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Learning To Launch Onto Different Circuit Thicknesses
Circuit materials are available in a wide range of thicknesses. Once a material has been selected for its electrical performance, the thickness is usually a function of the optimum circuit dimensions for a given frequency range since thinner printed-circuit-board (PCB) laminates yield finer conductor widths for a given relative dielectric constant. Thinner circuit boards may be attractive for their light weight and compactness, but fabricating those finer circuit dimensions may prove to be difficult. The choice of circuit thickness can be further compounded by the need to launch signals from a nonplanar transmission medium, such as waveguide or a connector, onto the planar transmission lines of an RF/microwave PCB, such as coplanar waveguide (CPW) or microstrip transmission lines. That transition, from a waveguide or coaxial connector to the PCB, is critical to the performance of the circuit, and the PCB’s thickness can impact how an end launch transition is made. Waveguide and coaxial connectors come in many shapes and sizes, as do PCB thicknesses, and matching the connector to the substrate thickness can play a large role in the overall performance and reliability of that design.
Coaxial end-launch connectors traditionally introduce RF/microwave energy to a PCB’s planar transmission lines. While waveguide launches still handle many of these transitions at millimeter-wave frequencies, coaxial end-launch connectors have improved over the years, to where 2.4-mm end-launch connectors can readily cover DC to 50 GHz with relatively low signal losses. Of course, optimal performance depends on making the transition from connector pin to PCB planar transmission line as seamless as possible, with minimal reflections, at an impedance junction that is matched in phase and magnitude.
That junction consists of the interface between the launch pin of the end-launch connector and the PCB’s planar transmission line, typically CPW or microstrip. At that interface, the connector’s launch pin rests on the planar transmission-line conductor metal, with the goal of achieving an extremely low-reflection transition from the launch pin to the planar transmission line.
What can be done to optimize this transition? Commercial computer-aided-engineering (CAE) software tools model this interface by considering that the end-launch connector’s pin, sitting atop the circuit trace of the planar transmission line, constitutes an increase in capacitance. One way to compensate for this increased parasitic capacitance is to add inductance to the PCB in some way. This is usually done with CPW and microstrip transmission lines by narrowing the circuit trace, thus creating a taper from the transmission line to the edge of the PCB where the transmission line contacts the launch pin of the connector. Parameters such as the length of the taper and the final width of the taper can be modeled in a commercial CAE program to determine an optimal amount of inductance for the capacitance boost from the connector’s pin.
Some end launch connectors feature designs that are optimized for this transition using, for example, a center launch pin with floating design to simplify the alignment of the pin to the planar transmission-line conductor in the x, y, and z directions for minimal electromagnetic (EM) discontinuities with any circuit material thickness. Many end launch connector manufacturers also offer a number of different configurations for their connectors, including with different diameter launch pins.
With these choices of different end launch options and circuit board thicknesses, the challenge is in selecting an optimum combination of end launch connector or waveguide and circuit-board thickness to achieve desired performance goals within a target cost budget. Although some end launch connectors have been designed for use with a wide range of circuit material thicknesses, some studies have shown that the best electrical performance is achieved by optimally matching the dimensions of an end launch connector’s launch pin to the thickness of the PCB material, with smaller launch pins working better with thinner circuit materials and larger launch pins better suited for thicker circuit materials.
Some of the dynamics between edge-launch connectors and PCB materials with different thicknesses are described in a study performed by Southwest Microwave (www.southwestmicrowave.com), “The Design and Test of Broadband Launches up to 50 GHz on Thin and Thick Substrates.” This report attempts to explain why the company offers end launch connectors with four different pin sizes. It compares what it calls thin (8 mils) and thick (30 mils) circuit board materials and how materials with different thicknesses might be better matched to end launch connectors with different size pins. For the dielectric constant of the materials studied, circuit materials thinner than 8 mils resulted in extremely fine (perhaps too fine) circuit dimensions, while materials thicker than 30 mils suffered higher losses at 50 GHz. Of course, these two circuit-board thicknesses are a sample of the available choices, but the idea of the study was to show that the dimensions of a coaxial launch should be matched to the thickness of the circuit substrate.
The study chose 8-mil-thick RO4003C™ and 30-mil-thick RO4350B™ laminates, both from Rogers Corporation (www.rogerscorp.com), to represent the two extremes of RF/microwave circuit-board thickness. Both are woven-glass-reinforced ceramic-filled thermoset materials that can be processed like standard FR-4 materials. Rogers RO4003C has a relative dielectric constant of 3.38 in the z direction at 10 GHz while RO4350B has a relative dielectric constant of 3.48 in the z direction at 10 GHz. Both materials are available in a wide range of thicknesses; both feature low loss and excellent dimensional stability. In the end, it found that an end-launch connector with a smaller-diameter launch pin was a better match for a thinner PCB substrate, while a connector with a larger-diameter pin matched better with the thicker substrate, in terms of achieving a good impedance match and a reflection-free transition.
The thickness of a circuit material such as RO4003C or RO4350B laminate may initially be determined by the desired circuit dimensions for the operating frequency range, along with other considerations, such as dimensional stability, loss, and the ease of fabricating plated through holes (PTHs) in a multilayer design. But the need to make a transition from a planar circuit to a coaxial or waveguide connection may require a different thickness to accommodate the mechanical and electrical needs of the connector.
This investigation of launching to RF/microwave circuits will continue in the next blog, with a look at somewhat more “exotic” transmission-line structures, including various types of PCB waveguide, such as substrate integrated waveguide (SIWG) which has shown to be an effective solution for high-speed on-chip communications in place of traditional RF/microwave transmission lines.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.