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RF Electronics Chapter 11: Circuit Manufacture Page 370 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. manufacturer has Prepreg sheets and bond sheets with properties that match their microwave laminates. For a combination of microwave and FR4 boards, the inner layer of the microwave board should be a ground plane so that the dielectric properties of the Prepreg sheet joining the microwave and FR4 are not so important. A good low cost option is to use the standard bond films used for FR4 for making multilayer PCBs. For bonding the laminates and Prepreg or bonding sheets are clamped at a pressure of typically 2.8 MPa and a temperature of typically 225C for more than one hour. That temperature varies with the properties of the Prepreg sheets. Since air has an r = 1, it is important to prevent air bubbles, particularly when high dielectric substrates or Striplines are used, since air-bubbles change the characteristic impedance of the Stripline at the point of the air-bubble and cause a discontinuity. Figure 11.6. Bonding laminates to form a multilayer board [2]. Non-Clad Substrates During the 1980's the microwave PCB substrates were not readily available and many microwave circuits had to be produced on Alumina substrates using thin film techniques. These non-clad substrates are now only used for precision microwave circuits using thin film techniques and for low cost thick film circuits at both RF and low frequencies. Companies such as Coors Ceramics [6] and Morgan Technical Ceramics [7] produce dielectric materials that can be used for such RF substrates. Ceramic substrates are used in many packaging applications. A paper by F. Bechtold [8] gives a good overview. Alumina Substrates Alumina is the dominant substrate material for thick and thin film applications. The dielectric constant is close to 10, with both the dielectric constant and cost increasing with the purity of the substrate. For lower cost applications, thick film substrate materials with 85% to 95% purity are used. In many cases these substrates are used "as fired" for low frequency applications or ground flat for low cost microwave applications. For precision, low-loss microwave applications 99.5% to 99.8% purity substrates are used and these may be ground and polished in order to remove any surface irregularities and ensure a constant thickness of the entire substrate. Alumina is ideal for demanding low loss applications, such as substrates for thin film circuits. Rogers Corporation [9] now have a range of metallised ceramic substrates in different substrate thicknesses. These are plated on both sides with copper, silver or gold. Their suggested applications are those involving high power. No RF properties of the substrates, like dielectric constant or the purity of the Alumina substrate, which affects the losses, is given. These substrates may possibly be used for RF applications. RF Electronics: Design and Simulation 370 www.cadence.com/go/awr

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