Method for treating a polyimide surface to improve the adhesion of metal deposited thereon, and articles produced thereby

Abstract: A method for treating a polyimide surface is disclosed, in which an adhesion-promoting compound such as thiourea contacts the surface prior to the electroless plating of metal thereon. Articles formed by such a method are also disclosed.

Description: This invention relates to a method of improving the adhesion of metal applied on the surface of a polyimide, and to articles formed by such a method. Printed circuit boards typically consist of a sheet of a dielectric, i.e., a nonconductive substrate formed from various filled or, unfilled synthetic materials such a phenolics, glass-impregnated epoxies, and the like. The substrate is provided with a pattern of thin metal foil which functions as a conductive path on one or both sides. The conductive paths or "traces" are usually formed of a conductive material such as copper, palladium, nickel, gold, and the like. These traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations on the board. Plastics such as polyimides are particularly suitable as materials for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and easy moldability. However, metallization does not adhere well to a polyimide surface. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during subsequent manufacturing steps or during use of the circuit board. Several attempts have been made to increase the adhesion of the conductive metal traces to the polyimide substrates. Adhesion, is generally measured as "peel strength", i.e., the force, under controlled conditions, required to peel the metal from the substrate surface. Various surface treatment techniques are often used to physically modify the as-molded polyimide surface and thereby improve the adhesion of metal subsequently applied thereto. For example, polyimide surfaces have been grit-blasted to provide a roughened surface which anchors the subsequently-applied metals. Chemical swelling agents or penetrants have been used to swell the surface, and chemical etching agents have been used to remove portions of the surface. While such methods do increase adhesion, they are often not entirely satisfactory for several reasons. Such techniques result in degradation of the molecules forming the polyimide surface, and may decrease both the tensile and impact strength of the substrate due to swelling and cracking of the entire substrate material, especially in those instances in which the polyimide contains fillers. Grit blasting requires cumbersome equipment and impedes the processing of printed circuit boards. Another method for increasing adhesion teaches the treatment of polyimides with compounds containing the NO- group in order to improve the adhesion of metals subsequently applied thereto. The primary object of the present invention is to provide a process for improving the adhesion of metal to a polyimide surface without substantial physical modification or degradation of the surface structure. An additional object of this invention is to provide a photopatternable polyimide surface to which a highly adherent metal layer may be electrolessly applied. Improved adhesion of a metallization layer on the surface of a polyimide substrate is achieved by treatment of the surface with an adhesion promoter, followed by electroless deposition of the metallization layer on the treated surface, and then heating of the resulting metallized article. The polyimide materials forming the substrate may be any of those well-known in the art, such as those described in the Kirk-Othmer Encyclopedia of Chemical Technology , Third Edition, Vol. 18. Polyimides may be produced by a variety of methods, e.g., by reacting carboxylic acid anhydrides with aromatic diamines. Especially preferred polyimides for the present invention are those commonly referred ,to as "polyetherimides" - for example, those formed by the reaction of bisphenol A dianhydrides (BPADA) with phenylene diamine. Examples of polyetherimides and methods for their preparation are described in U.S-A-3,983,093 and 4,360,633. The polyimides may contain therein various amounts of fillers or reinforcing agents, all of which are well-known in the art. Illustrative fillers and reinforcing agents are disclosed in the examples and in U.S-A-4,525,508. These materials include talcs, mica, aluminum silicates (clay), zinc oxide, calcium metaphosphate, titanium dioxide, carbon black, franklinite, precipitated or natural calcium carbonate, glass fibers, glass spheres, silicone-polyimide copolymeric materials, carbon fibres, as well as mixtures thereof. The process of the