Carbon-based direct plating process

Abstract

A method of preparing a non-conductive substrate to allow metal plating thereon and a two-component gel coating composition. The method comprises the steps of: a) contacting the non-conductive substrate with a pore-conditioning substance comprising a high molecular weight pore-conditioning agent; b) applying a carbon-based dispersion to the pore-conditioned substrate, wherein the carbon-based dispersion comprises adhesive carbon or graphite particles dispersed in a liquid solution; and c) etching the non-conductive substrate. The adhesive carbon black or graphite particles in the liquid carbon-based dispersion have a small particle size and a compact particle size distribution.

Description

Technical Field

[0001] The present invention generally relates to a carbon-based direct plating process for printed circuit board manufacturing. Background Art

[0002] A printed wiring board (also called a printed circuit board) is typically a laminate comprising two or more copper foil sheets separated from each other by layers of non-conductive material. Although copper is most commonly used as the electroplated metal in printed wiring boards, it may also be electroplated with other metals such as nickel, gold, palladium, silver, etc. The non-conductive layer typically comprises an organic material (such as an epoxy resin impregnated with glass fibers), but may also comprise a thermosetting resin, a thermoplastic resin, and mixtures thereof, either alone or in combination with reinforcing materials such as glass fibers and fillers. In order to accommodate additional circuits on the printed wiring board, additional metal (e.g., copper) layers may be sandwiched between layers of insulating material to produce a multilayer wiring board.

[0003] In many printed circuit board designs, electrical pathways or patterns need to be connected between separate metal layers (i.e., copper plates) at certain points in the pattern. This is usually achieved by drilling holes through the laminate of copper plates and non-conductive layers at the desired locations and connecting the separate metal layers. Metallization of the through-hole walls is necessary to achieve connections between two metal circuit patterns on each side of a printed circuit board and / or between inner layer circuit patterns of a multilayer board.

[0004] Although electroplating is an ideal method for depositing copper and other conductive metals on surfaces, electroplating cannot be used to coat non-conductive surfaces, such as untreated through-holes. Therefore, it is necessary to treat the through-holes with a conductive material to make the through-holes suitable for electroplating.

[0005] One process for making a via conductive involves physically coating it with a conductive film. The coated via is therefore conductive enough to be electroplated, but is generally not conductive enough and strong enough to form a permanent electrical connection between the circuit layers at either end of the via. Therefore, the coated via is electroplated to provide a permanent connection. Electroplating reduces the resistance of the via to a negligible level, which does not consume an appreciable amount of power or change the circuit characteristics.

[0006] The through-hole walls can be prepared for electroplating by a carbon-based process using a liquid carbon dispersion. The typical steps of the process are as follows:

[0007] 1) Drill and deburr the surface of the through hole. In the case of multi-layer printed circuit boards, the board may also be deburred or etched back to clean the internal copper butt surface of the through hole.

[0008] 2) Optionally but preferably subjecting the printed wiring board to a pre-cleaning treatment comprising applying a pre-cleaner to the surface of the printed wiring board to prepare the printed wiring board for receiving the liquid carbon black dispersion thereon.

[0009] 3) After applying the cleaning agent, the printed wiring board is rinsed in water to remove excess cleaning agent from the board, and the board is contacted with a pore-shaping material solution. The pore-shaping material solution ensures that substantially all of the through-hole wall surfaces are prepared to receive the subsequently applied continuous layer of carbon-based dispersion.

[0010] 4) Applying the liquid carbon-based dispersion to or in contact with a cleaned and conditioned printed wiring board. Preferred methods of applying the dispersion to a printed wiring board include dipping and spraying.

[0011] 5) Subjecting the carbon covered printed wiring board to a step in which substantially all (i.e., greater than about 95% by weight) of the water in the applied dispersion is removed and a dry deposit containing carbon is left in the through-holes and on other exposed surfaces of the non-conductive layer. This drying step can be accomplished by a variety of methods including, for example, room temperature volatilization, subjecting the printed wiring board to elevated temperature for a period of time, air knives, or other similar methods commonly known to those skilled in the art. To ensure complete coverage of the through-hole walls, the steps of immersing the board in the liquid carbon dispersion followed by drying can be repeated.

[0012] 6) Thereafter, the metal portion of the substrate is aggressively etched at a high spray pressure and a high total etch volume to fully remove the dried carbon coating from the metal portion of the substrate. This micro-etching step simultaneously performs two highly desirable tasks: (1) the micro-etching step removes substantially all excess carbon black or graphite material adhered to the exposed surfaces of the outer copper plates or copper foils and the inner copper plates or copper foils in the multilayer printed wiring board; and (2) the micro-etching step chemically cleans and slightly micro-etches the outer copper surface, thereby making the surface a good substrate for dry film application or electrolytic deposition of copper, followed by mechanical scrubbing of the printed wiring board.

[0013] The mechanism by which this micro-etching step works is not to directly attack the carbon material deposited on the copper foil, but rather to attack only the first few atomic layers of copper directly beneath which provide adhesion for the coating. The carbon black or graphite coated printed wiring board is contacted with a micro-etching solution to "peel" the carbon black or graphite from the copper surface, and then the carbon black or graphite is removed from the micro-etching bath by filtration or other similar methods.

[0014] The basic steps of this process are described in more detail in, for example, U.S. Pat. No. 4,619,741, the subject matter of which is incorporated herein by reference in its entirety. Various modifications and improvements to this process are shown in U.S. Pat. Nos. 4,622,107, 4,622,108, 4,631,117, 4,684,560, 4,718,993, 4,724,005, 4,874,477, 4,897,164, 4,964,959, 4,994,153, 5,015,339, 5,106,537, 5,110,355, 5,139,642, 5,143,592, 5,725,807, and 7,128,820, the subject matter of each of which is incorporated herein by reference in its entirety.

[0015] U.S. Patent 4,897,164 to Piano et al. describes a process in which, after a drying step and prior to microetching, the dried carbon black deposit in the through-hole is contacted with an aqueous solution of an alkali metal borate to remove loose or easily removable carbon black particles from the area of ​​the through-hole.

[0016] US Patent No. 4,964,959 to Piano et al. describes the addition of conductive polymers or combinations thereof to carbon black dispersions.

[0017] U.S. Patent No. 4,994,153 issued to Piano et al. describes a process for treating tool holes or slots that have been coated with a dispersion of carbon black in a non-conductive material, which process includes removing the carbon black with an aqueous solution containing: (a) an alkanolamine; (b) an anionic surfactant, which is a neutralized addition product of maleic acid and / or fumaric acid with a poly(oxy)alcohol; (c) a nonionic surfactant, which is an aliphatic monophosphate and / or diphosphate; and (d) an alkali metal or alkaline earth metal hydroxide.

[0018] US Patent 5,015,339 to Pendleton describes an electroplating pretreatment in which a non-conductive material is first contacted with an alkaline permanganate solution, then with a neutralizer / pore-regulating material solution, and then with a carbon black dispersion.

