Issue link: https://iconnect007.uberflip.com/i/1487920
DECEMBER 2022 I DESIGN007 MAGAZINE 49 laminate has a Df value of 0.0017 when tested at 10 GHz, a variety of substrate thicknesses, choices of copper type with different surface roughness, and very high thermal conductivity of 1.24 W/m∙K. TCDk is another related thermal stability issue that is very oen considered for circuits operating in environments with changing tem- peratures, or a circuit that will change temper- ature significantly due to changing duty cycles. As a general rule of thumb, a good TCDk for circuit material is 50 ppm/°C. e ideal TCDk would be 0 ppm/°C, but few circuit materials have numbers in that range. e TCDf issue is more difficult to accu- rately measure due to the influence of heat- ing on different mechanisms which are part of the test method, thereby affecting the results. TCIL is a better measure in this regard because it is typically more accurate and has the benefit of being a real-word mea- surement. TCDf considers the change of Df, which is related to dielectric losses only. However, TCIL shows performance change in terms of the overall loss of a circuit given a change in temperature. e TCDk, TCDf and TCIL are param- eters which show instantaneous changes in certain properties due to a temperature change. However, circuits and their materials can have changes in properties due to long- term aging. Long-term aging at room tem- perature is typically a minor issue, but if aged at elevated temperatures, circuit and material properties can change in shorter timeframes. A higher temperature will cause the circuit/ material to change aging properties quicker. As another general rule, circuits made with thermoplastic materials will have better long- term thermal aging performance than ther- moset materials. However, there are some thermoset materials which are formulated to be very well-behaved for long-term thermal aging. Moisture absorption is typically not consid- ered with thermal stability issues, but in some cases it should be. A circuit material with a high moisture absorption property can change Dk and Df due to the circuit being exposed to changing humidity levels in the operating environment. is effect can be exaggerated with higher temperatures and should be con- sidered. However, a possible exception for this issue is a circuit operating at or above water evaporation temperature. Even if the material has a high moisture absorption property, if the temperature is too high to allow water vapor to accumulate within the circuit material, the performance difference due to moisture uptake will be minimal. ermal stability of a PCB can be affected by many different circuit material proper- ties. Also, a circuit's design and construction will impact the thermal performance of the PCB in the end-use application. It is highly recommended to contact your material sup- plier when working with thermal management issues. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns, click here. Moisture absorption is typically not considered with thermal stability issues, but in some cases it should be.