GB2462822A

GB2462822A - Wire bonding

Info

Publication number: GB2462822A
Application number: GB0815094A
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
Anticipated expiration: 2028-08-18
Also published as: GB2462822B; GB0815094D0

Abstract

A method of snaking a connection between a wire and a contact pad using a wire bonding technique, wherein the wire and/or the substrate is/are coated with a composition that comprises one or more halo-hydrocarbon polymers at a thickness of from 1 nm to 2 pm. The coating may also comprise one or more monolayers of metal halide. The coatings allow bonding using metallic compositions which are susceptible to corrosion in non-inert environments containing oxygen. The removal of the coating material prior to bonding is optional. The bonding method is applicable to the formation ball/wedge and wedge/wedge bonds.

Description

S

WIRE BONDING

The present invention relates to a new wire bonding technique.

In general, wire bonding relates to a technique for joining electrical components in the absence of solder andlor flux. More specifically, wire bonding can be used to make an electrical connection between two or more electrical contacts using a conductive wire.

Thus, wire bonding is a method of making interconnections, typically between an integrated circuit in bare die form and the leadframe inside the integrated circuit or between the bare die and a PCB. The geometry of the wire may be of several different forms with typical cross sectional forms being circular or rectangular. Rectangular wires may also be known as ribbons. The wire used has traditionally been gold or aluminum but more recently there has been a considerable interest in using copper wire for a number of reasons including the significant cost differential with gold. In wire bonding, two jointing methods are commonly used, wedge bonding and ball bonding, both of which use different combinations of heat, pressure, and ultrasonic energy to make a weld at either or both ends of the wire. The terms "ball" and "wedge" refer to the geometry of the wire at the point where the connection is made.

The first step of ball/wedge bonding involves formation of a bond between a spherical ball at the end of the wire and a substrate, typically an electronic component. Bond formation is achieved using thermal and ultrasonic energy. This bond is known as the ball bond. The wire bonding apparatus then forms a loop with the wire of desired height and shape, until in the correct position for formation of a second bond to a substrate, or another component or structure. This second bond could be a wedge bond and is typically made to a conductive track or pad on a circuit board. Once the second bond has formed, the wire bonding apparatus normally cuts the wire to leave a free end that can be formed into a spherical ball, which can be used to form the next wire bond.

Wedge/wedge bonding involves formation of two wedge bonds, as described above, with a loop of wire connecting the bonds.

S

The wire used in wire bonding techniques can typically be gold, aluminium or copper, although other metals are also used for certain applications. Each material requires different wire bonding parameters and equipment. To achieve a good bond both the wire and the substrate, for examples a conductive track, pad or contact, to which it is bonded must be free of contaminants, including oxidation products. For example, copper is readily oxidized under normal atmospheric conditions, with the layers of copper oxide on the surface of the copper wire and/or substrate making formation of wire bonds difficult.

A further problem is that the elevated temperatures required for wire bonding, lead to increased oxidation.

Thus, in the past wire bonding methods have either not used copper wire, or have required the use of inert atmospheres to prevent oxidation. It has also been necessary to clean the copper wires immediately before wire bonding, to remove build up of copper oxide and other tarnishes on the surface of the copper. Cleaning of the wire and use of inert atmospheres introduce additional complications and expense to the wire bonding process, with the result that copper has not been commonly used in wire bonding.

The present inventors have found that coatings of compositions that comprise one or more halo-hydrocarbon polymers at a thickness of 1 nm to 2 tm on a wire and/or a substrate can prevent oxidation of the wire/substrate, whilst allowing formation of a good wire bond without the prior removal of the coating.

Thus, the present invention relates to a method of making a connection between a wire and a substrate using a wire bonding technique, wherein the wire and/or the substrate is/are coated with a composition that comprises one or more halo-hydrocarbon polymers at a thickness of from 1 nm to 2 jim.

The halo-hydrocarbon coating is preferably on both the substrate and the wire, but it may be only on the substrate or on only the wire. When the coating is only on one of either the substrate or the wire, it is preferably only on the substrate. The coating may only be on the areas of the surface of the wire and/or substrate where in the wire bond will be formed. Alternatively, the coating may be over all or substantially all of the wire and/or substrate.

In one embodiment, strong wedge bonds may be achieved by providing a substrate surface that is coated with a composition that comprises one or more halo-hydrocarbon polymers, so that the surface of the substrate is substantially free of oxide and/or other contaminants. The surface is also typically flat on a macro scale. Thus, the surface of the substrate has been protected by coating it with a composition comprising a halo-hydrocarbon polymer. The wire may also be coated with a composition comprising a halo-hydrocarbon polymer.

