Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/31—Coating with metals
  • C23C18/42—Coating with noble metals
  • C23C18/44—Coating with noble metals using reducing agents

Definitions

• the present invention relates to a liquor for use as an electroless gold deposition bath and to a process for depositing gold on a substrate using such a liquor.
• An electroless (autocatalytic) deposition system has two main advantages over conventional electroplating. It can deposit metal on electrically isolated or non-conducting substrates and it can deposit an even thickness of metal over a component, regardless of geometry. For some time the electronics industry has been searching for a truly autocatalytic gold deposition process. Most of those previously developed have proved unsuitable due to the instability of the solutions. A survey of previously proposed electroless gold deposition systems has been provided by H.O. Ali and I.R.A. Christie, "A review of electroless deposition processes", Gold Bull., 1984, 17 (4), pages 118-127 (the teaching in which article is incorporated herein by reference).
• autocatalytic deposition means that the metal already deposited on the workpiece acts as a catalyst for further deposition of the same metal from the solution onto the workpiece.
• galvanic deposition for example copper deposition from an acidic solution onto an iron substrate, the more noble metal (Cu) replaces the less noble metal (Fe) on the surface. However, once the surface is covered with copper the reaction stops.
• galvanic (immersion) gold deposition baths the more noble metal (Au) replaces less noble metal, in particular either copper or nickel, on the surface of the component, until it is covered with gold, whereupon the reaction ceases.
• the maximum thickness of gold deposited by this method is 0.1-0.2 ⁇ m.
• An autocatalytic bath will deposit more metal on the same metal substrate and, in theory, assuming that all the operating parameters are within their limits, will continue to deposit the desired metal to an unlimited thickness.
• metal ions are reduced to metal atoms by electrons provided by the oxidation of a reducing agent.
• the oxidation consists of two steps: (1) BH4 ⁇ + H2O ⁇ BH3OH ⁇ + H2 (2) BH3OH ⁇ + H2O ⁇ BO2 ⁇ + H2
• Electroless deposition baths are prone to spontaneous decomposition, which is the sudden precipitation of metal ions from the solution as metal particles (sometimes known as "plating up").
• stabilisers in particular alkali metal cyanides such as KCN.
• Metallic impurities in the bath may also cause this decomposition and so chelating agents are added to complex any dissolved metals. Accelerators are often added to increase deposition rates.
• electroless gold deposition systems have hitherto still suffered from the problems of instability, in particular a variable rate of deposition (typically varying from 0 to 3 ⁇ m per hour) and a tendency to precipitate gold in bulk suddenly. Furthermore, such systems cannot be replenished and, therefore, are usually operated to exhaustion or until precipitation occurs.
• an electroless system which is stable, replenishable and capable of depositing gold of high purity at an acceptable rate. Desirably, such a system should be suitable for wire bonding and should have a robustness approaching that of the electroless copper or nickel systems already available, in order to reduce the demand on operator skill.
• the present invention provides an aqueous liquor for use as an electroless gold deposition bath, comprising a source of gold and a reducing agent, which liquor also contains a reduction-stabilising agent selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures of any of these.
• a reduction-stabilising agent selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures of any of these.
• the present invention also provides a method for the electroless deposition of gold, onto a surface of a substrate, from an aqueous liquor comprising a source of gold and a reducing agent, wherein the liquor also contains a reduction-stabilising agent selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures of any of these.
• a reduction-stabilising agent selected from (a) a mixture of an alkali metal or ammonium ferrocyanide and an alkali metal or ammonium ferricyanide, (b) 1-H-tetrazole, (c) redox mediators and (d) mixtures of any of these.
• the present invention also provides an article having at least one surface onto which gold has been deposited by such a method.
• the aqueous liquor according to the present invention contains a source of gold in solution.
• any of the gold compounds, including salts and complexes, that have been used or proposed previously for use in electroless gold deposition (plating) solutions come into consideration, these including such compounds as MAu(CN)2, MAu(CN)4, MAu02 and MAu(0H)4 wherein M is ammonium or alkali metal, in particular potassium; M3Au(S03)2 wherein M is ammonium or alkali metal, in particular sodium; AuCN; and alkali metal gold imides, in particular potassium gold succinimide or potassium gold phthalimide.
