WO2025149599A1

WO2025149599A1 - A curable inkjet composition for the manufacturing of printed circuit boards

Info

Publication number: WO2025149599A1
Application number: PCT/EP2025/050497
Authority: WO
Inventors: Mitch MATTHYS; Hubertus Van Aert; Marion Sauvageot
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 2024-01-11
Filing date: 2025-01-10
Publication date: 2025-07-17
Anticipated expiration: 2026-07-11

Abstract

A curable inkjet composition for a printed circuit board comprising a photoinitiator and two or more (meth)acrylates, of which at least one (meth)acrylate comprises a tricyclic aliphatic group, characterized in that: - the total amount of (meth)acrylates comprising a tricyclic aliphatic group is at least 40 wt% relative to the total weight of the curable inkjet composition; and - the total amount of multi-functional acrylates is from 0 to 30 wt% relative to the total weight of the curable inkjet composition.

Description

A curable inkjet composition for the manufacturing of printed circuit boards

Technical Field

[0001] The present invention relates to a curable composition for use as an ink jet composition in the manufacturing of electronic devices. The invention also relates to a cured product of said composition.

Background Art

[0002] Printed Circuit Boards (PCBs) are traditionally manufactured in an extensive process including multiple photolithographic and etching steps, thereby generating a lot of waste. In order to reduce the amount of process steps, production costs, and waste, there is an increased interest in digitalizing the PCB manufacturing workflow.

[0003] Inkjet printing is a preferred digital manufacturing technology for several PCB production steps, such as the application of the etch resist and the solder mask, or printing of the legend.

[0004] The PCB solder mask, or solder resist, acts as an insulator between the copper traces, and prevents the formation of solder bridges. Moreover, it plays an important role in protecting the board against oxidation triggered by outer influences, such as weather conditions, temperature variations, and humidity. When mechanical defects occur in the solder mask due to exposure to these external conditions, the protective and insulating function of the solder mask can be negatively affected. Therefore, the main objective for producing an effective solder mask is to provide a good resistance against the conditions to which it is exposed.

[0005] During the production of a PCB solder mask using ink jet, it is often a problem that cracks appear in the solder mask. This is a result of thermal stress occurring while soldering electronic components on the PCB, or due to heat variations during the lifetime of the PCB. Because of this, the printed and cured solder mask needs to resist several thermal stress tests, such as IR reflow resistance.

[0006] EP-A 3778793 (Taiyo Ink Manufacturing) discloses an inkjet ink for solder mask printing, comprising a photopolymerizable monomer with a cyclic skeleton and a shrinkage of less than 10%, which results in an improved thermal resistance. [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. W02020/109769 (Electra Polymers) discloses an inkjet ink for solder mask printing comprising a reactive monomer, an oligomer of prepolymer containing at least one epoxy or oxetane functional group, a free radical polymerizable compound, a thermal cross-linking agent and a radical initiator.

[0008] EP-A 4215551 (Konika Minolta) discloses an inkjet ink for solder mask printing containing a (meth)acrylic monomer, a photopolymerization initiator, a thermosetting compound, and a gelling agent. This ink is improving adhesiveness and heat resistance and has a low relative dielectric constant.

[0009] However, there is still a need for inkjet inks for use in a PCB manufacturing process that have an improved resistance to thermal stress.

Summary of invention

[0010] It is an object of the invention to provide a curable composition for solder mask printing in the manufacturing of printed circuit boards (PCBs) having an improved resistance to thermal stress.

[0011] The object of the invention is realized by the curable composition as defined in claim 1.

[0012] Further objects of the invention will become apparent from the description hereinafter.

Description of embodiments

Definitions

[0013] The term “monofunctional” in e.g. monofunctional polymerizable compound means that the polymerizable compound includes one polymerizable group.

[0014] The term “difunctional” in e.g. difunctional polymerizable compound means that the polymerizable compound includes two polymerizable groups.

[0015] The term “polyfunctional” or "multifunctional” in e.g. polyfunctional polymerizable compound means that the polymerizable compound includes more than two polymerizable groups.

[0016] The term “alkyl” means all variants possible 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 tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2- methyl-butyl, etc.

[0017] Unless otherwise specified a substituted or unsubstituted alkyl group is preferably a C_x to C₆-alkyl group.

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

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

[0020] Unless otherwise specified a substituted or unsubstituted alkaryl group is preferably a phenyl or naphthyl group including one, two, three or more C_x to C₆-alkyl groups.

[0021] Unless otherwise specified a substituted or unsubstituted aralkyl group is preferably a C₇ to C₂₀-alkyl group including a phenyl group or naphthyl group.

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

[0023] Unless otherwise specified a substituted or unsubstituted heteroaryl group is preferably a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof.

[0024] The term “substituted”, in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms

[0025] Unless otherwise specified a substituted alkyl group, a substituted alkenyl group, a substituted alkynyl group, a substituted aralkyl group, a substituted alkaryl group, a substituted aryl and a substituted heteroaryl group are preferably substituted by one or more constituents selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tertiary- butyl, ester, amide, amine, ether, thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester, sulphonamide, -Cl, -Br, -I, -OH, -SH, -CN and -NO₂.

[0026] Unless otherwise specified an alkyl, a cycloalkyl or an aryl interrupted by a heteroatom means that a heteroatom in present in the carbon chain of said group, for example -C-O-C-C- or -C-S-C-C-.

Curable inkjet composition [0027] The curable composition according to the present invention comprises a photoinitiator and two or more (meth)acrylates, of which at least one (meth)acrylate comprises a tricyclic aliphatic group, characterized in that: the total amount of (meth)acrylates comprising a tricyclic aliphatic group is at least 40 wt% relative to the total weight of the curable inkjet composition; and the total amount of multi-functional acrylates is from 0 to 30 wt% relative to the total weight of the curable inkjet composition.

[0028] The composition is preferably a radiation-curable composition. Any type of radiation may be applied, but preferred radiation types 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 present invention comprises a mixture of UV-curable compounds and thermal crosslinking agents. Therefore, the curable composition according to the present invention is preferably also a thermally curable composition. Any type of heat source may be used for the thermal curing step, but preferably the thermal curing is performed in an oven. The two curing processes, thermal and UV, can take place simultaneously or sequentially.

[0030] The curable composition according to the invention is preferably applied as an inkjet ink.

[0031] 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 from 1 to 18 mPa.s at 45 ° C, and most preferably from 5 to 15 mPa.s at 45 ° C, all at a shear rate of 1000 s'¹.

[0032] A preferred jetting temperature is from 10 to 70 ° C, more preferably from 20 to 55 ° C, and most preferably from 25 to 50 ° C. 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.

(Meth)acrylates

[0033] The curable composition according to the present invention comprises two or more (meth)acrylates, of which at least one (meth)acrylate comprises a tricyclic aliphatic group.

[0034] The (meth)acrylates may be monomers, oligomers and/or prepolymers. These monomers, oligomers and/or prepolymers may possess different degrees of functionality, i.e. a different amount of free radical polymerizable groups. A mixture including combinations of mono-, di-, tri-and higher functional monomers, oligomers and/or prepolymers may be used. The viscosity of the curable inkjet ink may be adjusted by varying the ratio between the monomers and oligomers.

[0035] The (meth)acrylates may also comprise other functional groups, such as thiols, hydroxyls, amines, sulfonic acids, phosphoric acids, and carboxylic acids. Examples of hydroxyl-functionalized (meth)acrylates are those listed in paragraphs [0028] to [0029] in US20150090482A.

[0036] Examples of (meth)acrylates are those listed in paragraphs [0106] to [0112] in EP-A 1911814.

