GB2462824A

GB2462824A - Printed circuit board encapsulation

Info

Publication number: GB2462824A
Application number: GB0815096A
Authority: GB
Inventors: Frank Ferdinandi; Rodney Edward Smith; Mark Robson Humphries
Original assignee: CROMBIE 123 Ltd; Semblant Ltd
Current assignee: CROMBIE 123 Ltd; Semblant Ltd
Priority date: 2008-08-18
Filing date: 2008-08-18
Publication date: 2010-02-24
Also published as: GB0815096D0

Abstract

A Printed Circuit Board (PCB) is encapsulated by a multi-layer coating comprising one or more polymers, wherein the polymers are selected from halo-hydrocarbon polymers (e.g. PTFE, PCTFE, EPCTFE and other fluoroplastics) and non-halohydrocarbon polymers (e.g. polyalkenes such as polythene or polypropylene, vinyl polymers, phenolic resins or polyanhydrides), and the thickness of the multi-layer coating is from 1 nm to 10 microns. Solder connections to the conductive tracks on the PCB may be formed without removing the polymer coating which is locally dispersed and/or absorbed and/or vaporized during soldering. The encapsulation is flame retardant and prevents absorption of moisture, solvents and oxygen. Selected areas of the PCB may be coated separately to form a complex 3D encapsulation coating configuration. The wetting characteristic of the coating may be selectively modified using e.g. localized plasma treatment or liquid based chemical etching. The multi-layer coating may also be a graded layer.

Description

PRNTED CIRCUIT BOARDS

The present invention relates to articles such as those comprising printed circuit boards coated with polymers.

Printed circuit boards (PCBs) are used in the electronics industry to mechanically support and electrically connect electrical and electronic components. A PCB comprises a board or other substrate made of an insulating material on which conductive tracks, typically made of copper, lie. These conductive tracks function as wires between electrical components that are later attached to the board by, for example, soldering. A large proportion of PCBs are manufactured by depositing or otherwise adhering a layer of copper to the substrate board, and then removing unwanted copper by chemical etching to leave copper tracks in the required configuration. At this stage the blank PCBs may often be stored for variable periods of time, potentially up to several months, prior to attachment of the electronic components to the PCB by a soldering method.

The conductive tracks on the printed circuit board may be made from any conductive material. The preferred material for the tracks is copper. Copper is the preferred material for the conductive tracks mainly due to its high electrical conductivity, but unfortunately copper is readily oxidised in air leading to a layer of copper oxide, or tarnish, on the surface of the metal. This oxidation is particularly evident if a long period of time has elapsed between manufacture of the blank PCB and attachment of the electrical components. The components are attached by soldering, but the presence of an oxide layer on the copper tracks may reduce the effectiveness of soldering. In particular dry joints, which have a tendency to fail during operation of the device, and weak joints with low mechanical strength may be formed. Occasionally the joint will fail to make electrical contact altogether. Similar problems arise when the conductive tracks comprise conductive materials other than copper.

To minimise these problems, PCB manufacturers apply a range of coatings, or surface finishes, to the areas where soldering will be required. Metals such as tin, silver or a nickel/gold combination are frequently used. The processes for applying these finishes are all time consuming, requiring additional metals to be used, with consequent environmental issues. There are potential health issues associated with some of the processes and materials. Further, some of the metals used, such as gold, are expensive. A similar approach involves coating the tracks with a coating comprising organic compounds such as benzimidazoles and particles of solder-wettable metals or solder (see for example WO 97/39610), thus preventing exposure of the tracks to oxidative conditions. During soldering the organic layer is removed. These organic coatings typically do not survive multiple heat cycles and have a relatively short storage life before processing.

It is apparent that the techniques adopted by manufacturers up until now are either expensive or time consuming (involving extra steps in the manufacturing process), or both, and deplete non-renewable resources including precious metals. There is a need for a cheaper and/or higher performance method of preventing oxidation of the conductive tracks prior to attachment of electrical components by soldering.

A separate issue is that PCBs are often required in devices that are used in very harsh and corrosive environments. Under such conditions, the conductive tracks on the PCB may be corroded leading to a far shorter lifetime of the circuit board than would normally be expected. Such conditions may arise, for example, when a device is used in very humid environments, especially where microscopic droplets of water containing dissolved gases such as sulphur dioxide, hydrogen suiphide, nitrogen dioxide, hydrogen chloride, chlorine and water vapour form a corrosive solution. Additionally, droplets of moisture may form a thin film or corrosion deposits between conductive tracks on the PCB that may potentially cause short circuits. In circumstances where PCB manufacturers envisage the devices being used in hostile conditions, they have tended to coat the assembled PCB with a conformal coating of a polymer that forms a barrier to the environment. However such coatings are expensive to apply and require an extra step in the manufacturing process to apply the coating after the PCB has been assembled, and generally an extra step later to remove it. This may also cause problems when reworking a damaged or :.

failed PCB, or during testing to ascertain its performance and troubleshoot a problem. A lower cost and/or high performance method of environmentally protecting the completed PCB would be of great interest to manufacturers.

Another problem that may arise after soldering electronic components to a PCB is the formation of dendrites of metal compounds on the solderjoint. These dendrites can cause failure of the assembled PCB due to short circuits and electrical leakage between contacts. Dendrites are very fine metallic growths along a surface, resulting from electromigration, which form fern-like patterns. The growth mechanism for dendrites is well understood, unlike "tin whiskers", and requires the presence of moisture that generates metal ions that are then redistributed by electromigration in the presence of an electromagnetic field. The coating of the invention protects against the formation of dendrites by preventing moisture reaching the surface of the PCB, which is where dendrites normally grow and also provides a physical barrier between conductors. The coating provides additional protection as dendrite materials have low adhesion to the surface coating reducing the formation of dendrites between contacts and components.