In another embodiment of the invention, the halo-hydrocarbon coating on the surface of the substrate is displaced by the action and processes of wedge or ball bonding. In these bonding methods, energy is effectively coupled into the region of the bond, which facilitates the displacement of the coating on the substrate and enables formation of the bond itself. Wedge bonding relies on a combination of ultrasonic, frictional energy, with or without a contribution of additional thermal energy introduced by heating the wire, in order to achieve the bonding process. In contrast, ball bonding is a primarily a thermosonic process. For both wire bonding methods the halo-hydrocarbon coating can be displaced selectively in the region of the bond by a combination of frictional and thermal actions, resulting in the coating being displaced as either a solid material or by a phase change or by vaporisation.

In another embodiment of the invention, the halo-hydrocarbon coating on the surface of the wire is displaced by the action and processes of wedge or ball bonding. For both wedge and ball wire bonding methods the halo-hydrocarbon coating on the end of the wire can be displaced selectively in the region of the bond by a combination of frictional and thermal actions, resulting in the coating being displaced as either a solid material or by a phase change or by vaporisation. Similarly, the coating can be on the substrate and the wire.

In another embodiment of the invention, the halo-hydrocarbon coating on the surface of the substrate or wire can be removed selectively prior to the wire bonding process. This can be achieved by heating the substrate or wire in the region selected for bonding, or by laser ablation, or by plasma processing, or by a liquid chemical etching process in the area selected for bonding.

Alternatively the halo-hydrocarbon coating can be removed across the whole area of the substrate or wire prior to wire bonding and the protective halo-hydrocarbon coating can be replaced after the wire bond is formed. The coating can be removed from the general area of the substrate or wire by heating the substrate, or by laser ablation, or by plasma processing, or by a liquid chemical etching process.

In another embodiment the coating can be applied after wire bonding, either onto a clean substrate or a pre-coated substrate. Such a process might be considered for example, where long term stability is required with the option of subsequent processing or rework at a later time.

In another embodiment of the invention, strong ball bonds may be achieved by modifying the surface roughness of the coating and/or substrate aridlor wire used for bonding, to provide optimal combinations for various bonding applications. For example, the surface roughness of the wire or substrate may be modified prior to application of the coating.

The surface roughness of the coating (where present) may then also be modified.

The surface can be further controlled by controlling the macro (more than I tim) and micro (less than 1.tm) surface roughness of the coating, substrate and/or wire (in effect controlling their contact area and frictional characteristics) as well as the surface flatness of the coating, substrate and/or wire to couple energy into the bonding process in an efficient manner for type of bond required. This allows formation of strong ball bonds between the wire and the substrate surface.

The frictional characteristics of the coating, substrate or wire can be modified by gas plasma treatment, by liquid/acid etching, or by selection of appropriate precursor materials for the coating, or by mechanical treatment, or by combination of any of these techniques.

The bonds formed by the wire bonding method of the present invention preferably exhibit strong bond strengths. Preferably the bond strength exceeds the failure strength of the wire. A bond strength of 5 g to 12 g force to break the wire bond for a 25 i.tm wire is preferred. A bond strength of 7 g to 12 g is still more preferable. The strength of the bond may also be further improved by gas plasma cleaning prior to coating to provide a "super clean" surface. Activation and cleaning of the surface by gas plasma treatment can typically provide stronger joints.

The halo-hydrocarbon composition may be identical or substantially identical on both the wire and the substrate. Alternatively, different halo-hydrocarbon compositions may be used to coat the wire and the substrate.

The coating may also provide longer-term advantages. In particular, the coating provides long-term environmental stability, but may also be selectively removed/displaced in the area of the wire bond. The coating may be removed by the wire bonding method only in the local area of the bond, leaving the coating intact right up to the bond. Thus, in another embodiment the halo-hydrocarbon polymer coating remains intact everywhere except where the connection is made. This provides environmental protection of the substrate and/or wire up to the bond.

In a further embodiment of the invention, once the wire bond joint is formed, the substrate, wire and/or wire bond joint can be further protected by applying an additional overcoating of the halo-hydrocarbon coating.

The substrate is typically a conductive track, pad or contact on a printed circuit board.

The substrate may comprise any conductive material. Possible materials from which the substrate may be made are metals such as copper, gold, silver, aluminium or tin, or conductive polymers or conductive inks. The preferred material for the substrate is copper. Thus, in one preferred embodiment, the substrate will be copper conductive tracks on a printed circuit board.