[0037] Preferred (meth)acrylates are selected from the group consisting of 2- ethylhexyl (meth)acrylate, 1,10-decanediol di(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl (meth)acrylate, hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, polyethyleneglycol di(meth)acrylate, 2- phenoxyethyl (meth)acrylate, and acryloyl morpholine.

[0038] An especially preferred (meth)acrylate is 4-tert-butylcyclohexylacrylate. Using this monomer may positively affect the mechanical properties of the solder mask, because the cured polymer of 4-tert-butylcyclohexylacrylate has a relatively high Tg. Furthermore, the bulky tert-butylcycohexyl group may contribute to a cured polymer film having less shrinkage. Due to the low viscosity and the low volatility of this monomer, the properties of the curable composition may be further tuned to make it suitable for inkjet printing without compromising other properties. When 4-tert-butylcyclohexylacrylate is being used in the composition, it is preferably present in an amount from 1 to 40 wt%, preferably from 2.5 to 30 wt%, and most preferably from 5 to 20 wt% based on the total weight of the curable inkjet composition.

[0039] At least one (meth)acrylate of the curable inkjet composition comprises a tricyclic aliphatic group. A tricyclic aliphatic group is characterized in that three aliphatic rings together form a fused ring system, a bridged ring system, or a bridged fused ring system.

• In a fused ring system, at least two rings share two adjacent atoms. In other words, the rings share one covalent bond.

• In a bridged ring system, at least two rings share three or more atoms, separating the bridgehead atoms by a bridge containing at least one atom.

• Finally, a bridged fused ring system is a ring system where at least two of the rings constitute a fused ring system and the remaining rings are created by one or more bridges. [0040] In the case of a tricyclic aliphatic group, there are always 2 ring junctions, which may be both fused, both bridged, or a combination of fused and bridged.

[0041] A (meth)acrylate comprising a tricyclic aliphatic group is less flexible and has a higher structural rigidity because the atoms being part of the ring system are hindered in their movement by being joint to other rings, compared to conventional (meth)acrylate groups used in curable inkjet compositions for a printed circuit board. Next to this, a tricyclic aliphatic group is also considered a ‘bulky’ group, meaning that they are possibly affecting the structure of the cured polymer network by controlling polymer chain length and by creating distance in the polymer network. These properties are all believed to contribute to the thermal properties of the cured ink when a (meth)acrylate comprising a tricyclic aliphatic group is used instead of other conventional (meth)acrylates.

[0042] To observe the effect of the improved thermal properties in an IR reflow resistance test, the total amount of (meth)acrylates comprising a tricyclic aliphatic group must be at least 40 %, preferably at least 45 %, more preferably at least 50 % relative to the total weight of the curable inkjet composition. If the amount of (meth)acrylates comprising a tricyclic aliphatic group is lower than 40 %, the composition does not give rise to a sufficient improvement in the IR reflow resistance test.

[0043] Without being limited thereto, examples of (meth)acrylates comprising a tricyclic aliphatic group are tricyclodecanedimethanol diacrylate (CAS registry number 42594-17-2), dicyclopentanyl acrylate (CAS registry number: 7398- 56-3), tricylodecanedimethanol dimethacrylate (CAS registry number 43048- 08-4), tricyclodecanemethanol acrylate (CAS registry number 93962-84-6), dihydrodicyclopentadienyl acrylate (CAS registry number 903574-98-1), adamantyl (meth)acrylate, (Octahydro-4, 7-methano-lH-inden-5-yl)methyl 2- methyl-2-propenoate (CAS registry number 127823-23-8), (Octahydro-4, 7- methano-lH-inden-5-yl)methyl 2-propenoate (CAS registry number 127823- 21-6), tricyclodecyl methacrylate or dicyclopentyl methacrylate (CAS registry number 34759-34-7), dicyclopentanyloxyethyl acrylate (CAS registry number 99353-06-7), 2-propenoic acid, (octahydro-4, 7-methano-lH -indene-2,5- diyl)bis(methylene) ester (CAS registry number 116738-40-0), CAS registry number 30305-68-1, adamantyl methacrylate (CAS registry number 16887-36- 8), adamantyl acrylate (CAS registry number 129090-25-1), 2-propenoic acid, octahydro-4, 7-methano-lH-indenyl ester (CAS registry number 79637-74-4), dihydrodicyclopentadienyl methacrylate (CAS registry number 31621-69-9), dicyclopentenyloxyethy I acrylate (CAS registry number 68169-12-0, dicyclopentenyloxyethyl methacrylate (CAS registry number: 68169-03-9), 2- propenoic acid, 2-methyl-, 2-[(3a,4,5,6,7,7a-hexahydro-4,7-methano-lH- inden-5-yl)oxy]ethyl ester (CAS registry number 75662-22-5), 3-hydroxy-l- adamantyl methacrylate (CAS registry number 115372-36-6), 2-isopropyl-2- adamantyl methacrylate (CAS registry number 297156-50-4), 2-methyl-2- adamantyl methacrylate (CAS registry number 177080-67-0), 2-ethyl-2- adamantyl methacrylate (CAS registry number 209982-56-9), 3,5- dihydroxyadamantan-l-yl methacrylate (CAS registry number 115522-15-1), dicyclopentenyloxyethyl acrylate (CAS registry number 65983-31-5) or a combination thereof.

[0044] Preferred types of tricyclic aliphatic groups are tricyclodecane and tricyclodecene groups, and preferred (meth)acrylates comprising a tricyclodecane or a tricyclodecene group are tricyclodecanedimethanol di(meth)acrylate, tricyclodecanemethanol (meth)acrylate, dicyclopentanyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate.

These compounds comprise long aliphatic chains making them very hydrophobic which is useful for resisting aqueous processes such as ENIG. Long chains are normally negatively impacting the LED sensitivity, but it is believed by the inventors that because the chains are condensed in multiple rings, curing can be improved. Moreover, when comparing tricyclic acrylate monomers like dicyclopentanyl acrylate with dicyclic monomers such as isobornyl acrylate, the additional ring is believed to provide additional rigidity and a higher hydrophobicity, resulting in polymers with a higher glass transition temperature, higher mechanical robustness and/or better adhesion due to van der Waals interactions.

[0045] The curable inkjet composition of the invention may also comprise multifunctional acrylates. However, the total amount of multi-functional acrylates cannot be more than 30 wt% relative to the total weight of the curable inkjet composition. If the total amount of multi-functional acrylates in the curable inkjet composition is higher than 30 wt%, the crosslinking degree may become too high and the cured film could become too rigid, thereby compromising the thermal resistance.

[0046] If there are no multi-functional acrylates, or only a small amount of multifunctional acrylate in the inkjet composition, it is possible that the ink may not cure as well. Therefore, preferably, the curable inkjet composition of the invention comprises from 5 to 30 wt% of multi-functional acrylate relative to the total weight of the curable inkjet composition, more preferably from 10 to 30 wt%, even more preferably from 7.5 to 25 wt%, most preferably from 10 to 20 wt%.

[0047] In a preferred embodiment, the curable inkjet composition according to the invention comprises at least two (meth)acrylates comprising a tricyclic aliphatic group. When there are at least two (meth)acrylates comprising a tricyclic aliphatic group, the properties of the ink can be further optimized, such as viscosity and curability.

[0048] In an especially preferred embodiment, the curable inkjet composition according to the invention comprises at least one monofunctional (meth)acrylate comprising a tricyclic aliphatic group and at least one multifunctional (meth)acrylate comprising a tricyclic aliphatic group. When there is a multi-functional acrylate comprising a tricyclic aliphatic group present in the inkjet composition, the total amount of multi-functional acrylates may not be higher than 30 wt% relative to the total weight of the curable inkjet composition and the total amount of (meth)acrylates comprising a tricyclic aliphatic group must be at least 40 wt%.

