| This paper introduces the general developmental situation of the Computer-Aided Design (CAD) and the class and primary structure of the circuit CAD software. The developmental situation and class of the auto routing are also described, the theory of auto routing is studied synthetically. Compared grid routing and gridless routing, an efficient and fast gridless line-search routing algorithm - target-oriented line search routing method for automatic circuit layout - is proposed.The algorithm can be applied to connection routing in PCBs with various pad geometry and pad sizes, with different wire widths and variable distances between wires. The basic element in routing is the search line with a given destination. The main features of the router are the target orientation of the search lines, the routing efficiency is guaranteed by rationally designed data structures and by the ability of searching around obstacles, the small amount of working storage and the reasonably short routing time.According to the design structure of circuit CAD software, data are putted into the routing program by text file and are stored in data chain. In order to improve the routing paths, the refinements of routing are studied preliminarily.
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