Curable inkjet compositions for manufacturing printed circuit boards

Abstract

A curable inkjet composition for a printed circuit board comprising (a) one or more thermal crosslinkers selected from the group consisting of unblocked isocyanates, blocked isocyanates, and triazine compounds; and (b) at least two photopolymerizable compounds, characterized in that at least one polymerizable compound has the chemical structure # imgabs0 # according to general formula (I), wherein R1, R2, R3 and R4, independently of each other, represent hydrogen or a substituted or unsubstituted C1 to C10 alkyl group.

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. Moreover, it plays an important role in protecting the board from oxidation triggered by external influences such as weather conditions, temperature changes, and humidity. When mechanical defects occur in the solder mask due to exposure to these external conditions, the protective and insulating functions of the solder mask can be negatively affected. Therefore, the main goal in producing an effective solder mask is to provide good resistance to the conditions it is exposed to.

[0005] For the production of PCB solder masks via inkjet, an important requirement is that the solder mask must be able to withstand the operating conditions during typical finishing processes such as ENIG plating where harsh and varying conditions (pH and temperature) are used (ENIG plating resistance).

[0006] EP4032958 (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.

[0007] EP-A 1624001 (Taiyo Ink Manufacturing) discloses an inkjet ink for solder mask printing, comprising a (meth)acrylate monomer including a thermosetting functional group. WO2020 / 109769 (ElectraPolymers) discloses an inkjet ink for solder mask printing, comprising a reactive monomer, a prepolymer oligomer containing at least one epoxy or oxetane functional group, a free radical polymerizable compound, a thermal crosslinker, and a free radical initiator.

[0008] EP-A 3778793 (Taiyo Ink Manufacturing) discloses an inkjet ink for solder mask printing comprising a photopolymerizable monomer having a cyclic backbone and a shrinkage of less than 10%, which results in improved heat resistance prior to ENIG plating.

[0009] Finally, it is beneficial for inkjet inks to have good LED sensitivity. Inks with good LED sensitivity are desirable because improved curing efficiency will result in better image quality and line definition. Typically, good LED sensitivity also indicates good mechanical properties of the cured ink, such as hardness and strength. Therefore, there is a need for an inkjet ink for PCB manufacturing processes that has both good LED sensitivity and good resistance to ENIG plating. Summary of the invention

[0010] An object of the present invention is to provide an inkjet composition for PCB manufacturing process, which has good resistance to ENIG plating. This object of the present invention is achieved by a curable composition as defined in claim 1. Another object of the present invention is to provide an inkjet composition for PCB manufacturing process, which has good resistance to ENIG plating and good LED sensitivity.

[0011] Other objects of the present invention will become apparent from the following description. DETAILED DESCRIPTION

[0012] definition

[0013] The term "monofunctional", such as in a monofunctional polymerizable compound, means that the polymerizable compound contains one polymerizable group.

[0014] The term "difunctional", such as in a difunctional polymerizable compound, means that the polymerizable compound contains two polymerizable groups.

[0015] The term "polyfunctional" or "multifunctional," such as in a polyfunctional polymerizable compound, means that the polymerizable compound contains more than two polymerizable groups.

[0016] 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.

[0017] Unless otherwise stated, substituted or unsubstituted alkyl radicals are preferably C1 to C6 -alkyl radicals.

[0018] Unless otherwise stated, a substituted or unsubstituted alkenyl group is preferably a C2 to C6-alkenyl group.

[0019] Unless otherwise stated, a substituted or unsubstituted alkynyl group is preferably a C2 to C6 -alkynyl group.

[0020] Unless otherwise stated, a substituted or unsubstituted alkaryl group is preferably a phenyl or naphthyl group which contains one, two, three or more C1 to C6-alkyl groups.

[0021] Unless otherwise specified, the substituted or unsubstituted aralkyl group is preferably a C7 to C5 arylalkyl group containing a phenyl group or a naphthyl group. 20 -alkyl group.

[0022] Unless otherwise specified, a substituted or unsubstituted aryl group is preferably a phenyl group or a naphthyl group.

[0023] Unless otherwise specified, a 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.

[0024] The term "substituted" in, for example, a substituted alkyl group means that the alkyl group may be substituted with atoms other than the atoms normally present in such groups, namely carbon and hydrogen. For example, a substituted alkyl group may contain a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

[0025] Unless otherwise specified, substituted alkyl groups, substituted alkenyl groups, substituted alkynyl groups, substituted aralkyl groups, substituted alkaryl groups, substituted aryl groups and substituted heteroaryl groups are preferably substituted by one or more components selected from the group consisting of 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.

[0026] Curable inkjet composition

[0027] The curable composition according to the present invention comprises one or more thermal crosslinking agents selected from unblocked isocyanates, blocked isocyanates and triazine compounds; and at least two photopolymerizable compounds as described below.

[0028] 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.

[0029] In a preferred embodiment, the composition according to the invention comprises a mixture of a UV-curable compound and a thermal crosslinker. 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.