• the alkali metal gold cyanides are particularly preferred, especially potassium gold cyanide, which may be represented by the formula KAu(CN)2.
• the gold compound may be formed in situ .
• any gold in the solution will, in effect, be present as a cyanide complex.
• Au III will tend to be converted into Au I in the solution.
• the liquor will have a gold concentration (expressed as elemental gold) of up to 10 g/l and it has been found that the concentration is desirably at least 0.25 g/l in order to achieve an acceptable deposition rate.
• gold concentrations as high as 3.5 g/l have been tested satisfactorily, the gold concentration is preferably maintained at a level of between 0.8 and 1 g/l in order to minimise the effects of dragout and to ensure a good distribution of the deposit.
• the aqueous liquor according to this invention also contains a reducing agent in solution.
• reducing agents have been proposed in the prior art for use in electroless gold plating solutions, including hypophosphites, formaldehyde, hydrazine and boron-based compounds such as borohydrides and amine boranes (eg. isopropyl amine borane, di- or tri-ethylamine borane and di- or tri-methylamine borane), although the amine boranes can be unpleasant to use and are expensive.
• the alkali metal borohydrides, especially potassium borohydride are preferred, these being particularly effective in conjunction with the alkali metal gold cyanides.
• the reducing agent will usually be present in an amount of 1 to 25 g/l, preferably 5 to 15 g/l.
• Potassium borohydride for example, may be used typically at a level of 5 to 21.6 g/l.
• the aqueous liquor also contains a reduction-stabilising agent, that is to say an agent that is intended to stabilise the reaction(s) whereby the gold is reduced in order to reduce or inhibit the tendency to random deposition varying from the very slow to the very sudden or even to the spontaneous precipitation of the gold.
• a reduction-stabilising agent that is to say an agent that is intended to stabilise the reaction(s) whereby the gold is reduced in order to reduce or inhibit the tendency to random deposition varying from the very slow to the very sudden or even to the spontaneous precipitation of the gold.
• the reduction-stabilising agent is generally employed in an amount of from 0.25 to 100 g/l, more usually 1 to 10 g/l.
• the aqueous liquor in certain preferred embodiments may contain in solution an alkali metal or ammonium ferricyanide and an alkali metal or ammonium ferrocyanide; the potassium compounds, K3Fe III (CN)6 and K4Fe II (CN)6, are preferred.
• the potassium compounds, K3Fe III (CN)6 and K4Fe II (CN)6, are preferred.
• 0.25 to 3 parts by weight of alkali metal or ammonium ferricyanide more preferably 0.4 to 2.5 parts by weight thereof, will be used per part by weight of alkali metal or ammonium ferrocyanide.
• potassium ferrocyanide and potassium ferricyanide for example, may be used typically at levels of, respectively, 1 to 50 g/l and 0.5-20 g/l.
• the aqueous liquor contains 1-H-tetrazole as a reduction-stabilising agent.
• 1-H-tetrazole - which may be used typically at levels of 1 to 10 g/l - may be used as the sole such agent, useful results have also been obtained using it in the presence of an alkali metal ferricyanide, especially K3Fe III (CN)6.
• Redox mediators oxidation-reduction mediators
• Redox mediators are known in biochemistry and cell biology as chemicals that promote transfer of electrons essential to the analysis of enzymes and tissues and have recently been proposed as additives in microbial fuel cells as they couple rich sources of electrons within micro organisms to an electrode (see J. Bennetto, "Microbes come to power", New Scientist, 16 April 1987, pages 36-39).
• Redox mediators which come into consideration are, for example, thionine (at a level, for example, of 1 to 5 g/l), resorufin (at a level, for example, of 1 to 5 g/l) and 1,10-phenanthroline (at a level, for example, of 0.25 to 1 g/l).
• ferrocyanide/ferricyanide mixtures may act as electron reservoirs thereby reducing or eliminating the tendency to spontaneous decomposition and thus may, like 1-H-tetrazole, act in a similar manner to the redox mediators in stabilising the gold-reduction (Au I ⁇ Au°) process.