Another problem arises when the board or substrate and/or the conductive tracks that lie upon it are made of a material that absorbs water or solvent (including aqueous, organic, inorganic or mixed solvents) based chemicals in liquid, vapour or gaseous form. Most materials commonly used for printed circuit boards absorb water and solvent based chemicals to some degree and the extent of the detrimental affects depends upon this propensity to absorb water and solvent based chemicals. Materials commonly used for substrates that absorb water and solvent based chemicals include epoxy resin bonded glass fabrics, synthetic resin bonded paper, phenolic cotton paper, cotton paper and epoxy. Other potential substrate materials include paper, cardboard, textiles, natural and synthetic wood based materials. Absorbent materials for conductive tracks include nickel, conductive polymers, or printed conductive inks. Other absorbent materials that may be applied to substrates include magnetic structures, printed magnetic inks, etc. Indeed, the substrate or conductive track could be any porous material, or hydrophilic material wherein a natural tendency for water or, more generally, solvents may cause changes to that structure. The tendency of the material to interact with water or solvents in the liquid phase or through condensation from the gas phase, would include solid solvents. Absorption of this nature potentially causes a number of problems including: -Increased mechanical stresses during thermal cycles due to the difference in thermal expansion coefficients.

-Ability to alter adhesion properties.

-Alterations to the dielectric constant and loss tangent.

-Swelling of the structure rendering some materials unsuitable for plated through holes and also for use in some high humidity conditions, especially where high voltages are used.

-Corrosion of the conductive tracks at or around the interface between the track and the substrate.

-Loss of mechanical strength.

-Reordering of the material in the presence of water.

-Electrolysis in the presence of an applied field leading to corrosion and/or degradation.

This is a particular problem when conductive ink polymers are used to form the conductive tracks, since such conductive ink polymers are often water absorbing too.

Printed active devices, as used in plastic electronics, also suffer from absorption of water and solvent based chemicals which causes their characteristics to change.

A further problem for printed circuit boards is that in order to meet standards for fire safety, flame retardants compounds, particularly bromine based ones such as tetrabromobisphenol A (TBBPA), are incorporated in the materials used to make them.

However, bromine compounds are toxic and there are environmental consequences of their use and subsequent disposal is problematic and not desirable.

The present invention provides a printed circuit board to which a solder connection is to be made, the surface of said printed circuit board having a multi-layer coating comprising one or more polymers, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and the thickness of the multi-layer coating is from 1 nm to 10.im.

The coating structure may comprise one or more layers of discrete polymers. We take this definition to include both layers of different chemical compositions and layers of similar chemical composition but different structures, degrees of conjugation, different weight, physical structure etc. Thus, typically each layer may comprise a different polymer. Preferably there are two or more layers in the multi-layer coating. However, if there are more than two layers, two of the layers may comprise the same polymer provided that they are not adjacent to each other.

Alternatively, the multi-layer coating may comprise graded layers of different polymers, wherein adjacent layers are typically fused together, with polymers of intermediate chemical composition present between adjacent layers. Alternatively, the multi-layer coating may comprise polymer layers adjacent to metal fluoride layers Preferably the multi-layer coating has two to five layers, more preferably two to four layers, and most preferably two or three layers.

By combining different polymer compositions, different composite properties may be achieved. In a one embodiment, different areas of the printed circuit board may be coated with different polymers, or mixtures thereof, to achieve different properties in the different areas.

The multilayer coatings can be designed and engineered to offer varying, customised functional performance, optimising, for example, coating conductivity, oxidation resistance, environmental protection, and other physical and chemical properties. Other examples of properties that can be enhanced or optimised include reduction or virtual elimination of moisture absorption by the substrate or conductive tracks and printed components, dendrite prevention and flame retardancy.

Halo-hydrocarbon polymers and non-halohydrocarbon polymers of the invention are described below. The multi-layer coating typically has an overall thickness of from mm to 10 jim, more typically from I nm to 500 nm, still more typically from 3 nm to 500 nm, still more typically from 10 nm to 500 nm, and most typically from 10 nm to 250 nm.

S The multi-layer coating is preferably at a thickness of from 10 nm to 100 nm, with 100 nm being a preferred thickness. In another embodiment, the thickness of the coating is nm to 30 nm.

When the multi-layer coating comprises layers of discrete polymers, the ratio of the thicknesses of each layer can be varied to achieve different properties. For example, in one embodiment each discrete layer of the multi-layer coating may be of equal or approximately equal thickness. Alternatively, one layer may be thicker than other layers, so that the multi-layer coating has overall properties tuned to provide the desired combined functionality derived from contributions from each layer. Thus, typically one layer may comprise 60 to 90 % of the overall thickness of the coating, with the remaining layer(s) comprising 10 to 40%.

Similarly, for graded multi-layers, the proportions of the polymers in the multi-layer can be varied to achieve different properties. For example, in one embodiment the proportion of each polymer in the multi-layer coating may be of equal. Alternatively, there may be more of one polymer present, so that the multi-layer coating has the higher beneficial properties of that layer. Thus, typically one polymer may comprise 60 to 90 % of the coating, with the remaining polymer(s) comprising 10 to 40%.