Other polymerizable compounds

[0049] The curable inkjet composition preferably contains a polymerizable compound containing a vinyl group. More preferably, the vinyl group is a vinylether group, an N-vinyl amide group or an N-vinyl carbamate group.

[0050] Preferred examples of polymerizable compounds containing a vinylether group or an N-vinylamide group are those listed in paragraphs [0047] to [0056] in EP-A 3686251.

[0051] A particularly preferred vinylether containing polymerizable compound is 2-(2- vinyloxyethoxy)ethyl acrylate, as it is advantageous in providing a good balance between the curability and the viscosity of the curable inkjet composition, especially when it is 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-py rrol idone and N-vinylcaprolactam as they combine a high Tg with a good ink curability and a good adhesion of a cured ink layer to a recording medium.

[0053] The polymerizable compound containing an N-vinyl carbamate group is preferably a cyclic compound represented by General Formula I,

General Formula I wherein R4, R5, R6 and R7 independently from each other represent hydrogen, alkyl, cycloalkyl, or aryl and combinations thereof, any of which may be interrupted by heteroatoms. Any of R4 to R7 may represent the necessary atoms for forming a five-or six-membered ring.

[0054] Preferably R4, R5, R6 and R7 independently from each other represent hydrogen or a substituted or unsubstituted C_x to C₁₀ alkyl group.

[0055] Preferred compounds are disclosed in WO 2015/022228 (BASF) and US 4831153 (DOW CHEMICAL).

[0056] Cyclic compounds according to General Formula I are often referred to as oxazolidinones, such as N-vinyl-5-ethyl-2-oxazolidinone or 3-methyl-5-vinyl- 2-oxazolidinone and N-vinyl-5-methyl-2-oxazolidinone. A particularly preferred oxazolidinone is N-vinyl-5-methyl-2-oxazolidinone, also referred to as vinyl methyl oxazolidinone, or VMOX. Including VMOX improves the hardness and increases the Tg of the cured ink layer, especially when it is used in an amount from 1 to 25 wt%, preferably from 2.5 to 15 wt%, and most preferably from 5 to 10 wt% based on the total weight of the curable inkjet composition. Moreover, VMOX has a low viscosity and a more preferred toxicological profile compared to other N-vinyl compounds, which makes it especially suitable for ink jet printing. The amount of VMOX in the ink can be adjusted such that the viscosity of the inkjet ink is optimized.

[0057] The polymerizable compound including a vinylether, an N-vinylamide or an N- vinyl carbamate may be used singly or in a combination of one or more polymerizable compounds including a vinylether, an N-vinylamide or an N- vinyl carbamate.

Thermal cross-linking agent

[0058] The curable composition of the present invention comprises one or more thermal cross-linking agent selected from the group consisting of unblocked isocyanates, blocked isocyanates and triazine compounds. The presence of a thermal cross-linking agent may improve the adhesion after soldering or ENIG plating and the EN IG resistance of the obtained coating. The thermal crosslinking agent may be monofunctional, difunctional or multifunctional.

[0059] The inkjet composition may comprise a mixture of different thermal crosslinking agents.

[0060] Thermal cross-linking agents are for example oxiranes, oxetanes, melamineformaldehyde resins, urea-formaldehyde resins, benzoguanamine- formaldehyde resins, cyclic carbonate compounds, carbodiimides, isocyanates, blocked isocyanates, and combinations thereof.

[0061] Preferred thermal cross-linking agents are isocyanate compounds. Isocyanate compounds are preferably used in combination with a compound comprising active hydrogen functionalities, including, without limitation, alcohols, thiols, amines, water, or combinations thereof. Atmospheric moisture may also cause isocyanate cross-linking. When atmospheric moisture is reacting with an isocyanate, it may not be necessary to prepare an ink combining the isocyanate compound with a compound comprising active hydrogen functionalities.

[0062] The isocyanate compound may be an aliphatic, an alicyclic or an aromatic isocyanate. In case the isocyanate compound is a multifunctional isocyanate, it may comprise a combination of aliphatic, alicyclic or aromatic isocyanate functionalities.

[0063] Examples of aliphatic isocyanates include, without limitation, 1,6- hexamethylene diisocyanate (HDI or HMDI), isophorone diisocyanate (IPDI), l,3-(isocyanatomethyl)cyclohexane (hydrogenated XDI), lysine diisocyanate (LDI), 2,2,4-trimethyl hexamethylene diisocyanate (TMDI), and dimeryl diisocyanate (DDI).

[0064] Examples of alicyclic isocyanates include, without limitation, isophorone diisocyanate (IPDI), methylcyclohexane 2,4-(2,6)-diisocyanate (hydrogenated TDI), and 4,4'-methylenebis(cyclohexylisocyanate) (hydrogenated MDI).

[0065] Examples of aromatic isocyanates include, without limitation, toluene diisocyanate (TDI), 1,5-naphthalene diisocyanate (NDI), 4,4'-di-phenyl- methanediisocyanate (MDI) and xylylene diisocyanate (XDI).

[0066] Examples also include adducts (e.g. trimethylol propane adducts), uretdiones, biurets, and isocyanurates of the isocyanates listed above.

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

[0068] A blocked isocyanate can be obtained by reacting an isocyanate with a blocking agent of choice. Such a blocking agent is a protective group, which is cleaved off at elevated temperatures, for example during a thermal curing process. The blocking agent can be chosen such that it will cleave off at a certain temperature, the so-called de-blocking temperature. Using a blocked isocyanate typically improves the storage stability of the inkjet ink.

[0069] Examples of the blocking agent include alcohols such as ethanol, n-propanol, isopropanol, t-butanol, and isobutanol; phenols such as phenol, chlorophenol, cresol, xylenol, and p-nitrophenol; alkylphenols such as p-t-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; acid amides such as acetamide, acrylamide, and acetanilide; acid imides such as succinimide and maleic imide; 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 ketone or aldehyde, such as acetoxime, methyl ethyl ketone oxime, cyclohexanone oxime, and acetaldoxime; ethyleneimine; and bisulfite.

[0070] Hindered secondary amines may be used as blocking agents for toxicology reasons. Preferred hindered secondary amines are selected form the group consisting of ethyl-tert-butyl amine, diisopropyl amine, 2,6-dimethyl- piperidine, ethyl-isopropyl amine, di-tert-butyl amine and diisobutyl amine.

[0071] A preferred blocked isocyanate compound is a blocked HDI oligomer or a blocked IPDI oligomer. Such an oligomer can be for example a trimethylol propane adduct, a biuret, or an isocyanurate.

[0072] 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.

[0073] Thermal cross-linking agents with a triazine skeleton are particularly preferred in the present invention. The triazine moiety is believed to contribute to the mechanical properties and heat resistance of the cured film. Any triazine compound having thermal cross-linking properties may be used.

[0074] A preferred triazine compound has a chemical structure according to General Formula II,

General Formula II wherein X represents N, 0, S, P or C;

R5, R6 and R7 independently from each other represent a substituted or unsubstituted alkyl group;

X preferably represents 0 or C, most preferably 0.

Preferably, R5, R6 and R7 independently from each other represent a substituted or unsubstituted C1-C8 alkyl group. More preferably, R5, R6 and R7 independently from each other represent a group selected from the group consisting of methyl, ethyl, n-propyl, i-propyl, butyl, n-octyl, 2-ethyl hexyl.

[0075] Preferred triazine compounds according to General Formula II and their preparation method are disclosed in US5084541 (American Cyanamid Company).

[0076] Preferred triazine compounds according to General Formula II are commercially available from Allnex under the name Cymel® N F 2000 and from BASF under the name Larotact® 150.

[0077] The inkjet ink according to the present invention preferably includes a blocked isocyanate compound or a triazine compound according to General Formula II.

