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56 The PCB Design Magazine • November 2015 the PCB database. This improved database in- tegrity dramatically. PCB routers were developed using either the grid-based, gridless, shape-based or geo- metrical approaches. The first were maze and line searching routers that use an imaginary gridded workspace, while a gridless router uses a workspace with available polygon areas to ac- commodate the new paths. In a shape-based router, each entity on the board is represented as polygonal geometry with no reference to a specific routing grid. This enables the router to cope easily with boards in which there are SMT devices and fine-pitch BGAs with a variety of pitches and odd shapes. Also, unlike a grid- based router, a shape-based autorouter does not have to work at a particular resolution, so routing of high-density or fine-pitch boards is not significantly slower than for lower density work. Put another way, routing time depends only on the available memory, the number of objects on the boards and on the number of connections to be routed. Later, topology rout - ers allowed designers to plan the strategy for a set of nets with attributes to define routing lay- ers, bias and rules. The first autorouters were not very capable, limited by computing power and lack of mem- ory. They added too many vias, wasted space due to the strict XY bias, and the quality was poor compared to manual routing. I recall that I used to set up our Advanced Technology De- signer Star router to run on the MicroVax main- frame over the weekend, only to find it 50% complete by Monday morning. However, auto- routers evolved, like all technology, to include angle routes, reducing vias, push-and-shove al- gorithms, rip-up and retry, spreading and gloss passes. But so also has interactive routing. Probably the most popular shape-based rout- er, 20 years ago, was Cooper & Chyan Technol- ogy's Specctra router. The Specctra router was used by many PCB layout tools and interfaces to the router still exist today. Design constraints and routing strategies were setup in a "do file" which contained the sequence of commands. The routing was not graphically visible but the routing status was indicated and updated. Ca- dence's Specctra for OrCAD is still available to- day. In the mid 1990s, Intergraph Electronics (VeriBest Inc.) came up with arguably the best routing technology still available today. Mentor Graphics has made considerable improvements to the router since acquiring this technology, and it is now available in both the Xpedition and PADS flows. They provide a selection of routing tools with each optimized to perform a particular function. The fanout of a high-pin-count BGA is the beyond design WHY AuTOROuTERS DON'T WORk: THE MINDSET! Figure 2: Cross-probing between schematic and PCB.

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