present invention is also useful for polyimides which contain various other additives, such as pigments, ultraviolet radiation absorbing agents, impact modifiers, plasticizers, antioxidants, stabilizers, processing aids, and antistatic agents. The substrate of the present invention may be in a variety of shapes and sizes. For example, the substrate may be a printed circuit board having flat or curved surfaces as well as contact holes and vias. The term "adhesion promoter" as used herein means an adhesion-promoting organic double bond-containing compound characterized by the presence of a carbonyl or thiocarbonyl group and at least one other atom that has at least one pair of free electrons. When applied to a polyimide surface, the adhesion promoter substantially increases the peel strength, i.e., the adhesive strength, of an electrolessly deposited layer of metal as measured by standard adhesion tests, for example, a test measuring the force required to lift metal strips from a plastic surface. The adhesion promoter of the present invention is selected from the group consisting of 1,1,3-3-tetramethylurea, and salts or hydrates thereof; wherein each X is independently oxygen or sulfur; R¹ and R² are independently selected from the group consisting of hydrogen, alkyl groups containing 1 to 15 carbon atoms, aromatic groups containing 6 to 20 atoms, and alkoxy groups containing 1 to 10 carbon atoms; R³ and R⁴ are independently selected from the group consisting of R¹, S⁻, NH₂, NHR¹, NHNH₂, and R¹CO-; R⁵ is selected from the group consisting of R¹, NHR¹, and S⁻; R⁶ and R⁷ are each independently hydrogen or an alkyl group containing 1 to 10 carbon atoms; Y is selected from the group consisting of NH, NHNH, -S-S-, alkylene groups containing from 1 to 10 carbon atoms, and arylene groups containing 1 to 20 carbon atoms; and n is 1 or 2. In preferred embodiments, R¹ and R² are independently selected from the group consisting of hydrogen, lower alkyl groups, lower alkoxy groups, and benzene. A preferred group of these compounds is characterized by R³ and R⁴ independently selected from the group consisting of lower alkyl groups, NH₂, NHNH₂, lower alkylamine groups and CH₃CO-; R⁵ being NH₂CH₃ or NHCH₂CH₃; and R⁶ and R⁷ each independently being hydrogen or lower alkyl. "Lower" as used in this manner denotes 4 or less carbon atoms in the main chain of the group. Various substituents , such as hydroxy or halogen groups, can be attached to the R¹-R⁷ groups when appropriate. Families of compounds suitable for use as adhesion promoters are the following: ureas; dithiooxamides; acetamides; semicarbazides; dithiocarbamates; xanthates; and thiuramdisulfides. Illustrative adhesion promoting compounds within the scope of the present invention include thiourea; N-methyl thiourea; 1,3-diphenyl-2-thiourea; acetyl thiourea; 1,3-dimethylurea; phenylurea; acetylurea; urea; 1,1,3,3-tetramethylurea; diacetamide; diethyl dithiocarbamic acid, sodium salt; formamide; N-methyl formamide; N,N-dimethyl formamide; thiosemicarbazide hydrochloride; 2,4-dithiobiuret; semicarbazide hydrochloride; acetamide; N,N-dimethylacetamide; thioacetamide; malonamide; N-methyl acetamide; dithiobiurea; tetramethyl thiuram disulfide; biuret; semicarbazide; oxamide; oxanilide; urethane; O-ethyl xanthanic acid, potassium salt; sodium thiocarbonate; dimethyl carbamic acid, sodium salt diethylcarbamic acid, sodium salt; 3-(thiocarbamoyl) dithio carbazic acid, potassium salt; pyrrolidone; thiocyanuric acid; and alkyl dithiooxamides such as dimethyldithiooxamide and didodecyldithiooxamide A preferred group of adhesion promoters are thiosemicarbazide hydrochloride, 2,4-dithiobiuret, semi-carbazide hydrochloride, acetylthiourea, thiourea, acetamide, urea, N,N-dimethylacetamide, thioacetamide, and thiocyanuric acid. The most preferred adhesion promoter is thiourea, which significantly improves the adhesion of metal to the surfaces of various polyimides when used in accordance with the procedures described below. The active ingredient of the present invention, i.e., the adhesion-promoting compounds such as those described above, may be applied to the polyimide surface by a variety of techniques. For example, the adhesion-promoting ,compound can be applied directly to the surface or can be dissolved in or admixed with an aqueous or organic liquid and applied in the form of a solution or slurry. Some of the adhesion-promoting compounds, such as diethyldithiocarbamic acid, can be used in the form of a salt or hydrate. In one embodiment of the present invention, the adhesion-promoting compound may be incorporated into a non-aggressive solvent. As used herein, "incorporation" of