[0078] More preferably, the inkjet ink includes both a blocked isocyanate and a triazine compound according to General Formula II.

[0079] The total amount of thermal cross-linking agents is preferably from 0.5 to 15 wt%, more preferably from 1 to 10 wt%, most preferably from 2.5 to 7.5 wt%, all relative to the total weight of the inkjet inks.

[0080] The amount of the blocked isocyanate compound is preferably from 0.1 to 10 wt%, more preferably from 1 to 7.5 wt%, most preferably from 2 to 5 wt%, all relative to the total weight of the inkjet ink.

[0081] The amount of the 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.

[0082] When both a triazine compound and an isocyanate compound are present in the inkjet ink, the amount of the blocked isocyanate compound is preferably higher compared to the amount of the triazine compound. [0083] It has been observed that the presence of thermal cross-linking agents improves various solder resist properties such as resistance to heat, hardness, resistance to soldering heat, resistance to chemicals, electrical insulating properties, and resistance to electroless plating and immersion plating.

Photoinitiators

[0084] The curable inkjet composition comprises a photoinitiator, preferably a free radical photoinitiator.

[0085] A free radical photoinitiator is a chemical compound that initiates polymerization of monomers and oligomers when exposed to actinic radiation by the formation of a free radical. A Norrish Type I initiator is an initiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish type ll-initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a polymerization synergist or co-initiator. Both type I and type II photoinitiators can be used in the present invention, alone or in combination.

[0086] Suitable photoinitiators are disclosed in CRIVELLO, J.V., et al. Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization. 2nd edition. Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p.276- 293.

[0087] 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 isopropylthioxanthone; 2-hydroxy-2-methyl-l- phenyl propan- 1-one; 2-benzyl-2-dimethylamino- (4- morpholinophenyl) butan-l-one; benzyl dimethylketal; 2-methyl-l- [4- (methylthio) phenyl] -2- morpholinopropan-l-one; 2,2-dimethoxy-l, 2-diphenylethan-l-one or 5,7- diiodo-3- butoxy-6-fluorone.

[0088] A preferred photoinitiator is a thioxanthone compound, such as Darocur ITX, an isomeric mixture of 2- and 4-isopropylthioxanthone, or Genocure DETX, 2,4-diethylthioxanthone. Genocure DETX has a more favourable toxicological profile compared to Darocur ITX.

[0089] Another preferred photoinitiator is an acylphosphine oxide compound. The acylphosphine oxide compound may be selected from the group consisting of a mono-acylphosphine oxide and a di-acylphosphine oxide. Preferred acylphosphine oxide photoinitiators are diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide (TPO), ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate (TPO-L), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) (BAPO), bis (2,6- dimethyl-benzoyl)-2,4,4-trimethylpentylphosphine oxide and 2, 4,6-trim ethoxy benzoyl-dipheny I phosphine oxide.

[0090] Other preferred photoinitiators are a -hydroxy-ketone Type I photoinitiators such as for example oligo[2-hydroxy-2-methyl-l-[4-(l-methylvinyl- phenyl]propanone] available as Esacure® KIP IT from IGM resins.

[0091] A preferred amount of photoinitiator is from 0.2 to 20 wt%, more preferably from 0.5 to 10 wt%, most preferably from 1 to 8 wt%, particularly preferred from 1.5 to 6 wt%, all relative to the total weight of the curable inkjet composition.

[0092] In order to increase the photosensitivity further, the curable inkjet composition may additionally contain co-initiators. Suitable examples of coinitiators can be categorized in three groups:

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

(2) aromatic amines such as amylparadimethyl-aminobenzoate, 2-n- butoxyethy 1-4- (dimethylamino) benzoate, 2- (di methylamino) -ethyl benzoate, ethyl-4-(dimethyl-amino)benzoate, and 2-ethylhexyl-4- (dimethylamino)benzoate; and

(3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates (e.g., diethyl-aminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates (e.g., N- morpholinoethyl-acrylate).

[0093] Preferred co-initiators are aminobenzoates. A preferred low molecular aminobenzoate is Genocure® EPD from RAHN. Particularly preferred aminobenzoate co-initiators are selected from the group consisting of polymerisable, oligomeric and polymeric aminobenzoate co-initiators.

[0094] Polymerisable co-initiators are disclosed in EP-A 2033949 (Agfa Graphics N.V.).

[0095] In a more preferred embodiment, the aminobenzoate co-initiators are oligomeric aminobenzoate derivatives. Particularly preferred aminobenzoates are polyether derivatives of aminobenzoates, wherein the polyether is selected from the group consisting of poly(ethylene oxide), polypropylene oxide), copolymers thereof, and poly(tetrahydrofuran), ethoxylated or propoxylated neopentyl glycol, ethoxylated or propoxylated trimethylpropane and ethoxylated or propoxylated pentaerythritol. Preferred oligomeric aminobenzoates are disclosed in W01996/33157 (Lambson Fine Chemicals Ltd.) and W02011/030089 (Sun Chemicals B.V.). Typical examples of polyethylene glycol bis p-dimethylaminobenzoate are OMNIPOL ASA, commercially available from IGM Resins and Speedcure 7040, commercially available from Lambson Fine Chemicals.

[0096] Other oligomeric or polymeric co-initiators are for example ESACURE A198, a polyfunctional amine from IGM and SARTOMER® CN3755, an acrylated amine co-initiator from ARKEMA.

Inhibitors

[0097] The curable inkjet composition may contain at least one inhibitor for improving the thermal stability of the ink.

[0098] Suitable polymerization inhibitors include phenol type antioxidants, hindered amine light stabilizers, phosphor type antioxidants, hydroquinone monomethyl ether commonly used in (meth)acrylate monomers, and hydroquinone, t- butyl-catechol, pyrogallol, 2,6-di-tert.butyl-4-methylphenol (=BHT) may also be used.

[0099] Suitable commercial inhibitors are, for example, Sumilizer™ GA-80, Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.; Genorad™ 16, Genorad™18 and Genorad™ 20 from Rahn AG; lrgastab™UV10 and Irgastab™ UV22, Tinuvin™ 460 and CGS20 from Ciba Specialty Chemicals; Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) from Kromachem Ltd, Additol™ S range (S100, S110, S120 and S130) from Cytec Surface Specialties.

[0100] Since excessive addition of these polymerization inhibitors may lower the curing speed, it is preferred that the amount capable of preventing polymerization is determined prior to blending. The amount of a polymerization inhibitor is preferably lower than 5 wt%, more preferably lower than 3 wt% of the total curable inkjet composition.

Adhesion promotor

[0101] The curable inkjet composition may include an adhesion promoter to further optimize the adhesion of the cured composition to various surfaces, in particular a copper surface. [0102] Any adhesion promoter may be used, for example those disclosed in W02004/026977 and W02004/105 both from AVECIA; W02017/009097 and W02020/104302 both from Agfa-Gevaert; and W02018/087059, W02018087052 W02018087056 and W02018087055, all from Agfa- Gevaert/Electra Polymers.

[0103] The curable inkjet composition may include one adhesion promoter or a combination of two, three or more different adhesion promoters.

[0104] The total amount of adhesion promoters 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.

[0105] However, it has been observed that an inkjet composition according to the present invention may have a sufficient adhesion, even after soldering, gold or ENIG plating, in the absence of an adhesion promoter. As the presence of adhesion promoters may result in a worse 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 promotor is preferably less than 2.5 wt%, more preferably less than 1 wt%, most preferably less than 0.5 wt %, relative to the total weight of the composition.

Flame retardant

[0106] The curable inkjet composition preferably comprises a flame retardant.

