2021 Volume 60 Issue 1 Pages 104-107
The frequency bands used for the autonomous driving of cars and the 5G/6G communications are increasing. In response to the frequency increasing trend, the FPC (Flexible Printed Circuit board) with low transmission loss has been developed recently. Therefore, it is important to optimize the material factors, such as grain size, surface roughness, electric conductivity, and purity of the copper foil for low transmission loss in high frequency bands.
In this study, multiple copper foils having various material factors without surface treatment were examined to compare the influence of the material factors of the copper foils on the transmission characteristics.
As the results, it was found that the grains being more than 10 μm in diameter don’t affect high–frequency transmission characteristics. On the other hand, smaller surface roughness and the higher electrical conductivity at room temperature showed better high–frequency transmission characteristics. It is considered that the surface roughness is the major factor to affect the transmission loss, since the skin depth at several tens of GHz and surface roughness of copper foil for FPC are comparable.
