GB2057351A

GB2057351A - Laminated blanks

Info

Publication number: GB2057351A
Application number: GB8012343A
Authority: GB
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1979-04-30
Filing date: 1980-04-15
Publication date: 1981-04-01
Also published as: DE3013130C2; IT8048536A0; NL188674B; DE3013130A1; FR2455616B1; SE8003203L; AU5777780A; CH657571A5; AT384144B; IT1146956B; DE3012889C2; DK184980A; SE454125B; JPS564460A; ATA224680A; DE3012889A1; GB2057351B; NL8002514A; FR2455616A1; AU539984B2

Abstract

A blank and method for its manufacture which blank is useful in the preparation of printed circuit boards, comprises an insulating substrate, e.g., a glass fibre-reinforced epoxy-resin impregnated laminate. Superimposed and adhered to at least one surface of the substrate is a high temperature thermoplastic polymer foil or sheet having a substantially uniform thickness between about 10 and 500 microns. The thermoplastic polymer surface can be chemically treated, e.g., exposed to chemical agents to activate it and facilitate subsequent deposition of an adherent film of electrolessly deposited metal thereon. The printed circuits formed on said substrates are characterized by excellent adherence of the conductor pattern to the thermoplastic polymer surface, have excellent electrical properties and resist heat in continuous use or when soldered.

Description

SPECIFICATION Polysulfone surfaces laminated blanks The present invention relates to a blank or substrate suitable for use in the manufacture of printed circuit boards. More particularly, the present invention relates to a blank comprised of an insulating substrate having a thin, high temperature, thermoplastic polymer sheet or foil superimposed and adhered to at least one surface thereof and a method of its manufacture.

Printed circuit boards generally comprise an electrically insulating substrate associated with one or more electrically # electrically conductive circuit pattern(s). Typically, the insulating substrate comprises a synthetic resin composition reinforced with non-conductive fibrous materials, e.g., fibrous glass sheets or papers or webs or mats of glass fibers in either woven or unwoven form, or cellulose paper sheets; the electrically 'conductive circuit pattern may be a metal such as copper, nickel, cobalt, gold, silver or the like.

The use of insulating substrates to prepare printed circuits by electroless deposition techniques is well known, and it is also known to provide suitable substrates with a thermosetting rubber-resin film before electrolessly depositing a metal in order to improve adhesion. The insulating resinous film layer adhered to the base has uniformly distributed therein particles of a rubber oxidizable and/or degradable by suitable oxidizing chemicals.

This technique has been and is successfully employed in the printed circuits industry since a number of years. The surface resistance of printed circuits employing said techniques have been as low as 5000 megohms when conditions according to ASTM D618-61 procedure C and measured on an insulation resistance pattern as shown in IPC Test Method Number 5.8.1. (April 1973) (Institute for Interconnecting and Packaging Electronic Circuitry). As circuits have become more complex and conductors spaced closer together, relatively low surface resistance becomes a problem.

The prior art techniques for adhesion promotion can also be better understood by the type of substrates used. Organic coatings and materials whose surfaces may be provided with electroless metal deposits having commercially acceptable adhesion, that is, peel strengths of at least 1,2 newtons/mm of width, have heretofore fallen into distinct categorities according to the method of preparing them and the requisite chemical treatment for insuring sufficiently adherent electroless metal plating on them.

Materials of a first type used to form an adhesive coating on a suitable substrate typically comprise a dispersed phase of synthetic rubber such as butadiene or acrylonitrile butadiene copolymers in a matrix of materials such as epoxy/phenolic blends. The material ofthe dispersed phase of such substrates is readily degraded by oxidizing agents, such as chromic or permanganate etching solutions, while the matrix phase is less reactive to such agents. Following the oxidation treatment, the substrate surface is microporous and hydrophilic and is suitable for further processing in known electroless metal plating procedures combined or not with electro-plating steps.

Another general type of resinous substrates, such as certain epoxy- and polysulfone materials, sometimes referred to as singlephase materials, requires a mandatory step preceding the etching step for forming microporous surfaces; polar and strained sites or domaines that are selectively attacked in the oxidation steps must be created, usually by contacting the surface with an organic solvent, to permit preferential attack at these sites on exposure to the etchant. This process has become known as the "swell and etch" technique.

In the "swell and etch" technique, the surface of glass reinforced epoxy resin impregnated laminate is first treated with a solvent and then with a strong oxidizer, e.g., chromic acid, to etch away part of the surface and produce a microporous, hydrophilic surface suitable for adherent electroless metal deposition. This technique by itself did not give acceptable surface resistance because the oxidation process could penetrate deep enough to allow contamination of the glass cloth laminate core. To avoid this problem, special grades of laminates have been suggested having thick, epoxy resin "butter coates" over the glass fibres. Using such grades of laminates it has been possible to produce printed circuits with an insulation resistance of 100000 megohms.However, the variation of the cure of the epoxy "butter coat" from one manufacturer to another and from lot to lot of the same manufacturer requires the process to be redefined for each lot. For this reason, and because of difficulties in reliably achieving uniform "butter coates", attempts to achieve commercial production have not been successful. A further disadvantage of this process is the frequent failure of the bond in large areas of exposed metal during soldering.