Preferably, for coatings with layers of discrete chemical composition, the first coating applied to the substrate is continuous or substantially continuous, such that none, or substantially none, of the second or subsequent coatings comes into contact with the surface of the printed circuit board.

Typically there is no solder, or essentially no solder, between said coating composition and the conductive tracks of said printed circuit board.

In one embodiment, the first layer comprises a non-halo-hydrocarbon polymer, such as polythene or polypropylene. The advantage of having a polymer that does not contain a halogen or halide as the first layer is that no metal halide layer is formed on the printed circuit board. Further, during subsequent addition of a halo-hydrocarbon polymer layer, the halo-hydrocarbon polymer does not come into contact with the printed circuit board and thus no metal halide layer can form on the printed circuit board. Thus, in one embodiment of the invention the printed circuit board typically has no, or essentially no, metal halide layer on the surface of the printed circuit board. However, it may also be preferable under some circumstances to have a first layer comprising a halo-hydrocarbon polymer.

It may be preferable that all of the layers in the multi-layer coating are halo-hydrocarbon polymers. Alternatively, it may be preferably that all of the layers in the multi-layer coating are non-halo-hydrocarbon polymers.

The present invention also provides a method of making a connection to a printed circuit board, the surface of which has a multi-layer coating comprising one or more polymers, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and the thickness of the multi-layer coating is from I nm to 10 jim, which method comprises applying solder, and optionally flux, to the printed circuit board at a temperature and for a time such that the solder bonds to the metal and the composition is locally dispersed and/or absorbed andJor vaporised. Preferably, one or more of(a) the substrate characteristics, (b) the coating characteristics, (c) the solder/flux characteristics, (d) the soldering profile, including time and temperature, (d) the process to disperse the coating, or (e) the process to control solder flow around the joint, are selected such that there is good solder flow, solder covers the substrate (typically a conductive track or pad) on the printed circuit board and a strong solder joint is generated. 1.

The present invention further provides a method of modifying the wetting characteristics of a coating comprising one or more halo-hydrocarbon polymers on a printed circuit board through choice of the preferred material and/or by plasma etching, plasma activation, plasma polymerisation and coating, and/or liquid based chemical etching.

These modifying techniques can be applied to either (a) the substrate and conductive tracks before the coating is applied, or (b) the coating after it has been applied. In another embodiment, the method of modifying the wetting characteristics can be performed on the multi-layer coating described above.

The present invention also provides a printed circuit board comprising a substrate and conductive tracks, wherein the surfaces of said printed circuit board are completely or substantially encapsulated with coating of a composition comprising one or more halo-hydrocarbon polymers at a thickness of 1 nm to 10.tm. Alternatively, in another embodiment the coating may be a multi-layer coating as described above. Typically, the substrate of said printed circuit board comprises a water or solvent based chemical absorbent material, typically an epoxy resin bonded glass fabrics, synthetic resin bonded paper, phenolic cotton paper, or cotton paper and epoxy. Other potential substrate materials, particularly for printed electronics, could include card, paper, textiles, natural and synthetic wood based materials. Any substrates with an open structure are potential substrate materials when used in combination with the coatings of the invention.

The present invention further provides a method of preparing a printed circuit board, which comprises (a) providing a printed circuit board having an environmentally exposed surface, (b) cleaning the surface in an plasma chamber, using gases such as hydrogen, argon or nitrogen, and (c) applying to the surface a thickness of I nm to 10 j.im of a composition comprising a halo-hydrocarbon polymer by plasma deposition, said coating optionally following the 3D form of the printed circuit board.

The present invention further provides a method of preparing a printed circuit board, which comprises (a) providing a printed circuit board having an environmentally exposed surface, (b) cleaning the surface in an plasma chamber, using gases such as hydrogen, argon or nitrogen, (c) applying to the surface a thickness of I nm to 10 jim of a multi-layer coating comprising one or more polymers by plasma deposition, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and said multi-layer coating optionally following the 3D form of the printed circuit board.

The present invention also provides the use of a composition comprising a halo-hydrocarbon polymer as a flame-retardant coating for printed circuit boards.

By polymer we include polymers formed in-situ from single or multiple monomers, linear, branched, grafted and crosslinked copolymers, oligomers, multipolymers, multimonorner polymers, polymer mixtures, grafted copolymers, blends and alloys of polymers, as well as interpenetrating networks of polymers (IPNs).

The thickness of the coating is typically from mm to 10 jim, more typically from 1 nm to 500 nm, still more typically from 3 nm to 500 nm, still more typically from 10 nm to 500 nm, and most typically from 10 nm to 250 nm. The coating is preferably at a thickness of from 10 nm to 100 nm, in various gradients, with 100 nm being a preferred thickness. In another embodiment, the thickness of the coating is 10 nm to 30 run.

The polymer coatings may be continuous, substantially continuous (particularly over surfaces to be soldered and non-soldering surfaces between or adjacent to them, and more particularly over substantially all exposed and vulnerable surfaces of the PCB), or non-continuous. For a very high level of environmental protection, a substantially continuous coating may be required. However, a non-continuous coating may be sufficient for other purposes.

By halo-hydrocarbon polymer it is meant a polymer with a straight or branched chain or ring carbon structure with 0, 1, 2 or 3 halogen atoms bound to each carbon atom in the structure. The halogen atoms could be the same halogens (for example fluorine) or a mixture of halogens (for example fluorine and chlorine). The term "halo-hydrocarbon polymer" as used herein includes polymers that contain one or more unsaturated groups, such as carbon-carbon double and triple bonds, and polymer that contain one or more heteroatoms (atoms which are not C, H or halogen), for example N, S or 0. Preferably the halo-hydrocarbon polymer contains less than S % hetero atoms as a proportion of the total number of atoms in the polymer.

