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38 The PCB Design Magazine • September 2016 • Blind vias behave like a lumped capacitor with very little inductance. And because the mi- crovia hole is very small, it has less capacitance than a standard PTH via. • Removing non-functional via pads reduces via capacitance which in turn increases imped- ance. • Increasing the size of the antipad reduc- es capacitance but, at the same time, may well make Swiss cheese out of the reference plane in- creasing the DC drop and reducing the amount of instantaneous current available. • An oblong antipad still reduces the para- sitic capacitance significantly, but at the same time allows the return current to flow directly between the vias to reduce loop inductance and preserve the continuity of the reference plane. • A terminating element placed at the bot- tom end of the stub reduces signal reflection. • Lowering the dielectric constant, of the surround material, and tin plating the via barrel are possible but may not be practical solutions. PCBDESIGN References 1. Barry Olney's Beyond Design column, How to Handle Danglers - Part 1. 2. "PCB Vias, an Overview," by Bert Simon- vich. 3. Dell products patent, Stuart Allen Berke. 4. IBM Corporation patent, Bhyrav Mutnury. 5. "Via Optimization Techniques for High- speed Channel Designs," Altera. 6. High-Speed Signal Propagation: Advanced Black Magic, Howard Johnson, Martin Graham. Barry Olney is managing director of In-Circuit Design Pty Ltd (ICD) Australia. The company is a PCB design service bureau that special- izes in board-level simulation. ICD has developed the ICD Stackup Planner and ICD PDN Planner soft- ware, which is available here. To contact Barry, click here. HOW TO HANDLE THE DREADED DANGLERS, PART 2 Chemists at Friedrich-Al- exander-Universiät Erlangen- Nürnberg (FAU) have now succeeded in producing de- fect-free graphene directly from graphite for the first time. Graphene is two-dimen- sional and consists of a single layer of carbon atoms. It is particularly good at conduct- ing electricity and heat, transparent and flexible yet strong. Graphene's unique properties make it suitable for use in a wide range of pioneering technologies, such as in transparent electrodes for flexible displays. A common way of synthesising graphene is through chemical exfoliation of graphite. In this process, metal ions are embedded in graphite, which is made of carbon, resulting in what is known as an intercalation compound. The individ- ual layers of carbon - the graphene - are separated using solvents. The stabilised graphene then has to be sep- arated from the solvent and reoxidised. However, defects in the individual layers of car- bon, such as hydration and oxidation of carbon atoms in the lattice, can occur during this process. FAU researchers have now found a solution to this problem. By adding the solvent benzonitrile, the graphene can be re- moved without any additional functional groups forming - and it remains defect-free. "This discovery is a break-through for experts in the international field of reductive graphene syn- thesis,' Professor Hirsch explains. 'Based on this dis- covery we can expect to see major advancements in terms of the applications of this type of graphene which is produced using wet chemical exfoliation. An example could be cutting defect-free graphene for semi-conductor or sensor technology." Low-cost and Defect-free Graphene