The wire can be made of any conductive material, including conductive metals, conductive non-metallic materials or conductive polymer based materials.

More preferably, the wire comprises of a conductive metals such as gold, aluminium, copper, silver, nickel and palladium or an alloy of any of these materials and/or combinations of precious/rare metals. Other conductive metals that could be used include platinum, rhodium, iridium, tin, lead, germanium, antimony, bismuth, indium, gallium, cobalt, iron, manganese, chromium, vanadium, titanium, scandium, zirconium, molybdenum, tungsten, other transition metal elements, or an alloy of any of these materials and/or combinations of precious/rare metals. Gold, aluminium, copper and silver are the most preferred materials for the wire.

Metals or alloys that readily oxidise, for example, aluminium and copper benefit from the coating, since the coating prevents aluminium and copper oxidising or otherwise corroding, and therefore extends shelf life. Similarly, the coating also prevents silver tarnishing. Thus, the coating on the wire will extend shelf life where oxidation, tarnishing or corrosion can occur.

The cross section of the wire may have any shape, with circular or rectangular being preferred. When the wire is rectangular, it may be known as a ribbon. When the wire is circular, it typically has a diameter of 5 im to 1 mm. Preferably the diameter is in the range 10 jim to 200 jim and most preferably 15 jim to 75 jim. When the wire is rectangular, its dimensions have rectangular sides in the range 5 jim to I mm. Preferably the rectangular wire has sides of dimensions is in the range 10 jim to 200 jim and most preferably 20)tm to 75.1n1.

By polymer we include polymers formed in-situ from single or multiple monomers, linear, branched, grafted and crosslinked copolymers, oligomers, multipolymers, multimonomer 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 I nm to 2 jtm, 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 nm. In yet another embodiment, the coating is a monolayer (usually a few angstroms (A)). The coating may be in single or multiple layers.

However, the optimal thickness of the coating will depend on the properties that are required for the substrate or wire after the wire bonding has been carried out. For example, if very high environmental toughness is required (high corrosion and abrasion resistance), a thicker coating may be used. The thickness of the coating and the conditions for wire bonding can be optimised for a given requirement.

The halo-hydrocarbon coating may be continuous, substantially continuous 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 polymer contains less than S % heteroatoms as a proportion of the total number of atoms in the polymer.

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

The 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 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 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).

-PCTF'E (çolychlorotrifluoroethylene) and copolymers, oligomers, multipolyrners, 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 polychiorotrifluoroethylene) and copolymers, oligomers, multipolymers, multitnonomer 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 tetrafluoroethylerie), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), PFA (copolymer of tetrafluoroethylene and perfluorovinyl ether), PVDF (polymer of vinylidenefluoride), THV (copolymer of tetrafluoroethylene, hexafluoropropylene and vinylidenefluoride), PVDFHFP (copolymer of vinyl idene fluoride and hexafluoropropylene), MFA (copolymer of tetrafluoroethylene and perf]uoromethylvinylether), EFEP (copolymer of ethylene, tetrafJuoroethyl ene and hexafluoropropylene), HTE (copolymer of hexfluoropropylene, tetrafluoroethylene and ethylene) or copolymer of vinylidene fluoride and chiorotrifluoroethylene and other fluoroplastics.

Most preferably the polymer is a polytetrafluoroethylene (PTFE) type material, and in particular polytetrafluoroethylene (PTFE).

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 tthrough' the coating; chemical resistance to corrosive gases, liquids and salt solutions, particularly environmental pollutants; exhibit low or controlled surface energy and wettability'; to be stable inert material over the temperature range required for the device; have good mechanical properties, good mechanical abrasion resistance; and improved electrostatic protection.

The wire and/or substrate are typically coated with the halohydrocarbon shortly after manufacture, to prevent oxidation. The halo-hydrocarbon polymer coating may be applied 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 coating techniques such as liquid dipping, spray coating, spin coating and sol/gel techniques are further alternatives.

The preferred 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 preferred method of applying the halo-hydrocarbon polymer is plasma deposition, although all the other techniques mentioned above would also be applicable.

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. The substrates are coated in a vacuum chamber that generates a gas plasma comprising ioriised gaseous ions, electrons, atoms and neutral species. In this method, the wire or substrates are introduced into a vacuum chamber that is first pumped down to pressures typically in the range 10 to 10 mbar. A gas is then introduced into the vacuum chamber to generate a 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.

