Conductive adhesive and method for the production of a microcapsule type conductive filler for the conductive adhesive

Abstract: A method for the production of a microcapsule (MC) type conductive filler is provided. The method comprises (a) immersing minute conductive particles in an affinity agent thereby treating the surface of the minute conductive particles, (b) immersing and dispersing the surface-treated minute conductive particles in an epoxy monomer thereby forming a suspension, and (c) polymerizing the monomer in the suspension thereby forming a thermosetting insulating polymer on the surface of the minute conductive particles.

Description: This invention relates to a method for the production of a microcapsule (MC) type conductive filler and more particularly to a method for coating the surface of minute conductive particles with an insulating polymer and to an MC type adhesive agent having dispersed in an adhesive agent the coated MC type conductive filler. In the conventional method of adhesion, the adhesion effected by soft soldering or welding where the interface produced by this adhesion requires conductivity. The conventional method is effectively applicable only to a limited number of materials because of the heat factor. In contrast, the organic-inorganic composite conductive adhesive agent that is composed of a binder using a synthetic resin as a main component thereof and a conductive filler using a metal powder as a main component thereof finds utility in a wide variety of applications involving different kinds of materials subjected to adhesion. This adhesive agent, therefore, is an indispensable medium for conductive adhesion of plastic substances (such as epoxy and phenol resins) that do not adhere by soft soldering, for adhesion of NESA glass used in liquid crystal display devices, for adhesion of phosphor bronze with a carbon brush used in micrometers, and for adhesion of lead wires as in quartz oscillators and sdc meters, for example. Particularly, in the semiconductor industry, which has been enjoying significant growth recently, IC's and LSI's of increasingly high quality have been developed and mass produced. For the adhesion of these semiconductor chips (silicon wafers) to lead frames, though the method involving an Au-Sn eutectic once prevailed, conductive adhesive agents formed by kneading an epoxy resin with silver powder now have multiple applications utility owing to their ability to lower cost and enhance productivity. As a resin binder for conductive adhesive agents, while epoxy resin is generally used, polyimide type, phenol type, and polyester type resins are also used, though only partially. As a conductive filler, minute particles of such metals as gold, silver, and copper and amorphous carbon and graphite powder are generally used as well as metal oxides, though only partially. Silver powder is preferably used over the conductive fillers cited above because it is inexpensive, reliable and effective. The conductive adhesive agent is advantageous in various respects compared with conventional applications such as soft soldering and welding though it is not perfectly free from fault. When this conductive adhesive agent is used between an LSI chip and patterns for mounting component parts, for example, an increase in the amount of minute conductive particles that are incorporated in the conductive adhesive agent lowers insulation resistance as illustrated in Fig. 1 and increases the possibility of adjacent patterns forming electric continuity. A reduction in the amount of minute conductive particles reduces the electric continuity between the LSI and the patterns. Data indicate the necessity for rigidly controlling the amount of minute conductive particles to be used in the conductive adhesive agent. And at the same time, reveal the fact that the minute conductive particles cannot be used in large amounts. It is believed possible that this problem can be solved by a procedure that comprises preparing an MC type conductive adhesive agent having dispersed in an adhesive agent, an MC type conductive filler formed by coating the surface of minute conductive particles with an insulating polymer, applying the MC type conductive adhesive agent to the entire surface of the substrate of an IC or LSI chip, exerting pressure to bear on the interface between the chip and patterns deposited thereon, thereby rupturing the coating layer of the capsules and establishing electric continuity between the chip and the patterns, and meanwhile allowing the encapsulated minute conductive particles interposed between the adjacent patterns to remain intact and continue to insulate these patterns from one another. The insulating resins that are usable for coating the surface of minute conductive particles include thermoplastic resins and thermosetting resins as classified by kind. In terms of resistance to moisture absorption and electric insulating properties, thermosetting resins definitely excel thermoplastic resins. Since thermocompression bonding of a chip to a substrate is generally carried out at an elevated temperature of at least 170°C, the insulating