Process of manufacturing a heat-conductive material

Abstract: This invention provides heat-conductive materials which may receive evenly the heat as applied thereto, which have an improved heat-releasing effect, which are free from fine pores on the surfaces thereof and therefore have excellent adhesiveness to thin films of plating materials or brazing materials, which have excellent compatibility with materials to be applied thereto, such as chips or sealant resins, with respect to the thermal expansion coefficients of them, and have excellent thermal conductivity, and which may have any desired thermal expansion coefficient and thermal conductivity in accordance with the use and object of them. It also provides methods of producing the heat-conductive materials. One aspect of the heat-conductive materials is such that a two-layer sheet or three-layer sheet to be formed by previously welding under pressure a copper foil to one surface or both surfaces of a Kovar sheet followed by forming a number of small through-holes through the sheet has been welded under pressure to and integrated with one surface or both surfaces of a copper sheet as previously heated up to a temperature not lower than the recrystallizing temperature thereof with a heating device. Another aspect of them is such that a copper sheet as previously heated up to a temperature not lower than the recrystallizing temperature thereof has been welded under pressure to the lower surface of a Kovar sheet having a number of small through-holes therethrough, with a welding machine, and additionally another copper sheet has been welded under pressure to the upper surface of the same with a welding machine, all the welded sheets having been integrated together. Still another aspect of them is such that the materials of the first aspect and/or the second aspect have been laminated and integrated together. The heat-conductive materials of all the aspects have a high welding strength even though the welding of the constitutive sheets is effected at a small reduction ratio, and they may have determined thermal expansion coefficient and thermal conductivity without fluctuating the previously determined ratio of the copper exposing surface areas (through-hole areas) to the Kovar surface.

Description: This invention relates to a process of manufacturing a heat conductive laminated body which has oppositely facing major surfaces constituted by a metal having a high coefficient of thermal expansion, the body comprising a core sheet of a metal having a high coefficient of thermal expansion having pressure welded on at least one of its sides, a perforated sheet of a metal having a low coefficient of thermal expansion, the metal of the core sheet occupying the holes in the perforated sheet. The heat-conductive composite material obtained with the process of the present invention has a high lamination strength, and the thermal expansion coefficient and the thermal conductivity of the material are variable without varying the ratio of the thickness of the selected metal sheets and the ratio of the area of the through-holes. Accordingly, the capacity of receiving heat of the material is uniform and the heat-diffusibility of the material is improved. The surface of the material is free from fine pores so that any thin film may well be coated thereon by plating or brazing. Recently, integrated circuit chips (hereinafter referred to as "chips") of semiconductor packages, especially those in LSl or ULSl for large-scaled (super) computers, have been directed toward elevation of the integrated degree and acceleration of the operation speed, and therefore the quantity of heat generated by the operation of the devices has become extremely large due to increase of the electric power consumed. That is, the capacity of the chips has become large-scaled and the quantity of heat generated during the operation has also become large. Accordingly, if the thermal expansion coefficient of the substrate material is significantly different from that of the material of the chip of silicon or gallium arsenide, the chip will peel off from the substrate or be broken. For this reason, heat-diffusibility has to be taken into consideration in planning the semiconductor package, and therefore the substrate to which the chip is mounted must also have a proper heat-diffusibility. Accordingly, the substrate material is required to have a thermal expansion coefficient near to that of the chip and have a large thermal conductivity. Conventional semiconductor packages of various constitutions have been proposed. For instance, one constitution is mentioned, in which the substrate is provided with heat-release fins. Composite materials for heat-release substrates (heat sinks), such as clad boards or Cu-Mo or Cu-W alloys for ensuring the heat-releasability, have been proposed (Japanese Patent Application Laid-Open Nos. 59-141247 and 62-294147). The proposed composite materials meet the satisfactory conditions for practical use with respect to both the thermal expansion coefficient and the thermal conductivity. However, since Mo and W both have a high density and therefore are heavy and brittle, they must be shaped and machined by non-plastic machining such as grinding so as to obtain a shaped body having a determined dimension. Such machining needs a high machining cost so that the yield of the products from the materials is low. In resin sealed semiconductor packages, not only the lead frame is to be a path for electrical connection with the outside parts but also it has an important role as a heat-releasing path for releasing the heat to be generated in the chips therein. For the reason, a lead frame made of a copper alloy is much used. However, in the field of needing high reliability, copper alloys involve a danger of peeling the welded interface between a chip and an island, since they have a low mechanical strength and the thermal expansion coefficient of them is not compatible with that of chips. Therefore, in view of the compatibility of the thermal expansion coefficient of the lead frame material with that of a chip, a semiconductor package having an Ni-Fe alloy of low thermal expansion, such as 42% Ni-Fe alloy, has been proposed. However, since Ni-Fe alloys have a poor thermal conductivity, they could not yield a thermal releasability capable of satisfying the current requirement. In addition, since the difference in the thermal expansion between a chip and a sealant resin is extremely large, the compatibility between a