• the pH of the aqueous liquor may be adjusted, as appropriate, for the deposition to proceed properly.
• an alkali metal borohydride as the reducing agent
• the pH of the liquor has not been found to be a critical parameter: as long as there are sufficient hydroxide ions for the formation of BH3OH ⁇ ions, the deposition reaction will proceed.
• an alkaline pH usually at least 11 and conveniently between 11 and 11.5, will in general be maintained when using such a borohydride, and may also be appropriate if a different reducing agent is employed.
• the liquor may contain alkali metal hydroxide, eg.
• sodium or potassium hydroxide which latter, for example, may be used typically at a level of 5 to 22.4 g/l
• a buffer salt may be used, such a buffer salt being selected, for example, from the alkali metal (especially sodium or potassium) orthophosphates, pyrophosphates, citrates, tartrates, borates and metaborates.
• an organic chelating agent which serves to complex or otherwise combine with metal ions present as impurities in the plating solution and thereby prevent their interference with the deposition or plating process.
• chelating agents are generally employed in an amount of from 0.1 to 100 g/l, preferably 1 to 20 g/l, and include ethylenediaminetetraacetic acid (EDTA) and the alkali metal salts thereof, diethylenetriaminepentaacetic acid and the alkali metal salts thereof, nitrilotriacetic acid and the alkali metal salts thereof, ethanolamine and triethanolamine.
• EDTA has been found to be particularly effective in complexing nickel in solution and may be used typically at a level of 1 to 10 g/l.
• Ethanolamine and triethanolamine may be used typically at levels of, respectively, 25-75 ml/l and 5-50 ml/l.
• the aqueous liquor according to the present invention will contain an alkali metal cyanide, such as sodium, potassium or lithium cyanide, in order to improve the stability of the source of gold, in particular when a gold cyanide complex is used.
• alkali metal cyanide such as sodium, potassium or lithium cyanide
• such cyanides are employed in an amount of 0.1 to 50 g/l, preferably 1 to 25 g/l.
• potassium cyanide is particularly preferred and may be used typically at a level of 5 to 22 g/l.
• glycine which may typically be used at a level of 1 to 10 g/l, acts as a chelating agent within the aqueous liquor.
• the aqueous liquor of the present invention may also contain an accelerator in order to improve the rate of deposition.
• an accelerator in order to improve the rate of deposition.
• water-soluble salts of semi-metals and metals of Groups IIIB, IVB and VB of the Periodic Table especially those elements in the 4th, 5th and 6th periods, such as thallium, lead or arsenic, may be used for this purpose, although in certain embodiments the presence of such an accelerator has increased the sensitivity of the bath to the presence of nickel contaminants.
• the concentration of the accelerator (expressed as elemental metal or semi-metal) will generally be from 0.001 to 500 mg/l, more usually 0.001 to 100 mg/l.
• Preferred accelerators are thallium (eg. as thallium sulfate), which can be used typically at a level of 0.001 to 7 mg/l (calculated on the elemental thallium), and lead (eg. as lead sulfate), which can be used typically at a level of 2 to 50 mg/l (calculated on the elemental lead).
• any of the specified components may be constituted by a mixture of two or more compounds of the appropriate description.
• the corresponding ammonium compounds may also come into consideration; however, the potassium compounds are usually the least expensive and the most readily available in high purity.
• the aqueous liquor of the present invention may be employed in conventional manner as an electroless gold deposition bath.
• the substrate to be plated will be immersed in the bath for a period of time sufficient to achieve a deposit of gold of the desired thickness.
• the substrate should present a catalytically active surface, especially a surface of a metal such as nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, kovar and permalloy.
• a metal such as nickel, cobalt, iron, steel, palladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, kovar and permalloy.
• nickel undercoat If a nickel undercoat is deemed necessary, it is recommended that nickel-boron type (ENi-B) electroless nickel deposits be used as the undercoat in preference to nickel-phosphorus type (ENi-P) electroless nickel deposits (since the present formulation will not deposit gold on the latter). If an ENi-P undercoat has to be used, it should be flashed with ENi-B prior to the electroless plating with gold.