[0030] The two 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 the reason for the superior mechanical properties observed compared to individually cured polymer networks.

[0031] The curable composition according to the invention is preferably used as inkjet ink.

[0032] 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 between 1 and 18 mPa.s at 45°C, most preferably between 5 and 15 mPa.s at 45°C, all within 1000s -1 at a shear rate of .

[0033] Preferred spraying temperatures are between 10 and 70°C, more preferably between 20 and 55°C, most preferably between 25 and 50°C.

[0034] For good image quality and adhesion, the surface tension of the curable inkjet ink is preferably in the range of 18 to 70 mN / m at 25°C, more preferably in the range of 20 to 40 mN / m at 25°C.

[0035] Photopolymerizable compounds

[0036] The curable inkjet composition according to the present invention comprises at least one photopolymerizable compound having a chemical structure according to Formula I.

[0037]

[0038] 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 one of R1 to R4 may represent the necessary atoms to form a five-membered or six-membered ring.

[0039] Preferably, R1, R2, R3 and R4 are independently selected from hydrogen or substituted or unsubstituted C1 to C 10 Alkyl group.

[0040] Preferred compounds are disclosed in WO 2015 / 022228 (BASF) and US 4831153 (DOW CHEMICAL).

[0041] Cyclic compounds according to formula I are often referred to as oxazolidinones. A particularly preferred oxazolidinone is N-vinyl-5-methyl-2-oxazolidinone, also known as vinylmethyloxazolidinone or VMOX, formula II.

[0042]

[0043] Including compounds according to formula I or II will improve the LED sensitivity of the cured ink layer. The amount of the compound according to formula I or II is preferably 1 to 40 wt %, more preferably 2.5 to 30 wt %, and most preferably 5 to 20 wt %, based on the total weight of the curable inkjet composition. In addition, compared to other N-vinyl compounds, the compounds according to formula I or II have low viscosity, which makes them particularly suitable for inkjet printing.

[0044] The curable composition according to the present invention may also contain other photopolymerizable compounds.

[0045] The photopolymerizable 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 amounts of free radical polymerizable groups. A mixture comprising a combination of monofunctional, difunctional, trifunctional and higher 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 photopolymerizable 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 photopolymerizable compounds are those listed in EP-A 1911814, paragraphs

[0106] to

[0115] .

[0048] Particularly preferred monofunctional photopolymerizable compounds are selected from 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, isobornyl acrylate, 2-phenoxyethyl acrylate and acryloylmorpholine.

[0049] The curable inkjet composition preferably contains one or more multifunctional photopolymerizable compounds.

[0050] Preferred multifunctional photopolymerizable compounds are selected from polyethylene glycol diacrylate, hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, polyester acrylate oligomers and polyether acrylate oligomers.

[0051] The curable inkjet composition may further contain a photopolymerizable compound containing a vinyl group selected from a vinyl ether group and an N-vinyl amide group.

[0052] Preferred examples of photopolymerizable compounds containing vinyl ether groups or N-vinyl amide groups are those listed in paragraphs

[0047] to

[0056] of EP-A 3686251.

[0053] A particularly preferred vinyl ether-containing photopolymerizable 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. The amount of 2-(2-vinyloxyethoxy)ethyl acrylate is preferably at least 5 wt%, more preferably at least 7.5 wt%, and most preferably at least 10 wt%, based on the total weight of the curable inkjet composition.

[0054] Particularly preferred vinylamide-containing photopolymerizable 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.

[0055] Combinations of any of the above-described photopolymerizable compounds may be used.

[0056] Thermal crosslinking agent

[0057] The curable composition of the present invention comprises one or more thermal crosslinking agents selected from unblocked isocyanate, blocked isocyanate and triazine compounds. The presence of the thermal crosslinking agent can improve the adhesion of the coating film obtained after soldering or ENIG plating.

[0058] Thermal crosslinkers may be monofunctional, difunctional or polyfunctional.

[0059] The inkjet composition may contain a mixture of different thermal crosslinkers.

[0060] Typical thermal crosslinking agents are oxirane, oxetane, melamine-formaldehyde resin, urea-formaldehyde resin, benzoguanamine-formaldehyde resin, cyclic carbonate compounds, carbodiimide, isocyanate, blocked isocyanate, triazine compounds and combinations thereof.

[0061] Preferred thermal crosslinking agents are isocyanate compounds. Isocyanate compounds are preferably used in combination with compounds containing active hydrogen functions, 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 combining an isocyanate compound with a compound containing active hydrogen functions.

[0062] 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.

[0063] 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 dimeryl diisocyanate (DDI).

[0064] 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).

[0065] 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).

[0066] Examples also include adducts of the isocyanates listed above (eg, trimethylolpropane adducts), uretdiones, biuret and isocyanurates.

[0067] The isocyanate compound may be blocked or unblocked, preferably blocked.

