WO2008130081A1

WO2008130081A1 - Manufacturing method of conductive ball using eletroless plating

Info

Publication number: WO2008130081A1
Application number: PCT/KR2007/004143
Authority: WO
Inventors: Won Il Son; Jeong Hee Jin; Seok Heon Oh
Original assignee: Hanwha Chemical Corporation
Priority date: 2007-04-23
Filing date: 2007-08-29
Publication date: 2008-10-30
Also published as: TW200900531A; KR20080095047A; KR100892301B1

Abstract

The present invention relates to an eletroless process for preparing plated powder having excellent conductivity. More specifically, the invention relates to a process for preparing conductive powder via formation of a conductive transition metal plated layer on the surface of resin powder as a core and then eletroless displacement gold plating, wherein, at the early stage of the displacement gold plating, a gold-plated layer is formed by displacement reaction under acidic condition, and then a certain amount of reductant is used under basic condition to plate gold, to form a minute and homogeneous gold-plate layer while inhibiting elusion of excess nickel. According to the process, the deviation of electric resistance between individual particles of conductive balls upon being connected to a microcircuit is small, thereby providing high reliability.

Description

[DESCRIPTION]

[invention Title]

MANUFACTURING METHOD OF CONDUCTIVE BALL USING ELETROLESS PLATING 5

[Technical Field]

The present invention relates to an eletroless process for preparing plated powder having excellent conductivity.

More specifically, the invention relates to a process for

0 preparing conductive powder via formation of a conductive transition metal plated layer on resin powder as a core and then eletroless displacement gold-plating, wherein transition metal plated powder is gold-plated by displacement reaction under acidic condition, and then reductant is added to carry

5 out deposition gold-plating under basic condition to form a minute and homogeneous gold plate layer while inhibiting elusion of excess nickel. According to the process, one particle of the conductive ball has excellent conductivity upon being connected to a microcircuit , with high reliability

!0 with small deviation of electric resistance between particles.

[Background Art]

Resin particulate materials provided with conductivity are widely used as a subsidiary material for preventing static

!5 electricity of an electronic device or parts thereof, absorption of electric wave, shielding of electric wave, or the like. Recently, plated particles are used as a conductive material for electrically connecting the micro-portions of electronic instruments including a connection of an electrode of a liquid crystal display panel to a circuit substrate of a LSI chip for operation, and a connection between the electrode terminals in a micro-pitch.

Conventional processes for preparing plated powders have included a process for physically coating metal particles on the surface of resin particulates (Japanese Patent Application No. 1994-267328), a process for incorporating finely- pulverized metallic protrusions on the surface of base particulates (Japanese Patent Application No. 2003-253465), or the like. Recently, processes for preparing plated powders by using eletroless plating are preferentially used (Japanese Patent Application Nos. 2004-238730, 2004-265831, 2003-197028, and the like) .

A conventional technique by using an eletroless plating process comprises plating nickel as a nickel plated layer having thin film of 80~120 nm of thickness (hereinafter, referred to as "primer plated layre") via eletroless plating process; followed by plating with gold by using a displacement plating process. According to the process, a complexing agent is used in order to facilitate dissolution reaction of nickel and to solubilize the dissoluted nickel ion in the plating liquid (Japanese Patent application Nos . 2003-277942, 2003- 147542 and 2003-293147). However, as the displacement gold- plating reaction proceeds, metallic layer of the thin primer plated layer may be destroyed by total or partial excessive elution, or minute pinholes may be generated, so that the plated layer may be easily peeled off from the resin powder, or peeling occurs between the resin powder and the plated layer, upon compressing the layer to a substrate or an electrode terminal, thereby resulting in problems of lowering the conductivity.

