CA2203171A1 - Process for coating electrically non-conducting surfaces with connected metal structures - Google Patents

Abstract

According to the present invention, sharply defined metal structures can be produced on electrically non-conducting surfaces without the use of etching processes by using a process involving the following essential process steps: application of a catalyst suitable for currentless metal deposition; subsequent formation of interconnected structures on the surfaces using mask technology; currentless deposition of an initial thin metal layer onto catalytically coated surface regions which have been exposed following and by the action of structuring processes; electrolytic deposition of a second metal layer on the first metal layer which forms interconnected structures.

Description

Process For Coating Electrically Non-Conducting Surfaces with Connected Metal Structures Description The invention relates to a process for coating electrically non-conducting surfaces with metal structures.

Such processes have been known for a long time. In some processes, for example, metal coatings generated over an entire surface are structured on non-conducting surfaces.
These processes are used when metal structures are to be produced in order to enable electronic component groups to be conductively interconnected on carrier members.
This is used in particular for producing printed circuit boards for the electronics industry. For this purpose normally a metal coating on entire surface is deposited on the outer sides and on the aperture walls produced by drilling in the carrier material. The metal coating on the entire surface is then covered by a photoresist. The resist is exposed through an appropriate mask and then developed. In the so-called positive process, the exposed resist layers are dissolved off, and in the .

negative process the non-exposed layers are dissolved off. Thereafter further metal is electrolytically deposited on the revealed metal surfaces. Then the remaining resist can be entirely removed from the metal surfaces. Conductor tracks are then formed in that the thin base metal coating between the structures is removed in an etching procedure.

The named processes however have disadvantages as, dependent on the thickness of the first metal coating applied to the entire surface, the lateral areas of the metal structures are attacked to a greater or lesser degree during etching, so that under certain circumstances, due to undercutting of the metal structures and the forming of galvanic elements between various metals and the entailed irregular attack on the lateral surfaces of the metal coatings, well-defined edges of the metal structures cannot be produced. On the other hand, solutions result during etching which are damaging to the environment, and are complex to reprocess. Moreover, complex devices and careful monitoring of the process are necessary.

There is disclosed in the publication E~ O 328 944 A2 a process for conditioning a non-conductive substrate for .

the subsequent selective deposition of a metal from a electroless metallising bath, the non-conductive substrate first having its surfaces roughened, and then being treated with a colloidal dispersion of palladium/tin particles, the palladium/tin particles being activated by contact with an alkali hydroxide solution, thereafter a permanent photosensitive layer is laminated on and this is then exposed with a conduc~or image overlay and developed. The disadvantage of this process resides in the fact that the metal structures produced can only be formed by electroless metal deposition. Because of this, an extremely lengthy treatment ln an electroless metallislng bath is necessary. Moreover, this also entails complex monitoring of the bath and measures incurring extraordinarily extensive and high costs for reprocessing of the resultant waste liquids. It has further become apparent that it is extremely difficult to avoid a situation in which, during electroless deposition, metal is not only deposited on the areas provided for the metal structures, but also on the resist surface. This phenomenon, known as wild growth, can lead to short--circuits between individual metal structures, for example conductor tracks.

.

A method is known from DE 35 10 982 A1 for producing electrically conductive structures on non-conductors, in which among other things firstly a copper layer is applied by glow discharge on the non-conductive surface, the non-conductive surface then being coated with a photoresist, which is thereafter exposed with a conductor image overlay and developed. In the last process step copper is deposited from a chemically reductive copper bath. In this case also the non-conductor requires to be treated for a long time in the chemically reductive copper bath, in order to achieve the desired coating thickness of the conductor tracks. It is further indicated that the first metallisation with copper is again removed, i.e. is etched off, after the resist film is removed.

