Connecting material

Abstract: A connecting material for bonding and connecting elements each having electrodes thereon in a correpondingly confronted relation to each other, while attaining electroconductive connection between the corresponding electrodes, which material has a high heat resistance and can avoid occurrence of faulty electric conductance even in the case of bonding such elements as having a large number of electrodes arranged, thus, at a considerable small interval under such a condition that the bonded assembly is exposed to a service environment of high temperature or of high temperature and high humidity, wherein the said connecting material comprises a thermosetting resin and an inorganic filler and has, after having been cured, characteristic features comprising a modulus of elasticity of 1 - 12 GPa, a glass transition temperature Tg of 120 - 200 °C, a coefficient of linear expansion (α 1) of 50 ppm/°C or less at temperatures below the Tg and a coefficient of linear expansion (α 2) of 110 ppm/°C or less at temperatures above the Tg, wherein the difference (α 2 - α 1) does not exceed over 60 ppm/°C.

Description: FIELD OF THE INVENTION The present invention relates to a connecting material for bonding and connecting elements to be bonded together each having a plurality of electrodes opposite to those on the other element, in particular, a connecting material containing a thermosetting resin. DESCRIPTION OF THE RELATED TECHNIQUES For assembling a semiconductor, such as IC, LSI or so on, on a substrate circuit board, a practice has been employed, in which a semiconductor chip, such as bear chip, is mounted directly on the substrate circuit board using a connecting material. Here, the bonding of such a chip with the circuit board is performed by holding them in a posture in which the electrodes or terminals disposed on the chip and on the circuit board are in a correspondingly confronted relationship to each other, while interposing the connecting material therebetween, whereupon the connecting material is caused to harden to attain mechanical firm bonding of them and an assured electroconductive connection between the corresponding electrodes simultaneously. In such a connecting material, a thermosetting resin has been used as the fundamental constituent. The connecting material is interposed between the substrate circuit board and the semiconductor chip and they are held at such a posture that the electrodes or terminals to be electroconductively connected together disposed on them are in a correspondingly opposing relationship to each other, whereupon the resulting assemblage is heat-pressed from both sides by pressing it with heating to cause the thermosetting resin to set to thereby attain a firm bonding of them. Here, the mechanical bonding of the chip with the substrate circuit board is established by the bonding strength (adhesive strength) of the resin and the electroconductive connection between the corresponding electrodes or terminals is attained by a pressed friction contact of them secured by the heat setting of the resin. This electroconductive connection between the opposing electrodes may be attained by direct contact of the electrodes with each other or under intermediation by bridging therebetween by electroconductive particles contained in the connecting material in a dispersed state. The substrate circuit board having assembled thereon semiconductor chip(s) is required to have a heat resistance for withstanding the condition in the step of reflowing of the solder in which the substrate board is exposed to high temperature. The resulting assembly of the circuit board may be put in service under an environmental condition of relatively high temperature and relatively high humidity, so that it should withstand such a condition. However, the heat resistance and the fastness to environmental condition of conventional connecting materials are not enough and, thus, a problem has been brought about in that occurrence of defective electric connection in the assembly is sometimes detected not only in the examination tests for the heat resistance and for the fastness to environmental condition, such as heat shock test, pressure cooker test (PCT) and solder-reflowing test, but also in the practical production process steps, when the assembly is exposed to a high temperature. Such a faulty electroconductive connection occurs often in assemblies having semiconductor chips with a narrow-pitched pattern in which a large number of electrodes are arranged within a confined region. Examination of the reason therefor has reached a discovery that a discrepancy or displacement in the settled positions of the electrodes occurs on connecting two elements with each other, of which coefficients of linear expansion are different from each other, using a connecting material, when the connecting material is brought to a temperature above the glass transition temperature (Tg) of the adhesive resin in the connecting material, as in the case where the material is exposed to such a high temperature as in the reflowing of solder, whereby a faulty electroconductive connection is caused. Here, it is assumed that decrease in the fastness to environmental condition may be caused by a possible interlayer exfoliation of the bonding layer at the interfaces between the connecting material layer and the elements