AWR eBooks

RF Electronics Chapter 11: Circuit Manufacture Page 369 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Other Laminates Rogers Corporation also have other RF laminates, some examples are: 92ML Laminates and 92ML Prepreg sheets. They have a high thermal conductivity, are flame retardant and are suitable for high temperature applications. XtremeSpeed RO1200 series are designed for digital signals to keep their pulse shape when using many layer boards. In addition there are many other laminates on their web site [2]. Other manufacturers have a similar range of RF and Microwave laminates to the Rogers products described above. For example, Isola have several RF and Microwave laminates, Astra MT77, I-Tera MT40 (RF/MW) and IS-680 AG are examples [1]. AGC Multi Material also has several RF and Microwave laminates [3]. RF-35 is a fibreglass and PTFE/Ceramic laminate, similar to RO3035. It also has a dielectric constant of 3.5. AGC Multi Material TLY laminates use fibreglass for mechanical reinforcement and increased dimensional stability. It is similar to RT-duroid 58X0. The TL series has dielectric constants from 2.17 to 3.2 and a loss tangent from 0.0009 to 0.003. Panasonic have Megtron6 R-5775(N) laminates and R5670 Prepreg sheets [5]. Those laminates have a similar performance as RO4003, but with a higher dissipation factor. Multilayer Boards There are many applications where RF circuits require multilayer boards. One example is a Stripline RF circuit. Another is a multilayer board with an RF/Microwave substrate for an RF layout and an FR4 type substrate for power supply, control, and digital signal processing. A Prepreg sheet is a thin uncured sheet, containing fibreglass, thermoset resins, and fillers. The laminates are normally produced by gluing copper sheets to one or more Prepreg sheets, and curing them using pressure and heat. For multilayer boards, jigs with locating pins are used to ensure that each of the boards making up the multilayer board are in perfect alignment. For a multilayer board, the inner layers are etched. The boards are stacked with Prepreg sheets between the multilayer boards, to form the multilayer board. In many cases, the same Prepreg sheets that are used to make the double-sided boards are used. That will space all the copper layers evenly, as shown in figure 11.6. The vias in the boards are drilled and electro plated prior to the tracks being etched, so that all vias are connected together and electroplating of the vias is possible. The outer layers are then etched to produce the required tracks on them. The solder pads and solder mask are then applied, completing the PCB. It is possible to have vias on inner layers only. These are called blind or buried vias and are produced on a 2 or 4 layer board and then the location of these vias are not drilled in the outer copper layers. Buried vias offer space savings and provide some intellectual property protection by making it a little bit more difficult to trace the design, but they also make it more difficult to fault-find. Several types of adhesives can be used depending on the laminates to be bonded. Figure 11.6 shows a typical construction requiring an RO4003 Stripline circuit bonded to an FR4 multilayer construction. For constructing the Stripline circuit, a high frequency bond-film like the RO4450 "Prepreg" sheet from Rogers is required. The bonding sheets are close to 0.1 mm thick and have dielectric constants from 3.3 to 3.54 at 10 GHz, matching the dielectric constants of the RO4000 series laminates. The loss tangent of the Prepreg bonding sheets of 0.004 at 10 GHz is close to the 0.0037 and 0.0037 for the RO4000 laminates, so that no sudden changes in dielectric occur in the multilayer board. Each RF Electronics: Design and Simulation 369 www.cadence.com/go/awr

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