The Distribution of Power and Earth on Boards for High Speed Digital Logic

To provide an efficient interconnection system, compromises must be made on materials and packing density within acceptable limits, and designers must understand electrical characteristics and the functioning of signal generating devices. High speed digital logic devices are analysed with special attention given to signal swings. Solutions to the problem of line reflection currents are suggested, and the decoupling process is explained. Board design is vitally important and the power and earth distribution contribute strongly towards its success or failure. Power distribution on double-sided boards is examined, with a consideration of various arrangements of capacitors used for decoupling. After discussion of the effects of other tracks, it is evident that signal tracking lengths should be kept to a minimum, while line impedances are maximised. Guidelines for efficient tracking are provided and an explanation of the trackability factor of a PCB reveals power distribution past the ends of packages to be the most effective arrangement for achieving high packing densities. Double-sided boards should be used where possible in preference to multilayers.