lead frame and the sealant resin is bad so that complete prevention of cracks in the resin is difficult even though the compatibility of the thermal expansion coefficient between the lead frame and the chip is good. Further, in ceramic semiconductor packages, since sealing is effected by Al wire bonding or glass sealing, an Ni-Fe alloy provided with Al at the bonding area and the sealing position is much used as the lead frame. However, such an Ni-Fe alloy has a poor heat-releasability, as mentioned above, it involves a problem about the compatibility of the thermal expansion coefficient thereof with that of ceramics. In EP-A-432867, the present applicant already proposed a heat-conductive composite material, so as to solve the above-mentioned problem about the thermal expansion coefficient and/or the thermal conductivity, in which a metal foil of high thermal expansion has been laminated under pressure to the both surfaces of a core material comprising a metal sheet of low thermal expansion having necessary through-holes in the direction of the thickness, as integrated with a metal sheet of high thermal expansion in such a way that the metal of high thermal expansion is exposed out to the surface of the metal sheet of low thermal expansion through the said through-holes, the proportion of the thickness of these metal sheets and the proportion of the area of the through-holes having been selected suitably (Japanese Patent Application No. 2-40550). The proposed composite material is characterized in that the thermal expansion coefficient and the thermal conductivity is variable, that the capacity thereof of receiving heat is made even, that the heat-diffusing effect thereof is improved, that the surface thereof is free from fine pores and that the adhesiveness thereof to a thin film of a plating material or a brazing material is good. In order to obtain the proposed heat-conductive composite material, a metal sheet of low thermal expansion such as a Kovar sheet is first stamped by pressing to form a number of small through-holes to make it reticulate, then annealed and coiled to make a metal sheet coil of low thermal expansion, the coil is uncoiled with laminating onto the both surfaces thereof a metal sheet coil of high thermal expansion as being uncoiled from both the above and the below of the former coil, and the laminated sheets are rolled and welded under pressure with large-diameter rolls by cold rolling or warm rolling and annealed for diffusion to obtain a core material, then a metal foil of high thermal expansion, such as Cu or Al foil, is laminated onto the both surfaces of the core material with the coil of the metal foil being uncoiled from both the above and the below of the core material, and the laminated core material is rolled and welded under pressure with rolls by cold rolling or warm rolling and annealed for diffusion. In manufacturing the heat-conductive composite material, a metal foil of high thermal expansion such as Cu or Al foil is laminated onto the both surfaces of the core material and welded by cold rolling. In the step, if the reduction ratio is made too large so as to obtain a high welding strength, the circular or oval shape of the exposed area of the metal of high thermal expansion such as Cu or Al as exposed onto the surfaces of the core material would be a long oval shape whereby the selected proportion of the surface area between the metal of high thermal expansion and the metal of low thermal expansion would vary and accordingly the thermal expansion coefficient and/or the thermal conductivity of the laminated material would vary or the thermal expansion coefficient of the same would be anisotropic, defectively. In order to evade the defective problem, it may be considered to employ a plating method in place of the rolling method, for coating the metal foil of high thermal expansion such as Cu or Al foil over the surfaces of the core material. According to the alternative method, however, a plating solution would remain in the interface between the metal of high thermal expansion and the metal of low thermal expansion on the surfaces of the core material after the material has been dipped in a plating bath, and this would be gasified in the subsequent diffusion-annealing step to often cause swelling or peeling of the plated layer. In addition, the plating step is generally complicated and needs much time, unfavorably. One object of the present invention is to provide a heat-conductive material, which may receive heat evenly, which has an improved heat-diffusing effect, which is free from fine pores on the surface thereof, which has excellent adhesiveness to a thin film of plating or brazing material, which has excellent compatibility with materials to be welded therewith, such as chips or sealant resins, with respect to the thermal expansion coefficients of them, which has good thermal conductivity, and the thermal expansion coefficient and thermal conductivity of which may freely be selected in accordance with the use and object thereof. Another object of the present invention is to provide a method of producing the heat-conductive material by welding under pressure, without greatly varying the shape of the through-holes as provided through the metal sheet of low thermal expansion, to give the intended heat-conductive material having selected thermal expansion coefficient and/or thermal conductivity and having an improved welding strength to the respective metal of high thermal expansion and metal of low thermal expansion to be laminated. The heat-conductive material to be obtained by the present invention is a multi-layered material substantially composed of three or more layers. Accordingly, this invention provides a process of manufacturing a heat conductive laminated body which has oppositely facing major surfaces constituted by a metal having a high coefficient of thermal expansion, the body comprising a core sheet of a metal having a high coefficient of thermal expansion having pressure welded on at least one of its sides, a perforated sheet of a metal having a low coefficient of thermal expansion, the metal of the core sheet occupying the holes in the perforated sheet process being characterized in that prior to its being perforated, said sheet is welded to form a unitary structure with at least one foil of a metal having a high coefficient of thermal expansion, which unitary structure is then perforated with a multiplicity of through holes extending through the thickness thereof and distributed