ADHESIVE, METHOD FOR BONDING, AND ASSEMBLIES OF MOUNTED BOARDS

Abstract: The present invention is characterized by comprising a two-pack adhesive of an A agent selected from components, an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent and containing at least one or two of the acrylic monomer, the peroxide and the reducing agent, and a B agent containing all of the remaining components which are not selected in the A agent. The use of this adhesive makes it possible to stably obtain the bonding free from a thermal stress with the excellent heat resistance and the good reliability.

Description: The present invention relates to an adhesive, a bonding method and a mounting substrate assembly. More specifically, the present invention relates to a mixed adhesive comprising a redox polymerization-type acrylic adhesive and an epoxy adhesive, a bonding method using the same and a mounting substrate assembly. The adhesive and the bonding method according to the present invention can be used to replace a soldering method, and are especially useful for bonding functional elements, such as electronic parts, to a mounting substrate such as a wiring circuit substrate or a printed wiring substrate. In recent years, semiconductor technology has developed markedly. Functional elements such as high-performance IC and LSI and the like have been developed and mass production techniques have evolved. A bonding method for mounting these functional elements on a mounting substrate which method is, a soldering method has become known. However, with the development of semiconductor technology, various bonding methods other than the soldering method have been proposed, and are being put to practical use. As a method which is not a soldering method, a bonding method using a resin adhesive which can cope well with a fine circuit pattern formed on a mounting substrate, the working environment, the operational environment and the like and which can give rise to bonding with a high reliability has attracted much interest, and it has been put to practical use to some extent. Further, a conductive adhesive obtained by adding conductive fine particles to the above-mentioned adhesive is known. For such an adhesive, it is required that stable bonding is realized in which no thermal stress remains in a bonded portion after bonding. When bonding is conducted using the above-mentioned resin adhesive or conductive adhesive in place of soldering, there are the following advantages. Since a soldering material generally used in the soldering has a poor wettability with aluminum (Al) which is generally used as an electrode of a functional element to be bonded, a film for improving a wettability of a soldering material has to be formed in advance on the Al electrode. To this end, an extra film-forming step is required, involving a high cost. However, when the above-mentioned adhesive is used, such inconveniences are eliminated. Further, since a melting point of a general soldering material is 183°C, the soldering has to be conducted at a temperature of higher than 183°C, generally higher than approximately 200°C. For this reason, there is a problem that the bonded portion undergoes thermal stress. However, when, for example, an epoxy adhesive is used, the curing treatment can be conducted at approximately 150°C which is a temperature lower than the soldering temperature. Thus, it is possible to reduce a thermal stress that the bonded portion undergoes. A bonding method using a resin or conductive adhesive has, compared with the soldering method, a large number of advantages, for example, (1) thermal stress that a bonded portion undergoes with adhesive is lower than the thermal stress that a soldered portion undergoes in a soldering method, (2) since a heat treatment at 200°C or more as in the soldering method is not required, a heater capable of heating at this temperature is dispensed with, (3) it is unnecessary to design a mounting substrate so as to endure a high temperature, with the result that production cost can be reduced, and (4) the bonding can be conducted without having Pb in the soldering material, Pb having an adverse effect on environment. Nevertheless, when a functional element is bonded to a mounting substrate using an ordinary adhesive, a special device such as a press having a heating mechanism is needed. Further, the thermal stress on a bonded portion is greater than when the bonding at room temperature, though it is less than in the soldering. Especially, when a bonded portion is soft as in a substrate made of an organic material or a mounting substrate having a photosensitive resin laminated thereon, the substrate is much deformed at the high temperature encountered in the bonding. Consequently, the reliability of the bonded portion comes to decrease. Thus, for further improving the reliability of the bonded portion, it is required to more reduce the thermal stress that the bonded portion undergoes. To this end, an adhesive capable of bonding at room temperature and having a good reliability is needed. Therefore, an acrylic adhesive has been proposed previously comprising an agent A composed mainly of a methacrylic monomer made of ω-carboxy-polycaprolactone monoacrylate, an organic peroxide, a polymerization inhibitor and an elastomer and further containing an epoxy resin as a chlorine ion trapping agent and an agent B composed mainly of a reducing agent that forms a redox catalyst system with the organic peroxide of the agent A, the above-mentioned resin adhesive being capable of bonding at room temperature (Japanese Unexamined Patent Publication No. HEI 7(1995)-216310). However, it is known that the above-mentioned acrylic adhesive is inferior to the epoxy adhesive in its indirect fillability, heat resistance and the like and has a low curing rate at room temperature. Accordingly, it is lacking in the reliability and the productivity as an adhesive for bonding a functional element to a mounting substrate. Other acrylic adhesives which likewise lack the desired properties specified above are those described at DATABASE WPI Section Ch, Week 198028 Derwent Publications Ltd., London, GB; Class A14, AN 1980-49184C XP002172070 & SU 429 658 A (MECH ENG DES TECHN), 25 February 1980 (1980-02-25) and EP-A-0218364. In both an epoxy resin is a bulking agent in the adhesive and there is only one step curing at room temperature. In view of the above-mentioned problems, it is an object of the present invention to provide a novel adhesive in which, using a mixed adhesive obtained by combining a redox polymerization-type acrylic adhesive with an epoxy adhesive, a functional element can be fixed to a mounting substrate such as a wiring circuit substrate or the like at room temperature and then the adhesive is heat-cured to thereby minimize thermal stress at the bonded portion, the cured adhesive bond having excellent heat resistance and reliability as well as a bonding method and a mounting substrate assembly. Thus, according to the present invention, in one aspect, there is provided an adhesive which is composed of an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent