WO2017108510A1

WO2017108510A1 - Method for coating printed circuit boards

Info

Publication number: WO2017108510A1
Application number: PCT/EP2016/080879
Authority: WO
Inventors: Antonio Russo; Milan Wilhelm KRUMBE; Michael Dornbusch
Original assignee: Solvay Specialty Polymers Italy SpA
Current assignee: Syensqo Specialty Polymers Italy SpA
Priority date: 2015-12-23
Filing date: 2016-12-14
Publication date: 2017-06-29
Anticipated expiration: 2018-06-23

Abstract

A method of coating a printed circuit board which comprises the use of a solvent-based formulation comprising functional (per)fluoropolyether polymers and acrylic polymers is herein disclosed. Printed circuit boards manufactured with the method are also disclosed.

Description

Method for coating printed circuit boards Cross-reference to related application

[0001 ] This application claims priority to EP15202545.8, filed on December 23 2016 and EP16163628.7 filed on 04 Apr 2016, whose content is entirely incorporated herein by reference for all purposes.

Technical Field

[0002] The present invention relates to a method for coating printed circuit boards and to printed circuit boards obtained with such method.

Background Art

[0003] Printed circuit boards (PCBs) are supports used for connecting together different electronic components of an electronic circuit through conductive tracks, pads or other features etched from copper sheets laminated onto a non-conductive substrate.

[0004] PCBs are used in a variety of environments and often exposed to different agents conditions, like moisture, salts, chemicals and temperature changes, which can sometimes be harsh and, therefore, cause corrosion or damages or malfunctioning of the circuitry. In order to avoid such drawbacks, PCBs are covered with a "conformal coating", i.e. a thin polymeric film (typically 25 - 250 μηη thick) which perfectly adheres to and follows the shape of, the circuit's components.

[0005] The most common polymeric materials used in the manufacture of

conformal coatings are acryclic polymers, epoxy polymers, polyurethanes, silicones fluorinated and non-fluorinated poly-para-xylylenes and amorphous fluoropolymers. The polymer materials can be dissolved in an organic solvent or in water, optionally in combination with other additives, in order to provide solvent-based or water-based coating formulations. The choice of the coating formulation (solvent- or water-based formulation) depends on the application method of the same to the circuit, said method comprising curing and drying. Usually, conformal coatings are applied by air drying (at room temperature or with heating) or by UV curing, the latter being preferred when speed of application is required.

[0006] Among the requirements of conformal coatings are transparency, chemical inertness, resistance to moisture, low water absorption, ability to perfectly adhere to the circuit's components and absence of electrical interaction with the circuitry. The polymer materials are therefore selected according to their ability to provide such properties.

[0007] For instance, acrylic polymers (also referred to in the filed as "acrylic

resins") are chosen when ease of re-work, simple drying, good moisture resistance, high fluorescence value and ease of viscosity adjustment are required.

[0008] Compositions for conformal coatings may include, in addition to the

polymer material and solvent or water, other ingredients which promote drying and curing (e.g. UV-curing agents) or improve one or more properties of the finished coating.

[0009] EP 2093243 A (SAMSUNG ELECTRONICS CO., LTD.) 8/26/2009

discloses compositions comprising a functional or non-functional

(per)fluoropolyether (PFPE) and a PFPE-miscible polymer for inhibiting or preventing oxide film formation on electronic components, especially those made of copper, aluminium, iron and molybdenum. The functional PFPE comprise end groups selected from silanol, chlorosilane, carboxylic acid, alcohol, amine and phosphoric acid and derivatives thereof. This

document specifically discloses end groups of formula -Si(OEt)3, - C(O)NHCH₂CH₃ and -OP(O)(OH)₂. PFPE-miscible polymers are typically selected from the group consisting of polyvinyl alcohol, polyvinyl chloride, polyacrylic amide, polyethylene glycol, polyethylene oxide,

polymethylvinylether, polyethyleneimine, polyphenylenevinylene, polyaniline, polypyrrole and copolymers thereof. The compositions typically include water and are applied to the substrate by UV curing, therefore, they contain a UV-curing agent as additive. Preferred substrates onto which the compositions are applied are wiring pads of memory and processing microdevices, optical sensors, heat sinks for display devices, wirings and electrodes of Organic Thin Film Transistors, electrodes of display devices, and wirings and electrodes of battery devices.

[0010] DE 102008012895 A (ROBERT BOSCH GMBH) 9/10/2009 discloses fluorinated or partially fluorinated anti-corrosion gels for electronic or microelectronic devices which comprise a PFPE derivative which may comprise hydroxy, alkoxy, trialkoxysilane, amino, carboxyl, thiol, cyanate, isocyanate, isothiocyanate, phosphate, carboxamide or polyether alcohol groups. This document does not disclose gels which contain another polymer selected from acryclic polymers, epoxy polymers, polyurethanes, silicones fluorinated and non-fluorinated poly-para-xylylenes.

[001 1] In view of the continuing development of the electronic industry and

increasingly demanding market requirements in terms of performance, the need is still felt of providing improved conformal coating compositions for printed circuit boards, said compositions being applicable to printed circuit boards with conventional methods.

Summary of invention

[0012] The Applicant has surprisingly found out that functional

(per)fluoropolyether polymers can be mixed with acrylic polymers, organic solvents and, optionally, other ingredients, to provide solvent-based conformal coatings formulations able to form conformal coatings which are improved in terms of one or more of hydrophobicity, chemical resistance and conductivity and which can be applied to printed circuit boards with conventional methods.

