Since a polyimide and a precursor thereof have excellent durability, heat resistance, flame retardancy, and mechanical and electronic properties, they are frequently used as a base film of a printed circuit board, and a highly integrated semiconductor device or a cover film for highly integrated multilayered circuit board.
Recently, a photosensitive protective film (photoimageable coverlay) for a circuit board has been used in a photolithography process in order to improve in providing a fine circuit pattern and a precise position. In order to be used for a photosensitive protective film for a circuit board, the photosensitive resin composition is required to satisfy certain necessary requirements such as excellent durability, flame retardancy, and mechanical and electronic properties.
In general, the photosensitive protective film for a circuit board is prepared by thermal compressing a liquid or film type of photosensitive resin composition on a circuit of a copper clad laminate (CCL), exposing it to UV along the pattern, developing it with a developing agent, washing, drying, and then performing thermal curing. Then, fine holes required to connect a circuit can be formed precisely at a desired position.
The photosensitive resin composition used for a conventional dry film and the like has been obtained by adding an acrylate to an epoxy resin. However, such resin can be discolored or delaminated from the circuit in a soldering process due to poor flame retardancy and poor soldering heat resistance of the cured resin. Additionally, the resin composition is not suitable for a protective film of the circuit board because its insufficient flexibility and bending resistance easily cause cracks in repeated foldings.
To solve these problems, there is a large need for a polyimide photosensitive resin that has been used as a conventional protective film for circuit pattern and that has high heat resistance, bending resistance, and a good dielectric property. However, the most serious obstacle in using the photosensitive polyimide resin for a photosensitive protecting film of a circuit board is that it is exposed to a high temperature process.
In application of the polyimide photosensitive resin, a polyamic acid, which is a precursor of a polyimide, is used so as to increase the formability and easily express the photosensitivity. However, polyimidization of the polyamic acid requires a curing process that is normally carried out at 300˜350° C. or higher, or at least 250° C.
In the process of preparing a printed circuit board (PCB), the protective film is coated or laminated on a patterned copper circuit at 200° C. or lower because of oxidization and deterioration of the copper circuit. The polyamic acid that requires a high temperature curing process at 250□° C. or higher, has a limit in application in a process of preparing a PCB. Therefore, there is a need for a new method for lowering the temperature for curing the polyamic acid.