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54 DESIGN007 MAGAZINE I OCTOBER 2022 conductor. Most PCB laminates have a ther- mal conductivity of 0.2 to 0.4 W/m∙K. ere are some laminates which have higher ther- mal conductivity, and a laminate with one of the highest thermal conductivities comes from Rogers. is laminate has a thermal conduc- tivity of 1.24 W/m∙K and that is when test- ing the substrate only. In some evaluations for thermal conductivity, the laminate supplier will include the copper cladding and that will cause the reported thermal conductivity to be higher. Rogers does not test copper with the substrate and the thermal conductivity associ- ated with this particular laminate is the value of the substrate only. Lastly on this subject, hav- ing a combination of PTH grounding vias that are used in conjunction with a laminate having high thermal conductivity is advantageous for thermal management. A thermal management issue related to the circuit heating due to applied power is a dif- ferent thermal management issue than previ- ously discussed. In this case, the heat flow path through the circuit as previously mentioned still applies, however, there are other issues which need to be considered. In the case of the circuit being heated due to RF applied power, then the insertion loss becomes more impor- tant to consider. Basically, an increased inser- tion loss will cause an increase in heat gener- ated. One tradeoff to consider is between ther- mal conductivity and insertion loss. If a circuit material has low thermal conductivity, it may still be acceptable if the material has very low loss. If the insertion loss is very low, there will be less heat generated and the thermal conduc- tivity becomes less important. On the opposite end of that concern would be to use a material that has high thermal conductivity but is not as good for insertion loss. e poor insertion loss will generate more heat, but using a material with high thermal conductivity will have the heat move through the circuit efficiently. Ide- ally, it would be best to have a material that is very low for generating insertion loss and high for thermal conductivity. Another complication with RF heating of the circuit is that different thicknesses of the circuit dielectric material will cause differ- ences in insertion loss. A circuit using a thicker low loss substrate will generate less heat due to lower insertion loss, however the heat flow path through the material is now longer due to the substrate being thicker. e heat flow formula is thickness-dependent, and a thicker circuit will have a longer heat flow path and will not be more efficient at moving the heat through the circuit to the heat sink. Additionally, the dielectric constant of the material can have an impact on thermal man- agement. A circuit using a material with a low dielectric constant will cause the conductors to be wider. Having wider conductors will generate less insertion loss and less heat. Also, a wide conductor will have more surface area on the signal plane. Increased surface area on the signal plane will cause a wider heat flow path and the heat flow will be more efficient from the signal plane to the planes below. e Rogers circuit material mentioned ear- lier is a high-frequency circuit material that is formulated to have good qualities for thermal management. When the laminate is evaluated at 10 GHz, it has a Dk of about 3.5, and dissipa- tion factor of 0.0017. It has a thermal conduc- tivity of 1.24 W/m∙K, which is considered very good, and it is available in a variety of thick- nesses. Additionally, the laminate is available with different copper types, and the reverse- treated copper foil that is available has a rela- tively smooth copper surface. e smoother copper can also support lower insertion loss, which will help to decrease the heat generated and make thermal issues better. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns, click here.