The molecular weight of the halo-hydrocarbon polymer is preferably greater than 500 amu.

The halo-hydrocarbon polymer chains may be straight or branched, and there may be crosslinking between polymer chains. The halogen may be fluorine, chlorine, bromine or iodine. Preferably the halo-hydrocarbon polymer is a fluoro-hydrocarbon polymer, a chioro-hydrocarbon polymer or a fluoro-chioro-hydrocarbon polymer wherein 0, 1, 2 or 3 fluoro or chioro atoms are bonded to each carbon atom in the chain.

Examples of preferred halo-hydrocarbon polymers include: -PTFE, PTFE type material, fluorinated-hydrocarbons, chlorinated-fluorinated-hydrocarbons, halogenated-hydrocarbons, halo-hydrocarbons or co-polymers, oligomers, multipolymers, multimonomer polymers, polymer mixtures, blends, alloys, branched chain, grafted copolymers, cross-linked variants of these materials and also interpenetrating polymer networks (IPNs).

-PCTFE (polychiorotrifluoroethylene) and copolymers, oligomers, multipolymers, multimonomer polymers, polymer mixtures, blends, alloys, branched chain, grafted copolymers, cross-linked variants of this material and also interpenetrating polymer networks (IPNs).

-EPCTFE (ethylene copolymer of polychlorotrifluoroethyl ene) and copolymers, oligomers, multipolymers, multimonomer polymers, polymer mixtures, blends, alloys, branched chain, grafted copolymers, cross-linked variants of this material and also interpenetrating polymer networks (IPNs).

-Other fluoroplastics including the materials below and co-polymers, oligomers, multipolymers, multimonomer polymers, polymer mixtures, blends, alloys, branched chain, grafted copolymers, cross-linked variants of these materials as well as interpenetrating polymer networks (IPNs): ETFE (copolymer of ethylene and tetrafluoroethylene), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), PFA (copolymer of tetrafluoroethylene and perfluorovinyl ether), PVDF oIymer of vinyl i denefluori de), THV (copolymer of tetrafluoroethylene, hexafluoropropylene and vinylidenefluoride), PVDFHFP (copolymer of vinylidene fluoride and hexafluoropropylene), MFA (copolymer of tetrafluoroethylene and perfluoromethylvinylether), EFEP (copolymer of ethylene, tetrafluoroethylene and hexafluoropropylene), HTE (copolymer of hexfluoropropylene, tetrafluoroethylene and ethylene) or copolymer of vinylidene fluoride and chlorotrifluoroethylene and other fluoroplastics.

By non-halo-hydrocarbon polymer it is meant any polymer that does not contain halogen atoms. The non-halo-hydrocarbon polymer can have a straight or branched chain or ring carbon structure. The non-halo-hydrocarbon polymer may contain one or more unsaturated groups, such as carbon-carbon double and triple bonds, and one or more heteroatoms (atoms which are not C, H or halogen), for example N, S or 0. The molecular weight of the non-halo-hydrocarbon polymer is preferably greater than 500 amu. The non-halo-hydrocarbon polymer chains may be straight or branched, and there may be crosslinking between polymer chains. It is preferred that the non-halo-hydrocarbon polymer is a polymer that can be deposited by plasma deposition.

Preferably the non-halo-hydrocarbon polymer is a polyalkene, a polyester, a vinyl polymer, a phenolic resin or a polyanhydride. More preferably, the non-halo-hydrocarbon polymer is a polyalkene, typically polythene or polypropylene.

It is desirable that the coating composition have any one or more, and preferably substantially all, of the following properties: capable of being deposited as continuous films, free of cracks, holes or defects; relatively low gaseous permeability which provides a significant barrier to gaseous permeation and avoids gaseous corrosion and oxidation through' the coating; the ability to selectively solder through without the need for prior removal and to achieve good solder joints comparable to other currently available surface finishes; the ability to withstand multiple heat cycles; chemical resistance to corrosive gases, liquids and salt solutions, particularly environmental pollutants; exhibit low surface energy and wettability'; to be stable inert material over the temperature ranges where PCB's may be used; have good mechanical properties, including good adhesion to PCB materials and good mechanical abrasion resistance; improved electrostatic protection; relatively low liquid and salt solution permeability, to avoid liquid corrosion through' the coating; and generally be environmentally beneficial compared to existing processes when used in this application.

The polymer coating preferably provides a good barrier to the permeation of atmospheric gases and liquids, most importantly oxygen, which would normally react with the conductive tracks, typically copper tracks, to form a layer of tarnish, typically copper oxide, on the surface of the track, or in the case of liquids, cause swelling and undesirable changes to the substrate as previously described. As a result, the coated circuit board may be stored for long periods of time, up to several months or years, without damaging oxidation of the conductive tracks occurring. Optical microscopy, interferometry, scanning electron microscopy and back scattering electron imaging have been used to investigate the nature, continuity and thickness of the coating. Energy dispersive analysis by X-rays has been used to map the levels and distribution of halogens in the coating.

Measurements of the surface activation and surface wettability using chemical solvent solutions provide an indication of the potential to act as a protective coating.

