Low loss transmission lines on flexible COP substrate by standard lamination process

Wearable electronics are gaining more attention in the market. They usually demand a non-planar structure to conform the shape of human body. Flexible substrate provides a good basis to design electronics systems that can be non-planar and conformal. On the other hand, to support high speed data or high quality video transmission, the loss of transmission lines at microwave, even at millimeter wave band becomes an important performance index. Typical low loss flexible substrate such as liquid crystal polymer (LCP) requires processing temperature higher than 250 o C, this presents a challenge for standard flexible substrate processing lines. A low dielectric constant and low loss tangent (tanδ) material called cyclo-olefin polymers (COP) developed at ZEON Corporation, Japan, requires processing temperature under 200 o C while maintaining similar low loss characteristics as LCP materials that require high processing temperature. In the future, the insulation layer will be made of COP for the 2 layer flexible copper clad laminate (2L FCCL) structure. Coating process of the copper foil on COP may also be possible in the future. The coating process will be more cost-competitive than the current high temperature lamination process. As the material cost and processing cost are both reduced for this promising material, we therefore need a more thoroughly study of the high frequency characteristics of this material. In this study, both microstrip lines (MSL) and grounded co-planar waveguide (GCPW) are designed and fabricated on COP. The dielectric constant and loss tangent of COP is 3.0 and 0.005. To protect the copper traces, PI cover layer with thickness of 38um is also applied on top of COP. The dielectric constant and loss tangent of PI cover layer is 3.5 and 0.02. Simulation results show that with 40μm of COP and 12 μm thickness of copper trace, the loss at 60 GHz is about 1.0~1.2dB/cm for both types of transmission lines under 50Ω of characteristic impedance. Measurement results shows about 2.5dB/cm for MSL and 3.3dB/cm for GCPW. The extra loss may due to that some solvents are not fully evaporated during laminating process.