The ePTFE matrix functions as a scaffold to carry the resin and also provides the dielectric separation and electrical signal boost between the bonded innerlayers. Other prepreg varieties utilize a cyanate ester resin within the nonwoven, expanded PTFE structure. The dielectric constants of these prepregs range from 2.6 to 3.1, depending on resin content and ePTFE matrix thickness. Nonwoven fluoropolymer and resin matrices provide a very uniform and consistent dielectric constant compared to woven fiberglass and resin prepreg materials in controlled impedance PCB applications. SEM photo of ePTFE structure.
Dark spaces indicate resin filled section.
Applications Expanded PTFE based prepregs are used to increase signal speed, improve impedance control, reduce capacitance and crosstalk of transmission lines, increase wiring density and reduce overall PCB thickness. They also require fewer plies at a given impedance value because of the lower dielectric constant values. At high frequencies, electronic signal transmission in a PCB is primarily on the outside surfaces of the copper conductor and in the dielectric material between the signal and ground plane. Signals for a stripline or microstrip design are, therefore, influenced by the dielectric constant of the prepreg materials that encapsulate or surround the conductor lines. The dielectric constant of the prepreg is very critical to increasing signal speed and controlling electrical parasitics in the system.