[0019] In a variation of this basic process, the carbon coated wiring board is subjected to a fixing step prior to drying in order to remove excess carbon dispersion from the surface of the printed wiring board and to make the carbon dispersion more handleable, as described, for example, in U.S. Patent Publication No. 2010 / 0034965 to Retallick et al., the subject matter of which is incorporated herein by reference in its entirety. Fixing can be achieved by chemical fixing methods or by mechanical fixing methods.

[0020] In chemical fixing, a fixing solution is applied to the surface that has been wetted with the carbon dispersion, and the fixing solution removes excess carbon deposits, thereby smoothing the carbon coating on the recessed surface by eliminating lumps and making the coating more uniform. In physical fixing, the recessed or other surface of the substrate that has been wetted with the carbon dispersion is subjected to a mechanical force to remove excess deposits of the carbon coating before it dries, such as using a fluid jet or an air jet. For example, a fluid or air jet can be used to contact the surface coated with the carbon dispersion to blow away any excess carbon deposit accumulation and smooth the carbon coating on the recessed surface by eliminating lumps and making the coating more uniform.

[0021] Once the carbon coated printed wiring board has been microetched, the printed wiring board may be electroplated with a suitable conductive metal.

[0022] All of the processes described above include a step in which, prior to the microetching step, the carbon-coated printed wiring board is subjected to removal of substantially all (i.e., greater than about 95% by weight) of the water in the applied dispersion, so that a dry deposit containing carbon remains in the pores and on other exposed surfaces of the non-conductive layer. That is, in all of the above processes, the carbon-coated printed wiring board is dried prior to the microetching step.

[0023] In many cases, microetching can cause problems, especially when electroplating copper-dielectric interface areas. Specifically, frequent etching of copper can also strip the carbon coating from the dielectric area directly adjacent to the copper, thereby forming an insulating barrier to electrical continuity in subsequent electroplating steps. This barrier can then lead to poor plating and defects such as voids, bus lines, and plating folds. To avoid such defects, a less intensive microetching step is needed.

[0024] In order to adequately remove carbon black or graphite from the copper surface, large pumps, high pressures, large etching chambers and / or aggressive etching chemistries must generally be used to produce acceptable results. In addition, the equipment needs to be cleaned frequently to reduce nodulation of carbon black or graphite that has been stripped from the copper surface during the metal plating step.

[0025] It has also been found that carbon-based direct plating colloids / dispersions tend to have poor adhesion to dielectric substrates due in part to the size of the colloidal particles. A broad particle size distribution can also lead to poor adhesion due to the presence of larger sized colloidal particles.

[0026] Therefore, it is desirable to provide a direct plating process that provides reduced plating buildup and also does not require additional processing steps or conditions to produce good results. Additionally, it is desirable to provide a direct plating method that improves the adhesion of a carbon dispersion to a printed wiring board substrate. Summary of the invention

[0027] It is an object of the present invention to provide an improved direct plating process for preparing a printed wiring board to receive electroplating thereon.

[0028] Another object of the present invention is to provide an improved direct electroplating process capable of reducing plating nodules in the metal electroplating step.

[0029] Another object of the present invention is to provide a direct plating process with improved electroplating conditions.

[0030] It is yet another object of the present invention to provide a graphite or carbon black dispersion in which the carbon particles have a smaller average particle size to enhance adhesion of the particles to a printed wiring board.

[0031] It is yet another object of the present invention to provide a graphite or carbon black dispersion having a tighter particle size distribution to enhance adhesion of the particles to printed wiring boards.

[0032] Yet another object of the present invention is to provide a graphite or carbon black dispersion wherein the graphite or carbon black particles are coherent.

[0033] To this end, in one embodiment, the present invention is generally directed to a method of preparing a non-conductive substrate to allow metal plating thereon, the method comprising the steps of:

[0034] a) optionally but preferably contacting the non-conductive substrate with a pre-cleaner;

[0035] b) contacting the non-conductive substrate with a pore-forming material comprising a high molecular weight porogen;

[0036] c) applying a liquid carbon-based dispersion to the porous non-conductive substrate to form a carbon / porous substance gel coating on the porous non-conductive substrate, wherein the carbon-based dispersion comprises adherent carbon black or graphite particles dispersed in a liquid solution, wherein the carbon particles coagulate onto the porous substrate to form the carbon / porous substance gel coating; and

[0037] d) etching the carbon / porous material gel-coated substrate;

[0038] The adherent carbon black or graphite particles in the liquid carbon-based dispersion have a small particle size and a tight particle size distribution.

[0039] In one embodiment, the present invention is also generally directed to a two-component gel coat composition for preparing a non-conductive substrate to allow metal plating thereon, the two-component gel coat comprising:

[0040] a. A pore-forming material, the pore-forming material comprising:

[0041] i polyquaternium compounds having a molecular weight greater than 1,000,000 g / mol;

[0042] ii. pH buffer; and

[0043] iii. Surface tension reducing agents,

[0044] wherein the pore-forming material has a pH in the range of about 8 to about 10; and

[0045] b. a liquid carbon-based dispersion, wherein the liquid carbon-based dispersion comprises:

[0046] i. adherent carbon or graphite particles dispersed in a dispersant, and

[0047] ii. pH adjuster;

[0048] wherein the adherent carbon black or graphite particles have a small particle size and a tight particle size distribution, and wherein the pH of the liquid carbon dispersion is in the range of about 8 to about 10;

[0049] wherein when the pore-integrating substance and the liquid carbon-based dispersion are sequentially applied to a non-conductive substrate,

[0050] An adherent carbon / porous material gel coating is formed on the surface of the non-conductive substrate. DETAILED DESCRIPTION

[0051] The present invention generally relates to a carbon-based direct plating process for printed circuit board or printed wiring board manufacturing.

[0052] As used herein, "a," "an," and "the" refer to both the singular and the plural, unless the context clearly indicates otherwise.

[0053] As used herein, the term "about" refers to a measurable value, such as a parameter, amount, duration, etc., and is intended to include variations of + / -15% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less relative to the specifically stated value, as long as such variations are suitable for performing in the invention described herein. In addition, it should also be understood that the value to which the modifier "about" refers is itself specifically disclosed herein.

[0054] As used herein, for ease of description, spatially relative terms such as "below", "beneath", "lower", "above", "upper", "front", "rear", etc. are used to describe the relationship of one element or feature structure to another or more elements or features. It should also be understood that the terms "front" and "rear" are not intended to be limiting and are intended to be interchangeable under appropriate circumstances.

[0055] As used herein, the terms “includes and / or comprises” specify the presence of stated features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0056] In one embodiment, the present invention is generally directed to a method of preparing a non-conductive substrate to allow metal plating thereon, the method comprising the steps of:

[0057] a) optionally but preferably contacting the non-conductive substrate with a pre-cleaner;

[0058] b) contacting the pre-cleaned non-conductive substrate with a pore-regulating material comprising a high molecular weight porogen;

[0059] c) applying a liquid carbon-based dispersion to a pre-cleaned and porous non-conductive substrate to form a carbon / porous substance gel coating on the porous non-conductive substrate, wherein the carbon-based dispersion comprises adherent carbon black or graphite particles dispersed in a liquid solution, wherein the carbon particles coagulate onto the porous substrate to form the carbon / porous substance gel coating; and

[0060] d) etching the carbon / porous material gel-coated substrate;

[0061] The adherent carbon black or graphite particles in the liquid carbon-based dispersion have a small particle size and a tight particle size distribution.