It is also well known that certain plastic materials may be metal plated for decorative applications by first conditioning them in strong oxidizing acids. Among the plastic materials that have been successfully plated are acrylonitrile butadiene styrene copolymers, polyphenylene oxides, polysulfones, polycarbonates and nylon. Acrylonitrile butadiene styrene has been proposed for use as a film in the manufacture of printed circuit boards but was found to be not suitable: its bond strength was only 1 newtonimm and the respective printed circuit board could not withstand soldering temperatures.

Molded polysulfones have been used in very limited quantities as printed circuit base material, and only in high frequency applications where the low dielectric constant and dissipation factor of the polysulfone is of sufficient importance. Circuit base materials consisting of polysulfone have not achieved wide usage firstly because of the extreme processing difficulties and secondly the high price of said material. In processing molded polysulfone material for use as printed circuit substrates, it is necessary to anneal or stress relieve a minimum of 2 to 4 hours; 6 to 8 hours is preferred. These laborious steps are required two or more times during a typical circuit board manufacturing cycle. Over-annealing the polysulfone materials also must be avoided to prevent resulting embrittlement and other deleterious effects.

It is an object of this invention to provide improved methods for forming substrates for adherent metallization and improved substrates for the electroless deposition of metals thereon.

An object of this invention is to provide new and improved insulating blanks having a surface which can be activated to receive electrolessly formed metal deposits.

Another object of this invention is to provide rugged and durable metallized objects from such insulating blanks.

Afurther object of this invention is to make from such blanks printed circuit boards, including one-layer, two-layer and multi-layer boards, which are provided with conductive passageways.

An object of the present invention is to provide printed circuits utilizing such blanks, the circuits having high surface resistance, excellent bond strength between the surface of the circuit and the electrolessly deposited metal adhered thereto, excellent stability at soldering temperatures, reproducible methods of manufacturing and field repairability.

Another object of this invention is to provide a multi-layer circuit board with controlled impedence for high speed logic and other controlled impedence uses.

To achieve the foregoing objects, the present invention provides an improved blank and method for its preparation, an improved metal clad insulating substrate and method of its manufacture, improved method of producing printed circuit boards employing the improved blanks and the improved circuit boards formed thereby. As will be clear from the following description, the base material or blank of this invention suitable for the manufacture of printed circuit boards comprises a substrate having adhered to at least one surface thereof a preformed film or foil of a thermoplastic organic high-temperature polymer, said polymer having an aromatic backbone that does not liquefy or decompose at the temperature of mass soldering and during such soldering process.

In accordance with the present invention blanks for the manufacture of printed circuits are produced by the method comprising the steps of providing at least one of the surfaces of a substrate selected from metal blanks and insulating materials with a superimposed layer of a preformed film or foil of a thermoplastic polymer having an aromatic backbone that does not liquefy or decompose under exposure to mass soldering procedures; and consolidate the assembly so produced by heating under pressure.

It is a further object of the present invention to provide an assembly comprising a metallic or insulating substrate and on at least one of the surfaces of said substrate a preformed foil of said thermoplastic polymer adhered thereto by the application of heat and pressure.

It has been found that such assemblies do not embrittle during processing and use and provide printed circuit boards with excellent surface resistance values.

In another embodiment of the present invention printed circuit boards are produced by treating the surface or surfaces of the polymer foil after consolidation, such treatment comprising the steps of pretreating the polymer surface(s) with a polar solvent thus swelling the outer layer of said polymer and exposing the thus pretreated surface(s) to a strong oxidizing solution; and electrolessly depositing metal, followed or not by electrodeposition on at least portions of said surface(s), thus forming printed circuit patterns by methods known as such.

A further embodiment of this invention is directed to printed circuit boards having more than one circuit pattern on one side of the substrate and to manufacture of such boards by a method comprising the steps of providing at least one surface of a suitable base material with a first printed circuit, and providing at least the surface area provided with said circuit pattern with a layer consisting of a preformed foil of the thermoplastic polymer, and treating the surface of said foil with a solvent and subsequently with an oxidizing agent; and providing the thus prepared surface of said foil with a second printed circuit pattern; and repeating said sequence of applying preformed polymer foil and forming conductor pattern as many times as desired.

In order for connecting selected conductors of different levels holes intersecting said conductors and having metallized walls may be employed.

In another embodiment the areas of conductors of one level to be connected to conductors of a subsequent arranged level are kept free of the polymer foil, or are freed after applying said foil, and the respective conductors of said second level are formed in such a way that the metal deposit formed covers the exposed conductor areas of the respective conductors of the first level.

Other embodiments of the present invention will become obvious from the following description.

By "B-Stage" as used through the the specification and claims, is meant that condition of a composition where some but not all of the active molecules are cross-linked and the composition is still softened by heat.

By "C-Stage" as used throughout the specification and claims, is meant that condition where a composition has substantially reached the final stage of polymerisation where cross-linking becomes general and the composition, assumes a thermoset, is substantially insoluble and infusable.