In order to overcome the problem, conventional technique tried to use an elution inhibitor so as to prevent excessive elution of nickel plated layer at the time of displacement plating reaction in an acidic pH range from 4 to 6 (Japanese Patent Application No. 2005-307309, 1993-222542, 1993-331655) . When using an elution inhibitor, gold-plating is inhibited so that a desired thickness cannot be readily obtained, and a uniform gold-plated layer can be hardly obtained. In addition, the amount of gold to be used increases to plate gold to a desired thickness, because of inhibition of elution and excessive elution, thereby increasing production cost. The conductive balls as conductive powder thus prepared may cause high connection resistance or a short circuit, upon being connected to a micro-circuit. Since an elution inhibitor contains a reductant, disadvantages such as deterioration of stability during long-term use of the plating liquid and difficulties in inhibiting micropores in the primer plated layer may occur. Thus, the conductive balls manufactured according to conventional techniques lack reliability with very large deviation of electric resistance between particles of conductive balls upon connection of a micro-circuit.

Recently, conductive powder having highly excellent conductivity is required along with rapid advancement of electronic instruments and miniaturization of electronic parts, and thus the minute wiring on a substrate or the like.

[Disclosure]

[Technical Problem]

The present inventors performed intensive studies to develop conductive powder having excellent conductivity, and as a result, they discovered a process wherein, at the early stage of the displacement gold plating, a gold-plated layer is formed by displacement reaction under acidic condition, and then a certain amount of reductant is used under basic condition to plate gold, to form a minute gold plate layer while inhibiting elusion of excess nickel, so that high reliability can be obtained by reduced deviation of electric resistance between individual particles of conductive balls upon being connected to a microcircuit . On the basis of the discovery, completed was the present invention. Thus, the object of the invention is to provide a process for preparing plated powder, which can response to a minute wiring and provide high conductivity performance without problems in electric capacity upon connection. In order to achieve the object described above, the invention provides an eletroless process for plating gold on metal plated powder, wherein destroy of coating of primer plated layer or generation of micropores can be inhibited by adjusting pH and adding a reductant.

[Technical Solution]

The invention relates to a process for plating gold by using displacement gold-plating solution. The invention was completed by discovering a process for preparing conductive powder via eletroless formation of a transition metal plated layer on the surface of core and then formation of a gold plate layer on the top of the transition metal layer, wherein elution of metal (employed for said transition metal) is inhibited and a minute and homogeneous gold plate layer is formed by virtue of adjusting pH of the displacement gold- plating solution.

More specifically, the invention comprises a process for preparing conductive powder to form a metal plate layer on resin powder as a core in eletroless plating liquid, which comprises the steps of a) forming a conductive transition metal layer on a surface of the core; b) performing displacement gold-plating by dispersing the conductive powder, on which the conductive transition metal layer has been formed, in displacement gold-plating solution; and c) adjusting the displacement gold-plating solution to basic condition and adding a reductant to form a reductive gold-plate layer on said displacement gold plate layer.

[Description of Drawings]

Fig. 1 shows the results of change in nickel and gold content during the gold-plating process according to Example 1 and Comparative Example 1, which were analyzed by titration as time goes by.

Fig. 2 is a photograph (x 1,000) of the surface of the plated powder manufactured according to Example 1 of the invention, which was taken by a scanning electron microscope (SEM) . Fig. 3 is a photograph (x 20,000) of the surface of the plated powder manufactured according to Example 1 of the invention, which was taken by a scanning electron microscope (SEM) .

Fig. 4 is a photograph (x 40,000) of the surface of the plated powder manufactured according to Example 1 of the invention, which was taken by a scanning electron microscope (SEM) .

Fig. 5 shows measurements of contact resistance of ten conductive particulates manufactured according to Example 1 of the invention, for individual compression pressures.

Fig. 6 is a magnified photograph of the surface of the plated powder manufactured according to Example 2 of the invention, which was taken by a scanning electron microscope (SEM) . Fig. 7 is a magnified photograph of the surface of the plated powder manufactured according to Comparative Example 1 of the invention, which was taken by a scanning electron microscope (SEM) .

Fig. 8 shows measurements of contact resistance of ten conductive particulates manufactured according to Example 1 of the invention, for individual compression pressures.

