Curable inkjet compositions for manufacturing printed circuit boards

Abstract

A curable inkjet composition for a printed circuit board comprising more than one polymerizable compound and one or more thermal crosslinkers; the composition is characterized in that at least one polymerizable compound is methacrylate.

Description

Technical Field

[0001] The present invention relates to a curable composition for use as an inkjet composition in the manufacture of electronic devices. The present invention also relates to a cured product of the composition. Background Art

[0002] Printed circuit boards (PCBs) are traditionally manufactured in an extensive process that includes multiple photolithography and etching steps, generating a large amount of waste. In order to reduce the number of process steps, production costs, and the amount of waste, there is growing interest in digitizing the PCB manufacturing workflow.

[0003] Inkjet printing is the preferred digital manufacturing technology for several PCB production steps, such as applying resist and solder mask or printing legends.

[0004] PCB solder mask or solder resist acts as an insulator between copper traces and prevents the formation of solder bridges. In addition, it plays an important role in protecting the board from oxidation caused by external influences such as weather conditions, temperature changes, and humidity. When the solder mask develops mechanical defects due to exposure to these external conditions, the protective and insulating functions of the solder mask are negatively affected. Therefore, the main goal of producing an effective solder mask is to provide good resistance to the conditions to which it is exposed.

[0005] For the production of PCB solder masks by inkjet, the first requirement is that the cured ink has good adhesion on various substrates, such as copper and FR-4.

[0006] Adhesion on various substrates can be improved by adding adhesion promoters, as disclosed in WO2004 / 026977 and WO2004 / 105 (Avecia). WO2018 / 087056 (Agfa-Gevaert / Electra Polymers) discloses a combination of an adhesion promoter and a compound containing at least two phenolic groups. However, the presence of adhesion promoters may result in poor stability of the inkjet ink.

[0007] The second requirement is that the solder mask must be able to withstand the harsh conditions of typical finishing processes, such as soldering (solder resistance) and ENIG plating (ENIG plating resistance). In particular, the ENIG plating process, where harsh and variable conditions (pH and temperature) are used, places very stringent requirements on the adhesion of inkjet inks.

[0008] EP-A 4032958 (Agfa-Gevaert) discloses a radiation curable inkjet ink for preparing solder masks, wherein the ink comprises a polymerizable compound, a phenolic resin and a thermal crosslinker. The combination of these compounds ensures good adhesion and good resistance to ENIG plating.

[0009] EP-A 1624001 (Taiyo Ink Manufacturing) discloses an inkjet ink for solder mask printing, which comprises a (meth)acrylate monomer containing a thermosetting functional group. WO2020 / 109769 (ElectraPolymers) discloses an inkjet ink for solder mask printing, which comprises a reactive monomer, an oligomer of a prepolymer containing at least one epoxy or oxetane functional group, a free radical polymerizable compound, a thermal crosslinker, and a free radical initiator.

[0010] However, even with good adhesion and resistance to ENIG plating, cracks may still occur in the solder mask due to thermal stress generated when soldering electronic components on the PCB, or due to thermal changes during the service life of the PCB. Therefore, the printed and cured solder mask must withstand multiple thermal stress tests, such as immersion in a solder bath and IR reflow. These stress tests are especially important when performed after the ENIG plating process.

[0011] EP-A 3778793 (Taiyo Ink Manufacturing) discloses an inkjet ink for solder mask printing, which contains a photopolymerizable monomer having a cyclic skeleton and a shrinkage rate of less than 10%, thereby improving heat resistance before ENIG plating.

[0012] However, for inkjet inks used in the PCB manufacturing process, there is still a need to have good thermal stress test resistance performance before and after ENIG plating, while also having good resistance to ENIG plating and having sufficient ink stability. Summary of the invention

[0013] The object of the present invention is to provide a curable composition for solder mask printing in printed circuit board (PCB) manufacturing, combining good stability, good resistance to the ENIG plating process and good heat resistance before and after the ENIG plating process.

[0014] The objects of the invention are achieved by a curable composition as defined in claim 1 .

[0015] Further objects of the present invention will become apparent from the following description. DETAILED DESCRIPTION

[0016] definition

[0017] For example, the term "monofunctional" in the monofunctional polymerizable compound means that the polymerizable compound includes one polymerizable group.

[0018] For example, the term "difunctional" in a difunctional polymerizable compound means that the polymerizable compound contains two polymerizable groups.

[0019] For example, the term "polyfunctional" or "multifunctional" in a polyfunctional polymerizable compound means that the polymerizable compound contains more than two polymerizable groups.

[0020] The term "alkyl" refers to all possible variations for each number of carbon atoms in the alkyl group, i.e., methyl, ethyl; for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tert-butyl; for five carbon atoms: n-pentyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl and 2-methylbutyl, etc.

[0021] Unless otherwise specified, the substituted or unsubstituted alkyl group is preferably a C1 to C6 alkyl group.

[0022] Unless otherwise specified, the substituted or unsubstituted alkenyl group is preferably a C2 to C6 alkenyl group.

[0023] Unless otherwise specified, the substituted or unsubstituted alkynyl group is preferably a C2 to C6 alkynyl group.

[0024] Unless otherwise specified, the substituted or unsubstituted alkaryl group is preferably a phenyl group or a naphthyl group including one, two, three or more C1 to C6 alkyl groups.

[0025] Unless otherwise specified, the substituted or unsubstituted aralkyl group is preferably a C7 to C10 group including a phenyl group or a naphthyl group. 20 alkyl.

[0026] Unless otherwise specified, substituted or unsubstituted aryl is preferably phenyl or naphthyl.

[0027] Unless otherwise specified, the substituted or unsubstituted heteroaryl group is preferably a five-membered or six-membered ring substituted with one, two or three oxygen atoms, nitrogen atoms, sulfur atoms, selenium atoms or a combination thereof.

[0028] The term "substituted", such as in substituted alkyl, means that the alkyl group may be substituted with atoms other than the atoms normally present in the group (i.e., carbon and hydrogen). For example, a substituted alkyl group may include a halogen atom or a thiol group. Unsubstituted alkyl groups contain only carbon and hydrogen atoms.

[0029] Unless otherwise specified, substituted alkyl, substituted alkenyl, substituted alkynyl, substituted aralkyl, substituted alkaryl, substituted aryl and substituted heteroaryl are preferably substituted with one or more components selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl, esters, amides, amines, ethers, thioethers, ketones, aldehydes, sulfoxides, sulfones, sulfonates, sulfonamides, -Cl, -Br, -I, -OH, -SH, -CN and -NO2.

[0030] Unless otherwise stated, an alkyl, cycloalkyl or aryl group interrupted by a heteroatom means that the heteroatom is present in the carbon chain of the group, for example -COCC- or -CSCC-.

[0031] Curable inkjet composition

[0032] The curable composition according to the present invention comprises

[0033] - more than one polymerizable compound, wherein at least one polymerizable compound contains a vinyl group selected from vinyl ether groups, N-vinylamide groups and N-vinylurethane groups;

[0034] - one or more thermal crosslinking agents selected from unblocked isocyanates, blocked isocyanates and triazine compounds;

[0035] It is characterized in that at least one polymerizable compound is a methacrylate.

[0036] The composition is preferably a radiation curable composition. Any type of radiation may be applied, but preferred types of radiation are UV light and UV-LED light. Therefore, the curable composition according to the present invention is preferably a UV curable composition.

[0037] In a preferred embodiment, the composition according to the invention comprises a mixture of a UV curable compound and a thermal crosslinking agent. Therefore, the curable composition according to the invention is preferably also a heat curable composition. Any type of heat source can be used for the heat curing step, but heat curing is preferably carried out in an oven.

