In the fast-growing technology, printed circuit boards are the base foundation for any electronic gadget or device. They function from very low current to high power consumption application designs.
High-speed PCBs are different from simple PCBs in terms of design, material, stack-up, and performance. These have applications in industries such as defence, aerospace, and networking. The boards are engineered to meet high performance with minimal signal losses/reflections. We should get a comprehensive understanding of PCB material for high speed design and the properties of high speed PCB material before designing a high-speed PCB.
What is a High-Speed PCB?
High-speed PCBs as the name suggests are printed circuit boards which operate on high speeds ranging from a few Mhz to Ghz.
High-speed PCBs have to meet the high demand of complex applications, they need to be efficient in their fabrication. To make the high-speed boards less prone to EMI-EMC issues, the high speed PCB material selection criteria will be the game changer.
In this article, we will explore various high speed PCB material properties that will provide complete guidance to design engineers for selection criteria. They will get to know high speed PCB material knowledge and use this learning of to relate to project specifications.
High-Speed PCB Material Properties
There are different types of high speed PCB materials available in the market like FR4, PET, PTFE and many more. These materials have different thermal, mechanical, and electrical properties which should be considered while making high speed PCB material selection to achieve correct performance.
Thermal Properties
- Glass Transition Temperature (Tg): It is the temperature at which PCB base materials transition from a glassy, rigid state to a softened state. This change is reversible when the heat reduces.
- Decomposition Temperature(Td): The temperature range at which the material decomposes chemically, and PCB loses some percentage of total mass. Td should always be higher than Tg based on operational temperature.
- Coefficient of Thermal Expansion(CTE): CTE is the expansion rate of PCB substrate when it heats up beyond its Tg. The PCB material for high speed design will see a rise in CTE. As substrate has higher CTE than copper, it causes problems when additional heat is added.
- Thermal Conductivity(k): This is the heat dissipation property of a PCB material for high speed design. Generally speaking, the higher the thermal conductivity the better, since it helps to conduct heat quicker which is better to keep electronics functional.
Electrical Properties
- Dielectric Constant(Dk): It is equal to the ratio of E/E0 (electric permittivity of material/ electric permittivity of free space). Dk is also known as relative permeability. As frequency increases, the dielectric constant decreases. Dk ranges from 2.5 to 4.5 for most of the PCB material.
- Dissipation Factor(Df): The lower the Df, the less dielectric loss. It is also called a loss tangent (Tan δ) which is the phase angle between ESR and reactive current. For analog and digital circuits loss tangent has different effects. It is more important in analog signals.
- Volume Resistivity: It is denoted by (ρ). The resistance of PCB dielectric materials for high speed applications to insulation is volume resistance. The PCB dielectric materials for high speed applications should have high resistance to outside variables cold, heat, and moisture. Materials with high resistivity resist the movement of electric charges, while materials with low resistivity allow electric charges to move more freely.
Mechanical Properties
- Flexural Strength: It is also known as bend strength or modulus of rupture, which determines the ability of PCB dielectric materials for high speed applications to withstand physical stress without failure to breakage. It is measured in kg/m^2.
- Time to Delamination: The time taken by the PCB layers to separate from each other when exposed to a high-temperature threshold. IPC -650 defines a method to determine the time to delamination(TMA) of high speed PCB material.
- Density: PCB dielectric materials for high speed applications and PCB density are measured in grams per cubic cm and pounds per cubic inch.
Chemical Properties
- Moisture Absorption(%): PCB material for high speed design with low moisture absorption is suitable. Generally used FR4 grade has a 50% rating.
- Flammability Specification(UL94): As to the high speed PCB material, UL94 is a standard from UL company, it states PCB should not burn more than 10 seconds in high combustion.
Important Considerations for High-Speed PCB Material Selection
The high-speed boards aim for high performance of signal integrity, power integrity, thermal resistance, quality, and reliability. With so many options in high speed PCB materials, it is a challenge in itself for engineers to find the best high speed PCB materia. Each product has its unique application, field of operation, cost constraints, mechanical challenges, and regulatory compliance. The Following Points Should also be Kept in Mind while Analyzing the Best Choice of PCB Material.
- Matching Dk’s: In the PCB, different dielectric constants of high speed PCB material should be matched with each other. If we are matching the Dk’s there will be less likely chances of mismatched Dk’s resulting in repair and failure.
- Match CTE Coefficient: Like Dk’s, we should ensure that different substrates used in the fabrication process match the Thermal expansion coefficient, otherwise when the temperature rises, mismatched CTE’s impact the expansion rate causing the inconsistency.
This mismatching in CTE also impacts the Dk values.
- Avoid FR4 Material, Overlooking Low Cost: As to PCB dielectric materials for high speed applications, we should avoid FR4 as high speed PCB material. In the frequency range of less than 1 GHz, FR4 shows a high dielectric loss. We tend to select the readily available high speed PCB material for the fabrication process. It is suited for low-frequency boards and applications.
- Prefer Conductive Anodic Filament(CAF): CAF is a metallic film that causes PCB failures. It is advised to select the right conductive foil.
- Mechanical Stress: Every product has to be designed as a manufacturable product which withstands EMI/EMC issues, signal reflections, vibration, high temperature, flexibility, and shocks. In applications where physical stress is crucial, mechanical properties have to be judged.
- Strict Impedance Tolerance: When the PCB is designed to serve the purpose in high frequencies, signal reflection due to noises causes an imbalance in impedance matching.
To maintain the constant Dk value, we should ensure the impedance is controlled even when the frequency varies.
- Cost: Investing a good amount of money in the selection of high speed PCB material is always a good decision.
As we can ensure the failure and defect rates will be low and negligible. Substrates of reliable quality, such as high speed PCB material, come in handy as we pay. It is better to negotiate with the supplier if there is a budget constraint in the product design. Sometimes we can predict the post-sample quantity and ask the supplier to lower the fabrication cost, as the future holds bulk orders of high speed PCB material.
- Manufacturing Limitations: Collaboration with the fabrication house is to be taken as a priority step to discuss and find the solutions of etching, drilling, high speed PCB material layer-stackup challenges and impedance values. It is an important part of the manufacturing process to avoid any rework charges or warranty issues related to high speed PCB material.
Conclusion
With this selection guide of high speed PCB material, designers will find complete support in deciding which is right for high-speed boards. They can compare in detail each high speed PCB material’s properties. There are other factors like layer stackup, routing, and signal integrity which are also crucial for less losses in high-speed design. Simulation tools also provide support to model PCB performance.
Some of the special high speed PCB materials like PTFE-based laminates, Isola I-speed, Astra, polyimide boards and Megatron can support the high-speed PCB design requirements.
Meeting strict RoHS standards encourages the use of more heat-resistant and reliable high speed PCB materials. Halogen-free materials cut down chlorine and bromine content to around 900 ppm. Additionally, lead-free materials remove lead from PCBs by using a different curing process instead of the traditional FR-4 method.
There are also other high speed PCB material options to take guidance. We can use expert advice from more experienced industry manufacturers to streamline the design process.
