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RF Electronics Chapter 11: Circuit Manufacture Page 373 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. then printed on the board and finally the solder pads are dipped in solder for easy soldering of components onto the board. For RF layouts the designer needs to be aware that the etching process will slightly undercut the tracks. The amount of undercut will depend on the copper thickness and the etching chemicals used. The manufacturer will supply details of the amount of undercut. If a solder mask is applied to protect the tracks, then the thickness, dielectric constant and dielectric losses need to be taken into account during the design process. If the losses are too high, the PCB may need to be gold plated to prevent oxidation. Manufacturing using a PCB Milling Machine Many universities use a special PCB milling machine [17] to produce prototype PCBs. The ceramic filler used in RF substrates like RO4003 are much more abrasive than the epoxy, paper, and fibreglass used in FR3 and FR4 type substrates. As a result, the cost of milling bits is a major cost in the production of those boards. For milling the more flexible PTFE boards, like RT/Duroid, the substrate needs to be thick enough to be able to be held flat as part of the milling process, so that the more rigid RO4003 substrates are the best to use in PCB milling machines. Since the milling process can disperse copper and PCB substrate materials in the air, the appropriate workplace health and safety considerations must be considered. PCB milling allows a fast turn-around, for prototype RF boards needed for research and development, PCBs up to 229 x 305 mm can be accommodated. Typical position repeatability is ±5 µm when using the plotter's camera to detect registration marks. Tracks widths and spacing of 0.15 mm are possible. Manufacturing using Laser Ablation PCB cutters can also use green or UV lasers to remove the copper from PCB laminates by laser ablation [22, 23]. The laser vaporises the copper or gold, where the metal needs to be removed. Minimum track widths of 50 m and gaps between tracks of 15 m are possible on ceramic substrates like RO4003. Trace widths of 75 m are possible on FR4 or RT Duroid substrates. Since the heat absorption of ceramic substrates are different from FR4 or RT Duroid, the laser pulse duration is changed to ensure there is no heat damage to the substrate. The plotting accuracy is 2 m. This accuracy is better than what can be achieved reliably with conventional etching or PCB milling. The UV laser plotters can also be used for inscribing, drilling or cutting the PCB. An A4 size PCB can be produced in about 20 minutes. The larger machines are fast enough for commercial production. Since no hazardous chemicals are used, this is a relatively environmentally friendly means of producing PCBs. However, since the ablation process vaporises copper and PCB substrate materials, the appropriate workplace health and safety considerations must be considered. Some commercial PCB etching companies use these laser cutters to produce the masks used for putting solder paste onto conventional PCBs. Thin Film The polished Alumina substrates, used for the thin film circuit boards, have a very low dielectric loss. The thin film substrates have less thickness variation than RF PCB material. As a result microstrip transmission lines have a better impedance accuracy, which may be critical in a design. Thin film circuits can be made to better accuracies, with smaller track and spacing sizes than etched PCB's. RF Electronics: Design and Simulation 373 www.cadence.com/go/awr

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