[0013] Accordingly, the present invention relates to a method [method (M)] for coating a printed circuit board (PCB), said method comprising the following steps:

(a) providing a functional (per)fluoropolyether polymer ["polymer (P1 )" or " PFPE polymer (P1 )"] comprising at least one fully or partially fluorinated polyoxyalkylene chain [chain (R_Pf)], said chain having at least two ends, wherein at least one end bears an end group [group (E)] comprising at least one functional group [group (G)];

(b) mixing polymer (P1 ) with an acrylic polymer ["polymer (P2)" or "acrylic polymer (P2)"], an organic solvent and, optionally, other ingredients, to provide a solvent-based formulation [formulation (F)];

(c) applying formulation (F) to said PCB and

(d) drying and/or curing the formulation (F).

[0014] The invention further relates to a PCB obtained by coating a PCB with method (M) as defined above and to specific formulations (F) for performing said method.

Detailed description of the invention

[0015] For the purposes of the present invention:

- the determinate and indeterminate articles "the", "a", and "an" are intended to denote also plural forms, or the expressions, or "at least one" or "one or more", unless stated otherwise; for example, the expressions "a PFPE polymer (P1 )", "an acrylic polymer (P2)", "an organic solvent", are intended to mean "at least one polymer (P1 )", "at least one acrylic polymer (P2)", at least one organic solvent", etc...

- any reference back to a generic embodiment of the invention is intended to include each specific embodiment falling within that generic

embodiment, unless indicated otherwise;

- the use of brackets "( )" before and after symbols or numbers identifying compounds or formulae, e.g. "polymer (P1 )", "polymer (P2)", etc . , has the mere purpose of better distinguishing those symbols or numbers from the rest of the text; thus, said parentheses could also be omitted;

- when numerical ranges are indicated, range ends are included;

- the expression "functional group" has its general meaning as intended in organic chemistry and it encompasses atoms or combination of atoms bonded to the carbon skeleton of chain (R_Pf), either directly or by means of a bridging moiety and which confers to polymer (P1 ) specific chemical properties and reactivity. Examples of functional groups are those comprising one or more heteroatoms other than fluorine, said atoms being independently selected from O, S, N, P, Si and mixtures thereof. Preferred examples of functional groups are the hydroxyl (-OH) group and the α,β- unsaturated carbonyl (-C=O) group; - the adjective "aromatic" denotes any mono- or polynuclear cyclic group (or moiety) having a number of π electrons equal to 4n+2, wherein n is 0 or any positive integer; an aromatic group (or moiety) can be an aryl or an arylene group (or moiety);

[0016] - the acronyms "PFPE" and "PFPEs" stands for (per)fluoropolyether and (per)fluoropolyether respectively;

- an "aryl group" is a hydrocarbon monovalent group consisting of one core composed of one benzenic ring or of a plurality of benzenic rings fused together by sharing two or more neighboring ring carbon atoms, and of one end. Non limitative examples of aryl groups are phenyl, naphthyl, anthryl, phenanthryl, tetracenyl, triphenylyl, pyrenyl, and perylenyl groups. The end of an aryl group is a free electron of a carbon atom contained in a (or the) benzenic ring of the aryl group, wherein an hydrogen atom linked to said carbon atom has been removed. The end of an aryl group is capable of forming a linkage with another chemical group;

- an "arylene group" is a hydrocarbon divalent group consisting of one core composed of one benzenic ring or of a plurality of benzenic rings fused together by sharing two or more neighboring ring carbon atoms, and of two ends. Non limitative examples of arylene groups are phenylenes, naphthylenes, anthrylenes, phenanthrylenes, tetracenylenes,

triphenylylenes, pyrenylenes, and perylenylenes. An end of an arylene group is a free electron of a carbon atom contained in a (or the) benzenic ring of the arylene group, wherein an hydrogen atom linked to said carbon atom has been removed. Each end of an arylene group is capable of forming a linkage with another chemical group.

The PFPE polymer (P1 )

[0017] For the purpose of the present invention, the polymer (P1 ) is a functional (per)fluoropolyether, that is to say a polymer comprising at least one fully or partially fluorinated polyoxyalkylene chain [chain (R_Pf)] having at least two ends, in which at least one end bears an end group [end group (E)] comprising at least one functional group [functional group (G)]. [0018] Preferably, the PFPE polymer (P1 ) comprises one fully or partially fluorinated polyoxyalkylene chain (R_Pf) having two ends, in which and at least one end bears a group (E) comprising at least one functional (G). Preferably, group (G) is at least one hydroxyl group [group (G1 )] or at least one α,β-unsaturated carbonyl group [group (G2)]. Preferably, group (G2) complies with formula: -C(O)-CRH=CH2 in which RH is hydrogen or a straight or branched C1-C6 alkyl group. The other end of chain (R_Pf) can also bear an end group (E) comprising at least one functional group (G) as defined above or can bear a non-functional fluorinated alkyl group .

[0019] Preferably, chain (R_Pf) of polymer (P1 ) is a chain of formula: -O-D- (CFX^#)zi-O(Rf)(CFX#)z2-D-O- wherein:

z1 and z2, equal or different from each other, are equal to or higher than 1 , preferably from 1 to 10, more preferably from 1 to 3;

X^#, equal or different from each other, is -F or -CF3,

provided that, when z1 and/or z2 are higher than 1 , both X are -F;

D, equal or different from each other, is a divalent straight or branched aliphatic radical comprising from 1 to 20, more preferably from 1 to 6 and even more preferably from 1 to 3 carbon atoms, said alkyl chain being optionally substituted with at least one perfluoroalkyl group comprising from 1 to 3 carbon atoms.