resin to be used is required to be stable enough to resist this elevated temperature though few thermoplastic resins can endure this temperature. In contrast, most thermosetting resins can tolerate temperatures in the neighborhood of 200°C. For use as an insulating resin in the MC type conductive filler, thermosetting resins that are advantageous in various respects over thermoplastic resins are suitable. For the application of an insulating resin coating to the surface of minute conductive particles, however, the procedure that involves dissolving the resin in a solvent, spraying the solution on the surface of the minute conductive particles, and drying the applied coating of the solution is predominant though since thermosetting resins are insoluble in solvents, this procedure applied conventionally is difficult and the application of a thermosetting resin coating to the surface of minute conductive particles, therefore, necessitates development of a novel coating procedure. The prior techniques pertaining to the MC type conductive adhesive agent have been disclosed by Japanese Unexamined Patent Publications No. 176,139/1987, No. 76,215/1987, No. 47,943/1988, No. 54,796/1988, No. 103,874/1990, and No. 103,875/1990, for example. First, the disclosures of Japanese Unexamined Patent Publications No. 176,139/1987, No. 76,215/1987, No. 47,943/1988, and No. 54,796/1988 will be described. These patent publications disclose, as conductive adhesive agents, those produced by forming an intermediate conductive layer on spherical cores of resin and coating the intermediate layer with a surface layer of an insulating thermoplastic resin and those produced by coating the surface of minute spherical conductive particles with an insulating thermoplastic resin. Actual mounting of a chip on a substrate for a printed circuit by using such a conductive adhesive agent is attained by a procedure that comprises applying the conductive adhesive agent to the substrate and thermocompression bonding the chip to the substrate so that the intermediate layer or the minute conductive particles will discharge a conductive function and the insulating thermoplastic resin an adhesive function and an insulating function. The techniques disclosed by these patent publications differ from the method using the MC type conductive adhesive agent of the present invention and these patent publications do not mention using a thermosetting resin as an insulating resin for coating the surface of the minute conductive particles. Now, the disclosure of Japanese Unexamined Patent Publication No. 103,874/1990 will be described below. The invention of this patent publication pertains to an MC type conductive adhesive agent produced by dispersing in a film of an insulating adhesive agent serving as a binder an MC type conductive filler having minute conductive particles coated with an insulating thermoplastic resin or thermosetting resin. Conductive union of two given members using this MC type conductive adhesive agent is accomplished by depositing this adhesive agent on the two members and pressing the two members against each other while being heated state. Thus, in the part expected to form electric continuity, the impact of the pressure exerted as described above ruptures the insulating resin layer of the MC filler and establishes the desired electric continuity, whereas in the part requiring insulation, the MC type conductive filler is allowed to remain intact and, therefore, retain stable insulation. Incidentally, this MC type conductive filler is manufactured by plasma polymerization or plasma CVD polymerization and there are times when the insulating film of the MC type filler may be formed of a thermosetting resin. The number of kinds of thermosetting resins that can be manufactured by the plasma polymerization and the plasma CVD polymerization is very small because the number of kinds of gases usable for injection during the polymerization is not large. Further in accordance with this method of plasma polymerization or plasma CVD polymerization, the cost is sufficiently high to render the manufacturing thereof impracticable and productivity is inferior because the amount of MC type filler to be manufactured is small. The disclosure of Japanese Unexamined Patent Publication No. 103,875/1990 will be described below. The invention of this patent publication pertains to the use of an MC type conductive adhesive agent produced by coating minute conductive particles with an insulating thermoplastic resin or thermosetting resin. Actual mounting of a chip on a substrate for a printed circuit using this MC type conductive adhesive agent is attained by applying the conductive adhesive agent to the substrate and thermocompression bonding the chip to the substrate, with the intermediate layer or the minute conductive particles discharging a conductive function and the insulating resin on the surface of the minute conductive particles an adhesive function and an insulating function. Incidentally, this MC type conductive filler is manufactured by either plasma polymerization or plasma CVD polymerization. Thus, these