The laminated blanks of the present invention and methods of their manufacture represent an improvement over the substrates heretofore employed. The methods of this invention utilize thermoplastic, organic, high temperature polymers as the surface layers) of a blank. The surface layer has a thickness of preferably not less than 10 microns, e.g., above 25 microns and most preferably above 50 microns; generally, the thickness of the polymer surface is below 500 microns, preferably below 125 microns and most preferably below 75 microns. One or more plies of a thermoplastic polymer is superimposed and laminated onto one or more plies of a "B-Stage", resin impregnated reinforcement, such as glass-film, cloth or paper, under heat and pressure to form a rigid printed wiring board substrate.

The present invention provides a simple and economical method of preparing blanks having substantially planar surfaces which surfaces may be adapted to receive a layer or pattern of conductive metal by electroless deposition techniques. In one aspect, this invention relates to an insulating substrate suitable for use in printed circuits and the method of its preparation which method comprises:: providing thermoplastics films or sheets having a substantially uniform thickness between about 10 and about 500 lim, the thermoplastic material having an aromatic backbone that does not liquefy or decompose at a temperature of, e.g., 2450C after five seconds exposure at the temperature; - providing a fibrous sheet or web impregnated with a thermosettable resin or plies of the impregnated fibrous sheets or webs; - superimposing at least one of said films or sheets on at least one of said plies of thermosettable resin impregnated fibrous sheets of webs; and - consolidating, preferably between planar press plates, the assembly so produced and curing the thermosettable resin by heating under pressure.

In another aspect, this invention relates to a blank suitable for use in printed circuits which comprises: - an insulating substrate having adhered to a surface thereof or opposite surfaces thereof a thermoplastic organic high temperature polymer having a thickness between about 10 and about 500 microns, the polymer having an aromatic backbone that does not liquefy or decompose at a temperature of 2450C after five seconds exposure at the temperature.

In still another aspect, this invention relates to a laminate and the method of its preparation as subsequently described herein which laminate comprises the blank as described hereinabove and further including an electro-conductive metal layer superimposed on an adhered to the polymer surface layer(s).

The surface layer of polymer foil serves as an adhesive means between said metal layer and the reinforced thermoset substrate. Consequently, to laminate a metal to a reinforced polyester substrate, for example, a metal film and thin thermoplastic film may be pressed together with a reinforced polyester substrate to bond the three together or the thermoplastic film surface of the blank may be treated with an oxidizing media or a plasma to produce a hydrophilic surface receptive to subsequent metallization and provided with an electrolessly formed metal deposit.

In another aspect, this invention relates to a multi-layer printed circuit board and method of its preparation which method comprises the steps of: - providing an insulating substrate having a circuit pattern adhered to at least one surface thereof; - applying a layer of, e.g., a polysulfone foil over the circuit pattern(s); - treating the polysulfone surfaces with a solvent and oxidizing agent to render said surface(s) microporous and hydrophilic; and - electrolessly depositing a metal onto the treated surface(s).

Anythermosettable resin known for use in preparing insulating substrates for printed circuits may be employed in applicants' method and blank provided it or they produce, together with the other materials employed, the desired properties in the finished substrates. Examples are allyl phthalate, furane, allyl resins, glyceryl phthalates, silicones, polyacrylic esters, phenol formaldehyde and phenol furfural copolymer, alone or compounded with butadiene acrylonitrile compolymer or acrylonitrile butadiene styrene copolymers, ureaformaldehyde, melamine formaldehyde, modified methacrylic, polyester and epoxy resins. Phenol formaldehydes may be used if requirements of use are not stringent. Epoxy resins are preferred when stringent properties are required.For impregnating the fibrous webs or sheets utilized in applicants' methods, the thermosettable resin may be employed in any convenient form and manner, but a varnish is preferably employed wherein the resin is dispersed or dissolved in a suitable medium. The weight of resin solids in the varnish is not generally critical, but it is selected to achieve epoxy glass cloth composites comprising about 35 to 70%, e.g., about 35 to about 55 % resin solids by weight.

The insulating base of this invention need not be organic. Thus, it could be made of inorganic insulating materials, e.g., inorganic clays and minderals such as ceramic, ferrite, carborundum, glass, glass bonded mica, steatite and the like.

Furthermore, sheet metal may be used as the substrate to be covered with the preferred thermoplastic foil with or without an interlayer of a thermosetting resin or of a material impregnated with such resin.

Suitable thermoplastic foil materials are high temperature thermoplastic polymers having an aromatic backbone and which do not liquefy or decompose at a temperature of mass soldering, e.g., of 245"C, after five seconds exposure at such a temperature. Examples including polycarbonate, polysulfone having the following recurring unit:

polyethersulfone having the following recurring unit:

and polysulfone.

As can be seen in the structural formula set forth hereinabove, each aromatic unit in the polysul

fone is linked to its neighbor by an -S04- substituent, called a sulfone linkage.

Similarly, each aromatic unit in the polyethersulfone is linked to its neighbor by an -S04- substituent at one end, and an -0- substituent at the other end, called an ether linkage. Furthermore, it also can be seen that each substituent is separated by four carbon atoms of the aromatic unit; e.g., para substitution.