the adhesion-promoting compounds implies dissolving a solid adhesion-promoting compound in one or more solvents, or admixing a liquid adhesion-promoting compound with one or more liquids. Liquids which may be used in the present invention are those in which the adhesion-promoting compound or one of its salts or hydrates is stable and has sufficient solubility to produce an effective amount of the adhesion promoter in solution. An "effective amount" of adhesion-promoting compound means an amount sufficient to improve the adhesion of a metal layer applied to the polyimide surface as measured by the testing apparatus discussed below, as compared to an untreated surface. The liquid must also be non-aggressive to the polyimide of which the substrate is formed. A non-aggressive liquid as defined herein is one which will not discolor, weaken, or cause other deleterious effects to the bulk of the polyimide substrate under conditions of use, i.e., when the substrate is immersed in the liquid system for a period of up to 60 minutes. The bulk of the substrate as used herein is the region of the substrate which is about 15 micrometers or greater below the surface being plated by the various embodiments of the present invention. Among the suitable liquids for the adhesion promoter are water and alcohols such as methanol, ethanol, phenol, cyclohexanol, and ethylene glycol, with water being preferred. Mixtures of alcohols with water or with other organic compounds are also suitable. Illustrative mixtures are those of methanol and pyridine; water and N-methyl-2-pyrrolidone; methanol and methylene chloride; and methanol and dimethylformamide (DMF). Other suitable liquid systems include mixtures of water with various acids such as hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the adhesion-promoting compound in any of the liquids or liquid mixtures discussed above depends on the particular compound used, and typically ranges from 0.1% by weight to 50% by weight, based on total weight of the solution. The upper limit of the concentration is dependent on the saturation point of the compound in the solution. A preferred concentration range is from 1% by weight to 20% by weight. Higher concentrations within the above ranges are sometimes used when faster attainment of a particular level of adhesion is desired. Those skilled in the art are able to select an appropriate liquid for a particular adhesion promoter of the present invention without undue experimentation when the general guidelines disclosed herein are followed. Due to good solubility, ready availability, ease of handling, and low cost, water is the preferred solvent when the adhesion promoter is thiourea. Thiourea is advantageously dissolved in water at 1% by weight to 20% by weight, and more preferably, at 8% by weight to 15% by weight. The pH of the adhesion promoter solution is generally not critical to the present invention, and may range from 0 to 11.0. While a substantially neutral pH is preferred, i.e., between 6.5 and 7.5, the solution is advantageously made acidic in certain instances by the addition of acid. For example, copper already present on the substrate prior to treatment according to the present invention may tarnish due to oxidation when subsequently exposed to treatment solutions having a pH of greater than 4.5. In such an instance, it is desirable to maintain the solution containing the adhesion promoter at a pH less than 4.5, and more preferably, less than 1.0. Addition of any of various acids such as citric, phosphoric, sulfuric or hydrochloric acid, etc., is suitable for this pH adjustment. The temperature of the solution containing the adhesion promoter is preferably maintained at between 65°C and 85°C during treatment of the polyimide surface, although improved adhesion can also be obtained at temperatures as low as room temperature. Those skilled in the art can easily determine the optimal temperature, based in part on the solubility characteristics of a particular adhesion promoter in a particular liquid or liquid mixture. The liquid system containing the adhesion-promoting compound may be applied to the surface of the polyimide substrate by any art-recognized technique, e.g., spraying or brushing. In preferred embodiments, the substrate is immersed in the organic or aqueous solution for a preselected period of time. The immersion or treatment time will depend on several factors, including the particular adhesion promoter used, its concentration in the solution, the type of aqueous or organic liquid employed, as well as the particular polyimide material present. The appropriate time for treatment may be readily determined