WO2011104500A1

WO2011104500A1 - Plasma-polymerized polymer coating

Info

Publication number: WO2011104500A1
Application number: PCT/GB2011/000238
Authority: WO
Inventors: Timothy Von Werne; Rodney Edward Smith; Mark Robson Humphries
Original assignee: Semblant Global Limited
Priority date: 2010-02-23
Filing date: 2011-02-21
Publication date: 2011-09-01
Also published as: CN106916331A; TW201204203A; CN102791779A; KR20120129993A; EP2539392A1; GB201003067D0; BR112012021172A2; JP6085480B2; JP2013527973A; KR101778820B1; TWI547221B

Abstract

An electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at least one surface of the substrate, at least one electrical or electro-optical component connected to at least one conductive track, and a continuous coating comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

Description

PLASMA-POLYMERIZED POLYMER COATING

The present invention relates to a plasma-polymerized polymer coating, which is useful for coating electrical and electro-optical assemblies and components, and methods involving said coating.

BACKGROUND

Conformal coatings have been used for many years in the electronics industry to protect electrical assemblies from environmental exposure during operation. A conformal coating is a thin, flexible layer of protective lacquer that conforms to the contours of the PCB and its components. Conformal coatings protect circuits from corrosive chemicals (for example salt, solvents, petrol, oils, acids and environmental pollutants), humidity/condensation, vibration, current leakage, electromigration and dendritic growth. Current conformal coatings are typically 25 to 200 μπι thick and are generally epoxy resin, acrylic resin or silicone resin based. These materials are all deposited as liquids which must be applied and then cured on the assembly. Recently parylene, which is expensive, has also been used as a conformal coating. Parylene is typically deposited using conventional chemical vapour deposition techniques widely known to those skilled in the art.

There are a number of disadvantages associated with the current conformal coatings. The techniques used to deposit the coatings require that the contacts by which the assembly is connected to other devices are masked prior to coating, to prevent the conformal coating from covering the contacts. Coated contacts would not be able to make an electrical connection to other devices, since the conformal coating is thick and insulating.

Furthermore, it is very difficult and expensive to remove the current conformal coatings if reworking of the electrical assembly is required. There is no possibility of soldering or welding through the coatings, without prior removal. Additionally, due to the liquid techniques that are generally used to deposit these conformal coatings, there is a tendency for defects, such as bubbles, to form in the coating. These defects reduce the protective capabilities of the conformal coating. A further problem with prior art conformal coatings is that, due to the liquid techniques used during coating, it is difficult to deposit the coating underneath components on the assembly. l SUMMARY OF THE INVENTION

The present inventors have surprisingly found that plasma polymerized polymers can be used to form excellent conformal coatings on electrical and electro-optical assemblies. Not only are these coatings continuous and substantially defect free, but also they overcome the problems with existing coatings discussed above. Further, the plasma polymerized polymer coatings of the invention are easy to deposit on devices and are relatively cheap.

Thus, the present invention provides an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, at least one electrical or electro-optical component connected to at least one conductive track, and a continuous coating comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

The present invention further provides a method comprising (a) providing an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate and at least one electrical or electro-optical component connected to at least one conductive track, and (b) depositing by plasma-polymerization a continuous coating comprising a polymer which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

The present invention further provides an electrical or electro-optical assembly obtainable by the method defined above.

The present invention further provides an electrical or electro-optical component which is completely covered with a continuous coating comprising a plasma-polymerized polymer. The present invention further provides a method comprising (a) subjecting an electrical or electro-optical assembly as defined above to a plasma-removal process, such that the continuous coating is removed, and then (b) optionally reworking said electrical or electro- optical assembly, and then (c) optionally depositing by plasma-polymerization a replacement continuous coating comprising a polymer which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro- optical component.

The present invention further provides a method comprising soldering through the continuous coating of an electrical or electro-optical assembly as defined above, to form a solder joint between a further electrical or electro-optical component and at least one conductive track, the solder joint abutting the continuous coating.

The present invention further provides a method comprising (a) subjecting an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, a surface-finish coating comprising a halohydrocarbon polymer covering at least a portion of the plurality of conductive tracks and at least one electrical or electro-optical component connected to at least one conductive track through the surface-finish coating to a plasma-removal process such that the surface-finish coating is removed, and then (b) depositing by plasma-polymerization a continuous coating comprising a polymer as defined above which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component. The present invention further provides a method comprising (a) subjecting an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate and a surface-finish coating comprising a halohydrocarbon polymer covering at least a portion of the plurality of conductive to a plasma-removal process such that the surface-finish coating is removed, then (b) connecting an electrical or electro-optical component to at least one conductive track, and then (c) depositing by plasma-polymerization a continuous coating comprising a polymer as defined above which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component. The present invention further provides an electrical or electro-optical assembly which has a conformal coating comprising a plasma-polymerized polymer as defined above.

The present invention further provides use of a plasma-polymerized polymer as defined above as a conformal coating for an electrical or electro-optical assembly. The present invention further provides a method for conformally coating an electrical or electro-optical assembly comprising depositing a polymer as defined above by plasma- polymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 A shows the results of an X-ray photoelectron spectroscopic analysis of a plasma- polymerized fluoropolymer.

Figure IB shows the results of an X-ray photoelectron spectroscopic analysis of a

fluoropolymer obtained by standard polymerization techniques.

Figure 2A shows an electron microscope image of a plasma-polymerized fluoropolymer coating of the invention and the smooth physical nature of said coating.

Figure 2B shows an electron microscope image of a PTFE coating deposited by standard polymerization techniques, which has a structure in which fibrils are clearly visible. Figures 3 to 7 show electrical assemblies of certain embodiments.

Figure 8 shows an electro-optical assembly of certain embodiments.

