Deciphering datasheets for high-frequency circuit materials: Page 2 of 7

January 07, 2013 //By John Coonrod, Rogers Corporation
Data sheets for printed-circuit-board (PCB) materials carry a great deal of information. Understandably, these materials are the foundations for many circuits, and they are characterized by many different parameters, some related to applications, some to fabrication issues, some to environmental and mechanical concerns.
critical material parameter not always properly considered is thermal coefficient of ε r. It is an indicator of how much ε r will change as a function of temperature. The thermal coefficient of dielectric constant, or TcDk for short, is generally considered acceptable when its absolute value is less than 80 ppm/°C, with ideal behavior being a value of zero, or no change with temperature. Better performance for this parameter is needed for high-frequency circuit designs that are particularly sensitive to fluctuations in ε r. TcDk values can be either positive or negative, with positive values denoting an increase in the ε r value for positive changes in temperature, and negative values representing materials having a decrease in ε r value with a positive change in temperature.

As the name implies, volume resistivity is the amount of resistance exhibited by a material in a volume measurement, or how strongly that material opposes the flow of current. Resistivity is a material property, different than resistance, with a unique value for each material. It is often thought of as the resistivity of a material between two copper planes. Volume resistivity is related to conductance, as can be shown with the schematic representation of the microstrip transmission-line segment in Figure 2. The volume resistivity is the inverse of conductance G, with the inductance, resistance, and capacitance of this simple schematic represented by L, R, and C, respectively. Most high-frequency PCB materials have relatively high volume resistivity, so that leakage losses are typically not significant. Leakage losses can be a concern for RF/microwave applications using circuit materials having lower volume resistivity, such as those found in semiconductor-grade materials.

Figure 2: This schematic diagram represents a microstrip transmission-line segment, with values of inductance (L), resistance (R), capacitance (C), and conductance (G).

Electrical strength is associated with the dielectric strength of a circuit material. This parameter is presented in units of voltage per unit

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