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36 DESIGN007 MAGAZINE I FEBRUARY 2019 ductor plated thickness. For mmWave circuits, even small variations in these key material properties can impact electrical performance. Fortunately, flexible circuit technology is typi- cally well controlled for thickness compared to thicker, more rigid circuit board materials. As a simple example, a microstrip circuit us- ing 5-mil circuit material with a Dk of about 3 and a thickness variation of 1 mil will exhibit an impedance difference of about 6Ω at mi- crowave frequencies. A 6Ω impedance differ- ence may impact some applications; however, at mmWave frequencies, even small thickness variations can cause impedance differences that cause more reflections and many related issues. The 1-mil difference could be from a material with a thickness tolerance of only ±0.5 mil (±10% thickness tolerance). This is considered a very tight tolerance for a rigid circuit mate - rial; however, the thickness tolerance of flexible circuit materials can be much better controlled. Flexible circuit materials tend to provide highly consistent Dk that is nearly isotropic, as a func - tion of the material formulation. This is gener- ally true, although exceptions do exist. While flexible circuit materials offer many advantages for mmWave frequency designs, it should also be noted that they can suffer higher loss, dissipation factor (Df), and moisture ab - sorption than more traditional, rigid circuit ma- terials. The Df for many flexible circuit materi- als is in the range of 0.015–0.030 at 10 GHz, which is considered high. The Df of circuit ma- terials for mmWave frequencies usually needs to be less than about 0.003 because of the often- limited signal power at those high frequencies. Moisture absorption can also affect the electri - cal performance of a circuit material. Water va- por is polar in nature and can increase the Dk of the dielectric material that absorbs it. Still, as some circuit designers may not be aware, some circuit materials that can be used in flexible circuit applications also provide very good Df and moisture absorption characteristics, mak - ing them well suited for mmWave applications. An example of flexible circuit material for mmWave applications is the 5-mil RO3003 cir- cuit material. It provides material properties for mmWave circuit applications such as 77- GHz automotive radar sensors, and it is not a woven glass-reinforced material. The glass re- inforcement provides strength, but it can also limit circuit flexibility. RO3003 materials have an extremely low Df of 0.001 when tested at 10 GHz with low moisture absorption of 0.05%. The material's thickness is also tightly con- trolled, held to a tolerance of ±10%. As always, it's best to communicate with your material supplier well before beginning the design cycle. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. A University of South Carolina research team is inves- tigating the oxygen reduction performance of energy conversion materials called perovskites by using neu- tron diffraction at Oak Ridge National Labora- tory's Spallation Neutron Source. Perovskites are core c o m p o n e n ts of s o l i d oxide fuel cells, which can be utilized for dis - Neutrons: Fueling Better Power tributed power generation in remote areas or for backup power at data centers. Neutrons' sensitivity to light elements allows them to accurately probe the perovskites' structures and reveal how they influence the fuel cell's performance. Using a furnace in the VULCAN beamline, the team mimicked a fuel cell's typical environmental conditions. "VULCAN's unique high-temperature capability allowed us to see the perovskites' structures in their operating con- ditions," said USC's Kevin Huang, the corresponding author. (Source: Oak Ridge National Laboratory)

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