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2001-04-18

2004-09-07

Le, H. Thi (Department: 1773)

Stock material or miscellaneous articles

Coated or structually defined flake, particle, cell, strand,...

Particulate matter

Reexamination Certificate

active

06787233

ABSTRACT:

The present invention relates to electroconductive particles, in particular gold-coated polymer microbeads, and to a process for their preparation.
The use of electroconductive particles to achieve an anisotropic electrical connection between electronic components has been described in EP-A-265212 (Japan Synthetic Rubber) and in JP-B-5/19241 (Sumitomo Chemical). Metal coated polymer beads, distributed in an adhesive matrix are disposed between the component surfaces and serve to provide an electrical connection between those surfaces.
In JP-B-5/19241, gold, silver, copper, nickel and aluminum are said to be suitable electroconductive coatings and the polymer is stated to be a styrene copolymer, e.g. with an acrylic acid ester, a methacrylic acid ester, an unsaturated carboxylic acid, a nitrile or a diene. The copolymer must however contain styrene as its principal component and no more than 30 wt. % of the polymer can derive from another ethylenically unsaturated monomer. In the Examples a divinyl benzene:styrene (4:96 wt. ratio) polymer particle is either acid treated, treated with colloidal palladium chloride and then plated with silver or is coated with silver by vacuum vapour deposition.
EP-A-265212 describes the use of polystyrene, styrene/divinylbenzene/methacrylic acid (65:35:5 by weight), methyl methacrylate/divinylbenzene (50:50), and styrene/divinyl benzene/sodium styrene sulphonate (65:30:5) polymer particles. The surfaces of the particles were chemically etched, treated with colloidal palladium chloride and then plated with nickel or copper. Subsequent gold plating of such nickel plated styrene/divinyl benzene/sodium styrene sulphonate particles was also described and the resulting particles displayed the best results in terms of anisotropic conductivity. Indeed Sumitomo refer to gold plating as being especially preferred for obtaining a good stable and long lasting electrical connection.
The electroconductive particles are substantially spherical having an average particle diameter in the micrometer range, e.g. 1 to 50 &mgr;m. Gold plated polymer microspheres of this type are available commercially from Sekisui.
In application and subsequent use as electrical connectors, the electroconductive microparticles undergo mechanical and thermal stresses and for optimum performance the coatings should be substantially blemish free. Where inert gold coatings are used, to avoid contamination any underlying non-gold metal coating should not be exposed.
However when gold-plated polymer microspheres produced as described above are examined by electron microscopy, it may clearly be seen that many of the gold surfaces are blemished and incomplete.
We have now found that improved gold-plated styrene copolymer microspheres may be produced by using a styrene copolymer in which styrene is a minor component and/or which incorporates a functionalised comonomer and is palladinated by reaction with a solution of a palladium compound.
Thus viewed from one aspect the invention provides electroconductive particles comprising a styrene copolymer core and an external gold coating, wherein the copolymer core comprises less than 50 wt. % styrene monomers.
Viewed from a further aspect the invention also provides electroconductive particles comprising a styrene copolymer core and an external gold coating, wherein the copolymer core comprises a styrene copolymer produced using styrene and a functionalized comonomer and subsequently palladinated by reaction with a solution of a palladium compound. functionalized comonomer and subsequently palladinated by reaction with a solution of a palladium compound.
Preferably the polymer core in the particles of the invention comprises less than 50 wt. % styrene residues and comprises residues of a functionalized comonomer.
By “functionalized” it is meant that the comonomer contains reactive chemical groups that are not transformed into a non-reactive form by the polymerization reaction. Typically such groups will be pendant from the polymer backbone of the resultant copolymer. Exemplary functional groups include amine, thiol, hydroxyl, amide, ester and oxyacid groups, e.g. carboxyl, sulphonic, phosphoric and phosphonic groups. Preferred functional groups for the ready-to-palladinate particles include groups capable of coordinating palladium, e.g. amino, carboxyl, hydroxyl and sulphonic groups, especially amino groups.
The functional groups in the palladium treated particles need not be the same as in the comonomer used in the polymerization reaction, ie. they may be modified after the copolymer is prepared, e.g. by reaction with a bifunctional agent which serves to introduce the desired functional group. By way of example this may be achieved by reaction of a styrene:divinylbenzene: glycidylmethylmethacrylate copolymer with an aminating agent such as ethylene diamine.
The styrene content of the styrene copolymer in the palladium-treated particles is preferably 0.5 to 98.5 wt %, e.g. 20 to 95 wt. %, more preferably 10 to 75 wt. %, e.g. 25 to 70 wt. %. Where the styrene content is below 50 wt. %, it is preferably 20 to 49 wt. %, e.g. 25 to 45 wt. %, particularly 30 to 45 wt. %.
Where the styrene polymer is crosslinked, the most commonly used cross-linking comonomer is divinylbenzene (DVB). As used, however, DVB generally contains a relatively high proportion (about 20, 35 or 45% wt) of the monoethylenically unsaturated, styrene analog ethylvinylbenzene (EVB), and the styrene content of the polymer may be considered to be the sum of the styrene and the EVB.
The comonomers used with styrene to produce the polymer cores of the particles of the invention may be any comonomers capable of copolymerization with styrene. These will generally be compounds which are mono- or multiply ethylenically unsaturated; examples include acrylic acid esters (e.g. methyl, ethyl, butyl, glycidyl, etc. acrylate), methacrylate acid esters (e.g. methyl, ethyl, butyl, glycidyl, etc. methacrylates), acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, &agr;-methyl styrene, vinyl toluene, vinyl chloride, vinyl acetate, vinyl propionate etc. Acrylic and methacrylic acids and especially esters, and mixtures of two or more thereof are preferred. Other multiply ethylenically unsaturated monomers may be used to achieve a degree of crosslinking, e.g. dienes such as butadiene, divinyl benzene, trimethylolpropane acrylate or polyethylene glycol diacrylate, especially divinyl benzene. The proportion of multiply ethylenically unsaturated monomer used will depend on the resilience required of the polymer cores in use but their residues will generally make up from 2 to 30 wt. %, more preferably 5 to 20 wt. %, especially about 10 wt. % of the polymer. The proportion of the functionalized comonomer (e.g. an acrylate or methacrylate ester) will preferably be such that their residues make up 5 to 99.5 wt. %, e.g. 10 to 75 wt. %, more preferably 15 to 70 wt. %, especially 55 to 65 wt. % (e.g. 5 to 60 wt. %, for example 10 to 50 wt %). Particularly satisfactory results have been obtained with a styrene content of 25-35 wt %, a DVB content of 7 to 13 wt % and a methacrylate content of 55 to 65 wt %, and metal-plated particles having a core of such a polymer form a further aspect of the invention.
Particularly suitable copolymers include styrene: divinyl benzene:glycidyl methacrylate and styrene: divinyl benzene:glycidyl methacrylate:butyl methacrylate which may subsequently be aminated by treatment with ethylene diamine.
The polymer particles are preferably substantially spherical and also substantially monodisperse.
Thus the particle size distributions have a coefficient of variation CV of 20% or less, preferably 12% or less, more preferably 5% or less, still more preferably 3% or less, e.g. 1.5 to 2.5%. CV is determined in percent by the formula (standard deviation/mean)×100 where mean is the mean particle diameter and standard deviation is the standard deviation in particle size. CV is normally calculated by fitting a monomodal distribution curve to the detected particle size distribu

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