Figure 9 shows an electrical component of certain embodiments.

Figure 10 shows an example of an apparatus that can be used to form the plasma-polymerized polymer coating of the invention.

Figures 11 A, 1 IB and 11C are flow-charts showing the methods certain embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is concerned with electrical and electro-optical assemblies. An electrical assembly typically comprises at least one electrical component. An electro-optical assembly typically comprises at least one electro-optical component, and may optionally further comprise at least one electrical component. An electrical or electro-optical assembly is preferably a printed circuit board. The continuous coating of the present invention comprises a plasma-polymerized polymer. The continuous coating of the present invention may prevent environmental damage to electrical or electro-optical assemblies. Environmental damage is typically caused by corrosion due to atmospheric components (for example oxygen S0₂, H₂S and/or N0₂) and/or moisture at ambient or elevated temperatures. Additionally, the continuous coating of the present invention may continue to protect the electrical and electro-optical assemblies to which it is applied over a greater temperature range than current conformal coatings, which may be degraded by higher temperatures.

The continuous coating of the invention is preferably a conformal coating.

Plasma-polymerized polymers are a unique class of polymers which cannot be prepared by traditional polymerization methods. Plasma-polymerized polymers have a highly disordered structure and are generally highly crosslinked, contain random branching and retain some reactive sites. Plasma-polymerized polymers are thus chemically distinct from polymers prepared by traditional polymerization methods known to those skilled in the art. These chemical and physical distinctions are well known and are described, for example in Plasma Polymer Films, Hynek Biederman, Imperial College Press 2004.

A plasma-polymerized polymer is typically obtainable by a plasma-polymerization technique, as defined in further detail below.

The plasma-polymerized polymer is typically a plasma-polymerized hydrocarbon, halohydrocarbon, silicone, siloxane, silane, silazane or stannane. A plasma-polymerized hydrocarbon is typically a straight and/or branched polymer which optionally contains cyclic moieties. Said cyclic moieties are preferably alicyclic rings or aromatic rings, more preferably aromatic rings. Preferably a plasma-polymerized

hydrocarbon does not contain any cyclic moieties. Preferably a plasma-polymerized hydrocarbon is a branched polymer. A plasma-polymerized halohydrocarbon is typically a straight and/or branched polymer which optionally contains cyclic moieties. Said cyclic moieties are preferably alicyclic rings or aromatic rings, more preferably aromatic rings. Preferably a plasma-polymerized halohydrocarbon does not contain any cyclic moieties. Preferably a plasma-polymerized halohydrocarbon is a branched polymer.

Plasma-polymerized hydrocarbons and halohydrocarbons comprising aromatic moieties are plasma-polymerized aromatic hydrocarbons and aromatic halohydrocarbons (such as aromatic fluorohydrocarbons) repectively. Examples include plasma-polymerized polystyrene and plasma-polymerized parylene. Plasma-polymerized parylene is particularly preferred. The plasma-polymerized parylene may be unsubstituted or substituted with one or more substituents. Preferred substituents include halogens, with fluorine most preferred. A parylene substituted with one or more halogen atoms is a haloparylene. A parylene substituted with one or more fluorine atoms is a fluoroparylene. Unsubstituted parylene is most preferred.

A plasma-polymerized hydrocarbon optionally contains heteroatoms selected from N, O, Si and P. Preferably, however, the plasma-polymerized hydrocarbon contains no N, O, Si and P heteroatoms.

A plasma-polymerized halohydrocarbon optionally contains heteroatoms selected from N, O, Si and P. Preferably, however, the plasma-polymerized halohydrocarbon contains no N, O, Si and P heteroatoms.

An oxygen-containing plasma-polymerized hydrocarbon preferably comprises carbonyl moieties, more preferably ester and/or amide moieties. A preferred class of oxygen- containing plasma-polymerized hydrocarbon polymers are plasma-polymerized acrylate polymers.

An oxygen-containing plasma-polymerized halohydrocarbon preferably comprises carbonyl moieties, more preferably ester and or amide moieties. A preferred class of oxygen- containing plasma-polymerized halohydrocarbon polymers are plasma-polymerized haloacrylates polymers, for example plasma-polymerized fluoroacrylate polymers.

A nitrogen containing plasma-polymerized hydrocarbon preferably comprises nitro, amine, amide, imidazole, diazole, trizole and/or tetraazole moieties.

A nitrogen containing plasma-polymerized halohydrocarbon preferably comprises nitro, amine, amide, imidazole, diazole, trizole and/or tetraazole moieties Plasma-polymerized silicones, siloxanes, silanes and silazanes are optionally substituted with one or more fluorine atoms. However, silicones, siloxanes, silanes and silazanes are preferably unsubstituted. A preferred silazane is hexamethyl disilazane.

Preferably the plasma-polymerized polymer is a plasma-polymerized halohydrocarbon, more preferably a plasma-polymerized fluorohydrocarbon. Most preferably the plasma- polymerized polymer is a plasma-polymerized fluorohydrocarbon which is branched and contains no heteroatoms.

As used herein, the term halo is preferably fluoro, chloro, bromo and iodo. Fluoro and chloro are preferred, with fluoro most preferred. Halogen takes the same meanings.

The assemblies of the present invention may be prepared by depositing a polymer by plasma polymerization. Plasma-polymerization is generally an effective technique for depositing thin film coatings. Generally plasma-polymerization provides excellent quality coatings, because the polymerization reactions occur in situ. As a result, the plasma-polymerized polymer is generally deposited in small recesses, under components and in vias that would not be accessible by normal liquid coating techniques in certain situations.