with at least one of the acrylic monomer, the peroxide and the reducing agent being coated within its own microcapsules by means of a thermoplastic resin to provide said at least one of the acrylic monomer, the peroxide and the reducing agent out of contact with the other(s) of the acrylic monomer, the peroxide and the reducing agent up until the microcapsules are broken down at the time of use. In a second aspect, the invention provides a method of bonding two members, characterised in that there is coated between the two members an adhesive composition comprising an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent, at least one of the acrylic monomer, the peroxide and the reducing agent being coated with a thermoplastic resin to form microcapsules such that it remains out of contact with the other(s) of the acrylic monomer, peroxide and reducing agent until the microcapsules are broken down at the time of use, the two members are brought together with the acrylic monomer, peroxide and reducing agent coming into intimate contact with each other, the two members are fixed and the adhesive is then heat-cured. In a third aspect, this invention provides a mounting substrate assembly in which pad portions of a mounting substrate and lead terminals of a functional element are to be mounted thereon and electrically connected by means of an anisotropic conductive adhesive, characterized in that the anisotropic conductive adhesive is an adhesive which is composed of an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent with at least one of the acrylic monomer, the peroxide and the reducing agent being coated within its own microcapsules by means of a thermoplastic resin to provide said at least one of the acrylic monomer, the peroxide and the reducing agent out of contact with the other(s) of the acrylic monomer, the peroxide and the reducing agent up until the microcapsules are broken down at the time of use, the adhesive further containing a microcapsule type conductive filler. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figs. 1(a) and 1(b) are sectional views showing the principle of a bonding method useful for understanding, but not embodying, the present invention; Figs. 2(a) to 2(c) are sectional views showing in sequence a first working example of a bonding method which does not embody the present invention; and Figs. 3(a) and 3(b) are sectional views showing further steps of the first working example of the bonding method in sequence after Fig. 2. The adhesive and the bonding method of the present invention are suited for bonding two members, a functional element and a mounting substrate. The functional element is not particularly limited. For example, integrated circuit chips (IC, LSI and the like) may be mentioned. The mounting substrate is also not particularly limited. For example, a wiring circuit substrate and a printed wiring board and the like are mentioned. In particular, the bonding method of the present invention is used in the bonding of electronic parts and/or a mounting substrate containing members that are thermally weak to bring forth especially preferable effects. The acrylic monomer in the context of the present invention signifies both an acrylic monomer and a methacrylic monomer. Any acrylic monomer can be used in the acrylic adhesive, for example, an ω-carboxy-polycaprolactone mono(meth)acrylate, (meth)acrylic acid, alkyl (meth)acrylate, hydroxyalkyl (meth)acrylate, phenoxyalkyl (meth)acrylate, cycloalkyl (meth)acrylate, tetrahydrofurfuryl (meth) acrylate, ethylene glycol di(meth)acrylate, an epoxy(meth)acrylate obtained by the addition reaction of an epoxy compound with meth(acrylic acid), a urethane (meth)acrylate obtained by the addition reaction of urethane with (meth)acrylic acid, a polyester (meth)acrylate obtained by the addition reaction of a polyester with (meth)acrylic acid, and a (meth)acrylate of an alkylene oxide adduct of bisphenol A. In the above-mentioned illustration, methyl, ethyl, propyl, butyl, pentyl and hexyl are mentioned as the alkyl. Hydroxyethyl and hydroxypropyl are mentioned as the hydroxyalkyl. Cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl are mentioned as the cycloalkyl. These (meth)acrylic monomers may be used either singly or in admixture of two or more. Incidentally, in the present specification, (meth)acrylic is used as a term referring to both acrylic and methacrylic, and a (meth)acrylate as a term referring to both an acrylate and a methacrylate. Next, as the peroxide, any peroxide that can be used in an acrylic adhesive is to be used. For example, benzoyl peroxide, cumene hydroxyperoxide, methyl ethyl ketone peroxide, dicumyl peroxide, t-butyl peroxybenzoate, butyl hydroxyperoxide and the like are mentioned. These peroxides may be used either singly or in an admixture of two or more. Especially preferable peroxides are benzoyl peroxide and cumene hydroperoxide. Further, as the reducing agent, any reducing agent that is used in an acrylic adhesive is usable. For example, triethylamine, tripropylamine, tributylamine, ethylenediethanolamine, N,N-dimethylaniline, N,N-dimethyitoluidine, methylthiourea, diethylthiourea, acetylthiourea, tetramethylthiourea, ethylenethiourea, methyl ethyl ketone oxime, methyl isobutyl ketone oxime, acetophenone oxime, p,p'-dibenzoylquinone dioxime, cobalt naphthenate, copper naphthenate, manganese naphthenate, nickel naphthenate, iron naphthenate, vanadyl acetylacetonate, titanium acetylacetonate, cobalt sulfide, copper sulfide, manganese sulfide, nickel sulfide and ferrous sulfide may be mentioned. These reducing agents may be used either singly or in admixture of two or more. Especially preferable reducing agents are dimethylaniline and ferrous sulfide. Especially preferable combinations of the peroxide and the reducing agent are benzoyl peroxide and dimethylaniline, and cumene hydroperoxide and ferrous sulfide. The proportions of the (meth)acrylic monomer, the peroxide and the reducing agent used are determined, as required, depending on the types of (meth)acrylic monomer, peroxide and reducing agent. Usually, it is advisable to use, per 100 parts by weight of the (meth)acrylic monomer, from 0.01 to 100 parts by weight of the peroxide and from 0.01 to 100 parts by weight of the reducing agent. When the proportion of the peroxide used is less than 0.01 parts by weight, the (meth)acrylic monomer is not reacted satisfactorily. Thus, it is undesirable. When it is more than 100 parts by weight, the peroxide that does not participate in the reaction remains in a large amount. Thus, it is undesirable. Further, when the proportion of the reducing agent used is less than 0.01 parts by weight, the (meth)acrylic monomer is not reacted satisfactorily. Thus, it is undesirable. When it is more than 100 parts by weight, the reducing agent that does not participate in the reaction remains in a large amount. Thus, it is undesirable. The more preferable proportions of the peroxide and the reducing agent are between 1 and 20 parts by weight and between 1 and 20 parts by weight respectively. As the epoxy resin