A new diffusion joining method

Abstract: The present invention relates to a diffusion joining method of Cu or Cu alloy surface and a conductive paste used therefor, especially applied to a method for preparing a multilayer printed wiring board. The present invention is characterized by providing the contacting surfaces with a layer selected from the group comprising a noble metal thin layer, a metal oxide remover layer and a conductive paste layer mainly consisting of a Cu or Cu alloy particles and the metal oxide remover; and pressing the contacting metal surfaces at a temperature higher than 170 °C more or less whereat the Cu atom at interface of the Cu or Cu alloy surfaces to be joined becomes to be able to diffuse, to give a joined metal body.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a new diffusion joining method used for joining Cu or Cu alloy contacting surfaces at a low temperature for use in metallurgical technology and electronic engineering; the method may be applied particularly for preparing a multilayer printed wiring board. Herein is also described a paste used in the method. Description of the prior Art In order to join at least two contacting metal surfaces, there has hitherto widely been used a joining method such as a soldering. In the electronic engineering, there has been sometimes used a ultrasonic joining method, but in general a reliable electric connection could not be obtained by any method other than the soldering method. However, in the soldering method, a contacting metal component such as lead or tin different from the copper of the surfaces to be connected sometimes causes generation of corrosion in the joint and also causes formation of intermetallic compound, which results in the problems of sick joint in which cracks easily generate by repeating stress. The above ultrasonic joining does not offer the above problems, but offers the problem that there is obtained only a small connecting area which can not be applied to the usual metal joint. Still another typical but not practical method to to join the copper surfaces at low temperature is described by R. A. Nichting et al. in Journal of Materials Engineering and Performance, Vol. 1, No. 1 (1992). Here the surfaces of the copper to be joined are treated by means of sputter cleaning, and are joined in a high vacuum. The report by R. A. Nichting clearly shows the limitation of the former technology in that joining of the copper in an air atmosphere is not possible. Many kinds of resin dispersive type conductive pastes have been used in methods for connecting electronic parts or circuits and formating electroconductive paths. However, there is the problem that the resistivity of the contact or the path is relatively high and thus tends to indicate non-ohmic characteristics by which the resistance increases at low current ranges, so that use of the conventional conductive paste is limited. The methods used for connecting the via holes of a multilayer printed wiring board in the electronic circuits are classified into two groups. One is a method for forming a conductive path in the via hole by means of electroplating, the other a method for filling the via holes with the conductive paste which is referred to as Ag through hole method. The electroplating method can provide via holes superior in conductivity and reliability while it requires many processes and facilities for after-treating electroplating or washing drainage, which disadvantageously costs much. The other method can provide an easy to make and inexpensive Ag through hole while it leads to a relatively high resistance and thus tends to create non-ohmic characteristics by which the resistance increases in a low current range, so that use of the conventional conductive paste is limited. US patent 4,228,944 discloses substrates made of a metal or an alloy which are bonded in the air by coating a brazing flux containing powdered metal or powdered metal compound on the surface of the substrate having a melting point greater than 700 °C. The coated substrates are heated to a temperature higher than the melting point of the flux, but lower than the melting point of the metal or of the metal compound. This causes molten crystallization and mutual diffusion between the substrates and the metal activated by the molten flux. The brazing flux may be a commercial flux containing substantial amounts of KFB 4 and/or KF along with sodium borate or boric acid, or commercial fluxes mainly containing AgCl, CuCl or CoCl 2 . The substrates to be bonded may be of copper. In one experiment, a mixed flux was prepared by incorporating AgCl powder into a molten fluoride type flux. The mixed flux was coated on the treated surfaces of two substrates, one being a sintered Cu/W alloy, the other one being electrolytic copper. After contacting the substrates, they were heated in air to a temperature of 800 °C which is higher than the melting point of the flux, but lower than the melting point of Ag. US patent 4,383,363 discloses a method wherein a conductive paste is disposed in a through hole to provide communication between wiring patterns formed on both major surfaces of an insulator substrate. Conductive layers integral with the wiring patterns are formed on both major surfaces of the substrates so that the conductive layers extend across the through hole. The conductive layers come into contact with the conductive paste that is disposed in the through hole, and the layers function to enclose both ends