In addition, the in situ formation of the polymer may provide good adhesion to the surfaces to which the coating is applied, since the polymer generally reacts with the surfaces during deposition. Therefore, in certain situations, plasma-polymerized polymers can be deposited on materials which other conformal coatings cannot be deposited on. A further advantage of the plasma-polymerization technique of the invention is that there is no need to dry/cure the coating following deposition. Prior art techniques for coating require a drying/curing step, which leads to formation of curing/drying defects on the surface of the coating. Plasma-polymerization avoids formation of curing/drying defects.

Plasma deposition may be carried out in a reactor that generates a gas plasma which comprises ionised gaseous ions, electrons, atoms, and/or neutral species. A reactor may comprise a chamber, a vacuum system, and one or more energy sources, although any suitable type of reactor configured to generate a gas plasma may be used. The energy source may include any suitable device configured to convert one or more gases to a gas plasma. Preferably the energy source comprises a heater, radio frequency (RF) generator, and/or microwave generator.

In certain embodiments, an electrical or optical assembly may be placed in the chamber of a reactor and a vacuum system may be used to pump the chamber down to pressures in the range of 10^"3 to 10 mbar. One or more gases may then be pumped into the chamber and an energy source may generate a stable gas plasma. One or more precursor compounds may then be introduced, as gases anoVor liquids, into the gas plasma in the chamber. When introduced into the gas plasma, the precursor compounds may be ionized and/or decomposed to generate a range of active species in the plasma that polymerize to generate the polymer coating.

A plasma-polymerized fluorohydrocarbon is preferably obtained by plasma polymerization of one or more precursor compounds which are hydrocarbon materials comprising fluorine atoms. Preferred hydrocarbon materials comprising fluorine atoms are perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes, fluoroalkenes, fluoroalkynes. Examples include C₃F₆ and C₄F8.

Other preferred precursor compounds are fluorochloroalkanes, fluorochloroalkenes and fluorochloroalkynes. Examples include C₂F₃C1 and C₂F₄C1₂.

A plasma polymerised parylene is preferably obtainable by plasma-polymerization of di-p- xylylene, xylylenes or xylenes. The exact nature and composition of the plasma-polymerized polymer coating typically depends on one or more of the following conditions (i) the plasma gas selected; (ii) the particular precursor compound(s) used; (iii) the amount of precursor compound(s) (which may be determined by the combination of the pressure of precursor compound(s) and the flow rate); (iv) the ratio of precursor compound(s); (v) the sequence of precursor

compound(s); (vi) the plasma pressure; (vii) the plasma drive frequency; (viii) the pulse width timing; (ix) the coating time; (x) the plasma power (including the peak and/or average plasma power); (xi) the chamber electrode arrangement; and/or (xii) the preparation of the incoming assembly.

Typically the plasma drive frequency is 1 kHz to 1 GHz. Typically the plasma power is 500 to 10000 W. Typically the mass flow rate is 5 to 2000 seem. Typically the operating pressure is 10 to 500 mTorr. Typically the coating time is 10 seconds to 20 minutes. Pulsed plasma systems may also be used.

However, as a skilled person will appreciate, the preferred conditions will be dependent on the size and geometry of the plasma chamber. Thus, depending on the specific plasma chamber that is being used, it may be beneficial for the skilled person to modify the operating conditions.

The surface energy of the continuous coating can be controlled by careful selection of the precursors and the plasma processing conditions. Depending on the particular plasma polymer, the surface may be either hydrophilic or hydrophobic.

A hydrophobic coating preferably demonstrates a water contact angle of greater than 90 degrees, and more preferably greater than 105 degrees. A hydrophobic coating preferably demonstrates a surface energy of less than 35 dynes/cm and more preferably less than 30 dynes/cm. In certain situations, hydrophobic properties of the continuous coating may be highly desirable, since they could reduce the likelihood of damage to an assembly by moisture.

In some situations, however, a hydrophilic coating may be desirable. For example, hydrophilic coatings may be desirable if further coatings or labels (such as barcodes) are to be applied to the continuous coating. It is generally easier to adhere additional coatings to a hydrophilic coating. A hydrophilic coating preferably demonstrates a water contact angle of less than 70 degrees, and more preferably less than 55 degrees. A hydrophilic coating preferably demonstrates a surface energy of greater than 45 dynes/cm and most preferably greater than 50 dynes/cm.

As used herein, "continuous" means that the coating is substantially defect free. Possible defects include holes, fissures and breaks in the coating. A continuous coating may be achieved using a plasma-polymerized polymer formed in situ on an electrical or electro- optical assembly. Using the plasma polymerization method described below, a continuous coating may be formed on all surfaces that are in contact with the plasma gas. This can be a particular advantage when coating high aspect ratio features such as components typically found on an electronic or electro-optical assembly. Using a plasma-polymerization method may also allow underhangs to be covered by the coating as well.

The continuous coating typically has a mean-average thickness of 1 nm to 10 um, preferably 1 nm to 5 μηι, more preferably 5nm to 500 nm, more preferably 100 nm to 300 nm, and more preferably Γ50 nm to 250 nm, for example about 200 nm. The thickness of a continuous coating may be substantially uniform or may vary from point to point. In particular embodiments, the continuous coating may be deposited such that it conforms to the three- dimensional surface of the substrate, conductive tracks, and components.

A continuous coating completely covers at least one surface of a substrate, a plurality of conductive tracks, and at least one electrical or electro-optical component. Preferably a coating encapsulates the exposed portions of at least one surface of a substrate, a plurality of conductive tracks, and at least one electrical or electro-optical component. The continuous coating may therefore preferably be a conformal coating. A conformal coating may conformally coats at least one surface of a substrate, a plurality of conductive tracks, and the at least one electrical or electro-optical component. For the avoidance of doubt, references to coatings completely covering other items, as discussed below, are intended to take the same meanings as defined above. The area of the electrical or electro-optical assembly which is completely covered with the continuous coating might, under some circumstance, be a portion of a larger electrical or electro-optical assembly, the remainder of which may not be coated. A continuous coating may cause minimal changes to electrical and/or optical performance of an assembly to which it is applied. For example, the inductance of an electrical circuit in an assembly may only be minimally affected by the coating. In some situations, this may be highly advantageous as compared to other conformal coatings that typically alter the properties of the circuit significantly, which may require the altered properties of the assembly caused by other coatings to be considered when an assembly is designed. The coating of the present invention may remove this requirement in some situations.