precursor, any epoxy resin precursor that is used in an epoxy adhesive is usable. For example, a bisphenol A-type, bisphenol F-type or a naphthalene-type epoxy resin precursor may be mentioned. The epoxy resin precursor here means an epoxy resin having a relatively low molecular weight and which becomes a high-molecular-weight epoxy resin by crosslinking through curing. As the curing agent, any curing agent that is used in an epoxy adhesive is usable. For example, acids such as phthalic acid, hexahydrophthalic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, methylnadic acid, maleic acid, trimellitic acid, and pyromellitic acid and, imidazole and dicyanediamide may be mentioned. Preferably an above-mentioned acid is used as a mono- or di-anhydride. These curing agents may be used either singly or in an admixture of two or more. The proportions of the epoxy resin precursor and the curing agent used are determined in accordance with the types of the epoxy resin precursor and the curing agent. For example, it is advisable to use from 1 to 100 parts by weight curing agent per 100 parts by weight of the epoxy resin precursor. When the proportion of the curing agent used is outside the above-mentioned range, complete curing of the adhesive hardly takes place. Thus, it is undesirable. Especially preferable is that the curing agent is contained in a proportion of approximately 10 parts by weight per 100 parts of the epoxy resin precursor. In the bonding method of the present invention, the five essential components consisting of the acrylic monomer, the peroxide, the reducing agent, the epoxy resin precursor and the curing agent are mixed in use to form a mixed adhesive. Here, with respect to the mixing proportions of the three essential components of the acrylic adhesive, the acrylic monomer, the peroxide and the reducing agent and the epoxy adhesive composed of the epoxy resin precursor and the curing agent, the acrylic adhesive is preferably between 20 and 500 parts by weight, especially preferably between 100 and 200 parts by weight per 100 parts by weight of the epoxy adhesive. When the mixing figure for the acrylic adhesive is less than 20 parts by weight, no satisfactory fixing strength is obtained. Thus, it is undesirable. Meanwhile, when it is more than 500 parts by weight, the heat resistance of a bonded portion is poor, and reliability is decreased. Thus, it is undesirable. With respect to the (meth)acrylic monomer, the peroxide and the reducing agent which are components of the acrylic adhesive used in the present invention, at least one thereof is preferably coated with a thermoplastic resin. The adhesive can be cured when required by coating at least one thereof with the thermoplastic resin. As the above-mentioned thermoplastic resin for coating, a styrene resin, a (meth)acrylic resin, a polyester resin and a polyamide resin may be mentioned. The coating of the (meth)acrylic monomer, the peroxide and/or the reducing agent with the thermoplastic resin can be conducted by an interfacial polymerization method or an in-situ polymerization method. For example, the coating can be formed as follows. That is, first, an oily phase composed of a solvent, a.starting material (for example, a styrene monomer) for a thermoplastic resin, and a (meth)acrylic monomer, a peroxide or a reducing agent is formed. Separately from this, an emulsifying agent is added to water to form an aqueous phase. To this aqueous phase, the above-mentioned oily phase is gradually added dropwise, and then dispersed with a stirrer such as a homogenizer to form a suspension. Subsequently, heat and a catalyst are added to the suspension, and the polymerization is conducted while stirring the starting material for the thermoplastic resin on the surface of the (meth)acrylic monomer, the peroxide or the reducing agent to coat the (meth)acrylic monomer, the peroxide or the reducing agent with the thermoplastic resin. The coating of the (meth)acrylic monomer with the thermoplastic resin is conducted in the presence of a polymerization initiator that does not polymerize the (meth)acrylic monomer but selectively polymerizes the starting material of the thermoplastic resin and/or in the presence of a polymerization inhibitor that selectively inhibits the polymerization of the (meth)acrylic monomer. For the same reason as in the above-mentioned acrylic adhesive, it is preferable that the epoxy resin precursor and/or the curing agent, a component of the epoxy adhesive is coated with the thermoplastic resin. As the coating method, the same method as used with the acrylic adhesive can be employed. In the present invention, a microcapsule-type conductive filler obtained by coating the conductive fine particles with the insulating resin may be added to the mixed adhesive. When the microcapsule-type conductive filler is added, conduction can be achieved by exerting a pressure on a portion requiring conduction in the bonding to break the microcapsules. Incidentally, in a portion on which no pressure is exerted, microcapsules are not broken, thereby maintaining an insulating property. Such an adhesive containing the microcapsule-type conductive filler is called an anisotropic conductive adhesive. The conductive fine particles are not particularly limited. Fine particles of metals such as silver, gold, copper, nickel, palladium and rhodium, fine particles of alloys such as a silver-palladium alloy, and powders of amorphous carbon and graphite may be mentioned. Meanwhile, a thermosetting resin is preferably used as the insulating resin. Specifically, an epoxy resin and an epoxy-amine resin may be mentioned. With respect to the conductive filler, the particle diameter is preferably between 2 and 12 µm. When it is less than 2 µm, unevenness of intervals in plural portions requiring the conduction cannot be alleviated, and poor conduction occurs. Thus, it is undesirable. Meanwhile, when it is more than 12 µm, there is a fear that an unnecessary portion might be rendered conductive. Thus, it is undesirable. The coating of the conductive fine particles with the insulating resin can be formed as follows. An oily phase composed of a solvent, a starting material for an insulating resin and conductive fine particles is formed. Separately from this, an emulsifying agent is added to water to form an aqueous phase. To this aqueous phase, the above-mentioned oily phase is gradually added dropwise, and dispersed with a stirrer such as a homogenizer to form a suspension. Subsequently, heat and a catalyst are added to the suspension, and the polymerization is conducted while stirring the starting material of the insulating resin on the surfaces of the conductive fine particles, whereby the conductive fine particles can be coated with the insulating resin. A filler (for example, alumina and aluminum nitride) may be added to the mixed adhesive used in the bonding method of the present invention. The proportion of the filler used is preferably between 1 and 80% by weight based on the mixed adhesive. It is especially preferably between 20 and 60% by weight. To the mixed adhesive used in the bonding method