Certain grades ofthesethermoplastics in molded sheets, rods and/or foil form can be treated to render the surfaces of these materials receptive to adherent metal deposition. These materials have been used widely in the decorative, automotive, electronic component, medical appliance, food processing and dairy equipment industries. For illustrative purposes, the following discussion will be directed to certain grades of polysulfone. It is known that the various grades of polysulfone are characterized by toughness, low creap, and long term thermal and hydrolytic stability, including years of continuous service in boiling water or steam, and in air in excess of 150 C, with little change in properties.Polysulfones qualify for Underwriters' Laboratories Thermal Index ratings of 150 C; they maintain their properties over a temperature range from -1000C to above 150"C. They have a heat deflection temperature of about 1740C at 264 psi (1.8 MPa) and about 181"C at 6 psi (41 MPa). Long term thermal aging at 150 to 200"C has little effect on the physical or electrical properties of polysulfones.

Polysulfone may be prepared by the nucleophilic substitution reaction between the sodium salt of 2,2-bis(4-hydroxyphenyl)propane and 4,4'-dichlorodiphenyl sulfone. The sodium phenoxide and groups are reacted with methyl chloride to terminate the polymerization. This controls the molecular weight of the polymer and contributes to thermal stability.

The chemical structure of polysulfone is characterized by the diaryl sulfone grouping. This is a highly resonating structure, in which the sulfone group tends to draw electrons from the phenyl rings. The resonance is enhanced by having oxygen atoms para to the sulfone group. Having electrons tied up in resonance imparts excellent oxidation resistance to polysulfones. Also, the sulfur atom is in its highest state of oxidation: it increases the strength of the bonds involved and fixes this grouping spatially into a planar configuration. This provides rigidity to the polymer chain, which is retained at high temperatures.

The ether linkage imparts some flexibility to the polymer chain, giving inherent toughness to the material.

The sulfone and ether linkages connecting the benzene rings are hydrolytically stable. Therefore, as indicated previously hereinabove, polysulfones are resistant to hydrolysis and to aqueous acid and alkaline environments.

Suitable grades of polysulfone according to the present invention include, e.g., an unfilled grade such as the P-1700 series which is used for injection molding or extrusion; a higher molecular weight series for extrusion applications, such as P-3500 series; and a mineral-filled polysulfone useful for plating applications such as the P-6050 series (the P-1700, P-3500 and P-6050 series all commercially available from Union Carbide Corporation, 270 Park Avenue, New York, NY 10017).

Polycarbonates are linear, low-crystalline, high molecular weight (about 18.000) polymers in which the linking elements are carbonate radicals. Polycarbonates possess a combination of very useful properties including: (1) very high impact strength (16 ft-lb/in notch) combined with good ductility; (2) excellent dimensional stability combined with low water absorption (0.35 % immersed in water at room temperature; boiling water immersion does not cause dimensions to alter more than 0,001 in/in); (3) high heat distortion temperature of about 135 C; (4) superior heat resistance showing excellent resistance to thermal oxidative degradation; and (5) good electrical resistance.

Polyphenylene oxide may be prepared via oxidative coupling of phenols. By oxidative coupling is meant a reaction of oxygen with active hydrogens from different monomers to produce water and a dimerized molecule. If the monomer has two active hydrogens, oxidative coupling continues resulting in polymerization. The polymer structure of polyphenylene oxide is characterized by a high degree of symmetry, no strongly polar groups, rigid phenylene oxide backbone, a high glass transition temperature (210C) and no other observable transition in the range of - 273O to 21 00C.

Polyphenylene oxide possesses a combination of useful properties including: (1) a temperature range between about - 1 80"C and about 180 C; (2) excellent hydrolytic stability; (3) dimensional stability with very low water absorption, low creep and high modulus; and (4) excellent dielectric properties over a wide range of temperatures (- 180 C to 180 C).

The laminated thermoplastic polymer foils of this invention provide a high performance adhesive means suitable for printed circuit application with reliable properties and performance superior to that obtainable with the resin-rich and rubber thermoset adhesive blends of the prior art. The thermoplastic foil surface(s) of the blank of this invention have a substantially uniform thickness and can be chemically treated by techniques known in the art to achieve excellent adhesion of subsequent deposits of electroless metal during the manufacture of printed circuit boards.

It is generally known that these high temperature polymers, specifically polysulfones, when used by themselves require prolonged secondary annealing bakes to prevent stress cracking. Typical recommenda tins for annealing conditions are two to four hours and up to nine hours at 170"C prior to processing. An additional extended annealing cycle is required after machining the material prior to etching the surface for subsequent metal depositions. As subsequently described herein, annealing and production of the blank and/or laminate of this invention occur simultaneously in one step. It has been found that when the thermoplastic polymer foils of this invention, such as polysulfone, were laminated to an insulating substrate according to the present invention, the thermoplastic polymer foils are stress relieved during the laminating cycle.This eliminates the need for the previously mentioned laborious and time consuming secondary annealing steps.