If a very high degree of protection from the environment is required for the electrical or electro-optical assembly, then additional continuous coatings of plasma-polymerized polymers may be applied over the initial continuous coating. Thus, the electrical or electro- optical assembly may further comprise a first additional continuous coating comprising a plasma-polymerized polymer as defined above which completely covers the continuous coating, and optionally a second additional continuous coating comprising a plasma- polymerized polymer as defined above which completely covers the first additional continuous coating. Further additional continuous coatings of plasma-polymerized polymers as defined above, for example third to tenth continuous coatings may be applied, as necessary. The plasma-polymerized polymers used for each additional continuous coating may independently be the same as or different from the plasma-polymerized polymer of the initial continuous coating. Each additional continuous coating is typically deposited by the methods used to deposit the continuous coating. The exact nature of the initial continuous coating and the any additional continuous coatings can be selected to improve or optimise the performance required from the coated assembly. For example, it may be desirable to have a highly hydrophobic coating as the uppermost coating, in order to achieve good resistance to moisture.

A plasma-polymerized coating of the invention may also be used to provide extra

environmental protection to existing electrical or electro-optical assemblies which are coated with another conformal coating. This may be advantageous in situations where a water resistant outer coating is required. Thus, the electrical or electro-optical assembly may further comprise a coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene deposited between at least a portion of the continuous coating of a plasma-polymerized polymer and at least a portion of a substrate, a plurality of conductive tracks and at least one electrical or electro-optical component. In certain embodiments, a parylene coating may be deposited by a chemical vapour deposition method.

Thus, an electrical or electro-optical assembly may comprise a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, at least one electrical or electro-optical component connected to at least one conductive track, a coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene (which may be deposited by traditional chemical vapour deposition methods) on at least a portion of the substrate, and a continuous coating comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate, the plurality of conductive tracks, the at least one electrical or electro-optical component and the coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene.

Preferably the coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene is a conformal coating. Such an arrangement can be prepared by subjecting an electrical or electro-optical assembly comprising a coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene deposited on at least a portion of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component, to a coating method described herein.

A continuous coating may also be applied to electrical or electro-optical assemblies which carry a halohydrocarbon surface-finish coating as described in WO 2008/102113 (the content of which is incorporated herein by reference). Thus, an electrical or electro-optical assembly may comprise a surface-finish coating comprising a halohydrocarbon polymer deposited between (a) the continuous coating and (b) the at least one surface of the substrate and the plurality of conductive tracks, wherein the surface-finish coating covers at least a portion of the plurality of conductive tracks, and the at least one electrical or electro-optical component is connected to the at least one conductive track through the surface-finish coating.

Preferably the surface-finish coating comprises a fluorohydrocarbon polymer, more preferably a plasma polymerized fluorohydrocarbon polymer. An electrical or electro-optical assembly may also comprise a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, at least one electrical or electro-optical component connected to at least one conductive track, a surface-finish coating comprising a halohydrocarbon polymer deposited on at least a portion of the plurality of conductive tracks, and a continuous coating comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate, the plurality of conductive tracks, the at least one electrical or electro-optical component and the surface finish coating, wherein the at least one electrical or electro-optical component is connected to the at least one conductive track through the surface-finish coating.

Preferably, the electrical or electro-optical component is connected to the at least one conductive track via a solder joint, a weld joint or a wire-bond joint and the solder joint, weld joint or wire-bond joint abuts the surface-finish coating.

When an assembly includes a surface-finish coating or a coating of an epoxy resin, an acrylic resin, or a silicone resin, the assembly may be prepared by subjecting the assembly with the appropriate surface-finish coating or a coating of an epoxy resin, an acrylic resin, or a silicone resin to a coating method described above. Similarly, additional continuous coatings are typically deposited by to a coating method described above.

A further advantage of plasma-polymerized coatings is that, in some situations, they can easily be removed by a plasma-removal process. A plasma removal process may involve plasma etching of the coating to expose the underlying surface of the electrical or electro- optical assembly. The coating may have a thickness around 200nm. A traditional conformal coat applied to an electrical or electro-optical assembly using methods known in the prior art is typically between 25 and 200um thick. Removal of current conformal coatings can be time consuming and expensive using plasma etching because of the large volume of material to be removed. Thus, an electrical or electro-optical assembly as defined above can undergo a plasma-removal process. This plasma removal process typically removes substantially all of the continuous coating. Where present, it will typically remove substantially all of the additional continuous coatings and/or the surface-finish coating. A plasma-removal process typically involves placing the electrical or electro-optical assembly into a plasma chamber, and introducing a reactive gas plasma which chemically and/or physically bombards the surface of the coating to remove material and progressively etch back to the original underlying surface.

This process may be quick and inexpensive, and therefore advantageous. The electrical or electro-optical assembly from which the coating has been removed may then be re- worked, typically by addition of further components or replacement of existing components.

Alternatively, the connection between conductive tracks and components may be reworked, if said connection has become damaged through use. Once re- working is complete, a replacement continuous coating comprising a polymer can optionally be deposited by plasma-polymerization, which coating completely covers at least one surface of the substrate, a plurality of conductive tracks and at least one electrical or electro-optical component. Thus, in some situations, a damaged electrical or electro-optical assembly can be mended easily.