[0068] Blocked isocyanates can be obtained by reacting an isocyanate with a selected blocking agent. Such blocking agents are protective groups that cleave at elevated temperatures, such as during a thermal curing process. Blocking agents that will cleave at a certain temperature, the so-called unblocking temperature, can be selected. The use of blocked isocyanates will generally improve the storage stability of inkjet inks.

[0069] Examples of the blocking agent include alcohols such as ethanol, n-propanol, isopropanol, tert-butyl alcohol and isobutyl alcohol; phenols such as phenol, chlorophenol, cresol, xylenol and p-nitrophenol; alkylphenols such as p-tert-butylphenol, p-sec-butylphenol, p-sec-amylphenol, p-octylphenol and p-nonylphenol; basic nitrogen-containing compounds such as 3-hydroxypyridine, S-hydroxyquinoline and 8-hydroxyquinaldine; active methylene compounds such as diethyl malonate, ethyl acetoacetate and Acetylacetone; 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.

[0070] For toxicological reasons, hindered secondary amines may be used as capping agents. Preferred hindered secondary amines are selected from ethyl tert-butylamine, diisopropylamine, 2,6-dimethylpiperidine, ethylisopropylamine, ditert-butylamine and diisobutylamine.

[0071] Preferred blocked isocyanate compounds are blocked HDI oligomers or blocked IPDI oligomers. Such oligomers may be, for example, trimethylolpropane adducts, biuret or isocyanurate.

[0072] Particularly preferred blocked isocyanate compounds are Trixene Bl 7960 (an HDI biuret blocked with 3,5-dimethylpyrazole, commercially available from Lanxess) and Trixene BI7982 (an HDI trimer blocked with 3,5-dimethylpyrazole, commercially available from Lanxess).

[0073] Other preferred thermal crosslinking agents are triazine compounds 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.

[0074] A preferred triazine compound has a chemical structure according to formula III,

[0075]

[0076] wherein X represents N, O, S, P or C;

[0077] R5, R6 and R7 independently represent a substituted or unsubstituted alkyl group;

[0078] X preferably represents O or C, most preferably O.

[0079] Preferably, R5, R6 and R7 independently represent a substituted or unsubstituted C1-C8 alkyl group. More preferably, R5, R6 and R7 independently represent a group selected from methyl, ethyl, n-propyl, isopropyl, butyl, n-octyl, 2-ethylhexyl.

[0080] Preferred triazine compounds according to formula III and methods for their preparation are disclosed in US Pat. No. 5,084,541 (American Cyanamid Company).

[0081] Preferred triazine compounds according to the general formula III are available from Allnex under the name NF 2000A and from BASF under the name 150 was purchased commercially.

[0082] The inkjet ink according to the present invention preferably comprises a blocked isocyanate compound or a triazine compound according to the general formula III.

[0083] More preferably, the inkjet ink comprises both a blocked isocyanate and a triazine compound according to formula III.

[0084] The total amount of thermal crosslinking agent is preferably from 0.5 to 20% by weight, more preferably from 1 to 15% by weight, most preferably from 2 to 12% by weight, all relative to the total weight of the inkjet ink.

[0085] The amount of blocked isocyanate compound is preferably 0.1 to 12.5 wt%, more preferably 0.5 to 10 wt%, most preferably 1 to 8 wt%, all relative to the total weight of the inkjet ink.

[0086] The amount of triazine compound is preferably from 0.1 to 5 wt%, more preferably from 0.5 to 4 wt%, most preferably from 1 to 3 wt%, all relative to the total weight of the inkjet ink.

[0087] The presence of a thermal crosslinker has been observed to improve various solder resist properties such as heat resistance, hardness, resistance to soldering heat, resistance to chemicals, electrical insulation, and resistance to electroless and immersion plating.

[0088] Phenolic resin

[0089] The curable composition according to the present invention may comprise one or more phenolic resins.

[0090] The phenolic resin is preferably a phenol-based functionalized polymer selected from polyacrylate, polymethacrylate or polystyrene.

[0091] Preferred phenolic resins are poly(4-hydroxystyrene) and its derivatives.

[0092] Particularly preferred phenolic resins are disclosed in US20060099531 (DuPont Electronic Polymers L.P.), especially the phenolic resins having structures I, II, III and IV disclosed in paragraph 0018.

[0093] The amount of phenolic resin is preferably from 0.5 to 20 wt%, more preferably from 1 to 15 wt%, most preferably from 2.5 to 10 wt%, all relative to the total weight of the inkjet ink.

[0094] Photoinitiator

[0095] The curable inkjet composition preferably comprises a photoinitiator, preferably a free radical photoinitiator.

[0096] Free radical photoinitiators are chemical compounds that will initiate polymerization of monomers and oligomers by forming free radicals when exposed to actinic radiation. Norrish Type I initiators are initiators that will cleave upon excitation to immediately generate initiating free radicals. Norrish Type II initiators are photoinitiators that are activated by actinic radiation and form free radicals by abstracting hydrogen from a second compound, which becomes the actual initiating free radical. This second compound is known as a polymerization synergist or coinitiator. Both Type I and Type II photoinitiators can be used in the present invention alone or in combination.