There is disclosed in WO-A 88/03668 a process for ]~etallising a substrate in a predetermined pattern. For this purpose firstly a resist is applied with the pattern on the substrate surfaces in such a way that the areas of the substrate surface not covered by resist correspond to the pattern to be metallised. The substrate is then treated with a conditioning agent in order to increase the absorption of catalytically effective species for the following metallisation. Thereafter the substrate is .

treated with a further auxiliary agent, in order to reduce the absorption capacity of the resist surfaces for the catalytic species. Thereafter the surfaces are treated with a deactivating agent and are then catalytically activated and metallised in an electroless manner After metallisation, the resist is again removed from the substrate and further metal is deposited on the conductor track structures produced. The various treatment steps for activation and deactivation of the surfaces for the subsequent catalytic treatment serve to avoid wild growth on the resist surfaces without preventing electroless deposition at the required points.

This process is extraordinarily complex. In order to achieve sufficient coating thicknesses, in each case a total build-up of the metal structures by electroless deposition is necessary, so that in addition the disadvantages already mentioned above result.

There is disclosed in EP O 098 472 Bl a process for reducing faults in plated metal, in which a non-noble metal surface is coated with a coating of a noble metal, thereupon a photoconductive coating is applied and this latter is then exposed with the deslred pattern and developed. A metal is then deposited in an electroless .

manner in the areas which are exposed. This process serves to provide metal surfaces and electrically non-conductive surfaces with metal structures. In order to obtain metal structures which are electrically insulated ~rom one another, the basic metal coating over the entire surface upon which the metal structures have been produced, must be removed by an etching process at the points at which it is not required. Further, the metalstructures have to be produced exclusively by electroless deposition, for example in coating thicknesses between 25 ,um and 50 ~m. Thus the disadvantages mentioned above again arise.

l'here is described in US-~S 48 10 333 a process for clepositlng metal on the surface of a non-conductor, in which among other things a photoresist is formed on the conductive sulphide conversion layers, which are formed ~rom palladium/tin layers, formed on the non-conductive surfaces, said photoresist layer being exposed with a desired conductor image overlay and developed. Then, in the areas in the photoresist layer in which the conductive sulphide conversion layer is exposed, metal can be directly electrolytically deposited. In order to form metal structures, the conductive sulphide converslon layer however must be at least partly removed again in the surface areas in whlch the individual metal structures are to be separated in an insulating manner from one another, as otherwise short-circuit bridges would form between the metal structures. For this reason the process is suitable only for specific applications, preferably for the production of printed circuit boards in which copper layers are applied to the outer sides as a lamination. In this case the conductor track structures are formed by etching the copper coatings on the outer sides, so that the sulphide conversion layer formed on the copper coatings is at the same time removed .

3R-A 9105585 describes a process of producing payment cards for public telephone apparatus, in which a first conductive layer, preferably a nickel coating with a m~xl ml-m thickness of 0.3 um, having a higher resistance, i-s deposited over the entire surface by chemical methods, and thereafter a metal or alloy coating is deposited over the entire surface electrolytically with a thickness of 2 to 8 um, and with a clearly lower resistance and meLting point than that of the first coating, preferably a tin/lead coating, on a non-porous impermeable substrate.
~uring the electrolytic deposition, low current densities of 0.5 to 2 A/dm2 are to be used.

.

Therefore the problem underlying the present invention is to find a process by means of which the disadvantages of prior art can be removed, and with which it is possible to produce well-defined metal structures on the electrically non-conductive surfaces without using etchlng processes. A further problem resides in finding a process for producing means of irreversible information storage, preferably on electronic debit cards.

This problem is solved by claim 1. Preferred embodiments of the invention are given in the sub-claims.

The problem is in particular solved by a process with the following process steps:

- application of a catalyst suitable for electroless deposition of metals, - thereafter formation of structures interconnected by narrow webs on the surfaces by means of a masking technique, -- thereafter electroless deposition of a first thin metai ]ayer on the catalytically coated surface areas exposed in the previous step, - thereafter electrolytic deposition of a second metal coating on the first metal coating consisting of connected structures.