Once the manufacturer is ready to install components on the blank PCB, there is no requirement to remove the coating layer prior to the soldering process. This arises because, surprisingly, the polymers used in the present invention provide a composite coating that has the unusual property that it may be soldered through to form a solder joint between the conductive track on the board and the electrical component. Flux is generally required in this soldering technique. In the extreme, a soldering process using heat alone could be used to selectively "remove" the coating, for example laser soldering.

Welding, laser-enhanced welding, ulstrasonic welding or use of conductive adhesives are further alternatives. Another possible technique is wave soldering; this technique may require selective fluxing. The solder used may be leaded solder or lead-free solder. There is generally no reduction in the strength of the solder joint as might be expected, and indeed under certain circumstances the solder joint may be stronger than a standard solder joint. Furthermore, under certain circumstances, the present invention may prevent dendrite formation on the solder bonds, particularly when lead-free solder is used.

Thus, the present invention provides an alternative technique to applying surface coatings of metals (such as tin, silver, nickel and gold) to the conductive tracks of PCBs to prevent oxidation of the conductive tracks prior to soldering. The present invention has the advantage that it is based on a low cost process, it does not use toxic metals such as nickel, it is environmentally friendly and it is safer than current industrial metal plating processes. It also simplifies the PCB manufacturing process and is compatible with current industrial soldering processes. In addition it has the extra benefit of "solder through" properties, whereby the need to remove the coating before soldering is avoided.

A further feature of the polymer coating of the invention is that it is typically only removed in the areas where solder andlor flux is applied. Thus, in the areas of the PCB where components are not attached by selective soldering the coating, particularly the halohydrocarbon coating, remains intact, maintaining a protective layer over the board and conductive tracks, which provides a barrier to corrosion by atmospheric gases such as sulphur dioxide, hydrogen sulphide, nitrogen dioxide, hydrogen chloride, chlorine and water vapour and other corrosive materials, thus avoiding corrosion by environmental pollutants. The coating is also substantially impermeable to liquids and corrosive liquids.

Consequently it is possible to attach components to the circuit board in a series of steps with significant periods of time elapsing between each step; this could provide a number of advantages to the manufacturer. This coating is not destroyed by the soldering process other than in the selected solder areas, therefore in the non-soldered areas the PCB can be reprocessed and/or reworked by soldering at a later stage. Furthermore, once assembly of the PCB is completed, the unsoldered areas of the PCB remain coated with the polymer, which forms a permanent barrier to environmental corrosion. There is no need for further costly over-coating steps such as conformal coating.

The conductive tracks on the printed circuit board may comprise any conductive material.

Possible materials from which the conductive tracks may be made are metals such as copper, silver, aluminium or tin, or conductive polymers or conductive inks. The preferred material for the tracks is copper. Conductive polymers tend to absorb water and swell, and thus coating conductive polymers with a halo-hydrocarbon polymer layer can prevent water absorption.

The printed circuit boards of the invention may be manufactured by providing a blank printed circuit board having an environmentally-exposed surface, and which preferably has no solder, or essentially no solder, on said environmentally exposed surface, and applying to that surface to a thickness of a 1 nm to 10 pm of a composition comprising one or more polymers by a thin film deposition technique such as plasma deposition, chemical vapour deposition (CVD), molecular beam epitaxy (MBE), creation of inter-penetrating polymer networks (IPNs), surface absorption of monolayers (SAMs) of polymers of monomers to form in-situ polymers, polymer alloys, or sputtering. Plasma enhanced-chemical vapour deposition (PE-CVD), high pressure/atmospheric plasma deposition, metallo-organic-chemical vapour deposition (MO-CVD) and laser enhanced-chemical vapour deposition (LE-CVD) are alternative deposition techniques. Liquid 1.

coating techniques such as liquid dipping, spray coating, spin coating and sol/gel techniques are further alternatives.

The multi-layer or graded layer coating of the invention can be deposited on the substrate by varying the composition of the precursor gas or gases in the plasma deposition process. One or more precursor gases can be used to generate a mixture in the plasma chamber which can be used to generate graded layers. Multi-layers can be deposited by switching the precursor gas and modifying conditions in the plasma deposition process.

The composition can be controlled by a number of methods: l0 -Varying the composition of the precursor gas or the mixture of precursor gases.

-Changing the flow rates of the precursor gas or gases.

-Changing the pressure of the precursor gas or gas mixture.

-Changing the power used during the plasma process.

-Modifying the electrode to substrate geometry of the plasma chamber.

The preferred deposition method may depend on the thickness of coating that is required.

Liquid coating techniques may be preferred for thicker coatings, while plasma deposition techniques may be preferred for thinner coatings. The thickness of the coating is typically from I nm to 2 pm, more typically from 1 nm to 500 nm, still more typically from 3 nm to 500 nm, still more typically from 10 nm to 500 nm, and most typically from 10 nm to 250 nm. The coating is preferably at a thickness of from 10 nm to 100 nm, with 100 nm being a preferred thickness. The halo-hydrocarbon polymer is preferably a fluoro-hydrocarbon polymer, a chioro-hydrocarbon polymer or a fluoro-chioro-hydrocarbon polymer, which may also contain micro-pigments and small quantities of other performance additives (being a common practice in the polymer industry) and may for example be polytetrafluoroethylene (PTFE) type materials. The preferred method of applying the halo-hydrocarbon polymer is plasma deposition, although all the other techniques mentioned above would also be applicable. Non-halo-hydrocarbon polymers are also preferably deposited by a plasma deposition technique in the present invention. 1.