[0097] Suitable photoinitiators are disclosed in CRIVELLO, JV et al., Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization. 2nd edition. BRADLEY, G. ed. London, UK: John Wiley and Sons Ltd, 1998. p. 276-293.

[0098] 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.

[0099] Preferred photoinitiators are thioxanthone compounds, such as Darocur ITX, which is an isomer mixture of 2- and 4-isopropylthioxanthone.

[0100] 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), (2,4,6-trimethylbenzoyl)phenylphosphonic acid ethyl ester (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.

[0101] Other preferred photoinitiators are α-hydroxyketone type I photoinitiators, such as those available from IGM resins. Oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinylphenyl]propanone]] obtained by KIP IT

[0102] Preferred amounts of photoinitiator are from 0.2 to 20 wt%, more preferably from 0.5 to 10 wt%, most preferably from 1 to 8 wt%, particularly preferably from 1.5 to 6 wt%, all relative to the total weight of the curable inkjet composition.

[0103] In order to further improve the photosensitivity, the curable inkjet composition may also contain a co-initiator. Suitable examples of co-initiators can be divided into three categories:

[0104] (1) tertiary aliphatic amines such as methyldiethanolamine, dimethylethanolamine, triethanolamine, triethylamine and N-methylmorpholine;

[0105] (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

[0106] (3) (Meth)acrylated amines, such as dialkylaminoalkyl (meth)acrylates (eg, diethyl-aminoethyl acrylate) or N-morpholinoalkyl-(meth)acrylates (eg, N-morpholinoethyl-acrylate).

[0107] Preferred coinitiators are aminobenzoates.

[0108] A preferred low molecular weight aminobenzoate is from RAHN EPD.

[0109] Particularly preferred aminobenzoate co-initiators are selected from polymerizable, oligomeric and polymeric aminobenzoate co-initiators.

[0110] Polymerizable co-initiators are disclosed in EP-A 2 033 949 (Agfa Graphics NV).

[0111] In a more preferred embodiment, the aminobenzoate co-initiator is an oligomeric aminobenzoate derivative.

[0112] 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(tetrahydrofuran), ethoxylated or propoxylated neopentyl glycol, ethoxylated or propoxylated trimethylolpropane and ethoxylated or propoxylated pentaerythritol.

[0113] Preferred oligomeric aminobenzoates are disclosed in WO1996 / 33157 (Lambson Fine Chemicals Ltd.) and WO2011 / 030089 (Sun Chemicals BV). Typical examples of polyethylene glycol bis-p-dimethylaminobenzoate are OMNIPOL ASA available from IGM Resins and Speedcure 7040 commercially available from Lambson Fine Chemicals.

[0114] Other oligomeric or polymeric co-initiators are, for example, ESACURE A198 (a multifunctional amine from IGM) and CN3755 (an acrylated amine co-initiator from ARKEMA).

[0115] Inhibitors

[0116] The curable inkjet composition may contain at least one inhibitor to improve the thermal stability of the ink.

[0117] Suitable polymerization inhibitors include phenolic antioxidants, hindered amine light stabilizers, phosphorus antioxidants, hydroquinone monomethyl ether commonly used in (meth)acrylate monomers, and hydroquinone, tert-butylcatechol, pyrogallol, 2,6-di-tert-butyl-4-methylphenol (=BHT) may also be used.

[0118] Suitable commercially available 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 Cytec Surface Specialties TM S Series (S100, S110, S120 and S130).

[0119] Since excessive addition of these polymerization inhibitors may reduce the curing speed, it is preferred to determine the amount capable of preventing polymerization before blending. The amount of the polymerization inhibitor is preferably less than 5 wt %, more preferably less than 3 wt % of the total curable inkjet composition.

[0120] Adhesion Promoters

[0121] The curable inkjet composition may contain an adhesion promoter to further optimize the adhesion of the cured composition to various surfaces, particularly copper surfaces.

[0122] 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 / Electra Polymers).

[0123] The curable inkjet composition may comprise one adhesion promoter or a combination of two, three or more different adhesion promoters.

[0124] The total amount of adhesion promoter is preferably between 0.1 and 20 wt%, more preferably between 0.5 and 15 wt%, most preferably between 1 and 10 wt%, all relative to the total weight of the inkjet composition.

[0125] However, it has been observed that the inkjet composition according to the present invention can 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.

[0126] Flame retardants

[0127] The curable inkjet composition preferably comprises a flame retardant.

[0128] 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).

[0129] Preferred flame retardants are disclosed in WO2019 / 121098.

[0130] Colorants

[0131] The curable inkjet composition may be a substantially colorless inkjet ink, or may contain at least one colorant. For example, when the inkjet ink is used as an etch resist, the colorant will make the temporary mask clearly visible to the manufacturer of the conductive pattern, allowing visual inspection of the quality. 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.