~y means of this process it is possible to produce well-defined metal structures on the surfaces, in particular only one first thin metal coating being deposited in an electroless manner and by virtue of the fact that by means of the masking technique, interconnected structures are produced and then the metal structures are substantially produced in the electrolytïc manner.
Problems of wild growth such as occur in electroless deposition of thick metal coatings in this case do not occur.

A substantial advantage of the process resides in the fact that no etching processes are required to form the metal structures.

Thus there are eliminated additional process steps, the necessity for complex monitoring of an etching process, the necessity for reprocessing used and carried-over solutions from the etching process, and the problem of undercutting and of irregular attack of the etching medium on the sides of the metal structures. Metal structures with weLl-defined edges can also thereby be formed.

A further advantage arises from the fact that no alkaline solutlons must be brought in contact ~or a long period with the resist coatings required for structuring. In order to be able to form sufficiently conductive structures, it would as a rule be conventional to deposit these from a copper bath containing an alkaline formaldehyde. If the entire coating thickness were formed in this manner, the resist layers would be attacked. Resist layers which can be developed and removed by alkalis can in no way be used. Instead of this, resist layers should be used which can be developed and removed with organic solvents. These however should be avoided where possible for reasons of technical problems involved with waste liquids and air pollution.

.

F'inally it is also substantially simpler to deposit metal coatings with the required metallic/physical properties from an electrolytic metallising bath than from an electroless metallising bath. Usually the metal coatings deposited from electrolytic baths are more ductile and substantially free of impurities. The speed of deposition from electrolytic baths is clearly higher than that from baths which deposit in an electroless manner.
Moreover, in the case of electroless baths there are considerable restrictions as regards the type of metals which can be deposited. Metals such as tin and lead can only be deposited in extremely thin coatings, and chromium not at all in an electroless manner. Precise maintenance of the alloy composition during deposition is scarcely practicably possible.

A preferred embodiment of the process according to the invention resides in the fact that the mask applied by means of masking techniques, for example a photoresist, is again removed after deposition of the second metallic ,-oating.

~s particularly suitable metals for a first metal coating, a nickel alloy coating is deposited in an ~lectroless manner, and as a second metal coating tin/lead alloy coating is deposited electrolytically;
these have the lowest possible melting points. For reasons of operational reliability, there are used for the second coating metals with melting points below 350C, for example lead. In the case of nickel-phosphorus alloys, the melting point reduces from above 1400''C for nickel to below 900C for the alloy.

.

In one embodlment, the first metal coating can be formed by reaction of the metal ions contained in an electroless metallising bath with a reduction agent which is adsorbed on the surface to be coated. In a preferred variant, the tin (II) ions, in the form of the tin (II) hydroxides adsorbed during catalysation of the surfaces with a palladium/tin catalyst, are used as a reduction agent, and by means of this reduction agent, copper is deposited by reduction of copper ions.

The metal structures are in particular produced on surfaces of acrylnitrile butadiene-styrol copolymers lABS), polyvinyl chlorides or epoxy resins. However, basically other materials may be coated by the process, for example all inorganic or organic substrates such as glasses, ceramics and polymers, cyanate esters, polyimides, polyamides, polypropylene, polycarbonate, polystyrol, polyacrylic and polymethacrylic acid and its esters, fluoropolymers and others.

The process serves above all for producing means of irreversible information storage, preferably of automated cards for electronic debit, as it represents a cost-effective alternative process for this application.

.

For this purpose metal structures are produced on an ABS
plate according to the pattern shown in Figure 1! In order to store and read the stored information on the automated cards, for example telephone cards, the metal structures are inductively reacted on the card During the storage/reading procedure, a set of primary coils is Located with one coil above each field 1 of the metal ~structure. In order to store information, a magnetic ~ield is induced in individual primary coils by an alternating current, and thus in turn an alternating current is induced in the individual fields 1. If individual fields 1 are short-circuited by the bridges 2, the current in the primary coils for generating the magnetic field increases. If no bridge 2 is present, then the current in the primary coils is low.