of the conductive paste. It is an object of the present invention to provide a new method for joining contacting surfaces of Cu or Cu alloys in an air atmosphere, thereby achieving a joint having superior reliability. According to aspect of this object, the invention wants to provide a low cost method for preparing a multilayer printed wiring board, the wiring board having low resistance and exhibiting high reliability. This object is achieved by the method as defined in claims 1 and 5. Suitably and preferred embodiments are described in the respective subclaims. SUMMARY OF THE INVENTION As a result of our research, it has been surprisingly found that Cu atoms at the contacting interface begin to diffuse from one contacting surface into the other contacting surface in an air atmosphere even at a temperature as low as 170 °C, provided that no metal oxide layer is formed at the contacting surfaces. Therefore, utilizing the inventive concept according to a first aspect, the invention provides a method for joining at least two contacting Cu or Cu alloy surfaces in an air atmosphere at a low temperature according to claim 1. In an embodiment according to the present invention, it is preferred that one of the contacting metal surfaces is Cu or Cu alloy and coated with a coating layer of the metal oxide remover, while the other surface is of a noble metal or an alloy thereof or of any other metal provided with a thin layer of a noble metal or an alloy thereof by means of electroplating, evaporating or sputtering. In the present invention, the metal oxide remover is preferably selected from the group comprising carboxylic acids and polymers or oligomers having carboxyl groups. The electroconductive paste optionally used for joining the contacting metal surfaces at the low temperature by diffusing Cu atoms into an opposite metal surface from the Cu or Cu alloy surface in an air atmosphere comprises metal particles selected from the group comprising noble metal, Cu and alloy thereof, the metal oxide remover and if necessary a binding resin. In an embodiment of the electroconductive paste, any surface active agent available in the soldering technology may be used as the metal oxide remover and it is preferable that it is selected from the group comprising carboxylic acids and polymers of oligomers having carboxyl groups, especially it is more preferable that the metal oxide remover acts as the binding resin. The Cu or Cu alloy particle is preferably of spherical form. According to a second aspect of the present invention, the diffusion joining method is applied to a method for preparing a multilayer printed wiring board. The method is defined by claim 5. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: Fig.1 is a schematic view showing a joining process for preparing a double-layered printed wiring board according to the present invention; Fig.2(a) is a perspective view of the double-layered printed wiring board prepared by the process shown in Fig.1 and Fig.2(b) is a perspective view of the board after etching; Fig.3(a) is a schematic views showing a joining process for preparing a four layered printed wiring board from the double layered printed board shown in Fig.2(b); Fig.3(b) is a schematic views showing a joining process for preparing a six layered printed wiring board from the double layered printed board shown in Fig.2(b); Fig.4 is a schematic view showing a imprinting process with use of peelable films; Fig.5 is a graph of Cu diffusion of silver electroplated Cu/Silver interface (180°C, 30min, Pressed) at XMA analysis in Example 5; Fig.6 is a graph of Cu diffusion of deoxidized Cu/Silver interface (180°C, 30min, Pressed) at XMA analysis in Example 10; Fig.7 is a graph of Cu diffusion of deoxidized Cu/Silver interface (240°C, 30min, Pressed) at XMA analysis in Example 10. DESCRIPTION OF THE PREFERRED EMBODIMENTS The joining method of the present invention can be applied to contacting metal surfaces of Cu or an Cu alloy such as electrolytic copper, rolled copper, bronze, brass, beryllium copper, gun metal. The metal parts to be joined may be of any forms such as a plate, a foil, a wire, a rod and so on, but it is necessary that the surfaces to be joined are subjected to coating treatment with the metal oxide remover or noble metal before joining. In the case of noble metal plating, there are known many kinds of pretreatment for removing oxide layers before plating, which are an acid etching method, an mechanical grinding method and an heating method in a reducing atmosphere, from which a suitable method may be optionally selected depending on the shapes to be joined and the required characteristics and so on. For example, in the case of a continuous film such as a copper foil, a wet continuous process may be applied, while in the case of small parts such as one body elements, a reducing oven may be used. Following the oxide layer removing treatment, a thin noble metal layer is deposited. The noble metal layer prevents the surface from oxidizing. If not so, the metal oxide formed on the following heating process prevents Cu atoms from diffusing during the joining process. The conventional Ni plating for preventing the surface from oxidizing, also prevents Cu atoms from diffusing. Therefore, the noble metal plating on the clean Cu or Cu alloy surfaces is essential to make Cu atoms diffuse. Even a small thickness of the noble metal plating is sufficient to prevent the contacting surface from oxidizing and promote diffusion of