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50 DESIGN007 MAGAZINE I APRIL 2023 usually focused around using high-frequency circuit materials with low loss, high thermal conductivity, low CTE (coefficient of thermal expansion), and low TCDk (thermal coeffi- cient of Dk). e PCB design and fabrication are also considered for thermal management concerns and, when practical, design features like via farms or via fences are included. Most dielectrics used for high-frequency cir- cuit materials have poor thermal conductivity when compared to the excellent thermal con- ductivity of metals. As a simple comparison, the thermal conductivity of copper is about 400 W/m∙K and most PCB dielectric materi- als have a thermal conductivity around 0.3 W/ m∙K. Because of the big difference in thermal conductivity, some PCBs with thermal man- agement challenges will have cavities built into them, which will minimize the amount of dielectric material in the heat flow path. e heat flow path originates from the heat source that resides on the PCB or a RF trace on the circuit, and the migration of that heat to a heat sink that will absorb the heat. Over many years of dealing with thermal management issues, the PCB industry has informally adopted a rule of thumb that a dielectric with thermal conductivity of 0.5 W/ m∙K or higher is considered good for thermal management concerns. Many of the ceramic- filled high-frequency laminates have this ther- mal conductivity value or higher but there are a few special materials with significantly higher thermal conductivity. RF designers must consider the different properties of the high-frequency circuit mate- rials used in the board. For thermal manage- ment issues, a thicker substrate will increase the heat flow path, and that is not desirable. However, a thicker laminate is oen desired when operating at lower microwave frequen- cies because there will be less insertion loss. is assumes the use of a low-loss material, with a low dissipation factor. Insertion loss is directly related to heat generation due to RF power heating the circuit, and higher insertion loss will cause more heat to be generated. A thin circuit will have a shorter heat flow path and that is desired for good thermal manage- ment. However, a circuit using a thinner sub- strate will have higher insertion loss and more heated generated from the applied RF power. e tradeoffs for thermal conductivity are described here, but an application using a thick substrate should consider a material with high thermal conductivity and low dissipation fac- tor. Of course, an application using a thin sub- strate will also benefit from these properties, but typically more attention is put on minimiz- ing insertion loss so there is less heat generated. In minimizing insertion loss for a circuit based on a thin substrate, copper surface roughness is oen a consideration. Copper surface roughness at the substrate- copper interface can have significant impact on insertion loss and that is especially true for circuits based on thinner substrates. A rougher copper surface will increase conductor loss and conductor loss is usually a large portion of the overall insertion loss for a circuit based on thin material. Basically, when the copper planes are close together, which is the case for a circuit using a thin substrate, the effects of the cop- per surfaces will be more impactful on RF per- formance. e effects on RF performance are related to phase angle, wave velocity, effective Dk and, as already stated, insertion loss. Most dielectrics used for high-frequency circuit materials have poor thermal conductivity when compared to the excellent thermal conductivity of metals.