[0038] The two curing processes, thermal curing and UV curing, can be performed simultaneously or sequentially. This so-called dual curing principle leads to the formation of an interpenetrating polymer network, which the inventors believe is responsible for the superior mechanical properties observed compared to individually cured polymer networks.

[0039] The curable composition according to the present invention is preferably used as inkjet ink.

[0040] For reliable industrial inkjet printing, the viscosity of the curable inkjet ink is preferably no more than 20 mPa.s at 45°C, more preferably 1 to 18 mPa.s at 45°C, most preferably 5 to 15 mPa.s at 45°C, all at a shear rate of 1000 s -1 .

[0041] The preferred spraying temperature is 10 to 70°C, more preferably 20 to 55°C, most preferably 25 to 50°C.

[0042] For good image quality and adhesion, the surface tension of the curable inkjet ink is preferably from 18 to 70 mN / m at 25°C, more preferably from 20 to 40 mN / m at 25°C.

[0043] Polymerizable compounds

[0044] The curable inkjet composition according to the present invention comprises more than one polymerizable compound, wherein at least one polymerizable compound contains a vinyl group selected from a vinyl ether group, an N-vinyl amide group and an N-vinyl urethane group.

[0045] The polymerizable compound is preferably a free radical polymerizable compound. The free radical polymerizable compound can be a monomer, an oligomer and / or a prepolymer. These monomers, oligomers and / or prepolymers can have different functionalities, i.e. different numbers of free radical polymerizable groups. A mixture comprising a combination of single, two, three and more high functional monomers, oligomers and / or prepolymers can be used. The viscosity of the curable inkjet ink can be adjusted by changing the ratio between the monomer and the oligomer.

[0046] The polymerizable compound may also contain functional groups such as thiol, hydroxyl, amine, sulfonic acid, phosphoric acid and carboxylic acid. Examples of hydroxyl-functionalized polymerizable compounds are those listed in paragraphs

[0028] to

[0029] of US2015 / 0090482A.

[0047] Preferred polymerizable compounds are those listed in EP-A 1911814, paragraphs

[0106] to

[0115] .

[0048] Particularly preferred polymerizable compounds are selected from 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, isobornyl acrylate, hexanediol diacrylate, trimethylolpropane triacrylate, polyethylene glycol diacrylate, 2-phenoxyethyl acrylate and acryloylmorpholine.

[0049] At least one polymerizable compound in the curable inkjet composition contains a vinyl group selected from the group consisting of a vinyl ether group, an N-vinyl amide group, and an N-vinyl urethane group.

[0050] Preferred examples of polymerizable compounds containing vinyl ether groups or N-vinylamide groups are those listed in paragraphs

[0047] to

[0056] of EP-A 3686251.

[0051] A particularly preferred vinyl ether-containing polymerizable compound is 2-(2-vinyloxyethoxy)ethyl acrylate, as it is advantageous in providing a good balance between curability and viscosity of the curable inkjet composition, especially when used in an amount of 5 wt %, preferably at least 7.5 wt %, and most preferably at least 10 wt %, based on the total weight of the curable inkjet composition.

[0052] Particularly preferred vinylamide-containing polymerizable compounds are N-vinyl-2-pyrrolidone and N-vinylcaprolactam because they combine high Tg with good ink curability and good adhesion of the cured ink layer to the recording medium.

[0053] The polymerizable compound containing an N-vinyl carbamate group is preferably a cyclic compound represented by the general formula (I).

[0054]

[0055] Wherein R1, R2, R3 and R4 independently represent hydrogen, alkyl, cycloalkyl or aryl and combinations thereof, any of which may be interrupted by heteroatoms. Any of R1 to R4 may represent atoms necessary to form a five-membered or six-membered ring.

[0056] Preferably, R1, R2, R3 and R4 are independently hydrogen or a substituted or unsubstituted C1 to C 10 alkyl.

[0057] Preferred compounds are disclosed in WO 2015 / 022228 (BASF) and US 4831153 (DOW CHEMICAL).

[0058] Cyclic compounds according to formula I are generally referred to as oxazolidinones. A particularly preferred oxazolidinone is N-vinyl-5-methyl-2-oxazolidinone, also known as vinylmethyloxazolidinone or VMOX. The inclusion of VMOX improves the hardness of the cured ink layer, especially when it is used in an amount of 1 to 50 wt %, preferably 2.5 to 40 wt %, and most preferably 5 to 30 wt %, based on the total weight of the curable inkjet composition. In addition, VMOX has a low viscosity compared to other N-vinyl compounds, which makes it particularly suitable for inkjet printing.

[0059] The polymerizable compound containing vinyl ether, N-vinyl amide or N-vinyl carbamate may be used alone or in combination of one or more polymerizable compounds containing vinyl ether, N-vinyl amide or N-vinyl carbamate.

[0060] The curable composition according to the present invention comprises at least one methacrylate, i.e. any ester of methacrylic acid. The methacrylate may be a monofunctional, difunctional or multifunctional methacrylate. The methacrylate compound may also comprise other functional groups, such as but not limited to hydroxyl, thiol, amine, sulfonic acid, phosphoric acid and carboxylic acid. A mixture of methacrylates may be used.

[0061] Without limitation, preferred methacrylates are methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, hexyl methacrylate, ethylhexyl methacrylate, hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, 4-hydroxybutyl methacrylate, hydroxypentyl methacrylate, hydroxyhexyl methacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate. 、1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate, 1,12-dodecanediol dimethacrylate, triethylene glycol dimethacrylate, diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, 2-(2-vinyloxyethoxy)ethyl methacrylate, isopentyl methacrylate, stearyl methacrylate, lauryl methacrylate, octyl methacrylate, decyl methacrylate, isoamylstyl methacrylate, isostearyl methacrylate, 2-ethylhexyl methacrylate, methacryloyl morpholine, 2-ethylhexyl diglycol methacrylate, 2-methacryloyloxyethyl hexahydrophthalic acid, butoxyethyl methacrylate, ethoxydiethylene glycol methacrylate, methoxydiethylene glycol methacrylate, methoxypolyethylene glycol methacrylate, methoxypropylene glycol methacrylate, phenoxyethyl methacrylate, tetrahydrofurfuryl methacrylate, isobornyl methacrylate, 3,3,5-methacrylic acid trimethylcyclohexyl ester, 2-hydroxy-3-phenoxypropyl methacrylate, vinyl methacrylate, 2-methacryloyloxyethyl succinic acid, 2-methacryloyloxyethyl phthalic acid, 2-methacryloyloxyethyl-2-hydroxyethyl phthalic acid, lactone modified flexible methacrylate, tert-butyl cyclohexyl methacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, tripropylene glycol dimethacrylate, polypropylene glycol dimethacrylate, 1,4-Butanediol dimethacrylate, neopentyl glycol dimethacrylate, dimethylol tricyclodecane dimethacrylate, bisphenol A EO (ethylene oxide) adduct dimethacrylate, bisphenol A PO (propylene oxide) adduct dimethacrylate, hydroxy neopentyl glycol dimethacrylate, propoxylated neopentyl glycol dimethacrylate, alkoxylated dimethylol tricyclodecane dimethacrylate and polytetramethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, EO-modified trimethylolpropane trimethacrylate, tri(propylene glycol) trimethacrylate, caprolactone-modified trimethylolpropane trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, pentaerythritol ethoxylated tetramethacrylate, dipentaerythritol hexamethacrylate, ditrimethylolpropane tetramethacrylate, glycerol propoxylated trimethacrylate and caprolactam-modified dipentaerythritol hexamethacrylate.

[0062] Particularly preferred methacrylates are 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, 3,3,5-trimethylcyclohexyl methacrylate, isobornyl methacrylate, 1,6-hexanediol dimethacrylate, trimethylolpropane trimethacrylate, 2-(2′-vinyloxyethoxy)ethyl methacrylate and polyethylene glycol dimethacrylate.

