There are provided a resin composition for copper-clad laminates which comprises the following ingredients: (a) an epoxy resin mixture comprising an epoxy resin and a hardener therefor, (b) a maleimide compound, and (c) at least one solvent-soluble aromatic polymer having at least one functional group polymerizable with the epoxy resin or the maleimide compound, and also provided a resin-coated copper foil, a multilayered copper-clad laminate and a multilayered printed circuit board each using the resin composition.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a multilayered printed circuit board
 having excellent heat resistance and to a resin composition for
 copper-clad laminates, a resin-coated copper foil, and a multilayered
 copper-clad laminate each suitable for use in producing the circuit board.
 2. Prior Art
 Laminates for use in producing printed circuit boards in the electronic
 industry are frequently produced by impregnating a glass cloth, craft
 paper, nonwoven glass fabric, or the like with a thermosetting resin such
 as a phenolic or epoxy resin, semicuring the resin to obtain a prepreg,
 and laminating the prepreg on one or each side to a copper foil.
 Furthermore, a multilayered printed circuit board is also produced by
 forming a circuit on each side of the above copper-clad laminate to obtain
 an inner-layer material and cladding the same on each side with a copper
 foil by interposing a prepreg therebetween.
 With the trend toward the high densification of printed circuit boards, the
 formation of so-called via holes, which are minute holes not extending
 through the board thickness, has recently become very common. Such via
 holes are formed by means of a laser beam or plasma processing. Since use
 of a prepreg containing an inorganic ingredient, such as glass fibers, as
 an insulating layer results in poor processability in processing with a
 laser beam or plasma, resins containing no inorganic ingredients are
 frequently used as insulating layers. In this case, there are three
 methods for forming a resin layer using resins containing no inorganic
 ingredients, namely, the resin layer is formed by (1) directly applying a
 liquid resin to an inner circuit, (2) a resin film made of a semicured
 thermosetting resin, or (3) applying a resin to one side of a copper foil
 and semicuring the resin. The thus formed resin layer is laminated to a
 printed circuit board having a circuit (an inner-layer material), before
 the outer-layer copper foil is subjected to circuit formation and via hole
 formation to produce a multilayered printed circuit board.
 The method in which a liquid resin is directly applied onto an inner
 circuit has problems that it is difficult to apply the resin with good
 thickness precision, and that polishing and other steps require much labor
 when a circuit is formed through plating. In the case of using a resin
 film, which is produced by applying a resin composition to a plastic film,
 there is a problem that the plastic film, which is discarded after use, is
 costly. Hence, the method in which a resin-coated copper foil is used is
 more common. As the resin ingredient, an epoxy resin is frequently used.
 Epoxy resins can fully satisfy the property requirements in general
 printed circuit boards because they are excellent in electrical insulating
 properties and chemical resistance. However, epoxy resins have limited
 heat resistance and, hence, there have been cases where they cannot be
 used as a material for printed circuit boards required to have high heat
 resistance.
 The present inventors proposed the use of a resin composition comprising
 the following ingredients as a resin ingredient for a resin-coated copper
 foil (Japanese Patent Application No. 176565/1997).
 (1) An epoxy resin and a hardener therefor, in a content of 40 to 80 parts
 by weight per 100 parts by weight of the total amount
 (2) A maleimide compound in a content of 10 to 50 parts by weight per 100
 parts by weight of the total amount
 (3) A polyvinyl butyral resin having at least one polymerizable double bond
 as a functional group, in a content of 5 to 30 parts by weight per 100
 parts by weight of the total amount
 Due to the above makeup, not only the resin can have greatly improved heat
 resistance, but also the brittleness of the maleimide compound can be
 reduced.
 However, the above resin composition has a problem that it has an increased
 coefficient of thermal expansion at high temperatures because of the use
 of a polyvinyl butyral resin. An increase in the coefficient of thermal
 expansion may cause problems such as cracking in a severe thermal cycle
 test, etc. and position shifting after parts mounting.