of the present invention may be added as required, additives such as an elastomer and a (meth)acrylic resin for enhancing peel strength and impact strength, a paraffin wax for increasing surface curability of the adhesive, an antioxidant for increasing stability, a plasticizer, a filler (for example, alumina and aluminum nitride), a coupling agent for improving adhesion strength, and a coloring agent. Further, an organic solvent such as methylene chloride, carbon tetrachloride, methyl ethyl ketone, methyl isobutyl ketone, acetone, benzene, toluene, xylene, ethyl acetate, butyl acetate, n-hexane, cyclohexane or the like may be added as required. Bonding methods are described below. The first, described for assistance in understanding the present invention but not embodying it, is the case when none of the (meth)acrylic monomer, the peroxide and the reducing agent constituting the acrylic adhesive are coated with the thermoplastic resin. In this case, for preventing curing by the polymerization of the (meth)acrylic monomer, it is necessary that the (meth)acrylic monomer is not contacted with the reducing agent or the peroxide simultaneously until these are to be used. Accordingly, a method is used in which a two-pack adhesive of a component (agent A) containing the (meth)acrylic monomer and a component (agent B) containing the reducing agent or the peroxide is previously prepared and agent A and agent B are mixed only during the bonding. Further, it is advisable that the epoxy resin precursor and the curing agent constituting the epoxy adhesive are added separately to the above-mentioned agent A or agent B which is to contain therein. Incidentally, when the (meth)acrylic monomer and the peroxide are present at the same time, the curing of the (meth)acrylic monomer proceeds gradually. However, for controlling this curing, it is advisable to store the adhesive in a cold, dark position. Further, when the reducing agent, the peroxide and the curing agent are present at the same time, it is advisable to use them in such a combination that they are not reacted with one another. As such a combination, for example, a combination of dimethylaniline (reducing agent), benzoyl peroxide (peroxide) and imidazole (curing agent) may be mentioned. When the reducing agent and the peroxide are here present at the same time, the peroxide is converted to a radical. However, the present inventors have identified that this conversion to a radical does not influence a functioning of the peroxide as it cures the (meth)acrylic monomer. Specifically, there may be mentioned a procedure in which there is used (1) an agent A containing the (meth)acrylic monomer, the peroxide, the epoxy resin precursor and the curing agent and an agent B composed of the reducing agent. The procedure as shown in, for example, Fig. 1(a) is carried out with agent A 51 being coated on a functional element 1 and agent B 52 being coated on the mounting substrate 3. In this case, agent A and agent B are coated so as to cover bumps 2 (lead terminals) formed on the functional element and electrodes 4 (pad portions) formed on the mounting substrate 3. In the two-pack adhesive, the above-mentioned combination is not critical. For example, there may be used combinations of (2) an agent A composed of the (meth)acrylic monomer, the reducing agent, the epoxy resin precursor and the curing agent and an agent B composed of the peroxide, (3) an agent A composed of the (meth)acrylic monomer, the epoxy resin precursor and the curing agent and an agent B composed of the reducing agent and the peroxide, (4) an agent A composed of the (meth)acrylic monomer and the reducing agent and an agent B composed of the peroxide, the epoxy resin precursor and the curing agent, (5) an agent A composed of the (meth) acrylic monomer, the peroxide and the curing agent and an agent B composed of the epoxy resin precursor and the reducing agent, (6) an agent A composed of the (meth)acrylic monomer and an agent B composed of the peroxide, the reducing agent, the epoxy resin precursor and the curing agent. In case (1), in particular, such that each bump 2 of the above-mentioned functional element 1 is opposed to each corresponding electrode 4 of the wiring circuit substrate 3 as shown in Fig. 1(b), the functional element 1 is pressed on the mounting substrate 3 at room temperature. At this time, the peroxide of the agent A is contacted with the reducing agent of the agent B, and the peroxide is converted to a radical with the reducing agent. The (meth)acrylic monomer is polymerized by means of the radicals from the peroxide, and a cured adhesive 5 is obtained. Accordingly, there is no need to conduct pressing with heating as in the ordinary epoxy adhesive, and it is possible to bond the functional element 1 to the mounting substrate 3 at room temperature. Consequently, a conventional press fitted with a heating mechanism is not needed. In this bonding method only the pressing is conducted at room temperature as a first curing without heat treatment, so that there is no fear that the bonded portion might undergo a thermal stressing. Therefore, even when the bonded portion is made of a material that is easy to deform, there occurs no deformation by heating. Further, it is possible to widen the margin of the conditions for the first curing that can maintain the high reliability of the bonded portion. Besides a method in which the agent B is coated on the mounting'substrate and the agent A on the functional element followed by fixing, alternatively (a) agent A may be coated on the mounting substrate and agent B on the functional element followed by fixing, (b) agent A and agent B may be laminated and coated on the mounting substrate or the functional element and the functional element or the mounting substrate is fixed, or (c) agent A and the agent B may be previously mixed, and the mixture is coated on the functional element and/or the mounting substrate within a usable period of time followed by fixing, provided that the time between mixing and fixing is not so long that the mixture becomes no longer usable. Subsequently, at least the bonded portion is heated at a temperature appropriate for curing the epoxy resin precursor. The temperature appropriate for curing varies depending on the type of the epoxy resin precursor. It is usually between about 80 and 150°C. When solid imidazole is used here as the curing agent, it is advisable to conduct heating above a temperature at which imidazole melts. The curing agent is reacted with the epoxy resin precursor by this heating, and the epoxy resin precursor is polymerized by crosslinking in a second curing. Consequently, the bonded portion which is free from a thermal stress, has excellent heat resistance and good reliability and stability can finally be obtained. Now a method embodying the present invention, in which at least one of the (meth)acrylic monomer, the peroxide and the reducing agent constituting the acrylic adhesive is coated with a thermoplastic resin, will be described. In this case, the (meth)acrylic monomer does not undergo a curing reaction until the thermoplastic