Deciphering datasheets for high-frequency circuit materials: Page 5 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.
can be a concern when soldering or performing other operations at elevated temperatures. Figure 3 provides a graphical depiction of changes in CTE above and below a material’s Tg.

Figure 3: This CTE chart represents the behavior of a typical PCB substrate. Click image to enlarge.

The highest-temperature parameter listed in the thermal section of a PCB material’s data sheet is the thermal decomposition temperature (Td). This is the temperature at which a material starts to decompose. More about the importance of this parameter shortly.

A circuit material’s thermal conductivity is its tendency to act like a thermal conductor or resistor. This parameter is given in terms of power per distance per temperature (W/m/K), and most PCB materials exhibit thermal conductivity like that of a thermal insulator or resistor. FR-4 circuit materials, for example, have thermal conductivity values in the range of 0.25 to 0.35 W/m/K. Many PTFE-based substrates have thermal conductivity values in the same range. When ceramic is used as filler for those PTFE circuit materials, however, the thermal conductivity can be increased to the range of 0.5 W/m/K. Although this may not seem significant, it is double the thermal conductivity of those earlier examples. In general, a circuit material with thermal conductivity of greater than 0.5 W/m/K is considered good.

Water has a high value of εr and suffers excessive loss at RF/microwave frequencies, so that water absorption should be minimized for any circuit material intended for high-frequency use. A high value of ε r in itself is not bad, and some circuit materials are available with relatively high ε r values. But when a circuit is designed and tuned for performance at a particular ε r value, and environmental changes such as moisture can change ε r which in turn changes the impedance of fabricated circuits, that circuit will no longer perform as designed. For high-frequency designs, circuit materials with less than 0.2% moisture absorption are

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