In order to alter the stored information the respective bridge 2 must be destroyed, so that the fields 1 are separated from one another. For this purpose the alternating voltage in the primary coil is simply briefly increased.

Normally, the non-conductor surfaces must be pre-treated with suitable means in order to ensure adequate adhesion between the metal structures and the surface. This may be substantially effected by cleaning and mechanical or chemical roughening of the surfaces. Thus the surfaces are as a rule hydrophilised, i.e. made wettable with water. Further, also chemically functional groups in the surface may be modified by controlled chemical reactions.
Aqueous solutions containing wetting agents are generally used as cleaning solutions. The cleaning effect is in particular reinforced by mechanical movement of the treatment solution. A particularly effective measure is the added use of ultrasound. Various processes may be used for chemical roughening of the surfaces, which are adapted to the respective materials of the non-conductors. In particular, solutions containing chromic acid, such for example as chromosulphuric acid, have proved particularly suitable for ABS polymers and epoxy resins. It is also possible to use alkaline permanganate solutions, the same in conjunction with organic swelling solutions, concentrated sulphuric acid and, in the case of polyamides, alkaline solutions of organic solvents.

After pre-treatment of the non-conductor surfaces, these are coated with a layer which is a catalyst for the electroless deposition of metals. Both ionic and colloidal catalyst systems are known. Usually palladium is used as a catalytically active metal. Copper is only .

of restricted suitability, as it only has a low catalytic activity. There are used as ionic catalysts for example solutions with palladium ions complexed with complex ligands. The colloidal systems contain palladium in the oxidation stage zero and as a protective colloid either an organic compound such for example as polyvinylalcohol or tin (II) hydroxide. The last-named catalysts are formed by combination of palladium chloride and tin (II) chloride in a hydrochloric acid solution with subsequent heating.

Thereafter the interconnected structures are produced on one or a plurality of catalysed surfaces of the substrate, which is for example plate-shaped, by means of a masking technique. This is effected either with screen printing inks or with photoresists. Photoresists operating in the positive process and in the negative ~rocess can be used. After application, for example by dipping or laminating, the photoresists are exposed through an image overlay, in order to form a latent image in the resist. This latent image is then developed in appropriate solutions. The areas corresponding to the later metal structures are dissolved from the non-conductor surface during the development process. In .

this way the surface with the catalytic layer located thereon is exposed.

Thereupon there is deposited a thin first metal coating, for example with a maximum thickness of 3 ~m, preferably 0.01 to 1.0 um, exclusively on the areas in which the catalytically active layer has been exposed. No metal is deposited on the other areas. Nickel, copper, gold, palladium and cobalt come into consideration as metals which may be deposited in an electroless manner. Baths are preferred which are not alkaline, as in this case photoresist materials can be used which may be developed and removed in alkaline solutions.

For this purpose baths are usually used from which the metal is deposited by reduction by means of a reducing agent contained in the bath. For example copper can be deposited from alkaline baths containing formaldehyde, but also from acidic or neutral baths containing hypophosphite. Copper baths containing boronhydride compounds, for example sodium boronhydride and dimethylaminoborane, are known. Nickel and cobalt and lheir alloys are preferably deposited from solutions containing hypophosphite. In this case phosphorus alloys result. If hydrazine is used as a reduction agent, the .

pure metals can be deposited. With boronhydride compounds, boron alloys of the metals are obtained.
Noble metals are preferably deposited from baths containing boronhydride. Solutions containing formic acid and formiates are also known for deposition of palladium. Preferably, alloys with lower melting points, for example nickel-phosphorus alloys in a thin layer, are used.

Another possible method of electroless deposition of metals resides in the fact that the reduction agent is adsorbed on the surface to be coated, and reacts with the metal ions of the electroless bath. For example the tin (II) ions, which are present on the surface as tin (II) hydroxides, contained in a palladium/tin catalyst, can be used for this purpose.