after a prolonged service, since the internal stresses in the cured layer of the connecting material resulting from the hardening contraction of the adhesive resin, which is large for an adhesive resin exhibiting a high adhesive strength, are concentrated at these interfaces. Further connecting materials are disclosed in EP-A-387066 and EP-A- 914027. SUMMARY OF THE INVENTION An object of the present invention as claimed in claims 1,6 and 7 is to provide a connecting material for bonding and connecting elements each having electrodes thereon in a correspondingly confronted relation to each other, while attaining assured electroconductive connection between the corresponding electrodes, which connecting material exhibits a high heat resistance and will not suffer from occurrence of faulty electric connection even when the elements have a large number of electrodes arranged at a narrow interval and even if the connecting material is exposed to a condition of high temperature. Another object of the present invention is to provide a connecting material for bonding and connecting elements each having electrodes thereon in a correspondingly confronted relation to each other, which connecting material exhibits a high heat resistance and a high fastness to environmental condition and can maintain a highly reliable electroconductive connection even in services under an environmental condition of high temperature and high relative humidity. Thus, the present invention resides in the following connection material: (1) A connecting material for bonding and connecting elements each having electrodes thereon in a correspondiongly confronted relation to each other, comprising an adhesive component which comprises 20-75% by weight of an epoxy resin 25-80% by weight of silica and a thermoplastic resin in an amount of up to 40% by weight, the said material having, after having been cured, the characteristic features comprising a linear contraction of 0.25 % or less and a coefficient of linear expansion (α1) of 35 ppm/°C or lower at temperatures below the Tg of the cured material. (2) The connecting material as defined in the above (1), wherein it has, after having been cured, a moisture absorbability of 2 % or lower. (3) The connecting material as defined in the above (1) or (2), wherein it further comprises 0 - 30 %, based on the volume of the adhesive component, of electroconductive particles having an average particle size of 1 - 10 µm. DETAILED DESCRIPTION OF THE DISCLOSURE The connecting material of the present invention is excellent both in the heat resistance and in the fastness to environmental condition. For the elements to be bonded together by the connecting material according to the present invention, every pair of elements each having, on the face confronting another element, electrodes each opposite to corresponding one on the other element in a pair, in particular, a large number of electrodes to be connected electroconductively with corresponding counter electrodes on the other element may be served for the object of connection. The present invention may particularly be applicable to those in which each of the two elements to be bonded together has electrodes arranged within a confined region with a closer pitch, in a small width and at a narrow interval, as in the case of assembling a semiconductor chip, such as bear chip, on a substrate board. In many cases, a substrate board is used as the counter element for the above-mentioned semiconductor chip to be bonded. The connecting material according to the present invention can be used for assembling semiconductor chips and the like on the substrate board directly or under intermediation with, for example, interposer. Here, substrate boards made of any voluntary material may be used, for example, glass/epoxy substrate boards, resin boards, glass boards and flexible resin boards. The connecting material according to the present invention contains an adhesive component comprising an epoxy resin and silica. filler. The connecting material is interposed between the elements to be connected together and the elements are pressed onto each other from both sides so as to cause the electrodes disposed opposingly on the confronting face of each element to be brought into contact with each other, while holding the interspace between neighboring electrodes to be filled with the connecting material. In this state, the connecting material is caused to harden in order to attain electric connection and mechanical bonding simultaneously. The electric connection between the opposing electrodes may be realized either by a direct contact of the electrodes or under intermediation by electroconductive particles. If the surface area of thicker portion of the electrode, such as stud bump, is small (for example, 1,000 µm 2 or less), direct contact may be permitted, while a contact under intermediation with electroconductive particles is favorable for electrodes with greater surface areas. The electroconductive particles are incorporated in the connecting material in a form dispersed therein. As the epoxy resin, those of bisphenol type, those of epoxy-novolak type and those obtained from epoxy compounds having two or more oxirane groups in the molecule may be used. Commercial products of these epoxy resins may also be employed as such. While the main resin of the epoxy resin of the