Flexible multi-layer polyimide film laminates and preparation thereof

Abstract: Flexible multilayer polyimide metal-clad laminates, and their preparation, having at least one layer of aromatic polyimide bonded to at least one layer of a metallic substrate using a heat-sealable copolyimide adhesive containing repeating imide units derived from 4,4'-oxydiphthalic dianhydride and an aromatic ether diamine. The laminates are used in flexible printed circuits and tape automated bonding applications. Additionally, the metallic substrate may be directly coated with the copolyimide adhesive and used as a single-clad laminate for flexible printed circuits.

Description: BACKGROUND OF THE INVENTION This invention relates to flexible multilayer polyimide metal-clad laminates comprising at least one layer of an aromatic polyimide and at least one layer of a metallic substrate for use in flexible printed circuits and tape automated bonding applications. The invention also relates to a process for producing these laminates. Laminates comprising one or more layers of polyimide and one or more layers of metallic substrate material may be used for a variety of applications. For example, polyimide coated metal foils, due to the flexibility and outstanding mechanical, thermal and electrical properties of polyimides, can be used for printed electrical circuits. This is because the laminates are frequently exposed to high temperatures during further processing, for example, during soldering or drilling. The laminates also have to satisfy stringent requirements in regard to their electrical and mechanical properties. Laminates comprising only one substrate layer of metal or metal alloy and a layer of polyimide, so called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so called multi-clads or multilayer circuits, which comprise several metal layers and/or several polyimide layers. Laminates containing polyimides and metal substrates ore well-known in the art. Usually the polyimide layers are bonded to the metal substrate by a conventional adhesive. For example, U.S. 3,900,662 and U.S. 3,822,175 disclose bonding of polyimide to metal using an acrylate-based adhesive. However, it has been found that when conventional adhesives such as acrylates, epoxides, polyamides, phenolic resins etc. are used to bond the polyimide to the metal, the resulting laminates do not exhibit entirely satisfactory properties which meet the stringent demands often imposed. Conventional adhesives do not generally possess the high temperature heat stability of the polyimide material itself, and the strength of the adhesive bonds in multilayer laminar polyimide structures deteriorates rapidly when subjected to elevated temperatures. On account of the disadvantages of laminates comprising layers of conventional adhesives between polyimide and metal, multilayer laminates have been proposed in which the polyimide is bonded directly to metal, i.e. without a layer of adhesive. Thus, British Patent 2,101,526 discloses the bonding of a polyimide derived from biphenyltetracarboxylic dianhydride directly to metal foil by applying heat and pressure. In other words, the polyimide is formable. It has been found, however, that such formable polyimides have inferior thermal stability to conventional non-formable polyimides. Further, EP-A-0 276 922 discloses a crystalline polyimide polymer containing at least 30 mol% of repeating imide units of the formula: (in which X is a single bond, an oxygen or sulphur atom or a group -CO- or -SO₂-), optionally together with one or more other repeating imide units; the polymer having an inherent viscosity of from 0.1 to 5 dl/g as measured in a concentration of 0.5g/dl in concentrated sulphuric acid at a temperature of 30 ± 0.01 °C and EP-A-0 189 643 discloses an aromatic polyamic acid solution composition comprising an aromatic polyamic acid dissolved in an amount of 5 to 40% by weight in an organic polar solvent, said aromatic polyamic acid being prepared by polymerizing substantially equimolar amounts of an aromatic tetracarboxylic acid component comprising, based on the total amount of the aromatic tetracarboxylic acid component, 15 to 85 