prior techniques are described as allowing what is formed by coating the surface of minute conductive particles with a thermosetting resin. In spite of these disclosures, thermosetting resins should be unusable for the purpose of coating because they do not melt with heat and, therefore, are incapable of functioning as an adhesive. Even if a thermosetting resin is used, the method of manufacturing the MC type conductive filler entails a serious drawback as pointed out in Japanese Unexamined Patent Publication No. 103,874/1990. Practically all the prior techniques pertaining to the manufacture of an MC type conductive filler or the conductive adhesive agent using this filler invariably use a thermoplastic resin. Even when the patent publications mention usability of a thermosetting resin, methods of manufacturing using such a thermosetting resin are not disclosed with sufficient specificity or are devoid of practicability and thus, these methods cannot be actually used. This invention has been produced for the purpose of solving the problems encountered by the prior art as described above. According to this invention, a method for production of an MC type conductive filler comprises (a) a step of immersing minute conductive particles in an affinity agent to treat the surface of the minute conductive particles, said affinity agent inducing uniform adhesion of a monomer to the surface of the particles, the monomer being polymerisable to form a thermosetting insulating polymer, (b) a step of immersing and dispersing the surface-treated minute conductive particles in a solution of said monomer which is polymerisable to form a thermosetting insulating polymer, thereby forming a suspension, and (c) a step involving polymerizing the monomer in said suspension thereby forming a thermosetting insulating polymer on the surface of the minute conductive particles. The invention also provides an MC type conductive adhesive agent produced by dispersing the MC conductive filler obtained by the method of the invention in an adhesive agent. The invention will be better understood and objects and advantages other than those set forth above will become apparent when consideration is give to the following detailed description thereof. The description makes reference to the annexed drawings wherein: Fig. 1 is a flow sheet of the production of a microcapsule type conductive filler according to the method of this invention, Fig. 2 is a type diagram illustrating a growth model of a triazine thiol film on the surface of metal, and Fig. 3 is a type diagram illustrating the reaction mechanism of an epoxy monomer with trianzine thiol, Fig. 4 is a type diagram illustrating one example of a substrate, Fig. 5 is a type diagram illustrating one example of a glass chip, Fig. 6 is a type diagram illustrating sites for determination of electric continuity resistance and insulation resistance, Fig. 7 is a partially magnified diagram of Fig. 6, and Fig. 8 is a type diagram illustrating the state of union between a chip and a substrate. The affinity enhancer such as triazine thiol, which is used at the step (a) in the method of this invention, allows effective polymerization of the monomers because it is capable of inducing uniform adhesion of the epoxy resin monomer to the surface of the minute metallic particles and opening the heterocycles in the resin. As a result, the heretofore difficult coating of the surface of the minute conductive particles with the thermosetting resin can be easily attained by the method of this invention. Further, since the coating film of the thermosetting resin is superior to the thermoplastic resin in strength, the MC type conductive filler can be incorporated in a large amount in the adhesive agent and the MC type conductive adhesive agent consequently produced can effect an adhesive union of two given members with higher reliability than the conventional technique. The minute conductive particles to be used for this method of production of the filler are only required to be made of a conductive metallic material. The kind of metallic material is irrespective. For example, minute Cu particles having the surface thereof coated with Ag or minute Ag particles are preferably used. The minute conductive particles are preferably spheres or pseudospheres in shape. These minute conductive particles preferably have a diameter of not more than 50 µm. The insulation layer of a thermosetting resin for the MC type conductive filler is preferably made of a cured epoxy/amine or bismaleimide/amine type resin. The insulating layer of thermosetting resin of the MC type conductive filer preferably has a thickness of not more than 3 µm. This invention pertains in one aspect of an MC type conductive adhesive agent having dispersed in an adhesive agent the filler obtained as described above. The adhesive agent that can be effectively used in the MC type conductive adhesive agent is the same as mentioned above. For example, an epoxy type one-component polyimide or polyester adhesive agent is preferably used. The viscosity of the adhesive agent mentioned above is preferably not more than 150,000 cps. The