substantially evenly throughout the structure, and then said perforated unitary structure is united with said core sheet to form the laminated body by said pressure welding, during which step, the core sheet material penetrates said holes and integrates with each said foil. All the aspects of the heat-conductive material produced by the present invention which will be explained hereunder indispensably have the metal sheet of high thermal expansion having the above-mentioned characteristic feature and are therefore proposed in correspondence to the structure of other parts to be welded to and integrated with the said metal sheet of high thermal expansion. As a matter of course, all the heat-conductive materials of all the claimed aspects of the present invention attain the above-mentioned objects. As preferred embodiments of the heat-conductive materials of all the above-mentioned aspects of the present invention, the metal sheet of high thermal expansion is made of anyone of Cu, Cu alloys, Al, Al alloys, Ag and Ag alloys, the metal sheet of low thermal expansion is made of anyone of Mo, Ni-Fe alloys each containing 30 to 50 wt.% of Ni, Ni-Co-Fe alloys each containing 25 to 35 wt.% of Ni and 4 to 20 wt.% of Co, and W; and the metal foil of high thermal expansion is made of anyone of Cu, Cu alloys, Al, Al alloys, Ag and Ag alloys. As the preferred embodiment of the five-layer heat-conductive material of them, which is substantially composed of five layers, the thickness t 1 of the metal sheet of high thermal expansion, the thickness t 2 of the metal sheet of low thermal expansion and the thickness t 3 of the metal foil layer of high thermal expansion satisfy the following expressions: t 1 = 1t 2 to 5t 2 , t 3 ≤ 1/10 t 2 , t 1 + t 2 = 0.1 to 30 mm, t 3 = 2 to 100 µm. The process of the present invention further provides a heat-conductive material, which has a metal plate layer of anyone of Cu, Al, Ni and Sn, at desired position(s) of at least one main surface of anyone of the heat-conductive materials of the above-mentioned aspects of the present invention. For instance, a plating material of anyone of the above-mentioned metals or a brazing material is applied to desired position(s) of the metal foil of high thermal expansion to be disposed as the outermost layer of anyone of the heat-conductive materials of the above-mentioned aspects of the present invention, by pressing or lamination, to give heat-conductive materials of various uses, for example, for heat-releasing substrates (heat sinks) for mounting chips thereon and lead frames in ceramic packages or metal packages. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(A), 1(B), 1(C) and 1(D) are longitudinal explanatory views of each showing the heat-conductive material of the first aspect of the present invention. Fig. 2 is a partly-cut perspective explanatory view of showing the heat-conductive material of Fig. 1(A). Figs. 3 to 5 are perspective explanatory views of each showing one embodiment of the equipment for producing the heat-conductive material of the present invention. Characteristic features of the process of the present invention are mentioned in more detail hereunder. In the heat-conductive material obtained by the present invention, a metal sheet of low thermal expansion, at least the outer surface of which has been welded under pressure with a metal foil of high thermal expansion and which has a number of through-holes in the direction of the thickness thereof, has been welded under pressure onto one surface or both surfaces of a metal sheet of high thermal expansion, in such a way that the metal of high thermal expansion is exposed out of the surface of the metal foil of high thermal expansion through the through-holes so as to be integrated with the latter. The heat-conductive material may have a freely variable thermal expansion coefficient by essentially selecting the thickness ratio of the metal sheet of high thermal expansion. Using a metal of high thermal conductivity as the metal of high thermal expansion to be a core material and suitably selecting the exposing surface area ratio on the same surface between the metal of high thermal expansion as intruded into the through-holes and the metal sheet of low thermal expansion, the thermal conductivity of the heat-conductive material may freely be varied. Accordingly, by desired selection and combination of the material of the metal sheet of high thermal expansion and that of the metal sheet of low thermal expansion as well as by desired selection of the above-mentioned thickness ratio and exposing surface area ratio, any desired thermal expansion coefficient and thermal conductivity of the heat-conductive material of the present invention may be settled in accordance with various uses and objects thereof. Thus, heat-conductive materials of various kinds for various uses may be provided by the present invention. The method of producing the heat-conductive material according to the present invention comprises only a simple step of welding under pressure a metal sheet of low thermal expansion, to one surface of both surfaces of which a metal foil of high thermal expansion has been welded under pressure and which has a number of through-holes in the direction of the thickness thereof, to a metal sheet of high thermal expansion to give the intended heat-conductive material. In the case, since welding under pressure of the two metal sheets is effected only one time, deformation of the shape of the exposed area of the metal of high thermal expansion is small so that the exposing surface area ratio on the same surface between the previously selected metal sheet of high thermal expansion and metal sheet of low thermal expansion does not vary so much and, as a result, a heat-conductive material having a determined thermal expansion coefficient and a determined thermal conductivity can be obtained. In addition, in accordance with the method of producing the heat-conductive material according to the present invention, since the metal foil of high thermal expansion has previously been welded under pressure to the metal sheet of low thermal expansion prior to forming through-holes through the metal sheet of low thermal expansion, air is hardly intruded into the periphery of each through-hole during welding under pressure of the metal sheet of low thermal expansion and the metal sheet of high thermal expansion so that peeling of the metal foil of high thermal expansion may well be retarded. As a result, the heat-conductive material