resin coating(s) of the above-mentioned component(s) which together produce the acrylic adhesive. Accordingly, there is no need to separate the respective components as in the above-mentioned case in which these are not coated, nor is a method of mixing the components particularly limited. The encapsulation of any one component of the adhesive system provides it as a pack distinct from any other components whether or not the latter is/are encapsulated. While the mixed adhesive is interposed between the mounting substrate and the functional element, the functional element is pressed to physically break the coating with the thermoplastic resin, so that the (meth)acrylic monomer is cured (the first curing). Here, the coating may be broken by heating the whole at a temperature at which the coating thermoplastic resin is melted simultaneously with the pressing. Then, as in the case of not coating with the thermoplastic resin, at least the bonded portion is heated to a temperature appropriate for curing of the epoxy resin precursor to achieve the second curing. When there is coating of the epoxy resin precursor and/or the curing agent, the components of the epoxy adhesive, is coated with the thermoplastic resin, the thermoplastic resin is melted by the heating in the second curing, and the epoxy resin precursor is contacted with the curing agent to conduct the heat-curing. Further, when the microcapsule-type conductive filler obtained by coating conductive fine particles with insulating resin is added to the above-mentioned mixed adhesive, the functional element is pressed on the mounting substrate through the adhesive, and then heated, whereby the electrical connection between the fine functional element and the mounting substrate can be achieved satisfactorily and stably. The adhesive containing the microcapsule-type conductive filler is termed herein an anisotropic conductive adhesive, as noted above. In the present invention, there is also provided the assembly of the mounting substrate in which the pad portions of the mounting substrate and the lead terminals of the functional element mounted thereon are electrically connected by the anisotropic conductive adhesive and the bonding method. EXAMPLES Examples are specifically illustrated below with reference to Figs. 2(a) to 2(c) and Figs. 3(a) and (b) of the drawings which are sectional views of features which play a part in bonding methods described below. In the Examples only Examples 13 to 20 embody the present invention. Example 1 100 parts by weight of an acrylic monomer and 1 part by weight of benzoyl peroxide (peroxide) were uniformly mixed. Subsequently, an epoxy adhesive obtained by uniformly mixing 100 parts by weight of a bisphenol F-type epoxy resin precursor with 10 parts by weight of solid imidazole was mixed with the above-mentioned mixture of the acrylic monomer and benzoyl peroxide. Finally, approximately 250 parts by weight (approximately 49.9% by weight) of a filler formed of an alumina (AlO 3 ) powder having an average particle diameter of 0.5 µm were dispersed in the resulting mixed adhesive. The component obtained is called an agent A. Meanwhile, dimethylaniline was used as a reducing agent. This reducing agent is called an agent B. As shown in Figs. 2(a) and 2(b), the agent A in a two-pack adhesive composed of the agent A and the agent B was coated on a predetermined region 31 (agent A coating surface) on a wiring circuit substrate (mounting substrate) 3, and the agent B was coated on a bonding surface 11 (agent B coating surface) of an integrated circuit chip (functional element) 1. Subsequently, as shown in Fig. 2(c), electrodes 4 of the wiring circuit substrate 3 and bumps 2 of the integrated circuit chip 1 are aligned to be opposite to each other. In Fig. 2(c), reference numeral 53 indicates the agent A, and reference numeral 54 the agent B. Then, as shown in Fig. 3(a), the integrated circuit chip 1 was pressed on the wiring circuit substrate 3 in the direction shown by an arrow at room temperature and a pressure of from 5 to 7 kg/cm 2 with a pressing base 6a and a pressing portion 6b. At this time, the peroxide of the Agent A and the reducing agent of the B agent are brought into contact, and the peroxide was converted to a radical form with the reducing agent. The peroxide-derived radicals were reacted with the acrylic monomer to achieve the first curing of the adhesive 5. The integrated circuit chip 1 was fixed on the wiring circuit substrate 3 by this first curing at room temperature. Accordingly, it was possible to conduct the fixing without imparting a thermal stress to the bonded portion. The wiring circuit substrate 3 on which the integrated circuit chip 1 was fixed by the first curing was heated from the side of the integrated circuit chip 1 at 150°C for approximately 2 hours (refer to Fig. 3(b)). The epoxy resin precursor was heat-cured (second curing) by this heating. As a result, the bonded portion requiring conduction did not suffer from poor conduction, and an insulation resistance of 10 8 Ω - or more was exhibited in the bonded portion not requiring conduction. Further, the bonded portion was free from a thermal stress, excellent in the heat resistance, and good in the reliability. Moreover, a conduction test and an insulation test were conducted using the above-mentioned agent A and agent B. The test conditions are described below. First, the two-pack adhesive composed of the above-mentioned agent A and agent B in the combinations shown in Table 1 was coated on a predetermined surface of an integrated circuit chip 1 cm 2 square and a predetermined region of a wiring circuit substrate (having 238 electrode terminals mounted at a pitch of 125 µm) made of a glass epoxy respectively. Sample Component coated on a surface of an integrated circuit chip Component coated on a surface of a wiring circuit substrate No. 1 B agent Agent A No. 2 Agent A B agent The bumps on the coated surface of the integrated circuit chip and the electrodes on the coated surface of the wiring circuit substrate were aligned. Further, the integrated circuit chip was pressed on the wiring circuit substrate with a press at a pressure of from 5 to 7 kg/cm 2 to cure the acrylic monomer component of the above-mentioned adhesive and fix the integrated circuit chip and the wiring circuit substrate at room temperature. The case in which the agent A was coated on the integrated circuit chip side and the agent B on the wiring circuit substrate side was designated Sample No. 1, and the case in which the agent A was coated on the wiring circuit substrate side and the agent B on the integrated circuit chip side was designated Sample No. 2. Further, 20 pieces of each of Sample No. 1 and Sample No. 2 were produced. Then, each Sample No. 1 and Sample No. 2 was heated at 150°C for 2 hours to heat-cure the epoxy resin precursor. A conduction test and an insulation test were conducted on the resulting Samples No. 1 and Samples No. 2. The conduction test was conducted by applying a DC voltage of 5 V between the bumps and the electrodes of the integrated circuit chip and the wiring circuit