According to one method of the present invention, the blank is formed by arranging impregnated plies of the insulating substrate and preformed thermoplastic foils or sheets in the form of a laminate and laminating the same under heat and pressure, e.g., at 1600C and 1,4 MPa for up to 60 minutes. The lamination step can be carried out in a conventional press using conditions known for preparing thermosettable resin impregnated laminates with substantially planar surfaces. Suitable cure cycles are, e.g., 10 to 60 minutes at 1200Cto 180"C and 1,5 to 10 MPa.

Other methods of manufacturing the blank of this invention may be employed. E.g., a laminated insulating substrate may be dipped into a polysulfone adhesive to the preformed thermoplastic foil. It is well known that a 2 to 5% solution of polysulfone in methylene chloride can be used to achieve a strong bond at room temperature. A polysulfone foil, e.g., may be clad to, e.g., an insulating substrate by dipping the foil and the substrate in the polysulfone-methylene chloride solution, air drying for 15 seconds, and then assembling them in a jig and placing them under a pressure of about 500 psi for 5 minutes.

After removal from the press plates or the like employed in the lamination step described hereinabove, the blank thus formed may be employed in the manufacture of printed circuit boards.

In another preferred embodiment, a thin metal foil may be superimposed on one or more surfaces of the blank and adhered thereto to form a laminate.

Blanks of the type described hereinabove may be used to prepare one-layer, two-layer and multi-layer printed circuit boards with and without plated through holes in the manner more particularly described hereinafter.

In one method of producing printed circuit boards, the so-called "semi-additive" technique is employed.

The insulating blank of this invention is cut to size and holes are prepared therein by drilling, punching, or the like. The surface of the blank is subjected to a pre-etch solvent attack or an abrasive treatment thereon. It is believed that the surface of the blank may be mechanically roughened before the oxidizing treatment to replace the solvent pretreatment. Whereeverthe expression "solvent pretreatment" is used herein, it shall also mean the use of a mechanical roughening step instead of the treatment with a solvent. A typical mechanical roughening is grit blasting the surface of the blank with a slurry of abrasive particulate matter such as sand, aluminum oxide, quartz, carborundum, and the like, sized finer than 100 USA Sieve Series mash.

The solvent attacked board is then mechanically and chemically treated with an oxidizing solution to activate the surface of the blank.

A conventional electroless plating process is employed is deposit a thin conductive layer of copper on the activated surface of the blank and, if and when desired, on the hole walls.

A temporary, protective coating or resist layer is employed to silk screen print circuit pattern having 0,35 mm lines; the temporary resist is heat cured. The circuit pattern is built up by electroplating a metal onto the exposed areas of the substrate. The temporary resist is removed and the thin layer of electrolessly deposited metal which had been covered by the mask is etched away with an acid. Alternatively, the temporary protective coating may be a photoresist.

In another method of producing printed circuit boards, known as "fully additive" technique, a suitable insulating blank according to the present invention is prepared having a polysulfone, polyethylsulfone or polycarbonate surface layer laminated to a suitable substrate, e.g., such as an epoxy-resin fiber glass reinforced base. Holes are formed in the blank at preselected sites. The blank and walls of the holes are surface pretreated as described hereinbefore. A photoimaging technique is then employed. The blanks and holes are completely coated with an aqueous, ultraviolet light reducible, copper compound and dried. An ultraviolet light photoimage is formed by projection- or contact-printing on the sensitized substrate and a metal such as copper is electrolessly deposited onto the exposed pattern and in the holes until a circuit is built up to the desired thickness.

In general, it is desirable to pretreat the surface with an agent, e.g., dimethyl formamide or dimethvl sulfoxide before or during the etching process in an oxidizing agent.

Suitable solvents and blends thereof for swelling polysulfone in particular include dimethyl formamide, acetophenone, chloroform, cyclohexanone, chlorobenzene, dioxane, methylene chloride and tetrahydrofurane.

Depending upon the particular surface of the blanks, other ion exchange imparting materials may be utilized to effect the aforementioned temporary polarization reaction. For example, acidified sodium fluoride, hydrochloric and hydrofluoric acids, chromic acids, borates, fluoroborates and caustic soda, as well as mixtures thereof, have been found effective to polarize the various synthetic thermoplastic insulating materials described herein.

The acid oxidizing agents typically used for etching acrylonitrile butadiene styrene substrates are found to be satisfactory for polysulfone substrates. Atypical composition comprises on a weight basis: 60% H2S04 10% Cr03 30% H20 During etching, the chromium that comes in contact with the pretreated polysulfone surface is reduced from Cr+6 to Cr+3. When most of the chromium is reduced, the acid is no longer as effective in improving adhesion of metal coatings. For this reason, it is desirable to have as much chromium in the acid conditioner as possible. However, with dimethyl formamide as the pre-conditioner bath, chromic acid contents above about 3% result in macro-crazing and poor adhesion.A preferred acid conditioner for the polysulfone surface(s) is, therefore, on a weight basis: 55,9 % H2S04 (96%) 10,4% H3P04 (85 to 87 %) 30,7 % H20 3,0 % CrO3 In an alternative 'fully additive" technique after exposing the surface to render it polar and microporous, the blank and walls of the holes are activated using known seeding and sensitizing agents such as, e.g., stannous chloride - palladium chloride, activators. A permanent protective coating or resist is screened to produce a permanent background resist leaving the desired circuit pattern exposed. The resist is cured and copper is electrolessly deposited on the exposed pattern and in the holes.