adhesive component can be subjected to hardening usually by a concurrent use of a hardening agent, it is permissible to dispense with the use of hardening agent, when a substituent functional group facilitating the hardening reaction is present in the molecule of the main resin. As the hardening agent, there may be used those which can be subjected to the hardening reaction with the main resin by the action of heat or irradiation of a ray, for example, imidazoles, amines, acid anhydrides, hydrazides, dicyandiamides and isocyanates as well as modified products of them. Commercial products may also be employed. For such a hardening agent, preference is given to a latent hardening agent. A latent hardening agent will not be subjected to a curing reaction during processing operations and storage at normal temperature and upon drying at a relatively lower temperature (40 - 100 °C) but is subjected to a curing reaction under pressure with heating (heat-pressing) at a curing temperature or by the action of irradiation of a ray, such as UV ray. For such a latent hardening agent, particular preferance is given to one in which the above-mentioned hardening agent, such as an imidazole or an amine, is encapsulated in microcapsules, for which commercial products may also be employed as such. For activation by heating, those having a curing initiation temperature of 80 - 150 °C may be preferred. The silica to be incorporated in the connecting material according to the present invention serves for improving the heat resistance and, in particular, the coefficient of linear expansion of the connecting material by admixing to the adhesive component containing the epoxy resin. For practical use, products of crystalline silica, molten silica and synthetic silica may be employed with preference to those having an average particle size of 0.1 - 5 µm. According to the present invention, it is permissible to admix a thermoplastic resin to the adhesive component in order to impart to the connecting material an ability for being coated on substrates or for film forming. For such a thermoplastic resin, there may be used, for example, a phenoxy resin, a polyester resin, an acrylic resin, a polyurethane resin, a butyral resin or a rubbery polymer, such as NBR or SBR. The adhesive component of the connecting material according to the present invention may further contain other additives, such as surfactant, coupling agent, ion-adsorbent, antioxidant and so on. The connecting material according to the present invention comprising the adhesive component described above may or may not contain electroconductive particles. Thus, it may contain particles of a metal, such as a solder metal, nickel metal or so on; electric conductor-coated particles in which core resin particles are coated with an electroconductive material by way of plating or the like; and insulator-coated particles in which these electroconductive particles are coated with an insulating resin. The average particle size of these electroconductive particles may be in the range from 1 to 20 µm, preferably in the range from 2 to 10 µm. The adhesive component in the connecting material according to the present invention comprises, in addition to the epoxy resin and silica, on requirement, other resins and additives. The connecting material comprises the adhesive componment described above and, on requirement, the above-mentioned electroconductive particles. The connecting material according to the present invention is constituted of the adhesive component which is composed of the epoxy resin in an amount in the range from 20 to 75 % by weight, preferably in the range from 30 to 70 % by weight, the thermoplastic resin in an amount of up to 40 % by weight, preferably up to 25 % by weight, silica in an amount in the range from 25 to 80 % by weight, preferably from 30 to 70 % by weight, and other additive(s) in an amount of 0 - 10 % by weight, preferably 0 - 5 % by weight, with, if necessary, incorporation of electroconductive particles therein in a proportion of 0 - 30 %, preferably 0 - 20 %, based on the volume of the adhesive component. The connecting material according to the present invention may be provided as a product in a form of a paste or a sheet. For preparing a paste of the connecting material, suitable constituent ingredients among those given above may be chosen to form a paste without using any solvent, while, in general, it is practical to form the paste by dissolving or dispersing the constituent ingredients in a suitable solvent. As the solvent, there may be used, for example, alcohols, ketones, esters, ethers, phenols, acetal and nitrogen-containing hydrocarbons, among which toluene, MEK, ethyl acetate and cellosolve acetate may be exemplified. The amount of the solvent to be used may be about 20 - 40 %, based on the weight of the resin components. For preparing the connecting material in the form of a sheet, the above paste of the connecting material is coated on an exfoliative film in a layer, whereupon the solvent of the paste is volatilized to build up a sheet. The connecting material according to the present invention should be formulated from the above-given constituent ingredients so that the resulting connecting material after having been cured will have a linear contraction of 0.25 % or lower, preferably 0.22 % or lower, and a coefficient of linear