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 ionized/decomposed to generate a range of active species that will react at the surface of the wire or substrates, typically by a polymerisation process, to generate a thin halo-hydrocarbon coating. Preferred precursor compounds are perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes, fluoroalkenes, fluoroalkynes, fluoroch loroalkanes, fluorochioroalkenes, fluorochloroalkynes, or any other fluorinated and/or chlorinated organic material (such as fluorohydrocarbon s, fluorocarbons, chiorofluorohydrocarbons and chlorofluorocarbons). l0

In another aspect of the invention, the coating on the wire andlor substrate may comprise a very thin layer (for example 5nm or less) of metal halide (preferably a metal fluoride) directly in contact with the metal surface. In one embodiment, the metal halide layer may be a monolayer or substantially a monolayer, or a few monolayers, or comprise a metal halide zone of layers at the surface. Such a metal halide layer may be very robust and inert, and prevents formation of oxide layers or other tarnishes which prevent effective wire bonding. The metal halide layer may form when active species in the gas plasma react with the metal surface or may be enhanced using a higher concentration of fluorine species. The halo-hydrocarbon layer may then be deposited in combination with the metal halide layer. The two layers may be discrete, axially or spatially, or alternatively there may be a graded transition from metal halide to halo-hydrocarbon. It is possible that the metal halide layer protects the metal from oxidation, whilst the halo-hydrocarbon layer provides environmental protection from corrosive gases andlor liquids as well as oxidation protection. Furthermore, should the halo-hydrocarbon coating eventually be worn away by mechanical abrasion, the underlying metal fluoride layer will prevent oxidation build up, enabling contact to still be made.

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 contacts or electrodes; 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 angstronis (A)) to 2xm, depending on the above settings and conditions.

In a variant of the plasma process, it is possible to use the plasma method for in situ cleaning of the surface the wire or substrate prior to plasma deposition using an active gas plasma. In this variant, an active gas plasma is used typically in the same chamber for cleaning the electrodes or contacts 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 electrodes or contacts could be cleaned by the same material as to be deposited. For example, a fluorinated or chlorinated hydrocarbon such as tetrafluoromethane (CF4) or hexafluoroethane (C2F5) or hexafluoropropylene (C3F6) or octafluoropropane (C3F8) could be used in the plasma 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). This in situ cleaning aspect of the invention is particularly advantageous as the wire and/or substrate are cleaned of tarnish and other corrosive residues and then coated with a protective layer of halo-hydrocarbon polymer. Both aspects may be achieved in a single step, without removal from the plasma chamber.

The present invention also relates to use of a halo-hydrocarbon polymer to prevent oxidation and/or corrosion of a wire and/or a substrate prior to formation of a bond between the wire and the substrate by a wire bonding technique.

The present invention also relates to the use of a halo-hydrocarbon polymer to allow formation of a connection between a wire and a substrate under a non-inert atmosphere using a wire bonding technique. Thus, in contrast to prior art techniques, an inert atmosphere, such as nitrogen or argon is not necessary for the method of the present invention. A non-inert atmosphere is beneficial for cost and efficiency, but is not a prerequisite for the invention, since the invention will work well in an inert atmosphere.

The halo-hydrocarbon polymer forms a coating over the surface of the wire and/or the substrate, so that oxidation andlor corrosion of the wire caused by corrosive gases in the non-inert atmosphere is prevented. By non-inert atmosphere we mean any atmosphere containing gases which will corrode the substrate and/or wires during wire bonding.

Typically the non-inert atmosphere comprises oxygen.

S

Description of the drawings

Figure 1 shows wedge bonds of the invention formed between uncoated aluminium wire and a coated copper substrate.

Figure 2 shows wedge bonds of the invention formed between uncoated gold wires and a coated copper substrate.

Figure 3 shows wedge bonds of the invention formed between uncoated aluminium wires and a coated copper substrate.

Figure 4 shows a cross section through a wedge bond of the invention formed between an uncoated gold wire and a coated copper substrate.

Figure 5 shows a cross section through a wedge bond of the invention formed between an uncoated aluminium wire and a coated copper substrate.

Figure 6 shows wedge bonds of the invention formed between coated copper wire and a coated copper substrate.

Figure 7 shows a cross section through a wedge bond of the invention formed between a coated copper wire and a coated copper substrate.

Figure 8 shows ball bonds formed between uncoated gold wires and a coated copper substrate.

Figure 9 shows a cross section through a ball bond formed between an uncoated gold wire and a coated copper substrate.

