Even though printed circuit board routing is a venerable problem in computer-aided design, fully automatic routing of densely packed boards remains an elusive goal. In current industry practice, a program is used to make most connections automatically. The remainder is left for manual completion. This procedure is a poor second to fully automatic routing. It leaves the possibility for introducing errors in the routing of the final connections. More seriously, it is an investment in time and effort that makes subsequent logic changes more difficult. It is always easy to specify a routing problem that is too hard for a program to solve. One need only add wiring to the problem, or remove routing layers. In this sense, designing a completely automatic router is an impossible task. A better program will simply encourage engineers to design harder problems.
The only realistic goal for a routing program is to solve practical layout problems well enough that manual intervention is unnecessary. This report describes the printed circuit board router grr (greedy router). Grr was developed during the construction of the Titan computer [Nielsen 86], a high-performance scientific workstation designed at the Digital Equipment Western Research Laboratory. It was used to route all thirteen boards of the Titan, with run times of 5 to 30 minutes of VAX 11/785 CPU time and no failures.
