Designers working at microwave frequencies have a long list of circuit-board materials to choose from, some tailored for specific applications, such as amplifiers and antennas.
Electronic substrate materials are like the foundation in a building: rarely noticed, but critical to the overall architecture. For RF/microwave circuits, the printed-circuitboard (PCB) substrate or laminate material is an essential building block that must be stable over time and temperature with low signal losses at microwave frequencies.
Although PTFE has long been a substrate of choice at microwave frequencies, pure PTFE tends to suffer from variations over time and temperature. For that reason, suppliers offer PTFEbased circuit-board substrates that are reinforced with rigid filler materials, such as glass or ceramic fibers or particles.
Taconic for example, offers different PTFEbased materials with either ceramic or woven-glass fillers. The firm's TLC substrates, for example, are reinforced with woven glass and target low-cost RF and microwave applications. Different versions of the material exhibit dielectric constants of 2.75, 3.00, and 3.20. Arlon fortifies its CLTE-XT and lower-cost CLTE-AT ceramic/PTFE composites with woven glass. The CLTE-XT material boasts a dielectric constant of 2.94 at 10 GHz with a tolerance of 0.03, and loss tangent of 0.0012. Ideal for military applications, the material has a low z-axis coefficient of thermal expansion (CTE) of 20 PPM/C and thermal conductivity of 0.56 W/mK. The commercial version, CLTE-AT, has a dielectric constant of 3.0 at 200 MHz and 10 GHz and holds it to a tolerance of 0.04.
Some suppliers have developed lower-cost alternatives to PTFE based on thermoset materials. Along with its lines of PTFE-based materials, Rogers Corporation now offers RO4360 copper-clad laminates based on a ceramic-filled, thermoset resin system reinforced by glass fiber. With a dielectric constant of 6.15 and dissipative factor (loss) of 0.003 at 2.5 GHz, the laminates cater to the needs of RF/microwave amplifier designers trying to miniaturize their circuits. Compatible with standard printed-circuit-board (PCB) processes, the materials feature the low loss and excellent power-handling capabilities. They boast a CTE in the z axis (30 PPM/C) needed for reliable plated through holes (PTHs) in multilayer circuits. Rogers has also built upon its legacy of PTFE-based products with RT/duroid 5880LZ laminate materials. With its low dielectric constant of 1.96 and low loss, this particularly lightweight material (density of 1.37 g/ cm3) is ideal for airborne antennas and other weight-sensitive designs.
High-frequency designs based on ceramic substrates are still much in demand, with an increasing use of lowtemperature- cofired-ceramic (LTCC) substrates to form small components. LTCC circuit materials, such as those offered by Minicaps and Kyocera, feature a dielectric constant of 7.8 at 1 MHz with CTE of 5.8 PPM/C from +25 to +300C. For those wishing to get started with LTCC, some companies offer design/fab services, including Barry Industries, and LTCC Lab.
To assist designers working with LTCC, DuPont Microcircuit Materialspart of DuPont Electronic Technologieshas teamed with software supplier CAD Design Software to incorporate LTCC models and manufacturing processes into the software company's design tools. DuPont's Green Tape 941 and 943 materials are now part of CAD Design Software's Ceramic Design library. Users can customize line widths and spacings on circuit designs, or use preset files. This is one instance of CAE support for specific materials, as most commercial design tools allow users to enter parameters based on a material's characteristics