[0063] It was found that the cured layer from the curable inkjet composition containing a certain amount of methacrylate compound exhibited better heat resistance. This is due to the lower curing speed of the composition, resulting in lower shrinkage and better adhesion of the cured layer. In order to achieve this effect, the amount of methacrylate compound in the curable inkjet composition is preferably greater than 1 wt%, more preferably greater than 2.5 wt%, and most preferably greater than 5 wt%, based on the total weight of the curable inkjet composition.

[0064] However, when the amount of the methacrylate is too high, the cured solder mask may not be effectively cured, thereby reducing the heat resistance of the cured layer.

[0065] Therefore, the amount of the methacrylate compound in the curable inkjet composition is preferably not more than 15 wt%, more preferably not more than 12.5 wt%, most preferably not more than 10 wt%.

[0066] Thermal crosslinking agent

[0067] The curable composition of the present invention comprises one or more thermal crosslinking agents selected from unblocked isocyanates, blocked isocyanates and triazine compounds. The presence of the thermal crosslinking agent can improve the adhesion of the resulting coating film after soldering or ENIG plating.

[0068] Thermal crosslinkers may be monofunctional, difunctional or polyfunctional.

[0069] The inkjet composition may contain a mixture of different thermal crosslinkers.

[0070] Preferred thermal crosslinking agents are ethylene oxide, oxetane, melamine formaldehyde resin, urea formaldehyde resin, benzoguanamine formaldehyde resin, cyclic carbonate compounds, carbodiimide, isocyanate, blocked isocyanate and combinations thereof.

[0071] Preferred thermal crosslinking agents are isocyanate compounds. Isocyanate compounds are preferably used in combination with compounds containing active hydrogen functionality, including but not limited to alcohols, thiols, amines, water, or combinations thereof. Atmospheric moisture may also cause isocyanate crosslinking. When atmospheric moisture reacts with isocyanates, it may not be necessary to prepare an ink that combines an isocyanate compound with a compound containing active hydrogen functionality.

[0072] The isocyanate compound may be an aliphatic, cycloaliphatic or aromatic isocyanate. If the isocyanate compound is a polyfunctional isocyanate, it may contain a combination of aliphatic, cycloaliphatic or aromatic isocyanate functionalities.

[0073] Examples of aliphatic isocyanates include, but are not limited to, 1,6-hexamethylene diisocyanate (HDI or HMDI), isophorone diisocyanate (IPDI), 1,3-(isocyanatomethyl)cyclohexane (hydrogenated XDI), lysine diisocyanate (LDI), 2,2,4-trimethylhexamethylene diisocyanate (TMDI), and dimer fatty acid diisocyanate (DDI).

[0074] Examples of cycloaliphatic isocyanates include, but are not limited to, isophorone diisocyanate (IPDI), methylcyclohexane 2,4-(2,6)-diisocyanate (hydrogenated TDI), and 4,4'-methylenebis(cyclohexyl isocyanate) (hydrogenated MDI).

[0075] Examples of aromatic isocyanates include, but are not limited to, toluene diisocyanate (TDI), 1,5-naphthalene diisocyanate (NDI), 4,4′-diphenylmethane diisocyanate (MDI), and xylylene diisocyanate (XDI).

[0076] Examples also include adducts of the isocyanates listed above (eg trimethylolpropane adducts), uretdiones, biuret and isocyanurates.

[0077] The isocyanate compound may be blocked or unblocked, preferably blocked.

[0078] Blocked isocyanates can be obtained by reacting an isocyanate with a selected blocking agent. Such a blocking agent is a protective group that is cleaved at high temperatures, for example during a thermal curing process. The blocking agent can be selected so that it cleaves at a certain temperature, the so-called deblocking temperature. The use of blocked isocyanates generally improves the storage stability of inkjet inks.

[0079] Examples of the blocking agent include alcohols (e.g., ethanol, n-propanol, isopropanol, tert-butyl alcohol, and isobutyl alcohol); phenols (e.g., phenol, chlorophenol, cresol, xylenol, and p-nitrophenol); alkylphenols (e.g., p-tert-butylphenol, p-sec-butylphenol, p-sec-amylphenol, p-octylphenol, and p-nonylphenol); basic nitrogen-containing compounds (e.g., 3-hydroxypyridine, S-hydroxyquinoline, and 8-hydroxyquinaldine); active methylene compounds (e.g., diethyl malonate, ethyl acetoacetate, and acetyl pyridine); acetone); amides (such as acetamide, acrylamide, and acetanilide); imides (such as succinimide and maleimide); imidazoles (such as 2-ethylimidazole and 2-ethyl-4-methylimidazole); pyrazoles (such as pyrazole, 3-methylpyrazole, and 3,5-dimethylpyrazole); lactams (such as 2-pyrrolidone and 8-caprolactam); oximes of ketones or aldehydes (such as acetone oxime, methyl ethyl ketone oxime, cyclohexanone oxime, and acetaldehyde oxime); ethyleneimine; and bisulfites.

[0080] For toxicological reasons, hindered secondary amines can be used as capping agents. Preferred hindered secondary amines are selected from ethyl tert-butylamine, diisopropylamine, 2,6-dimethylpiperidine, ethylisopropylamine, di-tert-butylamine and diisobutylamine.

[0081] Preferred blocked isocyanate compounds are blocked HDI oligomers or blocked IPDI oligomers. Such oligomers may be, for example, trimethylolpropane adducts, biuret or isocyanurate.

[0082] Particularly preferred blocked isocyanate compounds are Trixene Bl 7960, a HDI biuret blocked with 3,5-dimethylpyrazole, commercially available from Lanxess, and Trixene BI7982, a HDI trimer blocked with 3,5-dimethylpyrazole, commercially available from Lanxess.

[0083] The present invention particularly prefers thermal crosslinking agents having a triazine skeleton. The triazine portion is believed to contribute to the mechanical properties and heat resistance of the cured film. Any triazine compound having thermal crosslinking properties may be used.

[0084] Preferred triazine compounds have a chemical structure according to formula II,

[0085]

[0086] Wherein X represents N, O, S, P or C;

[0087] R5, R6 and R7 independently represent a substituted or unsubstituted alkyl group;

[0088] X preferably represents O or C, most preferably represents O.

[0089] Preferably, R5, R6 and R7 are independently substituted or unsubstituted C1-C8 alkyl. More preferably, R5, R6 and R7 are independently substituted or unsubstituted C1-C8 alkyl.

[0090] US5084541 (American Cyanamid Company) discloses preferred triazine compounds according to the general formula II and methods for their preparation.

[0091] Preferred triazine compounds according to the general formula II may be named NF 2000 is commercially available from Allnex and is available under the name 150 is commercially available from BASF.

[0092] The inkjet ink according to the present invention preferably comprises a blocked isocyanate compound or a triazine compound according to the general formula II.

[0093] More preferably, the inkjet ink comprises both a blocked isocyanate and a triazine compound according to formula II.

[0094] The total amount of thermal crosslinking agent is preferably 0.5 to 20 wt%, more preferably 1 to 15 wt%, most preferably 2 to 12 wt%, all relative to the total weight of the inkjet ink.

[0095] The amount of the blocked isocyanate compound is preferably 0.1 to 12.5 wt %, more preferably 2.5 to 10 wt %, most preferably 5 to 8 wt %, all relative to the total weight of the inkjet ink.

[0096] The amount of the triazine compound is preferably 0.1 to 5 wt%, more preferably 0.5 to 4 wt%, most preferably 1 to 3 wt%, all relative to the total weight of the inkjet ink.

[0097] When both the triazine compound and the isocyanate compound are present in the inkjet ink, the amount of the blocked isocyanate compound is preferably higher than the amount of the triazine compound. The ratio of the amount of the triazine compound to the amount of the isocyanate compound is preferably 0.05 to 0.95, more preferably 0.15 to 0.7.

