Characterizing Dielectric Loss in Adhesiveless Polyimide Substrates
Adhesiveless polyimide substrates eliminate high-loss acrylic adhesives, dropping dielectric loss tangents to 0.002 at 10 GHz when paired with smooth rolled copper.
High-Frequency Flex
Material Property
Specialized substrate materials for flexible electronics allow the transmission of signals above five gigahertz with minimal dielectric loss. The use of high-frequency flex provides the mechanical versatility of a thin polyimide ribbon combined with the electrical performance of high speed laminates. It utilizes low loss adhesives or adhesive-less copper cladding to reduce signal attenuation.
This material class is essential for applications where a moving hinge or tight bend radius must carry microwave signals. It maintains signal integrity by providing a stable dielectric constant across a wide temperature range.
Stackup Configuration
Construction of these circuits involves liquid crystal polymers or modified polyimides that exhibit very low moisture absorption. Standard flex materials often absorb water, which increases the loss tangent and ruins the high frequency response. By using specialized materials, the designer ensures that the impedance of the differential pairs remains constant.
The fabrication process must avoid aggressive chemical treatments that roughen the copper surface, as surface roughness increases skin effect losses. Careful selection of the coverlay material further reduces the parasitic capacitance of the traces.
Transmission Test
Characterization of the circuit occurs through time domain reflectometry and insertion loss measurements. A well designed cable shows a flat response without significant dips in the power spectrum. Testing at the assembly level confirms that the transition from the rigid board to the flex tail does not introduce reflections.