[Best Mode]

In the process according to the invention, the type of the resin powder to be used as a core is not particularly restricted. As the resin, used can be a resin or a mixture of two or more resins selected from the group consisting of polyolefins such as polyethylene, polyvinylchloride, polypropylene, polystyrene and polyisobutylene; olefin copolymers such as styrene-acrylonitrile copolymer and acrylonitrile-butadiene-styrene terpolymer; acrylic acid derivatives such as polyacrylate, polymethyl methacrylate and polyacrylamide; polyvinyl compounds such as polyvinyl acetate and polyvinyl alcohol; ether polymers such as polyacetal, polyethyleneglycol, polypropylene glycol and epoxy resin; amino compounds such as benzoguanamine, urea, thiourea, melamine, acetoguanamine, dicyanamide and aniline; aldehyde resins such as formaldehyde, palladium formaldehyde and acetaldehyde resin; polyurethane; and polyester. According to the invention, mean particle diameter of the resin powder is 0.5 ~ 1000 μm. If the mean particle diameter is less than 0.5 μm, the conductive powder may not contact with the face of the electrode to be joined, and ill contact may occur when there is a gap between the electrodes. On the other hand, if the diameter is more than 1000 μm, minute conductive joining cannot be achieved. Thus, the mean particle diameter is restricted to the range described above, more preferably from 1 to 100 μm, still more preferably from 2 to 20 μm, and most preferably from 3 to 10 μm. The aspect ratio of the resin powder according to the present invention is less than 2, more preferably less than 1.2, still more preferably 1.06. If the aspect ratio is more than 2, since the particle diameter is not uniform, a large amount of non-contacted particles may occur when the conductive particulates are contacted between the electrodes. Thus, the ratio is restricted to the range as mentioned above. The resin powder to be used has the coefficient of variation (Cv) value of not more than 30%, preferably not more than 20%, more preferably not more than 5%. If the Cv value exceeds 30%, since the particle diameter is not uniform, a large amount of non-contacted particles may occur when the conductive particulates are contacted between the electrodes. Thus, the coefficient is restricted to the range as mentioned above . In the present invention, the coefficient of variation (Cv) is defined by following Equation 1: [Equation 1] Cv(%) = ( O /Dn) x 100 wherein, σ is standard deviation of the particle diameter, and Dn is number average particle diameter.

The standard deviation and the number average particle diameter can be calculated by using an Accusizer model 780 from Particle Sizing Systems, Inc.

In the step of forming a transition metal plate layer on the surface of resin powder, a conductive transition metal selected from Au, Ag, Co, Cu, Ni, Pd, Pt and Sn, or an alloy thereof can be used. Plating in a multi-layer comprising the same or different metal (s) can be adopted. Preferably, the plate comprises Ni or Ni-Au multi-layer plate. The Ni plate layer comprises intimate cohesion with base resin particles, to form an electrolytic plate layer having good peeling resistance. In addition, it is easy to plate Au in a multilayer on the top of Ni plate, to ensure steady cohesion with the plate layer. When forming a Ni-Au multi-layer, the conductivity performance can be further enhanced as compared to a mono-layer plate. In the process for plating transition metal, wet plating or dry plating may be used on the surface of resin powder.

Wet plating processes are divided into electroplating and eletroless plating processes. Electroplating is a process wherein electrodes are put in a plating liquid comprising metal ions, and current is applied thereto, thereby inducing reductive deposition of metal ions on the surface of the substrate as a cathode to form a metal plate layer. Eletroless plating is a process to deposit metal ions in a state of aqueous solution without employing electricity, which can be widely applied to a substrate such as non- conductive resin. Eletroless plating processes are divided again into displacement plating and reductive plating processes.

In displacement plating, movement of electrons due to ionization tendency between the plating solution and the substrate provides plating. But the displacement reaction is ceased when the reaction proceeds to form a plate layer of a certain thickness, so that no more plate can be formed. This also deteriorates intimate cohesion.