[0098] It has been observed that the presence of a thermal crosslinker can improve various solder resist properties such as heat resistance, hardness, resistance to soldering heat, resistance to chemicals, electrical insulation properties, and resistance to electroless and immersion plating.

[0099] Photoinitiator

[0100] The curable inkjet composition preferably includes a photoinitiator, preferably a free radical photoinitiator.

[0101] A free radical photoinitiator is a compound that initiates the polymerization of monomers and oligomers by forming free radicals when exposed to actinic radiation. A Norrish Type I initiator is an initiator that breaks upon excitation and immediately produces an initiating free radical. A Norrish Type II initiator is a photoinitiator that is activated by actinic radiation and forms free radicals by abstracting hydrogen from a second compound, which becomes the actual initiating free radical. This second compound is referred to as a polymerization synergist or coinitiator. Both Type I and Type II photoinitiators can be used in the present invention alone or in combination.

[0102] Suitable photoinitiators are disclosed in CRIVELLO, JV et al., Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization, 2nd edition, edited by BRADLEY, G., London, UK: John Wiley and Sons Ltd, 1998, pp. 276-293.

[0103] Specific examples of free radical photoinitiators may include, but are not limited to, the following compounds or combinations thereof: benzophenone and substituted benzophenones; 1-hydroxycyclohexyl phenyl ketone; thioxanthones, such as isopropyl thioxanthones; 2-hydroxy-2-methyl-1-phenylpropan-1-one; 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one; benzyl dimethyl ketal; 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one; 2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone.

[0104] Preferred photoinitiators are thioxanthone compounds, such as Darocur ITX, which is an isomeric mixture of 2- and 4-isopropylthioxanthone.

[0105] Another preferred photoinitiator is an acylphosphine oxide compound. The acylphosphine oxide compound can be selected from monoacylphosphine oxides and diacylphosphine oxides. Preferred acylphosphine oxide photoinitiators are diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate (TPO-L), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide and 2,4,6-trimethoxybenzoyl-diphenylphosphine oxide.

[0106] Other preferred photoinitiators are α-hydroxyketone type I photoinitiators, such as those available from IGM resins as Oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinylphenyl]propanone]] obtained from KIP IT

[0107] The preferred amount of photoinitiator is 0.2 to 20 wt%, more preferably 0.5 to 10 wt%, most preferably 1 to 8 wt%, particularly preferably 1.5 to 6 wt%, all relative to the total weight of the curable inkjet composition.

[0108] In order to further improve the photosensitivity, the curable inkjet composition may also contain a co-initiator. Examples of suitable co-initiators can be divided into three categories:

[0109] (1) Tertiary fatty amines, such as methyldiethanolamine, dimethylethanolamine, triethanolamine, triethylamine and N-methylmorpholine;

[0110] (2) aromatic amines, such as amyl p-dimethylaminobenzoate, 2-n-butoxyethyl-4-(dimethylamino)benzoate, 2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate and 2-ethylhexyl-4-(dimethylamino)benzoate; and

[0111] (3) (Meth)acrylated amines, such as dialkylaminoalkyl (meth)acrylates (eg, diethylaminoethyl acrylate) or N-morpholinoalkyl-(meth)acrylates (eg, N-morpholinoethyl-acrylate).

[0112] Preferred coinitiators are aminobenzoates.

[0113] Preferred low molecular weight aminobenzoates are those from RAHN EPD.

[0114] Particularly preferred aminobenzoate co-initiators are selected from polymerizable, oligomeric and polymeric aminobenzoate co-initiators.

[0115] Polymerizable co-initiators are disclosed in EP-A 2 033 949 (Agfa Graphics NV).

[0116] In a more preferred embodiment, the aminobenzoate co-initiator is an oligomeric aminobenzoate derivative.

[0117] Particularly preferred aminobenzoic acid esters are polyether derivatives of aminobenzoic acid esters, wherein the polyether is selected from poly(ethylene oxide), poly(propylene oxide), copolymers thereof, and poly(tetramethylene oxide), ethoxylated or propoxylated neopentyl glycol, ethoxylated or propoxylated trimethylolpropane, and ethoxylated or propoxylated pentaerythritol.

[0118] Preferred oligomeric aminobenzoates are disclosed in WO 1996 / 33157 (Lambson Fine Chemicals Ltd.) and WO 2011 / 030089 (Sun Chemicals BV). Typical examples of polyethylene glycol bis-p-dimethylaminobenzoates are OMNIPOL ASA (commercially available from IGM Resins) and Speedcure 7040 (commercially available from Lambson Fine Chemicals).

[0119] Other oligomeric or polymeric co-initiators are for example: ESACURE A198 (polyfunctional amine from IGM) and CN3755 (acrylated amine co-initiator from ARKEMA).

[0120] Inhibitors

[0121] The curable inkjet composition may contain at least one inhibitor for improving the thermal stability of the ink.

[0122] Suitable polymerization inhibitors include phenolic antioxidants, hindered amine light stabilizers, phosphorus antioxidants, hydroquinone monomethyl ether commonly used in (meth)acrylate monomers, hydroquinone, tert-butylcatechol, pyrogallol, 2,6-di-tert-butyl-4-methylphenol (=BHT) can also be used.

[0123] Suitable commercial inhibitors are, for example, Sumilizer manufactured by Sumitomo Chemical Co. Ltd. TM GA-80, Sumilizer TM GM and Sumilizer TM GS; Genorad from Rahn AG TM 16. Genorad TM 18 and Genorad TM20; Irgastab from Ciba Specialty Chemicals TM UV10 and Irgastab TM UV22, Tinuvin TM 460 and CGS20; Floorstab from Kromachem Ltd TM UV Series (UV-1, UV-2, UV-5 and UV-8), Additol from CytecSurface Specialties TM S Series (S100, S110, S120 and S130).

[0124] Since excessive addition of these polymerization inhibitors may reduce the curing speed, it is preferred to determine the amount capable of preventing polymerization before mixing. The amount of the polymerization inhibitor is preferably less than 5 wt %, more preferably less than 3 wt % of the total curable inkjet composition.

[0125] Adhesion Promoters

[0126] The curable inkjet composition may contain an adhesion promoter to further optimize the adhesion of the cured composition to various surfaces, particularly copper surfaces.

[0127] Any adhesion promoter may be used, such as those disclosed in WO2004 / 026977 and WO2004 / 105, both from AVECIA, WO2017 / 009097 and WO2020 / 104302, both from Agfa-Gevaert, and WO2018 / 087059, WO2018087052, WO2018087056, and WO2018087055, all from Agfa-Gevaert / ElectraPolymers.

[0128] The curable inkjet composition may include one adhesion promoter or a combination of two, three or more different adhesion promoters.

[0129] The total amount of adhesion promoter is preferably from 0.1 to 20 wt%, more preferably from 0.5 to 15 wt%, most preferably from 1 to 10 wt%, all relative to the total weight of the inkjet composition.

[0130] However, it has been observed that the inkjet composition according to the present invention can still have sufficient adhesion even after soldering, gold plating or ENIG plating in the absence of an adhesion promoter. Since the presence of an adhesion promoter may lead to poor stability of the inkjet ink, the curable inkjet composition according to the present invention preferably does not contain an adhesion promoter. The amount of adhesion promoter is preferably less than 2.5 wt %, more preferably less than 1 wt %, and most preferably less than 0.5 wt % relative to the total weight of the composition.

[0131] Flame retardants

[0132] The curable inkjet composition preferably comprises a flame retardant.

[0133] Preferred flame retardants are inorganic flame retardants such as alumina trihydrate and boehmite; organophosphorus compounds such as organophosphates (e.g. triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP), bisphenol A diphenyl phosphate (BADP), and tricresyl phosphate (TCP)); organophosphonates (e.g. dimethyl methylphosphonate (DMMP)); and organophosphinates (e.g. aluminum dimethylphosphinate).