Cu atoms from one contacting surface into the opposite contacting surface. As described below, a 100 Å thickness is sufficient for that purpose. A larger thickness does not prevent the contacting surfaces from joining by the Cu atom diffusion, but disadvantageously causes high costs. Further, in the case of using silver as the noble metal, silver thick layers cause silver migration, which results in lowering the reliability. As the noble metal plating method, there are known methods of electroplating, chemical plating, vacuum depositing, sputtering and so on, the suitable one thereof being optionally selected according to the shape to be joined. The noble metal includes gold, silver, platinum, palladium and rhodium. Considering the costs, gold and silver are suitable. In the case of silver, there sometimes occurs a color change of the plating surface by sulfidization, but such a change does not have a bad influence to the joint. In place of the noble metal plating, a metal oxide remover may be coated on the surface of Cu or Cu alloy when joining. The metal oxide remover means an active agent which reacts with Cu oxide and converts the Cu or Cu alloy's surface into one of pure metal. Especially, carboxylic acid and polymers or oligomers containing carboxyl groups are preferred. Examples includes linear or side chain type saturated fatty acids such as caproic acid, enanthic acid, caprylic acid, 2-ethylhexanoic acid, stearic acid; unsaturated fatty acids such as oleic acid, linoleic acid, linolenic acid; mono- or multi-basic carboxylic acids or amino carboxylic acids such as abietic acid, succinic acid, malonic acid, aspartic acid; and esters thereof. Examples of the polymer or oligomer include polymers or copolymers of acrylic acid, copolymers of maleic acid, alkyd resin, polybutadiene having terminated or pendant carboxyl groups. The remover is coated directly or in a solution to Cu or Cu alloy surfaces. The other examples include a conventional active agent used as flux in the soldering such as hydrochloric acid, formic acid, acetic acid, lactic acid, primary, secondary and tertiary amines and their salts such as dimethylamine hydrochloride. In the case of using inorganic active agent, cleaning for removing the agent is necessary after the joining process. While even thick oxide layer can be removed by solution action of the remover, it is preferred that the oxide layer has already removed before coating in a similar way to the electroplating case. The temperature for joining the Cu or Cu alloy surface requires more than about 170°C. At temperatures lower than that, Cu atoms cannot diffuse to the opposite surface, resulting in no bonding between the contacting surfaces. While there is no upper limit of the temperature, the joining process is preferably carried out at a temperature as low as possible since Cu or Cu alloys are easily oxidized. Higher temperatures in the practicable range cause prompt joining. If possible, an ultrasonic method may be applied at the same time of the heat-pressing process. The pressing for the contacting surfaces does not require an extreme pressure higher than that causing the opposite surfaces to come into contact with each other. It is preferred that the roughness at the contacting surface is as small as possible in order to lower the pressure needed for such a joining. Even if the contacting surface has a bad roughness, Cu or Cu alloy can be deformed to be able to be joined because of its low rigidity. Further, insertion of ductile metal such as gold or silver between the Cu or Cu alloy contacting surfaces causes the contacting area to be increased and the joint strength to be improved even by means of the lower pressing pressure. The joining method according to the present invention may be applied not only to plates or boards but also to powders or particles. Therefore, the same joining theory can be applied to a joint or connection formed by a conductive paste. That is, the conductive paste used for joining contacting metal surfaces at a lower temperature can be prepared by using the metal oxide remover as a binding resin of the conductive paste or adding the metal oxide remover to the binding resin together with Cu or Cu alloy particles. According to the conductive paste, the conductive paths can be formed by coating the paste on a substrate or by filling the via holes and heating it under the pressure and provides ohmic characteristics whereby the resistance never increases even in a lower current range. The conductive paste used in the present invention mainly consists of Cu or Cu alloy particles as the electroconductive component and the metal oxide remover for removing the surface oxide layer. As other components, if necessary, many kinds of binding resins and additives may be added in order to improve an printing property. The particles may be preferably of a spherical form in order to facilitate application of a local pressure on the contacting surface of the particles, but it is not necessary to have a true sphere because the shape acts only to realize the local contacting state. Examples of the metal oxide remover include linear or side chain type saturated fatty acids such as caproic acid, enanthic acid, caprylic acid, 2-ethyl hexanoic acid, stearic acid; unsaturated fatty acids such as oleic acid, linoleic acid; mono- or multi-basic carboxylic acids or amino carboxylic acids such as abietic acid, succinic acid, malonic acid, aspartic acid; and esters thereof. Examples of the