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Power Plane Noise Suppression for Next Generation High Speed Busses
Keywords: Power System Noise, Thin power layers, High speed digital system Signal Integrity
The use of thin Polyimide and FR4 laminate materials as power distribution layers in high speed multilayer Printed Wiring Boards has been investigated in a joint program initiated and sponsored by DuPont with the Georgia Institute of Technology Packaging Research Center (PRC). The PRC provided the design, modeling, simulation and testing and DuPont provided the materials and board fabrication. Test vehicles having common design features with three different power plane dielectrics, 18 and 25 micron thick Polyimide and 50 micron thick FR4 were fabricated and tested. High frequency electrical performance data was used to create models of the different structures and to perform simulations to determine the material properties offering the most effective power delivery noise suppression and signal integrity in high speed bus applications. At high data rates, parasitics of the power/ground planes and transmission lines become limiting factors for transmission of digital signals through long distances such as in backplanes. Transmission line losses start closing the eye diagram, while the spreading inductance of the planes degrades the performance of the power / ground system. Discontinuities in the return paths of high speed signals cause noise current coupling to the power/ground planes, which can propagate to any point on the board. This noise voltage on a power/ground plane can then couple to another high-speed signal. Hence the power/ground planes can be regarded as a major source of noise coupling. Properties of the dielectrics between the power/ground planes may introduce other problems by shifting the resonance frequencies. A thin dielectric helps to increase the capacitance between the planes and reduce the inductance. This is a very important property for the return current management of high-speed I/O lines. Multilayer test boards were designed with stripline and microstrip and single ended and differential transmission lines with different via structures for high frequency characterization to 20 GHz. Another set of test boards was designed to measure power plane input impedances and transfer impedances at various points on the board. These boards differed only in the material used for the dielectric between the power planes and were used to characterize structures in the time and frequency domain. Additional boards with the same features and construction with twenty-two inch long transmission lines driven by single ended and differential line drivers were designed and their performance characterized. Using the data generated in these test boards, simulations of different system bus configurations were performed. The details of the test program will be described along with measurement results showing a significant improvement in power system noise reduction through the use of thin Polyimide power plane laminates. The results of simulations for a high speed standard bus system configuration will also be described.
Daniel Amey, Research Fellow
E.I. DuPont Electronic Technologies
Research Triangle Park, NC

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