[0020] Preferably, each X is F and each D is methylene, ethylene or -CH(CF3)-.

More preferably, each X is fluorine and each D is methylene.

[0021] Typically, chain (Rf) is a fully or partially fluorinated chain comprising,

preferably consisting of, repeating units R°, said repeating units being independently selected from the group consisting of:

(i) -CFXO-, wherein X is F or CF₃;

(ii) -CFXCFXO-, wherein X, equal or different at each occurrence, is F or CF3, with the proviso that at least one of X is -F;

(iii) -CF2CF2CW2O-, wherein each of W, equal or different from each other, is F or H;

(iv) -CF2CF2CF2CF2O-;

(v) -(CF2)j-CFZ-O- wherein j is an integer from 0 to 3 and Z is a group of general formula -O-R(f_-a)-T, wherein R(f_-a) is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the following: -CFXO- , -CF2CFXO-, -CF2CF2CF2O-, -CF2CF2CF2CF2O-, with each of each of X being independently F or CF3 and T being a C1-C3 perfluoroalkyl group.

[0022] Preferably, chain (Rf) complies with the following formula:

-[(CFX¹O)gi(CFX²CFX³O)g2(CF2CF2CF2O)g3(CF2CF2CF2CF₂O)g₄]- wherein

- X¹ is independently selected from -F and -CF3,

- X², X³, equal or different from each other and at each occurrence, are independently -F, -CF3, with the proviso that at least one of X is -F;

- g1 , g2, g3, and g4, equal or different from each other, are independently integers≥0, such that g1 +g2+g3+g4 is in the range from 2 to 300, preferably from 2 to 100; should at least two of g1 , g2, g3 and g4 be different from zero, the different recurring units are generally statistically distributed along the chain.

[0023] More preferably, chain (Rf) is selected from chains of formula:

(Rf-IIA) -[(CF₂CF2O)al(CF₂O)a2]- wherein:

- a1 and a2 are independently integers≥ 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; both a1 and a2 are preferably different from zero, with the ratio a1/a2 being preferably comprised between 0.1 and 10;

(Rf-I I B) -[(CF₂CF₂O)bi (CF₂O)b2(CF(CF3)O)b3(CF2CF(CF₃)O)b4]- wherein:

b1 , b2, b3, b4, are independently integers≥ 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; preferably b1 is 0, b2, b3, b4 are > 0, with the ratio b4/(b2+b3) being >1 ;

(Rf-IIC) -[(CF2CF2O)ci(CF2O)c2(CF2(CF2)cwCF₂O)c3]- wherein:

cw = 1 or 2; d , c2, and c3 are independently integers≥ 0 chosen so that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; preferably c1 , c2 and c3 are all > 0, with the ratio c3/(c1 +c2) being generally lower than 0.2;

(Rf-IID) -[(CF₂CF(CF₃)O)_d]- wherein:

d is an integer > 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000;

[0024] Still more preferably, chain (Rf) complies with formula (Rf-lll) here below:

(Rf-lll) -[(CF₂CF2O)al(CF₂O)a2]- wherein:

- a1 and a2 are integers > 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000, with the ratio a1/a2 being generally comprised between 0.1 and 10, more preferably between 0.2 and 5.

[0025] Thus, PFPE polymers (P1 ) according to the present invention comply with general formula (P1 ) here below:

in which:

- R_Pf is as defined above,

- E is an end group as defined above or a non-functional fluorinated alkyl group.

[0026] Preferably, group E comprises from 1 to 3 functional groups (G).

[0027] According to a first embodiment, PFPE polymer (P1 ) comprises a chain (R_Pf) having at least two ends, wherein at least one end bears an end group (E) comprising at least one hydroxyl group (G-1 ) [end group (E-1 )]. This PFPE polymer (P1 ) will be herein after also referred to as "PFPE alcohol(s) (P1 )". Preferred PFPE alcohols (P1 ) are those wherein at least one end bears one end group (E-1 ) comprising one hydroxyl group (G-1 ).

[0028] Preferred groups (E-1 ) comprising one hydroxyl group (G-1 ) are those complying with formula (E-1A):

(E-1A) O-[CH₂CH(J)O]j-H

wherein: - J is independently selected from hydrogen, straight or branched alkyl and aryl, preferably from hydrogen, methyl, ethyl, ethyl and phenyl; more preferably, J is hydrogen and

- j is 0 or a positive number ranging from 1 to 50, more preferably from 1 to 15 and even more preferably from 4 to 7.

[0029] Preferred PFPE alcohols (P1 ) comply with general formula (P1 -A):

(P1 -A) T-O(Rf)(CFX)-CH₂O-[CH₂CH(J)O]_j-H

wherein:

- Rf is as defined above;

- X is as defined above, preferably F;

- J and j are as defined above;

and

- T is a non-functional group selected from -CF3, -C2F5 and -C3F7, or a functional group of formula -(CFX)-CH₂O[CH₂CH(J)O]j-H in which X, J and j are as defined above.

[0030] In end groups (E-1A) and in polymers (P1 -A), each [CH₂CH(J)O] unit can be equal to or different from one another; when j is equal to or higher than 2 and the units are different, they can be arranged randomly. Examples of polymers (P1 -A) are those comprising [CH₂CH(J)O] units arranged to form moieties complying with the following formulae (RM) - (RMII):

(CH₂CH₂O)j (R^j-I)

[CH₂CH(CH₃)O]j (R^j-ll)

[(CH₂CH₂O)ji(CH₂CH(CH₃)O)_j2]j (R^j-lll)

wherein:

- j is as defined above and

- j1 and j2 are equal to or higher than 1 , with the proviso that j1 +j2 is not higher than 50.

