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RF Electronics Chapter 11: Circuit Manufacture Page 372 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. leaded resistor or capacitor is 0.9 mm, with a pad size of 2 mm. To ensure that the circuit can be changed if needed, for prototyping boards a bigger pad size is advisable. For milling of PCBs, the tolerances are slightly different. For a typical milling machine [17], the tool alignment has a ± 5 µm tolerance. Since 0.15 mm wide milling bits are the smallest commonly used, a minimum recommended track size is 0.5 mm, with a 0.15 mm minimum spacing between tracks. Having a 0.25 mm spacing ensures that 2 milling cuts are made, thus reducing the chance of short circuits. For short track lengths, track widths of 0.2 mm are possible, however such thin tracks should be avoided for long tracks since tracks may lift due to the mechanical friction of the milling bits. When milling circuit boards, as much ground-plane as possible should be left on the board, to minimise the amount of milling that has to be done and minimise the milling time and cost. In addition, the extra ground-plane provides some rigidity for thin boards to prevent them from curling. The ground plane can be used to provide a heat sink to remove heat from the circuit. For RF boards the bottom conductor should be a high integrity ground-plane underneath any RF transmission lines to provide the correct Microstrip circuit configuration. Vias connected to tracks and leaded components affect the integrity of that ground-plane. As a result, surface mount components should be used in any RF design. If needed leaded components can be soldered onto pads on the top layer. To prevent performance changes as the RF circuit is mounted in an enclosure and to provide suitable mounting locations, it is desirable to provide an earthed strip around the RF board as shown in figures 7.66, 7.67 and 7.80 to 7.83. In figure 7.67, the big holes are used for mounting the board and the many small holes are vias, connecting the earthed area to the ground-plane. Note that this grounded area is also used to minimise EM coupling between the different parts of the circuit. Etching Most commercial printed circuit boards are etched. There are many commercial PCB manufacturers that will etch prototype boards with a short turn-around time and low cost [18, 19, 20]. The turnaround times for PCBs using RF and Microwave substrates are normally longer than for FR4 substrates. The Gerber files generated by the designer are used to produce the PCB. The laminate typically has rolled copper on both sides. Each side is then coated with Photo Resist and typically a laser plotter, operating on the Gerber file, shines a laser on the tracks and ground-plane where the copper needs to remain. That hardens the photoresist and the unexposed photo resist is washed off. The board is then placed in an etching acid, which removes the unprotected copper. The etching acid can be Ferric Chloride, Ammonium Persulfate, Sodium Persulfate or Cupric Chloride [21]. All of these are nasty chemicals since they must eat copper. For most, their fumes are toxic and their disposal is difficult. For multilayer boards, the inner layers are stacked and drilled. The holes for through hole pads and vias are electroplated to provide the connections between layers and then the top and bottom layers are etched. A solder mask in normally applied to protect the copper tracks and prevent them from oxidation. The solder mask material is UV hardening like photoresist. A thin layer of the solder mask material is applied to the whole PCB and the areas that are to remain are exposed to UV light. The areas that are not exposed are then washed away to provide the areas for solder pads and mounting the required components. Overlay text and marks are RF Electronics: Design and Simulation 372 www.cadence.com/go/awr

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