mole% of a biphenyltetracarboxylic acid or a functional derivative thereof and 15 to 85 mole% of a pyromellitic acid or a functional derivative thereof and an aromatic diamine component comprising, based on the total amount of the aromatic diamine component, 30 to 100 mole% of a phenylenediamine and 0 to 70 mole% of a diaminodiphenyl ether. The object of this invention is to provide flexible polyimide metal-clad laminates, which withstand high temperatures, have good adhesion, good thermal, mechanical and electrical properties and are chemically etchable (for TAB applications). SUMMARY OF THE INVENTION According to the present invention there is provided a flexible polyimide metal-clad laminate suitable for use in a flexible printed circuits and tape automated bonding comprising at least one layer of a metallic substrate and at least one layer of an aromatic polyimide, said layer of polyimide being bonded on at least one side to said layer of metallic substrate with a peel strength of at least 71.4 kg/m (4 pli) through an adhesive layer of a heat-sealable copolyimide comprising at least 30 mole % of repeating imide units of the formula and less than 30 mole % of other and different repeating imide units of the formula wherein R is an aromatic tetravalent organic radical and R' is a divalent radical of an aromatic or aliphatic diamine containing at least two carbon atoms, the two amino groups of said diamine each being attached to a separate carbon atom of said divalent radical. A further embodiment of the invention relates to a polyimide laminate comprising a layer of a metallic substrate coated on one or both sides with the aforesaid copolyimide adhesive suitable for use as a single-clad laminate for flexible printed circuits. A still further embodiment of the invention relates to a process for preparing an adherable all-polyimide laminate for use in making metal-clads comprising coating a copolyamic acid adhesive directly either on a fully cured polyimide base film, or on a partially cured polyimide gel or green film or by coextruding the copolyamic acid with the polyimide base film and then curing to form the copolyimide. DETAILED DESCRIPTION OF THE INVENTION The polyimide film metal-clad laminates of the present invention comprise at least one layer of a polyimide base film which on at least one of its two sides is bonded to a metal using a heat-sealable copolyimide adhesive as specifically defined hereinafter. Alternatively, the copolyimide adhesive may be directly bonded to one or both sides of the metal substrate. The polyimide base films used in the laminates of the invention are preferably 7.6 - 127 µm (0.3 to 5 mils) in thickness and can be obtained from polyamic acid precursors derived from the reaction of suitable diamines with suitable dianhydrides in the manner described in, for example, U.S. 3,179,614. Dianhydrides which can be used in the polyimide base film include: pyromellitic dianhydride; 3,4,9,10-perylene tetracarboxylic dianhydride; naphthalene-2,3,6,7-tetracarboxylic dianhydride; naphthalene-1,4,5,8-tetracarboxylic dianhydride; bis(3,4-dicarboxyphenyl) ether dianhydride; bis(3,4-dicarboxyphenyl) sulfone dianhydride; 2,3,2',3'-benzophenonetetracarboxylic dianhydride; bis(3,4-dicarboxyphenyl) sulfide dianhydride; bis(3,4-dicarboxyphenyl) methane dianhydride; 2,2-bis(3,4-dicarboxyphenyl) propane dianhydride; 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane; 3,4,3',4'-biphenyltetracarboxylic dianhydride; 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; phenanthrene-1,8,9,10-tetracarboxylic dianhydride; pyrazine-2,3,5,6-tetracarboxylic dianhydride; benzene-1,2,3,4-tetracarboxylic dianhydride; and thiophene-2,3,4,5-tetracarboxylic dianhydride. Diamines which can be used together with the dianhydrides in the polyimide base film include the following: meta-phenylenediamine; para-phenylenediamine; 2,2-bis(4-aminophenyl) propane; 4,4'-diaminodiphenylmethane; 4,4'-diaminodiphenyl sulfide; 4,4'-diaminodiphenyl sulfone; 3,3'-diaminodiphenyl sulfone; 4,4'-diaminodiphenyl ether; 2,6-diaminopyridine; bis(3-aminophenyl) diethyl silane; benzidine; 3,3'-dichlorobenzidine; 3,3'-dimethoxybenzidine; 