expansion (α1) at temperatures below the Tg of the cured connecting material of 35 ppm/°C or lower, preferably 33 ppm/°C or lower, by selecting suitable ingredients and their proportions. Here, the second connecting material may favorably have, after having been cured, a moisture absorbability of not higher than 2 % by weight, preferably not higher than 1.7 % by weight. The concrete procedures for the determination of the above-mentioned characteristic properties are as follows: Tg is determined as the temperature at the peak of tan δ on the determination of the elastic modulus. Linear contraction was determined by the method according to JIS K-6911. Moisture absorbability was determined by holding the sample at 85 °C, at a relative humidity of 85 % for 500 hours and by calculating from the weight difference of the sample observed before and after this treatment. THE BEST MODE FOR EMBODYING THE INVENTION The connecting material according to the present invention is interposed between two elements to be connected together, such as a substrate circuit board and a semiconductor chip, each provided on the confronting face with a plurality of electrodes, while holding these elements in a posture in which the electrodes on the confronting face of each of the elements are in a correspondingly confronted relation to each other, whereupon the elements are heat-pressed by pressing them together from both sides with heating to cause the thermosetting resin to be cured to form a solid assembly. In the case of using a paste of the connecting material, it is coated on one of the elements over a region to be bonded including the electrodes, whereupon the other one of the elements is placed on the so-coated face of the said one of the elements before or after drying the coated layer in such a position that the electrodes on both elements are in a correspondingly confronted relation to each other, followed by heat pressing of the assemblage to cause curing of the resin. In the case of using a sheet of the connecting material, it is interposed between the two elements to be connected together, followed by heat pressing of the assemblage to cause curing of the resin. The curing may be realized not only by heating but also by the action of irradiation of a ray, such as UV ray. In the connecting procedure described above, by pressing the connecting material interposed between the two elements to be bonded together from both sides onto each other with heating, the connecting material is first melted and expelled from the interspace between the confronting electrodes aside to the free space where no electrode is found, until the confronting electrodes on the elements have been brought into contact with each other to thereby the confronting electrodes be pressed together to build up an electroconductive frictional contact of them, while the thermosetting resin in the connecting material is then cured there to build up the solid assembly. In case electroconductive particles are included, some of these particles will remain in between the confronting electrodes upon the heat pressing and are pressed there by the confronting electrodes to build up a bridge of electroconductive frictional contact between these confronting electrodes. The portion of the connecting material expelled from the interspace between the confronting electrodes aside to the electrode-free space will be cured there to build up a solid fixed bonding between the two elements. In this manner, the electroconductive connection between the confronting electrodes and the mechanical bonding of the two elements are established simultaneously. In the case of installing a semiconductor chip, as one element to be bonded, on the substrate circuit board, as the counter element to be bonded, the former may be assembled on the latter by the connecting material according to the present invention, and the resulting assembly is superior in the fastness to service environment of high temperature and high humidity due to suppresion of accumulation of internal stresses, since the linear contraction and the coefficient of linear expansion α1 are low. During testing for examining the heat resistance by environmental tests, including heat shock test, PCT and solder-reflowing test, there occurs also no faulty electric connection of the electrodes. The connecting material according to the present invention is superior in the fastness to environmental condition of high temperature and high humidity in services, because of that the connecting material comprises an adhesive component comprising a thermosetting resin and an inorganic filler in such a combination and proportion that the linear contraction and the coefficient of linear expansion at temperatures below the Tg of the cured connecting material are each within a specific range. Below, the present invention will further be described by way of Examples and comparative Examples. Examples 1 to 11 and Comparative Example 1 〈〈 Preparation of the Connecting Material 〉〉 A paste of the connecting material was prepared by kneading the epoxy resin A given above or an epoxy resin C (a product of the firm Yuka Shell-Epoxy with trade name EP828) as the thermosetting resin, a hardening agent based on imidazole (a product of the firm Asahi Ciba with trade name HX-3941 HP) as the hardening agent, a phenoxy resin (a product of Tohto Kasei Co., Ltd. with trade name YP 50) as the thermoplastic resin, a commercial silica (a product of the firm Tatsumori with tradename SOE 2, with an average particle size of 0.5 µm) as the inorganic filler and electric conductor-coated particles (a product of Nippon Chemical Industrial Co., Ltd. with trade name 20GNR-4.6EH, with an average particle size of 5 µm) as the electroconductive particles in a proportion given in Table 2 together with toluene, whereupon the resulting paste was coated on an exfoliative film and the coating layer was dried to obtain a connecting material as a sheet having a thickness of 35 µm. The properties of the connecting material after having been cured are recited in Table 2. 