Circuit Material Performance for PCBs at Millimeter Wave Frequencies

John Coonrod
Rogers Corporation

Millimeter-wave (mmWave) frequency applications are becoming more common. There are applications utilizing PCB technology at 60 GHz, 77 GHz and many other mmWave frequencies. When designing a PCB for mmWave frequency, the properties of the circuit materials need to be considered since they can be critical to the success of the application. Understanding the properties of circuit materials at these frequencies is very important.

This paper will give an overview of which circuit material properties are important to mmWave frequency applications using PCBs. There will be data supplied which demonstrates why these properties are essential to the circuit material selection for mmWave applications. Some properties discussed will be dielectric constant (Dk) control, dissipation factor, moisture absorption, thickness control and TCDk (Temperature Coefficient of Dk). Measured comparisons will be shown for insertion loss and Dk versus frequency for different types of circuit materials up to 110 GHz. As part of the test data, the impact on circuit performance due to TCDk and moisture absorption will be shown at mmWave frequencies.

There are four significant circuit properties to consider for good PCB RF performance at mmWave frequencies. These properties are conductor width, copper thickness, dielectric constant and substrate thickness. Material properties that need to be considered are TCDk, moisture absorption and copper surface roughness. The designer needs to consider the circuit and material properties when designing circuits at millimeter wave frequencies.