the thermal, mechanical, electrical, and chemical properties of materials to choose the right fit for your application.
Thermally, you need to factor in the temperature durability of the components and the distribution of optimum functioning temperatures. Additionally, you should think of mechanical properties, like enclosure and form factor control, and the density, in the case of rigid-flex PCBs.
Electrical properties matter most when you need to balance the dielectric requirements such as impedance, signal quality, and resistances. Chemical factors like moisture absorption (prevalent in electronics functioning in agricultural IoT) and flammability are also essential for the proper performance of your PCB products.
PCB Materials Ideal for Multilayer PCBs
Boards are often stacked to save on space –multilayering. You need to consider the dimensional and electrical features while designing multilayer PCBs. Below are the materials that are ideal for building multilayer boards:
RO4000: It features the low-thermal coefficient of the dielectric constant to reduce phase variations and the CTE z-axis, closely related to copper. RO4000 materials are therefore suitable materials for making multilayer boards.
Flex-rigid materials: The RO3000 materials are particularly ideal for low-loss microwave boards, though they do not perform well in multilayer boards because of their dimensional and dielectric changes with temperature. However, they can perform well in multilayer PCBs if you work on their structural properties by integrating their electrical properties with the mechanical elements of polyimide materials.
PCB Materials to Use in High-Frequency Boards
Applications that require wireless connections, high speeds, and frequencies are living proof of the necessity of building boards with materials that offer excellent performance while maintaining signal strength and overcoming interference. Below are the ideal materials for building high-frequency PCBs.
Fluorine-series resin: PTFE materials display high dielectric performance and are suitable for making 5GHz frequency boards. They are the most expensive PCB materials, and they have high-temperature expansion coefficients. Nevertheless, they function well in environments plagued with high dielectric loss and humid conditions.
PPO or PPE resin: They are excellent for building PCBs of 1-10GHz frequencies and guarantee reliable performance throughout the PCB lifespan.
Modified epoxy resin: They are suitable for creating boards with frequencies of 1-10GHz. Generally, they are the cheapest high-frequency PCB materials.
How to Choose the Right Materials for High-Frequency Applications
Matching the CTE properties: This is an essential substrate quality regarding temperature – ensure that the CTE between two materials matches to avoid expansion of substrates at distinct speeds and uniformities. Irregular CTE values can cause defects during the manufacturing process, altering the physical structure of the materials and their DK values.
Tight material weave: You must weave your materials appropriately for them to interact well with the mesh.
Matching the dielectric constants: If the Dk values of your materials do not match, you are likely to experience issues. For instance, pairing a resin with a woven layer will result in mismatched Dk values.
FR-4 is not an alternative: As mentioned before, the FR-4 material is unsuitable for high-frequency boards. However, many designers use it because it is cheap, even when they are aware that it will affect the proper performance of their high-frequency circuits. Such circuits require low-loss materials.
The Future of PCB Materials
Although small developments in the silkscreen ink or the solder mask materials are not likely to hit the headlines, a lot of research and development focuses on improving the substrate materials for making different boards. The need for miniaturized and lightweight PCBs for consumer electronic products has amplified the demand for multilayer designs that offer more functionality and performance.
Advanced epoxy materials like Getek, Megatron, 4000-13, and FR-408, which boast better performance than FR-4 materials, will likely shape the future of PCBs. Additionally, we will experience wide use of high-performing materials like A-PPE, Nelco 600-21 Si, and Rogers 4350 because of their low dielectric properties, high impedance control, and minimal jitter.
Stringent RoHS standards will also boast the use of higher heat-resistant and reliable substrate materials. Halogen-free substrates reduce chlorine and bromine content in PCBs to around 900 ppm. Furthermore, lead-free substrates eliminate lead in PCBs by swapping the curing of traditional FR-4 materials with a PN curing method.
This article has offered a detailed analysis of PCB materials and the factors that you should consider when selecting materials for high-frequency circuits. We also touched on the future of PCB materials to keep you up-to-date with the current industry trends.