 Accordingly, an object of the present invention is to provide a resin
 composition for copper-clad laminates which has a low coefficient of
 thermal expansion and high heat resistance and has extremely high crack
 resistance even upon undergoing a mechanical or thermal shock, thereby
 eliminating the above-described technical problems of prior art
 techniques, and to provide a resin-coated copper foil made by using the
 resin composition.
 Another object of the present invention is to provide a multilayered
 copper-clad laminate and a multilayered printed circuit board both
 obtained using the resin composition for copper-clad laminates, which has
 such high heat resistance and high crack resistance, and using the
 resin-coated copper foil.
 SUMMARY OF THE INVENTION
 The present inventors have made extensive studies in order to eliminate the
 problems described above. As a result, the present invention has been
 achieved which can eliminate the above technical problems of prior art
 techniques by using a resin composition for copper-clad laminates which
 comprises an epoxy resin mixture comprising an epoxy resin and a hardener
 therefor, a maleimide compound not containing a hydroxyl group, and a
 solvent-soluble aromatic polymer having at least one functional group
 polymerizable with the epoxy resin or the maleimide compound.
 The resin composition for copper-clad laminates of the present invention is
 characterized by comprising the following ingredients: (a) an epoxy resin
 mixture comprising an epoxy resin and a hardener therefor, (b) a maleimide
 compound, and (c) at least one solvent-soluble aromatic polymer having at
 least one functional group polymerizable with the epoxy resin or the
 maleimide compound.
 The resin-coated copper foil of the present invention is characterized by
 being obtained by coating a copper foil on one side with the resin
 composition for copper-clad laminates of the present invention as an
 interlaminar insulating resin ingredient for a multilayered printed
 circuit board.
 The multilayered copper-clad laminate of the present invention comprises an
 insulating base layer, an inner circuit formed on one or each side of the
 insulating base layer, and a copper foil serving as a layer for an outer
 circuit and formed outside the inner circuit through an insulating resin
 layer, and is characterized in that the insulating resin layer interposed
 between the inner circuit and the copper foil serving as a layer for an
 outer circuit is a layer formed from the resin composition for copper-clad
 laminates of the present invention.
 The multilayered printed circuit board of the present invention comprises
 an insulating base layer, an inner circuit formed on one or each side of
 the insulating base layer, and an outer circuit formed outside the inner
 circuit through an insulating resin layer, and is characterized in that
 the insulating resin layer interposed between the inner circuit and the
 outer circuit is a layer formed from the resin composition for copper-clad
 laminates of the present invention.
 The resin composition for copper-clad laminates of the present invention
 will be explained below in more detail.
 Of the epoxy resin and the hardener therefor both used in the resin
 composition for copper-clad laminates of the present invention, the epoxy
 resin is not particularly limited. Any kind of epoxy resin may be used as
 long as it is for use as an electrical/electronic material. Examples
 thereof include bisphenol A epoxy resins, bisphenol F epoxy resins,
 novolak epoxy resins, cresol-novolak epoxy resins, tetrabromobisphenol
 resins, and glycidylamine epoxy resins. These epoxy resins may be used in
 combination of two or more thereof.
 The epoxy resin hardener is preferably a so-called latent hardener which
 has low activity at room temperature and cures upon heating, such as,
 e.g., dicyandiamide, imidazole or an analogue thereof, an aromatic amine,
 a phenolic novolak resin, or a cresol-novolak resin, because it is for use
 in producing a resin-coated copper foil. It is preferred to use a curing
 accelerator which accelerates the reaction of the epoxy resin with the
 hardener. Although a tertiary amine, imidazole, or an analogue thereof can
 be used as the curing accelerator, imidazole or its analogue is preferable
 because it functions also as a curing accelerator for the maleimide
 compound.
 The content of the epoxy resin mixture is desirably 40 to 80 parts by
 weight per 100 parts by weight of the whole resin composition. If the
 content thereof is lower than 40 parts by weight, impaired adhesiveness
 results. If the content thereof exceeds 80 parts by weight, the effect of
 improving heat resistance is not expected.
