Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-07-02
2003-09-16
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S413000, C428S414000, C428S418000, C428S473500, C428S332000, C428S522000, C428S901000, C174S254000, C174S255000, C174S256000, C174S258000, C174S259000
Reexamination Certificate
active
06620513
ABSTRACT:
This application claims priority of Japanese Application No. 2000-200961 filed Jul. 3, 2000.
BACKGROUND OF THE INVENTION
The present invention relates to a base sheet for a flexible printed circuit board or, more particularly, to a multiple-layered sheet material used for the preparation of a flexible printed circuit board.
As is known, a base sheet for a flexible printed circuit board is a multiple-layered laminated sheet material comprising an electrically insulating film of a plastic resin and a metal foil, e.g., copper foil, adhesively bonded to one surface of the insulating plastic resin film either directly or with intervention of an adhesive layer therebetween. A flexible printed circuit board is prepared by patterning the metal foil into the pattern of a desired electric wiring circuit. The patterned metal foil layer is usually protected by integrally laminating with a coverlay film made from a plastic resin film and an adhesive layer.
Along with the trend of various electronic instruments in recent years toward more and more compactness, lighter and lighter weight and higher and higher wiring density, flexible printed circuit boards are also required to be more and more upgraded in their properties and performance. To be in compliance with this trend, for example, flexible printed circuit boards are required to be improved relative to the interlayer adhesion, heat resistance, electric insulation and other properties. Namely, a flexible printed circuit board must have a greatly decreased weight, increased interlayer adhesion and good dimensional stability in order to accomplish finer and finer circuit patterning corresponding to the rapidly progressing recent trend toward a remarkably increased wiring density of the printed circuit board. While it is usual in the prior art that an insulating plastic resin film and a metal foil are laminated by using an adhesive such as epoxy resin/acrylic resin-based adhesives, epoxy resin/NBR-based adhesives and epoxy resin/polyester resin-based adhesives as well as polyimide-based adhesives for an increased fineness of the circuit pattern, development works are now proceeding for a two-layered base sheet for flexible printed circuit boards by directly laminating a polyimide resin film and a copper foil adhesively laminated without intervention of an adhesive layer therebetween.
In each of the above described methods by using an adhesive, one of the most conventional base ingredients in the adhesive is a so-called NBR having functional groups in the molecule because such an NBR-based adhesive exhibits excellent adhesive bonding strength. Disadvantageously, however, such an adhesive is poor in the dimensional stability not to be suitable in a base sheet material of high performance for flexible printed circuit boards when very fine circuit pattern of several tens of micrometers fineness is desired. it is therefore generally accepted heretofore in the base sheet for flexible printed circuit boards that the requirement for an excellent adhesive bonding strength and the requirement for good dimensional stability are incompatible each with the other. In the two-layer type base sheet without intervention of an adhesive layer, besides, a high adhesive bonding strength between layers can be accomplished only by undertaking a complicated surface treatment of the layers and consequently resulting in great difficulties and very high costs for obtaining a high adhesive bonding strength with reproducibility.
SUMMARY OF THE INVENTION
In view of the above described problems and disadvantages in the prior art, the present invention has an object to provide a high-performance three-layer type base sheet for flexible printed circuit boards in which an excellent adhesive bonding strength between layers can be obtained with good reproducibility. The present invention has been completed on the base of an unexpected discovery that, in a three-layered base sheet for flexible printed circuit boards, the most important requirement for obtaining a high-performance base sheet is that each of the layers has a specified thickness.
Thus, the present invention provides a base sheet for flexible printed circuit boards comprising (a) an electrically insulating base film of a plastic resin, (b) a layer of an adhesive formed on one surface of the base film (a) and (c) a foil of copper adhesively bonded to the base film (a) with intervention of the adhesive layer (b), in which the base film (a) has a thickness in the range from 10 to 30 &mgr;m, the adhesive layer (b) has a thickness in the range from 5 to 15 &mgr;m and the copper foil (c) has a thickness in the range from 5 to 15 &mgr;m.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the most characteristic feature in the inventive three-layered base sheet for flexible printed circuit boards is that each of the layers has a thickness within the specified range.
Namely, the electrically insulating film (a) of a plastic resin has a thickness in the range from 10 to 30 &mgr;m. When the thickness of the plastic resin film (a) is too small, a great decrease is caused in the mechanical properties of the three-layered base sheet prepared by using the same resulting in a decrease of the workability in the preparation of the base sheet and circuit patterning. When the thickness of the film (a) is too large, on the other hand, the flexibility of the three-layered base sheet thereof is unduly decreased.
The electrically insulating plastic resin of the film (a) is preferably selected from polyimide resins, polyester resins and polyphenylene sulfide resins of which polyimide resins are more preferable though not particularly limitative thereto. As to the mechanical properties of the plastic resin or the polyimide resin, it is desirable that the plastic resin film has an elastic modulus in the range from 3.0 to 8.0 GPa or, preferably, from 3.8 to 8.0 GPa or, more preferably, from 4.0 to 6.0 GPa. When the elastic modulus of the polyimide resin is too low, the plastic resin film (a) is poor in the dimensional stability eventually to cause undue shrinkage while, when the elastic modulus is too high, the plastic resin film is too rigid and sometimes brittle resulting in a decreased flexibility of the three-layered base sheet. Though optional, the surface of the plastic resin film (a) can be subjected to a surface treatment by utilizing low temperature plasma, corona discharging, sand blasting and the like with an object to improve adhesion with the adhesive layer (b) formed thereon.
A conventional process for the preparation of a polyimide resin film is that a film of a polyamide resin prepared by the reaction of an aromatic dicarboxylic acid dianhydride and an aromatic diamine compound is subjected to a thermal or chemical dehydrating imidation reaction. The mechanical properties including the elastic modulus of the polyimide resin film can be controlled by adequately selecting the combination of the aromatic dicarboxylic acid dianhydride and aromatic diamine compound as the starting reactants. For example, a polyimide resin film having an elastic modulus in the range from 3.8 to 8.0 GPa can be obtained by using pimellitic acid dianhydride as the aromatic dicarboxylic acid dianhydride and a combination of 4,4′-diaminodiphenyl ether and 1,4-phenylene diamine in a weight ratio of 90:10 to 40:60 as the aromatic diamine reactant although any polyimide films produced by a conventional method can be used in the present invention.
Typically, 4,4′-diaminodiphenyl ether and 1,4-phenylene diamine are dissolved in a weight ratio of 90:10 to 40:60 in a polar organic solvent such as N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone to give a solution of a total solid concentration of 5 to 30% by weight and, while the solution is kept at a temperature of 0 to 30° C. under an atmosphere of an inert gas such as nitrogen, the aromatic dicarboxylic acid dianhydride in a stoichiometric amount is added to the solution followed by continued agitation of the solution for 1 to 10 hours to give a sol
Arai Hitoshi
Eguchi Yoshitsugu
Yuyama Masahiro
Dawson Robert
Feely Michael J
Reed Smith LLP
Shin-Etsu Chemical Co. , Ltd.
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