[0132] The colorant may be a pigment or a dye.

[0133] Color 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.

[0134] The pigment particles in the inkjet ink should be small enough to allow the ink to flow freely through the inkjet printing device, especially at the jet nozzle. It is also desirable to use small particles to obtain maximum color intensity 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 ParticleSizer BI90plus based on the principle of dynamic light scattering.

[0135] Typically, 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 Blue3R (CASRN 325781-98-4).

[0136] Other preferred dyes include crystal violet and copper phthalocyanine.

[0137] Different colorants can be combined together 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 and yellow colorant. In a particularly preferred embodiment, a combination of Pigment Yellow 150 and Pigment Blue 15:4 is used.

[0138] 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.

[0139] Polymer Dispersants

[0140] If the colorant in the curable inkjet ink is a pigment, the curable inkjet ink preferably contains a dispersant, more preferably a polymeric dispersant, to disperse the pigment.

[0141] 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 the properties of the monomers and their distribution in the polymer. The copolymer dispersant preferably has the following polymer composition:

[0142] Statistical polymerization of monomers (e.g., monomers A and B polymerize to form ABBAABAB);

[0143] Alternating monomers (e.g., monomers A and B polymerize to form ABABABAB);

[0144] Gradient polymerization of monomers (e.g., monomers A and B polymerize to form AAABAABBABBB);

[0145] 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 to the dispersing ability of the polymeric dispersant;

[0146] Graft copolymers (graft copolymers consist of a polymer backbone and polymer side chains attached to the backbone); and

[0147] • Mixed forms of these polymers, such as block gradient copolymers.

[0148] Suitable polymeric dispersants are listed in EP-A 1 911 814 in the section "Dispersants", more particularly in

[0064] to

[0070] and

[0074] to

[0077] .

[0149] Commercially available examples of polymeric dispersants are as follows:

[0150] DISPERBYK™ dispersants available from BYK CHEMIE GMBH;

[0151] SOLSPERSETM dispersant available from NOVEON;

[0152] TEGOTM DISPERSTM dispersants from EVONIK;

[0153] From CHEMIE's EDAPLANTM dispersant;

[0154] ETHACRYLTM dispersant from LYONDELL;

[0155] GANEX™ dispersant from ISP;

[0156] DISPEXTM and EFKATM dispersants from CIBA SPECIALTY CHEMICALS INC;

[0157] DISPONERTM dispersants from DEUCHEM; and

[0158] · JONCRYLTM dispersant from JOHNSON POLYMER.

[0159] Surfactants

[0160] The curable inkjet composition may contain at least one surfactant, which may act as a wetting agent, dispersant or emulsifier.

[0161] Surfactants can be anionic, cationic, nonionic or zwitterionic.

[0162] 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 acetylene glycols 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).

[0163] Preferred surfactants are selected from fluorinated surfactants (such as fluorinated hydrocarbons) and silicone surfactants. The silicone surfactant is preferably a siloxane and may be alkoxylated, polyether modified, polyether modified hydroxy functional, amine modified, epoxy modified and other modifications or combinations thereof. Preferred siloxanes are polymeric, such as polydimethylsiloxane.

[0164] Preferred commercially available silicone surfactants include BYK TM 333, BYK TM 347 and BYK TM UV3510.

[0165] In a preferred embodiment, the surfactant is a polymerizable compound.

[0166] 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. Examples are BYK TM 3566 and Ebecryl 1360 from Allnex. A preferred commercially available acrylated surfactant is Ebecryl 1360 from Allnex.

[0167] In a preferred embodiment, the (meth)acrylated silicone surfactant is a polyether-modified (meth)acrylated polydimethylsiloxane or a polyester-modified (meth)acrylated polydimethylsiloxane.

[0168] 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.

[0169] Preparation of inkjet compositions

[0170] The preparation of pigmented curable inkjet inks is well known to the skilled person. A preferred preparation method is disclosed in paragraphs

[0076] to

[0085] of WO2011 / 069943.

[0171] Method for manufacturing electronic device

[0172] 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 on a substrate.

[0173] According to a preferred embodiment, the electronic device is a printed circuit board (PCB).

[0174] In a particularly preferred embodiment, the method of manufacturing a PCB comprises a step in which the solder mask composition is applied on a substrate via an inkjet printing step, followed by a UV curing step and a heat treatment step.

[0175] The substrate is preferably a dielectric substrate containing a conductive pattern, typically comprising conductive pads electrically connected to each other using traces.

[0176] The dielectric substrate of the 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 often used as a dielectric substrate.

[0177] The conductive pattern is generally made of any metal or alloy conventionally used in the preparation 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.