substrate under an environment of 85°C and a humidity of 85% for 500 hours, and then measuring the resistance of the conductive portion. As a result of the measurement, the adhesive was judged to be an appropriate one so long as the increase in the resistance of the conductive portion was within 10%. The insulation test was conducted by applying a voltage under the same conditions as in the above-mentioned conduction test and then measuring the resistance of the insulating portion. As a result of the measurement, the adhesive was judged to be appropriate so long as the resistance of the insulating portion was 10 8 Ω or more. The temperature cycle test was conducted by defining cooling at -50°C and heating at 125°C as one cycle, repeating this one cycle 500 times and then measuring the resistance of the conductive portion and the insulating portion. As a result of the measurement, the adhesive was judged to be an appropriate one so long as the increase in the resistance of the conductive portion was within 10% and the resistance of the insulating portion was 10 8 Ω or more. The results of the above-mentioned tests are shown in Table 2. Samples No. 1 No. 2 Results of conduction test (number of poor results/number of tests) 0/20 0/20 Results of insulation test (number of poor results/number of tests) 0/20 0/20 Results of temperature cycle test (number of poor results/number of tests) 0/20 0/20 As is apparent from Table 2, good results were obtained such that none of the samples showed poor conduction and insulation resistance was 10 8 Ω or more. In the results of the temperature cycle test too, poor conduction and poor insulation did not occur at all, and no poor bonded portion owing to deterioration was observed. Accordingly, bonding with quite good heat resistance was provided. Notwithstanding curing of the epoxy resin precursor having been conducted under heating conditions of 150°C and 2 hours in the above-mentioned bonding, the above-mentioned temperature cycle test was also conducted in which the heating conditions changed to 70°C and 2 hours, 70°C and 4 hours, 80°C and 2 hours, 80°C and 4 hours, 100°C and 2 hours, and 120°C and 2 hours. The results are shown in Table 3. Sample Heating conditions Results of temperature cycle test (number of poor results/number of tests) Temperature Time No. 1 70°C 2 hours 7/20 70°C 4 hours 8/20 80°C 2 hours 0/20 80°C 4 hours 0/20 100°C 2 hours 0/20 120°C 2 hours 0/20 150°C 2 hours 0/20 No. 2 70°C 2 hours 6/20 70°C 4 hours 9/20 80°C 2 hours 0/20 80°C 4 hours 0/20 100°C 2 hours 0/20 120°C 2 hours 0/20 150°C 2 hours 0/20 As is apparent from this Table 3, when the heating was at 70°C, poor conduction and poor insulation were noted owing to the heat deterioration in the bonded portion occurred in both Sample No. 1 and Sample No. 2. Accordingly, the heating temperature was found to be preferably 80°C. In a variant to the above procedures, tests were conducted in the same manner as mentioned above except that cumene hydroperoxide was used as a peroxide and ferrous sulfide (FeS) as a reducing agent. The results obtained were good as mentioned above. Example 2 One hundred parts by weight of an acrylic monomer, 5 parts by weight, per 100 parts by weight of the acrylic monomer, of a reducing agent, 100 parts by weight of an epoxy resin precursor and 10 parts by weight, per 100 parts by weight of the epoxy resin precursor, of solid imidazole as in Example 1 were uniformly mixed to obtain an agent A. Subsequently, a component B composed mainly of a peroxide (5 parts by weight per 100 parts by weight of the acrylic monomer) was obtained. The conduction test, the insulation test and the temperature cycle test were conducted as in Example 1 using this two-pack adhesive. Used as the peroxide and the reducing agent were a combination of benzoyl peroxide and dimethylaniline and a combination of cumene hydroperoxide and ferrous sulfide. In Example 2, as in Example 1, the bonded portion was free from thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and excellent heat resistance was obtained. Example 3 The conduction test, the insulation test and the temperature cycle test were conducted as in Example 2 except that only the acrylic monomer, the epoxy resin precursor and the curing agent were contained in agent A and the peroxide and the reducing agent were contained in agent B. In Example 3, as in Examples 1 and 2, the bonded portion was free from a thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and excellent heat resistance was obtained. Example 4 The conduction test, the insulation test and the temperature cycle test were conducted as in Example 2 except that only the acrylic monomer and the reducing agent were contained in the agent A and the epoxy resin precursor, the curing agent and the peroxide were contained in the agent B. In Example 4, as in Examples 1 to 3, the bonded portion was free from thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and excellent heat resistance was obtained. Example 5 The conduction test, the insulation test and the temperature cycle test were conducted as in Example 2 except that the acrylic monomer, the peroxide and the curing agent were contained in the agent A and the epoxy resin.precursor and the reducing agent were contained in the agent B. In Example 5, as in Examples 1 to 4, the bonded portion was free from thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and excellent heat resistance was obtained. Example 6 The conduction test, the insulation test and the temperature cycle test were conducted as in Example 2 except that the acrylic monomer was contained in the agent A and the epoxy resin precursor, the curing agent, the reducing agent and the peroxide were contained in the agent B. Also in Example 6, as in Examples 1 to 5, the bonded portion was free from thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and the excellent heat resistance was obtained. Example 7 One hundred parts by weight of an acrylic monomer (3921 made by Three Bond or 326 LVUV made by Nippon Locktight) and an epoxy adhesive obtained by uniformly mixing 100 parts by weight of an epoxy resin precursor with 10 parts by weight, per 100 parts by weight of the epoxy resin precursor, of the curing agent used in Example 1 were mixed. In the resulting mixture were further dispersed 40% by weight, in terms of a weight ratio, of a microcapsule-type conductive filler obtained by coating surfaces of silver fine particles having an average particle diameter of approximately 7 µm with an insulating resin obtained by the reaction of an epoxy resin with an amine compound such as tetraethylenepentaamine, hexamethylenediamine or the like and 30% by weight, in terms of a weight ratio, of a silica (SiO 2 ) powder to obtain an agent A. Meanwhile, a redox initiator (3941 made by Three Bond or TDS-7649 made by Nippon Locktight), a combination of a peroxide made of benzoyl peroxide and a reducing agent made of dimethylaniline or a combination of a peroxide made of cumene hydroxyperoxide and ferrous sulfide, was used as agent B. Subsequently, of the two-pack anisotropic conductive adhesive composed