A further advantage of a plasma-polymerized coating is that it may not be necessary to remove the coating prior to reworking. That is because, in some situations, it may be possible to solder through the coating. In some situations, it may also be possible to solder through an imtial coating and, where present, the first and second additional coatings and/or a surface-finish coating, of an electrical or electro-optical assembly, to form a solder joint between a further electrical or electro-optical component and at least one conductive track. The solder joint may abut the continuous coating and, where present, the first and second additional coatings and/or the surface-finish coating. A further application involves removal of a surface-finish coating as described above by a plasma-removal process, followed by deposition of a plasma-polymerization polymer as described above. Another application involves removal of a surface-finish coating as described above by a plasma-removal process, then connecting electrical or electro-optical components to conductive tracks, followed by deposition of a plasma-polymerization polymer as described above.

An electrical or electro-optical assembly may comprise a plurality of conductive tracks that may be electrically conductive tracks or optically conductive tracks. An electrically conductive track typically comprises any suitable electrically conductive material. Preferably, an electrically conductive track comprises gold, tungsten, copper, silver, aluminium, doped regions of semi-conductor substrates, conductive polymers and or conductive inks. More preferably, an electrically conductive track comprises gold, tungsten, copper, silver or aluminium.

Suitable shapes and configurations for the conductive tracks can be selected by a person skilled in the art for the particular assembly in question. Typically, an electrically conductive track is attached to the surface of the substrate along its entire length. Alternatively, an electrically conductive track may be attached to the substrate at two or more points. For example, an electrically conductive track may be a wire attached to the substrate at two or more points, but not along its entire length. An electrically conductive track is typically formed on a substrate using any

suitable method known to those skilled in the art. In a preferred method, electrically conductive tracks are formed on a substrate using a "subtractive" technique. Typically in this method, a layer of metal (e.g., copper foil, aluminium foil, etc.) is bonded to a surface of the substrate and then the unwanted portions of the metal layer are removed, leaving the desired conductive tracks. The unwanted portions of the metal layer are typically removed from the substrate by chemical etching or photo-etching, milling. In an alternative preferred method, conductive tracks are formed on the substrate using an "additive" technique such as, for example, electroplating, deposition using a reverse mask, and/or any geometrically controlled deposition process. Alternatively, the substrate may be a silicon die or wafer, which typically has doped regions as the conductive tracks.

An optically conductive track typically comprises any suitable optically conductive material. Preferably an optically conductive track is an optical wave guide, which typically comprises an optically transmitting material where a change in refractive index is used to propagate the electromagnetic radiation through the desired path. The waveguide may be created, for example, by applying a cladding or boundary layer to the optically transmitting material, wherein the cladding or boundary layer is made of a material of different refractive index. Alternatively, a waveguide may be created by doping or modifying the optically transmitting material to create regions of variable refractive index. The waveguides can therefore be standalone components, or features integrated into a substrate. Typical optically transmitting materials are glasses, doped glasses and plastics.

The plurality of conductive tracks may comprise only electrically conductive tracks, only optically conductive tracks or a mixture of electrically conductive tracks and optically conductive tracks. Where more than one electrically conductive track is present, each track may be made from the same material as defined above and/or have the same shape, or alternatively there may be a variety of track materials and/or track shapes. Where more than one optically conductive track is present, each track may be made from the same material as defined above and/or have the same shape, or alternatively there may be variety of track materials and/or track shapes.

The plurality of conductive tracks may further comprise at least one external-contact means. The continuous coating preferably completely covers the at least one external-contact means.

The exact nature of the external-contact means may depend 0n the nature of the assembly and the device to which contact is required. Suitable contacts can be routinely selected by a person skilled in the art. Typically the external-contact means is an electrical or optical contact. The external-contact means can be a portion of the plurality of conductive tracks. Alternatively, the external-contact means can be an additional component which is electrically or optically connected to the at least one conductive track.

A plasma-polymerized coating may allow an electrical connection to be made between (a) an external-contact means, preferably an electrical contact, and (b) corresponding contacts on an external device, without prior removal of the continuous coating. Similarly, a plasma- polymerizedcoating may allow an optical connection to be made between (a) an external- contact means, preferably an optical contact, and (b) corresponding contacts on an external device, without prior removal of the continuous coating. Therefore, it may not be necessary in either case to mask the external-contact means of the assembly prior to formation of the plasma-polymerized coating. In some situations, this may be advantageous since masking of external contact means may be time-consuming and expensive.

An electrical or electro-optical assembly may include a substrate that may comprise an insulating material. The substrate typically comprises any suitable insulating material that prevents the substrate from shorting the circuit of electrical or electro-optical assembly. Thus, in an electrical assembly, the substrate is preferably electrically insulating. In an electro-optical assembly, the substrate is preferably both electrically insulating and optically insulating.

A substrate preferably comprises an epoxy laminate material, a synthetic resin bonded paper, an epoxy resin bonded glass fabric (ERBGH), a composite epoxy material (CEM), PTFE (Teflon), or other polymer materials, phenolic cotton paper, silicon, glass, ceramic, paper, cardboard, natural and/or synthetic wood based materials, and/or other suitable textiles. The substrate optionally further comprises a flame retardant material, typically Flame Retardant 2 (FR-2) and/or Flame Retardant 4 (FR-4). The substrate may comprise a single layer of an insulating material or multiple layers of the same or different insulating materials. The substrate may be the board of a printed circuit board (PCB) made of any one of the materials listed above.

An electrical or electro-optical assembly comprises at least one electrical or electro-optical component.

An electrical component may be any suitable circuit element of an electrical assembly.

Preferably, an electrical component is a resistor, capacitor, transistor, diode, amplifier, antenna or oscillator. Any suitable number and/or combination of electrical components may be connected to the electrical assembly.