[0134] Preferred flame retardants are disclosed in WO2019 / 121098.

[0135] Colorants

[0136] The curable inkjet composition may be a substantially colorless inkjet ink or may include at least one colorant. For example, when the inkjet ink is used as an etch resist, the colorant allows the conductive pattern manufacturer to clearly see the temporary mask so that the quality can be visually checked. When the inkjet ink is used to apply a solder mask, it typically contains a colorant. The preferred color of the solder mask is green, but other colors, such as black or red, may also be used.

[0137] The colorant may be a pigment or a dye, preferably a pigment.

[0138] The colored pigments may be selected from those disclosed in HERBST, Willy et al. Industrial Organic Pigments, Production, Properties, Applications, 3rd edition, Wiley-VCH, 2004. ISBN 3527305769. Suitable pigments are disclosed in paragraphs

[0128] to

[0138] of WO 2008 / 074548.

[0139] The pigment particles in the inkjet ink should be small enough to allow the ink to flow freely through the inkjet printing equipment, especially at the nozzle. It is also desirable to use small particles to achieve maximum color strength and slow down sedimentation. Most preferably, the average pigment particle size is no greater than 150nm. The average particle size of the pigment particles is preferably determined using a Brookhaven Instruments Particle Sizer BI90plus based on the principle of dynamic light scattering.

[0140] Generally, dyes exhibit higher light fading than pigments, but do not pose a problem for jetting. Anthraquinone dyes have been found to exhibit only slight light fading under normal UV curing conditions used in UV curable inkjet printing. In a preferred embodiment, the colorant in the curable inkjet ink is an anthraquinone dye, such as Macrolex from LANXESS. TM Blue 3R (CASRN 325781-98-4).

[0141] Other preferred dyes include crystal violet and copper phthalocyanine.

[0142] Different colorants can be combined to obtain the desired color, or to improve dispersion stability. A preferred colorant combination to obtain a green solder mask is a combination of a blue colorant and a yellow colorant. In a particularly preferred embodiment, a combination of Pigment Yellow 150 and Pigment Blue 15:4 is used.

[0143] In a preferred embodiment, the colorant is present in an amount of 0.5 to 6.0 wt%, more preferably 0.75 to 2.5 wt%, based on the total weight of the curable inkjet ink.

[0144] Polymer Dispersants

[0145] If the colorant in the curable inkjet ink is a pigment, the curable inkjet ink preferably contains a dispersant, more preferably a polymeric dispersant, for dispersing the pigment.

[0146] Suitable polymer dispersants are copolymers of two monomers, but they may contain three, four, five or even more monomers. The properties of the polymer dispersant depend on both the properties of the monomers and their distribution in the polymer. The copolymer dispersant preferably has the following polymer composition:

[0147] Statistical polymerization of monomers (e.g. monomers A and B polymerize to form ABBAABAB);

[0148] Alternating monomers (e.g. monomers A and B polymerize to form ABABABAB);

[0149] Gradient polymerization of monomers (e.g. monomers A and B polymerize into AAABAABBABBB);

[0150] Block copolymers (e.g. monomers A and B polymerized to AAAAABBBBBB), where the block length of each block (2, 3, 4, 5 or even more) is important for the dispersing ability of the polymeric dispersant;

[0151] Graft copolymers (graft copolymers consist of a polymer backbone and polymer side chains attached to the backbone); and

[0152] • Mixed forms of these polymers, such as block gradient copolymers.

[0153] Suitable polymeric dispersants are listed in EP-A 1 911 814 under "Dispersants", more particularly

[0064] to

[0070] and

[0074] to

[0077] .

[0154] Commercial examples of polymeric dispersants are as follows:

[0155] ·DISPERBYK TM Dispersants, available from BYK CHEMIE GMBH;

[0156] ·SOLSPERSE TM Dispersants are available from NOVEON;

[0157] TEGO from EVONIK TM DISPERS TM Dispersants;

[0158] From CHEMIE's EDAPLAN TM Dispersants;

[0159] ETHACRYL FROM LYONDELL TM Dispersants;

[0160] GANEX from ISP TM Dispersants;

[0161] DISPEX from CIBA SPECIALTY CHEMICALS INC TM and EFKA TM Dispersants;

[0162] DISPONER from DEUCHEM TM dispersants; and

[0163] · JONCRYL from JOHNSON POLYMER TM Dispersant.

[0164] Surfactants

[0165] The curable inkjet composition may contain at least one surfactant, which may act as a wetting agent, dispersant or emulsifier.

[0166] Surfactants can be anionic, cationic, nonionic or zwitterionic.

[0167] Suitable surfactants include fluorinated surfactants, fatty acid salts, ester salts of higher alcohols, alkylbenzene sulfonates, sulfosuccinates and phosphates of higher alcohols (e.g., sodium dodecylbenzenesulfonate and sodium dioctylsulfosuccinate), ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylphenols, ethylene oxide adducts of polyol fatty acid esters, and acetylenic diols and ethylene oxide adducts thereof (e.g., polyoxyethylene nonylphenyl ether, and SURFYNOL available from AIR PRODUCTS & CHEMICALS INC. TM 104, 104H, 440, 465 and TG).

[0168] Preferred surfactants are selected from fluorinated surfactants (e.g., fluorinated hydrocarbons) and silicone surfactants. The silicone surfactant is preferably a siloxane, which may be alkoxylated, polyether-modified, polyether-modified hydroxy-functional, amine-modified, epoxy-modified, and other modifications or combinations thereof. Preferred siloxanes are polymers, such as polydimethylsiloxane.

[0169] Preferred commercial silicone surfactants include BYK TM 333, BYK TM 347 and BYK TM UV3510.

[0170] In a preferred embodiment, the surfactant is a polymerizable compound.

[0171] Preferred polymerizable silicone surfactants include (meth)acrylated silicone surfactants. Most preferably, the (meth)acrylated silicone surfactant is an acrylated silicone surfactant, since acrylates are more reactive than methacrylates. A preferred commercial acrylated surfactant is Ebecryl 1360 from Allnex.

[0172] In a preferred embodiment, the (meth)acrylated silicone surfactant is a polyether-modified (meth)acrylated polydimethylsiloxane or a polyester-modified (meth)acrylated polydimethylsiloxane.

[0173] Preferably, the surfactant is present in the curable inkjet composition in an amount of 0 to 3 wt %, based on the total weight of the curable inkjet composition.

[0174] Preparation of inkjet compositions

[0175] The preparation of pigmented curable inkjet inks is well known to those skilled in the art. Preferred preparation methods are disclosed in paragraphs

[0076] to

[0085] of WO2011 / 069943.

[0176] Method of manufacturing electronic device

[0177] The method of manufacturing an electronic device according to the present invention includes at least one step in which the curable inkjet composition as described above is jetted and cured onto a substrate.

[0178] According to a preferred embodiment, the electronic device is a printed circuit board (PCB).

[0179] In a particularly preferred embodiment, the method of manufacturing a PCB comprises a step in which the solder mask composition is applied to a substrate by an inkjet printing step, followed by a UV curing step and a heat treatment step.

[0180] The substrate is preferably a dielectric substrate comprising a conductive pattern, which typically includes conductive pads electrically connected to each other using traces.

[0181] The dielectric substrate of an electronic device can be any non-conductive material. The substrate is typically a paper / resin composite or a resin / fiberglass composite, a ceramic substrate, polyester or polyimide. FR-4 is an example of a material that is often used as a dielectric substrate.

[0182] The conductive pattern is generally made of any metal or alloy commonly used in the manufacture of electronic devices, such as gold, silver, palladium, nickel / gold, nickel, tin, tin / lead, aluminum, tin / aluminum and copper. The conductive pattern is preferably made of copper.