The blank according to the present invention may alternately be catalytic, e.g., having catalytic materials distributed throughout. In the aforementioned techniques for manufacturing printed circuit boards, this eliminates the need for a separate seeding or sensitizing step.

For printed circuits, among the materials which preferably are used as the insulating substrates for the blanks, may be mentioned insulating thermosetting resins, thermoplastic resins and mixtures of the foregoing, including fiber, e.g., fiber-glass, impregnated embodiments of the foregoing.

An adhesive layer can be on the blank. The blanks can include metal substrates such as aluminum or steel which are coated with insulating layers of preformed foils of thermoplastic polymers. If the conductive pattern is to include plated through holes, it may be preferable to first provide the metal blank with holes and coat the blank by powder fusing techniques such as fluidized bed with a suitable insulating layer.

Among its embodiments, the present invention contemplates metallized blanks in which the electroless metal, e.g., copper, nickel, gold or the like, has been further built up by attaching an electrode to the electroless metal surface and electrolytically depositing on it more of the same or different metal, e.g., copper, nickel, silver, gold, rhodium, tin, alloys thereof and the like. Electroplating procedures are conventional and well known to those skilled in the art.

The invention is more fully described hereinafter with reference to the accompanying drawings which illustrate certain embodiments of the invention and together with the specification serve to explain the principles of the invention.

Figure 1 to 3 illustrate procedures which can be used to produce printed circuit boards from insulating blanks produced in accordance with the teachings of the present invention.

Figure 4 illustrates a production process apparatus for making a blank in a roll to roll fashion following the teachings of this invention.

Figure 5 illustrates a production process apparatus for making a blank in a roll application of polysulfone to a rigid substrate.

Referring to Figure 1 A, there is shown an insulating blank 10 according to the present invention. The insulating blank 10 comprises a thermoset resin inner core 12 and outer surface layers of preformed polysulfone foil 14. The core 12 is catalytic to the deposition of electroless metal. The polysulfone foil 14, also, is catalytic to the electroless deposition of metal. In Figure 1 B holes 16 and 18 are drilled through the blank 10. The blank 10 is then immersed in a pre-treatment solvent followed by a chemical treatment with an acid etching solution such as 20 g/l CrO# 350 mg/l H2SO4 50 g/l NaF at a temperature between 45 and 65 C to expose the catalyst and activate the surface of the blank 10 as shown in Figure 1 C.A photoresist 24 is applied (shown in Figure 1 D) on a surface of the blank to mask areas not to be subsequently copper plated. Copper is then electrolessly deposited, by methods known in the art, on the walls of holes 16 and 18 and onto the exposed surface areas of the blank 10 to form a copper conductive pattern 22 about 35 um thick, as shown in Figure 1 E. The photo-resist 24 is then stripped as shown in Figure 1 F. A registered solder mask 30 may be applied over the circuit pattern leaving holes 16 and 18 exposed (Figure 1G).

In Figure 2, there is shown an additive method for manufacturing a multi-layer printed circuit board. In Figure 2A, printed circuit pattern 102 is adhered on insulating blank 100. A preformed polysulfone foil 104 is superimposed and bonded over the printed circuit pattern 102 (Figure 2B). A hole 106 is then drilled through polysulfone foil 104, printed circuit pattern 102 and the insulating blank 100 (Figure 2C). The surface of the polysulfone foil 104 is adhesion promoted employing the techniques described previously herein. The polysulfone foil 104 surface is activated by dipping in a palladium and tin solution. In Figure 2D, a photoresist image 110 is imposed on the outer surface of the polysulfone foil 104. The exposed foil surface 104 and the hole 106 are electrolessly plated with copper 112 to a thickness of about 35 um (Figure 2E).In Figure 2F, the photo-resist image 110 has been stripped providing the multi-layer printed circuit board.

In Figure 3, there is shown a semi-additive method of manufacturing a multi-layer printed circuit board. In Figure 3A, blank 200 is clad on opposite surfaces with copper 201. A first or interior circuit pattern 202 is produced by conventional etching technology and covered with a layer of a preformed polysulfone foil 204 (Figure 3B). A hole 216 is drilled through the blank 200. The blank 200 is adhesion promoted and catalysed to the reception of electrolessly formed metal layers and an electrolessly deposited copper film 211 is applied onto the polysulfone surface 204 and in the hole 216 to a thickness of, e.g., 2 um (Figure 3C). A photoresist image 210 is applied and additional copper 212 is electroplated to provide a copper layer having a thickness of about 35 cm (Figure 3D).In Figure 3E, the photoresist image 210 is removed and the copper film 211 under the photoresist 210 is etched away with a suitable etchant.

In Figure 4there is shown a method for making an insulating blank according to the present invention.

