Printed Circuit Design & Fab - January 2008 - (Page 21)

when it is one-half a wavelength long. EQUATION 2 shows the formula to find the wavelength. For example, at the 900 MHz the wavelength is only 33 cm and a half wavelength is 16.5 cm (approximately 5 inches). Clearly, many PCBs will be large enough to be efficient radiators at these frequencies. vp λ= f vp = c εr EQ. 2 where: λ= wavelength c = speed of light (300e8 m/s) f = frequency εr = relative dielectric constant A general rule-of-thumb is that a few microamps of noise current on an unshielded and efficient wire antenna can cause emissions that will be over the commercial EMC emissions limits. So depending on the amount of shielding, emissions from edge currents may be important. Technic offers its newest liquid photo-imageable coverlay/solder mask for flexible circuits. TechniFlex LCL 1000F is a cost-effective alternative to conventional coverlay films and an answer to the fine pitch requirements of today’s leading edge flexible circuits. Summary The distance between the trace and the edge of the ground-reference plane is important for EMI/EMC. Most of the return current will be below the trace, but some of this return current will spread out over the reference plane. If the trace is too close to the edge, then emissions will be higher than if the trace is further away from the edge. These emissions can easily be higher than allowed by commercial emissions standards. This does not mean that the routing channels near the edge of the board may not be used and must be left blank. Most boards need every routing channel possible. Use these routing channels near the edge of the reference plane for low speed signals and keep the high-speed clock, data, address, etc. signals away from the edge of the reference plane. PCD&F TechniFlex LCL 1000F • • • • • Passes UL 94V-0 & NASA Outgassing Specifications 2 mil Resolution for Today’s Most Advanced Technologies Easily Passes Industry Flexibility Testing (Over 10,000x) Halogen Free and RoHS Compliant Excellent Copper and Polyimide Adhesion 401-781-6100 info@technic.com www.technic.com REFERENCES 1. M.Y. Koledintseva, J. Drewniak, T. Van Doren, D. Pommerenke, M. Cocchini, D. Hockanson, “Method of Edge Currents for calculating mutual external inductance in a microstrip structure, Accepted for publication Progress ” in Electromagnetic Research (PIER), Feb 2008. DR. BRUCE ARCHAMBEAULT is an IBM Distinguished Engineer and IEEE Fellow; barch@us.ibm.com. Canada • China • England • France • Germany • Hong Kong Italy • Japan • Korea • Singapore • Taiwan • Thailand • USA http://www.technic.com http://www.technic.com

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - January 2008

Printed Circuit Design & Fab - January 2008 Contents Our Line Market Watch Around the World Happenings ROI PTH Reliability: Designing to Improve PTH Reliability EMC for the Real World PTH Reliability: The Survival and Long-Term Reliability of Lead-Free PCBs iNEMI's Emerging Technologies: Research Priorities for the Electronics Industry Printed Circuit Design & Fab Annual Buyers Guide Special Suppliers Section Guide to Products and Services Off the Shelf: Designcon Preview Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - January 2008

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