[0183] Prior to applying the solder mask composition, the substrate is preferably subjected to one or more pretreatment processes. These processes may be mechanical or chemical, or a combination of both. A preferred pretreatment process is chemical microetching, which typically results in microroughness on the substrate. A so-called anti-bleeding treatment may be additionally applied to the microetched surface to prevent ink from penetrating into the micropores and improve the print quality. Such an anti-bleeding treatment typically involves applying a coating to the substrate to adjust its surface energy, thereby creating a sharper contact angle and minimal ink penetration into the micropores. An anti-bleeding treatment is preferably used when printing low viscosity compositions.

[0184] The process of inkjet printing a solder mask layer on a dielectric substrate having a conductive pattern preferably includes one or more printing steps as listed below.

[0185] ·The printing of the so-called "ramp" consists in printing a line next to the copper trace. This preparatory printing step ensures adequate coverage of the Cu trace. If there is no printed ramp next to the copper trace, the cured solder mask may be too thin at the edge of the copper trace (also called the shoulder). The thickness of the printed ramp is related to the height of the plated Cu trace. For plated Cu traces with a large height, a higher ramp ink thickness is required in order to have adequate coverage at the Cu edge. For plated Cu traces with a lower height, a lower ramp thickness can be used. The ramp preferably has a thickness of 0 to 80 μm, more preferably 10 to 60 μm, and most preferably 20 to 40 μm.

[0186] Often so-called "dam" printing is done to indicate the outline of the entire solder mask layer. These dams are usually cured with higher curing energies to provide precise features and lines. The thickness of the dam is preferably at least the same as the thickness of the entire solder mask layer. The thickness of the dam is more preferably higher than the thickness of the entire solder mask layer to avoid possible flow of ink to open pads. The thickness of the dam is preferably 5 to 75 μm, more preferably 10 to 60 μm, and most preferably 20 to 40 μm.

[0187] • Finally, the entire board is printed within the outline of the dam, covering the Cu traces but leaving open Cu pads for soldering.

[0188] The curable inkjet composition can be cured by exposing it to actinic radiation (e.g., electron beam or ultraviolet (UV) radiation). Preferably, the curable inkjet composition is cured by UV radiation, more preferably by UV LED. In order to fix the curable composition on the substrate, a UV pinning curing step can be used immediately after printing. This UV pinning curing can improve the print quality.

[0189] The jetted and UV-cured curable inkjet composition is preferably heat-treated. The heat treatment is preferably performed at a temperature of 80°C to 250°C. The temperature is preferably not less than 100°C, more preferably not less than 120°C. In order to prevent the solder mask from being burned, the temperature is preferably not higher than 200°C, more preferably not higher than 160°C.

[0190] The heat treatment is usually carried out for 15 to 90 minutes.

[0191] The purpose of the thermal treatment is twofold: to cure the thermal crosslinker present in the curable composition and to further polymerize any radiation curable compounds that may have not reacted. Thus, a dense interpenetrating polymer network can be created.

[0192] The method for manufacturing a PCB may include two, three or more inkjet printing steps. For example, the method may include two inkjet printing steps, wherein a resist is provided on a metal surface in one inkjet printing step, and wherein a solder mask is provided on a dielectric substrate comprising a conductive pattern in another inkjet printing step.

[0193] A third inkjet printing step can be used for legend printing.

[0194] Inkjet printing equipment

[0195] The curable inkjet composition may be jetted by one or more print heads by ejecting small droplets through nozzles in a controlled manner onto a substrate that is moving relative to the one or more print heads.

[0196] A preferred print head for an inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied. Applying a voltage changes the shape of the piezoelectric ceramic transducer in the print head, thereby creating a void that is then filled with ink. When the voltage is removed again, the ceramic expands back to its original shape, ejecting ink drops from the print head. However, the inkjet printing method according to the present invention is not limited to piezoelectric inkjet printing. Other inkjet print heads can also be used, and they include various types, such as continuous types.

[0197] The inkjet print head usually scans back and forth laterally on the moving ink receiving surface (substrate). The inkjet print head often does not print on the way back. In order to obtain high area throughput, bidirectional printing is preferably adopted. Another preferred printing method is "single-pass printing method", which can be carried out by using a page-wide inkjet print head or a plurality of staggered inkjet print heads covering the entire width of the ink receiving surface. In the single-pass printing method, the inkjet print head usually remains stationary, and the ink receiving surface is transmitted below the inkjet print head.

[0198] Example

[0199] Material

[0200] Unless otherwise stated, all materials used in the following examples are readily available from standard sources, such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium). The water used was deionized water.

[0201] VEEA is 2-(vinylethoxy)ethyl acrylate, available from NIPPON SHOKUBAI, Japan.

[0202] PEA is 2-phenoxyethyl acrylate, which can be used as a sartomer TM SR339 was obtained from ARKEMA.

[0203] ACMO is acryloylmorpholine, available from RAHN.

[0204] VMOX is N-vinyl-5-methyl-2-oxazolidinone available from BASF.

[0205] EHMA is 2-ethylhexyl methacrylate.

[0206] TMCHMA is 3,3,5-trimethylcyclohexyl methacrylate, available from ARKEMA as Sartomer TM SR421A Acquisition

[0207] IBOA is isobornyl acrylate, available as Photomer 4012 from IGM resins.

[0208] IBOMA is isobornyl methacrylate, available from ARKEMA as Sartomer TM SR423D obtained.

[0209] HDDA is hexanediol diacrylate, available from ARKEMA as Sartomer TM

[0210] SR238 obtained.

[0211] HDDMA is hexanediol dimethacrylate, available from ARKEMA as Sartomer TM SR239 obtained.

[0212] TMPTA is trimethylolpropane triacrylate, available from ARKEMA as Sartomer TM SR351 obtained.

[0213] TMPTMA is trimethylolpropane trimethacrylate, available from ARKEMA as Sartomer TM SR350 obtained.

[0214] VEEM is 2-(2'-vinyloxyethoxy)ethyl methacrylate, available from NIPPON SHOKUBAI.

[0215] PEGDA is polyethylene glycol diacrylate, available from ARKEMA as Sartomer TM SR259 obtained.

[0216] DAROCUR ITX is a mixture of isomers of 2- and 4-isopropylthioxanthone from BASF.

[0217] BAPO is a bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide photoinitiator available from BASF as Irgacure TM 819 obtained.

[0218] BISAPHOS is a flame retardant containing a mixture of aromatic polyphosphates available from ADEKA.

[0219] Trixene BI7960 is a DMP terminated isocyanate crosslinker commercially available from LANXESS.

[0220] Cymel NF2000A is a triazine crosslinker commercially available from ALLNEX.

[0221] Ebecryl 1360 is a silicone hexaacrylate available from ALLNEX.

[0222] WET is a 1% solution of Ebecryl 1360 in VEEA.

[0223] Cyan is SUN FAST BLUE 15:4, a cyan pigment available from SUN CHEMICALS.

[0224] Yellow is CROMOPHTAL YELLOW D 1085J, a yellow pigment from BASF.

[0225] INHIB is a mixture of polymerization inhibitors having a composition according to Table 1.

[0226] Table 1

[0227] Components wt% DPGDA 82.4 p-Methoxyphenol 4.0 2,6-Di-tert-butyl-4-methylphenol 10.0 <![CDATA[Cupferron TM AL]]> 3.6

[0228] Cupferron TM AL is aluminum N-nitrosophenylhydroxylamine from WAKO CHEMICALS LTD.

[0229] PRECIP 162 is a dispersant and has been precipitated from Disperbyk 162, a dispersant solution from BYK (ALTANA).

[0230] DISP is a solution of 30 wt% PRECIP 162 and 1 wt% INHIB in VEEA.