without undue experimentation by those skilled in the art. Typically, the treatment time ranges from 5 minutes to 15 minutes. Another factor influencing the exposure time is the presence or absence of filler materials in the polyimide. For example, inorganic filler material near or protruding from the polyimide surface may affect the adhesion of subsequently applied metal layers, and may therefore alter the time necessary for treatment with the adhesion promoter to achieve a desired level of adhesion. Those skilled in the art can readily determine the effective treatment time for a particular filled polyimide, based on, trial adhesion values. In preferred embodiments, especially when the polyimide contains a filler such as those described above, the polyimide surface can be prepared by a pretreatment prior to treatment with the adhesion-promoting compound. The pretreatment often further improves the adherence of metals subsequently applied to the polyimide surface. After the surface has been cleaned by well-known methods, e.g., the use of a deareasing compound such as 1,1,2-trichlorotrifluoroethane, the pretreatment comprises first treating the surface with a mild etching compound, e.g., a solvent which is capable of mildly etching the polyimide. Examples of mild etching compounds suitable for use on a polyimide surface include concentrated sulfuric acid, dimethylformamide, N-methyl-2-pyrrolidone, pyridine, tetrahydrofuran, and methylene chloride. Although these compounds remove a layer of the polyimide surface as described below, they are considered mildly etching and nonaggressive because, when used according to the procedures described herein, they do not swell or crack the bulk of the polyimide substrate, as often occurs when swell and etch techniques of the prior art are employed. The mild etching compound may be applied by well-known methods, e.g., brushing, spraying, immersion and the, like, and in preferred embodiments, is followed by one or more rinsing steps with water, methanol or other alcohols. Treatment with the mild etching compound may be carried out for 15 seconds to 5 minutes, but is preferably carried out for less than 1 minute. After the mild etching compound has been applied, a residual film of varying thickness is present on the polyimide surface. The film remains on the surface even after rinsing. The film may be loosened from the surface by treatment with a basic solution by one of the methods described above for 30 seconds to 10 minutes. Suitable basic solutions include those formed by dissolving 0.1M to 10M of a compound such as sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, and mixtures thereof in a solvent such as water, methanol, ethanol, ethylene glycol, and mixtures thereof. Those skilled in the art will be able to select a particular solvent, solute, and solute concentration without undue experimentation. After treatment with the basic solution and rinsing of the surface again, the loosened residue may be removed by treatment with a solution comprising a mild etching Compound (as described above) and either an alcohol, water, or mixtures thereof. The mild etching compound should be one which will not react with the water and/or alcohol. For example, sulfuric acid and methanol react with one another and thus would not be a suitable combination for this step. Methanol is the preferred alcohol in this solution because of its ability to quickly remove the loosened residue. The solution ingredients and their respective proportions depend in part on the particular polyimide employed and on the amount of residual film present on the polyimide surface, and may be readily determined by those skilled in the art. Typically, the volume ratio of the water/alcohol component to the mild etching compound ranges from 20:80 to 80:20, with 50:50 being preferred. Treatment duration for this step ranges from 60 seconds to 20 minutes. Furthermore, treatment of the polyimide with the adhesion-promoting compound discussed above ray be combined with this pretreatment step by incorporating the adhesion promoter into the solution comprising the water/alcohol component and the mild etching compound. Although the specific mechanism is not completely understood, it is thought that the pretreatment further enhances adhesion of filled polyimides by removing impurities from the surface and by removing a surface layer, i.e., the residual film, which contains only polyimide, to expose an underlying surface containing both polyimide and filler. Furthermore, the effect of the mild etching compound and the presence of intermittent protrusions of filler material on the newly exposed surface