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January 2015 • The PCB Magazine 45 in the insulating resins. When polymers have covalent bonds between different atoms (e.g., a bond between oxygen and carbon), these two atoms have what's called different electro- negativities, which means that the electrons of the covalent bond are not equally shared by the two atoms. The atom with the higher electro- negativity tends to attract more than 50% of the shared electrons, causing a partial negative charge on the oxygen, and a partial positive charge on the carbon. This (partial) sepa- ration of charges forms what is called a dipole, comparable to a magnet with its positive and negative poles. If these dipoles are exposed to an electromag- netic field, as is the case in a conductor surrounded by a di- electric, the dipole will line up in the direction of the electro- magnetic field lines, assuming they have the mobility to do so. If the electromagnetic field flip-flops, as is the case with A/C current, the dipoles re- verse direction (i.e., they flip- flop too). This flip-flopping of the dipoles in the dielectric basically amount to an A/C current flowing through the dielectric, the so called leakage current. As the signal frequency goes up, the dipoles switch faster, increasing the leakage current, but ultimately there are some spacial constraints to this mo- tion and the dipoles may not be able to keep up. This is the cause of the frequency depen- dency of the dielectric loss. Given the factors that contribute to the bulk A/C conductivity of a dielectric (see above) we'd like to choose a dielectric with a low dielectric constant and a low-loss tangent. Such dielectrics either have no dipoles, or they have very weak dipoles in the polymeric structure, or they have dipoles with little mobility. As it turns out, low dielec- tric constant and low-loss tangent go hand in hand. Thus, we notice that polymers with a low dielectric constant such as PTFE (poly-tetraflu- oroethylene) or hydrocarbons also have a low loss tangent. Unfortunately, they also tend to be more expensive that the higher dielectric constant and higher loss tangent epoxies. PCB References 1. "Signal Integrity—Simplified," Eric Bo- gatin, Prentice Hall Modern Semiconductor Design Series, ISBN 0-13-066946-6, Copyright 2004 by Pearson Education, Inc. 2. "An Update on the Effects of Laminate Weave on Jitter and Skew in Gigabits and Higher Signals," Lee W. Richey, Cir- cuitree Magazine, July 2008, pg. 29. 3. "Spatial Impact of PCB Fabrication and Materials on Single-ended and Differen- tial Impedance Transmission Lines," Gary Brist and James McCall (Intel Corp.), PCB West 2003. 4. "Effect of Surface Fin- ish on High-Frequency Signal Loss Using Various Substrate Materials," Don Cullen et al, Proceedings, IPC Printed Cir- cuits Expo ® , 2001. 5. "The Impact of Con- ductor Surface Profile R rms on Total Circuit Attenuation in Microstrip and Stripline Trans- mission Lines," Seth J. Normyle, Thomas F. McCarthy, and David L. Wynants. 6. "Reduction of High-Frequency Signal Loss through the Control of Conductor Geom- etry and Surface Metallization," Gary Brist, Don Cullen et al, SMTAI Chicago, 2002. SIGNAL LOSS continues Karl Dietz is president of Karl Dietz Consulting llC. He offers consulting services and tutori- als in the field of circuit board and substrate fabrication tech- nology. To view past columns or to reach Dietz, click here. Dietz may also be reached by phone at (001) 919-870-6230. if the electromagnetic field flip-flops, as is the case with a/C current, the dipoles reverse direction (i.e., they flip-flop too). this flip-flopping of the dipoles in the dielectric basically amount to an a/C current flowing through the dielectric, the so called leakage current. " " karl'S tECh talk