An electrical component is preferably connected to an electrically conductive track via a bond. The bond is preferably a solder joint, a weld joint, a wire-bond joint, a conductive adhesive joint, a crimp connection, or a press-fit joint. Suitable soldering, welding, wire- bonding, conductive adhesive and press-fit techniques are known to those skilled in the art, for forming the bond. More preferably the bond is a solder joint, a weld joint or a wire-bond joint, with a solder joint most preferred.

An electro-optical component may be a component in which an electromagnetic signal, ie. an optical signal, is controlled electronically, for example in active switches, filters, modulators, amplifiers, and switchable elements. Alternatively, an electro-optical component may be a component which converts electromagnetic signals, ie. optical signals, to electronic signals and vice versa, such as light emitters, light detectors and detector arrays. An electro-optical component is thus preferably a light emitting diode (LED), a laser LED, a photodiode, a phototransistor, a photomultiplier, or a photoresistor.

As a skilled person will appreciate, an electro-optical component may have an electrical input/output and an optical input/output. The electrical input/output may be connected to electrically conductive track, preferably via a bond as defined above. The optical input/output may be connected to an optically conductive track, preferably via a bond. The assembly may optionally further comprise an optical component. An optical component may be a passive component. Passive components may include, for example, couplers, splitters, Y-splitters, star couplets, fibres and optical switches. The optical component is typically connected to an optically conductive track, preferably via a bond. The optical component and the bond, where present, are typically completely covered by a continuous coating.

An optical connection may be achieved through an active or a passive mechanical structure, which optically aligns the component and conductive track and mechanically holds these in place. Alternatively, an optical connection may be made using adhesives, optionally with selected/controlled refractive index. Alternatively, an optical connection may be created by fusing the component and conductive track together. Alternatively, the refractive index of the material may be modified by, for example, doping with new materials, to create a new connection. Alternatively in situ addition of suitable material may be applied to create new optical geometries.

In one preferred embodiment, an electrical assembly comprises a substrate comprising an insulating material, a plurality of electrically conductive tracks present at at least one surface of the substrate, at least one electrical component connected to at least one electrically conductive track, preferably by at least one bond as defined above, and a continuous coating comprising a plasma-polymerized fluoropolymer completely covering the at least one surface of the substrate, the plurality of electrically conductive tracks the at least one electrical component and, where present, the at least one bond. More preferably the electrically conductive tracks comprise at least one external contact means, which is typically at least one electrical contact, and the at least one external contact means is also completely covered by the continuous coating.

In another preferred embodiment, a printed circuit board comprises a substrate comprising an insulating material, a plurality of electrically conductive tracks present at at least one surface of the substrate, at least one electrical component connected to at least one electrically conductive track by at least one solder joint, weld joint or wire-bond joint, and a continuous coating comprising a plasma-polymerized fluoropolymer completely covering the at least one surface of the substrate, the plurality of electrically conductive tracks, the at least one electrical component and the at least one solder joint, weld joint or wire-bond joint.

In yet another preferred embodiment, a printed circuit board comprises a substrate comprising an insulating material, a plurality of electrically conductive tracks, which comprise at least one external contact means, present at at least one surface of the substrate, at least one electrical component connected to at least one electrically conductive track by at least one solder joint, weld joint or wire-bond joint, and a continuous coating comprising a plasma-polymerized fluoropolymer completely covering the at least one surface of the substrate, the plurality of electrically conductive tracks, the at least one external contact means, the at least one electrical component and the at least one solder joint, weld joint or wire-bond j oint.

A continuous plasma-polymerized polymer coating may be useful for coating electrical or electro-optical components. Thus, an electrical or electro-optical component may be completely covered with a continuous coating comprising a plasma-polymerized polymer as described above for an electrical or electro-optical assembly. This coated component can be prepared by subjecting the component to a coating method described above. A plasma- polymerized polymer coating may provide excellent environmental protection for the electrical or electro-optical component, and thus may be particularly useful in the case of high value components. A preferred embodiment is an electrical component completely covered with a continuous coating comprising a plasma-polymerized fluoropolymer.

The coated electrical or electro-optical components may be connected to at least one conductive track of an electrical or optical assembly without prior removal of the continuous coating, typically in the case of an electrical component by soldering or wire-bonding. In that case, substantially all of the continuous coating may remain intact, and provide environmental protection post-installation. Alternatively, the coating may be removed by a plasma-removal process, prior to installation in an assembly. In certain embodiments, a plasma-polymerized polymer as described above may be used to conformally coat an electrical or electro-optical assembly or an electrical or electro-optical component.

Aspects of the invention will now be described with reference to the embodiments shown in the accompanying figures, in which like reference numerals refer to the same or similar components, and with reference to the Examples

DESCRIPTION OF THE FIGURES Figure 1A shows the results of an X-ray photoelectron spectroscopic analysis of a plasma- polymerized fluoropolymer. This shows that the plasma-polymerized fluoropolymer comprises a high proportion of CF_3> CF and C-CF moieties, indicating a high degree of branching and cross-linking. Figure IB shows the results of an X-ray photoelectron spectroscopic analysis of a fluoropolymer obtained by standard polymerization techniques, namely commercially available PTFE. This shows that fluoropolymers obtained by standard polymerization techniques contain predominately the CF₂ moiety and negligible proportions of CF_3> CF and C-CF moieties, indicating a very low degree of branching and cross-linking. The method by wh ch these results were obtained is described in Example 1 Figure 2A shows an electron microscope image of a plasma-polymerized fluoropolymer coating of the invention and the smooth physical nature of said coating. Figure 2B shows an electron microscope image of a PTFE coating deposited by standard polymerization techniques, which has a structure in which fibrils are clearly visible. Figure 3 shows an electrical assembly comprising a substrate 1 comprising an insulating material, a plurality of conductive tracks 2 present at at least one surface of the substrate 1, electrical components 3 connected to at least one conductive track 2, and a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 2 and the electrical components 3.

