FIELD

[0001] Embodiments herein generally relate to manufacturing circuit boards using inkjet printing processes. This application describes new methods and inks for inkjet printing circuit boards.

BACKGROUND

[0002] Printed circuit boards (PCBs) are commonly made by forming a conductive sheet, for example copper metal, on a non-conductive substrate, masking portions of the conductive sheet, and etching the unmasked portions to leave a pattern of conductive traces on the non-conductive substrate. The masking is normally done using a resin material that is acid-resistant. The copper is normally etched using an acid solution. The mask material protects copper under the mask material from being etched by the acid. In one commonly-used process, the mask material is printed onto the conductive sheet using an inkjet printing process. A PCB product can include one substrate with a circuit printed on one or both sides, or may include a plurality of substrate with multiple circuits laminated together in a complex device.

[0003] In an inkjet process of forming a circuit pattern negative image on a PCB device, a primer material is coated onto the substrate, and then a material reactive with the primer is applied onto the primer by inkjet printing in a pattern. The reactive material reacts with the primer and is frozen in place by the reaction. The primer is usually a polycationic material such as polyethyleneimine, a divalent salt matrix, or vinyl pyrrolidone polymers. The pattern material is an acid-resistant material that reacts with the polycationic material. Examples of such materials include acrylic and styrene- acrylic resins. The primer is typically coated onto the metallic surface and dried. The pattern material is then applied in the pattern to react with the primer and freeze in place. The pattern material is usually applied in base form, with pH above 7.0 to react with the primer. Acid, such as HCI, can be added to the primer to enhance the reaction.

[0004] In many cases, the base included with the pattern material is volatile. Ammonia, for example, is used in some cases. Volatility of the base in the pattern material complicates use of the material as an inkjet material because as the volatile material weathers, the inkjet material changes composition and properties and applying the material by inkjet printing becomes unreliable. Viscosity of the material changes, resulting in imprecise application of the material to the substrate. As a result, the pattern formed in the conductive material is often out of tolerance. There is a need in the area of inkjet printed PCBs for new methods and materials for reliably patterning a conductive coating.

SUMMARY

[0005] Embodiments described herein provide a method, comprising depositing a conductive material on a substrate, applying a primer material soluble in water or aqueous acid onto the conductive material, inkjet printing an acid-resistant patterning material reactive with the primer material onto the primer material according to a pattern to form an acid-resistant mask, and exposing the substrate to an acid to etch exposed portions of the conductive material.

[0006] Other embodiments described herein provide a method, comprising depositing a conductive material on a substrate, inkjet printing a blanket of an acid-dissolvable primer material comprising a polycationic material onto an area of the conductive material, inkjet printing an acid-resistant patterning material comprising a polyanionic material onto the primer material according to a pattern to form an acid-resistant mask, and exposing the substrate to acid to remove portions of the conductive material according to the pattern.

[0007] Other embodiments described herein provide a method, comprising depositing a conductive material on a substrate, inkjet printing a blanket of an acid-dissolvable primer material comprising a polycationic material and a solvent onto an area of the conductive material, removing the solvent to solidify the primer material, inkjet printing an acid-resistant patterning material comprising a polyanionic material onto the solidified primer material according to a pattern to form an acid-resistant mask, and exposing the substrate to acid to remove portions of the conductive material according to the pattern. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments.

[0009] Fig. 1 is a flow diagram summarizing a method according to one embodiment.

[00010] Fig. 2 is a flow diagram summarizing a method according to another embodiment.

[00011] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

[00012] The processes for manufacturing a PCB described herein use inkjet processes to apply a patterned mask material to a substrate. The mask material is a bi-component material that is formed from stable precursors, at least one of which is printed in a pattern using an inkjet process. The bi-component material is a stable acid-resistant material that defines a circuitry pattern to be resolved in the conductive coating on the PCB.

[00013] Fig. 1 is a flow diagram summarizing a method 100 according to one embodiment. At 102, a primer is formed on a conductive material layer of a substrate. The substrate can be any material that provides a foundation for the conductive material layer. For example, the substrate can be a conventional circuit board blank made of a resin, optionally impregnated with polymer fibers. The substrate can also be a piece of glass, a polymer film, or any other material on which circuitry or microcircuitry is to be formed. The conductive material layer is typically a metal, or any conductive acid-dissolvable material. Copper is an example, and is commonly used.