apparently allows the subsequently applied metal to become anchored securely to the "roughened" surface. The pretreatment of the polyimide surface may also be performed when the polyimide does not contain a filler, but is especially beneficial for enhancing adhesion in those instances in which the polyimide surface does contain a filler. The solution containing the adhesion promoter may be prepared shortly before treatment of the polyimide surface. Alternatively, the solution may be prepared up to about several months prior to use in the present invention. The specific chemical mechanism by which the adhesion promoter enhances adhesion of metal subsequently applied to a polyimide substrate is not fully understood. It is thought that the adhesion promoter in part functions as a chelating agent because of the presence of polydentate ligands which can wrap around and complex the metal which is subsequently applied to the polyimide surface. While treatment of the polyimide with the adhesion promoter may involve some chemical alteration of the polyimide, such treatment does not degrade or weaken the bulk of the substrate, unlike swell and etch treatments of the prior art. After treatment of the polyimide surface by exposure to the adhesion-promoting compound, the substrate may be thoroughly rinsed with water or with an alcohol solution, and then activated for plating by methods well-known in the art and described, for example, in U.S-A-3,589,916. For example, the substrate ray be contacted with an acid solution of a precious metal, such as palladium chloride in hydrochloric acid, for a period of time sufficient to cause catalytic activation of the substrate. It is often useful to begin the activation of the substrate by treatment with an additive which aids in absorption of the plating catalyst. Such additives are well-known in the art. Exemplary aids to catalyst absorption include Shipley 1175A, a product of the Shipley Company, and Metex 9420, a product of the MacDermid Corporation. Immersion in 0.1% to 5% by volume of either of these agents in water for 1 minute to 10 minutes at a temperature of from 40°C to 80°C is generally sufficient. Although such a treatment is not deemed critical to the present invention, its use often enhances the uniform deposition of electrolessly-applied metals onto the substrate. After the substrate has been water-rinsed, activation for plating may be accomplished by methods which are well-known in the art and described, for example, in U.S-A-3,589,916. For example, the substrate may be contacted with an acid solution of a precious metal, such as palladium chloride in hydrochloric acid, for a period of time sufficient to cause catalytic activation of the substrate. Another suitable activation system involves the use of colloidal suspensions of precious metals, as taught in U.S-A- 3,011,920 and 3,841,881. For example, these substrates may be immersed in a solution of Shipley Cataprep® 404, a product of the Shipley Company. This solution provides a protecting agent for the plating catalyst subsequently applied, and comprises sodium bisulfate and various surfactants. The substrates may then be immersed in a solution of Shipley Cataposit® 44, which contains the Cataprep® 404 ingredients and also tin and palladium, the electroless plating catalyst. After a water rinse, the substrates may then be immersed in a solution of Shipley Cuposit® Accelerator 19, a fluoroboric acid-containing formulation used to separate tin from the plating catalyst. After another water rinse, electroless plating can be undertaken. Illustrative metals used to form the metallization layer include copper, palladium, nickel, and gold. Electroless baths are well-known in the art and are generally described in the Kirk-Othmer-Encyclopedia of Chemical Technology , 3rd Edition, Volume 8. The selection of a particular bath or electroless plating process is not critical to the present invention. The contents of the bath and the particular plating parameters, e.g., temperature, pH, and immersion time, will of course depend on the particular polyimide serving as the substrate, and also upon the particular metal being deposited thereon. One suitable plating bath is the Shipley Cuposit® 250 system. Another suitable bath is the Enthone 406 system. Furthermore, the Grubb et al. and Dumas et al. applications mentioned above list suitable electroless plating formulations. Immersion times, bath temperatures, and other operating parameters can be determined and controlled according to manufacturers' suggestions Those having ordinary skill in the plating arts will be able to determine the most appropriate plating regimen for a