After production of a first thin conductive metal coating, this latter can be reinforced by further electrolytic metallisation. The required coating thickness of 3 to 50 ~m is achieved by electrolytic metal deposition. By virtue of the fact that in electroless metalisation no metal has been deposited on the resist surfaces, in the electrolytic bath, no metal either can be deposited at these points. The metal structures are formed rapidly and with good physical/metallic properties. Depositable metals are copper, nickel, tin, lead, gold, silver, palladium and other metals and alloys of these metals.

Thereafter the photoresist layer can be removed. For this purpose, depending on the type OL resist, an aqueous alkaline solution or a solution of organic solvents is used.

It is self-evident that further process steps can be applied between the individual process stages, such for example as rinsing procedures, cleaning, conditioning and etching steps and heating stages of the substrate.

The following examples serve to explain the invention.

.Ex~ple 1:

A plate-shaped ABS substrate with a thickness of 350 ~m and a surface area of 45 cm x 60 cm (18 inch x 24 inch) was pickled for 10 minutes in a solution of 360 g/l sulphuric acid and 360 g/l chromium (VI) oxide. After rinsing off excess acid, any residues of chromium(VI)ions still adhering were reduced by dipping for 1 minute in 2~

.

by weight of sodium hydrogen sulphide solution. The now cleaned, hydrophilic and micro-roughened substrate was dipped for 1 minute in 30% by weight of hydrochloric acid solution, rinsed, and thereafter placed for 3 minutes in a solution formed from 250 mg/l palladium chloride, 340 g/l tin (II) chloride and 250 g/l sodium chloride (palladium colloid with tin II hydroxide as protective colloid). Thereafter catalytically active palladium clusters were produced on the surface. Excess treatment solution was thereafter removed from the surface by rinsing. Then colloidal tin hydroxide formed was dissolved ~rom the substrate sur~ace by dipping ~or 2 minutes in a solution o~ organic acids (citric acid, oxalic acid). The substrate was again rinsed and dried.

This pre-treatment was followed by image transfer: the substrate was coated by lamination on both sides at 105C
~ith a commercially available photoresist (for example Laminar HG 1.5 MIL of Morton International GmbH, Dietzenbach, Germany) and exposed on one side through an appropriate mask (Figure 1) (45mJoule/cm2 at 365 nm).
After removal of the protective film from the photoresist the latter was developed in a conventional way with 1% b~
weight of sodium carbonate solution. The catalysed substrate surface now exposed was covered wlth a .

nickel/phosphorus alloy layer by dipplng in an electroless depositing nickel bath with hydrophosphite as a reduction agent at 45C within 3 minutes (pH value about 7). Thereafter nickel electrolyte was rinsed off. Then, within 8 minutes a coating of a tin/lead alloy was electrolytically deposited at a current density of 3 A/dm2 from a tinilead electrolyte containing methane sulphonic acid. The deposited coat thickness came to 8um.

The metal structures produced on one side of the ABS
plate accordlng to the pattern shown in Figure 1 serve for irreversible information storage. Information can be stored by destruction of the narrow tin/lead bridges 2 by inductive heating. The number of bridges destroyed and not yet destroyed can likewise be determined by induction measurement.

F.Y~mrle 2:

An ABS film 0.3 mm thick was firstly freed of dust and fingerprints within 2 minutes in an alkaline cleaner at 50C. After rinsing of residues of cleaner from the ABS
film surfaces, the film was swelled in an aqueous solution of ethylene glycol derivates (Sweller Covertron, Atotech Deutschland GmbH, Berlin, Germany) for 9 minutes .

at ambient temperature. The surfaces were then treated for 6 minutes in an aqueous solution of 140 g/l sodium permanganate and 50 g/l sodium hydroxide at 60C and thus hydrophilised and roughened. Manganese dioxide resulting on the surfaces was thereafter removed by reduction. An aqueous solution of hydrogen peroxide and sulphuric acid was used as a reduction agent.