〈〈Material Property Test〉〉 Linear contraction A sheet sample (thickness 40 µm) of the dried uncured connecting material removed from the exfoliative film was cut into ribbons of a size of approximately 1 cm × 10 cm and the precise length of the resulting ribbon was determined before and after the ribbon was subjected to a heat treatment for heat-setting the thermosetting resin in an oven at 100 °C for one hour and at 150 °C for further one hour, wherefrom the percent linear contraction of the connecting material was calculated by the equation Linear contraction % = [1 - (L 1 /L 0 )] × 100 in which L 0 is the length of the ribbon before the heat setting and L 1 is the length thereof after the heat setting. Coefficient of linear expansion Using a sample in a form of a cylinder with a diameter of 5 mm and a length of 15 mm, which had been cured at 190 °C for one hour, the coefficeint of linear expansion of the cured connecting material was calculated from the difference in the expansion between this cured sample and a quartz glass sample of the same size on heating them from room temperature to 200 °C. Moisture absorbability A sample in a form of a cylinder with a diameter of 5 mm and a length of 15 mm, which had been cured at 190 °C for one hour, was caused to absorb moisture in an oven under a high temperature and a high relative humidity, whereupon the moisture absobability (%) was calculate by the following equation. Moisture absorbability (%) = [1 - (W 0 /W 1 )] × 100 in which W 0 is the sample weight before absorption of moisture and W 1 is the sample weight after absorption of moisture. 〈〈Fastness Evaluation Test〉〉 An IC chip having metal-plated bumps disposed at a pitch of 150 µm with a bump surface area of 1,000 µm 2 and a bump thickness of 20 µm as the electrodes was assembled on a substrate circuit board having printed pattern of gold-plated copper as the counter electrodes in a relation correspondingly confronted to those of the bumps under intermediation by the above connecting material sample sheet, whereupon the resulting assemblage was heat-pressed at 180 °C at a pressing force of 1.47 N (150 gf) per one bump for 20 seconds. The resulting assembly was examined by the following PCT and heat-shock test. The results are recited in Table 1. In Table 1. "Eval. A" is expressed by the evaluation of electroconductive connection in the connected assembly by a PCT of 300 hours by the evaluation criteria given below under a condition of 121 °C, 100 % relative humidity and a pressure of 2 atm., after the assembly has been passed twice a solder-reflowing test with the highest reachable temperature of 240 °C; "Eval. B" is expressed by the evaluation of electroconductive connection in the connected assembly by a heat-shock test by the evaluation criteria given below by repeating heat cycles in which the assembly is maintained at -55 °C and at +125 °C alternatively for each 15 minutes up to 1,000 cycles, after the assembly has been passed twice a solder-reflowing test with the highest reachable temperature of 240 °C; "Eval. C" is expressed by the evaluation of electroconductive connection in the connected assembly by a PCT by the evaluation criteria given below under the condition in which the assembly is subjected to absorption of moisture under a condition of 30 °C and 70 % relative humidity for 168 hours, followed by a solder-reflowing test with the highest reachable temperature of 240 °C repeated twice and a further PCT under a condition of 121 °C, 100 % relative humidity and a pressure of 2 atm. for 300 hours; and "Eval D" is expressed by the evaluation of electroconductive connection in the connected assembly by a heat-shock test by the evaluation criteria given below by repeating heat cycles in which the assembly is maintained at -55 °C and at +125 °C alternatively for each 15 minutes up to 1,000 cycles, after the assembly has been subjected to absorption of moisture under a condition of 30 °C and 70 % relative humidity for 168 hours and, then, passed twice a solder-reflowing test with the highest reachable temperature of 240 °C. Fastness Evaluation Criteria Grade Criterion ○ : All the observed electric resistances are lower than 1 Ohm. Δ : The highest observed electric resistance is not lower than 1 Ohm but lower than 3 Ohms. × : The highest observed electric resistance is not lower than 3 Ohms. From the results given in Table 2, it is seen that the connecting materials in Examples 1 to 11 are superior in the heat resistance and fastness, whereas the connecting material of Comparative Example 1 in which the content of inorganic filler is outside the value prescribed by the present invention is inferior in the fastness.