particular situation. The electrolessly-applied metallization layer usually has a thickness of 2.54 X 10⁻⁴ mm (0.01 mil) to 2.54 x 10⁻³ mm (0.1 mil). The polyimide surface is heated after electroless deposition of the metal. Oven heating of the entire article, i.e., substrate with metal thereon, is sufficient, although any heating method is suitable. Typically, this heat treatment is carried out at a temperature ranging from 60°C to 170°C for 5 minutes to 120 minutes, with higher temperatures within the above range generally compensating for shorter duration, and vice versa. Although the mechanism is not understood, the heat treatment appears to reduce the time required to attain optimal adhesion. If another layer of metal is to be applied on the surface, e.g., by electroplating, the above-described heat treatment can in some instance be omitted if a heat treatment is employed after the plating of more of the metal, as described below. However, preferred embodiments include the heat treatment prior to this deposition of more of the metal. Most preferred embodiments include a heat treatment prior to the deposition of more of the metal (i.e., after the electroless deposition), along with another heat treatment after deposition of more of the metal, as described below. Electroplating is the preferred application method for the second metal layer. The substrate is usually cleaned prior to immersion in the electroplating bath. Such baths are well-known in the art and are described, for example, in U.S-A-4,555,315, although the particular electroplating bath used is not critical to the present invention. Furthermore, those skilled in the art appreciate that the particular bath contents will depend upon some of the factors considered for the electroless deposition of metal described above. Typically, the electroplating bath is operated at a temperature ranging from 16°C to 38°C, with a cathode current density in the range of 1 mA/cm² (1 ASF) to 86 mA/cm² (80 ASF). A description of such baths is given in the Kirk-Othmer reference described above, in Vol. 8, beginning on page 826. When such a bath is used to apply a layer of copper to the electroless-applied layer, it typically includes an aqueous acidic copper electrolyte such as those of the acidic copper sulfate or acidic copper fluoroborate type; halide ions, such as chloride and/or bromide ions; and various other components well-known in the art. The thickness of this second metal layer will of course depend upon the particular end use of the metal-coated substrate. Typically, thicknesses range from 2.54 x 10⁻² mm (1 mil) to 0,127 mm (5 mils). A variation in thickness within this range does not appear to affect the adhesive strength of the metal to the underlying electroless layer and to the substrate. After the second metal layer has been applied to the first layer of metal on the substrate, the composite article can be heat-treated for 15 minutes to 48 hours at a temperature of from 25°C to 170°C. Higher temperatures within the above range generally compensate for shorter duration, and vice versa. An especially preferred heat treatment is from 75°C to 130°C for 1 hour to 16 hours. This heat treatment reduces the time required to produce optimal adhesion, especially when used in conjunction with a heat treatment prior to application of this second metal layer, as described above. When it is desired that the metal applied to a polyimide substrate be in the form of a pattern rather than a continuous layer, the substrate surface, having been pretreated with the adhesion-promoting compound, may then be patterned by well-known methods, as described, for example, in U.S-A- 3,562,605. Another patterning technique suitable for use herein is disclosed in the copending Grubb et al. application discussed above. This resistless technique involves the photopatterning of an organic substrate by first exposing the substrate to electromagnetic radiation, followed by treatment with a precious metal compound to activate the surface. The metallic pathway formed after plating of the activated surface will tightly adhere to the underlying polyimide substrate because of the adhesion-promoting treatment of the present invention. In those instances in which even greater adhesive strength between the metal pattern and the underlying polyimide substrate is desired, the substrate may be abraded prior to treatment with the adhesion promoting compound. Well-known abrasion methods, e.g., grit blasting, are suitable for the method of the present invention. The combined effects from abrading the substrate and then treating the substrate with the adhesion-promoting compound may result