[0062] In one embodiment, the present invention is also generally directed to a two-component gel coat composition for preparing a non-conductive substrate to allow metal plating thereon, the two-component gel coat comprising:

[0063] c. A pore-forming material, the pore-forming material comprising:

[0064] i. a polyquaternary ammonium compound having a molecular weight greater than 1,000,000 g / mol;

[0065] ii. pH buffer; and

[0066] iii. surface tension reducing agent,

[0067] wherein the pore-forming material has a pH in the range of about 8 to about 10; and

[0068] d. a liquid carbon-based dispersion, wherein the liquid carbon-based dispersion comprises:

[0069] i. adherent carbon or graphite particles dispersed in a dispersant, and

[0070] ii. pH adjuster;

[0071] wherein the adherent carbon black or graphite particles have a small particle size and a tight particle size distribution, and wherein the pH of the liquid carbon dispersion is in the range of about 8 to about 10;

[0072] When the pore-integrating substance and the liquid carbon-based dispersion are sequentially applied to a non-conductive substrate, an adherent carbon / pore-integrating substance gel coating is formed on the surface of the non-conductive substrate.

[0073] In one embodiment, the non-conductive substrate is a printed wiring board or printed circuit board substrate.

[0074] Once the etching step is performed and the liquid carbon coating is dried on the non-conductive substrate to form a conductive carbon coating on the printed wiring board substrate, the printed wiring board substrate can be electroplated with a suitable metal.

[0075] The inventors of the present invention have found that improved results can be obtained if the etching step is performed before the conductive carbon coating dries on the substrate.

[0076] Previously, it was believed necessary to dry the conductive carbon coating on the surface before contacting the substrate with the etching solution. Therefore, it has been previously understood that it is necessary to dry the conductive carbon coating on the surface of the substrate before etching the substrate in order to allow the carbon particles to adequately adhere to the non-conductive portions of the substrate, thereby facilitating the electroplating of the metal onto the non-conductive portions of the substrate. However, when the conductive carbon coating is dried prior to etching, the equipment may be contaminated by the carbon particles.

[0077] Therefore, in a preferred embodiment, the etching step is performed before the liquid conductive carbon coating is dried on the non-conductive substrate. In other words, the etching step is preferably performed before the drying step, and the conductive carbon coating that reaches the surface is not dried first.

[0078] Some benefits of performing an etching step prior to drying the conductive carbon coating on the substrate include the following:

[0079] 1) If the etching step is performed before the conductive carbon coating is dried, it is much easier to etch the metal portion of the substrate to remove the conductive carbon coating thereon. While the dry conductive carbon coating acts as a barrier to etching for the metal below, the wet conductive carbon coating does not significantly act as a barrier to etching;

[0080] 2) if the substrate is etched before the conductive carbon coating dries, the total etching amount and spray pressure required to obtain a clean substrate metal portion is greatly reduced compared to after the conductive carbon coating dries; and

[0081] 3) The cleanliness of the process and equipment used to apply the conductive carbon coating is greatly improved because the carbon particles are etched away from the metal portion of the substrate in the chamber or tank immediately after the chamber or tank contains the carbon suspension / colloid. Typically, this means that the outer surface of the substrate is completely free of carbon, so that the rollers and dryers commonly used after the carbon suspension solution remain at least substantially free of carbon particles.

[0082] Overall, the benefits of the process described herein also reduce the amount of plating nodules that can be generated in subsequent metal plating steps. In contrast, prior art processes allow more carbon particles to remain on the metal portion of the substrate due to the difficulty in removing the particles during the etching step and due to carbon contamination from dried carbon remaining on the equipment that can be redeposited onto the substrate surface.

[0083] In contrast, in the present invention, the presence of nodules in the metal plating step is greatly reduced because the wet carbon coating on the substrate is etched away from the metal portion of the substrate before the coating is dried, thereby more completely removing the carbon from the metal portion of the substrate. Therefore, there is less opportunity for the equipment to re-precipitate dried carbon particles back onto the surface, resulting in greatly reduced nodule formation in the metal plating step.

[0084] As described herein, in one embodiment, the printed wiring board is optionally but preferably contacted with a pre-cleaner before the whole hole step. If used, suitable pre-cleaners include standard acid cleaners for printed wiring boards, such as solutions of sulfuric acid and nonionic surfactants, commercial products of which can be purchased from MacDermidEnthone Inc. under the trade name Acid Cleaner 6A. The printed wiring board can be subjected to pre-cleaning by immersing the printed wiring board in the pre-cleaner (or otherwise contacting the printed wiring board with the pre-cleaner) and continuing for a suitable time to remove debris and / or contaminants from the surface of the substrate. For example, the printed wiring board can be immersed in the pre-cleaner at a temperature of about room temperature to about 55°C for between 30 seconds and 5 minutes, more preferably between one minute and two minutes.

[0085] Next, the printed wiring board is subjected to a pore-forming step in which the substrate is contacted with a pore-forming substance comprising a high molecular weight pore-forming agent, as described, for example, in U.S. Pat. No. 10,986,738, the subject matter of which is incorporated herein by reference in its entirety. In one embodiment, the printed wiring board is contacted with the pore-forming substance by immersing the printed wiring board in the pore-forming substance at a suitable temperature and for a suitable amount of time.

[0086] In one embodiment, the pore-forming material comprises a high molecular weight pore-forming agent, wherein the molecular weight of the pore-forming agent is greater than about 1,000,000 g / mol, more preferably greater than about 2,000,000 g / mol, and even more preferably greater than about 3,000,000 g / mol. The printed wiring board substrate is in contact with the pore-forming material for at least 20 seconds. In one embodiment, the printed wiring board substrate is in contact with the pore-forming material for between about 20 seconds and about 5 minutes. In addition, the pH of the pore-forming material is generally between 1 and 14, more preferably between 7 and 14, and more preferably between about 8 and about 10. In one embodiment, the pore-forming material is alkaline, and a pH of about 9 is suitable.

[0087] Examples of high molecular weight pore-forming agents used in the pore-forming materials described herein include, but are not limited to, polyquaternium compounds having a molecular weight greater than 1,000,000 g / mol or greater than 2,000,000 g / mol or greater than 3,000,000 g / mol. Polyquaternium is an international nomenclature for cosmetic ingredients used in the personal care industry and is used to define structures containing quaternary ammonium centers in polymers. Examples of polyquaternium compounds are listed in Table 1 below. Other polyquaternium compounds having a molecular weight greater than 1,000,000 g / mol and other similar structures can also be used in the practice of the present invention.