content of the MC type conductive filler in the MC type conductive adhesive agent is preferably not more than 60% by volume. Now, this invention will be described more specifically below with reference to working examples, which are illustrative of and not limitative in any sense of this invention. Example 1 An MC type conductive adhesive agent was produced with the following materials. Minute conductive particles: Minute Cu pseudospheres having the surface plated with Ag (Ag/Cu, average particle diameter 5 µm). Adhesive agent: A composition consisting of an epoxy resin as the main component and an acid anhydride as the curing agent). Affinity agent: Triazine thiol (RTD). Monomer: Bisphenol A type epoxy resin (BPA) (produced by Shell and marketed under trademark designation of "Epikote 828"). (1) Method for production of MC type conductive filler The minute metallic particles were subjected to a surface treatment. First, the minute metallic particles were washed with an acid and then with an alkali, and pretreated with Triclene to defat and clean the surface thereof. The cleaned minute metallic particles were immersed in a triazine thiol solution to be coated with a film of triazine thiol. This solution was prepared by dissolving triazine thiol in acetone in a concentration of 10 -4 mol/lit. No uniform film is obtained if the concentration is lower than this level and the speed of treatment is too high to be controlled as required if the concentration exceeds 10 -3 mol/l. The temperature of this treatment is not lower than 17°C. It is desired to be in the range of 20 ± 3°C because the speed of the treatment is too high to be controlled as desired if the temperature is unduly high. The time of treatment is desired to be in the range of 30 ± 5 minutes in due consideration of the relation between the concentration mentioned above and the temperature. It goes without saying that for such conditions as concentration, temperature, and time of the treatment, the magnitudes thereof to be selected should be optimum for obtaining a film having a suitable thickness and a suitable constitution depending on the purpose or use thereof. Then, the minute metallic particles were washed with the solvent used and methanol and the wet minute metallic particles were dried to complete the surface treatment. In a solution of 10 g of epoxy monomer (BPA) in 15 ml of ethyl acetate, 10 g of the surface-treated minute metallic particles were stirred with a homogenizer at 150 rpm as illustrated in Fig. 1 to form a suspension and induce a reaction to effect the coating of the surface of Ag/Cu particles with an insulating resin layer. Here, the principle of the production of the MC type conductive filler will be described below. When a suspension is formed by dispersing minute metallic particles in a solution of triazine thiol in an organic solvent, this triazine thiol reacts with the OH - group on the surface of the metallic particles to form a relevant salt. As a result, the surface of the minute metallic particles is coated with a triazine thiol film. When a suspension is formed by dispersing the minute metallic particles treated will triazine thiol in a solution of the monomer, the surface of the minute metallic particles undergoes a reaction. Consequently an MC type conductive filler having the surface of minute metallic particles coated with the polymer is obtained. Now, the reaction mechanism involved herein will be described below. The triazine thiol is a compound having a structural formula I shown below. (wherein R stands for a group represented by -SH, -N(CH 3 ) 2 , -NHC 6 H 5 , -N(C 4 H 9 ) 2 , -N(C 8 H 17 ) 2 , -N(C 12 H 25 ) 2 , -N(CH 2 CH=CH 2 ) 2 , -NHC 8 H 16 CH=CHC 8 H 17 , -NCH 2 C 6 H 4 CH=CH 2 (C 8 H 17 ), or -NHC 6 H 4 ). When the minute metallic particles are subjected to a surface treatment with this triazine thiol, there ensues a reaction path in which a monomolecular film of triazine thiol is formed on the surface of the minute metallic particles in the first step and the monomolecular film develops into a polymolecular film in the second step as illustrated in Fig. 2, with the result that the surface of the minute metallic particles will be coated with the triazine thiol film. When the minute metallic particles that have undergone the surface treatment are mixed with an epoxy monomer, the triazine thiol acts as a cross-linking agent for the epoxy monomer to undergo a reaction illustrated in Fig. 3 and gives rise to a cured product of epoxy. Consequently, an MC type conductive filler having a surface of the minute metallic particles coated with the epoxy resin is obtained. Here, it is necessary to pay attention to the following points. (1) The production of the triazine thiol film must be carried out in an atmosphere of nitrogen. (2) The triazine thiol concentration must be not more than 10 -3 mol/liter. (3) The reaction of the monomer must be carried out with the suspension stirred at a rate in the range between 50 and 250 rpm. The reason for (1) is that the minute metallic particles readily undergo corrosion in the presence of air because they have a large surface area. The