4,4'-diaminobenzophenone; N,N-bis(4-aminophenyl)-n-butylamine; N,N-bis(4-aminophenyl) methylamine; 1,5-diaminonaphthalene; 3,3'-dimethyl-4,4'-diaminobiphenyl; m-aminobenzoyl-p-aminoanilide; 4-aminophenyl-3-aminobenzoate; N,N-bis(4-aminophenyl) aniline; 2,4-bis(beta-amino-t-butyl) toluene; bis(p-beta-amino-t-butylphenyl) ether; p-bis-2-(2-methyl-4-aminopentyl) benzene; p-bis(1,1-dimethyl-5-aminopentyl) benzene; m-xylylenediamine; p-xylylenediamine; position isomers of the above, and mixtures thereof. The preparation of polyimides and polyamic acids is more fully described in U.S. Pat. No. 3,179,614 and U.S. Pat. No. 3,179,634. A particularly preferred polyimide base film is derived from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. The heat-sealable copolyimide adhesives used in the present invention may be either random or block and contain at least 30 mole % of repeating imide units of the formula and less than 30 mole % of other repeating imide units of the formula wherein R is an aromatic tetravalent organic radical and R' is a divalent radical of an aromatic or aliphatic diamine containing at least two carbon atoms, the two amino groups of the diamine each being attached to separate carbon atoms of the divalent radical. The heat-sealable copolyimide adhesive contains at least 30 mole %, preferably from 40 to 80 mole % and, most preferably, from 70 to 80 mole % of imide units derived from 4,4'-oxydiphthalic dianhydride (ODPA) and an aromatic ether diamine of the formula If the percentage of such imide units is lower than 30 mole %, it is difficult to obtain copolyimide adhesives having glass transition temperatures less than 250°C and good adhesion on the substrates. Representative aromatic ether diamines include: 1,2-bis(4-aminophenoxy)benzene 1,3-bis(4-aminophenoxy) benzene 1,2-bis(3-aminophenoxy)benzene 1,3-bis(3-aminophenoxy) benzene 1-(4-aminophenoxy)-3-(3-aminophenoxy) benzene 1,4-bis(4-aminophenoxy) benzene 1,4-bis(3-aminophenoxy) benzene 1-(4-aminophenoxy)-4-(3-aminophenoxy) benzene The addition of up to 30 mole %, preferably from 2 to 25 mole %, and most preferably from 5 to 15 mole % of additional imide units of the formula wherein R and R' are as previously defined above, to the copolyimide adhesive provides the requisite low glass transition temperature (<250°C) and thermal stability of adhesion properties [> 71.4 kg/m (>4 pli)] without reducing the good thermal, mechanical and electrical properties characteristic of the polyimide. Such additional imide units may be derived from dianhydrides and diamines which are the same or different from the 4,4'-oxydiphthalic dianhydride and aromatic ether diamines previously defined. Particularly preferred dianhydrides and diamines include the following: pyromellitic dianhydride; 4,4'-oxydiphthalic dianhydride; 3,3'4,4'-benzophenone tetracarboxylic dianhydride; 2,2',3,3'-benzophenone tetracarboxylic dianhydride; 3,3'4,4'-biphenyl tetracarboxylic dianhydride; 2,2',3,3'-biphenyl tetracarboxylic dianhydride; 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride; bis(3,4-dicarboxyphenyl) sulfone dianhydride; bis(3,4-dicarboxyphenyl) sulfide dianhydride; bis(2,3-dicarboxyphenyl) methane dianhydride; bis(3,4-dicarboxyphenyl) methane dianhydride; 1,1-bis(2,3-dicarboxyphenyl) ethane dianhydride; 1,1-bis(2,3-dicarboxyphenyl) propane dianhydride; 2,2-bis(3,4-dicarboxyphenyl) propane dianhydride; m-phenylene bis(trimellitate)dianhydride; hexamethylene diamine; heptamethylenediamine; 3,3'-dimethylpentamethylenediamine; 3-methylhexamethylenediamine; 3-methylheptamethylenediamine; 2,5-dimethylhexamethylenediamine; octamethylenediamine; nonamethylenediamine; 1,1,6,6-tetramethylhexamethylenediamine; 2,2,5,5-tetramethylhexamethylenediamine; 4,4-dimethylheptamethylenediamine; decamethylenediamine; meta-phenylenediamine; 4,4'-diaminobenzophenone; 4-aminophenyl-3-aminobenzoate; m-aminobenzoyl-p-aminoanilide; 4,4'-diaminodiphenylether; 3,4'-diaminodiphenylether; bis(4-aminophenyl)methane; 1,1-bis(4-aminophenyl) ethane; 2,2-bis(4-aminophenyl) propane; 4,4'-diaminodiphenyl sulfoxide; 3,3'-diaminobenzophenone; 1,3-bis(4-aminophenoxy)benzene; 2,2'-diaminobenzophenone; 1,2-bis(4-aminophenoxy)borone; 