• ENi-B nickel-boron type
• ENi-P nickel-phosphorus type
• a pre-dip containing potassium borohydride and potassium hydroxide (typically 5 g/l of each) has proven useful in accelerating the initiation of the gold deposition, protecting the gold bath from contamination and reducing the heat-sink effect of large components. Immersion of the substrate in the pre-dip for 1 minute at 80°C has been found to be suitable.
• Non-metallic substrates may be prepared for gold-plating in accordance with this invention by first rendering the surfaces thereof catalytically active, for example by the method described in US-A-3,589,916.
• the substrate is provided by an article that also comprises surfaces which do not require to be plated, it is possible to mask such surfaces in known manner, the masking material being removed after the plating step.
• Articles which may be gold-plated or gold-metallised in accordance with this invention include electronics components, especially those which have electrically isolated islands, pads and tracks, for example microwave components, chip carriers, printed circuit boards, integrated circuits and transistor headers.
• the gold deposition bath will normally be operated at a temperature of 80°C ⁇ 5°C. If the temperature exceeds 85°C, there may be a significant risk of spontaneous decomposition whereas below 75°C there is a significant reduction in the rate of gold deposition.
• the bath should be agitated, for example by magnetic stirring.
• the rate of stirring is from 200 to 600 rpm; higher rates of stirring may cause spontaneous decomposition, while lower rates of stirring will reduce the rate of gold deposition on the substrate.
• the components in the bath liquor may be replenished as appropriate.
• the source of gold will need to be replenished so as to maintain the concentration of gold at the required level.
• the gold compound added by way of replenishment need not be the same as the gold compound used in making up the initial liquor.
• Potassium borohydride decomposes rapidly at 80°C. Accordingly, when this reducing agent is used, it is desirable to analyse the bath every thirty minutes during operation and to replenish the potassium borohydride (the concentration of which typically decreases by 30% for each thirty minutes that the bath is at the working temperature), as appropriate.
• the accelerator if used, normally also requires to be replenished frequently.
• aqueous solutions were made up in a new, or at least scratch-free, beaker with carbon-treated demineralised water (the carbon treatment being effected in order to remove any colloidal polymer from the ion-exchange resin). Distilled water could have been used instead.
• a solution of the accelerator (if used), the reducing agent and a solution of the gold source were added, in that order and with stirring, to an aqueous liquor already containing the reduction-stabilising agent, chelating agent and auxiliary stabiliser(s). The solution was heated to 80°C and filtered if any particulate matter was visible therein.
• the exemplary formulations were investigated as electroless gold deposition baths using life tests on substrates provided (unless otherwise stated) as copper panels electroplated with pure gold to 2.5 ⁇ m and at a surface loading of 0.5 dm2/l, since at such a low loading any weaknesses in a system under test tend to be quickly revealed.
• the baths were operated at a temperature of 80°C ⁇ 4°C, with magnetic stirring with a PTFE coated stirrer at 400 rpm.
• the concentration of the reducing agent was analysed every 30 minutes and replenishment was effected with the required amount dissolved in the minimum quantity of carbon-treated demineralised water.
• the gold and other constituents were replenished after each 0.25 g of gold had been removed from solution. Filtration, through two glass-fibre filter papers under vacuum, was effected if particles of gold appeared on the bottom of the beaker or if the solution (initially an orange/yellow colour, turning to very pale yellow upon heating) turned light brown.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1 g/l potassium orthophosphate, K3PO4 12 g/l potassium cyanide, KCN 11 g/l potassium borohydride, KBH4 10.8 g/l monoethanolamine 50 ml/l potassium ferrocyanide 5 g/l potassium ferricyanide 2 g/l thallium (present as the sulfate) 2 mg/l
• the liquor was tested as an electroless gold-deposition bath and it was found that an acceptable rate of deposition (2.5-3.0 ⁇ m/h) could be achieved without precipitation (plating up) of gold into the liquor.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1 g/l potassium cyanide 11.0 g/l potassium hydroxide 11.2 g/l EDTA 5.0 g/l potassium borohydride 10.8 g/l potassium ferrocyanide 5.0 g/l potassium ferricyanide 2.0 g/l monoethanolamine 50 ml/l
• this liquor When tested, this liquor was found to provide a robust bath, which was resistant to nickel contamination.