Plasma deposition techniques are extensively used for deposition of coatings in a wide range of industrial applications. The method is an effective way of depositing continuous thin film coatings using a dry and environmentally friendly technique. The PCBs are coated in a vacuum chamber that generates a gas plasma comprising ionised gaseous ions, electrons, atoms and neutral species. In this method, the PCI3 is introduced into the vacuum chamber that is first pumped down to pressures typically in the range to 10 mbar. A gas is then introduced into the vacuum chamber to generate a stable gas plasma and one or more precursor compounds are then introduced into the plasma as either a gas or liquid to enable the deposition process.

For a halo-hydrocarbon polymer, the precursor compounds are typically halogen-containing hydrocarbon materials, which are selected to provide the desired coating properties. When introduced into the gas plasma the precursor compounds are also ionizedldecomposed to generate a range of active species that will react at the surface of the PCB, typically by a polymerisation process, to generate a thin halo-hydrocarbon coating. Preferred precursor compounds for a halo-hydrocarbon polymer are perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes, fluoroalkenes, fluoroalkynes, fluorochioroalkanes, fluorochioroalkenes, fluorochioroalkynes, or any other fluorinated andlor chlorinated organic material (such as fluorohydrocarbons, fluorocarbons, chlorofluorohydrocarbons and chiorofluorocarbons).

For a non-halo-hydrocarbon polymer, the precursor compounds are typically hydrocarbon material, which are selected to provide the desired coating properties. When introduced into the gas plasma the precursor compounds are also ionized/decomposed to generate a range of active species that will react at the surface of the PCB, typically by a polymerisation process, to generate a thin non-halo-hydrocarbon coating. Preferred precursor compounds for a non-halo-hydrocarbon polymer are alkanes, alkenes and alkynes.

If a halo-hydrocarbon layer that contains a halogen or halide based material is applied directly to the conductive track of the PCB, a very thin layer (for example 5nm or less) of metal halide (typically a metal fluoride, such as copper fluoride) may form on the metals surface. Such a metal halide layer may be very robust and inert, and prevents formation of oxide layers or other tarnishes which prevent effective soldering. However, there may be certain circumstances where a metal halide layer is undesirable if it results, for example, in intermetallics that are vulnerable to weakening under specific environmental conditions. In those cases, a formation of a first layer of a composition comprising non-halo-hydrocarbon, such as polythene or polypropylene, on the surface of the printed circuit board prevents formation of a metal fluoride layer during subsequent deposition of a halo-hydrocarbon layer, The nature and composition of the plasma deposited coating depends on a number of conditions: the plasma gas selected; the precursor compound used; the plasma pressure; the coating time; the plasma power; the chamber electrode arrangement; the preparation of the incoming PCB; and the size and geometry of the chamber. Typically the plasma deposition technique can be used to deposit thin films from a monolayer (usually a few angstroms (A)) to 10 microns (preferably to 5 microns), depending on the above settings and conditions. The plasma technique itself typically only impacts the uppermost surface of the PCB and is typically fully compatible with the PCB itself, causing no damage or other unwanted effects. An advantage of plasma coating techniques is that the coating deposited accesses all surfaces of the PCB, and thus vertical surfaces such as those only accessible through holes in the PCB and any overhangs will also be covered. Thereby, protection can be provided along the sides, edges and the points/areas at which the conductive tracks join the PCB substrate.

Thus, in one embodiment of the invention all the surfaces of the printed circuit board are completely or substantially encapsulated with the polymer coating. This provides good protection and, in particular, complete or substantial encapsulation stops or reduces aqueous absorption and wetting' of the PCB itself. This may significantly reduce any corrosive action from within the PCB substrate, or under or adjacent to the tracks. For example, it is known that epoxy based PCBs (and paper/card PCBs) will absorb water 1.

(and aqueous acids and corrosive materials) and are vulnerable to corrosion by this in-situ mechanism.

In a variant of the plasma process, the present invention provides in situ cleaning of the surface of the PCB prior to plasma deposition using an active gas plasma. In this variant, an active gas plasma is used typically in the same chamber for PCB cleaning ahead of introduction of the precursor compound for the plasma deposition stage. The active gas plasma is based on a stable gas, such as hydrogen, oxygen, nitrogen, argon, methane, ethane, other hydrocarbons, tetrafluoromethane (CF4), hexafluoroethane (C2F6), tetrachloromethane (Cd4), other fluorinated or chlorinated hydrocarbons, other rare gases, or a mixture thereof. In one particular embodiment, the PCB could be cleaned by the same material as to be deposited. For example, a fluorinated or chlorinated hydrocarbon such as tetrafluoromethane (CF4) or hexafluoroethane (C2F6) or hexafluoropropylene (C3F6) or octafluoropropane (C3F8) could be used in the plasma iS method both to clean the surface of the PCB and lay down a layer of a halo-hydrocarbon polymer and/or a layer of metal fluoride (or chloride).

In another further variant of the plasma process, it is possible to vary the chemical content of the precursor gases used in the plasma treatment process. In this way, it is possible to build-up the following types of surface coatings: -multi-layer structures comprising layers of discrete chemical composition; and -graded multi-layers, where individual layers fuse together with varying intermediate composition.

These coatings can be designed and engineered to offer varying, customised functional performance, optimising for example coating conductivity, oxidation resistance, environmental protection, cost and other physical and chemical properties. For example, a thick coating that is highly fluorinated may be required for very good environmental in certain embodiments, whilst in others a thin layer comprising less halide may be preferred. An example of the application of a multilayer coating would be a layer of 1.

