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62 The PCB Design Magazine • December 2017 • A stripline is any trace sandwiched between reference planes on both sides. • A microstrip has dielectric material and a plane on one side and air on the other. An embedded microstrip is similar but is covered in a conformal coating such as solder mask or another dielectric material. • Even if the trace widths are adjusted on each layer, so as the impedance is identical, the propagation speed of microstrip is always faster than stripline, typically by 13-17%. • The speed of propagation of digital signals is independent of trace geometry and impedance. • Trace delays can be matched so that either microstrip or stripline will arrive at the receiver simultaneously. • The dielectric constant of materials can vary as much a 10%, depending on the stability of the material, creating temperature dependent skew. • The temperature coefficient for microstrip traces is less severe than for stripline because it partially travels in air which does not vary with temperature. • Even if the delay of microstrip and stripline traces are perfectly matched, they will still vary independently with temperature. • For stripline, materials with similar dielectric constants should be chosen. This will ensure that the signal flight times are identical. • One would be well advised to only route critical signals on inner stripline layers. References 1. Barry Olney's Beyond Design columns: New Functionality Improves Designer's Produc- tivity, Faster than a Speeding Bullet. 2. Waves as Energy Transfer, Science Learn- ing Hub. 3. IEEE 802.3ap Backplane Ethernet, Joel Go- ergen, Manny Hernandez. 4. High-Speed Signal Propagation, Howard Johnson. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bu- reau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner. The software can be downloaded from www.icd.com.au. To contact Olney, or read past columns, click here. SIGNAL FLIGHT TIME VARIANCE IN MULTILAYER PCBS A research team in the Department of Electri- cal and Electronic Information Engineering and the Electronics-Inspired Interdisciplinary Research Insti- tute (EIIRIS) at Toyohashi University of Technology has developed an ultrastretchable bioprobe using Kirigami designs. The Kirigami-based bioprobe enables one to fol- low the shape of spherical and large deformable biological samples such as heart and brain tissues. The results of their research will be published in Ad- vanced Healthcare Materials (Ultrastretchable Kiri- gami Bioprobes). High stretchability and deformability are promis- ing properties to increase the applications of flex- ible film electronics including sensors, actuators, and energy harvesters. However, conventional elastomer-based stretchable devices require a large strain-force to stretch it, that arises from an intrinsic material property. This makes it impossible to follow the deformation of soft biological tissues, thereby preventing natural deformation and growth. "To realize the ultrastretchable bioprobe with low strain-force characteristic, we used a Kirigami design as the device pattern. The stretching mecha - nism is based on an out-of-plane bending of the thin film rather than stretching of the material; therefore, the strain-stress characteristic is extreme- ly low compared to that of elastomer-based stretch- able devices," explains the first author of the article, Ph.D. candidate Y usuke Morikawa. Revolutionizing Electronics Using Kirigami