reason for (2) is that the concentration of RTD (triazine thiol) must be kept below 10 -3 mol/liter because the amount of film is calculated from the amount of unreacted RTD. The reason for (3) is that the minute metallic particles settle and agglomerate during the reaction of the monomer when stirring is omitted. (2) Observation of cross section of capsule type minute metallic particles The produced filler was embedded in the epoxy resin, allowed to cure, and cut with a microtome to expose a cross section of the capsule type minute metallic particle to visual observation. (3) Confirmation of insulation with capsule type minute metallic particles The produced MC type conductive filler was agglomerated into a cohering mass and tested for insulation resistance with an insulation resistance meter used at freely selected points of measurement. (4) Production of conductive adhesive agent An MC type conductive adhesive agent was produced by mixing an MC type conductive adhesive agent with 20% by volume of the MC type conductive filler prepared in (1) above. The results of the test indicate the viscosity of the produced adhesive agent was so high as to jeopardize the workability if the voluminal proportion exceeded 20%, the produced adhesive agent was barely usable if the voluminal proportion was up to 60% of the MC type conductive filler, and the adhesive agent included parts allowing no electric continuity if the voluminal proportion was unduly small. Thus, the optimum content of the MC type filler is fixed at 20% by volume. Here, the adhesive agent used herein was a one-component type for facilitating the process of production. (5) Union of chip and substrate A substrate illustrated in Fig. 4 (128 pads, interval between pads 100 µm and size of pads 200 µm □) with 2 representing an electrode and 4 representing an electrode used for evaluation purposes, and to which the conductive adhesive agent produced in (4) above was applied and a glass chip (128 pins, 300 µm pitch, and electrode interval 100 µm) illustrated in Fig. 5 on which stud bumps were formed were subjected to thermocompression bonding at 175°C, 30 s, and g/bump. (6) Test for electric continuity and test for insulation Samples of the product of union indicated in (5) above were tested for electric continuity resistance by the four-terminal method using the points of measurement illustrated in Fig. 6 and Fig. 7 and tested for resistance with a resistance meter. (Results) (1) Observation of cross section of microcapsule type conductive filler The condition of the surface of minute conductive particles coated uniformly with an insulating resin as illustrated by a type diagram of Fig. 3 was confirmed. (2) Insulation resistance of microcapsule type conductive filler The magnitudes of insulation measured at all the points invariably exceeded a high level of 1 × 10 11 Ω. (3) Measurement of electric continuity resistance and insulation resistance between bonded chip and substrate The union between the chip and the substrate was obtained as illustrated by a type diagram in Fig. 8. The magnitudes of electric continuity resistance were satisfactory, invariably falling below 0.2 Ω per point of contact. Though the filler was incorporated in such a large proportion as 20% by volume, highly satisfactory insulation of 1 × 10 11 Ω was found between the adjacent patterns.This example represents one case of using tiazine thiol as an affinity agent. This invention is not limited to this particular affinity agent. Naturally, any compound possessing a reactive group that exhibits affinity for both the metal and the monomer intended to coat the metal can be used as an affinity agent. Example 2 In the production of capsule type minute metallic particles by the procedure of Example 1, the stirring of the suspension was carried out at varying rates of 30, 50, 250, and 300 rpm to determine the effect of the stirring speed on the stability of the suspension. (Results) Table 1 shows the effect of the stirring speed (30, 50, 250, and 300 rpm) on the stability of the suspension. The results indicate that the stirring speed must be in the range between 50 and 250 rpm for 'the sake of suspension stability. Relation between stirring speed and suspension stability Speed of stirring (rpm) 30 50 250 300 Suspension stability Sedimentation of minute conductive particles observed Stable Stable Adhesion of minute conductive particles to beaker wall observed Example 3 An MC type conductive filler and a capsule type conductive adhesive agent were produced by following the procedure of Example 1, except that alcohol was used in the place of acetone. They were evaluated in the same manner as in Example 1. (Results) In all the items of evaluation, the results were nearly the same as those obtained in Example 1. This invention is constructed as described above, it enables an MC type conductive filler coated with a thermosetting resin possessed of better characteristic properties than a thermoplastic resin to be produced easily at a low cost. Thus, this invention realizes a practical MC type conductive adhesive agent excellent in reliability and performance.

