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62 DESIGN007 MAGAZINE I OCTOBER 2019 More specifically, the copper surface rough- ness I'm referring to is the surface roughness at the substrate-copper interface of the high- frequency laminate. Also, my comment about the laminate hav- ing a low Df is somewhat subjective and very dependent on the type of circuit being used in the 5G system. Typically, the high-frequen- cy laminate should have a Df of 0.004 or less. This is a good general Df value to consider for the 5G sub-6 GHz applications; however, at mmWave frequencies, it is likely the Df value will need to be even lower. Another circuit material property that can be important for some 5G circuitry is thermal con- ductivity. Using a high-frequency laminate that has a high thermal conductivity can be very beneficial to thermal management for 5G ap- plications. As a general rule, a laminate with a thermal conductivity of 0.50 W/m/K is consid- ered good, and there are some high-frequen- cy, low-loss laminates with this property value. However, there are a few low-loss laminates with much higher thermal conductivity. For example, a new Rogers laminate, TC350 Plus, has an excellent combination of low-loss and high thermal conductivity. TC350 Plus laminate has a Df of 0.0017 when tested at 10 GHz and a thermal conductivity value of 1.24 W/m/K. The type of 5G circuit that is usually more sensitive to thermal management is the power amplifier. Also, feeding structures for the antenna elements and other circuits can present thermal management issues as well. The antenna structures for 5G typically use low-loss materials, which generally have low- er Dk values. As with all circuit functions, there are many tradeoffs, and antenna cir- cuits usually have a lower Dk that will allow more efficient radiation. With the many trad- eoffs considered, typically, antenna circuits will use a circuit material with a Dk value of about 3. TCDk can be an important property for 5G antenna applications because these circuits will be exposed to a range of temperatures. TCDk is a material property that is the char- acteristic of the material to change Dk with a change in temperature, and all materials have this property. A good TCDk value is 50 ppm/°C for a laminate, and a value closer to zero is ide- al. The RO4730G3 laminate has a TCDk value of 26 ppm/°C. There are many things to consider when de- signing 5G PCB applications, such as material properties and possible materials interactions with PCB fabrication. I highly recommend that the designer work with the material supplier when considering a high-frequency material to be used in 5G applications. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coon- rod, click here. Researchers used neutron scattering at Oak Ridge Na- tional Laboratory's (ORNL's) Spallation Neutron Source to probe the structure of a colorful new material that may pave the way for improved sensors and vivid displays. Most materials, including many biological photon- ic structures, exhibit structural colors as light moves through long-range periodic arrange- ments of elements in their microstruc- ture. Yet this material can produce striking colors using smaller, local ar- rangements of nanoplates. Thanks to this unique characteristic, researchers used the material to develop fluidic photonic art. "With neutrons, we saw firsthand how these nano- plates interact with light to form such spectacular col- ors," said Texas A&M researcher Zhengdong Cheng. "They were the perfect tools for developing an in-depth under- standing of this material's microstructure." This discovery could be a significant development in the quest for advanced photonic materials. The research was published in Proceedings of the National Academy of Sciences of the United States of America. (Source: ORNL) Neutrons: Lighting up Liquid Crystals

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