Reductive plating processes are divided into non- catalytic chemical-reductive plating and self-catalytic chemical-reductive plating processes. Since the former does not have self-catalytic property, plating in a thick layer is limited. Further, the reaction simultaneously occurs in the substrate and in the plating solution, so that the solution cannot be reused. The latter is self-catalytic eletroless plating processes which are widely used in the industrial field at present, wherein metal in the plating solution is reductively deposited by the action of a reductant .

Dry plating processes, being compared to wet plating processes, broadly include hot dip coating, thermal spray, vapor deposition, or the like, but generally refer to vapor deposition. Vapor deposition processes, wherein metal or compound is vapor-deposited in vacuo, include CVD (chemical vapor deposition) and PVD (physical vapor deposition) processes. Vapor deposition process is advantageous in that most of metals or metal compounds (including Al, Ti, or the like that cannot be electro-deposited in an aqueous solution) can be deposited. Via the plating process described above, a transition metal plate layer is formed on resin powder as the substrate, but the invention is not restricted thereto. The thickness of the plate layer is in the range of 10-200 nm in case of a mono-layer plate, and 10-300 nm in case of multi- layer plate, but the thickness is not limited thereto.

The displacement gold-plating solution according to the invention contains a water-soluble gold composition, a complexing agent and a reductant . Now, the individual components contained in the displacement gold-plating solution according to the invention are described.

In the displacement gold-plating solution according to the invention, any gold composition may be used without particular restriction, as long as it is water soluble. Gold cyanide complex salts are particularly preferable. Specific examples of gold cyanide complex salts include one selected from potassium gold cyanide and sodium gold cyanide, or a mixture thereof. The concentration of water-soluble gold composition in the displacement gold plating liquid according to the invention is not particularly restricted. If the concentration of gold composition is too low, the initial rate of deposition of gold plate is slow, and a long time is required to form gold plate layer of a certain thickness. On the other hand, if the concentration of gold composition is too high, the cost increases due to the increased amount of gold used. In view of these situation, the concentration of water-soluble gold composition in the displacement gold- plating solution is preferably in the range of 0.01 ~ 1 mol/L, but it is not restricted thereto. Deposition of gold from the displacement gold-plating solution according to the invention is carried out with two stages: by displacement reaction with the plated metal layer at the early stage of reaction, and then by reduction by the use of a reductant. Consequently, when gold plating is made on powder having conductive transition metal plate, gold is reduced in the transition metal phase to form a gold layer at the early reaction stage, and the metal of the primer plate layer is dissolved in the plating liquid. The complex agent which is blended with the plate liquid of the present invention facilitates the dissolution reaction of the conductive transition metal which was plated on the core, and solubilizes the dissoluted metal ions in the plating liquid.

In the plating liquid according to the present invention, any complexing agent may be used without particular restriction, as long as it can stably solubilize the metal ion in the plating liquid. Ethylenediamine derivatives can be particularly preferable. Specific examples of ethylenediamine derivatives which can be effectively used in the present invention include alkaline metal salt such as sodium or potassium salt, ammonium salt or the like, of a derivative having three to five substituents selected from hydroxyalkyl group, carboxyalkyl group, sulfonic acid group, or the like on the nitrogen atom of ethylenediamine or diethylenetriamine such as ethylenediamine tetraacetatic acid, N- hydroxyethylethylene diamine, tetrahydroxyethylethylenediamine, ethylenediaminetetraacetic acid, ethylenediamine tetramethyleneacenic acid. In the process according to the invention, any one complexing agent selected from the above list, or a mixture thereof may be used.

If the concentration of the complexing agent is too low, it is hard to stably solubilize the dissoluted metal ions. On the other hand, if the concentration is to high, the transition metal plated on the core is excessively dissoluted to cause peeling. The concentration of the complexing agent preferably is from 10 to 1000 g/L, more preferably from 100 to 600 g/L.