[00014] The primer is an ionic material that can be applied as a liquid and solidified into a layer. The primer material may be a polymer precursor or a polymeric material dissolved in a solvent. The primer polymer is typically a polycationic polymer (i.e. a polybase), which is a polymer that can accept one or more protons to become a cation. The polycationic polymer has a plurality of locations that can accept protons from a proton-donor to form an ionic bond, or to catalyze formation of a covalent or quasi- covalent bond. Examples of polymer materials that may be used in a primer material include polyethyleneimines, polyspermines, polyspermidines, polyputrescines (poly- butanediamine) and other related polyamine polymers, which may be substantially linear or cross-linked, polyamidoamines, polyvinylpyrrolidones, polydiallyldimethylammonium chloride, polylysines, polytriazines, polyaminals and polythioaminals, and natural and semi-synthetic polycationic materials such as chitosans, gelatins, celluloses, and starch derivatives such as dextrans and dextrins, pectin, polypeptides, and alginates. These can be used as homopolymer, copolymer, or multi-polymer, and may be inter-polymerized, cross-linked, or dendrimerized with other monomers and polymers such as vinyl species (i.e. polyamino-diacrylate co esters), epoxy species, and urethane species. To the extent polymerizables such as vinyl species and epoxy species are included in the primer precursor, activators such as radical activators and co-activators (for example both monomers of co polymerization systems like epoxy and urethane systems) may be included. The polymeric primer material is typically acid soluble, such that primer material not reacted with patterning material can be subsequently removed to expose conductive material for etching.

[00015] The polymeric primer material is applied using a water-based precursor containing the polymer primer material, which is applied to the conductive material of the substrate as a first component of a bi-component system for fixing a patterned acid-resistant mask on the conductive material. A liquid medium for applying the primer material typically includes water with an organic, water miscible co-solvent such as monoethylene glycol, diethylene glycol, propylene glycol, dipropylene glycols, polyethylene glycols, polypropylene glycols, glycerine, aliphatic and aromatic amides, carboxylic acids, ethers, esters, alcohols, organosulfides, organosulfoxides, sulfones such as sulfolane, carbitol, butyl carbitol, cellusolve, amino alcohols (i.e. amino methyl propanol), ketones, N-methylpyrrolidone, cyclohexylpyrrolidone, hydroxyethers, lactones, imidazoles, and mixtures thereof. The non-water component is typically present from 0% to about 50% by weight, and helps to dissolve any other additives that may be used, such as colorants (i.e. Bayscript blue dye), salts, chelating agents (i.e. Trilon B), and the like . Wetting agents such as BYK-345, BYK-307, BYK-306, BYK-308, BYK-333, and BYK-341 , available from BYK Chemie; Fluorad FC-120 or other Fluoro surfactant; Masurf FS-1620 available from Mason Chemical Company; Surfinol 104 PG and Dynol 604 available from Air Products, Inc.; and Silwet L77 available from Witco Chemicals; TEGO Wet 270 available from Evonik; and Triton X- 100, FS-30, FS-34, FS-35, and FS3100 available from The Chemours Company. The wetting agents may be present in amounts up to about 20% by weight of the total mixture.

[00016] The compositions above are typically applied to the conductive material of the substrate using a method that can achieve a thin, uniform coating of the primer material on the conductive material. One such method is inkjet printing. Other methods include spray, ribbon, slot, die, and gravure coating. The compositions are tuned for used in such methods, for example by adjusting viscosity for optimal application. The polymers above can be tuned according to molecular weight and cross-linking by adjusting types of monomers and catalyst or activator content or activity, for example, to provide an optimal loading of primer material at a target viscosity for the application method. The precursor is typically applied to a thickness of 2-50 pm in direct contact with the conductive material of the substrate, thus covering at least a portion of the conductive material to be processed for patterning into circuitry.

[00017] After application of the precursor material, the precursor material is solidified to complete formation of the primer material. The substrate may be subjected to a drying process, which may include elevated temperature of up to about 150°C, for example about 80°C, and/or reduced pressure, such as negative pressure up to about 250 Torr. In the event polymerizable components that require activation are included, the substrate may be exposed to ultraviolet light to activate polymerization of such components. The resulting primer is a solid coating over the conductive material of the substrate that is reactive with a second component of the bi-component system to be applied in a pattern to form the patterned mask.

