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76 The PCB Design Magazine • August 2016 Another material property which further complicates designing for significant tempera- ture variation is temperature coefficient of di- electric constant (TCDk). The TCDk is a prop- erty that all materials have and is a measure of how much the material will change dielectric constant (Dk) with a change in temperature. One example: If a circuit is using a material with a high TCDk, such as 400 ppm/°C, the dielectric constant of the material will change 0.020 with a 46°C change in temperature. This temperature change is equivalent to the temperature range previously mentioned from 75°F, then at 160°F. The Dk change will impact the impedance, phase response and other circuit attributes. This impact would be in addition to the impact of the physical dimensional change of small cir- cuit features which would occur over the tem- perature range if the circuit was designed for mmWave applications. There are also issues associated with circuit attenuation or RF losses, due to thermal chang- es. The analog of the TCDk to dissipation factor is the temperature coefficient of dissipation fac- tor (TCDf). Again, every material has this prop- erty and it is a measure of how much the dis- sipation factor (Df) will change with a change in temperature. An RF circuit using a material with a high TCDf will have more RF losses with an increase in operating temperature. Another material issue is sometimes ignored. Copper increases resistance or is less conductive as it is heated. An increase in temperature will cause copper to have more conductor losses for a RF circuit. Conductor loss is one component of the overall insertion loss of a RF circuit. Con- ductor loss is frequency-dependent as well as circuit thickness dependent. An increase in fre- quency will cause more conductor losses and a thinner circuit will be more sensitive to changes in conductor loss than a thicker circuit. As men- tioned earlier, mmWave applications typically use thin substrates and are operating at very high frequencies. Due to this combination, PCBs used in mmWave applications are sensi- tive to conductor loss. As the circuit increases temperature, the change in conductor loss worsens due to a de- crease in copper conductivity. There is an ad- ditional loss which occurs as well and that is an increase in dielectric loss due to the TCDf of the material. An increase in temperature will cause the material to have worse Df and that combined with the conductor loss is- sue can be very troublesome for PCBs used at mmWave applications operating at elevated temperatures. Some aging effects for substrates can be ac- celerated by extended elevated temperature ex- posures. Thermoset substrates which are used in the PCB industry will oxidize to some degree when evaluated over a long period of time. This assumes the substrate is in an environment with oxygen. The oxidation can cause a difference in the dielectric constant and dissipation factor of the material and the oxidation typically affects a relatively thin layer at the surface of the sub- strate. When the dielectric material is protected by copper, oxidation cannot occur, and there may be some small amount of oxidation edge effect. The edge effect is more noticeable when the conductor width is narrow and the oxida- tion can migrate under the conductor from the edge next to the conductor where the substrate is exposed. RF performance with edge-coupled circuits which use an exposed substrate can be influenced by the oxidation more than circuits which are transmission lines and stubs where the copper protects the substrate. PCBDESIGN John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns or to reach Coonrod, click here. DESIGNING FOR PROFITABILITY: DON'T OVER-MATERIALIZE " An RF circuit using a material with a high TCDf will have more RF losses with an increase in operating temperature. "