A high-density fine line structure mainly includes: two boards with similar structures and a dielectric film for combing the two boards. Semiconductor devices respectively in two boards are opposite to each other after the two boards are combined. The two boards each include a fine line circuit, an insulated layer on the same surface, and the semiconductor device installed above the fine line circuit. The surface of the circuit, which is not covered by a solder mask, is made into a pad. The pad is filled with the tin balls for electrically connecting with another semiconductor device. Electroplating rather than the etching method is used for forming the fine line circuit layer, and a carrier and a metal barrier layer, which are needed during or at the end of the manufacturing process, are removed to increase the wiring density for realizing the object of high-density.