metal oxide remover acting as the binding resin include polymers or oligomers having carboxyl groups. Examples of the binding resin include thermoplastic resins and thermosetting resins used in conventional conductive pastes such as acrylic resin, epoxy resin, phenolic resin. The polymer or oligomer having carboxyl groups may be used together with the binding resin, in which if it is epoxy resin, the polymer or oligomer acts as a hardening agent because of having carboxyl groups. The conductive paste may be used as an assisting agent on joining Cu or Cu alloy surface as well as use of joint in the electric purpose. Especially, if the contacting surface has a bad roughness, the paste also acts to make the surface flat. The joining method of copper and copper alloy according to the present invention can be used for the manufacture of multilayer printed wiring board. The following description will be directed to the manufacture of the multilayer printed wiring board with reference to the accompanying drawings. Referring to Fig. 1 (a), the electric conductive paste 3 mentioned above is imprinted into a hole 2 formed at a given position of the insulating adhesive 1 by using, for example, a squeegee 4. The insulating adhesive 1 is interposed between two copper foils 5 and 5' and then pressed at a temperature higher than 170 c° as shown in Fig. 1 (c). In such a way, it is possible to form a substrate which has copper foils applied to both sides thereof and which has many via holes formed at necessary position as shown in Fig. 2 (a). The substrate which has copper foils applied to both sides thereof is etched at unnecessary parts with an usual etching method and is formed into a substrate having circuits formed at the both sides thereof as shown in Fig. 2 (b). The electric conductive particles in the via hole 6 are joined by a metallic bond as shown above and has a low electrical resistance. Further, the electric conductive particles in the via hole are joined to the copper films through a metallic bond. Therefore, it is possible to obtain a double layered printed wiring board which is superior in the reliability. The double layered printed wiring board according to the present invention can be manufactured in a simple manner similarly to that of a silver through hole printed wiring board. In addition, the via holes are formed by using a metallic bond. Therefore, the via hole according to the present invention has a low electric resistance similarly to that of the via hole formed by an electric plating method and further, has a low electric resistance at a low current range, thereby permitting a wide use for any circuit. Plural double layered printed wiring board 8, 8' and 8'' are combined with plural electric insulating adhesives 9 and 9' having electric conductive paste imprinted at the necessary parts thereto and then pressed together at a temperature higher than 170 °C. to form a multilayer printed wiring board having four layers or six layers. A multilayer printed wiring board having more layers can be formed by increasing the number of the double layered printed wiring board and the insulating adhesives. The electric conductive paste used in the manufacture of the multilayer printed wiring board according to the present invention essentially consists of electric conductive particles such as noble metals or copper and their alloys and an metal oxide remover. In addition to these components, the electric conductive paste is incorporated with a binder resin and other necessary additives for improving the printing ability. It is necessary for the manufacture of the multilayer printed wiring board that the electric conductive paste including a solvent should be dried sufficiently because the paste may be sealed in the board. In connection with the manufacture of the multilayer printed wiring board according to the present invention, it is preferred that the electric conductive particles are of a spherical form which permits the contact point between conductive particles to be pressed predominantly. In view of the purpose to execute the predominant pressure at the contact point between the electric conductive particles, the spherical form is not necessary of a genuinely sphere. As mentioned above, carboxylic acids or polymers or oligomers including carboxyl groups can be used for the metal oxide remover for use in the manufacture of the multilayer printed wiring board according to the present invention. Since the electric conductive paste is closed air-tightly in the substrate during manufacturing of the multilayer printed wiring board according to the present invention, it is necessary to pay attention to the boiling point of the added remover. When the metal oxide remover having a low boiling point remains in the board, the vapor of the metal oxide remover destroys the conductive path in the via hole during the soldering step. Therefore, it is preferred that the metal oxide remover such as fatty acid having a low molecular weight is reacted with a resin to be a stable compound through a chemical reaction, such as epoxy resin and the like. There have been known various imprinting methods such as the screen printing method or the pin method for imprinting the electric conductive paste mentioned above into the hole formed at a given position of the insulating adhesive. Fig. 4 shows another method to use a peelable film 10. The peelable film 10 is applied to the insulating adhesive 1 and then is drilled together with the insulating adhesive 