1,3-bis(4-aminobenzoyloxy)benzene; 4,4'-diaminobenzanilide; 4,4'-bis(4-aminophenoxy)phenyl ether; 2,2'-bis(4-aminophenyl)hexafluoropropane; 2,2-bis(4-aminophenyl)-1,3-dichloro-1,1,3,3-tetrafluoropropane; 4,4'-diaminodiphenyl sulfone; 1,12-diaminododecane; 1,13-diaminotridecane; polysiloxane diamine of the formula wherein R₄ is C₁₋₃ alkylene, aralkylene or phenylene, R₅ is C₁₋₃ alkyl or phenyl, R₆ is C₁₋₃ alkyl or phenyl and m is 1 to 100. Particularly preferred copolyimide adhesives of the invention contain from 70 to 95 mole % of oxydiphthalic dianhydride, 5 to 30 mole % of pyromellitic dianhydride and 100 mole % of 1,3-bis(4-aminophenoxy)benzene and from 80 to 95 mole % of 1,3-bis(4-aminophenoxy)benzene, 5 to 20 mole % of hexamethylene diamine and 100 mole % of oxydiphthalic dianhydride. The copolyimide adhesives of the present invention exhibit excellent physical properties including an elastic modulus of under 27.6 x 10³ N/mm² (400 kpsi)to provide the necessary conformability, a low coefficient of thermal expansion (<100 ppm), dielectric constant (<3.5), dielectric strength (98.4-137.8 V/mm (2.5-3.5 volts/mil)), dissipation factor (0.001-0.01), water absorption (<2.5%) and solder resistance. Chemical etchability of the copolyimide adhesive can be obtained by chemically converting the precursor polyamic acid using conversion chemicals such as acetic anhydride and beta-picoline. The chemically converted copolyimide adhesive, when coated on chemically converted polyimide base film, provides a chemically etchable film laminate which can be used in TAB applications. Chemical etchability can also be achieved by incorporating ester functionality in the adhesive backbone using a diester diamine (RDEDA) or a diester dianhydride (MPBTDA) as comonomer. Ideally the adhesives are cured at temperatures ranging from 150°C to 400°C by raising the temperature in increments of 5°C/min to 300°C over 90 minutes and heating at 400°C for 5 minutes and then laminated at 350°C to obtain high bond strengths. Higher cure temperatures tend to reduce the bondability of the adhesive. Longer residence times at high temperatures also reduce the bond strength. For example, the adhesive ODPA/PMDA:RODA (80/20:100) when coated on polyimide green film and cured at 150-300°C and laminated to copper foil at 350°C for 20 min and 10.3 bar (150 psi) pressure gave a peel strength of 214,3 kg/m (12 pli). When the same film is cured at 400°C for an additional 5 minutes and laminated at 350°C, the peel strength dropped to 178,6 kg/m (10 pli). Longer than 10 minutes curing at 400°C may cause reduced bond strengths. The polyimide laminates of the present invention can be prepared by applying a thin coating of a copolyamic acid precursor of the desired copolyimide described above to the polyimide base film or directly to a metallic substrate and thereafter treating the coated polyimide base film or coated metallic substrate to completely convert the polyamic acid precursor to polyimide. Coating thicknesses range from 2.54 to 50.8 µm (0.1 to 2 mil). The base polyimide film will of course be different from the coating polyimide. The copolyamic acid precursors are prepared by reacting approximately equimolar amounts of the aforesaid dianhydrides and diamines in an inert organic solvent such as N,N-dimethylacetamide or N-methylpyrrolidone. Other organic solvents such as N,N-dimethylformamide, dimethyl sulfoxide, xylene, toluene and tetrahydrofuran can also be used as cosolvents. The polyamic acids can be converted to polyimides by either a thermal conversion process or a chemical conversion process using a dehydrating agent and a catalyst to give distinct products having different adhesive characteristics. Copolyimide adhesive coatings can be applied to fully cured polyimide base film or to one of the intermediate manufacturing stages such as to "gel" film or to "green" film. The term "gel" film refers to a sheet polyimide material which is laden with volatiles, primarily solvent, to such an extent that the polyimide is in a gel-swollen, plasticized, rubbery condition. The volatile content is usually in the range of 80 to 90 % by weight and the polymer content usually