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50 DESIGN007 MAGAZINE I AUGUST 2018 these materials and the properties mentioned. Following is my subjective rating as a list of material properties for this comparison in Table 1. The first item, Dk control (control of dielec- tric constant property), is very well controlled for high-frequency materials and not so well controlled for FR-4. For applications at lower frequencies, the lack of Dk control is often not a concern, but there are exceptions. Even lower-frequency circuits sometimes need well controlled impedance and having Dk that is tightly controlled can be advantageous for get- ting better yields with a controlled-impedance board. Having good control of the Df property for low-frequency applications is typically not a concern. However, when the designer is con- sidering FR-4 for an application where inser- tion loss is a concern, the inconsistency of FR-4 may preclude the designer from understanding the true limits of that material. Circuit fabrication can impact the cost of the circuit as well as the consistency of different circuit properties. FR-4 materials are defined very well in the PCB fabrication process and are typically not a concern. The high-frequency material chosen for this example is defined well in the PCB fabrication process, but there are different parameters which must be used for fabricating with this material, and these parameters can make the circuit fabrication costlier. Because the same FR-4 substrates are often manufactured differently at different manufac- turing sites, this can certainly impact the thick- ness control of the substrate. The lot-to-lot (or within lot) variation of thickness con- trol is typically much worse for FR-4 than for high-frequency materials. But thickness control of the substrate is extremely important to high-frequency designs. High-frequency materials are tightly controlled in the manufacturing process to provide consistent substrate thickness tolerance, because this tol- erance has a direct impact on critical high-frequency circuit functions such as impedance, phase response, bandwidth and insertion loss. High-frequency materials with their tightly controlled thickness toler- ance help improve yields for controlled-imped- ance boards, which leads to cost savings. Moisture absorption is another important aspect to ensure consistent high-frequency performance, which is well defined in the for- mulation of most high-frequency circuit mate- rials. Moisture absorption for a low-frequency application may or may not be important. In some cases, having a low moisture-absorbing material in a dynamic operating environment may not be an electrical concern. However, it could be a reliability concern. There are low- frequency applications that have no issues with electrical performance, but still utilize a high-frequency circuit material to avoid issues with moisture absorption. Several applications have a board-mounted chip or device, which generates heat during normal operation. Using high-frequency mate- rials with higher thermal conductivity can minimize thermal management concerns. The typical FR-4 material has a thermal conductiv- ity property of about 0.25 W/m/K as compared to RO4835, which has a thermal conductivity of 0.62 W/m/K. This is a difference of more than 2X, which can certainly have a positive impact on thermal management. Even with low-frequency circuits, when elec- trical consistency is required, the thermal coef- ficient of Dk (TCDk) must be considered. In a nutshell, TCDk is a measurement of how much the Dk of the substrate will change, given a change in temperature. FR-4 materials are not formulated with this property in mind and because of that, it is not uncommon for these Table 1: A subjective rating of material properties.