There are shown feed rollers 100, 102 and 104. Wound on roller 100 isa flexible support carrier 106 with a thickness of about 1,6 mm, the carrier being woven glass, non-woven glass, dacron, rayon, cellulose paper and the like impregnated with resins, preferably thermoset resins such as epoxy, but high temperature thermoplastics, e.g., polyimides and polycarbonates may also be used. Wound on feed roller 102 is a thermoplastic foil 108 having a thickness of 1 to 5 mm. Wound on feed roller 104 is also a thermoplastic foil having a thickness of about 1 to 5 mm. The thermoplastic foil may be, e.g., a polysulfone, polyether-sulfone or folycarbonate foil.

Also shown are combining-take-up rollers 110 which apply heat and pressure to the laminate passing therebetween. A temperature of about 160 to 200'C and a pressure of about 30 to 400 N/mm is typically applied between rollers 110. Exiting from the rollers is a flexible laminated base material 10 according to the present invention.

In Figure 5 there is shown a roll application to a rigid substrate 12, e.g., 8 mm thick epoxy glass cloth reinforced laminate. There are shown feed rollers 100, 102 and 104. Wound on rollers 102 and 104 are respective thermoplastic foils 108 having a thickness of 1 to 5 mm. Also shown are combining-take-up rollers 110 which apply heat and pressure to the laminate passing therebetween. A temperature of about 160 to 200 C and a pressure of about 30 to 400 N/mm is typically applied between rollers 110. The insulating substrate 12 passes between rollers 110 and the thermoplastic foils 108 are laminated to opposed surfaces of 12 under heat and pressure to form the blank 10 which is severed from the web after exiting from the rollers 110.Optionally, the insulating base 12 is coated with a polysulfone adhesive comprised of polysulfone dissolved in solvent prior to applying the thermoplastic foil.

The following examples illustrate at least one of the best modes of the insulating blanks, printed circuit boards and methods of the present invention.

Example 1 Eight plies of glass cloth impregnated with 45 to 55% by weight epoxy resin were placed in a laminating press with a sheet of preformed polysulfone foil, 50 um thick, both on top and bottom. The polysulfone foil was made from Udel P-1700RTM polysulfone resin. A laminating temperature of 175 C, a pressure of 600 psi (4.1 MPa) and a dwell time in the hot press of 15 minutes were employed. After 15 minutes, the press was cooled to room temperature and the blank was removed. The blank was processed into a printed circuit board employing the following steps: (1) Through-holes were drilled in the blank; (2) the blank was brushed to remove drilling debris (it is noted that no annealing or oven baking was required after drilling); (3) the blank was immersed in a dimethyl formamide water solution (specific gravity of 0.955 to 0.965) for 3 to 6 minutes; (4) the blank was rinsed in hot water for 45 seconds; (5) the surface of the blank was adhesion promoted at a temperature of 550C for a time period of 7 minutes with the following solution:: Cr03 20 g/l H3P04 100 ml/l H2S04 600 ml/l FC-98* 0,5 g/l *FC-98 is an anionic perfluoroalkyl sulfonate (6) the blank was rinsed in still water; (7) Cr(VI) was neutralized with a solution containing 10% H202 and 15% H2S04; (8)-(11) the blank was rinsed in water, immersed successively in 2,5- MHCI, a seeder solution described in Example 1 of patent and an accelerator (5% HBF4); (12) copper was electrolessly deposited onto the blank to a thickness of 2,5 um; (13)-(14) the copper clad blank was rinsed in water and dried at 1250C for 10 minutes.

A printed circuit board was manufactured using said copper clad blank and employing techniques well known in the art, e.g., a background resist image was printed, a copper circuit pattern was formed by electroplating, the resist was removed and the exposed copper etched away.

A peel strength of 1,7 N/mm was measured for the printed circuit board. A solder float test was also employed. A one-inch square copper pattern (the printed circuit board) produced according to this example was floated on 2600C molten solder for 10 seconds. The sample was removed for examination of potential blisters and/or delamination of the copper pattern from the blank. No blistering or delamination was detected.

Example 2 Example 1 was repeated except a laminating pressure of 400 psi (2.8 MPa) and a dwell time on the laminating press of 1 hour were used. A final peel strength of 2,4 N/mm was measured and a one-inch square copper pattern sample floated in 260"C molten solder for more than 10 seconds without blistering or delaminating.

Example 3 Example 1 was repeated except that a laminating pressure of 200 psi (1,4 MPa) and a dwell time in the laminating press of 5 minutes were employed. After laminating, the blank was stabilized at 1600C for 1 hour in a circulating hot air oven to prevent shrinkage and warping during processing. A final peel strength of 1,9 N/mm was measured.

Example 4 An epoxy glass laminate, G10 FRRTM was clad with 35 um thick copper foil top and bottom. A copper circuit was produced by conventional etching techniques. A polysulfone adhesive was prepared by dissolving pellets of Udel P-1700 NTRTM polysulfone resin in methylene chloride. The etched panel was dipped in the polysulfone solution and air dried. A 75,am thick preformed polysulfone foil was laminated to the adhesive coated sides of the panel in a press at 175#C for 10 minutes at 200 psi (1,4 MPa).