1. The electric conductive paste 3 is imprinted on the peelable film 10, and then the peelable film 10 is peeled off. This method prevents the unnecessary portion from being contaminated with the electric conductive paste. When the electric conductive paste consists of copper or its alloy, it is possible to etch off the electric conductive paste adhered to unnecessary portion other than the via hole 2. The pressing process can be carried out at a temperature higher than 170 °C, preferably higher than 180 °C. When the pressing process is carried out at a temperature lower than 170 °C, the lower temperature does not promote the metal atom diffusion process between the metal particles of the electric conductive paste. As a result, even if the via hole has a low electric resistance at the initial period, it shows break down or an increase in electrical resistivity during the reliability test. The necessary pressing pressure varies with the kind of the used metal particles. The lower pressure is allowable with the electric conductive particles having a higher ductility such as gold or silver. On the other hand, a higher pressure is required for the electric conductive powders hardly deformed such as copper or its alloys. The following description will be directed to preferred embodiments. (Example 1) A copper plate having a thickness of 0.5 mm is previously degreased and etched lightly with an aqueous solution of sulfuric acid and hydrogen peroxide. After water washing, the copper plate is electroplated with silver film by a conventional electroplating method. The plating solution comprises a non-cyanide type commercially available; the anode used is a silver plate; and the current density is 0.5 A/dm 2 . The resultant silver plating film has a thickness of 500 angstroms and is colored silver white at the copper surface. Two copper plates having silver plating film formed thereon are washed with water, dried and then overlapped to each other. The overlapped copper plates are interposed between steel plates having a hard chromium plating film formed thereon and then pressed in air at the following condition; the pressing process can be executed at 180°C for 30 minutes, at a pressure of 300kg/cm 2 . As a result, the two plates are strongly adhered to each other. The silver plating film on the copper plate where is not overlapped is exposed to air during the pressing work and then is colored due to the oxidation of the copper. The joining strength between the two copper plates shows 400 to 500 kg/cm 2 in accordance with the shearing peeling strength measurement of the tensile strength test machine. For comparison, the following experiment was carried out: two copper plates are etched lightly, washed with water, dried and then overlapped to each other. The overlapped copper plates are pressed in a similar condition as above but are not bonded to each other. The copper plate having silver plating film formed thereon is overlapped with the copper plate washed with water without silver plating. The overlapped two copper plates are pressed in a similar condition to that mentioned above and are not bonded to each other. The above experiments indicate clearly that the two copper plates are bonded strongly when tow copper plates are electrically plated with silver and pressed to each other at a heated condition. (Example 2) A copper foil of 18 µm in thickness is plated at a flat plane with silver film of 100 angstrom in a similar way to that of Example 1. The copper foil is overlapped on the copper plate having a silver plating film formed thereon obtained in Example 1 and then pressed in a similar condition of those of Example 1 except for the pressure of 750 kg/cm 2 . For the easy pressing work, the other side plane of the copper foil is overlapped with a copper plate of 1mm in thickness. The copper foil is joined to the copper plate having silver plating film formed thereon but is not joined to the overlapping copper plate by a pressing work. The pressing work permits the copper foil to join strongly to the copper plate. The peeling test causes the copper foil to be broken and then it is not possible to measure the joining strength. (Example 3) In order to know the effect of the contamination of the surface, the surface of the copper plate having a silver plating film formed thereon is coated with an epoxy resin (The epoxy resin comprises epichlorohydrin - bisphonol type of epoxy equivalent of 180) including no curing agent and pressed in a condition similar to that of Example 1. The result shows a joining strength the same as that of Example 1. It is concluded that even if the surface of the copper plate having a silver plating film formed thereon is contaminated with resin, there is no effect on the peeling strength of the resultant bonding. (Example 4) A silver plate of 0.5mm thickness is overlapped with a copper plate having a silver plating film formed thereon and then pressed in a condition the same as that of Example 1. This case shows a strong joint the same as that of Example 1. It is found at the XMA analysis, as shown in Fig. 5 that the copper atoms diffuse to a surface of the silver plate in the depth of a few microns. This result clearly shows the bonding mechanism of the present invention. (Example 5) In this comparative example, the combination the same as that of Example 1 is pressed at 170°C for one hour instead of 180°C and 30 min. used in Example 1. The two copper plates join to each other but the strength is very