AWR eBooks

RF Electronics Chapter 11: Circuit Manufacture Page 374 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Thin film circuits can be produced using semi additive processes, where a 1 to 2 m seed- layer of gold is applied to an Alumina substrate. A photographic mask is then applied to the places where no track is required and the tracks are then electroplated to the correct thickness of several tens of m. The photo-resist is then removed and a quick etch removes the seed-layer, where no tracks should be, but does not completely remove the tracks. The required solder-pads are then added, together with dielectric dams to prevent the lead from solder leaching into the gold. Laser ablation can be used to remove metal cladding from Alumina substrates [23], so they can be used to produce thin film circuits. Microfab [25] specialise in the design and production of thin film circuits, predominantly for space and military applications and use a subtractive process where the whole substrate is covered with copper or gold and then ion beam etching is used to remove the conductive layer as required. This process achieves etching tolerances of 0.2 m, compared to 2 m for PCB based technology. The designer may have to adjust the track widths in the design, to compensate for the changes in track width caused by the production processes. For a 35 m conductor thickness, corresponding to 1 oz. copper per square foot, then the additive process of depositing tracks will expand the tracks by tens of m as plating will also occur on the sides of the tracks, where they are not confined by the photoresist. If subtractive processes are used then some etching on the sides will occur and the tracks will reduce in width by tens of m in addition to the other manufacturing tolerances. Ion beam etching or laser ablation does not under-etch and will thus result in more accurate track widths. Once the amount of over-etching or under-etching is known, then the track widths can be adjusted to compensate for this. If the line widths are made as variables, then this correction factor can be set to zero for simulation and set for the required correction when the layout is exported to the plotter. Figure 11.7. 20 W Class C amplifier hybrid MHW820 (Motorola). Thin film circuits are much more expensive than RF and microwave PCB, circuits. Thin film circuits are firstly used where the accuracy and reliability requirements demand its use, such as military or space applications [25, 26]. Secondly, Alumina has a very good thermal conductivity. As a result, thin film techniques are often used for power applications, where the Alumina substrate is in contact with a heat-sink. A typical RF Electronics: Design and Simulation 374 www.cadence.com/go/awr

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