 The maleimide compound is preferably a compound such as
 N,N'-(4,4-diphenylmethane)bismaleimide,
 bis(3-ethyl-5-methyl-4-maleimidophenyl)methane, or
 2,2-bis[4-(4-maleimidophenoxy)phenyl]propane. The content of the maleimide
 compound is desirably 10 to 50 parts by weight per 100 parts by weight of
 the whole resin composition. If the content thereof is lower than 10 parts
 by weight, sufficient heat resistance cannot be ensured. If the content
 thereof exceeds 50 parts by weight, the resin composition gives a cured
 article which is extremely brittle and has significantly impaired crack
 resistance.
 The solvent-soluble aromatic polymer having at least one functional group
 polymerizable with the epoxy resin or the maleimide compound may be a
 polyethersulfone resin, aromatic polyamide resin, polyamide-imide resin,
 or the like which each has at least one phenolic hydroxyl, carboxyl, or
 amino group in the molecule or at a molecular end. These aromatic polymers
 may be used in combination of two or more thereof. Although these resins
 are not particularly limited in Tg (glass transition temperature) or
 molecular weight, they desirably have a Tg of 150.degree. C. or higher in
 order to pass a thermal cycle test in the range of from -65.degree. C. to
 +150.degree. C. Since these resins are used as substitutes for polyvinyl
 acetal resins used for reducing the brittleness of resin compositions,
 they need to have the same tensile strength and film-forming properties as
 the polyvinyl acetal resins.
 The concentration of functional groups polymerizable with the epoxy resin
 or the maleimide compound should be at least one per molecule from the
 standpoint of ensuring sufficient heat resistance. The solvent-soluble
 aromatic polymer having at least one functional group polymerizable with
 the epoxy resin or the maleimide compound is used desirably in an amount
 of 5 to 30 parts by weight per 100 parts by weight of the whole resin
 composition. If the amount thereof is smaller than 5 parts by weight, the
 effect of improving crack resistance cannot be produced. If the amount of
 the aromatic polymer exceeds 30 parts by weight, it has impaired
 compatibility and polymerizability with the epoxy resin and the
 bismaleimide compound.
 Besides the essential ingredients described above, other resin ingredients
 may be added unless this does not depart from the spirit of the present
 invention. Examples of such optional resin ingredients include
 thermosetting polyimide resins, urethane resins, phenolic resins, and
 phenoxy resins. The addition of these resins is effective in improving
 flameproofing properties, resin flowability, etc.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will be explained below by reference to Examples and
 Comparative Example.

EXAMPLE 1
 1-1) Epoxy resin
 An epoxy resin was obtained by blending bisphenol A epoxy resin Epomic
 R-140 (trade name; manufactured by Mitsui Petrochemical Industries Ltd.)
 with o-cresol-novolak epoxy resin Epo Tohto YDCN-704 (trade name;
 manufactured by Toto Kasei K.K.) in a weight ratio of 100:100.
 1-2) Epoxy resin hardener
 Mirex XL-225 (trade name; manufactured by Mitsui Toatsu Chemicals, Inc.)
 was added to the above epoxy resin in a ratio of 1:1.
 1-3) Epoxy resin curing accelerator
 Curezol 2PZ (trade name; manufactured by Shikoku Kasei Co., Ltd.) was added
 in an amount of 1 part by weight to the epoxy resin.
 The epoxy resin, epoxy resin hardener, and epoxy resin curing accelerator
 were dissolved in dimethylformamide in a concentration of 50% to prepare
 an epoxy resin mixture.
 2) Bismaleimide compound
 Bis(3-ethyl-5-methyl-maleimidophenyl)methane was used.
 3) Aromatic polymer having functional group polymerizable with epoxy resin
 or maleimide compound
 A polyethersulfone resin was used which had a hydroxyl equivalent of about
 1,500 and a glass transition temperature of about 230.degree. C.
 These ingredients were mixed in the proportion shown in Table 1 to prepare
 a resin composition.