[0031] According to a preferred embodiment, J is hydrogen and j ranges from 4 to 7.

[0032] Preferably, in polymers (P1 -A), chain (Rf) complies with formula (Rf-lll) as defined above.

[0033] Preferred polymers (P1-A) are those wherein T is -(CFX)-

CH₂O[CH₂CH(J)O]j-H in which X, J and j are as defined above [herein after "bifunctional polymers (P1 -A)"]. Still more preferred are bifunctional polymers (P1 -A) wherein X is F, J is hydrogen and j ranges from 4 to 7. However, such polymers are always in admixture with monofunctional polymers (P1 -A), i.e. polymers wherein T is a non-functional group as defined above and also with traces of non-functional species, i.e. PFPEs wherein both ends of the polymer chain bear non-functional groups as defined above. Thus, for the purposes of the present invention, the expression "bifunctional polymers (P1 -A)" identifies PFPE alcohols (P1 -A) having an average functionality (F), i.e. an average number of -OH groups per polymer molecule, at least equal to 1.80, more preferably at least equal to 1.85 and still more preferably at least equal to 1.94. Functionality (F) can be calculated by means of ¹H-NMR and ¹⁹F-NMR analyses according to methods known in the art, for example following the teaching of US 5910614 (AUSIMONT SPA) .

[0034] PFPE alcohols (P1 -A) are known in the art. In particular, polymers (P1 -A) in which chain (Rf) complies with formula (Rf-lll), j is 0 and T is - CF2CH2OH are available on the market from Solvay Specialty Polymers Italy S.p.A. with trademark Fomblin^®, while a polymer (P1 -A) in which chain (R_f) complies with formula (RHII) and T is -(CF2)-CH₂O(CH2CH₂O)j- OH wherein j is comprised between 1 and 2 is available on the market from Solvay Specialty Polymers Italy S.p.A. with tradename Fluorolink^® E 10H. Polymers (P1 -A) in which j is an integer ranging from 1 to 50 can be advantageously manufactured according to the method disclosed in WO 2014/090649 (SOLVAY SPECIALTY POLYMERS ITALY SPA) .

[0035] According to a second embodiment, polymer (P1 ) comprises a chain (R_Pf) having at least two ends, wherein at least one end bears an end group (E) comprising at least one group (G2) as defined above, preferably a -C(O)- CRH=CH2 in which RH is hydrogen or a straight or branched C1 -C6 alkyl group. This polymer (P1 ) will be herein after referred to as acrylic polymer (P1 ).

[0036] Such acrylic polymer (P1 ) preferably complies with formula (P1 -B):

(P1 -B) T'-O(Rf)(CFX)-CH2O[CH₂CH(J)O]_j-E^*

wherein: - Rf, X and, J and j are as defined above and

- E* is an end group comprising at least one group (G2) as defined above, preferably a group -C(O)-CRH=CH2 in which RH is hydrogen or

a straight or branched C1 -C6 alkyl group, and

- T is -(CFX)-CH₂O[CH₂CH(J)O]j-E* or a group selected from -CF₃, -C2F5

[0037] Preferred groups E* are those complying with formulae (E^*1 ) - (E^*3) here below:

(E^*1 ) -CO-CR_H=CH₂

(E^*2) -CO-N H-CO-CR_H=CH₂,

(E^*3) -CO-R^A-CR_H=CH₂,

wherein

RH is H or a C1 -C6 alkyl group;

R^A is selected from the group consisting of:

(j) -N H-R^B-O-CO- (jj) -N H-R^B-N HCOO-R^B-OCO-;

R^B being a divalent group selected from the group

consisting of C1 -C10 aliphatic group, Cs-Ci₄

cycloaliphatic group; C6-Ci₄ aromatic or alkylaromatic group.

[0038] Preferably, in polymers (P1 -B), group E* complies with formula (E^*1 ), as defined above; preferably, in formula (E^*1 ), RH is hydrogen or methyl.

[0039] Preferably, in polymers (P1 -B), chain (Rf) complies with formula (Rf-lll) as defined above.

[0040] Such polymers (P1 -B) can be manufactured according to known methods by reaction of polymers (P1), preferably polymers (P1 -A) as defined above, with an α,β unsaturated carbonyl halide.

[0041] Suitable examples of α,β-unsaturated carbonyl halides are compounds having the following general formulae:

X-CO-CR_H=CH₂

X-CO-N H-CO-CRH=CH₂ and

X-CO-R^A-CR_H=CH₂

wherein X is halogen atom, preferably CI, and

RH and R^A have the same meaning defined above.

[0042] Preferred α,β-unsaturated carbonyl compounds are for example acryloyi chloride, methacryloyl chloride and the like.

[0043] In another preferred embodiment, group E^* is a group (Ε^*') comprising two or three two or three groups (G2) as defined above. Such group E^* comprises a hydrocarbon moiety [moiety (U)] optionally comprising oxygen and/or nitrogen atoms and/or (hetero)cycloalkylene groups, said moiety having two or three bonding sites for the carbonyl group of group (G2).