[0088] Table 1. Polyquaternary ammonium salt compounds :

[0089]

[0090]

[0091] In addition to the high molecular weight pore-enhancing agent, the pore-enhancing material may also contain a pH buffer such as borax and optionally but preferably a surface tension reducing agent such as a nonionic surfactant, an example of which is available from Evonik Industries AG under the trade name Tomadol 91-6.

[0092] When contacting the printed wiring board with the pore-forming substance, the temperature of the pore-forming substance may be maintained at a temperature between room temperature and about 150°F, more preferably between about 80 and about 130°F, even more preferably between about 85 and about 110°F, or about 95°F.

[0093] When the carbon dispersion contacts and then the surface of the pre-absorbed pore-integrating substance is coated on the surface of the substrate, the large molecular weight of the pore-integrating substance causes the formation of a gel. The flocculated graphite (or carbon black) and the pore-integrating substance containing the high molecular weight pore-integrating agent together form a gel-like solid-liquid coating, which allows the gel-like coating to be hard-sprayed during the subsequent micro-etching step without being washed away. The gel-like coating is formed between the pore-integrating substance and the graphite (or carbon black) coating because the pore-integrating substance has a very high molecular weight and contains a polyquaternary ammonium compound, and also because the carbon black or graphite particles are adhesive. The polyquaternary ammonium compound neutralizes the negative charge on the graphite (or carbon black) colloid, and then the pore-integrating substance containing the high molecular weight polyquaternary ammonium compound makes the resulting flocculant a gel-like solid-liquid coating that is not easily washed away and can withstand the micro-etching step even if the carbon coating is not dried first.

[0094] Once the pore shaping step is complete, the printed wiring board is contacted with the carbon-based dispersion, as further described herein.

[0095] The carbon-based dispersion comprises a source of conductive adhesive carbon particles and one or more binders and / or dispersants capable of dispersing conductive adhesive carbon particles. Preferred methods for applying liquid carbon dispersions to printed wiring boards include dipping and spraying, as well as other methods of applying chemicals used in the printed circuit board industry. When preparing the liquid carbon-based dispersion, the ingredients and any other preferred ingredients are mixed together to form a stable dispersion. This can be achieved by subjecting the concentrated form of the dispersion to wet grinding or grinding to fully mix the ingredients. Suitable wet grinding and grinding processes include, for example, ball milling, colloid grinding, high shear grinding, etc., ultrasonic techniques or other similar procedures. Importantly, wet grinding or grinding techniques can produce carbon dispersions with small particle sizes and tight particle size distributions and can produce adhesive carbon particles. Although there are many methods to reduce the particle size of the carbon-based dispersion, it is also important to control the particle size distribution. The dispersion can then be diluted later to the concentration required for the working bath with water or other dispersants.

[0096] An important factor is the consistency of particle size, and it is desirable that the dispersion contain no large particles, such as particles greater than 800 nm in diameter for carbon black and greater than 3,500 nm in diameter for graphite. In addition, it is desirable that the milling process produce a consistent particle size for the dispersion of the present invention without waste and without the need for centrifugation.

[0097] Thus, this combination of dispersant and / or binder selection with the grinding process produces a much improved product that is easier to manufacture, more efficient, less costly, more stable, less wasteful, and it adheres better to the surface of the PCB so that it can be hard sprayed with a microetching solution before drying and still not washed off. In addition, while graphite and carbon black particles are generally considered to be lubricating, the grinding process of the present invention surprisingly produces carbon black or graphite particles that are adhesive / sticky in nature. Thus, when dispersed in a dispersant, the adhesive carbon particles of the present invention are able to adhere more firmly to the substrate and are therefore less likely to be washed off.

[0098] Unlike prior art processes in which the carbon dispersion is dried prior to the micro-etching step, the gel-like solid-liquid coating formed from the carbon dispersion containing the adhesive carbon black and / or graphite particles and the porogen described herein can be subjected to high pressure spraying prior to drying. This allows the adhesive carbon particles to adhere more tightly to the through-hole walls and to withstand the spray etching step even before the coating is dried.

[0099] It has surprisingly been shown that controlling and / or reducing the particle size and particle size distribution of carbon-based colloids / dispersions affects the adhesion of carbon particles to the dielectric substrate of a printed circuit board. Preparing a colloid or dispersion with a smaller particle size and a tighter particle size distribution than is currently available in the art surprisingly facilitates subsequent adhesion of the particles to the substrate in a direct plating process to render the printed circuit board conductive. This adhesion is particularly good when the circuit board is treated with a direct plating process in which an etching step is performed before the coating is dried. Typically, when such a wet carbon-containing coating is spray-etched before drying, the particles are sprayed off the substrate and do not have adequate adhesion.

[0100] The carbon-based colloid or dispersion comprises adhesive graphite or carbon black particles, and wherein the particles are reduced to below a certain size. For colloids / dispersions containing carbon black, the particles are reduced to a D50 less than 120nm, preferably less than a D50 of 110nm, more preferably less than a D50 of 100nm, and less than a D99 of 400nm, preferably less than a D99 of 300nm, more preferably less than a D99 of 250nm. For colloids / dispersions containing graphite, the particles are reduced to less than 350nm, preferably less than 325nm, more preferably less than a D50 of 300nm and less than a D99 of 2500nm, preferably less than a D99 of 2,000nm, more preferably less than a D99 of 1,800nm. The particle size is measured with a BLUEWAVE particle size analyzer purchased from Microtrac MRB. BLUEWAVE uses a laser diffraction analyzer to measure the volume, number and area distribution and percentiles of particles.

[0101] Typically, small graphite particles have a D50 of about 1,000 nm. However, the D50 of the graphite particles of the present invention is significantly smaller, about less than 350 nm or less than 325 nm or less than 300 nm. This is due in part to the way the graphite is ground into smaller particles and the dispersants used therein.

[0102] Examples of conductive carbons that can be used in the carbon dispersion include, for example, carbon black and graphite. Various types of carbon can be used, including, for example, carbon black, furnace black, and graphite.

[0103] As for carbon black, preference is given to using carbon black that is initially acidic or neutral, i.e., carbon black that when slurried with water has a pH between about 1 and about 7.5, more preferably between about 2 and about 4. Preferred carbon black particles are also very porous and typically have a surface area of ​​about 45 m2 / g to about 1100 m2 / g, and preferably about 300 m2 / g to about 600 m2 / g, as measured by the BET method (Brunauer-Emmert-Teller method).

[0104] Examples of some commercially available carbon blacks suitable for use in the present invention include Cabot XC-72R Conductive, Cabot Monarch 800, Cabot Monarch 1300 (all available from Cabot Corporation of Boston, Massachusetts). Other suitable carbon blacks include Columbian T-10189, Columbian Conductiex 975 Conductive, Columbian CC-40,220, and Columbian Raven 3500 (all available from Columbian Carbon Company of New York, New York). Suitable graphites include Showa-Denko UFG (available from Showa-Denko KK, 13-9 Shiba Dmon 1-Chrome, Minato-Ku, Tokyo, 105 Japan), Nippon Graphite AUP (available from Nippon Graphite Industries, Ishiyama, Japan), and Asbury Micro 850 (available from Asbury Graphite Mills of Asbury, New Jersey).

