Some years back, my new neighbor decided to paint his house. He didn’t know at the time that using darker paint color than the house’s vinyl siding would cause it to warp and buckle from heat absorption. Sadly, the home looked much worse after the paint job and required new siding to fix the problem.
Proper materials management is essential in all aspects of our lives, from cooking ingredients to choosing the right paint colors for a house. It’s also crucial in manufacturing printed circuit boards, especially when those PCBs are operating at high frequencies. Here are some PCB material management for high-frequency considerations to keep in mind for your next design.
Circuit Board Composition and Materials
Circuit boards combine materials designed to conduct electrical signals along designated metal paths and insulate those metal paths from coming into contact with each other. Multilayer circuit boards use dielectric materials for a core and laminate, which serve as the foundation for the copper foil making up the metal paths. The dielectric materials used throughout the board have specific properties and characteristics that have to be considered for the frequency and operating environment of the PCB.
One material commonly used in circuit board manufacturing is flame retardant type-four woven glass-reinforced epoxy laminate, otherwise known as FR-4. This material has excellent electrical insulation characteristics, is sturdy, easy to work with, and is cost-effective, making it the ideal choice for most PCB applications. However, FR-4 does have some property and characteristic limitations. If a circuit board using FR-4 exceeds its rated heat, voltage, or power limitations, it is most likely that the board will eventually exhibit circuitry performance problems.
One of the characteristics of FR-4 is its glass transition temperature, or “Tg.” At this temperature, the base fiberglass material becomes amorphous or shifts from a solid rigid state to a rubbery state. This temperature is a concern for both PCB fabrication when heat is applied for the prepreg and during regular operation. If the PCBs operating temperature exceeds its rated Tg for an extended period, the base materials’ shift to a rubbery state will affect the board’s performance. For circuit boards operating at higher frequencies, or in harsh environments, or both, it may be better to look into more robust materials.
High-Frequency Design Requirements
As we have discussed, common circuit board materials such as FR-4 have operating limitations that may affect their ability to serve as a reliable substrate for high-frequency designs. If the Tg for the dielectric material is exceeded, its electrical insulation properties may diminish, and it will start conducting electricity instead. FR-4 has additional limitations that affect its suitability for high-frequency designs compared to other dielectric materials:
- Uncontrollable impedance: FR-4 is rated at a dielectric constant of ± 10% or more, which may not give high-frequency designs the tolerance they need on their controlled impedance routing.
- Signal loss: The signal loss of materials like FR-4 has a higher dissipation rate at high-frequencies than other high-speed materials.
- Moisture absorption: At nearly 50%, FR-4 has a high rating of moisture absorption. Even a small amount of absorbed moisture in high-frequency circuits can change the dielectric properties of the circuit board and affect its electrical performance.
- Thermal management: As stated earlier, FR-4 has limitations on how much heat it can conduct before it reaches its Tg rating. For high-temperature operating conditions, another material would be a better choice.
Fortunately, circuit board materials have been developed specifically for high-frequency and harsh operating environments, which we will look at next.
PCB Material Management for High-Frequency Designs
Many different materials are available for high-frequency circuit board manufacturing that fall under various brand names, types, and capabilities. These materials can be broken into the following general categories:
- Epoxy resins: This first category includes FR-4, which despite its weaknesses at higher frequencies, it is still the default material for most circuit board manufacturing
- Enhanced epoxy: These materials are taking standard FR-4 and related materials to new levels of performance.
- Polyimide: These materials are often used for flexible and rigid-flex PCB layers as they can withstand a lot of manipulation without impacting their performance. They are very robust materials with excellent thermal and mechanical properties and are chemical and moisture resistant, making them ideal in harsh environments.
- PTFE (Polytetrafluoroethylene): These materials are becoming standard alternatives to FR-4 in high-frequency RF applications. Also known as “Teflon,” these materials have a very low dielectric loss factor with improved impedance control and signal performance, making them a good choice for 5G wireless communication systems.
- Thermoset hydrocarbon laminates: These materials are easily manufactured and offer good mechanical stability for high-speed circuit boards but may experience dielectric loss at higher frequencies.
As you can see, there are many choices for your high-frequency applications. Before you choose one of these materials, though, there are some other details that you should consider first:
- These advanced materials do come with a higher price tag, and the benefits of the materials must be balanced with their increased cost.
- These materials can also be more challenging to work with, and the increased fabrication and assembly costs must also be factored in. Some of the PTFE laminates, for example, require non-standard fabrication processes.
- Not all of these materials give the same benefits, and you may have to balance some trade-offs between them.
- Different materials and fabrication temperatures may dictate changes in solder types and cleaning processes, affecting the board’s price and manufacturability.
These considerations can generate many questions, which you shouldn’t be shy about presenting to your PCB contract manufacturer. At VSE, we have been helping our customers work through these issues for years and understand the challenges you face and how best to assist you.