How to Select High-Frequency PCB Materials

Posted July 30, 2022

The PCB industry has been using high-frequency board materials for several reasons and for decades. These materials have several unique attributes when compared to those of the more traditional PCB materials. Rush PCB Inc recommends a proper understanding of the attributes so that OEMs and PCB fabricators can both benefit greatly.

Most high-frequency applications require PCB materials with emphasis on multiple attributes rather than specifying one item of interest. Moreover, a specific application will usually have one specification that is paramount, while others are secondary. With a proper understanding of the needs and attributes of high-frequency materials, OEMs and PCB fabricators can select the optimum material for their specific application.

Attributes Necessary in High-Frequency PCB Materials

Dielectric Constant (Dk)

Most high-frequency materials need a very consistent dielectric constant. The consistency required is not only within a sheet of circuit material but also between lots. This is necessary so that the electronic circuitry on the board will offer the same performance in large batches.

If the application is intended to operate over a wide range of frequencies, the value of the dielectric constant must be consistent over the range.

The dielectric constant must also remain consistent over the range of temperatures in which the application is likely to operate.

Coefficient of Thermal Expansion (CTE)

Although CTE is not important for high-frequency PCB materials, it can be critical. When the ambient temperature goes up, the PCB will want to expand. If the CTE for the board material is significantly different from the copper on the board, the expansion can place solder joints on stress. This effect will be significantly pronounced if the thermal change is dynamic and cycles up and down. The different materials expanding and shrinking in different amounts can lead to a separation from the solder connecting the circuits.

CTE along the z-axis is more important for board materials with vias. With circuit material having a higher z-axis CTE, higher temperatures may damage the via plating, thereby degrading the electrical connection.

Moisture Absorption

For high-frequency operation, it is important that the PCB material exhibits minimal moisture absorption. This is important to keep a consistent dielectric constant, as only a small amount of moisture ingress is enough to change the electrical performance of the board.

High-Frequency PCB Materials

High-frequency PCB materials, therefore, need a tight tolerance for dielectric constant, a low thermal coefficient of dielectric constant, a stable dielectric constant over frequency, and a low z-axis CTE.

While there are several high-frequency circuit materials available in the market, only a select few fit all the above requirements.

PCB fabricators use the following materials for building high-frequency PCBs:

FR4, 370HR, FR408HR, Rogers 4350 series, Rogers 4003 series,

High-speed material: Tachyon, Megtron series

Their high-frequency characteristics are as follows:

Material	Dielectric Constant	Loss Tangent	CTE Z-Axis	Frequency
FR4	3.8 – 4.8	0.0160	175	< 1 GHz
370HR	3.92	0.0250	45	100 MHz to 10 GHz
FR408HR	3.39	0.0095	55	1 GHz to 10 GHz
Rogers 4350B	3.48	0.0031	32	2.5 GHz to 40 GHz
Rogers 4003C	3.38	0.0021	46	2.5 GHz to 40 GHz
Tachyon 100G	3.02	0.0021	15	2 GHz to 40 GHz, data rate > 100 Gb/s
Megtron 6	3.4	0.0020	45	2 GHz to 10 GHz

FR4

PCB design at the fabricator begins with an insulating substrate, and they typically use FR4 as its core. The FR in the name refers to the flame retardant properties of the material, while type 4 represents the woven glass-reinforced epoxy laminate in the substrate. FR4 is highly insulating and rigid, and the laminate typically forms the base material for the PCB.

To successfully design RF PCBs on an FR4 laminate, it is necessary to understand its material properties. This includes the dielectric and thermal properties. When using standard FR4 for RF applications, its dielectric stability may not be consistent across higher frequencies. FR4 also has a higher dissipation factor and higher insertion loss at microwave frequencies, leading to larger attenuation at high frequencies. Most fabricators use high-performance FR4 for circumventing these problems with RF circuits.

370HR

370HR is a high-performance FR4 material from ISOLA. Compared to the regular FR4 material, 370HR has advanced properties such as better dielectric and thermal properties, along with higher CAF resistance and UV blocking. Made from E-glass fabric reinforced with multifunctional epoxy resins, 370HR has low CTE and high Tg, while retaining the processability of FR4.

As the mechanical and thermal performance of 370HR exceeds that of the traditional FR4 material, its design is specially formulated for multilayer high-performing PCB applications.

FR408HR

Using E-glass fabric reinforced with ISOLA’s patented high-performance multifunctional resin system, the FR408HR laminates offer over 30% improvement in z-axis expansion, while offering more than 24% electrical bandwidth compared to competitive products. The superior moisture resistance at reflow helps to bridge the gap from both electrical and thermal perspectives.

Reinforcing with E-grade glass fabric allows the high-performance multifunctional resin system in FR408HR to offer a low dielectric constant of 3.39. This allows wider trace widths and reduces the skew caused by the difference in dielectric constant between glass and resin.

Rogers 4000 Series

Designed for performance, the RO4000 series materials are made of reinforced hydrocarbon/ceramic laminates rather than PTFE. With low dielectric tolerances and low loss factor, the RO4000 material offers excellent electrical performance at higher operating frequencies. Their stable electrical properties versus frequency make the RO4000 series ideal for broadband applications.

Tachyon 100G

Designed for very high-speed digital applications, the Tachyon 100G lamination materials allow speeds up to and beyond 100 Gb/s. Exhibiting exceptionally stable electrical properties over a broad frequency and temperature range, Tachyon 100G laminates are suitable for upgrading existing products to the next generation. The low z-axis CTE allows using Tachyon 100G materials in high-layer count PCBs with multiple 2 oz planes and for use with BGAs of pitch 0.8 mm or lower.

Tachyon 100G materials use spread glass and copper with reduced profile to improve rise times, mitigate skew, and reduce jitter.

Megtron 6

This is an advanced material from Panasonic, offering low dielectric constant and low loss substrates for high-speed digital applications. The electrical properties of Megtron 6 laminates are competitive with PTFE-based materials, but with significant improvements in processability. Megtron 6 family of materials offers the designer significant benefits in system performance when used for telecommunications, routers/switching, high-speed computing, and transfer applications.

Conclusion

With the increasing use of PCBs in all areas over the years, it becomes very important to choose the correct PCB material. The right PCB material for an application not only affects the functionality and features, but also the overall cost of the board. Fabricators and designers select the materials on the basis of the application requirement, environmental factors, and most other constraints the PCB will likely encounter.

You can contact Rush PCB Inc for any PCB requirements. Being industry leaders, we will assist you not only in helping you select the most appropriate materials, but will also fabricate the circuit board to your satisfaction.