of the agent A and the agent B, the agent A was coated on a predetermined region of a wiring circuit substrate, and the agent B was coated on a bonding surface of an integrated circuit chip. Then, electrodes of the wiring circuit substrate and bumps of the integrated circuit chip were aligned to be opposite to each other, and the integrated circuit chip was pressed on the wiring circuit substrate with by pressing at room temperature with a pressure of from 5 to 7 kg/cm 2 . At this time, the microcapsule portion of the microcapsule-type conductive filler in the Agent A held between the electrodes of the wiring circuit substrate side and the integrated circuit chip side was broken, and the fine silver particles therein flowed out of the microcapsules. As a result, the fine silver particles were held between the electrodes of the wiring circuit substrate side and the bumps of the integrated circuit chip side which were selectively bonded conductively. The peroxide was converted to radicals as a result of reaction with the reducing agent of the agent B and the radicals were reacted with the acrylic monomer of the Agent A to conduct a first curing in which the integrated circuit chip was fixed on the wiring circuit substrate at room temperature. The fixed portion did not undergo thermal stressing. Thereafter, the wiring circuit substrate on which the integrated circuit chip was fixed by the first curing was heated at 100°C for 2 hours. The thermoplastic resin coating on the curing agent was heat-dissolved by this heating to cause the curing agent to flow out, and the curing agent was reacted with an epoxy resin precursor to cure the epoxy resin precursor (second curing). Consequently, as in Example 1, the bonded portion requiring conduction was free from poor conduction, and an insulation resistance of 10 8 Ω or more was shown to exist in the bonded portion not requiring conduction. Further, the bonded portion was free from thermal stress, had excellent heat resistance, and also had good reliability. The conduction test, the insulation test and the temperature cycle test were conducted in the same manner as in Example 1. Consequently, as in Example 1, the bonded portion was free from thermal stress. As a result of tests, bonding with good conduction, good insulation resistance and the excellent heat resistance was obtained. Examples 8 to 12 The conduction test, the insulation test and the temperature cycle test were conducted as in Example 7 except that the acrylic monomer, the peroxide, the reducing agent, the epoxy resin precursor, the curing agent and the microcapsule-type conductive filler were combined as mentioned below. Consequently, as in Example 1, the bonded portion was free from thermal stress, and as a result of the tests, bonding with good conduction, good insulation resistance and excellent heat resistance was obtained. The adhesive system components were provided as follows: (1) two-pack anisotropic adhesive of agent A composed of the acrylic monomer, the epoxy resin precursor, solid imidazole, the peroxide and the microcapsule-type conductive filler and agent B composed of the reducing agent (Example 8) (2) two-pack anisotropic adhesive of agent A composed of the acrylic monomer, the epoxy resin precursor, solid imidazole, the reducing agent and the microcapsule-type conductive filler and agent B composed of the peroxide (Example 9) (3) two-pack anisotropic conductive adhesive of agent A composed of the acrylic monomer and the reducing agent and the agent B composed of the epoxy resin precursor, solid imidazole, the peroxide and the microcapsule-type conductive filler (Example 10) (4) two-pack anisotropic conductive adhesive of the agent A composed of the acrylic monomer, the peroxide and the microcapsule-type conductive filler and the agent B composed of the epoxy resin precursor, solid imidazole and the reducing agent (Example 11) (5) two-pack anisotropic conductive adhesive of agent A composed of the acrylic monomer and the microcapsule-type conductive filler and agent B composed of the epoxy resin precursor, solid imidazole, the reducing agent and the peroxide (Example 12) Examples 13 to 19 Seven types of adhesives having the following compositions were prepared. (1) the acrylic monomer coated with the thermoplastic resin, benzoyl peroxide (peroxide), dimethylaniline (reducing agent), the epoxy resin precursor, solid imidazole and alumina (filler) (Example 13) (2) the acrylic monomer coated with the thermoplastic resin, benzoyl peroxide coated with the thermoplastic resin, dimethylaniline, the epoxy resin precursor, solid imidazole and alumina (Example 14) (3) the acrylic monomer coated with the thermoplastic resin, benzoyl peroxide coated with the thermoplastic resin, dimethylaniline coated with the thermoplastic resin, the epoxy resin precursor, solid imidazole and alumina (Example 15) (4) the acrylic monomer coated with the thermoplastic resin, benzoyl peroxide, dimethylaniline coated with the thermoplastic resin, the epoxy resin precursor, solid imidazole and alumina (Example 16) (5) the acrylic monomer, benzoyl peroxide coated with the thermoplastic resin, dimethylaniline, the epoxy resin precursor, solid imidazole and alumina (Example 17) (6) the acrylic monomer, benzoyl peroxide coated with the thermoplastic resin, dimethylaniline coated with the thermoplastic resin, the epoxy resin precursor, solid imidazole and alumina (Example 18) (7) the acrylic monomer, benzoyl peroxide, dimethylaniline coated with the thermoplastic resin, the epoxy resin precursor, solid imidazole and alumina (Example 19) The amounts of the respective components were such that per 100 parts by weight of the acrylic monomer, the peroxide was 10 parts by weight, the reducing agent 10 parts by weight, the epoxy resin precursor 100 parts by weight, and solid imidazole 10 parts by weight. Further, alumina was added in an amount of 50% by weight based on the overall adhesive. With respect to the above-mentioned adhesives, the conduction test and the insulation test were conducted as in Example 1. The results are shown in Table 4. Sample Results of conduction test (number of poor results/number of tests) Results of insulation test (number of poor results/number of tests) Adhesive (1) 0/20 0/20 Adhesive (2) 0/20 0/20 Adhesive (3) 0/20 0/20 Adhesive (4) 0/20 0/20 Adhesive (5) 0/20 0/20 Adhesive (6) 0/20 0/20 Adhesive (7) 0/20 0/20 As is apparent from Table 4, the good results were provided such that none of the samples showed poor conduction and the insulation resistance was 10 8 or more. Further, the conduction test and the insulation test were conducted as mentioned above except that the epoxy resin precursor was heat-cured under heating conditions shown in Table 5. Heating conditions Temperature Time 120°C 2 hours 100°C 2 hours 80°C 2 hours 80°C 4 hours 70°C 2 hours 70°C 4 hours As a result, when the curing was conducted at 70° C, slightly poor insulation and slightly poor conduction occurred. Accordingly, it was found that the heat-curing was preferably conducted at 80°C or more. The tests were conducted in the same manner as mentioned