[0044] For the purpose of the present invention, in moiety (U), optional oxygen atoms can be ethereal oxygen atoms or can form oxygen-containing groups such as carbonyl, ester and carbonate groups; the nitrogen atoms can be comprised as primary, secondary or tertiary amino groups or, together with oxygen and/or nitrogen atoms, can form groups like amido, urea or urethane groups. Preferably, moiety (U) comprises urethane groups. For the purpose of the present invention, the optional

cycloalkylene groups in moiety (U) are typically C5-C10 cycloalkylene rings, optionally substituted with one of more straight or branched alkyl groups, preferably straight or branched C1 -C3 alkyl groups; when the cycloalkylene groups comprise heteroatoms, said heteroatoms are preferably one or more oxygen and/or nitrogen atoms, wherein oxygen atoms can be ethereal oxygen atoms or can form oxygen-containing groups such as carbonyl, ester and carbonate groups; the nitrogen atoms can be comprised as primary, secondary or tertiary amino groups or, together with oxygen and/or nitrogen atoms, can form groups like amido, urea or urethane groups.

[0045] A preferred moiety (U) complies with formula (U-1 ) here below:

(U-1 )

A ^xCONH— R^ulb-0— # in which one of R^{U 1 A} is a straight or branched C2-C10 hydrocarbon chain and each R^{u1 b}, equal to or different from one another, is a C2-C10 hydrocarbon group, the ^* represents the bonding site to the right side oxygen atom in the -CH2O[CH2CH(J)O]j- moiety of polymer (P1-B) and the # represents the bonding site to the carbonyl group of group (G2).

A preferred example of group (Ε^*') comprising a moiety of formula (U-1 ) is the one complying with formula (E^*'ui) here below:

(E^*'ui)

wherein RH is as defined above, preferably hydrogen or methyl and the ^* is as defined above.

Preferred examples of polymers (P1 -B) comprise a chain (Rf-lll) as defined above and groups (E^*'ui) as defined above wherein RH is hydrogen of methyl. A convenient example of such polymers (P1 -B) is available from Solvay Specialty Polymers Italy S.p.A. as Fluorolink^® MT 70 and complies with general formula:

wherein:

- (Rf) is a chain (Rf-lll) as defined above and

- T' is a non-functional group selected from -CF3, -C2F5 and -CsF₇ or a group of formula (Tui):

(T'ui)

[0048] Polymers (P1 -B) in which group (Ε^*') is a group (E*'ui) as defined above can be conveniently obtained by reaction of a polymer (P1 ) wherein group G bears two hydroxy groups with a isocyanate of formula:

O=C=N-R^{U 1 B}-OCR_H=CH₂

in which R^{U 1 B} and RH are as defined above according to methods known in the art.

[0049] Another preferred group of moieties (U) complies with formula:

(U-2)

_* C(0)NH (¾U2a _^ _ _C(0)0 _ _RU2b _₀ _ _#

HN

^NC(0)0— RU2b_0 #

wherein:

- R^U2A is a straight or branched C2-C10 hydrocarbon group, optionally comprising one or more oxygen and/or nitrogen atoms and/or

(hetero)cycloalkylene groups;

- each R^U2B, equal to or different from one another, is a C2-C10

hydrocarbon group bearing and

- the ^* and # are as defined above.

[0050] Preferred moieties (U2) are those in which R^U2A is a straight or branched C2-C10 hydrocarbon group comprising (hetero)cycloalkylene groups.

[0051] A preferred example of group (Ε^*') comprising a moiety (U2) complies with formula (E*'u2) here below:

wherein R^U2B and RH are as defined above and the ^* is as defined above. [0052] Preferred examples of polymers (P1 -B) comprise a chain (Rf-lll) as defined above and groups (E*'u2) as defined above wherein RH is hydrogen of methyl and R^U2b is an ethylene moiety (-CH2CH2-).

[0053] A convenient example of such polymers (P1 -B) complies with formula:

wherein:

- (Rf) is a chain of formula (Rf-lll) as defined above;

- R^U2b is an ethylene moiety (-CH2CH2-);

- RH is hydrogen and T is a group of formula (Tu2):

(T'u2):

or a non-functional group selected from -CF3, -C2F5 and -C3F7.

[0054] This polymer is available from Solvay Specialty Polymers Italy S.p.A. as Fluorolink^® AD 1700 in solution with ethylacetate and butylacetate (70%wt polymer in a 1 : 1 wt. mixture of ethylacetate and butylacetate).

[0055] According to a preferred embodiment of the invention, polymer (P1 -B) is Fluorolink^® AD 1700 PFPE.

[0056] Polymers (P1 -B) in which group (Ε^*') is a group (E*'u2) as defined above can be conveniently obtained by reaction of a PFPE alcohol (P1 -A) as defined above, preferably a bifunctional PFPE alcohol (P1 -A) with a isocyanate of formula (I^s):

(I^s)

in which R^U2a, R^U2b and RH are as defined above according to methods know in the art.

[0057] In method (M) according to the present invention, PFPE polymer (P1) can be used at concentrations ranging from 0.1 % to 20% wt with respect to the weight of formulation (F), preferably at concentrations ranging from 0.1 to 6% wt, more preferably at concentrations ranging from 0.1 %wt to 5% wt.

The polymer (P2)

[0058] For the purpose of the present invention, an acrylic polymer (P2) is a

polymer obtained by polymerization of α,β-unsaturated carboxylic acids or reactive derivatives thereof, said acids or reactive derivatives being equal to or different from one another. Preferably, acrylic polymers (P2) are obtained by polymerization of α,β-unsaturated carboxylic acids or reactive derivatives thereof comprising a -CH=C-C(O)- moiety (acrylic acid moiety) and/or a -C(CH3)-C(O)- moiety (methacrylic acid moiety).

