The orientation of modules based on graph decomposition

In the layout stage of VLSI and printed circuit board (PCB) design, after all circuit modules (rectangular) are placed, it is possible to flip the modules so as to reduce the total net length. The authors formulate the orientation of modules as a graph problem and prove it to be NP-complete. The orientation problem is shown to be equivalent to finding a minimum cut of a graph with some arcs of negative capacities. In many cases, the graph can be decomposed into subgraphs to reduce the search space for optimum orientation. Experiments with real cases show that module orientation reduces the total net length and improves the routability