[0178] Before applying the solder mask composition, the substrate is preferably subjected to one or more pre-treatment processes. These processes can be mechanical or chemical, or a combination of the two. A preferred pre-treatment process is chemical micro-etching, which usually causes micro-roughness on the substrate. The so-called anti-bleeding treatment can be additionally applied to the micro-etched surface to prevent the ink from penetrating into the micro-pores and improve the print quality. This anti-bleeding treatment usually includes applying a coating on the substrate to adjust its surface energy, thereby producing a more acute contact angle and minimizing the penetration of ink into the micro-pores. Anti-bleeding treatment is preferably used when printing low-viscosity compositions.

[0179] 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.

[0180] ·The printing of the so-called "ramp" consists in printing a line next to the copper trace. This preliminary printing step ensures adequate coverage of the Cu trace. If the ramp is not printed next to the copper trace, the cured solder mask may be too thin on the edge (also called shoulder) of the copper trace. The thickness of the printed ramp is related to the height of the Cu plated trace. For Cu plated traces with a large height, a higher ramp ink thickness is required in order to have sufficient coverage on the Cu edge. For Cu plated 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, most preferably 20 to 40 μm.

[0181] Printing of so-called "dams" is usually performed 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 ink flow to the open pads. The dam preferably has a thickness of 5 to 75 μm, more preferably 10 to 60 μm, most preferably 20 to 40 μm.

[0182] Finally, the entire board is printed within the outline of the dam, covering the Cu traces but leaving openings for the Cu pads for soldering.

[0183] The curable inkjet composition can be cured by exposing it to actinic radiation, such as 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.

[0184] It is preferred to apply heat treatment to the jetted and UV-cured curable inkjet composition. The heat treatment is preferably performed at a temperature between 80°C and 250°C. The temperature is preferably not less than 100°C, more preferably not less than 120°C. In order to prevent scorching of the solder mask, the temperature is preferably not higher than 200°C, more preferably not higher than 160°C.

[0185] The heat treatment is usually carried out for 15 to 90 minutes.

[0186] The purpose of the thermal treatment is twofold: to cure the thermal crosslinkers 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.

[0187] 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 containing a conductive pattern in another inkjet printing step.

[0188] A third inkjet printing step can be used for legend printing.

[0189] Inkjet printing device

[0190] The curable inkjet composition may be jetted by one or more print heads that eject small droplets through nozzles in a controlled manner onto a substrate that moves relative to the print heads.

[0191] The preferred print head for the 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 to the piezoelectric ceramic transducer. The application of voltage changes the shape of the piezoelectric ceramic transducer in the print head, creating a gap that is then filled with ink. When the voltage is removed again, the ceramic expands to its original shape and ink drops are ejected 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 be used, and include various types, such as continuous types.

[0192] The inkjet print head generally scans back and forth laterally across the moving ink-receiving surface (substrate). Typically the inkjet print head does not print on the return pass. In order to obtain high areal throughput, bidirectional printing is preferred. Another preferred printing method is the "single pass printing method", which can be performed by using page-wide inkjet print heads or multiple staggered inkjet print heads covering the entire width of the ink-receiving surface. In the single pass printing method, the inkjet print head generally remains stationary while the ink-receiving surface is transported under the inkjet print head.

[0193] Example

[0194] Material

[0195] 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.

[0196] VMOX is N-vinyl-5-methyl-2-oxazolidinone, available from BASF.

[0197] DPGDA is dipropylene glycol diacrylate, which can be Sartomer TM SR508 is commercially available from ARKEMA.

[0198] VEEA is 2-(vinylethoxy)ethyl acrylate, available from NIPPON SHOKUBAI, Japan.

[0199] PEGDA is polyethylene glycol diacrylate, which can be Sartomer TM SR259 was obtained from ARKEMA.

[0200] EHMA is 2-ethylhexyl methacrylate.

[0201] CTFA is cyclic trimethylolpropane formal acrylate, which can be Sartomer TM SR531 was obtained from ARKEMA.

[0202] DAROCUR ITX is a mixture of isomers of 2- and 4-isopropylthioxanthone from BASF.

[0203] BAPO is a bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide photoinitiator that can be used by Irgacure TM 819 was obtained from BASF.

[0204] BISAPHOS is a flame retardant containing a mixture of aromatic polyphosphates available from ADEKA.

[0205] Cymel NF2000A is a triazine crosslinker commercially available from ALLNEX.

[0206] Trixene BI7960 is a DMP-terminated isocyanate crosslinker commercially available from LANXESS.

[0207] Cyan is SUN FAST BLUE 15:4, a cyan pigment available from SUN CHEMICALS.

[0208] Yellow is CROMOPHTAL YELLOW D 1085J, a yellow pigment from BASF.

[0209] INHIB is a mixture forming polymerization inhibitors having a composition according to Table 1.

[0210] Table 1

[0211]

[0212]

[0213] Cupferron TM AL is aluminum N-nitrosophenylhydroxylamine from WAKO CHEMICALS LTD.

[0214] PRECIP 162 is a dispersant precipitated from Disperbyk 162, a dispersant solution available from BYK (ALTANA).