Figure 4 shows an electrical assembly comprising a substrate 1 comprising an insulating materia], a plurality of conductive tracks 2 present at at least one surface of the substrate 1, electrical components 3 connected to at least one conductive track 2 by bonds 5, and a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 2, the electrical components 3 and the bonds 5.

Figure 5 shows an electrical assembly comprising a substrate 1 comprising an insulating material, a plurality of conductive tracks 2 present at at least one surface of the substrate 1, electrical components 3 connected to at least one conductive track 2, a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 2 and the electrical components 3, and a first additional continuous coating 7 comprising a plasma-polymerized polymer completely covering the continuous coating 4.

Figure 6 shows an electrical assembly comprising a substrate 1 comprising an insulating material, a plurality of conductive tracks 2 present at at least one surface of the substrate 1, electrical components 3 connected to at least one conductive track 2, and a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 2 and the electrical components 3, and a coating 8 of an epoxy resin, acrylic resin, or silicone resin deposited between at least a portion of the continuous coating 4 and at least a portion of the substrate 1, the plurality of conductive tracks 2 and the electrical components 3.

Figure 7 shows an electrical assembly comprising a substrate 1 comprising an insulating material, a plurality of conductive tracks 2 present at at least one surface of the substrate 1, electrical component 3 connected to at least one conductive track 2 by a bond 5, a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 2, the electrical components 3 and the bond 5, and asurface-finish coating 6 comprising a halohydrocarbon is deposited between the continuous coating 4 and the at least one surface of the substrate 1 and the plurality of conductive tracks 2. The electrical components 3 are connected to the conductive track 2 through the surface-finish coating 6 via a bond 5 which abuts the surface-finish coating 5. Figure 8 shows an electro-optical assembly comprising a substrate 1 comprising an insulating material, a plurality of conductive tracks 17, 18 present at at least one surface of the substrate 1, electro-optical components 19 connected to at least one conductive track 17, 18, and a continuous coating 4 comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate 1, the plurality of conductive tracks 17, 18 and the electrical electro-optical components 19. The conductive track 17 is an optically conductive track, such as an optical fibre. The conductive track 18 is an electrically conductive track. The refractive index of the region of the coating 20 by the optical internconnect 21 is controlled.

Figure 9 shows an electrical component 15 which is completely covered with a continuous coating comprising a plasma-polymerized polymer 16.

Figure 10 shows an example of an apparatus which can be used to form the plasma- polymerized polymer coating of the invention. In this example, the reactor 9 has a chamber 10 connected to a vacuum system 11 and an energy source 12. Precursor compounds are ionized/and or decomposed by the plasma to form active species 13 which then react at the surface of the assembly 14 to form a continuous plasma-polymerized polymer coating.

Figures 11 A, 1 IB and 11C are flow-charts showing certain embodiments of the methods described above.

EXAMPLES

Example 1 - XPS analysis of a plasma-polymerized fluorohydrocarbon An epoxy laminate substrate was coated with a plasma-polymerized fluorohydrocarbon. This laminate was cut to yield a sample size of approximately 1 cm square and introduced to the sample chamber of a Thermo- Scientific ESCALAB 250 X-Ray Photoelectron Spectrometer. The chamber was pumped down to an operating pressure of 10 Torr and then the sample was transferred to the analyzing chamber. A monochromatic X-ray beam was incident on the surface and the photoelectrons emitted by the sample were collected and analyzed.

A broad signal scan was conducted to capture all of the elements on the surface, and then a further high resolution scan of the Cls peak was conducted to determine the fine structure of the peak and the chemical structure of the sample.

The results are displayed in Figure 1 A.

Example 2— preparation of coated assemblies

Assemblies were coated in Runs 1 to 10 using the precursors and plasma-polymerization conditions displayed in Table 1 below.

TABLE 1

Example 3 - plasma removal process

An electrical assembly that had been coated with a plasma-polymerized fluorohydrocarbon was introduced to a plasma chamber. The chamber was pumped down to an operating pressure of 250 mTorr and oxygen gas was introduced at a flow rate of 2500 seem. The gas was allowed to flow through the chamber for 30 seconds and then the plasma generator was switched on at a frequency of 40 kHz and a power of 3 kW. The assembly was exposed to the active plasma for a time period of 5 minutes, after which the plasma generator was switched off and the chamber brought back to atmospheric pressure.

The assembly was removed from the plasma chamber and the removal of the plasma polymer coating was verified using a Bruker FTER. spectrometer. The absence of the characteristic C- F stretching peak at 1250nm indicated that the fluoropolymer had been completely removed.

Claims

1. An electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, at least one electrical or electro-optical component connected to at least one conductive track, and a continuous coating comprising a plasma-polymerized polymer completely covering the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

2. An electrical or electro-optical assembly according to claim 1, wherein the plasma- polymerized polymer is a plasma-polymerized hydrocarbon or halohydrocarbon.

3. An electrical or electro-optical assembly according to claim 1 or claim 2, wherein the plasma-polymerized polymer is a plasma-polymerized fluorohydrocarbon.

4. An electrical or electro-optical assembly according to any on of the preceding claims wherein the plasma-polymerized fluorohydrocarbon is obtainable by plasma polymerizing one or more precursor compounds selected from perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes, fluoroatkenes, fluoroalkynes, fluoroacrylates, fluoroesters, fluorosilanes, fluorochloroalkanes, fluorochloroalkenes fluorochloroalkynes,

fluorochloroacrylates, fluorochloroesters and fluorochlorosilanes.