in even greater adhesive strength than when the abrasion step is omitted. Furthermore, the abrading step may also be used in conjunction with the pretreatment step described above. However, since the abrasion step may alter the appearance of certain polyimides, its effect on the physical appearance of the particular polyimide employed should be determined beforehand. As is apparent from above, the present invention includes within its scope a polyimide substrate (optionally, pretreated) treated with, an adhesion promoter as described above, and then electrolessly plated with a metal such as copper. The article is generally characterized by an adhesive strength of at least 893 g/cm (5,0 pounds/inch), as measured by the technique described below. Furthermore, another metal layer may be disposed on top of the electrolessly-deposited layer. The following specific examples provide novel embodiments of the present invention. They are intended for illustrative purpose only and should not be construed as a limitation upon the broadest aspects of the invention. All liquid ratios are by volume, unless otherwise indicated. In these example, adhesion of the metal to the substrate was evaluated by measuring the force necessary to peel strips of the metal from the substrate surface. In the test, the metal surface of each plated sample is etched into 3.175 mm (1/8 inch) strips. An end of each strip is clipped to an Ametek digital force measuring gauge which is connected to a computer processor. Force values required to lift the metal strips from the substrate are converted by the computer into g/cm (pounds per inch) peel values. Multiple peel values for each strip are obtained and then averaged. Example 1 Samples 1-19 were substrates formed from a polyetherimide material having 30 % by weight milled glass dispersed therein. Unless otherwise indicated, each sample was treated according to the following regimen : Samples 16-19 served as controls. Sample 16 was subject to the steps listed above but was not treated with an adhesion promoter in step J. Samples 17-19 were treated in step J with compounds outside the scope of the present invention. After being cooled to room temperature, the samples were tested for adhesion by the method described above. The results are shown in Table 1. Adhesion Results Sample No. Adhesion promoting compound Adhesion value average (lbs/in) Kg/cm 1 diacetamide (7.3) 1.303 2 1,1,3,3-tetramethylurea (7.6) 1.357 3 diethyldithiocarbamic acid, sodium salt (7.7) 1.375 4 formamide (8.0) 1.428 5 pyrrolidone (8.1) 1.446 6 thiosemicarbazide hydrochloride (8.8) 1.571 7 2,4-dithiobiuret (8.9) 1.589 8 Semicarbazide hydrochloride (9.0) 1.607 9 acetylthiourea (9.1) 1.625 10 thiourea (9.3) 1.660 11 acetamide (9.8) 1.750 12 urea (9.9) 1.768 13 N,N-dimethylacetamide (9.9) 1.768 14 thioacetamide (10.1) 1.803 15 thiocyanuric acid (10.6) 1.893 16 NONE (6.4) 1.143 17 n-butyl amine (1.0) 0.178 18 methyl ethyl ketone (5.1) 0.910 19 sodium sulfide (5.3) 0.946 The above results demonstrate that treatment of polyimides with the adhesion-promoting compounds of the present invention improves adhesion, as compared to samples without any treatment, or those treated with compounds outside the scope of the present invention. Example 2 Samples of polyetherimide filled with various materials were created according to the steps listed in Example 1. The adhesion promoting compound was thiourea at 10% by weight in water. Filler concentrations are given in weight percent. The results are listed in Table 2: Treatment with Adhesion Promoter on Filled and Unfilled Samples Sample No. Filler Material Adhesion value average (lbs/in) Kg/cm 20 NONE (11.3) 2.018 21 20% glass (12.3) 2.196 22 30% glass (10.6) 1.893 23 15% Siltem® (9.7) 1.732 24 30% franklin fibers (10.7) 1.911 25 30% glass/15% TiO₂ (8.0) 1.428 26 20% mica (6.8) 1.214 27 30% calcium metaphosphate fibers (5.2) 0.928 28 30% glass, 1.5% carbon black (10.5) 1.875 Example 3 Samples of polyetherimide filled with 30% by weight glass fibers were treated as in Example 1, except that the sulfuric acid treatment time for step C was varied. The adhesion promoter employed was thiourea, in water at 10% by weight. Average peel strengths for the various samples are listed in Table 3. Variation in Sulfuric Acid Treatment Time Sample No. H₂SO₄ Treatment Time Adhesion value average (lbs/in) Kg/cm 29 15 sec. (11.4) 2.035 30 30 sec. (12.0) 2.143 31 1 min. (13.0) 2.321 32 3 min. (13.6) 2.428 33 5 min. (17.6) 3.143 The above results indicate that, in certain embodiments of this invention, an increase in the time of exposure to the mild etching solvent, H₂SO₄, results in an increase in adhesion.