[00018] At 104, an optional transition material is applied to the primer material. The transition material prepares the surface of the primer to accept, and optimally bond to, the patterning material to be applied subsequently. For example, a solvent removal material may be applied to the primer material to speed removal of solvent species slow to evaporate by dissolving them in a more volatile material. As another example, an adhesion promoter may be applied. The adhesion promoter may include functionalities that can bind with the primer material and the patterning material. Examples of adhesion promoters that may be used include peptides and silane coupling agents with acid and base reactive functionalities. The transition material may be similar to the primer material in function, but different in composition, physical properties, or chemical properties. For example, a second polymeric primer material may be applied that is the same composition, but different in molecular weight or reactive site density. Alternately, the second polymeric primer material may be a different polymer from the underlying polymeric primer material. The optional transition material is optionally used to increase bonding of the patterning material to the primer material. The transitional material can also be used to adjust any thickness, density, or surface elevation non-uniformities in the primer material.

[00019] At 106, a patterning material is applied to the transition material, or directly to the primer, to react with the primer. The patterning material is applied in a pattern to form a patterned, acid-resistant mask material that covers portions of the conductive material while leaving other portions uncovered. The patterning material is applied as a liquid using a method of patterned application such as printing, for example inkjet printing or other liquid printing method. The patterning material contains a material that reacts with the polycationic species of the primer to form an acid-resistant polymer. The material is thus the second component of the bi-component system comprising the primer material and the primer-reactive material. The primer-reactive material is an anionic or polyanionic material (i.e. a polyacid), such as polyacrylic acid or poly methacrylic acid. Mixed poly alkylacrylic acids, as copolymers or multipolymers, or as mixtures of homopolymers and/or copolymers and multipolymers, can also be used. Other suitable polyanionic materials include polyanionic cellulose (i.e. ANTISOL polyanionic cellulose from Dow Chemical Co.), and polystyrene sulfonate. The polyanionic species is used in a water solution, which may also contain wetting agents, colorants, and the like, as described above.

[00020] A base that is at least somewhat soluble or miscible in water is added to the patterning material to activate the polyanionic species in the patterning material. The strong base increases reactivity of the patterning material with the primer material or the primer-plus-transition material. The strong base is believed to remove protons from the polyanionic species, generating anions that are more reactive with the primer material or the primer-plus-transition material.

[00021] The base should have pH of 7.2-12 and relatively low volatility such that the ink does not change composition appreciably during application to the substrate. Low toxicity is also a plus. Suitable materials for use as a strong base include alkyl amines NR ¹ R ² R ³ , where at least one of R ¹ , R ² , and R ³ is made of carbon and hydrogen, and one or more of R ¹ , R ² , and R ³ may be hydrogen alone, having the general formula C _x HyN _z , where x is 3-6 (i.e. alkyl amines having 3-6 carbon atoms), z is 1 or 2, and y is 2x+2+z; alkanolamines NR ⁴ R ⁵ R ⁶ , where at least one of R ⁴ , R ⁵ , and R ⁶ is a hydroxyalkyl group C _a H _2a OH, and where one or more of R ⁴ , R ⁵ , and R ⁶ may be hydrogen alone and one or more of R ⁴ , R ⁵ , and R ⁶ may be an alkyl group C _a H2a+i; and organocyclic amines such as pyrrole, pyrrolidine, piperidine, imidazole, pyrazole, pyridine, purine, diazines, and triazines. Example materials are isopropylmethylamine, butylamine, 3-dimethylamino-2-propanol, and triethanolamine. Small amounts of ammonia, for example not more than about 10% by weight of the entire composition, may be included with the above materials a stabilizer at levels where loss of ammonia from vaporization will not significantly change the properties of the patterning material. Bases that contain carbon have reduced volatility that contributes to thermal stability of the patterning material. Lower molecular weight bases can be used with lower temperatures that minimize volatility of the base and composition change of the patterning material. Where higher temperatures are encountered, higher molecular weight strong bases can be used.