[0215] DISP is a solution of 30 wt % PRECIP 162 and 1 wt % INHIB in VEEA.

[0216] GD is a green dispersion prepared as follows:

[0217] A concentrated green dispersion GD having a composition according to Table 2 was prepared.

[0218] Table 2

[0219]

[0220] GD was prepared as follows: using DISPERLUX TM Distributor 138g VEEA, 2g INHIB, 30g DISP, 30g Cyan and 30g Yellow are mixed. Stirring is continued for 30 minutes. The container is connected to a NETZCH MiniZeta mill 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), and the rotation speed of the mill is 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.

[0221] Evaluation Method

[0222] Viscosity

[0223] 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 .

[0224] For industrial inkjet printing, at 45°C and 1000s -1 The viscosity at a shear rate of 5.0 to 15 mPa.s is preferably between 5.0 and 15 mPa.s. More preferably, at 45°C and 1000 s -1 The viscosity at the shear rate is less than 15mPa.s.

[0225] LED sensitivity

[0226] To evaluate the LED sensitivity of the ink, the inkjet ink was coated on a PET substrate using a rod coater and a Braive automatic film applicator. The rod coater produced a 10 μm thick coating, which was cured using a Fusion LED station (UV LED lamp 12W). The coating was passed through the LED lamp using a belt set to move at 50 m / min, and the LED lamp power was set to 4W.

[0227] After coating and curing, the ink was rated using a Q tip. The coating was rubbed with the Q tip and a rating was given based on:

[0228] 0: No wiping marks can be seen on the coating

[0229] 1: Wiping marks can be seen on the coating, but there is no ink on the Q tip

[0230] 2: Wipe marks can be seen on the coating and there is ink on the Q tip, but there is no damage to the coating

[0231] 3: There is ink on the Q tip and the coating is slightly damaged

[0232] 4: There is ink on the Q tip and there is significant damage to the coating

[0233] 5: The ink is completely removed after wiping with Q tip

[0234] ENIG resistance

[0235] ENIG simulations were performed using the following procedure:

[0236] - Immerse the board in a bath of acid cleaner (Umicore cleaner 865) for 4 minutes at 40° C. Then remove the board and immerse in a rinse bath of deionized water (DW) at room temperature (RT) for 90 seconds.

[0237] Immerse the board in a microetching bath containing 8.5 wt% Na2S208 and ±3.2 wt% H2SO4 (98%) in water for 100 seconds at a temperature between 26-33° C. The board is then removed and rinsed in DW for 90 seconds at RT.

[0238] The panels were immersed in a 2.5 wt% H2SO4 aqueous solution for 30 seconds at RT, then the panels were removed and rinsed in DW for 90 seconds at RT. They were immersed again in the same solution for 60 seconds and then transferred to the palladium bath.

[0239] The panels were immersed in a palladium activator bath (Accemulta MKN 4) at a temperature of around 30°C for 90 seconds and then immersed in a 5 wt% H2SO4 aqueous solution at RT for 75 seconds. The panels were then removed and immersed in a rinse bath of DW at RT for 90 seconds.

[0240] • The panels were then immersed in a nickel bath (Nimuden NPR 4) at a temperature of around 85°C for 35 minutes. The panels were then removed and immersed in a rinse bath of DW at RT for 90 seconds.

[0241] • Finally, the plate was immersed in a gold bath (Gobright TAM 55) for 12 minutes at a temperature of around 80° C. The plate was then removed and immersed in a rinse bath of DW for 90 seconds at RT.

[0242] After the ENIG treatment, so-called blistering may occur, especially around the open pads. In these areas, the ENIG solution appears to penetrate and lift up the solder mask layer. In addition to blistering, the so-called "halo effect" may be observed, whereby 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 the detachment of this layer.

[0243] The occurrence of blistering and halo effects was evaluated under the microscope using a digital microscope Dino-LITE and scored using a value from 0 to 5 depending on the density of blistering and the width of the halo observed:

[0244] 0: No bubbles and no halo

[0245] 1: The bubble density is less than 5% of the ink layer and / or the halo width is less than 1 mm.

[0246] ·2: The bubble density is less than 5-15% of the ink layer and / or the halo width is 1mm to 2mm.

[0247] ·3: The bubble density is 15-35% less than the ink layer and / or the halo width is 2mm to 3mm.

[0248] 4: The bubble density is 35-65% less than the ink layer and / or the halo width is 2mm to 4mm.

[0249] 5: The bubble density is greater than 65% of the ink layer and / or the halo width is greater than 4 mm.

[0250] Comparative Examples 1 to 2 and Examples 1 to 5

[0251] Curable inkjet compositions Ex-1 to Ex-5 and comparative curable inkjet compositions Comp-1 to Comp-2 were prepared according to Table 3. The weight percentages are based on the total weight of the curable inkjet composition.

[0252] Table 3

[0253]

[0254]

[0255] The LED sensitivity was evaluated as described above and the results are further shown in Table 4.

