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46 The PCB Design Magazine • May 2017 THE DARK SIDE – RETURN OF THE SIGNAL • As the frequency increases, the current is forced into the outer surface of the copper, due to the skin effect. • Skin depth decreases with increased frequency. • At high frequencies, the thickness of the copper plane is irrelevant—½ oz. and 3 oz. copper will have the same surface conduction area. • There is a sweet spot where the total energy stored in the electromagnetic field surrounding the trace is optimized. • The signal return currents generate EM fields. Those EM fields, in turn, induce voltages (crosstalk) into other signals. • The easiest way to reduce crosstalk is to increase the spacing between the signals in question. • Crosstalk can also be controlled by varying the trace height, above the plane. A tight coupling (less height) results in less crosstalk. • The return current distribution of two parallel traces shows an overlap of current in the surface of a microstrip plane. • To minimize crosstalk do not to intermingle dissimilar technologies but rather keep them isolated. • Synchronous buses, as typically used in DDRx designs, benefit from an extraordinary immunity to crosstalk. • Ensure that the required setup and hold times are provided at the receiver. References 1. Barry Olney's Beyond Design columns: Return Path Discontinuities, The Dumping Ground, Controlling the Beast, Effects of Sur- face Roughness on High-speed PCBs, Uncom- mon Sense. 2. High-Speed Digital System Design, by Ste- ven H. Hall, Garrett W. Hall, and James A. McCall 3. High-Speed Digital Design, by Howard Johnson and Martin Graham Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner, is a PCB design service bureau and special- izes in board-level simulation. The software can be downloaded from To contact him or read past columns, click here. Researchers of Karlsruhe Institute of Technology (KIT) have now developed a method to erase the ink used for 3D printing. In this way, small struc- tures of up to 100 nm in size can be erased and rewritten repeatedly. This development opens up many new applications of 3D fabrication in biol- ogy or materials sciences. "Developing an ink that can be erased again was one of the big challenges in di- rect laser writing," Professor Chris- topher Barner-Kowollik of KIT's Institute for Chemical Technology and Polymer Chemistry says. The process was developed in close cooperation with the group of Professor Martin Wegener at the Institute of Applied Physics and the Institute of Nanotechnology of KIT. Structures written with erasable ink can be in- tegrated into structures made of non-erasable ink: Support constructions can be produced by 3D printing, which are similar to those used when building bridges and removed later on. Recently, such structures were designed by KIT to grow cell cultures in three dimensions on the laboratory scale. "During cell growth, parts of the 3D microscaffold could be removed again to study how the cells react to the changed environment," We- gener explains. According to the sci- entists, it is also feasible to produce reversible wire bonds from erasable conducting structures in the future. Erasable Ink for 3D Printing

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