[00022] The patterning material is applied in a precision patterned deposition process such as inkjet printing. In inkjet printing, the patterning material is applied in individual droplets to form features of uniform size as low as 10 pm. Thus, a masking pattern can be formed on the conductive material of the substrate. Precision patterned deposition of liquids can be performed using inkjet printers available from Kateeva, Inc., of Newark, California, or using systems available from other manufacturers.

[00023] The patterning material is frozen in place by substantially instantaneous reaction with the primer to form rigid or semi-rigid polymer features in a pattern. The polymer features formed by reaction of the patterning material with the primer material are acid-resistant, so the polymer features can be used as a mask in an acid treatment to remove the unreacted primer material and exposed conductive material. If any components can be activated by radiation, the patterning material can optionally be exposed to UV radiation to build hardness or rigidity. For example, following deposition of the patterning material on the primer material, the substrate may be irradiated with 395 nm light for 30 seconds to increase hardness of the patterned polymer material. Increasing hardness can increase acid resistance of the patterned polymer material.

[00024] At 108, the substrate is treated with an acid to remove the primer material and expose portions of the underlying conductive material. A weak aqueous acid, such as acetic acid or citric acid, can be used. Strong acids, such as HCI, optionally including salts such as ferric chloride and cupric chloride, acetic acid, nitric acid, chloric acid, perchloric acid, iodic acid, bromic acid, and sulfuric acid, optionally including suitable salts (i.e. ferric or cupric salts), or a strong acid can also be used. Suitable acids have a pH in water of 1.75 or higher. The conductive material not covered by patterned polymer material is exposed, while the conductive material covered by patterned polymer material remains covered, since the patterned polymer material is acid- resistant. The substrate may be dipped in a solution of the acid, or the acid, or solution thereof, may be sprayed onto the surface of the substrate bearing the primer material. The solution may be as concentrated as 1 M, but lower concentrations can be used. An electric potential may be applied in some cases to enhance etch rate. For example, an electrode can be attached to opposite edges of the conductive material, and a DC or AC electric field applied to enhance reaction of the metal with ions in the etchant solution. In some cases, the treatment of 108 may partially remove the primer material, leaving a thin coating to protect the conductive material from environmental factors, for example in the event the substrate is not further processed right away. When using particularly strong acids, care should be taken not to damage the acid resistant pattern or non-uniformly etch the conductive material.

[00025] The acid treatment process can be done in one application, or optionally in two applications. For example, a first acid application can be performed using a weak acid, and a second acid application can be performed using a strong acid. In another example, primer material can be removed using a water treatment, without acid, and then etching is done using an acid application. In another example, a strong acid can be applied, followed by a weak acid or water. In this case, the strong acid develops the pattern and etches exposed conductive material, and the weak acid or water application removes etching byproducts and can also remove remaining pattern material.

[00026] At 110, etching is discontinued, and the substrate is rinsed in water to remove etchants from the substrate. Alcohol, such as isopropyl alcohol, may be included in solution with the water as a rinse solution. The patterned polymer may remain on the PCB in some cases, or may be removed by rinsing with water, which may be accelerated by elevated temperature and/or pH. Protic co-solvents such as alcohol and ammonia can also speed dissolution. The underlying primer material is also usually water soluble, but removal may be enhanced using elevated temperature and/or polar aprotic solvents such as pyridine and N-methylpyrrolidone.

[00027] The resulting PCB will be the substrate with conductive microcircuitry patterned thereon. The microcircuitry can be encapsulated at this point. Alternately, a second resin substrate can be laminated over the first substrate with microcircuitry such that a three dimensional circuitry structure can be constructed. [00028] Fig. 2 is a flow diagram summarizing a method 200 according to another embodiment. The method 200 is a method of forming a patterning ink for making microcircuitry on a PCB. At 202, a water miscible base is added to a volume of water to form a base mixture. The base is an alkylamine, alkanolamine, or an organic heterocyclic amine. The base generally has low volatility, for example boiling point between room temperature and about 180°C, pH in water of 7.5-12, and low or no toxicity is a plus, but not required.