to be obtained may have an outer surface which is homogeneous and has an excellent property. Therefore, the material does not need any further cladding or plating of a metal foil of high thermal expansion thereover. In producing the heat-conductive material of the first aspect of the present invention mentioned above, if the metal sheet of high thermal expansion is heated up to a temperature not lower than the recrystallizing temperature thereof when the metal sheet of high thermal expansion is welded under pressure to the metal sheet of low thermal expansion, one surface or both surfaces of which have been welded with a metal foil of high thermal expansion and which has a number of through-holes in the direction of the thickness thereof, and additionally, since the metal sheets of high thermal expansion, each of which has been welded under pressure with a metal sheet of low thermal expansion, are welded together under pressure, a high welding strength may be obtained even under a low rolling pressure so that deformation of the shape of the through-holes as provided through the metal sheet of low thermal expansion is small and the previously selected exposing surface area ratio (through-hole area ratio) on the same surface between the metal of high thermal expansion as intruded into the through-holes and the metal sheet of low thermal expansion is not varied. As a result, the heat-conductive material to be obtained by the method may have the determined thermal expansion coefficient and thermal conductivity. In particular, by heating the metal sheet of high thermal expansion to a temperature not lower than the recrystallizing temperature thereof in welding it under pressure to a metal sheet of low thermal expansion, one surface or both surfaces of which have been welded under pressure with a metal foil of high thermal expansion and which has a number of through-holes in the direction of the thickness, the metal of high thermal expansion may well penetrates into the through-holes of the metal sheet of low thermal expansion even under a low rolling pressure so that the former is exposed out to the surface of the metal foil of high thermal expansion and additionally the welding strength is noticeably elevated. Moreover, because of the welding effect of the same material of the metal foil of high thermal expansion as welded under pressure to the metal sheet of low thermal expansion and the heated metal sheet of high thermal expansion, a high welding strength may well be obtained even under a lower rolling pressure so that the rolling ratio (reduction ratio) may be made small and therefore the variation of the above-mentioned exposing surface area ratio may also be made small. Where the material of the metal foil of high thermal expansion and that of the metal of high thermal expansion as exposed out to the surface of the foil are the same, since the metal sheet of high thermal expansion to be exposed out in welding under pressure thereof has previously been heated up to a temperature not lower than the recrystallizing temperature thereof, integration of the metal sheet of high thermal expansion with the metal foil of high thermal expansion is possible to give an extremely excellent surface property of the composite material to be formed. In addition, in the process according to the present invention, through-holes are partly or wholly provided through a metal sheet of low thermal expansion in the direction of the thickness thereof at determined intervals to have a desired pattern, prior to laminating the metal sheet of low thermal expansion, one surface or both surfaces of which have been welded under pressure with a metal foil of high thermal expansion, onto the whole of one surface or both surfaces of a metal sheet of high thermal expansion. For instance, therefore, the size and shape of each through-hole and the pattern of disposition of the through-holes may be varied, or notches penetrating or not penetrating through the direction of the thickness of the metal sheet may be provided in consideration of the deformation of the sheet by rolling, in order that the thickness ratio of the metal sheet of the core material and/or the exposing surface area ratio between the metal of high thermal expansion as exposed out to the surface of a metal sheet of low thermal expansion and the metal of low thermal expansion may suitably be selected. By combining the above-mentioned means, any desired thermal expansion coefficient and thermal conductivity may well be settled in the whole or the desired parts of the heat-conductive composite material to be obtained, in accordance with the use and object of the material. For instance, any desired heat-conductive composite material may thus be obtained, having desired compatibility of the thermal expansion coefficient of itself with that of other materials, which are to be applied to the composite material, such as determined metals, ceramics, semiconductors such as Si, or plastics, and having a desired thermal conductivity. Any desired heat-conductive composite material may thus be obtained, having desired compatibility of the thermal expansion coefficient of itself with that of other materials, which are to be applied to the composite material, such as determined metals, ceramics, semiconductors such as Si, or plastics, and having a desired thermal conductivity. In the heat-conductive materials of all the abovementioned aspects, since the metal sheet of high thermal expansion is filled under pressure into the inside of the through-holes of the metal sheet of low thermal expansion by welding under pressure, the former is desired to be a material of high ductility and elongation and high thermal conductivity, such as Cu, Cu alloys, Al, Al alloys, Ag or Ag alloys. As the metal sheet of low thermal expansion for the heat-conductive materials, usable is a material of high ductility, such as Mo, Ni-Fe alloys containing from 30 to 50 % by weight of Ni, Ni-Co-Fe alloys containing from 25 to 35 % by weight of Ni and from 4 to 20 % by weight of Co, and W. As the metal foil of high thermal expansion to be the outermost layer of them, usable is a material of Cu, Cu alloys, Al, Al alloys, Ag or Ag alloys. In consideration of the use and the material of a thin film layer to be welded thereto, the foil may well be selected from various materials same as or different from those of the metal sheet of high thermal expansion of the core material. In addition, for the purpose of improving the brazing property or corrosion resistance