 TABLE 1
 Proportion
 Epoxy resin mixture 60 parts by weight (on solid
 basis)
 Bismaleimide 20 parts by weight
 compound
 Polyethersulfone 20 parts by weight
 N-Methylpyrrolidone to adjust the solid content of
 the whole composition to 40 wt. %
 The resin composition was applied to a matte surface side of an
 electrolytic copper foil having a thickness of 18 .mu.m. The coating was
 air-dried and then heated at 140.degree. C. for 5 minutes to obtain a
 resin-coated copper foil having a semicured resin layer. The thickness of
 the resin layer was 100 to 105 .mu.m. This resin-coated copper foil was
 heated at 200.degree. C. for 4 hours at ordinary pressure. After cooling,
 the copper foil was removed by etching to obtain a cured resin film.
 An inner-layer material FR-4 having a prescribed circuit (core thickness,
 0.5 mm; copper foil thickness, 35 .mu.m) was sandwiched between two sheets
 of the resin-coated copper foil in such a manner that the resin layer of
 each resin-coated copper foil was in contact with a surface of the
 inner-layer material. The resultant assemblage was hot-pressed at 20
 kgf/cm.sup.2 and 200.degree. C. for 4 hours to obtain a multilayered
 copper-clad printed circuit board having four copper foil layers.
 EXAMPLE 2
 A resin-coated copper foil, a cured resin film, and a multilayered printed
 circuit board were obtained in the same manner as in Example 1, except
 that an aromatic polyamide resin having a hydroxyl equivalent of 8,000 and
 a Tg of 203.degree. C. was used in place of the polyethersulfone used in
 Example 1.
 COMATIVE EXAMPLE 1
 1-1) Epoxy resin
 An epoxy resin was obtained by blending bisphenol A epoxy resin Epomic
 R-140 (trade name; manufactured by Mitsui Petrochemical Industries Ltd.)
 with o-cresol-novolak epoxy resin Epo Tohto YDCN-704 (trade name;
 manufactured by Toto Kasei K.K.) in a weight ratio of 100:100.
 1-2) Epoxy resin hardener
 Mirex XL-225 (trade name; manufactured by Mitsui Toatsu Chemicals, Inc.)
 was added to the above epoxy resin in a ratio of 1:1.
 1-3) Epoxy resin curing accelerator
 Curezol 2PZ (trade name; manufactured by Shikoku Kasei Co., Ltd.) was added
 in an amount of 1 part by weight to the epoxy resin.
 The epoxy resin, epoxy resin hardener, and epoxy resin curing accelerator
 were dissolved in dimethylformamide in a concentration of 50% to prepare
 an epoxy resin mixture.
 2) Bismaleimide compound
 Bis(3-ethyl-5-methyl-4-maleimidophenyl)methane was used.
 3) Polyvinyl acetal resin having functional group polymerizable with epoxy
 resin or maleimide compound
 A carboxylated polyvinyl acetal resin was used which had a degree of
 acetalization of 80, an acetaldehyde/butyraldehyde ratio of 50/50 (by
 mol), a hydroxyl concentration of 17 wt. %, and a carboxyl concentration
 of 1 wt. % and which had been obtained from a polyvinyl alcohol having a
 degree of polymerization of 2,400.
 These ingredients were mixed in the proportion shown in Table 2 to prepare
 a resin composition.
 TABLE 2
 Proportion
 Epoxy resin mixture 60 parts by weight (on solid
 basis)
 Bismaleimide 20 parts by weight
 compound
 Polyvinyl acetal 20 parts by weight
 resin
 Methyl ethyl ketone to adjust the solid content of
 the whole composition to 30 wt. %
 The resin composition was applied to a matte surface side of an
 electrolytic copper foil having a thickness of 18 .mu.m. The coating was
 air-dried and then heated at 120.degree. C. for 5 minutes to obtain a
 resin-coated copper foil having a semicured resin layer. The thickness of
 the resin layer was 100 to 105 lam. This resin-coated copper foil was
 heated at 200.degree. C. for 4 hours at ordinary pressure. After cooling,
 the copper foil was removed by etching to obtain a cured resin film.