[0231] GD is a green dispersion which is prepared as follows:

[0232] A concentrated green dispersion GD having a composition according to Table 2 was prepared.

[0233] Table 2

[0234] Components wt% Cyan 7.5 Yellow 7.5 DISP 15 INHIB 1 VEEA 69

[0235] GD was prepared as follows: DISPERLUX TM Distributor mixes 138g VEEA, 2g INHIB, 30g DISP, 30g Cyan and 30g Yellow. Stirring is continued for 30 minutes. The container is connected to a NETZCH MiniZeta mill, which is equipped with 900g 0.4mm yttrium stabilized zirconia beads ("high wear resistant zirconia grinding media" from TOSOH Co.). The mixture is circulated in the mill for 120 minutes (residence time is 45 minutes) at a mill speed of about 10.4m / s. During the entire grinding procedure, the contents in the mill are cooled to keep the temperature below 60°C. After grinding, the dispersion is discharged into a container.

[0236] Evaluation Method

[0237] Viscosity

[0238] The HAAKE RotoVisco 1 was used at 45 °C and 1000 s -1 The viscosity of the ink is measured at a shear rate of .

[0239] For industrial inkjet printing, at 45°C and 1000s -1 The viscosity at a shear rate of 1000 s is preferably less than 15 mPa.s. More preferably, at 45°C and 1000 s -1 The viscosity at a shear rate of 5.0 to 15 mPa.s.

[0240] The viscosity was measured again after 7 days. Inks with a viscosity change of less than 20% were considered stable.

[0241] ENIG resistance

[0242] The ENIG simulation was performed using the following procedure:

[0243] • Immerse the board in a 40°C acidic cleaner (Umicore cleaner 865) bath for 4 minutes. The board is then removed and immersed in a room temperature (RT) deionized water (DW) rinse bath for 90 seconds.

[0244] The board was immersed in a micro-etching bath for 100 seconds, which contained 8.5wt% Na2S208 and ±3.2wt% H2SO4 (98%) in water at a temperature of 26-33° C. The board was then removed and rinsed in DW, 2.5wt% aqueous H2SO4 solution and DW, respectively, all at room temperature for 90 seconds.

[0245] The panels were immersed in a 2.5 wt% aqueous H2SO4 solution at room temperature for 30 seconds, then removed and rinsed in DW at room temperature for 90 seconds. They were then immersed again in the same solution for 60 seconds.

[0246] The panels were immersed in a palladium activator bath (Accemulta MKN 4) at a temperature of about 30°C for 90 seconds and then immersed in a 5 wt% aqueous H2SO4 solution at room temperature for 75 seconds. The panels were then removed and immersed in a DW rinse bath at room temperature for 90 seconds.

[0247] The panels were then immersed in a nickel bath (Nimuden NPR 4) at a temperature of approximately 85°C for 35 minutes. The panels were then removed and immersed in a DW rinse bath at room temperature for 90 seconds.

[0248] • Finally, the board was immersed in a gold bath (Gobright TAM 55) at a temperature of approximately 80°C for 12 minutes. The board was then removed and immersed in a DW rinse bath at room temperature for 90 seconds.

[0249] After the ENIG treatment, so-called blistering may occur, especially around the open pads. In these areas, the ENIG solution appears to have penetrated and lifted the solder mask layer. In addition to blistering, the so-called "halo effect" may be observed, i.e. the area around the open pads has a different appearance. Both phenomena may create weak adhesion points of the solder mask to the substrate and may lead to layer delamination.

[0250] The occurrence of blistering was evaluated under a microscope using a digital microscope Dino-LITE and scored using a value from 0 to 5 depending on the density of blistering observed:

[0251] 0: No foaming.

[0252] 1: The bubble density is less than 5% of the ink layer.

[0253] ·2: The foaming density is less than 5-15% of the ink layer.

[0254] ·3: The foaming density is less than 15-35% of the ink layer.

[0255] ·4: The foaming density is less than 35-65% of the ink layer.

[0256] 5: The bubble density exceeds 65% of the ink layer.

[0257] The occurrence of the halo effect was evaluated under a microscope using a digital microscope Dino-LITE and scored using a value of 0 to 5 depending on the width of the halo observed.

[0258] 0: No halo effect is visible.

[0259] 1: The width of the halo is less than 1 mm.

[0260] 2: Halo width from 1mm to 2mm.

[0261] 3: Halo width from 2mm to 3mm.

[0262] 4: Halo width from 3mm to 4mm.

[0263] 5: Halo width exceeds 4 mm.

[0264] Using Tesatape TM 4104 PVC tape was used to evaluate adhesion after ENIG plating by cross-hatch adhesion test. The following criteria were used for evaluation. Adhesion on both Cu and FR-4 was evaluated.

[0265] 0: Nothing removed, perfect adhesion.

[0266] 1: Only a small part of the cured layer falls off, and the adhesion is almost perfect.

[0267] ·2: Small portion of the cured layer is removed by the tape, good adhesion.

[0268] 3: Part of the cured layer is removed by the tape, poor adhesion.

[0269] 4: Most of the cured layer is removed by the tape, and the adhesion is poor.

[0270] 5: The cured layer is completely removed from the substrate by the tape, with no adhesion.

[0271] Solder resistance

[0272] The solder resistance of the inkjet ink was evaluated using a SPL600240 digital dynamic solder oven obtained from L&M PRODUCTS, which was filled with TSC puralloy SN100C solder alloy obtained from SOLDER CONNECTION. The solder temperature was set to 290°C.

[0273] Apply flux SC7560A from SOLDER CONNECTION to the surface of the sample using a Q-tip to clean the surface. Allow the flux to dry by placing the sample over a soldering oven for 1 minute on each side. Wipe off any remaining flux with a soft paper towel.

[0274] Place the sample in the soldering oven for 10 seconds. Repeat three times in total, then allow the sample to cool for at least 10 minutes.

[0275] The adhesion of the inkjet ink after the solder dip test was evaluated after the printed samples were cooled at room temperature. They were then evaluated by visual observation for peeling / delamination. If no peeling / adhesion loss was observed, the sample was fine. If solder was observed on top of the printed layer, this is a sure sign of a failed solder dip test (solder mask peeling, solder may be attached to the copper underneath it). Based on visual inspection, the printed samples after the solder dip test were given the following scores:

[0276] 0: No peeling, no damage to the ink layer.

[0277] 1: The ink layer peels off slightly. Less than 5% of the ink layer peels off.

[0278] ·2: 5% to 15% of the ink layer has peeled off.

[0279] ·3: 15% to 35% of the ink layer has peeled off.

[0280] 4: 35% to 65% of the ink layer has peeled off.

[0281] 5: More than 65% of the ink layer has peeled off.

[0282] The occurrence of blistering after the solder dipping test was evaluated under the microscope using the digital microscope Dino-LITE and scored using a value from 0 to 5 depending on the density of blistering observed:

[0283] 0: No foaming.

[0284] 1: The bubble density is less than 5% of the ink layer.

[0285] ·2: The foaming density is less than 5-15% of the ink layer.

[0286] ·3: The foaming density is less than 15-35% of the ink layer.

[0287] ·4: The foaming density is less than 35-65% of the ink layer.

[0288] 5: The bubble density exceeds 65% of the ink layer.

[0289] IR Reflow

[0290] A 40 μm coating on FR-4EM825 was prepared by applying a 20 μm layer of ink and curing it 4 times using a 12W UV LED lamp. Then, a second 20 μm layer was applied and cured in the same way. Finally, the 40 μm coating was placed in an oven at 150°C for 1 hour.

