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Design007-Aug2019

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60 DESIGN007 MAGAZINE I AUGUST 2019 is a measure of how much the Dk will vary given a change in temperature. Most lower- grade materials have TCDk values in the range of 200–400 ppm/°C. A high-frequency materi- al formulated for good TCDk will usually have values less than 50 ppm/°C. Another property related to Dk variation is moisture absorption. Water vapor can be ab- sorbed into a circuit by the moisture absorp- tion property of the material. Water has a Dk value of about 70, but that value depends a lot on the frequency of which the data is collect- ed. Regardless, the higher Dk of water will in- crease the Dk of the high-frequency circuit due to the moisture absorption property of the ma- terial, and that change in Dk can certainly al- ter the RF or high-speed digital performance of the circuit. In many cases, having a circuit material with tight Dk control is mandatory, but in other cas- es, Dk control is not as important as a designer may assume. It is common for a PCB fabricator to test a circuit for impedance values as quality control, and it is pretty common for the target impedance to be 50 ohms. Designers often as- sume that tight Dk control is critical for imped- ance, but in reality, the Dk value is the lesser concern of several properties that influence the impedance of the circuit. If designers do simple impedance mod- eling for a basic microstrip circuit, they will find that the hierarchy of influences on im- pedance is (1) the circuit conductor width, (2) substrate thickness, (3) copper thickness, and (4) Dk. The Dk value and its variation is the least impactful variable for the impedance of the circuit. However, if designers did the same modeling and looked at these four influences on the impact of phase angle response, they would find that Dk is a much more important property for phase angle consistency than it is for impedance. Designers should be aware of the circuit property that is most important and consider the appropriate influences which can impact variation of that property. Resources There are a lot of excellent resources in the industry for high-frequency circuit materials. For example, the www.IPC.org website has a lot of information about these materials in terms of specifications and test methods. Also, IPC will often offer training that can include material properties and the influence these properties have on circuit fabrication and per- formance. The www.IEEE.org website is another ex- cellent resource for RF and high-speed digital applications. This website is more focused on the performance of electrical structures than materials. Although, there are many papers on the IEEE website related to material char - acterization of high-frequency circuit materi- als, and these references can be very useful to understand electrical properties of these materials. The high-frequency material suppliers in the industry also provide technical informa- tion, which is typically more tailored to their products and less of a general education about these materials. Rogers Corporation provides educational in- formation about high-frequency materials and how they are used in specific applications and general information as well. Rogers has a Tech- nology Support Hub with free calculators, vid- eos, articles, and other content, along with the ability to contact a local engineer for techni- cal assistance, at www.rogerscorp.com/techub. DESIGN007 John Coonrod is technical market- ing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. Water vapor can be absorbed into a circuit by the moisture absorption property of the material.

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