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Selecting mmWave PCB Materials

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Dielectric losses offset the performance of mmWave applications, and the losses increase in direct proportion with frequency.

  • The dielectric loss, thermal management requirements, thermal stability, power handling capabilities, and layer count determine the mmWave PCB material suitable for a given application. 

  • Ceramic-filled Polytetrafluoroethylene (PTFE) material without woven glass reinforcements is a good material choice, as it offers a minimal thermal coefficient of dielectric constant property. 

 PTFE

PTFE material without woven glass reinforcement is a good material choice for mmWave applications

Aerospace and defense systems are used for a wide variety of applications, expanding the requirements of the RF and mmWave printed circuit boards that are used in these systems. One such requirement is the appropriate selection of mmWave PCB materials. PCB material selection depends on factors such as performance, cost, and flexibility. In this article, we will discuss what to look for during the selection process for mmWave PCB materials and cover some common materials in use today. 

PCB Effects and the Selection of mmWave PCB Materials

Most emerging electronics systems operate with multiple RF and mmWave PCBs. The mmWave PCBs incorporate antenna substems, high-speed digital subsystems, and amplifiers in their high-density designs with integrated packaging.

New mmWave-based systems require higher bandwidth, enhanced carrier aggregation, higher frequencies, and massive MIMO support. This increase in technical requirements aggravates PCB effects such as:

All of these factors impact the performance of mmWave systems. Let’s take a closer look at some of these PCB effects.

Dielectric Losses

In mmWave systems, dielectric material properties are influenced by application frequency, noise, size, power, and types of components. The dielectric losses offset the performance of mmWave applications, and the losses increase in direct proportion with frequency. 

Surface Roughness of Copper

The surface roughness of copper induces increased attenuation at mmWave frequencies, and the conductor losses in such PCB materials are greater than the dielectric losses. When the frequency of an mmWave system is around 77GHz, the skin depth becomes extremely small and currents confine only to the surface. The skin effect in mmWave circuits is influenced by the roughness of the copper surface. 

Passive Intermodulation

Most modern electronic systems utilize multiple frequencies such as RF waves, microwaves, mmWaves, etc. In such systems, passive intermodulation (PIM) is a common phenomenon. PIM occurs when waves superimpose and create harmonics that either add, subtract, or cancel the desired signal.

To meet the demands of the present mmWave market without being influenced by PCB effects, the PCB design should be modified from the antenna to control functions to amplifier circuits. To ensure high functionality in densely-packed mmWave PCBs, PCB materials should be selected so that they support the performance needed at the mmWave frequencies.

The dielectric loss, thermal management requirements, thermal stability, power handling capabilities, and layer counts determine the mmWave PCB material suitable for the given application. The mechanical properties of the material, dimensional stability, stability under environmental conditions, and cost are some of the other factors that must be considered when selecting mmWave PCB materials. 

Desirable PCB Material Properties for mmWave Applications

Here are some properties that you’ll want to look for in a mmWave PCB material:

  • Low dissipation factor - To ensure maximum power delivery and suitability for high-power applications.

  • Low dielectric constant  - For rapid signal propagation.

  • Consistency of dissipation factor and dielectric constant over the operating bandwidth of intended application - To prevent phase distortion and maintain constant transmission line impedance. 

  • Consistency of dissipation factor and dielectric constant with changes in temperature - To ensure thermal stability in operation. 

PTFE: A Common mmWave PCB Material 

PCB materials are the base platform on which mmWave circuits are fabricated, and mmWave circuit performance is sensitive to PCB material properties. Using FR-4 materials is not good for mmWave circuit performance. Ceramic-filled Polytetrafluoroethylene (PTFE) material without woven glass reinforcements is a good material choice, as it offers a minimal thermal coefficient of dielectric constant property.

The minimal thermal coefficient of dielectric materials indicates that PTFE dielectric properties are stable over temperature. The mechanical properties of PTFE are excellent with low elastic modulus, variable thermal expansion (high expansion), and good dimensional stability. However, PTFE material is prone to time-dependent permanent deformation, called creep, which makes it unsuitable for harsh environments in automotive and industrial applications. 

New mmWave PCB Materials 

The use of thermoset materials for mmWave PCBs is increasing, as they behave as standard FR-4 materials with excellent electrical and thermomechanical properties. Thermoset materials offer high glass transition temperatures, a controlled coefficient of thermal expansion, and thermal reliability. Compared to PTFE, dielectric property variations are minimal in thermoset polymers. The excellent adhesion exhibited by thermoset polymers to metals enables the use of copper with extremely smooth surfaces in mmWave circuits. Switching to thermoset polymers from PTFE can reduce costs as well.

In mmWave PCB material selection, designers should look for materials with the desirable properties we have detailed above. Cadence’s suite of design and analysis tools can help you design high-performing mmWave circuits.

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