In the process according to the invention, the reductant is used for reducing the water-soluble gold salt contained in the displacement gold-plating solution. By virtue of using the reductant, an eletroless displacement gold-plating solution having appropriate deposition rate and excellent stability can be obtained. According to the present invention, one compound selected from the group consisting of an erythorbic acid compound such as L-ascorbic acid salts; a hydrazine compound such as p-hydrazinebenzenesulfonic acid and hydrazine sulfate; a hydroquinone compound such as methyl hydroquinone, chlorohydroquinone and methoxy hydroquinone; or a mixture thereof can be used as a reductant. The concentration of the reductant in the plating liquid according to the invention is not particularly restricted. If the concentration is too low, gold can be hardly deposited, while it is too much, the cost increases. From this point of view, concentration of the reductant preferably is from 0.01 to 50 g/L, more preferably from 0.1 g to 20 g/L on the basis of 0.5 g/L of gold ion to be reduced in the plating liquid.

The gold-plate according to the invention is initially formed by displacement reaction with the transition metal plated layer under acidic condition, and after pH of the displacement gold-plating solution is adjusted to basic condition, reductant is incorporated thereto to form a gold- plate via reduction.

Thus, pH of the initial displacement gold-plating solution is in the acidic range such as from 3 to 8, more preferably from 4 to 6, and then pH of the liquid is adjusted to 9 to 14 before incorporating the reductant.

If pH of the plating liquid for displacement at the early reaction stage is lower than 3, cyanide ion in the plating liquid is discharged to the atmosphere to lower the stability of the plating liquid, and the conductive transition metal is excessively eluted to cause peeling. On the contrary, if the pH is higher than 7, gold-plating cannot be carried out because the initial reaction of displacement plating does not take place. Accordingly, the pH suitably is from 3 to 7, preferably from 4 to 6. As the pH controlling agent, used can be organic acids such as, but not limited to, citric acid, acetic acid and oxalic acid, or salts thereof such as sodium salt or potassium salt. The concentration of the organic acid may be determined to reach desired pH, but preferably being in a range from 10 to 200 g/L per liter of the displacement plating liquid.

Then, pH of the liquid is adjusted to 9~14 before adding the reductant . If the pH is less than 9, metal ions eluted from the primer transition metal plated layer due to the displacement plating is reduced by the reductant, and gold ion in the plating liquid is also reduced, to make the plate layer uneven. Further, an alloy is formed with the transition metal of the primer plate layer to make an oxidative film, thereby causing increase of the electric connection resistance. On the other hand, if the pH exceeds 14, it is not preferable since the increase of the amount of pH controlling agent cannot achieve corresponding effect. Thus, the basic condition preferably comprises pH in the range from 9 to 14, more preferably from 10 to 14. As the pH controlling agent to obtain the basic condition, inorganic salt such as sodium hydroxide and ammonium chloride may be used, preferably in a concentration of 10-200 g/L per liter of the displacement plating liquid.

The temperature of eletroless displacement gold-plating solution preferably is not less than 40^°C. If the temperature is too high, decomposition of the plating liquid may readily occur, and it is difficult to maintain the concentration of the components contained in the plating liquid because of severe evaporation of water. Thus, the temperature of the plating liquid preferably is from 20 to 80^°C.

Therefore, the conductive powder prepared according to the present invention has excellent conductivity and uniform and intimate cohesion of plated layer, so that it may cope with more minute wiring, and provides a high quality conductive eletroless displacement gold-plating solution and plating process which would not cause a problem of electric capacity upon connection.

As described above, the process according to the invention, wherein a gold plate layer of a certain thickness is formed by displacement reaction in the early stage of gold-plating, by using a certain amount of reductant during the displacement gold-plating reaction, and then pH is adjusted to deposit gold by the action of the reductant, can reduce production cost. According to the process, a minute and uniform gold-plate layer is formed while inhibiting excessive nickel elution, so that the deviation of electric resistance between individual conductive balls is small when being connected to a microcircuit, to give high reliability. Thus, high utility in the industrial field is anticipated. Examples

Examples are described for more specific explanation of the present invention, but the invention is not limited to those Examples. [Example 1]