poor. A slight mechanical shock separates the two copper plates. (Example 6) The two copper plates are electroplated with plating silver film of thickness 100 angstroms different from that of the Example 1. Then, the two copper plates are treated in a similar manner to that of Example 1. It is possible to obtain the joining strength the same as that of Example 1. This indicates that even the thin film of plated silver results in giving a sufficiently strong joining strength therebetween. (Example 7) A phosphor bronze plate of 0.2 mm thickness is electroplated with a silver in a similar way to that of Example 1. The electroplated silver film has a thickness of 500 angstrom. The two phosphor bronze plates having silver film plated thereon is overlapped to each other and pressed at 180°C, for 30 min. at a pressure of 2000 kg/cm 2 . The two phosphor bronze plates are joined to each other. However, the strength is so weak that the two phosphor bronze plates are easily separated from each other upon bending at the joint portion. When the pressing is carried out at 220°C, it is possible to obtain a strong joint without the loss of the elasticity of phosphor bronze. (Example 8) A copper foil on the printed circuit board is plated with gold. The plated gold film has a thickness of 200 angstrom. After removing the solder plating layer of the IC lead wire of QFP package by etching the lead wire is plated with copper and further with silver. The plated copper film has a thickness of 5 µm and the plated silver film as a thickness of 500 angstrom. After the IC lead wire is positioned at a given positioned of the printed circuit board, the printed circuit board is heated at 220 °C for one minutes under the lead wire being compressed by a pressing jig. As a result, the lead wire can be strongly joined to the circuit wiring board. A heating and cooling cycle test is carried out with the circuit obtained by the present invention and the circuit obtained by the conventional-soldering method. The test result shows that the circuit obtained by the present invention shows a durability longer 2 times than that of the circuit obtained by the conventional soldering method. Further, the circuit obtained by the conventional soldering method is broken upon being heated at 200°C while the circuit obtained by the present invention does not show any change upon being heated at 200°C. (Example 9) A copper plate of 2 mm thickness is ground at the surface with a sand paper of #1200, washed with water and dried. After that, the copper plate is coated with 5% abietic acid acetone solution and is dried to the treated copper plate. The two treated copper plates are overlapped to each other and are pressed at 380 kg/cm 2 , for 30 minutes at 180°C in air to be joined together. A test piece is prepared from the joined copper plates and is tested with a tensile test apparatus. The joined copper plates show a shear extension strength of 240 to 260 kg/cm 2 and are joined to each other strongly. For a comparison, the copper plates mentioned above are ground with a sand paper of #1200, washed with water and dried. The two copper plates are overlapped to each other and are pressed in a similar condition to that of the above procedure. The result shows no joint between the two copper plates. It is confirmed that when the copper plates are coated with 5% abietic acid acetone solution and pressed at a heated condition in accordance with the present invention, the two copper plates can be joined together strongly. (Example 10) The treated copper plate of the Example 9 is overlapped with a silver plate of 2 mm thickness. The overlapped copper plate with silver plate is pressed in a condition similar to that of Example 9. The both plates are joined strongly to each other. The tensile strength test shows that the joined plate of copper plate and silver plate has a shear strength of 540 to 600 kg/cm 2 . The observation by an XMA analyzer indicates that silver and copper diffuse by several microns to the copper layer and to the silver layer, respectively at the boundary of the joined portion as shown in Fig. 6. The results of an XMA analysis of 240°C, 30 min. pressed copper/silver interface is also shown in Fig. 7. (Example 11) A copper round bar having a diameter of 5 mm is ground at the heading and is coated with abietic acid solution at the flat plane of the heading. Then, after drying, the copper round bar butts at the flat plane against a silver disc having a thickness of 0.5 mm and a diameter of 5 mm. A combination of the copper bar butting against the silver disc is pressed by a pressure of 680 kg/cm 2 for 30 minutes at 195°C. The resultant copper round bar is tested with a tensile strength test. The testing result shows that the copper round bar butting against the silver disc has a tensile down strength of 830 kg/cm 2 . It is verified that the copper round bar is strongly joined to the silver disc in accordance with the present invention. (Example 12) After phosphor bronze plate is coated with abietic acid, gold wire having a diameter of 1 mm is placed on the phosphor bronze plate. The combination of phosphor bronze plate and gold wire is pressed at 195°C for 15 minutes. As a result, the gold wire joins to the phosphor bronze plate. The peeling test of the gold wire indicates a peeling strength of 4.0 kg/mm 2 . It is found that the gold wire joins strongly to the phosphor bronze plate in accordance with the present invention. (Example 13) This Example uses N-laurel aspartic