AWR eBooks

RF Electronics Chapter 11: Circuit Manufacture Page 375 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. example is the 20 W Class C 800-900 MHz amplifier hybrid, shown in figure 11.7. The whole circuit including heat sink is 56 mm long. The bond-wires for the large output transistor on the right can clearly be seen. Thick Film Thick film techniques have a slightly higher circuit set up cost as several screen masks need to be made, but they have a lower increment cost as no etching or milling of the PCB's are required. In thick film techniques, a substrate, which normally is an Alumina substrate, has conductive, resistive or insulating inks applied to them using precision screen-printing technology. The substrate is then fired at typically 850 C to harden the tracks and thus provide the correct circuit connections. Dielectric layers can be made, so that tracks can cross each other. For a multilayer track circuit, several separate firings may need to be made. Similarly, capacitors can be made by firstly using a conductive ink to form the one electrode and firing that. Then a dielectric layer is applied and fired, followed by another conductive layer for the other electrode. Resistors can be made using resistive inks and inductors can be made by printing conductive spirals. This results in a high-density circuit board of low cost. The low thermal coefficient of expansion (TCE) of ceramic materials guarantees mechanical stability and is closely matched to that of ICs for bare chip assembly. The printing screen has a photographically produced mask applied to it, with the screen being open where the tracks need to go and being blocked everywhere else. The printing screen is placed in contact with the substrate and the ink pastes are applied with a squeegee to the screen. The inks can also be applied using other techniques, such as spraying, dipping using the appropriate screening techniques to ensure that the inks are only deposited where they are supposed to be. Figure 11.8. Thick film circuits before and after a protective coating has been applied. Figure 11.8 shows some typical thick film circuits. The circuit on the left shows, that thick film circuits can have a high component density. After the circuit is tested, the whole circuit is protected with a protective coating as shown on the right of figure 11.8. This makes thick film circuits very suitable for hazardous or wet environmental conditions. Many companies, such as [27, 29], specialise in the design and production of thick film circuits, or the supply of materials for them. Printing Screens The ink is normally applied using a squeegee. When the ink is applied, the thickness of the ink is close to the thickness of the screen. By varying the thickness of the screen, the thickness of the tracks can be varied. RF Electronics: Design and Simulation 375 www.cadence.com/go/awr

• Cover