Claims: A method for improving adhesion of a metallization layer on a polyimide surface characterized in that it comprises the steps of: treatment of the surface with an adhesion-promoting organic double bond-containing compound selected from the group consisting of 1,1,3,3-tetramethylurea, and salts or hydrates thereof; Wherein each X is independently oxygen or sulfur; R¹ and R² are independently selected from the group consisting of hydrogen, alkyl groups containing 1 to 15 carbon atoms, aromatic groups containing 6 to 20 atoms, and alkoxy groups containing 1 to 10 carbon atoms; R³ and R⁴ are independently selected from the group consisting of R¹, S⁻, NH₂, NHR¹, NHNH₂ and R¹CO-; R⁵ is selected from the group consisting of R¹, NHR¹, and S⁻; R⁶ and R⁷ are each independently hydrogen or an alkyl group containing 1 to 10 carbon atoms; Y is selected from the group consisting of NH, NHNH, -S-S-alkylene groups containing from 1 to 10 carbon atoms, and arylene groups containing 1 to 20 carbon atoms; and n is 1 or 2; electroless deposition of the metallization layer on the treated surface; and heating of the resulting metallized surface. The method of claim 1 wherein the metallization layer is copper. The method of claim 1 or 2 characterized in that R¹ and R² are independently selected from the group consisting of hydrogen, alkyl groups containing 4 or less carbon atoms in the main chain, alkoxy groups containing 4 or less carbon atoms in the main chain, and benzene. The method of claim 3 characterized in that R³ and R⁴ are independently selected from the group consisting of alkyl groups containing 4 or less carbon atoms on the main chain, NH₂, NHNH₂, alkylamine groups containing 4 or less carbon atoms in the main chain and CH₃CO-; R⁵ is NH₂CH₃ or NHCH₂CH3; and R⁶ and R⁷ are each independently hydrogen or an alkyl group containing 4 or less carbon atoms in the main chain. The method of claim 4 characterized in that the adhesion promoting compound is selected from the group consisting of thiosemicarbazide hydrochloride, 2,4-dithiobiuret, semicarbazide hydrochloride, acetylthiourea, thiourea, acetamide, urea, N,N-dimethylacetamide, thioacetamide, and thiocyanuric acid. The method of claim 5 characterized in that the adhesion promoting compound is thiourea. The method of claim 1 characterized in that the adhesion promoting compound is incorporated into a non-aggressive liquid at a concentration in the range of 0.1 % by weight to 50 % by weight. The method of claim 7 charaterized in that the liquid containing the adhesion promoter is maintained at a pH less than 4.5. The method of claim 7 characterized in that the liquid containing the adhesion promoter is maintained at a pH of 6.5 to 7.5. The method of claim 8 characterized in that thiourea as the adhesion promoter is dissolved in water at 1 % by weight to 20 % by weight. The method of claim 10 characterized in that the solution of water and thiourea is maintained at a temperature of 65°C to 85°C and a pH of 0 to 11.0 during treatment of the polyimide surface. The method of claim 11 characterized in that the pH is less than 4.5. The method of claim 11 characterized in that the polyimide surface is prepared by mild etching, washing with a basic solution, and then treatment with a composition consisting essentially of a mild etching compound and either an alcohol, water, or mixtures thereof. The method of claim 13 characterized in that the mild etching compound is selected from the group consisting of concentrated sulfuric acid, dimethylformamide, and N-methyl-2-pyrrolidone, and the basic solution comprises a compound selected from the group consisting of sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, and mixtures thereof, each of which may be dissolved in a solvent selected from the group consisting of water, methanol, ethanol, ethylene glycol, and mixtures thereof. The method of claim 14 characterized in that the polyimide contains a filler. The method of claim 1 characterized in that the polyimide surface having the metallization layer thereon is heat treated at a temperature of from 60°C to 170°C for 5 minutes to 120 minutes prior to the deposition of more of the metal by electroplating. The method of claim 16 characterized in that the polyimide surface is heat-treated after electroplating, at a temperature of from 25°C to 170°C for 15 minutes to 48 hours. The method of claim 1 characterized in that the heating of the articles is performed after the deposition of more of the metal by electroplating, at a temperature of from 25°C to 170°C for 15 minutes to 48 hours. The article produced by treating a polyimide surface according to the method of claim 1, characterized by an adhesive strength of at least 893 g/cm (5.0 pounds per inch). The article of claim 19 wherein the metal is copper.

Inventors: DUMAS WILLIAM VINCENT, FOUST DONALD FRANKLIN, KARAS BRADLEY ROSS, LAMBY EDWARD JOHN, DUMAS, WILLIAM VINCENT, FOUST, DONALD FRANKLIN, KARAS, BRADLEY ROSS, LAMBY, EDWARD JOHN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1988
Main classification: C23C18
IPC codes: C23C18, C08J7, C09J5, H05K3