In the past few decades, there has been a tremendous expansion of the wireless industry, which transformed itself from being predominantly military-driven to a consumer-driven, cost-conscious commercial market. Simultaneously, wireless applications are venturing further into the frequency spectrum. For instance, personal communications systems that earlier operated on 1.9 GHz, are being replaced by newer designs operating on 5.8 GHz.
With applications moving up in the frequency scale, there is renewed interest in high frequency, high performance, but low-cost substrates. Earlier, only military applications considered polytetrafluoroethylene (PTFE) as a substrate that satisfied the above technical requirements. PTFE is also commonly known as Teflon.
Why Use PTFE?
With a melting point of 327°C, Teflon has high temperature resistance, negligible water absorption, good resistance to processing chemicals, and a very low loss tangent. That makes the PTFE substrate suitable for all the technical requirements of RF/Wireless designs. However, commercial applications also demand low costs along with high performance, but the conventional PTFE substrates were eight to ten times more expensive than regular FR-4 epoxy glass laminates are. In addition, processing of PTFE substrates was difficult because of the inert and soft nature of PTFE.
Ceramic Filled PTFE Substrates
New research has established ceramic filled PTFE substrate, such as RF-35, as not only satisfying the commercial price requirements, but also exceeding all electrical and mechanical property that manufacturers seek in a PCB substrate for high frequency applications. The dielectric constant of RF-35 has a value of 3.5±0.1, and the material is available in thicknesses of 10, 20, 30, and 60 mil.
Substrates such as RF-35 are unique as the variation in their thickness and dielectric constant within a sheet are minimal. For instance, a sheet of RF-35 shows a standard deviation for dielectric constant as 0.01, while that for thickness as only 0.00023” within the sheet. Within a range of 500 MHz to 11.2