Issue link: https://iconnect007.uberflip.com/i/1487920
48 DESIGN007 MAGAZINE I DECEMBER 2022 As a general statement, the amount of power that can be applied to a PCB is related to the temperature rise it will cause; this is critical to ensure the circuit temperature does not violate the maximum operating temperature (MOT). e MOT is the maximum temperature at which the circuit can operate without degrada- tion to critical properties of the circuit. ere are many things that will impact the heating of a circuit, including the removal of the heat through good heat flow management and the use of heat sink technology. A typical circuit thermal management example is to have heat generated on the top signal plane and have a heat sink attached to the ground plane on the bottom of the cir- cuit. e heat flow path will originate at the signal plane, migrate through the substrate, and terminate at the ground plane below. e ground plane is at the same thermal poten- tial as the heat sink, which is attached to that plane. Basic heat flow concepts suggest that using a thinner substrate will shorten the heat flow path and move the heat more effectively from the signal plane to the heat sink, which enables a lower circuit temperature. Another option for optimizing heat flow is to increase the size of copper areas. Because copper is an extremely good thermal conductor, the larger the copper area between the signal plane and the ground plane, the wider the heat flow path(s), which assists in maintain- ing cooler circuit temperatures. Additionally, since the heat flow path is going through the substrate in this example, the thermal con- ductivity of the substrate can also be signifi- cant. A circuit material with higher thermal conductivity will increase the heat flow and aid in lower circuit temperatures. A simple rule of thumb is that a thermal conductivity of 0.5 W/m∙K or greater is considered good for a circuit material. To complicate circuit heating matters fur- ther, due to RF power being applied to the circuit, insertion loss is a major concern. Basi- cally, an increase in insertion loss will give an increase to the heat generated for the circuit. In some cases, designers will ignore the heat flow concerns previously mentioned and consider using very low loss circuit material to generate less heat. As thermal management has become increasingly demanding, this simple approach is oen not adequate. Material choices may result in undesirable consequences due to interactions between the multiple thermal management properties. For example, using a thinner circuit material to increase heat flow typically means higher insertion loss and more heat generated. For this thermal management tradeoff and many oth- ers, a good thermal simulation model is needed to understand the different relationships at play. at aside, if a thinner substrate is used, which has very low loss (low Df ), using cop- per with a smooth surface (which gives lower conductor loss) with high thermal conductiv- ity creates an optimum scenario for good heat flow and minimizing circuit temperature. e thin substrate gives a short heat flow path and its low Df gives lower insertion loss (i.e., gener- ates less heat); by combining this with smooth copper, which also gives lower insertion loss, and high thermal conductivity, the heat gener- ated is moved to the heat sink very effectively. Rogers has a material formulated with these optimal thermal management properties. is There are many things that will impact the heat- ing of a circuit, including the removal of the heat through good heat flow management and the use of heat sink technology.