Acrylic powder crosslinked by methacrylic acid having 3.6 μm of mean diameter, 5% of Cv and 1.06 of aspect ratio, and triethylene glycol dimethacrylate, was used. Ten (10) grams of the powder was dispersed in 100 g of a mixed solution prepared from 10 g of CrC>₃ and 200 g of sulfuric acid in 1000 g of ultra-pure water, and the mixture was treated by an ultrasonic washer for 30 minutes. After treating, the powder was settled at 60^°C for 10 minutes, and then washed with deionized water. Then, it was settled in an aqueous solution of SnCl₂ (1.0 g/1) for 3 minutes, and washed with cold deionized water. After being settled in an aqueous solution of PdCl₂ (0.1 g/1) for 3 minutes, it was washed several times with cold deionized water, to obtain slurry. <Primary eletroless nickel-plating process>

Aqueous phosphite (NaH₂PO₂) solution (0.5 M) was prepared as IL of dispersion. After warming the dispersion to

60^°C, the slurry (10 g) obtained above was charged thereto with stirring. To the resultant solution, nickel sulfate solution (1 M, 50 ml) and hypophosphite salt (NaH₂PO₄, 2M, 50ml) as a reductant were slowly added by using a micro- quantitative pump at a rate of 1 ml/min.

After complete incorporation of nickel sulfate and the reductant, the mixture was vigorously stirred until the bubbling of hydrogen ceased. Then eletroless nickel plating was performed while maintaining the temperature at 60 ^°C and pH

6.0.

The nickel plated powder thus obtained was washed with IL of ultra-pure water three times, and the residual water was completely removed by displacement of 100 ml of alcohol. Vacuum drying at 80 ^°C gave nickel-plated powder. The thickness of nickel plated layer thus prepared was about 120 nm.

<Gold-plating process> Potassium gold cyanide (10.0 g) , ethylenediaminetetraacetic acid (150 g) and ammonium citrate (70 g) were completely dissolved in 3 L of deionized water, to prepare a displacement gold-plating solution. The solution had pH of 5.2. The plating liquid thus prepared was warmed to 60^°C, and the nickel plated powder (20 g) obtained from the nickel plating was added thereto. The resultant mixture was reacted with stirring and dispersing for 10 minutes to obtain the gold thickness of about 0.08 μm. While adjusting the pH to 13.0 by adding aqueous sodium hydroxide (NaOH) solution (10 M) dropwise thereto by using a quantitative pump, hydrazine dihydrate (98%, 10 g) as reductant was added by using a micro-quantitative pump at a rate of 1 g/min for 10 minutes. Then the reaction was carried out for 35 minutes, and the change of nickel and gold content versus time of gold-plating is shown in Fig. 1.

The plated powder thus obtained was washed with 1 L of deionized water five times, and the residual water was completely removed by displacement by alcohol. Vacuum drying at 80 ^°C gave gold-plated powder. The cross-section of the gold-plated powder thus prepared was cut by using forced ion beam (FIB) , and measured by using a scanning electron microscope (SEM) . As a result, thickness of the gold-plate layer was about 20 nm. The plated powder thus prepared was subjected to the tests as follows. Minuteness and intimate cohesion are shown in Table 1. The change of nickel and gold content during the course of gold- plating by using nickel plated powder was analyzed by titration over reaction time, of which the result is shown in Fig. 1. Fig. 2a, 2b and 2c are magnified SEM photographs (2a: x 1000; 2b: x 20,000; 2c: x 40,000) of the surface in order to determine surface uniformity of the plated powder prepared from Example 1. Fig. 3 shows contact resistance of 10 particles for individual compression pressure for the conductive particulates prepared by using the process for measuring conductivity as written below. [Example 2]

By using the nickel-plated powder prepared from Example 1, displacement gold-plating was carried out for 10 minutes. While adjusting pH to 13.0 by adding aqueous sodium hydroxide solution (10 M) dropwise thereto by using a quantitative pump, L-ascorbic acid (20 g) as a reductant dissolved in 100 ml of ultra-pure water was added by using a micro-quantitative pump at a rate of 1 ml/min, to proceed gold-plating for 35 minutes. The plated powder thus obtained was washed with 1 L of deionized water five times, and the residual water was completely removed by displacement by alcohol. Vacuum drying at 80 ^°C gave gold-plated powder.