in the range of 10 to 20 % by weight of the gel film. The film becomes self-supporting in the gel film stage and can be stripped from the support on which it was cast and heated. The gel film generally has an amic acid to imide ratio between 90:10 and 10:90. The gel film structure can be prepared by the method described in U.S. 3,410,826 by mixing a chemical converting agent and a catalyst, such as a lower fatty acid anhydride and a tertiary amine, respectively, into the polyamic-acid solution at a low temperature, followed by casting the polyamic-acid solution in film-form, on a casting drum and mildly heating the cast film at, for example, 100°C to activate the conversion agent and catalyst for transforming the cast film to a polyamic acid-polyimide gel film. Another type of polyimide base film which can be coated with the copolyimide adhesive is "green film" which is all polyamic acid or which has a very low polyimide content. Green film contains generally about 50 to 75 % by weight polymer and 25 to 50 % by weight solvent and is sufficiently strong to be self-supporting. Green film can be prepared by casting the polyamic acid solution into film form onto a suitable support such as a casting drum or belt and removing the solvent by mild heating at up to 150°C. A low proportion of amic acid units in the polymer, e.g., up to 25%, may be converted to imide units. Application of the copolyamic acid coating can be accomplished in any number of ways, such as by slit die, dipping or kiss-roll coating, followed by metering with doctor knife, doctor rolls, squeeze rolls or air knife. It may also be applied by brushing or spraying. Using such techniques, it is possible to prepare both one- and two-side coated structures. In preparation of the two-side coated structures, one can apply the coatings to the two sides either simultaneously or consecutively before going to the curing and drying stage. In another embodiment, the polyamic acid adhesive solution can be directly extruded onto one or both sides of the polyamic acid base film and the polyamic acid layer(s) subsequently cured by heat treatment. In a further embodiment, the polyamic acid adhesive may be coated on a fully cured polyimide base film or directly on a metal substrate and subsequently imidized by heat treatment. The polyimide base film may be prepared by either a chemical or thermal conversion process and may be surface treated, e.g. by chemical etching, corona treatment, laser etching etc., to improve adhesion. A single polyimide metal-clad of the present invention comprises a flexible copolyimide layer which adheres to a metal foil such as copper, aluminum, nickel, steel or an alloy containing one or more of these metals as a substantial constituent, or to a foil of amorphous metal. The copolyimide layer adheres firmly to the metal and has a high peel strength of 4 pli and higher. The metals do not have to be used as elements in pure form, i.e. it is also possible to use substrates of metal alloys, such as alloys containing nickel, chromium or iron or nickel and copper, or of amorphous alloys containing iron. Particularly suitable metallic substrates are foils of rolled, annealed or electrodeposited copper or rolled, annealed copper alloy. In many cases, it has proved to be of advantage to pretreat the metallic substrate before coating. The pretreatment may consist of a chemical treatment or a mechanical roughening treatment. It has been found that this pretreatment enables the adhesion of the copolyimide layer and, hence, the peel strength to be further increased. Apart from roughening the surface, the chemical pretreatment may also lead to the formation of metal oxide groups, enabling the adhesion of the metal to the copolyimide layer to be further increased. It has been found that approximately a 3.57 kg/m (0.2 pli) increase in peel strength results for each 25 nm microinch increase of surface roughness. A polyimide multi-clad of the present invention comprising a double side copper clad can be prepared by laminating copper foil to both sides of an adhesive coated dielectric polyimide film. The construction can also be made by laminating adhesive coated copper