acid β-laurel ester as a copper oxide remover instead of abietic acid used in Example 9. In this case, the remover is scattered on the heated copper plate and is melted. The two copper plates are joined to each other using the melted remover. It is possible to obtain the joining strength similar to that of Example 9. (Example 14) A polymer is prepared by stirring and heating a mixture of acrylic acid monomer and 0.5% of benzoyl peroxide at 60°C. An 5% aqueous solution of the resulting polymer is used as an oxidized layer remover instead of abietic acid used in Example 9. In this case, it is possible to obtain the strong diffusion joint in a similar way to the of Example 9. (Example 15) In this comparative example, the combination the same as that of Example 9 is pressed at 170°C for one hour instead of 180°C and 30 min. of Example 1. The two copper plates join to each other but the strength is very poor. A slight mechanical shock separates the two copper plates. (Example 16) An electric conductive paste is prepared by dispersing 75g copper particles of a spherical form having a diameter 4 to 5 µm (Tanaka kikinzoku Co. CRYSTAL COPPER) in a solution of 35g cellsolve acetate and 25g rosin. This electric conductive paste is imprinted into a through hole of a diameter 0.5 mm formed at a given position of a glass-epoxy prepreg and dried. The glass-epoxy prepreg is interposed between two copper foils having a thickness of 35 µm and then is pressed into a double layered printed circuit board of a thickness of 0.5 mm, The pressing is carried out at a pressure of 150 kg/cm 2 at 180°C for 30 minutes. The board is etched for the purpose of formation of a given pattern. Then, the through hole is measured with a direct current resistance. The testing result indicates that one through hole has an electric resistance less than one mΩ. This electric resistance is kept at a current range from a high current to 10 µA. For comparison, the conventional silver through hole circuit board has a through hole resistance of 10 mΩ which increases when the electric current is less than 10 mA. (Example 17) An electric conductive paste is prepared by dispersing 75g of spray type copper particles having a diameter 2 to 15 µm in a solution containing 7.5g of dimethylor propionic acid and 15g of an epoxy resin (epoxy equivalent 180) comprising epichlorohydrin bisphenol A type. This electric conductive paste is imprinted in a through hole of a diameter 0.5 mm formed at a given position of a glass-epoxy prepreg. The glass-epoxy plate is interposed between two copper foils having a thickness of 35 µm and then is pressed to give a double layered printed wiring board of a thickness of 0.5 mm. The pressing is carried out at a pressure of 150 kg/cm 2 at 180 °C for 30 minutes. The board is etched for purpose of formation of a given pattern. Then, the through hole is measured with a direct current resistance. The testing result indicates that one through hole has an electric resistance less than one mΩ. This electric resistance is kept at a current range from a high current to 10 µA. The circuit substrate is subjected to a heat cycle test from 260 °C to 25 °C. The test result indicates that the electric resistance does not change up to 300 test cycles. For comparison, the conventional silver through hole circuit board has a through hole resistance of 10 mΩ which increases when the electric current is less than 10 mA. The heat cycle test indicates that the conventional silver through hole circuit board changes in the electric resistance more than 20 % through 100 cycles. This testing result clearly indicates that the double layered printed circuit board of this embodiment has copper foils at top and bottom planes joined electrically completely to each other. (Example 18) An epoxy prepreg is drilled to make holes of 0.5 mm. The electric conductive paste prepared in Example 17 is applied to an epoxy prepreg. The epoxy prepreg is interposed between the two of the double layered printed wiring board obtained by Example 17 and is pressed at a heated condition in order to form a four layer printed wiring board. The resultant four layer printed wiring board has an electric connection among four layers, which is characterized by a stable and a low electric resistance in a similar way to that of Example 17. It is verified that a simple process of hot pressing shown in this embodiment makes it possible to manufacture a multilayer printed wiring board having superior characteristic in the electrical connection. (Example 19) An electric conductive paste is prepared by dispersing 75 g of a spray type copper powder having a diameter 2 to 15 µm in a solution of 25 g cellsolve acetate and 25 g resin. This electronic conductive paste is imprinted in a through hole of a diameter 0.5 mm formed at a given position of a glass-epoxy prepreg and dried. The glass epoxy plate is interposed between two copper foils having a thickness of 35 µm and then is pressed into a double layered printed circuits board of a thickness of 0.5 mm. The pressing is carried out at a pressure of 150 Kg/cm 2 at 180 °C for 30 minutes. The board is etched for the purpose of formation of a given pattern. Then, the through hole is measured with a direct current resistance. The testing result indicates that one through hole has an electric resistance less than 0.1 mΩ. This electric resistance is kept at a current range from a high current to 10 µA. (Example 20) An electric conductive paste is prepared by using silver