Making The Most Of Microwave Materials

Understanding key dielectric material parameters can help design engineers to match a circuit-board substrate to a particular high-frequency application and its environment.

Microwave materials provide the foundations for highfrequency circuits and packaging. Dielectric circuit materials, often referred to as laminates because of the added copper for etching transmission lines, have made progress in recent years in terms of consistency and stability as well as flexibility, with many suppliers offering a wide range of materials in support of designs from RF through millimeter-wave frequencies.

High-frequency circuit designers have an unprecedented choice in substrate materials for printed circuit boards (PCBs), from basic FR-4 materials to high-performance polytetrafluoroethylene (PTFE)-based substrates. Suppliers offer their PCB materials in various panel sizes and thicknesses, in addition to laminated materials with different weights of copper.

Dielectric substrate materials are differentiated by their permittivity or relative dielectric constant, er, "relative" because it is referenced to the value of the dielectric constant of a vacuum (unity or 1). The relative dielectric constant, which is always greater than 1, refers to a material's capabilities of storing charge and serving as a insulator. High-frequency materials have dielectric constants between about 2 and 10.3, measured through the thickness or z axis. Materials with lower dielectric constant provide high isolation with low loss. Materials with higher dielectric constants have greater capacity to generate electromagnetic (EM) fields, but with less isolation and higher loss. Some dielectric substrate suppliers, including Arlon, Rogers Corporation, and Taconic Advanced Dielectric Division, offer materials for specific applications, such as amplifiers and antennas.

A number of dielectric material parameters are related to either the stability of the dielectric constant or the dimensional stability of the material. Variations in the dielectric constant across the length, width, and thickness of a substrate can result in inconsistent impedance values for transmission lines. The changes in impedance will cause signal reflections and deviations in frequency compared to computeraided- engineering (CAE) predictions. Materials suppliers typically specify their products in terms of a tolerance for the dielectric constant, such as 3.2 0.05, and some even suggest a value of the relative dielectric constant for use with CAE simulators.

The thermal coefficient of dielectric constant describes changes in a substrate material's relative dielectric constant as a function of temperature. It is defined in terms of changes in the dielectric constant in parts per million (PPM) per change in temperature in degrees Celsius (C). It can be either positive or negative, and is usually specified over a wide range of temperatures.

Another critical material parameter is dissipation factor or loss tangent, which is the ratio of the material's loss to its charge capacity. Dissipation factor is a unit less, such as 0.0030, referenced to a specific frequency. Lower values are important for minimizing insertion loss in passive circuits and optimizing gain in active circuits. Lower values are also critical when minimizing the generation of heat in high-power circuits.

One last parameter to consider when selecting a PCB substrate is the coefficient of thermal expansion (CTE), which describes physical changes in a dielectric material with changes in temperature. In the x and y directions, the CTE values of many PCB materials are designed to match the CTE of copper. In some cases, a material may be needed with CTE that matches a different material, such as the ceramics used in packaging. In the z direction, the CTE is usually designed to support reliable plated through holes (PTHs), especially for materials intended for multilayer applications. Low values of CTE usually result in excellent dimensional stability for a dielectric material.

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