[0105] When the coating composition is applied to a substrate, the conductive carbon particles should be present in an amount effective to provide a conductive coating. The carbon can be present in a concentration ranging from about 0.1% to about 20% by weight of the composition, or from about 0.5% to about 10% by weight, or from about 1% to about 7% by weight, or greater than about 4% to about 6.5% by weight.

[0106] In one embodiment, the lower limit of the total concentration of solids in the coating composition is between about 1.5 wt % and about 5 wt %, more preferably between about 2.0 wt % and about 4.5 wt % solids, this value includes graphite and / or carbon black particles themselves and dispersants and / or one or more binders, and any buffer and / or pH regulator, to produce the concentration of total solids in solution. The upper limit of the total concentration of solids in the coating composition is partly based on cost. Usually, the upper limit of the total concentration of solids is about 10 wt % solids, more preferably no more than 5 wt % solids. In one embodiment, the total concentration of solids in the coating composition is no more than 3 wt % solids, or in the range of about 2.0 wt % to about 2.5 wt % solids.

[0107] The normal percentage of graphite to other ingredients is about 50 to 60 wt %, so as an example, 1.5 wt % total solids would be 0.75 wt % graphite particles. This ratio of graphite / carbon to other ingredients (such as dispersants, binders, buffers, pH adjusters, etc.) can be about 33 to 80 wt % of the total solids as graphite / carbon black.

[0108] Common binders and / or dispersants used in prior art carbon dispersions are starch or polysaccharides, such as corn starch, potato starch, dextrin, gum arabic, etc. However, it has been found that starch does not produce foam or reduce the surface tension of the solution, and it is also a poor dispersant for preparing small particles and tight particle size distribution and stable colloids as described herein. Although it is believed that starch and polysaccharides help dry particles adhere more effectively to the surface of non-conductive substrates, the inventors of the present invention have found that in the process described herein, such starch and polysaccharides do not help to prepare small particles and stable colloids. In addition, compositions containing starch or other polysaccharides are more susceptible to bacterial or fungal growth than the compositions of the present invention. Therefore, in one embodiment, the carbon dispersion described herein does not contain any starch or polysaccharide or contains no more than trace amounts of any starch or polysaccharide.

[0109] The selection of dispersants and / or binders is an important feature of the present invention. In one embodiment, the dispersant has a negative charge, which imparts a negative charge to the particles that contributes to stability. In addition, the carbon dispersion must also exhibit desirable properties, including dispersants and / or binder materials that allow carbon particles to agglomerate onto the pore-enhanced wall surfaces to form a gel-like carbon / pore-enhancing material coating.

[0110] In one embodiment, the dispersant includes a wetting agent such as an anionic, nonionic or cationic surfactant (or a combination thereof, such as an amphoteric surfactant). The dispersant should be soluble, stable and preferably non-foaming in the liquid carbon black dispersion. Generally speaking, for a polar continuous phase (such as in water), the surfactant should preferably have a high HLB value (8-18). The preferred type of surfactant will depend primarily on the pH of the dispersion.

[0111] If the total dispersion is alkaline (i.e., has an overall pH in the alkaline range), it is preferred to use anionic or nonionic surfactants. Anionic surfactants include, for example, sodium or potassium salts of naphthalenesulfonic acid, such as DARVAN No.1 (RT Vanderbilt Co.), ECCOWET LF (Eastern Color and Chemical), PETRO AA, PETROULF (Petro Chemical Co., Inc.), and AEROSOL OT (American Cyanamid). Other anionic surfactants include neutralized phosphate ester surfactants, such as MAPHOS 55, 56, 8135, 60A, L6 (Mazer Chemicals Inc.). A preferred anionic surfactant for liquid carbon black dispersions is MAPHOS 56. Suitable nonionic surfactants include ethoxylated nonylphenols such as POLY-TERGENT B series (Olin Corporation) or alkoxylated linear alcohols such as POLY-TERGENT SL series (Olin Corporation).

[0112] If the total dispersion is acidic (i.e., having an overall pH in the acidic range), it is preferred to use a selected anionic or cationic surfactant. Examples of such anionic surfactants include, for example, sodium or potassium salts of the above-mentioned naphthalenesulfonic acid. Examples of suitable cationic surfactants include, for example, hexadecyldimethylbenzyl ammonium chloride, such as AMMONYX T (Onyx Chemical Corporation); ethanolated alkylguanamine complexes, such as AEROSOL C-61 (American Cyanamid); microencapsulated calcium (lipocals); dodecyldiphenyl oxide disulfonic acid (DDODA), such as DOWFAX 2Al (Dow Chemical); sodium salts of DDODA, such as STRODEX (Dexter Chemical Corporation); and salts of complex organic phosphates. Examples of preferred surfactants include amphoteric potassium salts of complex amino acids based on fatty amines such as MAFO 13 and cationic ethoxylated soy amines such as MAZEEN S-5 or MAZTREAT (Mazer Chemicals Inc.). Combinations of surfactants may be used.

[0113] If used, the binder can be any natural or synthetic polymer, polymerizable monomer, or other viscous or solid material (or its precursor) that is capable of adhering to the carbon particles and capable of receiving the anionic dispersant. Alternatively, the binder can disperse the carbon particles to which it adheres in the aqueous medium of the dispersion. For example, the binder can be a water-soluble or water-dispersible material selected from the group consisting of monosaccharides and polysaccharides (or more broadly, carbohydrates) and anionic polymers.

[0114] It is contemplated that polysaccharide (for purposes of the present invention, it includes disaccharides and higher sugars) binders for this paper include corn starch, other starches and polysaccharide gums.It is contemplated that polysaccharide gums for this paper include agar, gum arabic, xanthan gum (for example, KELZAN technical grade xanthan gum, purchased from Kelco Div.of Merck&Co, Inc.of Rahway, NJ), pectin, alginate, tragacanth gum, dextran and other gums.It is contemplated that the derived polysaccharide for this paper includes cellulose acetate, cellulose nitrate, methylcellulose and carboxymethyl cellulose.It is contemplated that the hemicellulose polysaccharide for this paper includes d-glucosyl-d-mannan, d-galactose-d-glucosyl-d-mannan etc.As mentioned above, in one embodiment, the carbon dispersion does not contain any starch or polysaccharide or contains any starch or polysaccharide that is no more than a trace amount.

[0115] Anionic polymers contemplated herein include alkyl or carboxyalkyl celluloses, their low and medium viscosity alkali metal salts (e.g., sodium carboxymethyl cellulose or "CMC"), cellulose ethers, and nitrocellulose. Examples of such anionic polymers include KLUCEL hydroxypropyl cellulose; AQUALON CMC 7L sodium carboxymethyl cellulose and NATROSOL hydroxyethyl cellulose, all of which are commercially available from Aqualon Company of Hopewell, VA; ethyl cellulose, available from Hercules of Wilmington, Del.; METHOCEL cellulose ethers, available from Dow Chemical Co., Midland, Mich.; and nitrocellulose, also available from Hercules.

