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a prediction with any level of accuracy, but these fullwave simulation tools are extremely useful to help the engineer analyze specific parts of the design in order to better understand the physics of the specific feature under study. The engineer can use this knowledge to make the correct design decisions and trade offs. provide advice similar to what would be received from a human expert. The basic underlying assumptions for the algorithms are sometimes very limited in scope, and when applied to a real world PCB, the algorithm ‘breaks’ without letting the user know. Tools that are in this category are often a great emotional comfort but of little actual value. Tool Box Approach is Required No single modeling/simulation technique will be the most efficient and accurate for every possible model needed. Unfortunately, most commercial packages specialize in only one technique and try to force every problem into a particular solution technique. The PCB design engineer and EMI engineer have a wide variety of problems to solve, requiring an equally wide set of tools. The “right tool for the right job” approach applies to EMI engineering as much as it does to building a house or a radio. You would not use a putty knife to cut lumber or a soldering iron to tighten screws, so why use an inappropriate modeling technique? Some of the various simulation techniques require a deep level of understanding electromagnetics. The Finite Element Method (FEM) and the Method of Moments (MoM) are two such techniques. Others, like the FiniteDifference Time-Domain (FDTD) technique and the Finite Integration Technique (FIT) are much simpler to learn and to use. The reader is advised to understand how each technique works before purchasing any software. Know the Software Tool’s Assumptions! Just as it is important to know the limitations of the simulation techniques, it is important to know and to understand the basic assumptions the vendor has used in the specific software tools. Many times, the important factors are not displayed to the user (so the tool looks easier to use and less confusing). However, these factors can have an enormous impact in the accuracy of the final results. Always remember: The tool will give you a very accurate answer to whatever question you ask it, even if the question is wrong! Proper Simulation Validation In the early years of EM simulation, the practitioners were experts in EM theory and simulation techniques and often wrote their own programs to perform the simulations. However, modeling and simulation are no longer restricted only to experts. The commercially available codes are diverse, easy to use and provide the user with convenient means to display results. New users can begin using these codes quickly without the requirement of being an expert. The hidden danger is the need to validate the simulation results. It is not sufficient to simply believe a particular software tool provides the correct answer. Some level of confidence in the results are needed beyond a religiouslike trust in a software tool simply because others use it, because the vendor assures their customers of the tool’s accuracy or because others have validated their results in the past. Quasi-Static Simulation Software When an object is electrically very small (compared to the wavelength of the highest frequency of interest) then quasi-static simulation tools can be used. The fundamental assumption is that there is no propagation delay between elements within the model. Quasi-static tools are very useful for creating an equivalent circuit of inductance, capacitance and resistances that can be solved with circuit solvers, such as SPICE. Matrices of many elements can be used for including complex PCB connectors in signal integrity simulations. Summary Many different software tools are available for PCB designers to aid in meeting EMI emissions and immunity requirements. There is no one tool that can do everything, and multiple tools, often at different complexity levels, are required. Automated EMI rule checking tools can provide quick and specific analysis of PCB CAD designs, while more complex fullwave simulation tools can provide very accurate and fundamental understanding for limited portions of the overall PCB and/or system. pCd&f dr. BrucE archaMBEault is distinguished engineer with IBM and can be reached at barch@us.ibm.com. Other Software Tools There are a wide variety of software tools available to do specific tasks. The user must carefully consider if the software tool will do the type of analysis that is required. For example, some vendors offer simulation software that will read complete CAD files and then predict the far field emissions level based on the simple loop formed by a microstrip and the return/ground plane. This simplifying assumption is too simple for most applications, since the far field emissions are most often directly controlled by the metal shield (and the openings), as well as long attached wires not directly from the traces on the board. The metal shield and/or cables creates a dominant effect that is often ignored by these tools and can lead to dangerous and disastrous decisions when used incorrectly. Beware of so-called ‘Expert System’ software. These software tools claim to replace the human expert and to DECEMBER 2008 whErE to Go For MorE inForMation: 1. PCB rule checking software tools; www.MossBayeDa.com 2. eM Modeling/simulation Techniques, “eMI/eMC Computational Modeling Handbook” http://www.springer.com/engineering/electronics/book/978-0-7923-7462-6 3. eM Modeling/Simulation Software; http://emclab.mst.edu/csoft.html Model Validation; http://www.ewh.ieee.org/cmte/tc9/ printEd CirCuit dESign & fAB 17 http://www.MossBayeDa.com http://www.springer.com/engineering/electronics/books/978-0-7923-7462-6 http://www.springer.com/engineering/electronics/books/978-0-7923-7462-6 http://emclab.mst.edu/csoft.html http://www.ewh.ieee.org/cmte/tc9/

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

Printed Circuit Design & Fab - December 2008 Contents Our Line Market Watch Around the World Happenings ROI Global Sourcing EMC for the Real World Interconnect Strategies On the Forefront Final Finish Forum Test and Inspection Electronic System Design Data Management 101 Designers Take on Technology Challenges in 2008 PCB Signal Integrity, Power Integrity and EMC Challenges What’s in a Name? Ad Index PCB Dielectric Materials for High-Speed Applications Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - December 2008

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