Claims: A connecting material for bonding and connecting elements each having electrodes or terminals thereon in a corresponding-positioned in relation to each other, comprising an adhesive component which comprises 20-75% by weight of an epoxy resin, 25-80% by weight of silica and a thermoplastic resin in an amount of up to 40% by weight, the said material having, after having been cured, the characteristic features comprising a linear contraction of 0.25% or less; and a coefficient of linear expansion (α1) of 35ppm/°C or lower at temperatures below the Tg of the cured connecting material. The connecting material as claimed in claim 1, which, after having been cured, is capable of absorbing no higher than 2% by weight of moisture. The connecting material as claimed in claim 1 or claim 2, wherein the thermoplastic resin is a phenoxy resin. The connecting material as claimed in claim 1 or claim 2, which further comprises 0-30%, based on the volume of the adhesive component, of electroconductive particles having an average particle size of 1-10µm. An article comprising at least two elements having on opposing faces thereof correspondingly-positioned electrodes or terminals which are bonded together by a connecting material as defined in any of claims 1-3. A process for manufacturing a connecting material for bonding and connecting elements each having electrodes or terminals thereon in a correspondingly positioned relation to each other comprising the steps of: applying an adhesive component which comprises 20-75% by weight of an epoxy resin, 25-80% by weight of silica and a thermoplastic resin in an amount of up to 40% by weight, to an element to be bonded to form a coating layer; drying the coating layer to give a connecting material; and curing the connecting material; wherein: a second element is placed on the so-coated face of the said element either before or after drying of the coated layer; said material, after having been cured, has a linear contraction of 0.25% or less; and the cured product has a coefficient of linear expansion (α1) of 35ppm/°C or lower at temperatures below the Tg of the cured connecting material. A process for manufacturing a connecting material for bonding and connecting elements each having electrodes or terminals thereon in a correspondingly positioned relation to each other comprising the steps of: applying an adhesive component which comprises 20-75% by weight of an epoxy resin, 25-80% by weight of silica and a thermoplastic resin in an amount of up to 40% by weight, to an exfoliative sheet; drying the coating layer to give a sheet of connecting material; interposing the sheet of connecting material between two elements to be bonded; and curing the connecting material; wherein: said material, after having been cured, has a linear contraction of 0.25% or less; and the cured product has a coefficient of linear expansion (α1) of 35ppm/°C or lower at temperatures below the Tg of the cured connecting material.

Inventors: SHINOZAKI JUNJI, TAKEICHI MOTOHIDE, YAGI HIDEKAZU, SHINOZAKI, JUNJI, TAKEICHI, MOTOHIDE, YAGI, HIDEKAZU
Applicants: SONY CHEMICALS CORP, SONY CHEMICALS CORPORATION
Application year: 2000
Main classification: H05K3
IPC codes: C09J7, H05K3, C09J9