Claims: A method for the production of a microcapsule (MC) type conductive filler comprising (a) a step involving immersing minute conductive particles in an affinity agent to treat the surface of said minute conductive particles, said affinity agent inducing uniform adhesion of a monomer to the surface of the particles, the monomer being polymerisable to form a thermosetting insulating polymer, (b) a step involving immersing and dispersing said surface-treated minute conductive particles in a solution of said monomer which is polymerisable to form a thermosetting insulating polymer, thereby forming a suspension, and (c) a step involving polymerizing the monomer in said suspension thereby forming a thermosetting insulating polymer on the surface of said minute conductive particles. A method according to Claim 1, wherein said step (a) is preceded by a step involving cleaning the surface of the minute conductive particles. A method according to Claim 1 or 2, wherein the minute conductive particles are extracted after said step (b) and are subjected to a step involving washing and drying prior to said step (c). A method according to Claim 1, 2 or 3, wherein said affinity agent is triazine thiol. A method according to any one of Claims 1 to 4, wherein said affinity agent is dissolved in a solvent during said step (a). A method according to Claim 5, wherein said solvent is acetone or an alcohol. A method according to Claim 4, wherein the concentration of triazine thiol is not more than 10 -3 mol/litre. A method according to any one of Claims 1 to 7, wherein the immersion temperature at said step (a) is not lower than 20°C. A method according to any one of Claims 1 to 8, wherein the immersion time during said step (a) is 30 ± 5 minutes. A method according to any one of Claims 1 to 9, wherein the immersion treatment is carried out in an atmosphere of nitrogen. A method according to any one of Claims 1 to 10, wherein said affinity agent reacts on the surface of minute metallic conductive particles to form a salt of the metal and deposit the salt as a coating film on the surface of said minute metallic conductive particles. A method according to any one of Claims 1 to 11, wherein stirring of said suspension during said step (c) is carried out at a rate in the range between 50 and 250 rpm. A microcapsule (MC) type conductive adhesive agent having an MC type conductive filler of minute conductive particles coated with an thermosetting insulating polymer, and produced by the method of any preceding claims dispersed in an adhesive agent. An MC type conductive adhesive agent according to Claim 13, wherein said adhesive agent is an epoxy type one-component adhesive agent. An MC type conductive adhesive agent according to Claim 13 or 14, wherein said minute conductive particles have a diameter of not more than 50 µm. An MC type conductive adhesive agent according to Claim 13, 14 or 15, wherein said minute conductive particles are either Cu particles having the surface thereof coated with Ag or Ag particles. An MC type conductive adhesive agent according to any one of Claims 13 to 16, wherein said minute conductive particles are spheres or pseudospheres. An MC type conductive adhesive agent according to any one of Claims 13 to 17, having a thermosetting type insulating resin layer of said MC type conductive filler which is formed of an epoxy resin in particular an epoxy/amine type curing substance or a bismaleimide/amine type curing substance. An MC type conductive adhesive agent according to any one of Claims 13 to 18, which is in the form of a conductive filler having an thickness of not more than 3 µm. An MC type conductive adhesive agent according to any one of Claims 13 to 19 whose viscosity is not more than 150,000 cps. An MC type conductive adhesive agent according to any one of Claims 13 to 20, wherein the content of said MC type conductive filler in said MC type conductive adhesive agent is not more than 60% by volume.

Inventors: DATE HIROAKI, HOZUMI YUKO, USUI MAKOTO, WATANABE ISAO, DATE, HIROAKI, HOZUMI, YUKO, USUI, MAKOTO, WATANABE, ISAO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01L21
IPC codes: H01L23, H05K3, H01L21, H01B1