PTFE and a layer of PCTFE, where the PCTFE give improved oxidation prevention and the PTFE layer give improved environmental protection.

In another embodiment, where the different layers are applied to different surface areas of a PCB, it would be possible to apply for example (a) PVDF in certain areas to enable its piezo-electric or electrorestrictive properties to be used in those areas, and (b) another halo-hydrocarbon polymer on the intervening areas to provide an electrically insulating barrier.

Another variant of the plasma manufacturing process, is to use selective area coating and deposition techniques to build-up more complex 3-D PCB coatings, offering different physical and chemical properties in different 3-D areas of the PCB. Possible methods that can be used to provide selective area plasma deposition for individual or multiple layers include: -masking the surface of the PCB to deposit plasma coatings only in non-masked areas; -using laser ablation techniques to remove the coating in selective areas; -using UV lightlmask ablation techniques to remove the coating in selective areas; -using post-treatment liquid based etching solutions to remove the coating in selective areas; -using photo-assisted plasma deposition techniques (e.g. laser or UV light assisted) to deposit the coating in selective areas e.g. photo assisted plasma deposition; and -using metallo-organic chemical vapour deposition (MOCVD) type precursor chemicals, such as metal-alkyls and carbonyl precursors for example.

In a further embodiment of the invention, the wetting characteristics of the surface of the coating on the PCB could be modified by one of the following techniques: -Plasma etching. This uses reactive gas plasmas, such as carbon tetrafluoride (CF4). 1.

-Plasma activation. This uses plasma gases selected to provide the desired surface activation, for example hydrogen, oxygen, argon, nitrogen gas plasmas, or gas plasmas based on mixtures of the above gases.

-Plasma polymerisation and coating. This use variants or mixtures of halo-hydrocarbons, for example fluro-hydrocarbons and chioro-hydrocarbons, or alternatively non halo-containing hydrocarbons e.g. polyethylene or polypropylene.

-Liquid Based Chemical Etching. This technique modifies the surface activation and surface roughness of the coating using, for example, strong acids such as sulphuric acid, nitric acid or oxidizing agents such as hydrogen peroxide.

Wetting means either (a) the wetting of the surface of the coated PCB by liquids such as water, or (b) wetting of the coating by solder aridlor flux during the soldering process, or (c) the wetting of the copper tracks and pads by the solder after the coating has been removed. Either of these types of wetting can be controlled as described.

A connection to a printed circuit boards of the invention can be made by applying solder and optionally flux to the printed circuit board at a temperature and for a time such that the solder bonds to the metal and the composition is locally dispersed andlor absorbed and/or vaporised andlor dissolved and/or reacted. The action of flux and increased temperature alone will generally interact with the halo-hydrocarbon polymer to remove the coating locally from the area of the PCB to which the flux is applied. The temperature is typically 200°C to 300°C, preferably 240°C to 280°C, and most preferably 260°C. In one embodiment, the halo-hydrocarbon polymer may be dissolved and/or absorbed and/or degraded by the flux. We have found that there is often a balance between the time and/or temperature required and the acidity or other aggressiveness of the flux. Thus, milder fluxes may suffice if higher temperatures are used, and vice versa. In another embodiment, we can take advantage of the self fluxing action of copper fluoride at the copper surface and any decomposition of the polymer coating to release fluorine and/or HF to initiate fluxing (self fluxing). In the extreme, we have found that in certain cases the invention may dispense with a flux if a sufficiently high temperature is used and so localised heating could be applied. Surprisingly, the composition is generally only 1.

removed specifically from the area where solder and/or flux is applied, and therefore the composition remains attached to the surface of the PCB right up until the solder joint.

This provides advantageous environmental protection of the conductive tracks of the PCB right up to the solder joint.

The present invention enables the formation of good solder joints on the conductive tracks of a substrate or printed circuit boards coated with the polymer coating. A number of factors need to be controlled to achieve good quality strong solder joints. These include: -The wetting characteristics of the coated substrate or printed circuit board.

-The surface roughness of the coated substrate or printed circuit board.

-The surface roughness of the underlying copper substrate of the coated substrate or printed circuit board.

-The surface preparation, pre-treatment and cleanliness of the underlying copper substrate of the coated substrate or printed circuit board. This may include surface treatments such as sulphuric acid! hydrogen peroxide liquid treatment and gas plasma treatments.

-The composition of the solder paste including controlling the composition of the solder metals, active agents and solvents.

-The temperature profile of the solder process. Optimising profile temperatures and residence times, can improve wetting performance of the solder paste and active components.

-The size and geometry of the pads and conductive tracks on the coated substrate or printed circuit board.

-Soldering conditions including aperture sizes and the thicknesses of the solder paste stencil, to control the quantities, position, wetting and spread of the solder paste dispensed on the pads and conductive tracks on the coated substrate or printed circuit board.

-The particle size of components present in the solder paste. 1.

-The viscosity and surface tension of the solder paste with temperature, to control the wetting and flow on contact pads and conductive tracks.

-The viscosity and surface tension of the solder paste with temperature, to control the capillary action caused by electronic components present on contact pads and conductive tracks which will tend to act to displace the solder paste from its desired location. This is particularly important for Fine Pitch and Ball Grid Array (BGA) soldering.

-Controlling the composition and/or chemical stability and/or thickness of the halo-hydrocarbon coating in order for the solder paste to selectively remove or displace the coating on the surface of the substrate or printed circuit board.

