Patent ID: 6346298
Filing Date: 2002-02-12
Classification: H05K,Y10T

Abstract:
A method of forming a flexible board, comprising:forming a first polyimide precursor film over a metal foil, the first polyimide precursor film having a content of residual volatile fractions within the range of 20 to 30 wt %; forming a second polyimide precursor film over the first polyimide precursor film, the second polyimide precursor film having a content of residual volatile fractions within the range of 30 to 50 wt %; forming a third polyimide precursor film over the second polyimide precursor film, the third polyimide precursor film having a content of residual volatile fractions within the range of 30 to 50 wt %; and heat treating the first polyimide precursor film, the second polyimide precursor film and the third polyimide precursor film at a temperature of 210Â° C. to 250Â° C. to set the content of residual volatile fractions of the first polyimide precursor film, the second polyimide precursor film and the third polyimide precursor film at 7 to 10 wt % and to bring the imidation ratio of the first polyimide precursor film, the second polyimide precursor film and the third polyimide precursor film to 50% or lower, to form a corresponding first polyimide-based layer, second polyimide-based layer and third polyimide-based layer, wherein said flexible board satisfies the equation k1>k3>k2 where k1 is the coefficient of linear thermal expansion of the first polyimide-based resin layer on the metal foil side, k2 is the coefficient of linear thermal expansion of the second polyimide-based resin layer, and k3 is the coefficient of linear thermal expansion of the third polyimide-based resin layer.