[0059] For the avoidance of doubt, an acrylic polymer (P2) comprising acrylic acid moieties or methacrylic acid moieties does not comprise acrylic polymers (P2).

[0060] Reactive derivatives preferably include halides and esters. Examples of such α,β-unsaturated carboxylic acids and reactive derivatives thereof include, but are not limited to, acrylic acid, acryloyl chloride, methyl acrylate, ethyl acrylate, ethyl acryloyl chloride, butyl acrylate, ieri-butyl acrylate, ethyl methacrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethyl 2-ethyl acrylate, 2-ethylacryloyl chloride, 2- ethylhexyl acrylate, ethyl 2-propyl acrylate, 2-(trimethylsilylmethyl)acrylate, hexyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, isobutyl acrylate. [0061] The acrylic polymer (P2) can optionally comprise reactive functional groups able to undergo cross-linking reactions in the presence of a cross- linker (or cross-linking agent). For the purpose of the present invention, such acrylic polymers (P2) will be selected in particular when formulation (F) is hardened by curing (thermal curing or UV-curing). Such polymers (P2) can be obtained either by polymerizing α,β-unsaturated carboxylic acids or derivatives thereof comprising such functional groups or by polymerizing such acids in the presence of non-acid monomers comprising such functional groups. Advantageously, acrylic polymers (P2) comprising reactive functional groups are acrylic polyols.

[0062] In method (M) according to the present invention, polymer (P2) can be used in amounts typically up to 40%wt with respect to formulation (F), advantageously from 15% to 40%wt with respect to formulation (F).

[0063] A person skilled in the art will be able to select the kind and amount of polymer (P2) according to the specific method whereby formulation (F) is applied to the PCB.

The organic solvent

[0064] For the purpose of the present invention, an organic solvent is a non-polar or polar aprotic aliphatic or aromatic solvent or a mixture thereof.

[0065] Preferably, the solvent is an aromatic solvent selected from one or more of toluene, xylene and naphthalene or an aliphatic solvent selected from a ketone or an ester or a mixture thereof, whererein the term "ester" is intended to include also esters of (poly)oxylakylene alcohols. Examples of suitable ketones are methylethylketone (MEK), methylisobutylketone (MIBK ). Examples of suitable ester are ethyl acetate, butyl acetate, isobutyl acetate, 2-ethoxy ethylacetate, 2-methoxy propylacetate, 2- methoxy-1 -methylethyl acetate, ethyleneglycol diacetate, butyleneglycol diacetatepolyoxyethylene monoethyl-ether acetate, polyoxyethylene monobutylether acetate, polyoxy butylene mono-ethyl-ether acetate, polyoxy-butylene monobutylether acetate, polyoxyethylene diacetate, polyoxybutylene-diacetate. Advantageously, the solvent is butyl acetate or 2-methoxy- 1-methylethyl acetate. [0066] The kind and amount of solvent or nnixture of solvents will be selected by the person skilled in the art according to the selected polymers (P1 ) and (P2) and the application method of formulation (F) to the PCB.

[0067] However, the solvent can be used in amounts ranging from 15% to 90%wt, advantageously ranging from 15% to 50%wt with respect to the weight of formulation (F).

Optional ingredients

[0068] Formulations (F) can include optional ingredients conventionally used in conformal coating formulations; advantageously, in formulations (F) the optional ingredients are selected from cross-linkers, such as

polyisocyanates, photo-initiators and catalysts.

[0069] The person skilled in the art will be able to select the kind and amount of optional ingredients according to the selected polymers (P1 ), (P2) and solvents and according to the method whereby formulation (F) is applied to the PCB.

[0070] The overall amount of the optional additional ingredients can range from 0.01 % to 50% wt based on the total weight of formulation (F). For example, each additional ingredients can be used in amounts ranging from 0.01 % to 8 wt.%, preferably from 0.01 to 5 wt.% based on the total weight of formulation (F).

Formation of a conformal coating on a PCB

[0071] As stated above, the formation of a coating on a PCB (also designated as conformal coating) is made through a method (M) which comprises:

(a) providing a functional PFPE polymer (P1 ) as defined above;

(b) mixing polymer (P1 ) with an acrylic polymer (P2) as defined above an organic solvent and optional ingredients to provide a formulation (F);

(c) applying formulation (F) to said PCB and

(d) drying and/or curing the formulation (F).

[0072] In one embodiment, formulation (F) does not comprise additional

ingredients or comprises additional ingredients that are not cross-linkers; this kind of formulation can dry in the air and belongs to a class of conformal coating formulations commonly referred to as "1 K" formulations. [0073] In another embodiment, formulation (F) comprises at least one additional ingredient which is a cross-linker and polymer (P2) comprises functional groups able to react with the cross-linker, thereby allowing hardening of the coating. This kind of formulation is commonly referred to as "2K" formulation.

[0074] Step (c) can be carried out according to conventional methods, including brushing, spraying and dipping, or by selective coating via a robot. Step (d) can be carried out according to conventional methods. According to a first embodiment, formulation (F) does not contain a cross-linker and step d) is carried out by air drying, at room temperature or in an oven;

according to another embodiment, formulation (F) contains a cross-linker and step d) is carried out by curing.

[0075] A person skilled in the art will be able to select the proper method of

application of formulation (F) and whether drying or curing the formulation on the PCB according to the specific need. Typically, drying is carried out by air drying, but UV-curing shall be preferred when speed is required.