The cross-section of the gold-plated powder thus prepared was cut by using forced ion beam (FIB) , and measured by using a scanning electron microscope (SEM) . As a result, thickness of the gold-plate layer was about 22 nm.

Conductivity, minuteness and intimate cohesion were measured for the plated powder thus prepared, and shown in Table 1.

Fig. 4 shows a SEM photograph (x 40,000) of the surface in order to determine the surface uniformity of the plated powder thus prepared.

[Comparative Example 1]

Displacement gold-plating was carried out according to the same procedure as Example 1, but reductant was not used during the gold-plating process, and pH was maintained at 5.2 for the reaction for 45 minutes.

Conductivity, minuteness of plating and intimate cohesion of the plated powder thus obtained was shown in

Table 1. Fig. 5 shows a SEM photograph to determine the uniformity of plating. The change of nickel and gold content during the course of gold-plating by using nickel plated powder was analyzed by titration over reaction time, of which the result are shown in Fig. 1. Fig. 6 shows contact resistance of 10 particles for individual compression pressure for the conductive particulates prepared by using the process for measuring conductivity of the gold-plated powder obtained from Comparative Example 1.

[Comparative Example 2]

With using the same displacement gold-plating solution as Example 1, pH was adjusted to 13.0 by using aqueous 10 M sodium hydroxide (NaOH) solution. Hydrazine dehydrate (98%,

Wako Chemical) (10 g) was added by using a micro-quantitative pump at a rate of 1 g/min for 45 minutes.

The plated powder thus obtained was washed with 1 L of deionized water five times, and the residual water was completely removed by displacement by alcohol. Vacuum drying at 80 ^°C gave gold-plated powder.

The physical properties of Table 1 were measured according to the methods written below: 1) Uniformity of plating The surface of the plated powder obtained was magnified by using a SEM to confirm the plate uniformity of the powder.

2) Measurement of conductivity

By using a particulate compression electric resistance meter (Fischer, HlOOC) , total 10 measurements of electric resistance of one particle under 10 mN compression were carried out, and the average and standard deviation were calculated.

3) Minuteness of plate Minuteness of the plate was examined by measuring the size of metal particles with magnifying the plated surface of the plated powder by 50,000 times by using a SEM. The smaller the metal particle is, the more minute plate layer is obtained.

4) Measurement of intimate cohesion

The plated powder obtained (1.0 g) and zirconia beads having 5 mm of diameter (10 g) were charged to a 100 ml glass bottle. After adding 10 ml of toluene, the mixture was stirred for 10 minutes at 400 rpm. Then, the zirconia beads were separated, and the condition of the plated layer was evaluated by using an optical microscope. The results are shown below.

[ Table 1]

Minuteness of Intimate Conductivity plate (nm) cohesion (Ω)^*

Example 1 75 O 29.1 ± 2.7

^*Electric resistance of one particle under 1OmN of compression

O: No peeling from the plate layer was observed. Δ: Some peeling from the plate layer was observed. X: Peeling from the plate layer was observed.

As can be clearly seen from the results shown in Table 1 and Figs. 1 to 6, at the early stage of the displacement gold plating, a gold-plated layer is formed by displacement, and then a reductant is used to deposit gold, to form a minute and homogeneous gold plate layer while inhibiting elusion of excess nickel. Since gold is reduced by reductant to form a uniform gold layer, and at least 99% of gold ion in the plating liquid is reductively deposited, so that the production cost can be advantageously lowered.

