RF and microwave laminates are used as substrates for radio frequency (RF) and microwave communication systems and electronics. They are designed to meet the demanding performance standards for higher-frequency applications. RF and microwave laminates are usually made of thermoplastics, thermoset resins, or polytetrafluoroethylene (PTFE), a synthetic fluoropolymer with excellent dielectric properties and an extremely low coefficient of friction. To produce multiple-layer products, PTFE is processed at very high temperatures with fluorinated ethylene propylene (FEP) or chlorotrifluoroethylene (CTFE). PTFE and woven glass can also be bonded to double-sided or multi-layer FR4, a flame retardant laminate impregnated with epoxy resin. An exact thickness-distribution of resin and glass fabric helps provide a uniform dielectric constant and dielectric loss factor. Although microwave base materials that use thermoset resins require the addition of flame retardants for UL-V0, PTFE and woven-glass products do not.
RF and microwave laminates are usually clad with copper, but are also available with heavy-metal claddings such as brass, copper, and aluminum. Dimensions such as length and thickness are measured in millimeters (mm) or inches (in). In terms of performance specifications, values include dielectric constant, dielectric breakdown, and dissipation factor; arc resistance, volume resistivity, and surface resistivity; Young’s modulus, Compressive modulus, and Poisson’s ratio; insertion loss, inductive loss; and copper conductive loss; as well as peel strength and moisture absorption. Flexural strength, tensile strength, specific heat, specific gravity (density), thermal conductivity, and outgassing are additional parameters to consider. With RF and microwave laminates, the coefficient of thermal expansion is expressed as a percentage and measured across three axes (x, y and z).
Most RF and microwave laminates have a low dissipation factor, low levels of moisture absorption, and a low dielectric constant. They also feature high thermal conductivity for better power-handling capabilities, and low thermal expansion for better mechanical properties. As a result, some RF and microwave laminates are used in products such as power amplifiers, filters, and couplers. Others are designed for aerospace or U.S. military applications, and comply with standards such as MIL-S-13949H. Non-reinforced microwave substrates can be used to create structures with very low-losses. Thin-core RF and microwave laminates can be used in high-speed digital and RF multi-layer applications. Products that are tested in accordance with IPC-TM 650 may be shipped with a certificate of conformance.