5. An electrical or electro-optical assembly according to any one of the preceding claims, wherein the at least one electrical or electro-optical component is an electrical component and the at least one conductive track is an electrically conductive track.

6. An electrical or electro-optical assembly according claim 5, wherein the electrical component is connected to the at least one conductive track by at least one bond and the continuous coating completely covers the at least one bond.

7. An electrical or electro-optical assembly according claim 6, wherein the at least one bond is a solder joint, a weld joint, a wire-bond joint, a conductive adhesive joint, a crimp connection or a press-fit joint.

8. An electrical or electro-optical assembly according claim 7, wherein the at least one bond is a solder joint, a weld joint or a wire-bond joint.

9. An electrical or eletro-optical assembly according to any one claims 1 to 4, wherein the at least one electrical or electro-optical component is an electro-optical component and the at least one conductive track is an electrically conductive track or an optically conductive track.

10. An electrical or eletro-optical assembly according to any one of the preceding claims, wherein the plurality of conductive tracks further comprise at least one external-contact means and the continuous coating completely covers the at least one external-contact means.

11. An electrical or electro-optical assembly according to claim 10 wherein the at least one external-contact means is an electrical contact.

12. An electrical or electro-optical assembly according to claim 10 wherein the at least one external-contact means is an optical contact.

13. An electrical or electro-optical assembly according to any one of the preceding claims, which further comprises an optical component which is connected to an optically conductive track.

14. An electrical or electro-optical assembly according to any one of the preceding claims, which further comprises a first additional continuous coating comprising a plasma- polymerized polymer as defined in any one of claims 1 to 4 which completely covers the continuous coating, and optionally a second additional continuous coating comprising a plasma-polymerized polymer as defined in any one of claims 1 to 4 which completely covers the first additional coating.

15. An electrical or electro-optical assembly according to any one of the preceding claims, which further comprises a coating of an epoxy resin, an acrylic resin, a silicone resin or a parylene deposited between at least a portion of the continuous coating of a plasma- polymerized polymer and at least a portion of the substrate, the plurality of conductive tracks and the at least one electrical or optical component

16. An electrical or electro-optical assembly according to any one of the preceding claims, which further comprises a surface-finish coating comprising a halohydrocarbon polymer deposited between (a) the continuous coating and (b) the at least one surface of the substrate and the plurality of conductive tracks, wherein the surface-finish coating covers at least a portion of the plurality of conductive tracks, and the at least one electrical or electro- optical component is connected to the at least one conductive track through the surface-finish coating.

17. An electrical or electro-optical assembly according to claim 16 when dependent on claim 8, wherein the solder joint, the weld joint or a wire-bond joint abuts the surface-finish coating.

18. An electrical or electro-optical assembly according any one of the preceding claims which is a printed circuit board.

19 A method for preparing an electrical or electro-optical assembly as defined in any one of the preceding claims, said method comprising (a) providing an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate and at least one electrical or electro- optical component connected to at least one conductive track, and (b) depositing by plasma- polymerization a continuous coating comprising a polymer as defined in any one of claims 1 to 3 which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

20. A method according to claim 19 wherein the depositing by plasma-polymerization involves plasma polymerizing one or more precursor compounds as defined in claim 4.

21. An electrical or electro-optical assembly obtainable by the method of any one of claims 19 and 20.

22. An electrical or electro-optical component which is completely covered with a continuous coating comprising a plasma-polymerized polymer as defined in any one of claims 1 to 4.

23. A method comprising (a) subjecting an electrical or electro-optical assembly as defined in any one of claims 1 to 14, 16 to 18 or 21 to a plasma-removal process, such that the continuous coating and, where present, the first and second additional continuous coatings and/or the surface-finish coating are removed, and then (b) optionally reworking the resultant electrical or electro-optical assembly, and then (c) optionally depositing by plasma- polymerization a replacement continuous coating comprising a polymer which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

24. A method comprising soldering through the continuous coating and, where present, the first and second additional continuous coatings and/or the surface-finish coating of an electrical or electro-optical assembly as defined in any one of claims 1 to 14, 16 to 18 or 21, to form a solder joint between a further electrical or electro-optical component and at least one conductive track, wherein the solder joint abuts the continuous coating and, where present, the first and second additional continuous coatings and/or the surface-finish coating.

25. A method comprising (a) subjecting an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate, a surface-finish coating comprising a halohydrocarbon polymer covering at least a portion of the plurality of conductive tracks and at least one electrical or electro-optical component connected to at least one conductive track through the surface-finish coating to a plasma-removal process such that the surface-finish coating is removed, and then (b) depositing by plasma-polymerization a continuous coating comprising a polymer as defined in any one of claims 1 to 4 which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

26. A method comprising (a) subjecting an electrical or electro-optical assembly comprising a substrate comprising an insulating material, a plurality of conductive tracks present at at least one surface of the substrate and a surface-finish coating comprising a halohydrocarbon polymer covering at least a portion of the plurality of conductive to a plasma-removal process such that the surface-finish coating is removed, then (b) connecting an electrical or electro- optical component to at least one conductive track, and then (c) depositing by plasma- polymerization a continuous coating comprising a polymer as defined in any one of claims 1 to 4 which completely covers the at least one surface of the substrate, the plurality of conductive tracks and the at least one electrical or electro-optical component.

27. An electrical or electro-optical assembly which has a conformal coating comprising a plasma-polymerized polymer as defined in any one of claims 1 to 4.

28. Use of a plasma-polymerized polymer as defined in any one of claims 1 to 4 as a conformal coating for an electrical or electro-optical assembly.

29. A method for conformally coating an electrical or electro-optical assembly comprising depositing a polymer as defined in any one of claims 1 to 4 by plasma- polymerization.