 An inner-layer material FR-4 having a prescribed circuit (core thickness,
 0.5 mm; copper foil thickness, 35 .mu.m) was sandwiched between two sheets
 of the resin-coated copper foil in such a manner that the resin layer of
 each resin-coated copper foil was in contact with a surface of the
 inner-layer material. The resultant assemblage was hot-pressed at 20
 kgf/cm.sup.2 and 200.degree. C. for 4 hours to obtain a multilayered
 copper-clad printed circuit board having four copper foil layers.
 The resin films and multilayered printed circuit boards produced in
 Examples 1 and 2 and Comparative Example 1 were evaluated for the
 following properties.
 (1) Resin film
 (i) Measurement of Tg (glass transition temperature) with an apparatus for
 measuring dynamic viscoelasticity.
 (2) Multilayered printed circuit board
 (i) Measurement of the coefficient of thermal expansion at 150 to
 200.degree. C. by TMA.
 (ii) Crack resistance was evaluated by forming a copper circuit having a
 line width of 100 .mu.m and via holes each having a via hole diameter of
 150 .mu.m and a land diameter of 350 .mu.m in an outer-layer copper foil
 of each circuit board by etching, subsequently subjecting the circuit
 board to either a thermal shock test consisting of 300 cycles each ranging
 from -50.degree. C. (30 minutes) to +125.degree. C. (30 minutes) (thermal
 shock resistance A) or a thermal shock test consisting of 1,000 cycles
 each ranging from -65.degree. C. (30 minutes) to +150.degree. C. (30
 minutes) (thermal shock resistance B), and then examining a section
 thereof for resin cracks.
 (iii) Oil-dip heat resistance test consisting of 100 cycles each including
 10-second immersion in a 260.degree. C. oil and air cooling.
 The results obtained in the above tests are given in Tables 3 and 4.
 These test results show that in the resin-coated copper foils of the
 present invention, the resin layers had high heat resistance and low
 coefficients of thermal expansion. The multilayered printed circuit boards
 produced using these copper foils were satisfactory in heat resistance and
 thermal shock resistance.
 TABLE 3
 Coefficient of
 thermal expansion
 Tg (.degree. C.) (ppm/.degree. C.)
 Example 1 225 57
 Example 2 203 55
 Comparative 186 80
 Example 1
 TABLE 4
 Thermal Thermal Oil-dip
 shock shock heat
 resistance resistance resistance
 A B C
 Example 1 .circleincircle. .circleincircle.
 .largecircle.
 Example 2 .circleincircle. .circleincircle.
 .largecircle.
 Comparative .largecircle. X .largecircle.
 Example 1
 .circleincircle.: excellent,
 .largecircle.: no cracks in resin layer,
 X: microcracks in resin layer
 [Effects of the Invention]
 The resin-coated copper foil of the present invention can be produced by
 applying the resin composition of the present invention, obtained by the
 method described above and having excellent resistance to heat and
 cracking, to one side of a copper foil as an interlaminar insulating resin
 ingredient for a multilayered printed circuit board.
 The multilayered copper-clad laminate of the present invention, which
 comprises an insulating base layer, an inner circuit formed on one or each
 side of the insulating base layer, and a copper foil serving as a layer
 for an outer circuit and formed outside the inner circuit through an
 insulating resin layer, can be produced by using the resin composition of
 the present invention, obtained by the method described above and having
 excellent resistance to heat and cracking, to form the insulating resin
 layer interposed between the inner circuit and the copper foil serving as
 a layer for an outer circuit.
 Furthermore, the multilayered printed circuit board of the present
 invention, which comprises an insulating base layer, an inner circuit
 formed on one or each side of the insulating base layer, and an outer
 circuit formed outside the inner circuit through an insulating resin
 layer, can be produced by using the resin composition of the present
 invention, obtained by the method described above and having excellent
 resistance to heat and cracking, to form the insulating resin layer
 interposed between the inner circuit and the outer circuit.