foil to both sides of a dielectric polyimide film or to an adhesive coated dielectric polyimide film. The advantageous properties of this invention can be observed by reference to the following examples which illustrate, but do not limit, the invention. All parts and percentages are by weight unless otherwise indicated. EXAMPLES 1-30 Polyamic acid solutions were prepared by reacting the appropriate molar equivalents of the monomers in dimethylacetamide (DMAC) solvent. Typically, the diamine(s) (0.05 mole) dissolved in DMAC were stirred under nitrogen and the dianhydride(s) (0.05 mole) were added as solid over a period of several minutes. Stirring was continued to obtain maximum viscosity of the copolymer. The viscosity was adjusted by controlling the amount of dianhydride in the polyamic acid composition. The copolyamic acids were coated on a fully cured corona treated polyimide base film derived from pyromellitic dianhydride and 4,4'-diaminodiphenyl ether and converted to copolyimide either by a thermal conversion process or by a chemical conversion process using acetic anhydride as dehydrating agent and betapicoline as a catalyst to provide distinct products having different adhesive characteristics. The copolyamic acids were coated on the base polyimide film using a coating bar to a thickness of 12.7 µm (0.5 mils)and the solvent removed by heating at 80°C for 20 minutes. The coated polyimide films were placed on a pin frame and Examples 1 to 19 were cured at 160 to 220°C and Examples 20 to 30 were cured at 160 to 260°C for 90 minutes. The coated polyimide films were subsequently laminated to roll-annealed copper at temperatures of 250°C, 300°C and 350°C to form the copper-clads. Roll clads could also be made by continuous lamination of the adhesive coated dielectric film to copper foil using a high temperature double belt press or a high temperature nip roll laminator. Peel strength results of the, polyimide copper-clad laminates are given in Tables I and II. PEEL STRENGTH OF THERMALLY CONVERTED ADHESIVE ON CORONA TREATED 2 mil PMDA/ODA POLYIMIDE FILM Example No. Adhesive Composition (mole %) Peel Strength (pli; 1 pli ≙ 17.86 kg/m) 250°C 300°C 350°C Tg(°C) ODPA/PMDA:RODA 20 80/20:100 7.7 6.5 6.3 225 ODPA/PMDA:RODA/SiODA 21 80/20:95/5 b 9.0 10.0 215 22 80/20:90/10 7.5 9.5 213 23 80/20:97.5/2.5 4.9 6 212 ODPA:RODA/HMD 24 100:90/10 7 209 ODPA/6FDA:RODA 25 80/20:100 5.0 5.1 5.2 227 ODPA/PMDA:RODA/HMD 26 80/20:80/20 6 204 ODPA/BTDA:RODA 27 80/20:100 5.6 Film Breaks ODPA/PMDA:RODA/DABA 28 80/20:80/20 9.0 7.2 236 ODPA/BTDA:RODA/SiODA 29 80/20:95/5 4.6 5.5 209 ODPA/ PMDA:RODA/DABA/ 30 80/20:75/20/5 3.5 8.0 215 EXAMPLES 31-55 Polyamic acid solutions were prepared as described in Examples 1 to 30 and coated on a polyamic acid "green film" derived from reaction of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether. The coated polyimide film was heated at 80°C for 20 minutes to remove most of the solvent, placed on a pin frame and dried and cured at 160-260°C for 90 minutes followed by 5 minutes at 400°C. Copper-clad polyimide laminates were prepared as previously described and peel strengths were determined. Results are given in Table III. EXAMPLE 56 A copolyamic acid containing 80 mole % ODPA, 20 mole % PMDA and 100 mole % RODA was prepared as previously described and coated and cured on a "green" film, a "gel" film and a fully cured polyimide film derived from reaction of PMDA and ODA. Properties of the ODPA/PMDA/RODA adhesive are given in Table IV. Polyimide copper-clad laminates were prepared using both roll-annealed (RA), electrodeposited (ED) and brass-treated (JTC) copper foils. Peel strength results are given in Table V. PROPERTIES OF ADHESIVE ODPA/PMDA/RODA FILM Tensile Strength 758.4 N/mm² 11 kpsi Elongation 23% Modulus 345 Shrinkage at 200°C 0.2% Glass Transition Temperature 220°C Dielectric Constant 3.38 Dielectric Strength 116.5 V/mm (2.96 volts/mil) Dissipation Factor 0.0016 Coefficient of Thermal Expansion (CTE) 60 PPM Water Absorption 0.93% Density 1.362 Tear Strength 228.3 g/mm 5.8 g/mil Moisture Content 0.163% PEEL STRENGTH OF ODPA/PMDA:RODA ADHESIVE ON VARIOUS COPPER FOILS ADHESIVE COATED ON AND CURED PEEL