[0107] Preferred flame retardants are inorganic flame retardants, such as Alumina Trihydrate and Boehmite; organo-phosphor compounds, such as organophosphates (e.g. triphenyl phosphate (TPP), resorcinol bis (diphenylphosphate) (RDP), bisphenol A diphenyl phosphate (BADP), and tricresyl phosphate (TCP)); organo-phosphonates (e.g. dimethyl methylphosphonate (DMMP)); and organophosphinates (e.g. aluminium di methyl phosphinate).

[0108] Preferred flame retardants are disclosed in WG2019/121098.

Colorants

[0109] The curable inkjet composition may be a substantially colourless inkjet ink or may include at least one colorant. For example, when the inkjet ink is used as etch resist, the colorant makes the temporary mask clearly visible to the manufacturer of conductive patterns, allowing a visual inspection of quality. When the inkjet ink is used to apply a solder mask it typically contains a colorant. A preferred colour for a solder mask is green, however other colours such as black or red may also be used.

[0110] The colorant may be a pigment or a dye, preferably a pigment.

[0111] A colour pigment may be chosen from those disclosed by 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 W02008/074548.

[0112] Pigment particles in inkjet inks should be sufficiently small to permit free flow of the ink through the inkjet-printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation. Most preferably, the average pigment particle size is not larger than 150 nm. The average particle size of pigment particles is preferably determined with a Brookhaven Instruments Particle Sizer BI90pl us based upon the principle of dynamic light scattering.

[0113] Generally, dyes exhibit a higher light fading than pigments, but cause no problems on jettability. It was found that anthraquinone dyes exhibit only minor light fading under the 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™ Blue 3R (CASRN 325781-98- 4) from LANXESS.

[0114] Other preferred dyes include crystal violet and a copper phthalocyanine dye.

[0115] It is possible to combine different colorants to obtain the desired colour, or to improve the dispersion stability. A preferred combination of colorants to obtain a green solder mask is a combination of a blue and a yellow colorant. In a particularly preferred embodiment, a combination of pigment yellow 150 and pigment blue 15:4 is used.

[0116] 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.

Polymeric dispersants

[0117] If the colorant in the curable inkjet ink is a pigment, then the curable inkjet ink preferably contains a dispersant, more preferably a polymeric dispersant, for dispersing the pigment.

[0118] Suitable polymeric dispersants are copolymers of two monomers but they may contain three, four, five or even more monomers. The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Copolymeric dispersants preferably have the following polymer compositions:

• statistically polymerized monomers (e.g. monomers A and B polymerized into ABBAABAB);

• alternating polymerized monomers (e.g. monomers A and B polymerized into ABABABAB);

• gradient (tapered) polymerized monomers (e.g. monomers A and B polymerized into AAABAABBABBB);

• block copolymers (e.g. monomers A and B polymerized into AAAAABBBBBB) wherein the block length of each of the blocks (2, 3, 4, 5 or even more) is important for the dispersion capability of the polymeric dispersant;

• graft copolymers (graft copolymers consist of a polymeric backbone with polymeric side chains attached to the backbone); and

• mixed forms of these polymers, e.g. blocky gradient copolymers.

[0119] Suitable polymeric dispersants are listed in the section on “Dispersants”, more specifically [0064] to [0070] and [0074] to [0077], in EP-A 1911814.

[0120] Commercial examples of polymeric dispersants are the following:

• DISPERBYK™ dispersants available from BYK CHEMIE GMBH;

• SOLSPERSE™ dispersants available from NOVEON;

• TEGO™ DISPERS™ dispersants from EVONIK;

• EDAPLAN™ dispersants from MUNZING CHEMIE;

• ETHACRYL™ dispersants from LYON DELL;

• GAN EX™ dispersants from ISP;

• DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;

• DISPONER™ dispersants from DEUCHEM; and

• JONCRYL™ dispersants from JOHNSON POLYMER.

Surfactants

[0121] The curable inkjet composition may contain at least one surfactant, which may act as wetting agent, dispersant or emulsifier.

[0122] The surfactant can be anionic, cationic, non-ionic, or zwitter-ionic.

[0123] Suitable surfactants include fluorinated surfactants, fatty acid salts, ester salts of a higher alcohol, alkylbenzene sulfonate salts, sulfosuccinate ester salts and phosphate ester salts of a higher alcohol (for example, sodium dodecylbenzenesulfonate and sodium dioctylsulfosuccinate), ethylene oxide adducts of a higher alcohol, ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of a polyhydric alcohol fatty acid ester, and acetylene glycol and ethylene oxide adducts thereof (for example, polyoxyethylene nonylphenyl ether, and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS & CHEMICALS INC.).

[0124] Preferred surfactants are selected from fluorinated surfactants (such as fluorinated hydrocarbons) and silicone surfactants. The silicone surfactants are preferably siloxanes and can be alkoxylated, polyether modified, polyether modified hydroxy functional, amine modified, epoxy modified and other modifications or combinations thereof. Preferred siloxanes are polymeric, for example polydimethylsiloxanes.

[0125] Preferred commercial silicone surfactants include BYK™ 333, BYK™ 347 and BYK™ UV3510 from BYK Chemie.

[0126] In a preferred embodiment, the surfactant is a polymerizable compound.

[0127] Preferred polymerizable silicone surfactants include a (meth)acrylated silicone surfactant. Most preferably the (meth)acrylated silicone surfactant is an acrylated silicone surfactant, because acrylates are more reactive than methacrylates. A preferred commercial acrylated surfactant is Ebecryl 1360 from Allnex.

[0128] In a preferred embodiment, the (meth)acrylated silicone surfactant is a polyether modified (meth)acrylated polydimethylsiloxane or a polyester modified (meth)acrylated polydimethylsiloxane.

[0129] 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.

Preparation of the inkjet composition

[0130] The preparation of pigmented curable inkjet inks is well-known to the skilled person. Preferred methods of preparation are disclosed in paragraphs [0076] to [0085] of WO2011/069943.

Method of manufacturing an electronic device

[0131] The method of manufacturing an electronic device according to the present invention includes at least one step wherein a curable inkjet composition as described above is jetted and cured on a substrate. [0132] According to a preferred embodiment, the electronic device is a Printed Circuit Board (PCB).

[0133] In a particular preferred embodiment, the method of manufacturing a PCB includes a step wherein the solder mask composition is applied on the substrate via an inkjet printing step, followed with a UV-curing step and a heat treatment step.

[0134] The substrate is preferably a dielectric substrate containing an electrically conductive pattern, which typically comprises conductive pads electrically connected with each other using traces.

[0135] The dielectric substrate of the electronic device may be any non-conductive material. The substrate is typically a paper/resin composite or a resin/fibre glass composite, a ceramic substrate, a polyester or a polyimide. FR-4 is an example of a material frequently used as dielectric substrate.

[0136] The electrically conductive pattern is typically made from any metal or alloy which is conventionally used for preparing electronic devices such as gold, silver, palladium, nickel/gold, nickel, tin, tin/lead, aluminium, tin/aluminium and copper. The electrically conductive pattern is preferably made from copper.

[0137] 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 thereof. A preferred pre-treatment process is chemical micro-etching, which typically results in micro-roughness on the substrate. A so-called anti-bleeding treatment can be additionally applied to the micro-etched surface to prevent bleeding of the ink into the micro-pores and to improve print quality. This anti-bleeding treatment typically comprises applying a coating layer on the substrate in order to adjust its surface energy, resulting in sharper contact angles and minimal bleeding of ink into micro-pores. Anti-bleeding treatments are preferably used when printing low-viscous compositions.

[0138] The process of inkjet printing the solder mask layer on a dielectric substrate with an electrically conductive pattern preferably comprises one or more printing steps as listed below.

