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PCBD-Dec2015

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58 The PCB Design Magazine • December 2015 these test methods need to determine Dk and Df relatively fast, have good repeatability, and be used for quality control. A common test method used is the clamped stripline resonator, where a clamping fixture is used to form a stripline struc- ture; the layer structure of a stripline is ground- signal-ground. This test method determines the Dk and Df of the material in the clamped fixture and more specifically, it is reporting these values related to the thickness axis of the material. Other tests used in high-volume testing include SPDR (split-post dielectric resonator), rectangular cavity and open cavity resonance methods. All three of these methods have elec- tric fields oriented perpendicular to the mate- rial, which means these test methods will evalu- ate the x-y plane of the material and not the z-axis. In the case of the common FR-4 material which is a resin-glass composite, the Dk num- ber can be very different in the x-y plane than in the z-axis due to the impact of the glass. Returning to the original example, where a material is tested and found to have a Dk of 3.5 and then another test is done on the same material and the Dk is found to be 3.8, both of these numbers can be correct when using two different test methods. These numbers are actu- ally based on real life experience when testing high frequency laminates that are PTFE based with ceramic filler and have woven-glass rein- forcement. With this type of material it is pos- sible to have the same piece of material tested in the clamped stripline test and get a value of 3.5 and then tested in SPDR and obtain a result of 3.8. Since the clamped stripline test is evalu- ating the z-axis of the material and the SPDR is evaluating the x-y plane of the material, both results are obviously different but still correct. Essentially these measurements show to some degree how anisotropic the material is, where the z-axis Dk of the material is 3.5 and the x-y plane Dk of the material is 3.8. Knowing the anisotropic Dk values of a laminate is typically critical for RF applications with edge coupled features. In the case of high- speed digital circuits, these values can be im- portant for differential pair structures. Having these values can be important, but also having a modeling software which can incorporate these values into predicted circuit performance is an important supplement to the design process. It is always recommended to contact your material supplier if you have questions about the Dk or Df of a laminate. You should ask which test method is used and which axis or axes of the material is being evaluated. Another good question to ask your material supplier is the frequency at which these values are gener- ated, because the Dk and Df of a material is fre- quency dependent. Having the most accurate laminate information for the design phase of a project is critical to its success. PCBDESIGN John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns, or to reach Coonrod, click here. More than a decade ago, theorists predicted the possibility of a nanolens. But scientists had been unable to build and arrange many nano- lenses over a large area. "That's where we came in," said Xiaoying liu, senior research scientist at the university of Chicago's Institute for Molecular engineering. They aligned three spherical gold nanopar- ticles of graduated sizes in the string-of-pearls arrangement predicted to produce the focusing effect. The scientists are already exploring using this "hot spot" for high-resolution sensing using spectroscopy. "If you put a molecule there, it will interact with the focused light," said liu. New Nanomanufacturing Technique Advances Imaging, Biosensing Technology lightning speed laminates WHY Do DIFFERENT TEST METHoDS YIELD DIFFERENT ELECTRICAL VALUES?

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