[0116] In one embodiment, the dispersant includes an anionic surfactant such as ethoxylated phosphate esters, ethoxylated and propoxylated phosphate esters, ethoxylated tristyrylphenol phosphate esters, and combinations of one or more of the foregoing. These commercial products include, for example, TETRAPOL® available from Stepan Company. TSP-16PE30 and purchased from Solvay SA FLK. Other examples of acceptable anionic surfactants include sodium or potassium salts of naphthalenesulfonic acid, such as DARVAN No. 1 (commercially available from Eastern Color and Chemical), PETRO AA and PETRO ULE (commercially available from Petro Chemical Co., Inc.), and AEROSOL OT (commercially available from American Cyanamid). Preferred anionic surfactants include neutralized phosphate ester surfactants, such as MAPHOS 55, 56, 8135, 60A and L6 (commercially available from BASF Chemical Co.). The surfactant should be soluble, stable and preferably non-foaming in the liquid carbon dispersion.

[0117] The dispersant is selected to reduce surface tension and produce more stable and smaller suspended particles. The inventors of the present invention believe that a suitable dispersant is one that can be attached to the carbon particles by the grinding process, and the additional dispersant is in solution. The dispersant also allows the graphite (or carbon black) particles to adhere to the printed wiring board.

[0118] In one embodiment, the conductive carbon black dispersion of the present invention consists essentially of: (a) anionic surfactant or dispersant; (b) adhesive carbon black and / or graphite particles; (c) pH adjuster, wherein the pH adjuster is a hydroxide; (d) binder, wherein the binder does not include starch or polysaccharide; and (e) balance water, wherein the adhesive carbon black and / or graphite particles are ground in a grinding process capable of producing carbon particles of small particle size and tight particle size distribution. In one embodiment, the conductive carbon black dispersion of the present invention consists essentially of: (a) anionic surfactant or dispersant; (b) adhesive carbon black and / or graphite particles; (c) pH adjuster, wherein the pH adjuster is a hydroxide; and (d) balance water, wherein the adhesive carbon black and / or graphite particles are ground in a grinding process capable of producing carbon particles of small particle size and tight particle size distribution. "Essentially consisting of..." means that the conductive carbon dispersion does not contain any additional elements that would reduce the ability of the carbon dispersion to adhere to a printed wiring board. In a preferred embodiment, the conductive carbon dispersion of the present invention consists of the listed ingredients.

[0119] It is also important to have an alkaline pH, and the pH is preferably in the range of about 8 to about 13, more preferably in the range of about 8 to about 10. Higher pH causes more carbonate to be absorbed into the solution from carbon dioxide in the air, so too high a pH is avoided for this reason. In a preferred embodiment, a pH adjuster may be used, and suitable pH adjusters include hydroxides, such as potassium hydroxide and sodium hydroxide. In contrast, some prior art products use ammonia as a pH adjuster, which is a volatile pH adjuster and is not preferred because it evaporates and makes control more difficult. Therefore, in one embodiment, the carbon dispersion is at least substantially free of ammonia.

[0120] The liquid carbon dispersion is typically placed in a container and the printed circuit board is immersed in the liquid carbon dispersion, sprayed with the liquid carbon dispersion, or otherwise contacted with the liquid carbon dispersion. The temperature of the liquid dispersion in the immersion bath should be maintained between about 60°F and about 95°F during immersion, and preferably between about 70°F and about 80°F. The immersion time is advantageously in the range of about 15 seconds to about 10 minutes, more preferably in the range of about 30 seconds to 5 minutes.

[0121] The desired thickness of the carbon coating is a thickness sufficient to allow copper or other metal films to be electroplated onto a printed circuit board in a direct plating process. The upper limit of this thickness is determined by the ability to remove the carbon coating from the copper surface. If the carbon does not fall off the copper surface, defects may occur in the circuit board, including poor copper-copper contact in the inner layers of the circuit board. This is also referred to as an "interconnect defect". In one embodiment, the thickness may be in the range of about 0.05 microns to about 0.25 microns.

[0122] However, as mentioned above, it is important that the thickness is sufficient to allow metal plating in a direct plating process without any defects.

[0123] The problem of carbon particles adhering to the surface of the printed wiring board arises in particular when the micro-etching spray etching step is performed before the drying of the coating. In many prior art processes, after coating the printed wiring board with a carbonaceous dispersion, the coating is dried before spray etching is used to clean the copper surface. However, the inventors of the present invention have found that the order of the steps can be varied and that good results can be obtained if the micro-etching step is performed before the drying step.

[0124] On the other hand, the typical system for performing the direct plating process is a horizontal processing machine, and the sequence of steps in a horizontal processing system cannot be easily changed or modified. Therefore, it is not possible to change the sequence of steps in the process, in which case the drying step may have to be performed after the drying step. However, due in part to the careful selection of the high molecular weight porogen and the characteristics of the carbon dispersion, including the small particle size and tight particle size distribution, the adhesion of the carbon coating composition to the substrate can be optimized.

[0125] In one embodiment, the printed wiring board is further exposed to compressed air to unclog any through holes that may contain blockages of the dispersion.

[0126] The carbon black or graphite dispersion on the printed wiring board not only coats the drilled through-hole surface (which is desirable), but also completely coats the metal (i.e., copper) plate or foil surface (which is undesirable). Therefore, all the carbon black or graphite must be removed from the copper (or other metal) plate and / or foil surface before subsequent operations.

[0127] A micro-etching step is used to achieve removal of carbon black or graphite, particularly from copper (or other metal) surfaces (including especially the edges of the drilled holes), while leaving the coating intact on the epoxy surfaces of the glass fibers and hole walls.

[0128] The process described herein also reduces the tendency of hole wall peeling, in which the electroplated copper plated on the direct plated coating is pulled from the hole, slot or micro-via wall surface after soldering or thermal cycling. The increased carbon particle adhesion due to the use of the process described herein reduces or eliminates this problem.

[0129] Microetching solutions for removing excess graphite and / or carbon black are typically based on oxidants (such as hydrogen peroxide) or persulfates (such as sodium persulfate). For example, a suitable microetching solution is a sodium persulfate-based microetching solution that is mixed with sufficient sulfuric acid to prepare a microetching bath comprising 100 to 300 grams of sodium persulfate per liter of deionized water and about 1 to 10 weight percent sulfuric acid.

[0130] However, any etchant suitable for the metal being plated can be used in the practice of the present invention. For example, for copper plating, sodium persulfate-based etchants, peroxide-sulfuric acid-based etchants, cupric chloride-based etchants, and ferric iron-based etchants are all suitable. However, any oxidant capable of oxidizing copper metal to copper ions is sufficient and can be used in the process described herein.

[0131] In one embodiment, the printed wiring board is contacted with the microetchant by spraying the microetchant, and the microetchant can be sprayed at a pressure in the range of about 20 psi to about 50 psi, more preferably about 30 psi to about 40 psi and a temperature in the range of about 20° C. to about 45° C., more preferably about 30° C. to about 35° C.