above except that cumene hydroperoxide was used as a peroxide and ferrous sulfide (FeS) as a reducing agent. Consequently, the results were good as mentioned above. Example 20 The conduction test and the insulation test were conducted in the same manner as in Example 7 except that a microcapsule-type conductive filler formed by coating surfaces of fine silver particles having an average particle diameter of from approximately 5 to 10 µm with an insulating resin obtained by the reaction of an epoxy resin and an amine compound such as tetraethylenepentaamine, hexamethylenediamine or the like was added to each of the adhesives in Examples 13 to 19 in an amount of 60% by weight in terms of a weight ratio. In all the samples, good results were obtained such that no poor conduction was provided and the insulation resistance was 10 8 Ω or more. As is apparent from the foregoing description, the adhesive according to the present invention is a mixed adhesive containing an acrylic adhesive and an epoxy adhesive, and in the bonding method of the present invention, the mixed adhesive is used by being applied as a coating between two members to be bonded. The (meth)acrylic monomer contained in the acrylic adhesive is redox-polymerized by pressing this adhesive at room temperature to conduct a first curing. In this curing, the heating which was conducted so far is unnecessary as usual, so that the bonded portion can be fixed without thermal stressing. Moreover, when an anisotropic conductive adhesive containing the microcapsule-type conductive filler is used, the microcapsules were broken by the pressing in the bonding, and a conductive bonded portion free from thermal stress can be formed with the conductive filler which flows out. In addition, the epoxy resin precursor contained in the epoxy adhesive is contacted with the curing agent by heating after the first curing to conduct the second coating. This second coating makes it possible to obtain finally stable bonding free from thermal stress with excellent heat resistance and good reliability. Accordingly, it is quite advantageous that the bonding method of the present invention is applied to a bonding step (especially a fine pitch bonding) of a functional element such as an integrated circuit chip and a mounting substrate such as a wiring circuit substrate. Further, since the bonding free from thermal stress with excellent heat resistance can be realized at room temperature, it can be applied to a wide variety of bonding usages other than the above-mentioned usage, for example, in the fields in which a step of easily bonding components of a component assembly at room temperature is required. Excellent results are achieved in practice. Moreover, a mounting substrate assembly can be obtained in which the pad portions of the mounting substrate and the lead terminals of the functional element mounted thereon are electrically connected by the above-mentioned anisotropic conductive adhesive and bonding method.

Claims: An adhesive which is composed of an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent with at least one of the acrylic monomer, the peroxide and the reducing agent being coated within its own microcapsules by means of a thermoplastic resin to provide said at least one of the acrylic monomer, the peroxide and the reducing agent out of contact with the other(s) of the acrylic monomer, the peroxide and the reducing agent up until the microcapsules are broken down at the time of use. The adhesive as claimed in claim 1, wherein the adhesive is an anisotropic conductive adhesive further containing a microcapsule-type conductive filler obtained by coating conductive fine particles with an insulating resin. The adhesive as claimed in claim 1 or 2, wherein the curing agent is a solid imidazole. The adhesive as claimed in any one of claims 1 to 3, wherein the acrylic monomer is mixed in a proportion of from 0.1 to 500 parts by weight per 100 parts by weight of the epoxy resin precursor. The adhesive as claimed in any one of claims 1 to 4, which further contains a filler. The adhesive as claimed in any preceding claim, wherein the peroxide is benzoyl peroxide or cumene hydroperoxide, the reducing agent is dimethylaniline or ferrous sulphide and the curing agent is imidazole. A method of bonding two members, characterised in that there is coated between the two members an adhesive composition comprising an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent, at least one of the acrylic monomer, the peroxide and the reducing agent being coated with a thermoplastic resin to form microcapsules such that it remains out of contact with the other(s) of the acrylic monomer, peroxide and reducing agent until the microcapsules are broken down at the time of use, the two members are brought together with the acrylic monomer, peroxide and reducing agent coming into intimate contact with each other, the two members are fixed and the adhesive is then heat-cured. The bonding method as claimed in claim 7, wherein the heat-curing is conducted at a temperature in the range from 80 to 150°C. The bonding method as claimed in claim 8, wherein the adhesive is an anisotropic conductive adhesive further containing a microcapsule-type conductive filler obtained by coating conductive fine particles with an insulating resin. The bonding method as claimed in any one of claims 7 to 9, wherein the curing agent is coated with the thermoplastic resin to form microcapsules. The bonding method as claimed in any one of claims 7 to 10, wherein the acrylic monomer is mixed in a proportion of from 0.1 to 500 parts by weight per 100 parts by weight of the epoxy resin precursor. The bonding method as claimed in any one of claims 7 to 11, wherein the adhesive further contains a filler. A mounting substrate assembly in which pad portions of a mounting substrate and lead terminals of a functional element are to be mounted thereon and electrically connected by means of an anisotropic conductive adhesive, characterized in that the anisotropic conductive adhesive is an adhesive which is composed of an acrylic monomer, a peroxide, a reducing agent, an epoxy resin precursor and a curing agent with at least one of the acrylic monomer, the peroxide and the reducing agent being coated within its own microcapsules by means of a thermoplastic resin to provide said at least one of the acrylic monomer, the peroxide and the reducing agent out of contact with the other(s) of the acrylic monomer, the peroxide and the reducing agent up until the microcapsules are broken down at the time of use, the adhesive further containing a microcapsule type conductive filler.

Inventors: DATE HIROAKI, IMAIZUMI NOBUHIRO, MOTOYAMA YUKO, TOKUHIRA HIDESI, USUI MAKOTO, DATE, HIROAKI, IMAIZUMI, NOBUHIRO, MOTOYAMA, YUKO, TOKUHIRA, HIDESI, USUI, MAKOTO, DATE, Hiroaki Fujitsu Limited 1-1, Kamikodanaka, IMAIZUMI, Nobuhiro Fujitsu Limited, MOTOYAMA, Yuko Fujitsu Limited, TOKUHIRA, Hidesi Fujitsu Limited, USUI, Makoto Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1998
Main classification: C09J163
IPC codes: H05K3, H05K1, C09J4, H01L21, C09J5, C09J163, C09J9