The talk about creating high-speed digital circuits is happening across the world due to the development of fifth-generation (5G) cellular communication systems. As technology becomes more advanced, engineers are looking for the right ways to convey signals and frequencies through standard materials that are available today for printed circuit boards (PCBs), as these PCBs need to handle the mechanical and electrical properties required without hindering operational capabilities.
A major issue with high-speed printed PCB designs is maintaining integrity of the digital signal for a wide range of high-speed frequencies. Not only will the PCB still handle millimeter wave frequencies of 6GHz or less, they will also have to handle the higher frequencies of up to 30GHz and more. These signals will act as transmission lines with an electrical length between the transmitters and the receivers. Crosstalk, impedance, and higher temperatures are just a few issues that will need to be addressed based on the types of materials that will be put into use.
Example of a printed circuit board manufactured with high-speed PCB materials.
Factors Required for High-Speed PCB Materials
A standard circuit board design consists of substrate core layers that are non-conductive dielectric as well as dielectric laminated layers. The laminate layers will be the base for the copper foil traces as well as the power planes. These layers are made from a variety of materials based on their characteristics as they act as insulation between the copper's conductive layers while allowing the conduction of the electricity.
To determine the appropriate materials for the substrate core layers and laminate layers, several specific parameters are used to evaluate the thermal properties and electrical properties of the materials. In addition, other factors such as chemical properties and mechanical properties must also be evaluated based on the specific application, as in some instances the PCB will be used in machinery and components that will experience higher levels of moisture or placed into tight spaces that require more flexible PCBs.
To figure out the electrical performance of the high-speed PCB material, a dielectric constant (Dk) measurement is used. The dielectric constant of materials helps to figure out the amount of potential electrical energy that the material will have. Almost everything has a dielectric constant, even the air. When it comes to PCB layers, you are looking for a material that has low Dk values to act as insulation for the copper traces and power planes. In addition, the chosen material should also maintain as much of a constant Dk as possible during its lifecycle for different frequency ranges. Signal integrity and impedance are the factors that help to figure out the electrical performance of the PCBs dielectric materials.
Another important aspect for the high-speed circuit board involves the dissipation factor (Df). The dissipation factor is another measurement used by engineers to determine the absorption rate of the dielectric material to AC energy as well as electromagnetic fields. The PCB should also have electrical strength to prevent electrical breakdown over a period of time, surface resistance of the material, and offer high volume resistivity to electricity.
Heat will be generated along the PCB while conducting electricity. This heat will cause thermal stress on the dielectric materials, transmission lines, and components, causing materials to decompose at different rates. In addition, the heat can cause certain materials to expand, which can be detrimental to the PCB as this heat can form cracks and failure.
When looking at materials for high speed PCBs, you want to evaluate the thermal conductivity of the material as it allows the heat to transfer and dissipate safely without impacting the PCBs operations. In addition, you want to also evaluate the thermal expansion rate of the materials, called the coefficient of thermal expansion (CTE), the decomposition temperature (Td) rates to figure out the temperature threshold when the materials start to lose their mass, and the glass transition temperature.
The glass transition temperature (Tg) involves materials that will change characteristics when heat is applied. When the temperatures reach a certain level, the PCB materials will become softer. As the temperature drops, the materials will return to a solid state.
Chemical and Mechanical Properties
When evaluating chemical resistivity, the type of environment where the circuit board will be used plays an important factor. You want to select a material with low moisture absorption and high chemical resistance (methylene chloride resistance). Engineers should also look for materials with flame retardant properties, where the materials will not burn longer than 10 seconds to 50 seconds when there is flame combustion.
Rigid-flex and flexible PCBs require certain mechanical properties to be able to bend and fold around components without compromising the material's integrity. If the laminate layers are bent, the copper layers may start to peel away. Also, the material must have enough flexural strength to not only be bent in a certain way but to also maintain this angle and physical stress for the life of the circuit board. Lastly, the layers of the PCB can begin to unravel at certain temperatures, as the time of delamination must be determined.
What You Want in High-Speed PCB Materials
Due to the many factors that PCB materials will have, there are basic, specific benefits that you desire for high-speed PCB materials. The benefits to look for include:
- Dimensional Tolerance Stability: The high-speed PCB should have materials that provide mechanical stability when experiencing different temperature ranges, vibrations, shocks, and electric surges.
- Superior Thermal Management: The PCB material should provide excellent heat transfer and dissipation without having the layers start to peel away, delaminate, or decompose at high rates.
- Enhanced Signal Performance: Signal performance should be constant across the board with little signal loss even when the frequencies increase. The materials should have a low Df to provide this advantage.
- Tight Impedance Control: High-speed circuits will require tight control of the impedance routing to maintain a constant Dk when there are varying frequency ranges.
- Moisture and Chemical Resistance: Select materials that have low absorption rates of moisture and chemicals so there will be little changes to the electrical performance of the PCB.
Materials to Use
The materials that will be selected should be based on the specific high-speed PCB application, their costs, and their manufacturability. The standard material out on the market today that is used for PCBs is flame retardant type-4 (FR-4). This material consists of epoxy laminate that is reinforced with woven glass. It is often used for low- to medium-speed circuit boards as it is cost-effective for high-volume production runs. While it can be chosen for high-speed PCB applications, the standard FR-4 is susceptible to excessive heat, power, and voltage rates. It also does not have a stable Dk for high-speed frequencies as signal loss can become more frequent.
If FR-4 is not an option, thermoset hydrocarbon laminates, PTFE laminates and polyimide may be acceptable alternatives. All these materials are readily available, but at a higher cost.
- Thermoset hydrocarbon laminates: Thermoset hydrocarbon PPE and PPO offer good mechanical stability, ages well and can be easily fabricated. It offers the mechanical stability designed for high-speed PCBs. However, there may be an increase in dielectric loss at higher frequencies.
- PTFE laminates: Polytetrafluoroethylene (PTFE) materials are becoming standard alternatives to FR-4. This material has excellent dielectric properties for microwave frequencies and millimeter wave frequencies, especially for those now being used for 5G wireless communication systems. It also provides low signal loss, improved impedance control, and good signal performance.
- Polyimides: Polyimide materials are usually seen in flexible and rigid-flex PCB layers as they can be bent and folded around components without impacting their performance. This material also offers excellent thermal management, moisture resistance, and chemical resistance.
Selecting the Right High-Speed PCB Materials
It's true that high-speed PCB materials are expensive, which may be cost prohibitive based on the application. Consider both advantages as well as the tradeoffs during the initial production of the PCBs as well as the long-term benefits that may be gained. The higher costs of specific high-speed PCB materials may be worth the initial costs for certain applications. Otherwise, your operations may still find that FR-4 is the best option in certain cases.
Speaking with an engineer who is experienced with the design and manufacturing of high-speed circuit boards, whether they are standard rigid boards, rigid-flex boards, or flexible boards, can allow you to evaluate the types of materials that will be best suited for the application. In addition, the engineer can also inform you about possible issues that certain PCB materials will have to other materials in the application. Compatibility issues can slow down manufacturing processes and your time-to-market deadlines. By providing comprehensive specification details with the engineer, we can provide the best material options that are suitable for the application and that can offer the cost savings that you require.