-The chemical action of the active component in the solder paste with the halo-hydrocarbon coating to facilitate its selective removal or displacement. The quantity and composition of the active components can be optimised to facilitate this action.

Thus, in one embodiment of the invention, one or more of the above factors is optimised to good quality strong solder joints.

The flux used in the invention could be a resin/rosin flux, an organic flux, an inorganic flux, a halide free flux, a no-clean flux, a no-residue flux or a low solids flux. A resin/rosin flux could for example be a synthetic resin or a natural rosin. An organic flux could for example be: an organic acid such as lactic acid or an acrylic acids; an organic salt such as dimethylammonium chloride (DMA HCI); or an organic amine such as urea.

An inorganic flux could for example be: an inorganic salt such as zinc chloride, sodium chloride, potassium chloride or sodium fluoride; or an inorganic acid such as hydrochloric acid or nitric acid. An example of a no-clean flux is a rosin flux. Other fluxes used more widely for industrial applications such as general soldering, brazing and welding, or to clean or etch a metal surface (for example borax) could also be used in the present invention. The flux used in this method is typically a mild flux such as a "no-clean" flux that does not require a subsequent step of cleaning the PCB. The flux may optionally be part of a soldering paste. The choice of flux may depend on the nature of the coating, particularly the thickness and composition of the coating. A thicker more resistive coating may require use of a more aggressive flux. A composition comprising 1.

the active ingredient or ingredients of flux that remove the composition from the board could also be used in the present invention in place of flux.

A further feature of the ha1ohydrocarbon coating is that it provides flame retardant properties to the PCB. It is thereby possible to reduce or eliminate the amount of bromine based compounds used in the manufacture of PCBs.

Claims

CLAIMSI. A printed circuit board to which a solder connection is to be made, the surface of said printed circuit board having a multi-layer coating comprising one or more polymers, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and the thickness of the multi-layer coating is from I nm to pm.
2. A printed circuit board according to claim I wherein there is no solder, or essentially no solder, between said coating first coating composition and the conductive tracks of said printed circuit board.
3. A printed circuit board according to claim 1 or 2 wherein multi-layer coating comprises one or more layers of discrete polymers.
4. A printed circuit board according to claim I or 2 wherein the multi-layer coating comprises, graded layers of different polymers.
5. A printed circuit board according to any one of the preceding claims wherein the multi-layer coating comprises two or more layers.
6. A printed circuit board according to any one of the preceding claims wherein the first layer, which is contact with the surface of the printed circuit board, comprises a non-halo-hydrocarbon polymer.
7. A printed circuit board according to any one of the preceding claims wherein there is no or essentially no, metal halide layer on the surface of the printed circuit board.
8. A method of making a connection to a printed circuit board having a multi-layer coating comprising one or more polymers, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and the thickness of the multi-layer coating is from 1 nm to 10 jim, which method comprises applying solder, and optionally flux, to the printed circuit board at a temperature and for a time such that the solder bonds to the metal and the composition is locally dispersed and/or absorbed andlor vaporised.
9. A method of modifying the wetting characteristics of a coating comprising one or more halo-hydrocarbon polymers on a printed circuit board by plasma etching, plasma activation, plasma polymerisation and coating, and/or liquid based chemical etching.
10. A method of modifying the wetting characteristics of a multilayer coating as defined in any one of claims 1 to 7 by plasma etching, plasma activation, plasma polymerisation and coating, and/or liquid based chemical etching.
11. A printed circuit board comprising a substrate and conductive tracks, wherein the surfaces of said printed circuit board are completely or substantially encapsulated with either (a) a coating of a composition comprising one or more halo-hydrocarbon polymers, or (b) a multi-layer coating as defined in any one of claims 1 to 7, at a thickness of 1 nm to 10 pm.
12. A printed circuit board according to claim 11, wherein the substrate comprises a material that absorbs water or solvent based chemicals.
13. A printed circuit board according to claim 12, wherein the substrate comprises epoxy resin bonded glass fabrics, synthetic resin bonded paper, phenolic cotton paper, cotton paper, epoxy, paper, cardboard, textiles, or natural or synthetic wood based materials.
14. A method of preparing a printed circuit board, which comprises (a) providing a printed circuit board having an environmentally exposed surface, (b) cleaning the surface in a plasma chamber, using gases such as hydrogen, argon or nitrogen, and (c) applying to the surface a thickness of I nm to 10 ini of a composition comprising a halo-hydrocarbon polymer by plasma deposition, said coating optionally following the 3D form of the printed circuit board.
15. A method of preparing a printed circuit board, which comprises (a) providing a printed circuit board having an environmentally exposed surface, (b) cleaning the surface in an plasma chamber, using gases such as hydrogen, argon or nitrogen, (c) applying to the surface a thickness of 1 nm to 10 im of a multi-layer coating comprising one or more polymers by plasma deposition, wherein the polymers are selected from halo-hydrocarbon polymers and non-halo-hydrocarbon polymers, and said multi-layer coating optionally following the 3D form of the printed circuit board.
16. Use of a composition comprising a halo-hydrocarbon polymer as a flame-retardant coating for printed circuit boards.
17. A method according to claim 8 wherein one or more of(a) the substrate characteristics, (b) the coating characteristics, (c) the solder/flux characteristics, (d) the soldering profile, including time and temperature, (d) the process to disperse the coating, or (e) the process to control solder flow around the joint, are selected such that there is good solder flow, solder covers the substrate (typically a conductive track or pad) on the printed circuit board and a strong solder joint is generated.