S Examples

Example I -formation of wedge bonds These results were achieved using a Kullicke & Soffe 4523 wedge bonder and a Kullicke & Soffe BT22 pull strength tester.

Wire material (diameter m) Nominal coating thickness Average bond strength (g) (nm) Gold (25prn) -50 5.60 Gold (25jim) -80 8.46 Aluminium (251.im) -30 7.65 Aluminium (25j.tm) -50 10.87 Aluminium (25j.tm) -80 7.00 Copper (25p.m) -60 8.60 Copper (25jim) -40 6.65 The settings for manual wedge bonding on the K&S 4523 were as follows: -Electronics settings: Long time interval.

-First Bond: Power 2.20, Time 4.0, Force 3.0 = 60g -Second Bond: Power 2.20, Time 3.0, Force 3.0 60g.

Note that in all of the above pull strength tests, the point of eventual failure was the wire breaking rather than the bond failing. Thus the figures indicated in the table provide effectively a lower limit of average bond strengths.

In the wedge bond data shown for gold and aluminium wires, the wires were used as supplied. The copper wire used was coated with the halo-hydrocarbon coating. Prior to coating, the copper wire was pre-treated using a 2 minute hydrogen plasma process.

I

In the examples, shown the copper substrates used were copper contact pads on PCBs, which had been coated with the halo-hydrocarbon coating. Prior to coating, the copper substrates had been pre-treated with a liquid based sulphuric acid/hydrogen peroxide solution.

Example 2 -formation of ball bonds Ball bonded joints shown in Figures 8 and 9 were formed using gold wire bonded to copper substrates. The copper substrates had previously been coated with the halo-hydrocarbon coating.

In these ball bond cases the gold wires were used as supplied.

In the first case, the copper substrates used were copper contact pads on PCBs, which had been coated with the halo-hydrocarbon coating. Prior to coating, the copper substrates had been pre-treated with a liquid based sulphuric acid/hydrogen peroxide solution and allowed to dry, followed by a hydrogen plasma treatment. The halo-hydrocarbon coating was then deposited.

In the second case, the copper substrates used were copper contact pads on PCBs, which had been coated with the halo-hydrocarbon coating. Prior to coating, the copper substrates had been pre-treated with a liquid based sulphuric acid/hydrogen peroxide solution and allowed to dry. The halo-hydrocarbon coating was then deposited onto the treated copper substrate, which was post-treated with a hydrogen plasma.

Claims

CLAIMSI. A method of making a connection between a wire and a substrate using a wire bonding technique, wherein the wire andlor the substrate is/are coated with a composition that comprises one or more halo-hydrocarbon polymers at a thickness of from 1 nm to 2 pm.
2. A method according to claim 1, wherein the wire bonding technique is ball/wedge bonding.
3. A method according to claim 1, wherein the wire bonding technique is wedge/wedge bonding.
4. A method according to any one of the preceding claims wherein the wire comprises gold, aluminium, silver, copper, nickel or iron.
5. A method according to claim 5 wherein the wire comprises copper.
6. A method according to any one of the preceding claims wherein the substrate comprises copper, gold, silver, aluminium or tin, conductive polymers or conductive inks.
7. A method according to claim 6 wherein the substrate comprises copper.
8. A method according to any one of the preceding claims wherein only the wire is coated with a composition that comprises one or more halo-hydrocarbon polymers at a thickness of from 1 nm to 2 j.tm.
9. A method according to any one of claims ito 7 wherein only the substrate is coated with a composition that comprises one or more halo-hydrocarbon polymers at a thickness of from I nm to 2 pin.
10. A method according to any one of the preceding claims where in the coating is at a thickness of from lOnm to lOOnm.
11. A method according to any one of the preceding claims wherein the halo-hydrocarbon polymer is a fluoro-hydrocarbon.
12. A method according to any one of the preceding claims wherein the halo-hydrocarbon polymer coating remains intact after wire bonding except in the area where the connection is made.
13. A method according to any one of the preceding claims wherein the halo-hydrocarbon polymer coating is removed andlor dispersed by the action of the wire bonding process, without the coating being removed in a separate pre-processing step.
14. A method according to any preceding claim wherein an additional coating as set out in any of the preceding claims is applied after formation of the connection.
15. Use of a halo-hydrocarbon polymer to prevent oxidation andlor corrosion of a wire and/or a substrate prior to formation of a bond between the wire and the substrate by a wire bonding technique.
16. Use of a halo-hydrocarbon polymer to allow formation of a connection between a wire and a substrate under a non-inert atmosphere using a wire bonding technique.