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DesignCon 2011 Stack-up and routing optimization by understanding microscale PCB effects
| Content Provider | Semantic Scholar |
|---|---|
| Author | Romo, Gerardo Baek, Seung-Won |
| Copyright Year | 2011 |
| Abstract | The high-frequency content of state-of-the-art digital signals acts like a magnifying glass for microscale PCB effects like glass weave and copper surface roughness. In this paper, we present an experimental, numerical, and analytical investigation of the important role that these effects play in defining the characteristic impedance of interconnects, the associated attenuation and phase constant, as well as the appearance of resonances in the insertion and return loss. By means of 3D EM simulations, macromodels are developed and correlated to measurements up to 50 GHz using a test vehicle designed using different prepregs, laminates and copper foils. As a result, recommendations and guidelines are provided for proper material selection and trace routing that mitigate such effects. Summary: PCB laminates are inhomogeneous structures constructed from multiple dielectric and metal layers. Thus, the anisotropic nature of PCBs introduces a variation of the dielectric characteristics of the substrate with position and may also originate resonances that considerably degrade the performance of interconnects. Despite this, it is a common practice for design engineers to solely rely on vendor published ε and TanD parameters obtained by a Bereskin and/or Stripline-resonator technique applied to smooth resin cast of homogenous material. Similarly, the properties of metal layers are typically characterized by bulk properties; at best, modified based on the model developed by Hammerstad and Jensen to account for the increase on the attenuation due to copper roughness. However, as communication speeds continue to rise, it becomes imperative to analyze the impact of micro-scale effects not only on the attenuation constant, but also on the phase constant and the characteristic impedance of the interconnects so that accurate representations can be achieved for reliable circuit design. In this paper, we study these micro-scale effects in detail. In particular, we analyze the dependence of the electrical properties of interconnects with respect to their position on the weave as well as the effects that result from the periodicity of the fiber glass bundles, which can give rise to resonances. Furthermore, we discuss the mechanisms that produce such resonances and propose an analytical equation for their prediction, validated by 3D electromagnetic simulations and measurements. For this purpose, a 3D fabric weave model based on actual prepreg layer measurements is created and simulated using an EM simulator. A PCB test vehicle was fabricated with sets of traces running at various angles. The predicted, simulated, and measured resonances are shown to be in excellent agreement. The focus of our investigation of copper roughness is not only on the additional attenuation introduced by the roughness, but also on the additional time delay, which can be directly extracted from our time-domain simulations. The investigation starts with the common approach where the copper surface bumps are assumed to be periodic. A parameterized model is developed which takes given surface statistics as input. The statistical data to generate such models are taken directly from optical profile meter measurements of copper foils. Bear in mind, however, that the large aspect ratio between the roughness feature size and the overall interconnect length makes the use of these 3D models currently unfeasible. For this reason, we propose a computationally inexpensive macromodel based on a frequency-dependent surface impedance model. After the models are developed, we use them to predict the effects of copper roughness on signal propagation and to characterize the copper foil by simulation. Our models are then correlated to existing models and measurements. For the experimental validation of the modeling results presented in this paper, we determine the complex propagation constant (γ) and characteristic impedance (Zc) of various high speed test lines. In our experiments we determine γ and Zc from line-line measurements, which is desirable for a more accurate and systematic characterization of the micro-scale effects on high-speed interconnects. Finally, as a result of our micro-scale investigations of PCBs, we provide guidelines and recommendations for material selection, stack-up optimization and trace routing not only for taking these effects into consideration but also for mitigating them as much as possible. Authors Biography Gerardo Romo obtained the Ph.D. degree in electrical engineering from Carleton University, Ottawa, ON, in 2005. From 2005 to 2008, he was a Sr. Hardware Engineer at the Systems Research Center of Intel Mexico, working on R&D of high-speed interconnects and electronic packages. His main research focused on exploring alternative technologies for high-speed interconnects such as Substrate Integrated Waveguides and non-linear Transmission lines. In 2009, he joined CST of America as an Application Engineer with emphasis on signal and power integrity applications. Chudy Nwachukwu is an Electrical Application Development Engineer at Isola Group, responsible for designing test vehicles and engineering applications to meet the requirements of OEMs in the high speed digital market. He obtained his bachelor ́s degree in Mathematics and Computer Science at Southwest Minnesota State University, Marshall MN in 2006. He completed his M.S in Electrical Engineering at Saint Cloud State University, Saint Cloud MN in 2009 and published his graduate Thesis on Design optimization of components in High Speed PCBs. He worked for Force10 Networks in San Jose, CA from 2007 2009 specializing in signal integrity, 3D modeling and Electromagnetic analysis of line card and backplane channels. Reydezel Torres-Torres is a senior researcher in the Microwave Research Group of the National Institute for Research in Astrophysics, Optics, and Electronics (INAOE) in Mexico. He has authored 30 journal and conference papers and directed one PhD. and 6 M.S. theses, all in high-frequency characterization and modeling of materials, interconnects, and devices for microwave applications. He received his PhD from INAOE and has worked for Intel in Mexico and IMEC in Belgium. Seung-Won Baek is an application engineer at CST of America. He has previously worked for LG Electronics, KMW(Korea MicroWave), and KBSI(Korea Basic Science Institute). His previous experiences include a variety of vacuum tubes, appliances and RF components designs. He holds fourteen International Patents and sixteen Domestic Patents on vacuum tubes. Martin Schauer received the Dipl.-Ing. degree in Electrical Engineering from the Technische Universität Darmstadt in 1999. In the same year, he joined the Computational Electromagnetics Laboratory (TEMF) for Theory of Electromagnetic Fields, where he earned his Ph.D. in 2005. Since 1999 he is with Computer Simulation Technology (CST), where he developed 3D electromagnetic simulation software until 2005. Currently he is working as a principal application engineer and technical key account manager for CST of America in the San Francisco Bay area. His main interests are numerical methods and their application towards low and high frequency electromagnetic problems. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.isola-group.com/wp-content/uploads/Stack-up-and-routing-optimization-by-understanding-micro-scale-PCB-effects.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