STRENGTHS (pli; 1 pli ≙ 17.86kg/m) RA RA-JTC ED ED-JTC ODPA/PMDA:RODA (80/20:100) "GREEN" FILM 11 7.5 24 12.5 "GEL" FILM 9.5 11.5 22 20 FULLY CURED FILM 8 EXAMPLES 57-61 Various copper foils were used to make polyimide laminates with an ODPA/PMDA:RODA (80/20:100) adhesive as shown in Table VI. The results show that the arithmetic average roughness (AA) of the foil influences the measured peel strength. Over the range of roughness tested, the ODPA/PMDA:RODA adhesive showed a 3.57 kg/m (0.2 lb/in) increase in peel strength for each microinch increase in AA. The oxide and brass treatments both produced a similar increase in peel strength over that measured with an untreated surface. EFFECT OF COPPER SURFACE TREATMENT ON PEEL STRENGTH OF ODPA/PMDA : RODA (80/20:100) ADHESIVE Ex. No. Copper Foil AA (Microinches) Peel Strength (pli; 1 pli ≙ 17.86 kg/m) --ODPA/PMDA:RODA(80/20:100)-- Green Film Gel Film Cured Film 57 RA/oxide (Brightside) 6.7 4.5 -- -- 58 RA/oxide 6.6 11 9.5 8.0 59 RA/brass 10.8 7.5 11.5 -- 60 ED/brass 51.3 12.5 20 -- 61 ED/oxide 72.6 24.0 22 -- EXAMPLES 62-65 Polyamic acid solutions were prepared as described in Examples 1 to 30 and coated on a polyamic acid "gel film" prepared by chemical conversion of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether. The coated gel film was heated at 80°C for 20 minutes to remove the solvent, placed on a pin frame and dried at 160-260°C for 90 minutes. Copper-clad laminates were prepared as previously described and peel strengths were determined. Results are given in Table VII. PEEL STRENGTH OF ADHESIVES ON PMDA/ODA GEL FILM Example Adhesive composition (mole %) Peel Strength (pli; 1 pli ≙ 17.86 kg/m 300°C 350°C 62 ODPA:RODA/HMD 100:80/20 8.0 6.5 63 ODPA/BPDA:RODA 80/20:100 5.5 64 ODPA:RODA/PPD 100:80/20 4.5 8.0 65 ODPA/PMDA:CODA 80/20:100 6.0 7.0

Claims: A flexible polyimide metal-clad laminate suitable for use in a flexible printed circuits and tape automated bonding comprising at least one layer of a metallic substrate and at least one layer of an aromatic polyimide, said layer of polyimide being bonded on at least one side to said layer of metallic substrate with a peel strength of at least 71.4 kg/m (4 pli) through an adhesive layer of a heat-sealable copolyimide comprising at least 30 mole % of repeating imide units of the formula and less than 30 mole % of other and different repeating imide units of the formula wherein R is an aromatic tetravalent organic radical and R' is a divalent radical of an aromatic or aliphatic diamine containing at least two carbon atoms, the two amino groups of said diamine each being attached to a separate carbon atom of said divalent radical. The flexible polyimide metal-clad laminate of claim 1 wherein the aromatic polyimide is derived from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. The flexible polyimide metal-clad laminate of claim 1 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 70 to 95 mole % oxydiphthalic dianhydride, 5 to 30 mole % pyromellitic dianhydride and a diamine comprising 100 mole % of 1,3-bis(4-aminophenoxy) benzene. The flexible polyimide metal-clad laminate of claim 2 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 70 to 95 mole % oxydiphthalic dianhydride, 5 to 30 mole % of pyromellitic dianhydride and a diamine comprising 100 mole % of 1,3-bis(4-aminophenoxy) benzene. The flexible polyimide metal-clad laminate of claim 3 wherein the metallic substrate is copper. The flexible polyimide metal-clad laminate of claim 1 or 2 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 80 mole % oxydiphthalic dianhydride and 20 mole % pyromellitic dianhydride, and a diamine comprising 90 to 98 mole % of 1,3-bis(4-aminophenoxy) benzene and 2 to 10 mole % of polysiloxane diamine of the formula wherein R₄ is C₁₋₃ alkylene, aralkylene or phenylene, R₅ is C₁₋₃ alkyl or phenyl, R₆ is C₁₋₃ alkyl or phenyl and m is 1 to 100. The flexible polyimide metal-clad laminate of claim 1 or 2 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 100 mole % oxydiphthalic dianhydride, and a diamine comprising 80 to 95 mole % of 1,3-bis(4-aminophenoxy) benzene and 5 to 20 mole % hexamethylene diamine. The flexible polyimide