or for the purpose of improving the adhesiveness to Au or Ag plating layer in accordance with the use of the heat-conductive material to be produced by the present invention, Cu, Al, Ni, Sn, Soldar or the like may be coated over the material by any known coating means such as plating, metal vapor deposition, ion plating or CVD (chemical vapor deposition) or Dipping in Molten Metal or Fusing and Solidification in Furnace at any desired positions, or the material may also be coated with soldering material, Ag-brazing material, ceramics or glass layer also at any desired positions. The metal sheet of low thermal expansion of the heat-conductive material, to the surface of which metal sheet is welded under pressure a metal foil of high thermal expansion, has through-holes in the direction of the thickness thereof. The through-holes may be formed by mechanical operation such as press-stamping or by chemical operation such as etching. The distance between the respective through-holes is desired to be narrow, since the distribution of the products to be obtained may advantageously be reduced. In general, therefore, it is 3 mm or less, preferably 1 mm or less, more preferably 0.5 mm or less. The shape of the through-holes is not specifically defined. For instance, the cross-sectional shape of them may be either circular or polygonal; and the longitudinal-sectional shape of them may be either straight or tapered. A tapered shape is preferred, since intrusion of a metal into the inside of the through-holes is easy and the welding strength may be elevated. The through-holes of the metal sheet of low thermal expansion in question in the direction of the thickness thereof may be such that may well be filled with a metal of high thermal expansion after welding and rolling under pressure. For instance, therefore, notches of penetrating through the non-rolled metal sheet of low thermal expansion or not penetrating but just before penetrating therethrough in the determined direction of the thickness of the sheet may be formed; or notches of various shapes may suitably be formed into the both surfaces of the metal sheet from various directions to have the same disposition as that of the above-mentioned through-holes. In the case, the shape of the notches is not specifically defined but may be selected from various shapes of, for example, "-", "+" and "<". In addition, wedge-wise notches, such as trigonal pyramid-like notches, may also be formed in the determined direction of the thickness of the metal sheet. In the present invention, the reduction ratio is needed to be about 60 % for cold rolling. Where a metal sheet of high thermal expansion is heated, bonding of metals of high thermal expansion to each other is to be extremely favorable crystallographically so that the reduction ratio may be lowered to about 20 %. Preferably, the reduction ratio is from 30 to 50 %. The heat-conductive materials of the present invention as well as methods of producing them are mentioned below in detail with reference to the drawings attached hereto. Figs. 1(A), 1(B), 1(C) and 1(D) are longitudinal explanatory views of each showing the heat-conductive material obtained by the process of the present invention, in each of which the part to be welded under pressure and integrated with a metal sheet of high thermal expansion comprises a metal sheet of low thermal expansion having a metal foil of high thermal expansion as provided on the surface thereof and having a number of through-holes in the direction of the thickness thereof. Fig. 2 is a partly-cut perspective explanatory view showing the heat-conductive material of Fig. 1(A). With reference to these drawings, one embodiment of having a copper sheet as the metal sheet of high thermal expansion and having a Kovar (Fe-Co-Ni alloy) sheet as the metal sheet of low thermal expansion is mentioned below. The heat-conductive material of Fig. 1(A) has such a constitution that the both surfaces of copper sheet (1) are welded under pressure with Kovar sheet (2) having metal foil layers of high thermal expansion (3)(3) as provided on the both surfaces thereof and having a number of through-holes (4)(4) . . . in the direction of the thickness thereof. The Kovar sheet (2) having the metal foil layers of high thermal expansion (3)(3), which is welded under pressure to the both surfaces of the copper sheet (1), has through-holes (4)(4) . . . each having the same dimension in the direction of the thickness thereof to thereby give circular copper exposing areas (5)(5) . . . on the surface of the metal foil layer of high thermal expansion (3) (refer to Fig. 2). Where the metal foil layer of high thermal expansion (3) is the same material of copper as the copper sheet (1), the copper exposing areas (5)(5) . . . are diffused to and integrated with the sheet (1) in such a degree that the former areas could not be differentiated from the latter sheet. The Kovar sheet (2) has through-holes (4)(4) . . . each having the same dimension in the direction of the thickness thereof. The through-holes (4)(4) . . . may be tapered so that the diameter of each hole is different from each other in the both surfaces, and they may be arranged in such a way that the adjacent through-holes each have a different diameter and that the arrangement comprises combination of large and small through-holes. By suitably selecting the thickness of the Kovar sheet (2) to be welded under pressure to the both surfaces of the copper sheet (1) via the metal foil layer of high thermal expansion (3) and also by selecting the ratio of the copper exposing areas (5)(5) . . . and the dispersed condition of them, the thermal characteristics of the respective main surfaces may be made near to the desired ones. Since the metal foil layer of high thermal expansion (3) of the outermost layer is selected from Cu, Cu alloys, Al, Al alloys, Ag and Ag alloys in consideration of the use and the material of a thin layer to be coated thereover, the composite material may have improved properties of uniform heat receivability, high heat diffusibility, high weldability to materials to be welded thereto and high adhesiveness to a thin film to be coated thereover. The heat-conductive material of Fig. 1(B) has such a constitution that Kovar sheet (2) having metal foil layers of high thermal expansion (3)(3) as provided on the both surfaces thereof and having a number of through-holes (4)(4) . . . in the direction of the thickness thereof is welded under pressure to only one surface of copper sheet (1). The heat-conductive material of Fig. 1(C) has such a constitution that Kovar sheets (2)(2) each having metal foil layer of high thermal expansion (3) as provided on only one surface thereof and having a number of throughholes (4)(4) . . . in the direction of the thickness thereof are welded under pressure to the both surfaces of copper sheet (1) in such a way that the metal foil layers of high thermal expansion (3)(3) are positioned on the both outer surfaces. The heat-conductive material of Fig. 1(D) has such a constitution that Kovar sheet (2) having a metal foil layer of high thermal expansion (3) as provided on one surface thereof and having a number of through-holes (4)(4) . . . in the direction of the thickness thereof is welded under pressure to only one surface of copper sheet (1) in such a way that the metal foil layer of high thermal expansion (3) is positioned on one outer surface. In producing the heat-conductive material of Fig.1 (A) which comprises a copper sheet, Kovar sheets and metal foils, copper foils (6)(6) are previously welded under pressure to the both surfaces of Kovar sheet (2) having determined dimension and thickness with press-welding rolls (7)(7), the laminated sheet of a determined length is stocked in a looper device, and thereafter the sheet is stamped with stamping machine (8), with which, for example, a number of small through-holes are made through the sheet to give a reticulate pattern to the sheet, then the sheet is annealed, surface-treated and coiled up, as shown in Fig. 3. Alternatively, after pressure welding with the press-welding rolls (7)(7), the welded sheet may then be coiled up and thereafter uncoiled to be conveyed to the stamping machine where the uncoiled sheet is stamped. In producing the heat-conductive materials of Figs. 1(B), 1(C) and 1(D), similarly to the production of the heat-conductive material of Fig. 1(A), the thermal characteristics of the respective main surfaces may suitably be adjusted by selecting the thickness etc. of Kovar sheet (2) or copper sheet (1). Next, as shown in Fig. 4, copper sheet (1) having determined dimension and thickness is uncoiled and heated in heating device (10) up to a temperature not lower than the recrystallizing temperature thereof, and the previously prepared stamped three-layer sheets (9)(9) each as uncoiled from above and below are laminated over the heated sheet (1) and welded together under pressure with press-welding rolls (7)(7). Since the copper sheet is previously heated up to a temperature not lower than the recrystallizing temperature thereof prior to pressure welding of the sheet (1) and the stamped three-layer sheets (9)(9), the copper sheet (1) may intrude into the through-holes of the Kovar sheet (2) under a low pressure so as to be exposed out to the surface of the copper foil (6) of the Kovar sheet (2); and since the copper foil (6) between the copper sheet (1) and the Kovar sheet (2) and the copper sheet (1) as well as the copper foil (6) of the outermost layer and the copper sheet (1) are well diffused and integrated together, a high welding strength may be obtained even under a small pressure. As a result, deformation of the shape of each through-hole as provided through the Kovar sheet (2) is small so that the composite material thus obtained may have determined thermal expansion coefficient and thermal conductivity without varying the previously determined exposing surface area ratio between the exposed copper sheet (1) on the surface of the Kovar sheet (2) and the Kovar sheet (2) and without causing anisotropy in thermal expansion of the material. In the above-mentioned pressure welding, the heating atmosphere for heating the metal sheet of high thermal expansion and the welding atmosphere of the laminated sheets are desired to be non-oxidizing ones. For instance, employable as the atmosphere is N 2 , Ar, H 2 or a mixed gas atmosphere of them, as well as an ammonia-decomposed gas or propane combustion gas (DX) atmosphere. The heating device may suitably be selected in accordance with the material of the metal of high thermal expansion. In general, usable is a tubular furnace column, a light beam heating device, a laser heating device, a high frequency heating device, or a plasma heating device. The heating temperature for the metal sheet of high thermal expansion may suitably be selected from a temperature range of from the recrystallizing temperature of the selected metal material to lower than the melting point of the same. Preferred is a temperature falling within the range of from a temperature lower than the melting point of the metal material to be heated by 100°C to lower than the melting point thereof. For instance, for heating a copper or copper alloy sheet, the heating temperature is from 950 to 1050°C; for heating an Al or Al alloy sheet, it is from 550 to 650°C; and for heating an Ag or Ag alloy sheet, it is from 800 to 900°C. In the above-mentioned process of Fig. 4, where the stamped sheet (9) is applied to the uncoiled copper sheet (1) as previously heated up to a temperature not lower than the recrystallizing temperature thereof only from either above or below, a heat-conductive material of Fig. 1(B) as mentioned above is obtained. In place of the stamped three-layer sheet (9) composed of the Kovar sheet (2) to the both surfaces of which the copper foils (6)(6) have been welded under pressure, a stamped two-layer sheet (11), which is obtained by welding under pressure copper foil (6) onto the upper surface of Kovar sheet (2) with press-welding rolls (7)(7), stocking the welded sheet of a determined length in a looper device and thereafter stamping it with stamping machine (8), as shown in Fig. 5, may be worked in the same manner as in Fig. 4 where the Kovar sheet (2) of each of the stamped two-layer sheets (11)(11) is laminated onto the both surfaces of copper sheet (1) as previously heated up to a temperature not lower than the recrystallizing temperature thereof and welded together under pressure. In the case, a heat-conductive material of the above-mentioned Fig. 1(C) of the present invention is obtained. Alternatively, after the laminated sheets have been welded together under pressure with the press-welding rolls (7)(7), then coiled up and thereafter uncoiled to be conveyed to the stamping machine (8), and stamped there. In the above-mentioned process, where the Kovar sheet (2) of the stamped two-layer sheet (11) is laminated onto only one surface of the copper sheet (1) as previously heated up to