Through-holes were drilled in the panel and the debris removed by brushing. The panel was converted into a multi-layer printed circuit board following the procedure of Example 1.

Example 5 A single layer of epoxy impregnated glass cloth was placed between two sheets of 25 um polysulfone foil and laminated in a press at 400 psi (2,8 MPa) at 175"C for 10 minutes. This produced a flexible blank useful in the manufacture of printed circuit boards.

Claims

1. A base material or blank suitable for the manufacture of printed circuit boards which comprises a substrate having adhered to at least one surface thereof a preformed foil or film of a thermoplastic organic high temperature polymer, said polymer having an aromatic backbone that does not liquefy or decompose at a temperature of mass-soldering and during such soldering process.

2. The blank of claim 1 wherein the polymer does not liquefy or decompose at a temperature of 245"C after 3 x 5 seconds exposure time.

3. The blank of claim 1 wherein said preformed foil or film has a thickness of 10 to 500 um.

4. The blank of one or more of claims 1 to 3 wherein the polymer of the preformed foil or film is selected from polysulfone, polycarbonate and polyethersulfone.

5. The blank of claim 1 wherein said substrate is an insulating substrate.

6. The blank of claim 1 wherein said substrate is a metal.

7. The blank of claims 1 to 5 wherein said substrate is a laminate comprising fiber impregnated thermosettable resins.

8. The blank of claims 1 to 5 and 7 further comprising a layer of metal superimposed and adhered to at least a portion of the surface of the polymer film or foil.

9. The blank of claim 8 wherein the metal layer is formed by electroless deposition in combination with electroplating.

10. A method for the manufacture of printed circuit boards comprising the steps of providing at least one of the surfaces of a substrate selected from metal blanks and insulating material with a superimposed layer of a preformed film or foil of a thermoplastic polymer having an aromatic backbone that does not liquefy or decompose under exposure to mass soldering producing and consolidating the assembly so produced by heating under pressure.

11. The method of claim 10 wherein the preformed polymer foil superimposed has a thickness of 10 to 500 um and preferably less than 125um.

12. The method of claims 10 and 11 wherein the substrate is composed of thermosettable resin impregnated fibrous sheets or webs.

13. The method of claim 12 wherein curing ofthethermosettable resin is performed concurrent with the consolidation of the assembly comprising the preformed foil of the thermoplastic polymers.

14. The method of claims 10 or 13 wherein the consolidation step takes place at a temperature of 120 to 180"C and a pressure of 1,5 to 10 MPa.

15. The method of one or more of claims 10 to 14 further comprising treating the consolidated assembly, said treatment comprising the steps of pretreating the polymer surface with a polar solvent suitable for swelling the outer layer of said polymer foil and exposing the thus pretreated surface to a strong oxidizing solution; and catalyzing at least portions of the thus prepared surface to render it receptive to electrolessly formed metal deposits; and electrolessly depositing metal on said areas.

16. The method of claim 15 wherein the polar solvent is a dimethylformamide solution and the oxidizing agent is chromic acid.

17. The method of claim 10 wherein the substrate is provided, at least one one surface, with a first circuit pattern employing known methods and applying a layer of a preformed foil of a thermoplastic copolymer over said surface or surfaces provided with a circuit pattern, and treating the surface(s) of the copolymer foil with a solvent and an oxidizing agent and providing the thus pretreated surface or surfaces with an additional circuit pattern or patterns and, if so desired, repeating the steps of providing the formed circuit pattern or patterns with layers of preformed foils of the thermoplastic polymer; and providing the surface(s) of said polymer foil with an additional circuit pattern or patterns and repeating said procedure until all circuit pattern layers are formed.

18. The method of claim 17 wherein the conductors of said circuit patterns of different layers are connected as desired by providing holes at selected areas and forming a metal deposit on the walls of said holes thus producing electrical connections between the respective conductors.

19. The method of claim 17 wherein the preformed foil is removed from areas of conductors to be connected to conductors to be formed on the surface of said foil and producing a connection between said conductors by forming a metal deposit reaching from the respective conductor on the foil surface to the exposed area of the respective conductor of the underlying layer, the said metal deposit thus covering at least a part of said exposed area of said conductors.

20. The method of claim 17 wherein the circuit pattern is formed by catalysing the surface of the polymer foil to render it catalytic to the reception of electrolessly deposited metal.

21. The method of claim 18 wherein the circuit pattern or patterns is or are formed by catalysing the surface of the polymer foil to render it catalytic to the reception of electrolessly formed metal deposits; and providing the pattern areas with a metal deposit formed by electroless metal deposition alone or in combination with electroplating.

22. The method of claim 21 wherein the total surface of the polymer foil is catalysed and metallized and the conductor pattern is formed employing known methods of masking and etching.

23. The method of claims 20 and 21 wherein the polymer foil comprises an agent which is catalytic to the reception of electrolessly formed metal deposits.

24. The method of one of claims 12 to 17 wherein the substrate is provided with an agent which renders it catalytic to the reception of electrolessly formed metal deposits.