metal-clad laminate of claim 1 or 2 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 100 mole % oxydiphthalic dianhydride, and a diamine comprising 70-95 mole %, 1,3-bis(4-aminophenoxy) benzene and 5-30 mole % paraphenylene diamine. The flexible polyimide metal-clad laminate of claim 1 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 70-95 mole % oxydiphthalic dianhydride and 5-30 mole % biphenyltetracarboxylic acid dianhydride and a diamine comprising 100 mole % 1,3-bis(4-aminophenoxy) benzene. The flexible polyimide metal-clad laminate of claim 2 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 70-95 mole % oxydiphthalic dianhydride, 5-30 mole % benzophenonetetracarboxylic acid dianhydride and a diamine comprising 100 mole % 1,3-bis(4-aminophenoxy) benzene. A flexible polyimide metal-clad laminate comprising a metallic substrate bonded on at least one side with a peel strength of at least 71.4 kg/m (4 pli) to a heat-sealable copolyimide adhesive comprising at least 30 mole % of repeating imide units of the formula and less than 30 mole % of other and different repeating imide units of the formula wherein R is an aromatic tetravalent organic radical and R' is a divalent radical of an aromatic or aliphatic diamine containing at least two carbon atoms, the two amino groups of said diamine each being attached to a separate carbon atom of said divalent radical. The flexible metal-clad laminate of claim 11 wherein the metallic substrate is copper. The flexible metal-clad laminate of claim 12 wherein the heat-sealable adhesive is a copolymer of a dianhydride comprising 70 to 95 mole % oxydiphthalic dianhydride and 5 to 30 mole % pyromellitic dianhydride and a diamine comprising 100 mole % of 1,3-bis-(4-aminophenoxy) benzene. A process for preparing an adherable polyimide laminate comprising coating at least one surface of an aromatic polyimide base film with a copolyamic acid precursor of a copolyimide and subsequently treating the copolyamic acid coated polyimide base film to completely convert the copolyamic acid precursor to copolyimide wherein the copolyimide comprises at least 30 mole % of repeating imide units of the formula and less than 30 mole % of other and different repeating imide units of the formula wherein R is an aromatic tetravalent organic radical and R' is a divalent radical of an aromatic or aliphatic diamine containing at least two carbon atoms, the two amino groups of said diamine each being attached to a separate carbon atom of said divalent radical. The process of claim 14 wherein the copolyamic acid precursor is thermally converted to copolyimide by heat treating at 160 to 260 °C for 90 minutes. The process of claim 14 wherein the copolyamic acid precursor is chemically converted to copolyimide by mixing a chemical converting agent and a catalyst with the copolyamic acid at a temperature of 80 °C and then heat treating at 160 to 220 °C for 90 minutes. The process of claim 16 wherein the chemical converting agent is a lower fatty acid anhydride and the catalyst is a tertiary amine. The process of claim 14 wherein the polyimide base film is a completely cured polyimide comprising pyromellitic dianhydride and 4,4,-diaminodiphenyl ether. The process of claim 14 wherein the polyimide base film is a partially converted polyimide gel film comprising pyromellitic dianhydride and 4,4,-diaminodiphenyl ether containing from 20 to 80 % by weight reaction solvent. the process of claim 14 wherein the polyimide base film is a substantially unconverted polyamic acid green film comprising pyromellitic dianhydride and 4,4'-diaminodiphenyl ether containing from 20 to 80 % by weight polyamic acid polymer and from 20 to 80 % by weight reaction solvent. The process of any of claims 18 to 20 wherein the copolyimide comprises from 70 to 95 mole % oxydiphthalic dianhydride and 5 to 30 mole % pyromellitic dianhydride as dianhydride component and 100 mole % 1,3-bis(4-aminophenoxy) benzene as diamine component.

Inventors: KANAKARAJAN KUPPUSAMY, KREUZ JOHN ANTHONY, KANAKARAJAN, KUPPUSAMY, KREUZ, JOHN ANTHONY
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: B32B27
IPC codes: B32B27, B32B15, H05K1, C08G73, H05K3