• the deposition rates were, in general, about 1 ⁇ m/h. Initiation of deposition on nickel was found to take, in general, from 5 to 15 minutes; the contact with the nickel surface in this test did not cause spontaneous decomposition of the liquor.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1 g/l potassium cyanide 11.0 g/l potassium hydroxide 11.2 g/l EDTA 5.0 g/l potassium borohydride 10.8 g/l potassium ferrocyanide 5.0 g/l potassium ferricyanide 2.0 g/l triethanolamine 10.0 ml/l glycine 4.5 g/l
• this liquor provided a robust bath, resistant to nickel contamination.
• the gold turnover was between 150 and 200%, with a mean deposition rate of 2.5 ⁇ m/h. Initiation of the deposition on nickel was immediate; the contact with the nickel surface in this test did not cause spontaneous decomposition of the liquor.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1 g/l potassium cyanide 11 g/l potassium hydroxide 11.2 g/l potassium borohydride 10.8 g/l monoethanolamine 50.0 ml/l EDTA 5.0 g/l potassium ferrocyanide 5.0 g/l potassium ferricyanide 2.0 g/l thallium (present as the sulfate) 2.0 mg/l
• this aqueous liquor provided a robust, general purpose bath which proved particularly suitable for depositing gold on copper and copper alloys.
• the bath could normally be operated to 200-250% gold turnover, with a mean deposition rate of 2-2.5 ⁇ m/h, at a low loading of 0.5 dm2/l, the gold being replenished with further KAu(CN)2.
• Such replenishment was found to have no adverse effect on the free cyanide concentration.
• the potassium ferrocyanide, potassium ferricyanide, thallium and monoethanolamine were replenished as the bath was used, at rates of 1.25 g, 0.5 g, 0.5 mg and 2.5 ml respectively, per 0.25 g gold removed by deposition.
• the bath liquor did not spontaneously decompose when presented with a nickel surface. Initiation of the deposition on nickel was immediate.
• the bath liquor was, however, sensitive to soluble nickel contamination, it being found that a concentration of nickel of 10 mg/l could cause spontaneous decomposition. This sensitivity could be overcome by omitting the thallium from the solution.
• the deposits obtained from the baths of Examples 2, 3 and 4 were 99.9% pure gold (with 0.1% K) having a density of 19.3 g/cm3 and a hardness of between 90 and 95 HV.
• the deposits which were up to 30 ⁇ m in thickness were matt and lemon yellow (Examples 2 and 4) or orange-yellow (Example 3) in colour.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1.0 g/l potassium cyanide 11.0 g/l potassium hydroxide 11.2 g/l EDTA 5.0 g/l potassium borohydride 10.8 g/l monoethanolamine 50.0 ml/l thallium (present as the sulfate) 2.0 mg/l 1-H-tetrazole 1.0 g/l
• the liquor When tested as a gold plating bath, the liquor reached 188% gold turnover, with a mean deposition rate of 2.07 ⁇ m/h.
• the gold deposit was comparable in properties to those of the preceding Examples.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1.0 g/l potassium cyanide 11.0 g/l potassium hydroxide 11.2 g/l EDTA 5.0 g/l potassium borohydride 10.8 g/l monoethanolamine 50.0 ml/l potassium ferricyanide 2.0 g/l thallium (present as the sulfate) 2.0 mg/l 1-H-tetrazole 1.0 g/l
• the liquor When tested as a gold plating bath, the liquor reached 225% gold turnover, with a mean deposition rate of 2.05 ⁇ m/h.
• the gold deposit was comparable in properties to those of the preceding Examples.
• An aqueous liquor was prepared with the following formulation: gold (present as KAu(CN)2) 1.0 g/l potassium cyanide 11.0 g/l potassium hydroxide 11.2 g/l potassium borohydride 10.8 g/l

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• 1988
  • 1988-05-25 GB GB888812329A patent/GB8812329D0/en active Pending
• 1989
  • 1989-05-12 EP EP89304821A patent/EP0343816A1/fr not_active Withdrawn
  • 1989-05-22 JP JP1126824A patent/JPH0219473A/ja active Pending
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