[0132] As described herein, the steps of the direct plating process can be performed in various orders. For example, the printed wiring board panel can be dried before or after the spray etching step. Although it is generally preferred to perform the drying step after the spray etching step, there is still the benefit of achieving improved adhesion when the spray etching step is performed after the drying step.

[0133] After the micro-etching step and the subsequent water rinse, the printed wiring board can be advanced to the photo-imaging process and then subjected to electroplating or directly subjected to full-board electroplating. After the above-mentioned micro-etching step, the printed wiring board can be further cleaned with, for example, citric acid or benzotriazole anti-oxidation discoloration solution or another acidic cleaning agent solution or both. The printed wiring board so treated is then ready for an electroplating operation, which includes immersing the printed wiring board in a suitable electroplating bath to plate a copper (or other metal) coating on the through-hole walls of the non-conductive layer.

[0134] As described herein, in a preferred embodiment, the carbon coating is not dried prior to etching. In addition, the metal plating step can also be performed without first drying the carbon coating.

[0135] After the etching step (or etching and plating steps), the printed circuit board is dried for a period of time to remove water. In one embodiment, the printed circuit board is dried at an elevated temperature for a period of time of about 20 seconds to about 90 seconds, more preferably about 30 seconds to about 60 seconds. The elevated temperature may be between about 125°F and about 200°F, more preferably between about 150°F and 175°F.

[0136] The plated metal is typically copper. However, the present invention is not limited to copper plating, and the plated metal may be, for example, nickel, rhodium, platinum, cobalt, gold, tin, lead, and alloys of any of the foregoing metals. Other metals are also known to those skilled in the art and may be plated using the process described herein.

[0137] The invention will now be discussed in conjunction with the following non-limiting examples.

[0138] Example 1 :

[0139] The printed circuit board with through holes is processed as follows:

[0140] 1) Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0141] 2) Rinse the circuit board with tap water for 30 seconds.

[0142] 3) The circuit board was immersed in a carbon black dispersion according to the present invention. The particles were adhesive carbon black particles with a particle size of 80 nm for D-50 and 377 nm for D-99. The dispersant used was an ethoxylated phosphate ester. A colloid was prepared at 20% solids and then diluted to 3% solids for use, which had a pH of 9.2.

[0143] 4) Use 50g / L sodium persulfate etchant and spray etch the circuit board at 40psi

[0144] 30 seconds. Etches 15 micro inches of copper from the copper surface.

[0145] 5) The panels were spray washed using tap water at 40 psi.

[0146] 6) Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0147] PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added to

[0148] 20A / ft 2 The board was directly plated with copper for 5 minutes.

[0149] When inspecting copper plated printed circuit boards, there are no pin holes present in the plated copper deposit.

[0150] Comparative Example 1 :

[0151] The printed circuit board with through holes is processed as follows:

[0152] 1) Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0153] 2) Rinse the circuit board with tap water for 30 seconds.

[0154] 3) The circuit board was immersed in a carbon black dispersion prepared by mixing carbon black powder with a surfactant having a particle size of D-50 equal to 180 nm and D-99 equal to 743 nm. A dispersion of 16.7% solids was prepared and then diluted to 3% solids for use, which had

[0155] pH of 9.2.

[0156] 4) Use 50g / L sodium persulfate etchant and spray etch the circuit board at 40psi

[0157] 30 seconds. Etches 15 micro inches of copper from the copper surface.

[0158] 5) The panels were spray washed using tap water at 40 psi.

[0159] 6) Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0160] PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added to

[0161] 20A / ft 2 The board was directly plated with copper for 5 minutes.

[0162] When inspecting the copper plated printed circuit board, there is essentially no plating in the holes of the board because the carbon coating has been washed off during steps 4 and 5.

[0163] Example 2 :

[0164] The printed circuit board with through holes is processed as follows:

[0165] 1) Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0166] 2) Rinse the circuit board with tap water for 30 seconds.

[0167] 3) The circuit board was immersed in a graphite dispersion according to the invention prepared in such a way that the particle size was 237 nm for D-50 and 1853 nm for D-99. The dispersant used was an ethoxylated phosphate ester. A colloid was prepared at 21% solids and then diluted to 3% solids for use, which had a pH of 9.2.

[0168] 4) The board was spray etched using 50 g / L sodium persulfate etchant at 40 psi for 30 seconds. 15 micro inches of copper were etched from the copper surface.

[0169] 5) The panels were spray washed using tap water at 40 psi.

[0170] 6) Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0171] A bath of PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added at 20 A / ft 2 The board was directly plated with copper for 5 minutes.

[0172] When inspecting copper plated printed circuit boards, there are no pin holes present in the plated copper deposit.

[0173] Comparative Example 2 :

[0174] The printed circuit board with through holes is processed as follows:

[0175] 1) Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0176] 2) Rinse the circuit board with tap water for 30 seconds.

[0177] 3) The circuit board was immersed in a graphite dispersion prepared by mixing graphite powder with a surfactant having a particle size of D-50 equal to 1050 nm and D-99 equal to 4288 nm. A 22% solids dispersion was prepared and then diluted to 3% solids for use, having a pH of 9.5.

[0178] 4) The board was spray etched using 50 g / L sodium persulfate etchant at 40 psi for 30 seconds. 15 micro inches of copper were etched from the copper surface.

[0179] 5) The panels were spray washed using tap water at 40 psi.

[0180] 6) Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0181] A bath of PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added at 20 A / ft 2 The board was directly plated with copper for 5 minutes.

[0182] When inspecting the copper plated printed circuit board, there is essentially no plating on the holes in the board because the graphite coating has been washed off during steps 4 and 5.

[0183] Example 3 :

[0184] The printed circuit board with through holes is processed as follows:

[0185] 1) Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0186] 2) Rinse the circuit board with tap water for 30 seconds.

[0187] 3) The circuit board was immersed in a graphite dispersion according to the invention prepared in such a way that the particle size was 297 nm for D-50 and 1968 nm for D-99. The dispersant used was ethoxylated tristyrylphenol phosphate. A colloid was prepared at 19% solids and then diluted to 3%

[0188] The solid was used with a pH of 9.3.

[0189] 4) The board was spray etched using 50 g / L sodium persulfate etchant at 40 psi for 30 seconds. 15 micro inches of copper were etched from the copper surface.

[0190] 5) The panels were spray washed using tap water at 40 psi.

[0191] 6) Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0192] A bath of PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added at 20 A / ft 2 The board was directly plated with copper for 5 minutes.

[0193] When inspecting copper plated printed circuit boards, there are no pin holes present in the plated copper deposit.

[0194] Comparative Example 3 :

[0195] The printed circuit board with through holes is processed as follows:

[0196] 1. Immerse the printed wiring board in a bath of a whole-well material containing a high molecular weight polyquaternium compound having an average molecular weight of about 3,200,000, a pH buffer, and a surfactant at 95°F and pH 9.0 for 30 seconds.

[0197] 2. Rinse the circuit board with tap water for 30 seconds.