particles of spherical from having a diameter of 1 µm in replace of copper particles used in Example 17. The double layered circuit board is prepared by using the electrical conductive paste mentioned above. A hot pressing in this embodiment is carried out at a pressure of 50 Kg/cm 2 at 190 °C. This embodiment also can produce a double layered circuit board superior in the reliability and the electric conductivity. A microscopic observation at the through hole area of the double layered circuit board of this embodiment indicates that the silver particles are formed into one body of silver pillar and that silver and copper execute a mutual diffusion at the boundary of the copper foil and the silver pillar. (Example 21) In place of a solution of dimethylol propionic acid and an epoxy resin (epoxy equivalent 180) comprising epichlorohydrin-bisphenol A type used in Example 17, this embodiment uses styrene-maleic acid (1:1) copolymer. In this case, methylethylketone is used for a solvent of the electric conductive paste. This embodiment can produce a double layered circuit board superior in the electric connection in accordance with the present invention. (Example 22) In place of a solution of dimethylol propionic acid used in Example 17, this embodiment uses N-stearoil aspartic acid mono-stearyl ester achieves a double layered circuit board superior in the electric connection between the circuits on the both sides. (Example 23) In this comparative example, the pressing temperature in Example 17 is changed to 170 °C. The pressing time in Example is changed to one hour. The double layered circuit board obtained in such a condition has a electric conduction between both sides similarly to that of Example 17 but shows a break down at 20 cycles of heat cycle test. These data show that the allowable pressing temperature is 170 °C as a lowest limit. As described in these embodiments, the method for manufacturing the multilayer printed wiring board according to the present invention is simple in a similar way to that of the silver through hole method, but achieves multi-layer circuit board superior in the electrical conductivity across the both sides similar to that of a electroplating method.

Claims: A method for joining at least two contacting surfaces of Cu or a Cu alloy in an air atmosphere at a low temperature by (a) coating the contacting surfaces with a joint layer, wherein said joint layer is selected from the group consisting of a noble metal thin layer, a metal oxide remover layer, and a conductive paste layer containing said metal oxide remover and metal particles selected from the group consisting of noble metal particles and Cu or Cu alloy particles; and (b) heating and pressing said surfaces to cause diffusion of Cu atoms from the Cu or Cu alloy surface into an opposite surface, wherein said heating and pressing is conducted at a temperature whereat said Cu atoms become capable to diffuse, the temperature being higher than about 170 °C and ranging up to 240 °C. The method of claim 1, wherein one of the contacting metal surfaces is Cu or a Cu alloy and is coated with a layer of the metal oxide remover, while the other surface is of a noble metal or alloy thereof or of any other metal provided with a thin layer of a noble metal or an alloy thereof by means of electroplating, evaporating or sputtering. The method of claim 1, wherein the contacting surfaces are disposed on a multilayer printed wiring board. The method of claim 1, wherein the metal oxide remover is selected from carboxylic acids, amino carboxylic acids, polymers or oligomers having carboxyl groups, and primary, secondary and tertiary amines end their salts. A method for preparing a multilayered printed wiring board (8, 8', 8''), the method comprising the steps of providing at least two foils (5, 5') of Cu or Cu alloy; interposing an insulating adhesive layer (1, 9) between each pair of the foils (5, 5'), wherein said insulating layer (1, 9) is previously treated to have via holes (2, 6) at desired positions and said via holes (2, 6) are imprinted with a conductive paste (3), characterized by heat pressing said foils (5, 5') and said insulating adhesive layer (1, 9) at a temperature of higher than about 170 °C, whereat said Cu atoms become capable to diffuse, to make a multilayered printed wiring board (8, 8', 8''), wherein said conductive paste (3) comprises a metal oxide remover and conductive metal particles selected from the group consisting of Ag, Cu and alloys thereof. The method of claim 5, characterized in that the metal oxide remover is selected from carboxylic acids, amino carboxylic acids, polymers or oligomers having carboxyl groups, and primary, secondary end tertiary amines and their salts. The method of claim 6, charcterized in that the metal oxide remover acts as a binding resin. The method of claim 5, characterized in that the conductive metal particles to have a spherical form. The method of claim 5, characterized in that the step of imprinting the via holes (2) with said conductive paste (3) comprises a step of drilling the insulating adhesive layer (1) sandwiched between a pair of peelable films (10); a step of imprinting the conductive paste (3) on the peelable film (10); and a step of peeling off the peelable films (10).

Inventors: HASEGAWA YO, NAGASAWA MASAHIRO, TSUKAMOTO MASAHIDE, HASEGAWA, YO, NAGASAWA, MASAHIRO, TSUKAMOTO, MASAHIDE
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: B23K35
IPC codes: H05K3, B23K20, B23K35, H05K1