[0076] The invention will be illustrated in the following experimental section by means of non-limiting examples.

[0077] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Experimental section

Materials

[0078] The following polymers (P1 -A) were used in the tests:

1 ) a PFPE complying with formula:

HO(CH2CH2O)nCH2CF2O(CF2CF2O)al(CF2O)a2CF2CH2(OCH₂CH2)OH, in which n is 7 and a1 and a2 are integers as defined above, selected in such a way that the average number molecular weight is 2,200 (in the following, "PFPE P1 -A1 or P1-A1 );

2) P1 -A2) a PFPE complying with formula:

HO(CH2CH2O)nCH2CF2O(CF2CF2O)al(CF2O)a2CF2CH2(OCH₂CH2)OH, in which n is 4.7 and a1 and a2 are integers as defined above, selected in such a way that the average number molecular weight is 2,000 (in the following, "PFPE P1 -A2 or P1-A2);

3) Fluorolink^® AD 1700 PFPE, PFPEs (P1 -A1 ) and (P1 -A2) were manufactured from PFPE diols of formula:

HOCH2CF2O(CF2CF2O)ai(CF2O)a2CF2CH₂OH

according to the procedure disclosed in WO2014090649.

[0079] Fluorolink^® AD 1700 PFPE is available on the market from Solvay

Specialty Polymers Italy S.p.A.

[0080] For the obtainment of 1 K formulations, the selected PFPE polymer (P1 ) was mixed with a commercially available acrylic polymer (P2) and butyl acetate, while for the obtainment of 2K formulations, the selected polymer (P1 ) was mixed with a commercially available acrylic polymer (P2), a cross-linker and 2-methoxy-1 -methyl acetate. Blank 1 K and 2K

formulations were prepared likewise without PFPE polymer (P1 ).

Methods

Pendulum hardness test

[0081] The pendulum hardness test was carried out according to ASTM D4366 - 14.

Determination of the contact angle versus water

[0082] The contact angle versus water was determined according to DIN/ ISO 55660.

Evaluation of the chemical resistance

[0083] Chemical resistance was determined according to AAMA 2605- 13.

Water uptake

[0084] Water uptake was determined according to ASTM G85-1 1.

Conductivity measurements

[0085] Conductivity measurements were carried out in a two-electrode set up (with a stainless steel as reference electrode) in a 3% NaCI electrolyte, according to DORNBUSCH, M., et al. Characterization of the water uptake and electrolyte uptake of organic coatings and the consequences by means of electrochemical impedance spectroscopy and UV-VIS

spectroscopy. Pro. Org. Coat. 2015, vol.89, p.322-343.

Results and discussion

Pendulum hardness test

[0086] The results of the pendulum hardness test are reported in Tables 1 and 2 below.

[0087] Measurements were made in triplicate. The number reported for each run indicates the number of pendulum swings; the higher the value, the higher the hardness.

[0088]

Table 1

Formulation Run 1 Run 2 Run 3 Average

1 K formulation 82 86 90 86

(blank)

1 K formulation 109 1 17 1 14 1 1 1 .3

+ Fluorolink^®

AD 1700 (0.5%

wt)

1 K formulation 46 55 50 50.3

+ P1 -A1 (3%

wt)

[0089]

Table 2

Formulation Run 1 Run 2 Run 3 Average

2K formulation 161 159 161 160.3

(blank) 2K formulation 162 162 162 162

+ Fluorolink^®

AD 1700 PFPE

(0.5% wt)

2K formulation 175 175 175 175

+ P1-A1 (0.5%

wt)

[0090] The results show that acrylic polymers (P1) increase the hardness of 1 K formulations and do not modify the hardness of 2K formulations, while PFPE alcohols (P1 ) increase the hardness of 2K formulations.

Determination of the contact angle versus water

[0091] The results are reported in Tables 3 and 4 below. The higher the contact angle value, the higher the hydrophobicity.

[0092]

Table 3

[0093]

Formulation Run 1 Run 2

2K formulation (blank) 83.3° 83.5° 2K formulation + Fluorolink^® 109.7° 109.8°

AD 1700 PFPE (0.5% wt)

2K formulation + P1-A1 107.9° 107.7°

(0.5% wt)

The results show that acrylic polymers (P1 ) increase the hydrophobicity of coatings obtained both with 1 K and 2K formulations, while PFPE alcohols (P1 ) increase the hydrophobicity of coatings obtained with 2K

formulations.

Evaluation of the chemical resistance

The results of the tests on chemical resistance are reported in Tables 5 and 6 below on a 0 to 5 scale. The lower the value, the higher the chemical resistance.

Table 5

[0097]

Table 6

Formulation Treatment with MEK Treatment with 10% Treatment with 10%

NaOH HCI 2K formulation 4 1 2 (blank)

2K formulation + 4 1 0

Fluorolink^® AD 1700

PFPE (0.5% wt)

2K formulation + P1- 4 1 0

A1 (0.5% wt)

The results show that both acrylic polymers (P1 ) and PFPE alcohols (P1 ) increase the resistance to bases and acids of coatings obtained both from 1 K and 2K formulations, acrylic polymers (P1 ) performing better when used in 1 K formulations.

Water uptake

[0099] The result of the water uptake tests are reported in Tables 7 and 8 here below.

