Electrical characterization of the interconnects inside a computer

Abstract

Calculations of the electrical characteristics of the signal lines and other interconnects in computer modules have, in the past, often relied upon simplifying assumptions. For instance, the quasi-TEM approximation or circuit models are often applied without proper consideration of the operating frequencies and coupling between package features. Though such approximations in most cases are probably valid, it is difficult and time consuming to verify them. A better approach would be to employ a solution technique that does not rely on such approximations in the the first place. This paper describes a rigorous approach to such modeling, employing a full-wave, Maxwell's equation solution for determining the propagation characteristics of packages. Arbitrarily shaped, 3-D signal lines and their discontinuities can be analyzed; structures may include finite-size dielectric regions, with material composition that may be anisotropic. The approach involves a method-of-moments solution technique, with rooftop basis functions used to represent the surface current on the conductors and the polarization current in the dielectrics. To account for the dielectric regions, the dielectric is first replaced by an array of interlocking thin-wall sections; the electric field boundary conditions are later applied through the use of surface impedances. In the paper, the signal propagation concerns in computer packages are first reviewed. Reflections off discontinuities, proximity effects associated with nearby but nontouching other signal lines and vias (OLVs), and crosstalk and its sensitivity to various package parameters are discussed. The modeling approach is then described, and used to characterize the TCM (Thermal Conduction Module) used in the IBM 3090 mainframe, which involves an essentially homogeneous dielectric medium. Results are compared to both measurements on actual hardware and to results from other approaches. Results for microstrip and dielectric waveguides are also presented, as a demonstration that other segments of the package, which may involve inhomogeneous dielectric regions, can be handled. Throughout the paper, advances in the understanding of related package phenomena and improvements in the algorithms are discussed.