Development of new dielectric material to reduce transmission loss

The 5G products and service with widened band and high frequency including mm wave have been commercially started. General dielectric materials like polyimides or epoxy resins have large transmission loss in high frequency region. The loss is divided into mainly two factors, dielectric loss and conductor loss. In order to reduce the transmission loss, dielectrics need to have lower dissipation factor and conductor such as copper foil needs to have lower surface roughness. Thus, we simulate the impact of dielectric loss by dissipation factor and conductor loss by surface roughness on transmission loss. Furthermore, we experimentally confirmed the accuracy of the simulation by the transmission measurement with various core films. The simulation revealed that applying lower dissipation factor of core film is the most effective method to reduce the transmission loss. Simulation and experiment of transmission loss trend are well matched with multi-layer dielectrics. From such knowledge, we have developed new dielectric materials with low dissipation factor along with strong adhesion to copper foil with smooth surface. Adhesive usually needs to have polar chemical structure for better adhesion yet the polarity tends to increase dielectric constant and loss tangent. Therefore, satisfy adhesion and low dielectric constants at the same time was difficult. Recent work at JSR discovered a chemical structure that keep dielectric constants low while exhibit polarity, which enhances adhesion. This new material can be used as an adhesive between polyimide and copper foil with smooth surface. Flexible printed circuit was fabricated using our new adhesive along with the conventional polyimide film and copper foil with smooth surface. Measurement of transmission line of the circuit revealed significant improvement of transmission loss at mm wavelength range. We hereby report the details of the simulation and measurement results of the novel dielectric adhesive with low loss tangent.