• The printing of so-called “ramps” comprises printing lines next to the copper traces. This preparative printing step ensures a sufficient coverage of the Cu traces. If no ramps are printed next to the copper traces, the cured solder mask may be too thin on the edges (also called shoulders) of the copper traces. The thickness of the printed ramps is related to the height of the plated Cu traces. For plated Cu traces having a large height, a higher ramp ink thickness is required in order to have sufficient coverage on the Cu edges. For plated Cu traces with a lower height, a lower ramp thickness may be used. The ramps preferably have a thickness of 0 to 80 pm, more preferably from 10 to 60 pm, most preferably from 20 to 40 pm.

• The printing of so-called “dams” is usually done to indicate the contours 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 dams is preferably at least the same as the thickness of the full solder mask layer. The thickness of the dams is more preferably higher than the thickness of the full solder mask layer, in order to avoid possible ink flow towards the open pad. The dams preferably have a thickness of 5 to 75 pm, more preferably from 10 to 60 pm, most preferably from 20 to 40 pm.

• Finally, the entire board is printed within the contours of the dams, covering the Cu traces, but leaving open the Cu pads for soldering.

[0139] The curable inkjet composition may be cured by exposing the composition to actinic radiation, such as electron beam or ultraviolet (UV) radiation. Preferably the curable inkjet composition is cured by UV radiation, more preferably using UV LED curing. To fixate the curable composition on the substrate, a UV pin curing step may be used immediately after printing. This UV pin curing may improve the print quality.

[0140] A heat treatment is preferably applied to the jetted and UV-cured curable inkjet composition. The heat treatment is preferably carried out at a temperature of from 80 ° C and 250 ° C. The temperature is preferably not less than 100° C, more preferably not less than 120 ° C. To prevent charring of the solder mask, the temperature is preferably not higher than 200 ° C, more preferably not higher than 160 ° C.

[0141] The thermal treatment is typically carried out from 15 to 90 minutes.

[0142] The purpose of the thermal treatment is two-fold: curing thermal cross-linking agents present in the curable composition, and further polymerizing potentially non-reacted radiation curable compounds. A dense interpenetrating polymer network can thus be created.

[0143] The method of manufacturing a PCB may comprise two, three or more inkjet printing steps. For example, the method may comprise two inkjet printing steps wherein an etch resist is provided on a metal surface in one inkjet printing step and wherein a solder mask is provided on a dielectric substrate containing an electrically conductive pattern in another inkjet printing step.

[0144] A third inkjet printing step may be used for legend printing. Inkjet printing devices

[0145] The curable inkjet composition may be jetted by one or more print heads ejecting small droplets in a controlled manner through nozzles onto a substrate, which is moving relative to the print head(s).

[0146] A 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 thereto. The application of a voltage changes the shape of the piezoelectric ceramic transducer in the print head creating a void, which is then filled with ink. When the voltage is again removed, the ceramic expands to its original shape, ejecting a drop of ink from the print head. However, the inkjet printing method according to the present invention is not restricted to piezoelectric inkjet printing. Other inkjet print heads can be used and include various types, such as a continuous type.

[0147] The inkjet print head normally scans back and forth in a transversal direction across the moving ink-receiving surface (substrate). Often the inkjet print head does not print on the way back. Bi-directional printing is preferred for obtaining a high areal throughput. Another preferred printing method is by a “ single pass printing process”, which can be performed by using page wide inkjet print heads or multiple staggered inkjet print heads which cover the entire width of the ink-receiving surface. In a single pass printing process, the inkjet print heads usually remain stationary and the ink-receiving surface is transported under the inkjet print heads.

Examples

Materials

[0148] All materials used in the following examples were readily available from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium) unless otherwise specified. The water used was deionized water.

[0149] SR833s is tricyclodecanedimethanol diacrylate available as Sartomer™ SR833S from ARKEMA with CAS registry number 42594-17-2.

[0150] SR834 is tricylodecanedimethanol dimethacrylate available as Sartomer™ SR834 from ARKEMA with CAS registry number 43048-08-4.

[0151] DPGDA is dipropylenediacrylate, available as Sartomer™ SR508 from ARKEMA.

[0152] HDDA is hexanediol diacrylate available as Sartomer™ SR238 from ARKEMA. [0153] CHDMDA is 1,4-cyclohexyldimethanol diacrylate with CAS number 67905-41- 3, available as Sartomer™ CD406 from ARKEMA.

[0154] SR789 is tricyclodecanemethanol acrylate available as Sartomer™ SR789 from ARKEMA with CAS registry number 93962-84-6.

[0155] FA513AS is dicyclopentanyl acrylate with CAS number 79637-74-4 available from Resonac.

[0156] DCPA is dihydrodicyclopentadienyl acrylate, commercially available as Laromer DCPA from BASF with CAS registry number 903574-98-1.

[0157] PEA is 2-phenoxyethyl acrylate available as Sartomer™ SR339 from ARKEMA.

[0158] VMOX is N-vinyl-5-methyl-2-oxazolidinone available from BASF.

[0159] IBOA is isobornyl acrylate available as Photomer 4012 from IGM resins.

[0160] TBCH is 4-tert-butylcyclohexylacrylate available as LAROMER TBCH from BASF.

[0161] DETX is a 2,4-diethylthioxanthone photoinitiator available as Genocure DETX from Rahn.

[0162] BAPO is a bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide photoinitiator available as Irgacure™ 819 from BASF.

[0163] BISAPHOS is a flame retardant containing a mixture of aromatic polyphosphates available from ADEKA.

[0164] Trixene BI7960 is a DMP-blocked isocyanate cross-linking agent commercially available from LANXESS.

[0165] Cymel N F2000A is a triazine cross-linking agent commercially available from ALLN EX.

[0166] Ebecryl 1360 is a silicone hexa-acrylate available from ALLNEX.

[0167] WET is a 1% solution of Ebecryl 1360 in VEEA.

[0168] Cyan is SUN FAST BLUE 15:4, a cyan pigment available from SUN CHEMICALS

[0169] Yellow is CROMOPHTAL YELLOW D 1085J, a yellow pigment from BASF.

[0170] INHIB is a mixture forming a polymerization inhibitor having a composition according to Table 1.

Table 1 [0171] Cupferron™ AL is aluminium N-nitrosophenylhydroxylamine from WAKO CHEMICALS LTD.

[0172] PRECIP 162 is a dispersing agent and has been precipitated from Disperbyk 162, a dispersant solution available from BYK (ALTANA).

[0173] DISP is a solution of 30 wt% PRECIP 162 and 1 wt% INH IB in VEEA.

[0174] GD is a green dispersion prepared as follows:

A concentrated green dispersion, GD, was prepared having a composition according to Table 2.

Table 2

GD was prepared as follows: 138 g of VEEA, 2 g of INHIB, 30 g of DISP, 30 g of Cyan and 30 g of Yellow were mixed using a DISPERLUX™ dispenser. Stirring was continued for 30 minutes. The vessel was connected to a NETZCH MiniZeta mill filled with 900 g of 0.4 mm yttrium stabilized zirconia beads (“high wear resistant zirconia grinding media” from TOSOH Co.). The mixture was circulated over the mill over 120 minutes (residence time of 45 minutes) and a rotation speed in the mill of about 10.4 m/s. During the complete milling procedure, the content in the mill was cooled to keep the temperature below 60° C. After milling, the dispersion was discharged into a vessel.

Evaluation methods

LED sensitivity

[0175] To evaluate the LED sensitivity of the inks, the inkjet inks were coated using a barcoater and a Braive automatic film applicator on a PET substrate. The bar coater resulted in coatings of 10 pm thick which were cured using a Fusion LED station (UV LED lamp 12 W). The coatings were passed through the LED light using a belt which was set at moving 50 m/min and the LED light power was set at 4 W. [0176] After each pass through the LED light (max. 4 passes), the inks were evaluated using a Q tip. The Q tip was used to wipe the coating and a score was given according below:

• 0: No wipe visible on coating

• 1: Wipe visible on coating but no ink on the Q tip

• 2: Wipe visible on coating and ink on Q tip but no damage on the coating

• 3: Ink on Q tip and minor damage on the coating

• 4: Ink on Q tip and major damage on the coating

• 5: Ink completely removed after wiping with Q tip

Viscosity

[0177] The viscosity of the inks was measured at 45 ° C and at a shear rate of 1000 s'¹ using a HAAKE RotoVisco 1.