[0198] 3. The circuit board was immersed in a carbon black dispersion prepared by mixing carbon black powder with water and an oil-based hydroxyethyl imidazoline surfactant having a particle size of D-50 equal to 385 nm and D-99 equal to 1503 nm. A dispersion of 14% solids was prepared and then diluted to 3% solids for use, having a pH of 9.2.

[0199] 4. Using 50g / L sodium persulfate etchant, spray etch the board at 40psi for 30 seconds. Etch 15 micro inches of copper from the copper surface.

[0200] 5. Spray wash the board using tap water at 40 psi.

[0201] 6. Use 80g / L copper sulfate pentahydrate, 200g / L sulfuric acid, 60ppm chloride ion and 1%

[0202] A bath of PC606 additive (MacDermid Enthone Inc, Waterbury, CT) was added at 20 A / ft 2 The board was directly plated with copper for 5 minutes.

[0203] When inspecting the copper plated printed circuit board, there is essentially no plating on the holes in the board because the carbon black coating has been washed off during steps 4 and 5.

[0204] Thus, it can be seen that the process described herein produces a carbon dispersion that adheres more tightly to the printed wiring board substrate, which in turn produces an adherent copper deposit without pinholes or other defects in the deposit.

[0205] Finally, it is to be understood that the following claims are intended to cover all generic and specific features of the invention described herein, together with all statements of the scope of the invention which might literally fall between them.

Claims

1. A method for preparing a non-conductive substrate to allow metal plating thereon, the method comprising the steps of: a) optionally but preferably contacting the non-conductive substrate with a pre-cleaner; b) contacting the non-conductive substrate with a pore-forming material comprising a high molecular weight porogen; c) applying a liquid carbon-based dispersion to a porous non-conductive substrate to form a carbon / porous substance gel coating on the porous non-conductive substrate, wherein the carbon-based dispersion comprises adherent carbon or graphite particles dispersed in a liquid solution, wherein the carbon particles condense onto the porous substrate to form the carbon / porous substance gel coating; as well as d) etching the carbon / porous material gel-coated substrate; The adherent carbon black or graphite particles in the liquid carbon-based dispersion have a small particle size and a tight particle size distribution.

2. The method according to claim 1, wherein the etching step is performed before the liquid carbon-based dispersion is dried on the non-conductive substrate, and the method further comprises a step of drying the substrate and the carbon-based dispersion after step d) to form a conductive carbon coating on the substrate.

3. The method according to claim 2, further comprising the step of electroplating a conductive metal on the substrate after step d). The method according to claim 1 , wherein the pore-stabilizing agent is a polyquaternary ammonium salt compound.

5. The method of claim 4, wherein the polyquaternium compound has a molecular weight of at least 1,000,000 g / mol.

6. The method of claim 5, wherein the polyquaternium compound has a molecular weight of at least 2,000,000 g / mol.

7. The method of claim 6, wherein the polyquaternium compound has a molecular weight of at least 3,000,000 g / mol.

8. The method of claim 1, wherein the substrate comprises a printed circuit board or a printed wiring board.

9. The method of claim 1, wherein the substrate is contacted with the pore-permeating substance by immersing the substrate in the pore-permeating substance for at least about 20 seconds.

10. The method of claim 3, wherein the carbon-based dispersion is dried after the etching step and before the electroplating step.

11. The method of claim 3, wherein the carbon-based dispersion is not dried prior to the etching step or the plating step.

12. The method of claim 3, wherein the carbon-based dispersion is dried after the etching step and after the electroplating step.

13. The method of claim 1, wherein the carbon-based dispersion comprises: a. Dispersant; b. an optional binder; c. a conductive carbon source, wherein the conductive carbon source is selected from carbon black and / or graphite particles; d. a pH adjuster, wherein the pH adjuster is a hydroxide; and e. The remaining amount of water, and The carbon black and / or graphite particles are ground in a grinding process which produces coherent carbon black and / or graphite particles having a small particle size and a tight particle size distribution.

14. The method of claim 13, wherein the conductive carbon is graphite and the graphite particles have a D50 of less than 350 nm and a D99 of less than 2500 nm.

15. The method of claim 14, wherein the graphite particles have a D50 of less than 300 nm.

16. The method of claim 13, wherein the conductive carbon is carbon and the graphite particles have a D50 of less than 100 nm and a D99 of less than 400 nm.

17. The method of claim 13, wherein the dispersant is an anionic surfactant selected from the group consisting of ethoxylated phosphate esters, ethoxylated and propoxylated phosphate esters, ethoxylated tristyrylphenol phosphate esters, and combinations of one or more of the foregoing.

18. The method of claim 1, wherein the concentration of the carbon or graphite particles in the carbon-based dispersion is between about 2% and about 5% by weight.

19. The method of claim 1, wherein the substrate is dried at a temperature between about 125°F and about 200°F for about 20 seconds to about 90 seconds.

20. The method of claim 19, wherein the substrate is dried for about 30 seconds to about 60 seconds.

21. The method of claim 19, wherein the substrate is dried at a temperature between about 150°F and about 175°F.

22. The method of claim 1, wherein the metal portion of the substrate is etched with an etchant selected from the group consisting of a sodium persulfate-based etchant, a peroxide-sulfuric acid-based etchant, a cupric chloride-based etchant, and a ferric iron-based etchant.

23. A two-component gel coat composition for preparing a non-conductive substrate to allow metal plating thereon, the two-component gel coat comprising: a. A pore-forming material, the pore-forming material comprising: i polyquaternium compounds having a molecular weight greater than 1,000,000 g / mol; ii. pH buffer; and iii. surface tension reducing agent, wherein the pore-forming material has a pH in the range of about 8 to about 10; and b. a liquid carbon-based dispersion, wherein the liquid carbon-based dispersion comprises: i. adherent carbon or graphite particles dispersed in a dispersant, and ii. pH adjuster; wherein the adherent carbon black or graphite particles have a small particle size and a tight particle size distribution, and wherein the pH of the liquid carbon dispersion is in the range of about 8 to about 10; wherein when the pore-integrating substance and the liquid carbon-based dispersion are sequentially applied to the non-conductive substrate, an adherent carbon / pore-integrating substance gel coating is formed on the surface of the non-conductive substrate.

24. The two-component gel coat composition of claim 23, wherein the conductive carbon is graphite and the graphite particles have a D50 of less than 350 nm and a D99 of less than 2500 nm.

25. The two-component gel coat composition of claim 24, wherein the graphite particles have a D50 of less than 300 nm.

26. The two-component gel coat composition of claim 23, wherein the conductive carbon is carbon and the graphite particles have a D50 of less than 100 nm and a D99 of less than 400 nm.

27. The two-component gel coat composition of claim 23, wherein the pH adjuster of the liquid carbon-based dispersion is a hydroxide.

28. A printed circuit board coating comprising a plurality of through holes, wherein the through-hole is prepared for electroplating by applying the two-component gel coat composition of claim 23 to form an adherent carbon / porous material gel coating on the surface of the through-hole.

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