Influence on LSI Package Wireability of Via Availability and Wiring Track Accessibility

Abstract

In this paper, experiments are reported which use automatic global and line-packing wiring routines, supplemented by a restricted maze runner, to evaluate the overall influences of several important physical variables upon the wireability of several logic-circuit package types. The logic circuits are contained in subpackages (e.g., modules carrying chips), which are inserted, using pins, into the carrier package in a regular array of holes otherwise available as vias interconnecting the wiring planes. Over a range of connection counts from several hundred to several thousand, it is found that “overflows” (connections not wired by the program) amounting to as much as 10 or 15% of the wires can be substantially reduced in number by careful design. This can be done by using a sufficient number of programmable vias (greater than one per used pin) and by using a track grid ensuring maximum global track accessibility to all pins, or by a combination of both of these tactics in conjunction with suitable wiring algorithms. Some simple theoretical arguments are given which characterize the design problem in the light of the results.