[0178] For industrial inkjet printing, the viscosity at 45 ° C and at a shear rate of 1000 s^-1 is preferably from 5.0 to 15 mPa.s. More preferably the viscosity at 45° C and at a shear rate of 1 000 s^-1 is less than 15 mPa.s.

IR Reflow

[0179] A 40 m coating on FR-4 EM 825 is prepared by coating a 20 pm layer of the ink and curing 4 times using a 12W UV LED lamp with a belt moving at 20 m/min. Then, a second 20 pm layer was applied and cured in the same way. Finally, the 40 pm coating is put in an oven for 1 hour at 150 ° C.

[0180] Next a crosshatch was applied to the coating. This will serve a possible starting point for crack formation and is considered a harsh test for the coatings to pass.

[0181] IR reflow evaluation is done 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 samples. The 40 pm coating is put into the IR eC-reflow- mate V4 reflow oven and placed above the central temperature sensor. The reflow cycle begins by heating up the oven to 260 ° C. Once the sample temperature has reached 260 ° C, the oven holds the temperature for 10 seconds after which the sample is cooled down at open air. The duration of one cycle is 4min 30s. The ink coating undergoes up to 10 IR reflow cycles (Cl to CIO). [0182] The evaluation of the samples is done visually by counting the number of cracks after each cycle. When there are 3 or more marks observed, the coating has failed the test and the test is stopped.

EN IG Resistance

[0183] An EN IG simulation was carried out using a procedure as described below:

• The boards were dipped in a bath of acid cleaner (Umicore cleaner 865) at 40 ° C during 4 min. The boards were then removed and dipped in a rinsing bath of deionized water (DW) at room temperature (RT) during 90 seconds.

• The boards were dipped in a microetching bath comprising 8.5 wt% Na₂S₂0₈ and ± 3.2 wt% H₂SO₄ (98 %) in water at a temperature between 26-33 ° C for 100 s. The boards were then removed and rinsed in respectively DW, a 2.5 wt% aqueous H₂SO₄ solution and DW, all at RT during 90 seconds.

• The boards were dipped in 2.5 wt% aqueous H₂SO₄ solution at RT during 30 seconds after which they were removed and rinsed in DW at RT during 90 seconds. They were then dipped again in the same solution during 60 seconds.

• The boards were dipped in a palladium activator bath (Accemulta MKN 4) at a temperature around 30 ° C for 90 s followed by dipping in a 5 wt% aqueous H₂SO₄ solution at RT during 75 seconds. The boards were then removed and dipped in a rinsing bath of DW at RT during 90 seconds.

• Then the boards were dipped in a nickel bath (Nimuden N PR 4) at a temperature around 85 ° C for 35 min. The boards were then removed and dipped in a rinsing bath of DW at RT during 90 s.

• Finally, the boards were dipped in a gold bath (Gobright TAM 55) at a temperature around 80 ° C for 12 min. The boards were then removed and dipped in a rinsing bath of DW at RT during 90 s.

[0184] After ENIG treatment, it is possible that a so-called halo effect occurs, especially around open pads. In these areas, it appears that the ENIG solution penetrates and lifts the solder mask layer, which can be observed as an area around an open pad having a different appearance. This phenomenon could create weak adhesion points of the solder mask onto the substrate and may result in peeling off from the layer.

[0185] The occurrence of halo effect was evaluated microscopically with a digital microscope Dino-LITE and scored with a value from 0 to 5, depending on the observed width of the Halo. • 0: No Hallo effect visible.

• 1: Halo width less than 1 mm.

• 2: Halo width from 1 mm to 2 mm.

• 3: Halo width from 2 mm to 3 mm.

• 4: Halo width from 3 mm to 4 mm.

• 5: Halo width more than 4 mm.

[0186] The adhesion after EN IG plating was evaluated with cross hatch adhesion test using Tesatape™ 4104 PVC tape. The evaluation was made in accordance with a criterion described below. The adhesion was evaluated both on Cu and FR-4.

• 0: Nothing removed, perfect adhesion.

• 1: Detachment of only very small parts of the cured layer, almost perfect adhesion.

• 2: Minor parts of the cured layer were removed by the tape, good adhesion.

• 3: Parts of the cured layer were removed by the tape, poor adhesion.

• 4: Most of the cured layer was removed by the tape, poor adhesion.

• 5: The cured layer was completely removed from the substrate by the tape, no adhesion.

Examples 1 to 3 and Comparative Examples 1 to 2.

[0187] The curable inkjet composition Ex-1 to Ex-3 and the comparative curable inkjet composition Comp-1 to Comp-2 were prepared according to Table 5. The weight percentages are based on the total weight of the curable inkjet composition.

[0188] In these examples, the amount of bifunctional HDDA is steadily increased, while the amount of monofunctional SR789 is decreased accordingly. All other compounds are kept constant.

Table 3

[0189] The LED sensitivity and IR reflow resistance were evaluated as described above and the results are shown below in Table 4.

Table 4

[0190] Inks containing at least 40 wt% of a tricycloalkane acrylate monomer such as SR789 have a good IR reflow resistance. When there is less than 40 wt% SR789, the IR reflow resistance becomes worse. It is also clear that an increasing amount of difunctional acrylate has a positive impact on LED sensitivity.

Examples 4 to 10 and Comparative Examples 3 to 4.

[0191] The curable inkjet composition Ex-4 to Ex-10 and the comparative curable inkjet composition Comp-3 to Comp-4 were prepared according to Table 5. The weight percentages are based on the total weight of the curable inkjet composition.

[0192] In these examples, the amount of acrylate monomers comprising a tricyclic aliphatic group the amount is kept constant at 52.4 wt%, with a variation in monofunctional and difunctional acrylate. The amount of difunctional acrylate SR833s is steadily increased, while the amount of monofunctional SR789 is decreased accordingly. All other compounds are kept constant. Only in Ex-10, the total amount of acrylate monomers comprising a tricyclic aliphatic group is higher, to compensate for the absence of low-viscous monomers VMOX and TBCH. Table 5

[0193] The LED sensitivity and IR reflow resistance were evaluated as described above and the results are shown below in Table 6.

Table 6

[0194] It is again clear from the above results that inks comprising tricycloalkane acrylate monomers such as SR833s and SR789 have a good IR reflow resistance. However, when the amount of diacrylate SR833s increases above 30.0 wt% (Comp-3 and Comp-4), the IR reflow resistance becomes worse. It is believed that above 30.0 wt% of diacrylates, in this case SR833s, the curing results in a high shrinkage stress, resulting in a reduced adhesion, having a negative impact on the IR reflow resistance. Moreover, above 30.0 wt% of diacrylates, it is possible that the cured layer becomes too brittle, which is also negatively impacting IR reflow resistance. Therefore, it is important that the ink does not contain more than 30 wt% diacrylates.

[0195] Preferably, the inks have a good curability after 1 or 2 passes. It is clear from the table that when there is no SR833s (Ex-4) or only 5 wt% SR833s (Ex-5), the LED sensitivity is less good than when there is more SR833s present.

[0196] Preferably, the inks have a viscosity of between 6 and 12 mPa.s. When there is no VMOX and no TBCH, the viscosity is higher than 15 mPa.s.

Examples 11 to 15 and Comparative Examples 5 to 9.

