To date, almost all FPC manufacturing processes have been processed using the subtractive method (etching method). According to Carbon Technology, copper-clad laminate is usually used as the starting material, and a resist layer is formed using photolithography, and the unwanted copper surface is etched away to form a circuit conductor. Due to problems such as side etching, the etching method has processing limitations for fine circuits.
Based on the difficulty of processing with the subtractive method or the difficulty of maintaining a high pass rate for fine circuits, people believe that the semi-additive method is an effective method, and people have proposed various semi-additive method solutions. An example of fine circuit processing using the semi-additive method. The semi-additive process uses polyimide film as the starting material. First, a liquid polyimide resin is cast (coated) on an appropriate carrier to form a polyimide film. Then, a sputtering method is used to form a crystal planting layer on the polyimide base film, and then a photolithography method is used to form an anti-etching layer pattern of the reverse pattern of the circuit on the crystal planting layer, which is called a plating resistance layer. The conductor circuit is formed by electroplating in the blank part. Then the anti-etching layer and unnecessary crystal planting layer are removed to form the first layer of circuit. Photosensitive polyimide resin is coated on the first layer of circuit, and holes, a protective layer or an insulating layer for the second layer of circuit layer are formed by photolithography, and then a crystal planting layer is sputtered thereon as the base conductive layer of the second layer of circuit. Repeating the above process can form a multi-layer circuit. According to Carbon Technology, this semi-additive method can be used to process ultra-fine circuits with a pitch of 5um and a through hole of 10um. The key to using the semi-additive method to make ultra-fine circuits lies in the performance of the photosensitive polyimide resin used as the insulating layer.
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