Similarly to Example 1, the ABS ~ilm was then catalysed and dried (ambient air drying: 10 minutes at 50C).
Thereafter a photoresist (Riston 4615, T~ade Mark o~
Du~ont de Nemours, Inc., Wilmington, Delaware, USA;
laminating temperature 115C, laminating speed 1 m/minute) was laminated on both sides of the film. The resist was exposed through an appropriate image overlay (60mJoule/cm2 at 365 nm) and then developed as in Example 1.

Similarly to the preceding ExampLe, the exposed catalysed ABS surfaces were then covered for 5 minutes by means of an electroless depositing nickel bath with hypophosphite as the reduction agent (Nichem 6100 AF, Atotech ~eutschland GmbH, Germany) at 45C and pH 7 with a nickel/phosphorus alloy layer. Then, also similarly to Example 1, a tin/lead alloy coating 8 ~m thick was .

deposited (treatment time 5 minutes, temperature 22C, current density 4 A/dm2).

E~ample 3:

A PVC film 0.25 mm thick was roughened in a brushing machine. The sur~ace was then freed of impurities by rinsing. Thereafter the surfaces were conditioned by dipping into an aqueous solution o~ a quaternised polyamine (concentration 6 g/l) ~or 5 minutes at 45C.
After short immersion in a pre-dipping solution containing 250 g/l sodium chloride and 7 ml/l concentrated hydrochloric acid, the surfaces were catalysed in a catalyst of colloidal palladium (concentration 250 g/l), tin (II) chloride (concentration 340 g/l) and sodium chloride (concentration 250 g/l) for 4 minutes at 45C. Following this treatment the surfaces were treated for 2 minutes in an aqueous solution c,ontaining 15 g/l tartaric acid, 4 g/l copper sulphate and 1 mol/l alkalihydroxide. Thus the tin compounds adsorbed on the surface were again removed. Copper was deposited in an electroless manner. Thereafter the surfaces were again rinsed and dried ~or 10 minutes at 45C.

.

Then a liquid positive resist (Ozatec PL 177, Trade Mark of Morton International GmbH, Dietzenbach) was applied by dipping to the surfaces (laminating speed 40 cm/min).
Thereafter the resist was dried for 10 minutes at 80C.
The resist was exposed with light with a wavelength of 365 nm and a power of 90 mJoule/cm2 through an appropriate image overlay, and developed as conventionally. The surfaces were then provided with a tin/lead coating in the way already described in E~amples 1 and 2.

Claims (6)

Claims

1. Means of irreversible information storage with securely adhering metal structures with well-defined edges from a first metal layer applied to a substrate with non-conductive surfaces of acrylnitrile-butadiene-styrol copolymer, polycarbonate, polystyrol, polyacrylic or polymethacrylic acid or their esters, and, applied thereon, a second alloy metal coating melting at temperatures below 350°C and forming eutectics, the metal structures substantially comprising fields located adjacent to one another and bridges short-circuiting these, obtainable by a process without the use of metal etching processes with the essential process steps:

- application of a catalyst suitable for electroless deposition of metals, from an aqueous solution, - thereafter formation of a pattern as an image of the interconnected structures on the surfaces by means of photoresists which may be developed and removed in an alkaline manner, - thereafter electroless deposition of the first thin metal layer on catalytically coated surface areas exposed after and by structuring by means of a deposition bath with a pH value in the neutral or acidic range, - thereafter electrolytic deposition of the second metal coating on the first metal coating.

2. Means of irreversible information storage according to claim 1, characterised in that the photoresists are again removed after deposition of the second metal coating.

3. Means of irreversible information storage according to one of the preceding claims, characterised in that a coating containing nickel is deposited as the first metal coating.

4. Means of irreversible information storage according to one of the preceding claims, characterised in that a tin/lead alloy coating is deposited as a second metal coating.

5. Means of irreversible information storage according to one of the preceding claims, characterised in that the reduction agent required for deposition of the first metal coating is adsorbed on the surfaces.

6. Process for producing means for irreversible information storage, preferably of automated cards for electronic debit, according to one of the preceding claims.