[0197] The curable inkjet composition Ex-11 to Ex-15 and the comparative curable inkjet compositions Comp-5 to Comp-9 were prepared according to Table 7. The weight percentages are based on the total weight of the curable inkjet composition.

[0198] In these experiments, the tricycloalkane monomers of the previous experiments are replaced by alternatives. Ex-11 is a repetition of Ex-9. In Ex- 12, Comp-5, Comp-6, and Comp-7, SR833s was replaced with other bifunctional (meth)acrylates, while keeping the amount of monofunctional SR789 constant. In Comp-5, SR833s was replaced by DPGDA, a linear diacrylate commonly known and used in inks for printed circuit boards.

In Comp-6, SR833s was replaced by HDDA, a linear diacrylate also commonly known and used in inks for printed circuit boards. In Comp-7, SR833s was replaced by CHDMDA, a diacrylate having a single aliphatic ring in its skeleton.

[0199] In Ex-13, 5 wt% of monofunctional SR 789 was replaced with 5 wt% of the bifunctional methacrylate SR834. Further, in Ex-14, Ex-15, Comp-8, and Comp-9, SR789 was replaced with other monofunctional acrylates, while keeping the amount of SR833s constant. In Ex-14, SR789 was replaced by FA513S (dicyclopentanyl acrylate), which is a tricyclodecane acrylate. In Ex- 15, SR789 was replaced by DCPA (dihydrodicyclopentadienyl acrylate), which is a tricyclodecane acrylate. In Comp-8, SR789 was replaced by IBOA, which has a single bridged bicyclic structure in its skeleton. In Comp-9, SR789 was replaced by PEA, which comprises a single aromatic ring. Table 7

[0200] The LED sensitivity and IR reflow resistance were evaluated as described above and the results are shown below in Table 8.

Table 8

[0201] It is clear from the above results that exchanging an acrylate compound comprising a tricyclic aliphatic group with an alternative acrylate compound always results in a worse IR reflow resistance, even when the inks cured well.

[0202] The LED curing outcome was not significantly influenced in these experiments, most inks cured well. This is most likely related to the fact that the total amount of diacrylate and monofunctional acrylate was not changed. The only exception is Ex-12, which only cured well after 4 passes. This is likely because the diacrylate SR833s is entirely exchanged for its dimethacrylate variant, SR834. Methacrylates are known to cure less efficiently than their acrylate counterparts.

Examples 16 to 20 and Comparative Examples 10 to 11.

[0203] The curable inkjet composition Ex-16 to Ex-20 and the comparative curable inkjet composition Comp-10 to Comp-11 were prepared according to Table 9. The weight percentages are based on the total weight of the curable inkjet composition.

[0204] In these examples, thermal crosslinkers are varied within the compositions, to verify that the IR reflow resistance results are only related to the presence of acrylates comprising a tricyclic aliphatic group. In Ex-16, Ex-20, Comp-10, and Comp-11, the amount of bifunctional SR833s is increased, while the amount of monofunctional SR789 is decreased accordingly. All other compounds are kept constant. In Ex-17 to Ex-19, two types of thermal crosslinkers were added separately or together.

Table 9

[0205] The LED sensitivity and IR reflow resistance were evaluated as described above and the results are shown below in Table 10.

Table 10

[0206] From these results, it is clear that the absence or presence of thermal crosslinkers does not influence the IR reflow resistance, nor the LED sensitivity. Comp-10 and Comp-11, which comprise more than 30 wt% of the diacrylate SR833s in their composition, show a less good IR reflow resistance.

[0207] Preferably, the inks also have a good adhesion and EN IG resistance. It was shown that the presence of the thermal crosslinkers improves the adhesion and/or the EN IG resistance of the cured solder mask layers, as is clear from Table 11 below.

[0208] The inkjet inks were printed using a MicroCraft CPS2013D (Printhead Konica Minolta KM1024iS, UV LED 395 12 W total output of the lamp) on a checkerboard (copper plated on FR4) substrate from Eurocircuits to obtain a soldermask layer having a final thickness of +/- 22 pm. The checkerboards include a 35 pm copper layer that was roughened by chemical etching. In the chemical etching step, the substrate was transported through a Bungard Sprint 3000 Conveyorized Spray Etch machine at a speed of 0.4 m/min while spraying with the chemical etchant CZ2001 (available from MEC) heated at 30° C. After a rinsing step with demineralized water an extra spraying step with IM HCL was carried out, followed again by a rinsing step with demineralized water and finally a drying step in an Air 2000 dryer (available from Bungard). The substrate was printed within 24 h after this pretreatment.

[0209] After printing the samples were baked in an oven at 150° C for 1 hour.

[0210] Different printing protocols were used:

Print-1 (Pl): An image having a resolution 1440 dpi in the x-direction and 1440 dpi in the y-direction was printed and cured. The UV energy applied corresponded 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 full energy of the 12 W lamp).

Print-2 (P2): Compared to Print 1, the image was printed in the same resolution but the UV energy applied corresponded to 100 % of the total power of the 12 W lamp. A final cure was then carried out as described for Print-1.

Table 11

[0211] When there are two types of thermal crosslinkers present, both the adhesion and the ENIG resistance improve.

Claims

Claim 1. A curable inkjet composition for a printed circuit board comprising: i. a photoinitiator, ii. one or more thermal crosslinker selected from the group consisting of an unblocked isocyanate, a blocked isocyanate and a triazine compound, iii. and two or more (meth)acrylates, of which at least one (meth)acrylate comprises a tricyclic aliphatic group; characterized in that: the total amount of (meth)acrylates comprising a tricyclic aliphatic group is at least 40 wt% relative to the total weight of the curable inkjet composition; and the total amount of multi-functional acrylates is from 0 to 30 wt% relative to the total weight of the curable inkjet composition.

Claim 2. A curable inkjet composition according to claim 1, wherein at least two (meth)acrylates comprise a tricyclic aliphatic group.

Claim 3. A curable inkjet composition according to claim 1 or claim 2, comprising at least one monofunctional (meth)acrylate comprising a tricyclic aliphatic group and at least one multifunctional (meth)acrylate comprising a tricyclic aliphatic group.

Claim 4. A curable inkjet composition according to any of the preceding claims wherein the tricyclic aliphatic group is a tricyclodecane or a tricyclodecene group.

Claim 5. A curable inkjet composition according to any of the preceding claims, wherein the total amount of multi-functional acrylates is from 10 to 30 wt% relative to the total weight of the curable inkjet composition.

Claim 6. The curable inkjet composition according to any of the preceding claims, wherein the total amount of thermal crosslinkers is more than 5 wt% relative to the total weight of the curable inkjet composition.

Claim 7. The curable inkjet composition according to any of the preceding claims, wherein at least one thermal cross-linking agent is a blocked isocyanate.

Claim 8. The curable inkjet composition according to any of the preceding claims wherein at least one thermal cross-linking agent is a triazine compound, having a chemical structure according to General Formula II

General Formula II wherein

X represents N, 0, S, P or C;

R5, R6 and R7 independently from each other represent a substituted or unsubstituted alkyl group.

Claim 9. The curable inkjet composition according to any of the preceding claims further comprising N-vinyl-5-methyl-2-oxazolidinone and/or 4-tert- butylcyclo hexyl aery I ate.

Claim 10. The curable inkjet composition according to any of the preceding claims having a viscosity of 5 to 15 mPa.s measured a 45° C at a shear rate of 1000 s’¹.

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

Claim 12. The method according to claim 12 also comprising a heating step.

Claim 13. The method according to claim 12 or 13 wherein the heating step is carried out at a temperature from 80° C to 250° C.

Claim 14. The method according to any one of claims 12 to 14 wherein the substrate is a dielectric substrate provided with an electrically conductive circuitry.

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