[0291] The IR reflow evaluation was performed using an eC-reflow-mate V4 reflow oven. The temperature inside the reflow oven is measured by three sensors. One at the top, one at the bottom and finally a central sensor to measure the temperature of the sample. The 40μm coating was placed in the IR eC-reflow-mate V4 reflow oven and placed above the central temperature sensor. The reflow cycle was started by heating the oven to 260°C. Once the sample temperature reached 260°C, the oven maintained this temperature for 10 seconds and then the sample was cooled down outdoors. The duration of one cycle was 4 minutes and 30 seconds. The ink coating underwent up to 6 IR reflow cycles (C1 to C6).

[0292] Evaluation of the samples was done visually by counting the number of cracks after each cycle.

[0293] Examples 1 to 6 and Comparative Examples 1 to 5

[0294] Curable inkjet compositions Ex-1 to Ex-6 and comparative curable inkjet compositions Comp-1 to Comp-5 were prepared according to Table 3. The weight percentages are based on the total weight of the curable inkjet composition.

[0295] Table 3

[0296]

[0297]

[0298] Inkjet ink was printed on a checkerboard (copper plated on FR4) substrate from Eurocircuits using MicroCraft CPS2013D (Printhead Konica Minolta KM1024iS, UV LED 395, total lamp output 12W) to obtain a solder mask with a final thickness of + / -22μm. The checkerboard contains a 35μm copper layer, which is roughened by chemical etching. In the chemical etching step, the substrate is transported through a Bungard Sprint 3000 conveyor spray etcher at a speed of 0.4m / min while spraying with a chemical etchant CZ2001 (available from MEC) heated to 30°C. After a rinsing step with demineralized water, an additional spraying step with 1M HCL is performed, followed by a rinsing step with demineralized water, and a drying step is performed in an Air 2000 dryer (available from Bungard). The substrate is printed within 24 hours after pretreatment.

[0299] After printing, the samples were baked in an oven at 150 °C for 1 h.

[0300] Different printing schemes were used:

[0301] Print-1 (P1): Print and cure an image with a resolution of 1440 dpi in the x direction and 1440 dpi in the y direction. The UV energy applied was equivalent to 10% of the total power of the 12 W lamp. A final cure was used to further cure the printed solder mask layer (4 passes at full power of the 12 W lamp).

[0302] Print-2 (P2): The image was printed and cured in two passes to achieve a certain thickness compared to Print-1, and the applied UV energy was equivalent to 100% of the total power of the 12 W lamp. The final curing was then carried out as described for Print-1.

[0303] The inks were tested for ENIG resistance and IR reflow resistance as described above. The results are shown in Table 4.

[0304] Table 4

[0305]

[0306] It is clear from the above results that the curable ink comprising a polymerizable compound and a thermal crosslinker has good stability and good ENIG resistance. The addition of methacrylate improves IR reflow resistance.

[0307] All inks had perfect soldering furnace resistance after ENIG plating (not shown in the table).

[0308] Examples 7 to 11 and Comparative Example 6

[0309] Curable inkjet compositions Ex-7 to Ex-11 and comparative curable inkjet composition Comp-6 were prepared according to Table 5. The weight percentages are based on the total weight of the curable inkjet composition.

[0310] Table 5

[0311] Comp-6 Ex-7 Ex-8 Ex-9 Ex-10 Ex-11 GD 6.6 6.6 6.6 6.6 6.6 6.6 VEEA 17.8 15.3 12.8 10.3 7.8 5.3 PEA 31 31 31 31 31 31 ACMO 5 5 5 5 5 5 VMOX 5 5 5 5 5 5 EHMA 2.5 5 7.5 10 12.5 PEGDA 15 15 15 15 15 15 DAROCUR ITX 0.6 0.6 0.6 0.6 0.6 0.6 BAPO 4.7 4.7 4.7 4.7 4.7 4.7 BISAPHOS 2 2 2 2 2 2 Trixene BI7960 7 7 7 7 7 7 Cymel NF2000A 2.4 2.4 2.4 2.4 2.4 2.4 WET 2 2 2 2 2 2 INHIB 0.9 0.9 0.9 0.9 0.9 0.9

[0312] The inkjet ink was printed in the same manner as in Examples 1-6.

[0313] After printing, the samples were baked in an oven at 150 °C for 1 h.

[0314] The printing scheme used was as follows:

[0315] An image with a resolution of 1440 dpi in the x direction and 1440 dpi in the y direction was printed and cured. The UV energy applied was equivalent to 10% of the total power of the 12 W lamp. A final cure was used to further cure the printed solder mask layer (4 passes at full power of the 12 W lamp).

[0316] As described above, the ENIG resistance and solder resistance of the ink were tested and evaluated. The results are shown in Table 6.

[0317] Table 6

[0318]

[0319] It is clear from the above results that the curable ink containing 2.5 wt% or less of 2-ethylhexyl methacrylate has poor ENIG resistance and good solder resistance. When 2-ethylhexyl methacrylate is 10 wt% or more, solder resistance is poor but ENIG resistance is good.

Claims

1. A curable inkjet composition for a printed circuit board, comprising - more than one polymerizable compound, wherein at least one polymerizable compound contains a vinyl group selected from vinyl ether groups, N-vinylamide groups and N-vinylurethane groups; - one or more thermal crosslinking agents selected from unblocked isocyanates, blocked isocyanates and triazine compounds; It is characterized in that At least one polymerizable compound is a methacrylate.

2. The curable inkjet composition according to claim 1, wherein the at least one polymerizable compound is selected from 2-(2-vinyloxyethoxy)ethyl acrylate, N-vinyl caprolactam, N-vinyl-2-pyrrolidone and N-vinyl-5-methyl-2-oxazolidinone.

3. The curable inkjet composition according to claim 1 or 2, wherein at least one thermal crosslinking agent is a blocked isocyanate.

4. The curable inkjet composition according to claim 3, wherein the blocked isocyanate is a blocked HDI or IPDI oligomer, and wherein the oligomer is selected from the group consisting of biuret, trimethylolpropane adduct and isocyanurate.

5. The curable inkjet composition according to any one of the preceding claims, wherein at least one thermal crosslinking agent is a triazine compound.

6. The curable inkjet composition according to claim 5, wherein the triazine compound has a chemical structure according to Formula II in X represents N, O, S, P or C; R5, R6 and R7 independently represent a substituted or unsubstituted alkyl group.

7. The curable inkjet composition according to any one of the preceding claims, wherein the amount of the methacrylate is at least 5 wt% relative to the total weight of the inkjet composition.

8. A curable inkjet composition according to any one of the preceding claims, wherein the one or more polymerizable compounds are selected from 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, isobornyl acrylate, hexanediol diacrylate, trimethylolpropane triacrylate, polyethylene glycol diacrylate, 2-phenoxyethyl acrylate and acryloylmorpholine.

9. The curable inkjet composition according to any one of the preceding claims, wherein the methacrylate is 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, 3,3,5-trimethylcyclohexyl methacrylate, isobornyl methacrylate, hexanediol dimethacrylate, trimethylolpropane trimethacrylate, 2-(2'-vinyloxyethoxy)ethyl methacrylate or polyethylene glycol dimethacrylate.

10. A curable inkjet composition according to any one of the preceding claims, having a thermal conductivity of 1000 s at 45°C. -1 The viscosity is measured at a shear rate of 5 to 15 mPa.s.

11. A cured product obtained by applying and curing a curable inkjet composition as defined in any one of the preceding claims on a substrate.

12. A method for manufacturing a printed circuit board comprising an inkjet printing step, wherein a curable inkjet composition as defined in any one of claims 1 to 10 is jetted and cured on a substrate.

13. The method of claim 12, further comprising a heating step.

14. The method of claim 13, wherein the heating step is performed at a temperature of 80°C to 250°C.

15. A method according to any one of claims 12 to 14, wherein the substrate is a dielectric substrate provided with an electrically conductive circuit.

16. A PCB board comprising a solder mask, wherein the solder mask is obtained using the curable inkjet composition defined in any one of claims 1 to 10.

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