[00029] Suitable alkylamines have the structure NR ¹ R ² R ³ , where at least one of R ¹ , R ² , and R ³ is carbon and hydrogen, and one or more of R ¹ , R ² , and R ³ may be hydrogen alone, having the general formula C _x H _y N _z , where x is 3-6 (i.e. alkyl amines having 3-6 carbon atoms), z is 1 or 2 (monoamines and diamines), and y is 2x+2+z. Examples include isopropylmethylamine, diethylamine, triethylamine, and trimethylamine. Suitable alkanolamines have the general structure NR ⁴ R ⁵ R ⁶ , where at least one of R ⁴ , R ⁵ , and R ⁶ is a hydroxyalkyl group C _a H _2a OH, and where one or more of R ⁴ , R ⁵ , and R ⁶ may be hydrogen alone and one or more of R ⁴ , R ⁵ , and R ⁶ may be an alkyl group C _a H2a+i _. Examples include triethanolamine and 3-dimethylamino-2- propanol. Suitable organocyclic amines include pyrrole, pyrrolidine, piperidine, imidazole, pyrazole, pyridine, purine, diazines, and triazines. Mixtures of any of these classes of compounds can be used, and ammonia can be included in the mixture in small amounts. The base mixture is generally prepared to a pH of 7.5-12.

[00030] At 204, a polyanionic material is added to the base mixture. The polyanionic material is a polymer with a plurality of sites with removable protons. The polyanionic material can be one or more of polyacrylic acid, poly methacrylic acid, a mixed poly alkylacrylic acids, as copolymer or multipolymer, or as a mixture of homopolymers and/or copolymers and multipolymers, a polyanionic cellulose (i.e. ANTISOL polyanionic cellulose from Dow Chemical Co.). Typically, the polyanionic material is added as a solid or a water dispersion.

[00031] At 206, the polyanionic material is dissolved in the base mixture. The base mixture with added polyanionic material is mixed for a period of time such as 2-24 hours. Heat may be applied to warm the mixture 10-20 °C above ambient temperature. [00032] At 208, further solvent, surfactant, or co-solvent may optionally be added to adjust viscosity, pH, surface tension, or other properties for processing. Water can be used, along with other solvents such as alcohols, for example glycols. It should be noted that any suitable solvent can be used to make mixtures according to the concepts described herein. The base is selected, along with the solvent, to dissolve the polyanionic material according to the deprotonation strength of the base in the solvent.

[00033] Example formulations made according to the method 200 are shown below. Example formulations are numbered across the top of the table and components are numbered down the side. The components are as follows:

Component 1 - Joncryl 8085 (polyacrylic acid in ammonium hydroxide solution); Component 2 - Joncryl 682 (polyacrylic acid); Component 3 - propylene glycol; Component 4 (base) - amino methyl propanol; Component 5 (base) - isopropylmethylamine; Component 6 (base) - ethanolamine; Component 7 (base) - isobutylamine; Component 8 (base) - 1 -amino-2 - propanol; Component 9 (base) - sec-butylamine; Component 10 (base) - 3- dimethylamino-2-propanol; Component 11 - ethylenediaminetetraacetic acid (EDTA); Component 12 - Bayscript Cyan; Component 13 - Bayscript Blue; Component 14 -TEGO Wet 500; Component 15 - deionized water.

In each example, the base is added to the water to form a base mixture, and then the polyacrylic acid component is added to the base mixture to form a dispersion mixture. The dispersion mixture is stirred for a period to dissolve the polyanionic material. Afterwards, the other ingredients are added in no particular order. The example formulations are as follows:

[00034] The ink made in the method 200 of Fig. 2 is usable as a patterning material in the method 100 of Fig 1. Such inks can be made prior to use in forming a circuitry pattern, and can be stored for, or used over a period of, up to a month. The ink may be continuously mixed while in use to form patterned materials, or may be intermittently mixed between uses.

[00035] The amount of base needed depends on the dissociation constants of the base, the relative acid strength of the conjugate anions, and the polyanionic material. Stronger bases generate more anions with lower acid strength. Larger molecular weight polyanions need more and/or stronger anions to dissolve them. Bases having low toxicity can be selected for convenience, but any suitable base can be selected according to its ionic properties. The base is generally selected according to its solubility in the solvent or solvent mixture used for the ink along with its ability to attract protons from the polyanionic material. The amount of base needed depends on the concentration of acid groups on the polyanionic material and on the dissociation constant of the base. Ideally, the base also does not react substantially with the polycationic material used as the primer, or the amount of base used is such that very little excess base remains after interacting with the polyanionic material.

[00036] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.