New insulating adhesive film for future high-frequency wearable devices

Wearable devices are the current trend in technology. The wearable technology is moving toward lighter, thinner, more flexibly shaped features. Continuing growth of such highly-functional electronic devices are increasing the volume and speed for data transmission required. In addition, communication between the devices will be at high frequencies. This suggests that device component suppliers will need to offer the products with low transmission loss at high frequencies. The materials commonly used for conventional adhesive films are polyimide (PI) and liquid crystalline polymer (LCP). However, PI has some issues in dielectric properties and LCP in workability (high temperatures required for adhesion). Considering those advanced applications and the difficulties that the conventional films are facing, we have successfully developed a thin insulating adhesive film with a low dielectric constant. Our new film provides significant advantages in terms of transmission loss, impedance matching, transmission speed, and multi-layered thinner design. The film that we developed has resin structure. It allows for low dielectric properties (dielectric constant: 2.5, dielectric loss tangent: 0.0025), low modulus (less than 1 GPa), low moisture absorptivity (0.9% at 85oC/85% RH for 1000 hours), high peel strength (8 N/cm using copper), and good interlayer insulation (5-μm layer). These properties confirm the advantage of our film. This resin structure also leads to stable dielectric properties at a high-frequency range (1 to 65 GHz), high flexibility, lower loss in the conductor and dielectric, and no directional dependence, which can facilitate circuit design. Furthermore, our new film is excellent in adhesion to a smooth-surfaced copper foil, which will allow for micro wiring and reduce the conductor loss (due to skin effect). To further examine the performance of our newly developed film, circuit evaluation was conducted. PI, LCP, and the new film were compared by measuring S-parameters of a microstrip line. Transmission losses (S21) of a 70-mmlong, 50-ohm microstrip line using PI, LCP, and our film were - 3.77 dB, -2.70 dB, and -2.58 dB at 20 GHz; -6.00 dB, -4.17 dB, and -4.05 dB at 40 GHz, respectively. The evaluation result proves that our new film has low loss in transmission line circuits, compared with PI and LCP. With many benefits, this new film is being used as a component for high-frequency devices (e.g. antenna cable board). We are aiming to further reduce the dielectric loss and are confident that this film is promising for future higherfrequency, higher-flexibility applications.