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RF Electronics Chapter 11: Circuit Manufacture Page 365 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Ceramic substrates are used for many RF circuits. This requires gold or copper conductor tracks to be deposited onto the ceramic. Those tracks are typically electro-plated. To ensure that all tracks have the same copper or gold thickness plated on them, all tracks must be electrically connected during the electroplating process. That requires a seed layer, of approximately 1 to 2 m of conductive material, often copper or gold to cover the entire substrate before electro-plating. This is normally achieved by using a series of chemical baths, to coat the substrate with this seed layer. The seed layer can also be produced, by placing the ceramic substrates in a vacuum chamber and spluttering gold onto them. The areas where no tracks are required are covered with a protective coating, using screen-printing or photoresist. Electroplating is then used to build up the tracks to the required thickness. The protective coating or photoresist is then removed, followed by and a light etch to remove the seed layer. The additive process minimises the amount of copper that is to be etched away and thus results in less copper and etching waste that needs to be recycled. Additive processes are required to produce the vias in double and multi layered printed circuit boards. Printed Circuit Board Materials Most PCBs are made by having thin copper sheets glued to an insulating substrate material, called a laminate. The copper thickness depends on the applications. Typical thicknesses are 2, 1, ½ and 1/4 oz. copper per square foot, which corresponds to a copper thickness of 70 m, 35 m, 17 m or 9 m respectively. The copper thickness together with the track width must satisfy the current carrying capability and resistive loss requirements of the tracks. Subtractive processes are used to produce the required circuit boards. Conventional PCB laminates consist of a filler or reinforcing material and a thermoset resin. These sheets are cured at high temperature and pressure. The mechanical and dielectric properties of the laminates depend on the composition of the laminate. For very low cost and low frequency applications, paper pulp or cotton and epoxy resins are used. FR2 and FR3 boards are examples of this. For higher mechanical strength, woven fibreglass mats are used as reinforcement for the thermoset resins. The temperature range over which the board can be used, and the dielectric properties are governed by the type of resin used. By using resins containing Polytetrafluoroethylene (PTFE or Teflon) and/or ceramics such as Alumina, PCBs with low losses at microwave frequencies can be obtained. In some applications, the fire retardant properties of the laminates must also be considered in the laminate selection process. The AWR DE Libraries AWR Web site Parts by Type Substrates contain details of PCB substrates. The number of manufacturers of substrates has reduced during the last few years and the manufacture of RF PCB substrates is now dominated by the Isola Group [1], Rogers [2] and AGC Multi Material (Taconic) [3]. Conventional PCB Substrates Paper and Resin Substrates; FR2 and FR3 FR2 and FR2 substrates consist of a mixture of paper pulp and a resin. Phenolic resin for FR2 and epoxy for FR3. They are mainly used in low cost consumer products, and are not normally used for multilayer boards. FR3 has a dielectric constant of approximately RF Electronics: Design and Simulation 365 www.cadence.com/go/awr

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