[industrial Applicability]

Resin particulate materials provided with conductivity are widely used as a subsidiary material for preventing static electricity of an electronic device or parts thereof, absorption of electric wave, shielding of electric wave, or the like. Recently, plated particles are used as a conductive material for electrically connecting the micro-portions of electronic instruments including a connection of an electrode of a liquid crystal display panel to a circuit substrate of a LSI chip for operation, and a connection between the electrode terminals in a micro-pitch. The conductive balls prepared according to conventional techniques have a problem in reliability owing to very large deviation of electric resistance between each ball upon being connected to a micro-circuit. Recently, conductive powder having highly excellent conductivity is required along with rapid advancement of electronic instruments and miniaturization of electronic parts, and thus the minute wiring on a substrate or the like.

The present invention relates to an eletroless process for preparing plated powder having excellent conductivity. More specifically, the invention relates to a process for preparing conductive powder via formation of a conductive transition metal plated layer on resin powder as a core, and then eletroless displacement gold plating, wherein transition metal plated powder is gold-plated by displacement under acidic condition, and then reductant is added to carry out deposition gold-plating under basic condition to form a minute and homogeneous gold plate layer while inhibiting elusion of excess nickel. According to the process, one particle of the conductive ball has excellent conductivity upon being connected to a microcircuit, with high reliability with small deviation of electric resistance between particles Thus, high utility in the industrial field is anticipated.

Claims

[CLAIMS]

[Claim l]

A process for preparing conductive powder to form a metal plate layer on resin powder as a core in eletroless plating liquid, which comprises the steps of a) forming a conductive transition metal layer on a surface of the core; b) performing displacement gold-plating by dispersing the conductive powder, on which the conductive transition metal layer has been formed, in displacement gold-plating solution; and c) adjusting the displacement gold-plating solution to basic condition and adding a reductant to form a reductive gold-plated layer on said displacement gold plate layer.

[Claim 2]

A process for preparing conductive powder according to claim 1, wherein pH of the displacement gold plating liquid in step b) is maintained in a range from 4 to 6.

[Claim 3]

A process for preparing conductive powder according to claim 2, wherein pH of the displacement gold plating liquid in step c) is maintained in a range from 9 to 14. [Claim 4 ]

A process for preparing conductive powder according to claim 3, wherein the gold plating in step b) and step c) is performed at a temperature from 20 to 80^°C.

[Claim 5]

A process for preparing conductive powder according to claim 4, wherein the core comprises a resin or a mixture of two or more resins selected from the group consisting of polyethylene, polyvinylchloride, polypropylene, polystyrene, polyisobutylene, styrene-acrylonitrile copolymer, acrylonitrile-butadiene-styrene terpolymer, polyacrylate, polymethyl methacrylate, polyacrylamide, polyvinyl acetate, polyvinyl alcohol, polyacetal, polyethyleneglycol, polypropylene glycol, epoxy resin, benzoguanamine, urea, thiourea, melamine, acetoguanamine, formaldehyde resin, palladium formaldehyde resin, acetaldehyde resin, polyurethane and polyester.

[Claim β]

A process for preparing conductive powder according to claim 5, wherein particle diameter of the core is 0.5 -1000 μ m; the aspect ratio is less than 2; and the coefficient of variation (Cv) value of the particle diameter as defined by following Equation 1 is not more than 30%: [Equation 1] Cv(%) = ( a /On) x 100 wherein, σ is standard deviation of the particle diameter, and Dn is number average particle diameter.

[Claim 7]

A process for preparing conductive powder according to claim 6, wherein the conductive transition metal to be plated on the surface of the core is a metal selected from Au, Ag, Co, Cu, Ni, Pd, Pt and Sn, or an alloy thereof.

[Claim 8]

A process for preparing conductive powder according to claim 7, wherein the conductive transition metal layer is formed by eletroless plating.

[Claim 9]

A process for preparing conductive powder according to claim 1, wherein the reductant is one or more compound (s) selected from the group consisting of erythorbic acid compounds, hydrazine compounds, hydroquinone compounds, boron compounds and phosphoric acid compounds.

[Claim 10] A process for preparing conductive powder according to claim 9, wherein the reductant is used in an amount of 0.01-50 g/L on the basis of 0.5 g/L of gold ion in the plating solution .