[0256] The inkjet ink was then printed on a chessboard (copper plated on FR4) substrate from Eurocircuits using a MicroCraft CPS2013D (print head Konica Minolta KM1024iS, UV LED 395, total lamp output of 12W) to obtain a solder mask layer with a final thickness of + / -22μm. The chessboard includes a 35μm copper layer that is roughened by chemical etching. Prior to chemical etching, the chessboard is immersed in an acid cleaner bath activated by air bubbling (MEKLEEN MAC-5330RTU, which is a solution of about 12-15% H2SO4 and about 4-6% H2O2 in demineralized water, commercially available from MEC Europe) for 30 seconds, followed by rinsing with demineralized water. In the chemical etching step, the substrate is transported through a Bungard Sprint 3000 conveyor spray etcher at a speed of 0.4 m / min, while spraying with a chemical etchant CZ2001 (available from MEC) heated to 30° C. After the rinsing step with demineralized water, an additional spraying step with 1M HCl is performed, followed by a rinsing step with demineralized water and an anti-bleeding treatment step, wherein the substrate is immersed in an anti-bleeding solution (25 volume / volume % CL-8320C in demineralized water, pH 7.5, commercially available from MEC Europe) for 30 seconds. The substrate is then rinsed with demineralized water again, and finally dried in an Air 2000 dryer (available from Bungard). The substrate is printed within 24 hours after this pre-treatment.

[0257] After printing, the samples were baked in an oven at 150 °C for 1 h.

[0258] Use different printing schemes:

[0259] Print-1(P1): 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 applied to further cure the printed solder mask layer (4 passes at the full power of the 12 W lamp).

[0260] Print-2(P2):The image was printed at the same resolution as Print-1, but with the difference that the UV energy applied was equivalent to 100% of the total power of the 12 W lamp. Final curing was then performed as described for Print-1.

[0261] Print-3(P3): Compared to Prints 1 and 2, the image was printed in two passes to achieve a certain thickness. In the first pass, an image with a resolution of 1440 dpi in the x-direction and 1080 dpi in the y-direction was printed and cured. The UV energy applied was equivalent to 100% of the total power of the 12 W lamp. In the second pass, an image with a resolution of 720 dpi in the x-direction and 1080 dpi in the y-direction was printed and cured. The UV energy applied was equivalent to 0% of the total power of the 12 W lamp. Then a final cure as described for Print-1 was performed.

[0262] The ENIG resistance was evaluated as described above. The results are also given in Table 4.

[0263] Table 4

[0264]

[0265] From Table 4, it is clear that the inks containing VMOX (Ex-1 to Ex-5) have better ENIG resistance than the ink without VMOX (Comp-1).

[0266] Table 4 also shows that the presence of VMOX improves LED sensitivity while maintaining good ENIG resistance. For inks with 5 to 30 wt% VMOX, a combination of improved LED sensitivity and good ENIG resistance is observed.

[0267] Comp-2 has the same composition as Ex-3, the only difference being that Comp-2 contains CTFA instead of VMOX. Comp-2 has good ENIG resistance but poor LED sensitivity, even at 3rd and 4th pass.

Claims

1. A curable inkjet composition for a printed circuit board, the curable inkjet composition comprising (a) one or more thermal crosslinking agents selected from unblocked isocyanates, blocked isocyanates and triazine compounds; and (b) at least two photopolymerizable compounds characterized in that At least one photopolymerizable compound has a chemical structure according to formula I wherein R1, R2, R3 and R4 are independently hydrogen or a substituted or unsubstituted C1 to C 10 Alkyl group.

2. The curable inkjet composition according to claim 1, wherein the amount of the photopolymerizable compound according to Formula I is 5 to 30 wt% relative to the total weight of the inkjet composition.

3. The curable inkjet composition according to claim 1 or 2, wherein the photopolymerizable compound according to Formula I is N-vinyl-5-methyl-2-oxazolidinone.

4. The curable inkjet composition according to any one of claims 1 to 3, wherein at least one thermal crosslinking agent is a blocked isocyanate.

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

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

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

8. The curable inkjet composition according to any one of the preceding claims, wherein at least one photopolymerizable compound is a multifunctional polymerizable compound.

9. The curable inkjet composition according to claim 8, wherein the multifunctional photopolymerizable compound is selected from the group consisting of 2-(2'-vinyloxyethoxy)ethyl acrylate, polyethylene glycol diacrylate, hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, polyester acrylate oligomers and polyether acrylate oligomers.

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

11. A method of manufacturing a printed circuit board, the method comprising an inkjet printing step, wherein a curable inkjet composition as defined in any one of the preceding claims is jetted and cured on a substrate.

12. The method of claim 11, further comprising a heating step.

13. The method of claim 12, wherein the heating step is performed at a temperature of 80°C to 250°C.

14. A method according to any one of claims 11 to 13, wherein the substrate is a dielectric substrate provided with an electrically conductive circuit.

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

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