[00100]

Table 7

Formulation Water uptake (% wt with respect to the coating)

1 K formulation (blank) 6

1 K formulation + 5% wt P1-A2 1

1 K formulation + 5% wt Fluorolink^® AD 0

1700 PFPE

1 K formulation + 5% wt P1-A1 0

[00101]

Table 8

coating)

2K formulation (blank) 0

2K formulation + 5% wt P1 -A1 0

2K formulation + 5% wt Fluorolink^® AD 0

1700 PFPE

[0102] The above results show that acrylic polymers (P1 ) and PFPE alcohols (P1 ) are able to prevent water uptake of coatings obtained both with 1 K and 2K formulations.

Conductivity measurements

[0103] The results of conductivity measurements on coatings obtained from the 1 K formulation at two different concentrations of polymer (P1 ) are reported in Table 9 below.

Table 9

The results reported in Table 9 show that formulations (F) according to th present invention comprising a polymer (P1 ) at different concentrations maintain the same dielectric properties as formulations that do not comprise a polymer (P1 ). [0105] The results of conductivity measurements on coatings obtained from a the 2K formulation are reported in Table 10 below:

Table 10

[0106] The results reported in Table 10 show that formulations (F) according to the present invention comprising a polymer (P1 ) maintain the same dielectric properties as formulations that do not comprise a polymer

Claims

1 . A method [method (M)] for coating a printed circuit board (PCB), said method comprising the following steps:

(a) providing a functional (per)fluoropolyether polymer [polymer (P1 )] comprising at least one fully or partially fluorinated polyoxyalkylene chain

[chain (R_Pf)] having at least two ends, wherein at least one end bears a group (E) comprising at least one functional group [group (G)] selected from a hydroxy group or an α,β-unsaturated carbonyl group;

(b) mixing polymer (P1 ) with an acrylic polymer [polymer (P2)] and an organic solvent to provide a solvent-based formulation [formulation (F)];

(c) applying formulation (F) to said PCB and

(d) drying and/or curing formulation (F).

2. The method according to claim 1 in which polymer (P1 ) is a polymer

comprising a chain (R_Pf) having two ends, wherein at least one end bears a group (E) comprising at least one hydroxyl group.

3. The method according to claim 2 in which polymer (P1 ) complies with formula:

(P1 -A) T-O(Rf)(CFX)-CH₂O-[CH₂CH(J)O]_j-H

wherein:

- (Rf) is a fully or partially fluorinated polyoxyalkylene chain complying with formula:

(Rf-lll) -[(CF₂CF2O)al(CF₂O)a2]- wherein a1 and a2 are integers > 0 such that the number average molecular weight is between 400 and 10,000;

- X is F or CF₃;

- J is independently selected from hydrogen, straight or branched alkyl and aryl;

- j is 0 or an integer ranging from 1 to 50;

and

- T is a non-functional group selected from -CF3, -C2F5 and -C3F7 or a functional group of formula -(CFX)-CH₂O[CH₂CH(J)O]j-H in which X, J and j are as defined above.

4. The method according to claim 3 in which in polymer (P1 ) J is hydrogen or methyl.

5. The method according to claim 4 in which in polymer (P1 ) J is hydrogen and j ranges from 4 to 7.

6. The method according to claim 1 in which polymer (P1) is a polymer

comprising a chain (R_Pf) having two ends, wherein at least one end bears a group (E) comprising at least one α,β-unsaturated carbonyl group.

7. The method according to claim 6 in which polymer (P1 ) complies with formula (P1 -B) T'-O(Rf)(CFX)-CH2O[CH₂CH(J)O]_j-E^*

wherein:

- X is F or CF₃;

- j is 0 or an integer ranging from 1 to 50;

- E^* is an end group comprising one α,β-unsaturated carbonyl group and

- T is -(CFX)-CH₂O[CH₂CH(J)O]j-E^*

or a group selected from -CF3, -C₂Fs and -C3F7.

8. The method according to claim 7 in which group E^* in polymer (P1 -B) is

selected from the groups complying with formulae (E^*1 ) - (E^*3) here below: (E^*1 ) -CO-CR_H=CH₂,

(E^*2) -CO-NH-CO-CR_H=CH₂,

(E^*3) -CO-R^A-CR_H=CH₂,

wherein

RH is H or a C1-C6 alkyl group;

R^A is selected from the group consisting of:

(j) -NH-R^B-O-CO-

(jj) -NH-R^B-NHCOO-R^B-OCO-;

R^B being a divalent group selected from the group consisting of C1-C10 aliphatic group, Cs-Ci₄

cycloaliphatic group; C6-Ci₄ aromatic or alkylaromatic group.

9. The method according to claim 7 in which group E^* in polymer (P1 -B) is a

group (Ε^*') comprising an alkyl or alkylene moiety [moiety (U)] optionally comprising oxygen and/or nitrogen atoms and/or (hetero)cycloalkylene groups, said moiety having two or three bonding sites for the carbonyl group of the α,β- unsaturated carbonyl group.

10. The method according to claim 9 in which polymer (P1-B) complies with

formula:

wherein:

- R^U2b is an ethylene moiety;

- RH is hydrogen and

- T' is a non-functional group selected from -CF3, -C2Fs and -C3F7 or a group of formula (Tu2):

(T'U2)

in which R^U2B is an ethylene moiety and RH is hydrogen.

1 1. The method according to any one of claims 1 to 10 in which polymer (P1 ) is used concentrations ranging from 0.1 % to 20% wt with respect to the weight of formulation (F).

12. The method according to any one of claims 1 to 1 1 wherein step d) is carried out by air drying.

13. The method according to any one of claims 1 to 12 wherein step d) is carried out by curing.

14. A coated printed circuit board obtained by coating a printed circuit board with the method (M) as defined in any one of claims 1 to 14.