a temperature not lower than the recrystallizing temperature and welded together under pressure, a heat-conductive material of the abovementioned Fig. 1(D) is produced. Since the Kovar sheets (2)(2) are welded under pressure to the both surfaces of the copper sheet (1) in preparing the above-mentioned heat-conductive material having the stamped two-layer sheet (11), the welded sheet is desired to be annealed at a low temperature so as to obtain the welding strength comparable to that of the heat-conductive material having the three-layer sheet (9) where welding have been made between the constitutive copper materials. Since the copper foil (6) of the outermost layer of the heat-conductive material has previously been welded under pressure to the Kovar sheet (2) in both cases of using the stamped three-layer sheet (9) and the stamped two-layer sheet (11) as the constitutive part, the heat-conductive materials to be obtained in the both cases have the same merit of involving few defects of peeling or the like. The present invention will be explained in more detail by way of the following examples, which, however are not intended to restrict the scope of the present invention. EXAMPLE 1: A Kovar sheet (29Ni-16Co-Fe alloy) having a thickness of 0. 3 mm and a width of 30 mm was annealed at 900°C and then wire-brushed, and a Cu foil having a thickness of 0.05 mm and a width of 30 mm was welded under pressure to the both surfaces of the sheet to give a three-layer sheet having a Cu foil thickness of 30 µm and a total thickness of 0.25 mm. Then, a number of throughholes each having a diameter of 1.0 mm were formed through the sheet with the distance between the holes in the rolled direction being 2.0 mm and the distance between them in the width direction being 2.0 mm. A Cu sheet having a thickness of 0. 5 mm and a width of 30 mm was wire-brushed and then heated up to 800°C, and the previously obtained three-layer sheet was welded under pressure to the both surfaces of the Cu sheet with a welding machine, in the manner as shown in Fig. 4, to thereby obtain a heat-conductive material of Fig. 1(A) substantially composed of five layers and having a thickness of 0.4 mm, in which copper intruded into the through-holes of the Kovar sheet and it was partly exposed out and integrated with the copper foil of the outermost layer in the determined positions. The reduction ratio was 60 %. The shape of each of the Cu-exposing areas in the main surface of the thus obtained heat-conductive material was almost circular in the rolled direction, and the distance between the through-holes was 2.0 mm in the rolled direction. The ratio of the Cu-exposing surface areas to the Kovar sheet was 35%. The thermal conductivity of the thus obtained material was 230 w/m.K in the thickness direction, and the thermal expansion coefficient in each main surface was 8 x 10 -6 /°C. In the heat-conductive material thus obtained, the thickness of the Cu sheet (t 1 ) was 0.15 mm, the thickness of each Kovar sheet (t 2 ) was 0.115 mm, and the thickness of each surface Cu foil (t 3 ) was 0.01 mm (refer to Fig. 2). The heat-conductive material having a thickness of 0.4 mm was cut into a determined size. Two of them were laminated to give a heat-releasing substrate (heat sink). Using the heat-releasing substrate, a ceramic package was prepared. The package was verified to have good heat-releasability and good thermal compatibility. The heat-conductive material having a thickness of 0.4 mm was annealed in a hydrogen atmosphere for 5 minutes at 1000°C and then cold-rolled to have a thickness of 0.15 mm. In the heat-conductive material thus obtained, the thickness (t 1 ) of the Cu sheet of constituting the core was 0.068 mm, the thickness (t 2 ) of each Kovar sheet was 0.038 mm, and the thickness (t 3 ) of each surface Cu foil was 0.003 mm. Next, this was worked into a lead frame by a known method, with which a semiconductor package was formed. In the package thus formed, there was neither peeling in the adhering interface between the chip and the island nor cracking of the sealant resin. This had good heat diffusibility similar to that of a lead frame made of a conventional copper alloy. EXAMPLE 2: A Kovar sheet (29Ni-16Co-Fe alloy) having a thickness of 0.5 mm and a width of 30 mm was annealed at 900°C and then wire-brushed, and a Cu foil having a thickness of 0.2 mm and a width of 30 mm was welded under pressure to one surface of the sheet while a Cu foil having a thickness of 0.05 mm and a width of 30 mm to the other surface thereof, to give a three-layer sheet having a Cu foil thickness of 30 µm and a total thickness of 0.29 mm. Then, a number of through-holes each having a diameter of 1.0 mm were formed through the sheet with the distance between the holes in the rolled direction being 2.0 mm and the distance between them in the width direction being 2.0 mm. A Cu sheet having a thickness of 0.35 mm and a width of 30 mm was wire-brushed and then heated up to 900°C, and the Cu sheet was welded under pressure to the previously obtained three-layer sheet on the surface thereof having a thinner Cu foil, with a welding machine, to thereby obtain a heat-conductive material of Fig. 1(B) substantially composed of three layers and having a thickness of 0.4 mm, in which copper intruded into the through-holes of the Cu/Kovar composite sheet and it was partly exposed out and integrated with the copper foil of the outermost layer in the determined positions. The reduction ratio was 62 %. The shape of each of the Cu-exposing areas in the main surface of the thus obtained heat-conductive material was almost circular in the rolled direction, and the distance between the through-holes was 2.0 mm in the rolled direction. The ratio of the Cu-exposing surface areas to the Kovar sheet was 35 %. The thermal conductivity of the thus obtained material was 250 w/m.K in the thickness direction, and the thermal expansion coefficient in each main surface was 9 x 10 -6 /°C. EXAMPLE 3: A Kovar sheet (29Ni-16Co-Fe alloy) having a thickness of 0.5 mm and a width of 30 mm was annealed at 900°C and then wire-brushed, and a Cu sheet having a thickness of 0.2 mm and a width of 30 mm was welded under pressure to one surface of the sheet to give a two-layer sheet having a Cu layer thickness of 85 µm and a total thickness of 0.3 mm. Then, a number of through-holes each having a diameter of 1.0 mm were formed through the sheet with the distance between the holes in the rolled direction being 2.0 mm and the distance between them in the width direction being 2.0 mm. A Cu sheet having a thickness of 0.35 mm and a width of 30 mm was wire-brushed and then heated in an N 2 atmosphere up to 900°C, and the Cu sheet was welded under pressure to the previously obtained two-layer sheet on the surface of the Kovar side thereof, with a welding machine, to thereby obtain a heat-conductive material of Fig. 1(C) substantially composed of three layers and having a thickness of 0.25 mm, in which copper intruded into the through-holes of the Cu/Kovar composite sheet and it was partly exposed out and integrated with the copper foil of the outermost layer in the determined positions. The reduction ratio was 73 %. The shape of each of the Cu-exposing areas in the main surface of the thus obtained heat-conductive material was almost circular in the rolled direction, and the distance between the through-holes was 2.0 mm in the rolled direction. The ratio of the Cu-exposing surface areas to the Kovar sheet was 35 %. The thermal conductivity of the thus obtained material was 250 w/m.K in the thickness direction, and the thermal expansion coefficient in each main surface was 9 x 10 -6 /°C. EXAMPLE 4: A Kovar sheet (29Ni-16Co-Fe alloy) having a thickness of 0.5 mm and a width of 30 mm was annealed at 900°C and then wire-brushed, and a Cu foil having a thickness of 0.2 mm and a width of 30 mm was welded under pressure to one surface of the sheet to give a two-layer sheet having a Cu foil thickness of 60 µm and a total thickness of 0.28 mm. Then, a number of through-holes each having a diameter of 1.0 mm were formed through the sheet with the distance between the holes in the rolled direction being 2.0 mm and the distance between them in the width direction being 2.0 mm. The mean thermal expansion coefficient of the Kovar sheet at 30 to 200°C was 5.2 x 10 -6 /°C. A Cu sheet having a thickness of 0.35 mm and a width of 30 mm was wire-brushed and then heated up to 900°C, and the Cu sheet was welded under pressure to the previously obtained two-layer sheet on the surface of the Kovar side thereof, with a welding machine, to thereby obtain a heat-conductive material of Fig. 1(D) substantially composed of three layers and having a thickness of 0.25 mm, in which copper intruded into the through-holes of the Cu/Kovar composite sheet and it was partly exposed out and integrated with the copper foil of the outermost layer in the determined positions. The reduction ratio was 60 %. The shape of each of the Cu-exposing areas in the main surface of the thus obtained heat-conductive material was almost circular in the rolled direction, and the distance between the through-holes was 2.0 mm in the rolled direction. The ratio of the Cu-exposing surface areas to the Kovar sheet was 35 %. The mean thermal expansion coefficient of the Cu sheet at 30 to 200°C was 17.2 x 10 -6 /°C. The thermal conductivity of the thus obtained material was 250 w/m.K in the thickness direction, and the thermal expansion coefficient in each main surface was 9 x 10 -6 /°C. EXAMPLE 5: A Kovar sheet (29Ni-16Co-Fe alloy) having a thickness of 0.6 mm and a width of 50 mm was annealed at 900°C and wire-brushed, and a Cu foil having a thickness of 0.25 mm and a width of 30 mm was welded under pressure to one surface of the sheet to give a two-layer sheet having a Cu foil thickness of 90 µm and a total thickness of 0.33 mm. A number of through-holes each having a diameter of 1.0 mm were formed through the thus prepared two-layer sheet with the distance between the throughholes in the rolled direction being 2.0 mm and the distance between them in the width direction being 2.0 mm. The mean thermal expansion coefficient of the Kovar sheet at 30 to 200°C was 5.2 x 10 -6 /°C. A Cu sheet having a thickness of 0.45 mm and a width of 30 mm was wire-brushed, and this was welded under pressure to the previously obtained two-layer sheet on the side of the Kovar sheet, with an welding machine, to thereby obtain a three-layer heat-conductive material of Fig. 1(D) composed of substantially three layers and having a total thickness of 0.3 mm, in which copper intruded into the through-holes of the Cu/Kovar composite sheet and it was partly exposed out and integrated with the copper foil of the outermost layer in the determined positions. The shape of each of the Cu-exposing areas in the main surface of the thus obtained heat-conductive material was almost circular in the rolled direction, and the distance between the through-holes was 2.5 mm in the rolled direction. The ratio of the Cu-exposing surface areas to the Kovar sheet was 33 %. The mean thermal expansion coefficient of the Cu sheet at 30 to 200°C was 17.2 x 10 -6 /°C. The thermal conductivity of the thus obtained heat-conductive material was 270 w/m.K in the thickness direction, and the thermal expansion coefficient thereof in each main surface was 9 x 10 -6 /°C. In this example, the Cu sheet was not heated prior to being welded under pressure to the Cu/Kovar composite sheet but was welded thereto at a room temperature. In the case that the amount of copper to be intruded into through-holes of a composite sheet is relative small, or in the case of using a part usable even though it has a relatively large thermal expansion coefficient or using one having such a constitution that the amount of copper therein is large relative to the whole amount thereof, a heat-conductive material having a desired dimension may be obtained without imparting a large load thereto. Therefore, in the case, a welding means to be effected at a room temperature may be employed. However, even though the material mentioned above is to be obtained, the copper metal sheet of high thermal expansion may be heated up to a temperature not lower than the recrystallizing temperature thereof, prior to welding, thereby to elevate the welding strength and to minimize the deformation of the through-holes. In producing the heat-conductive materials according to the present invention, the metal sheet of high thermal expansion, which is disposed essentially for improving the thermal conductivity of the materials, is welded under pressure to other parts to be integrated therewith, at a room temperature or after the sheet has been heated up to a temperature not lower than the recrystallizing temperature thereof. As a result, the welding strength may be elevated, and additionally, the thermal expansion coefficient and thermal conductivity of the heat-conductive material thus produced may be varied to any desired ones without varying the thickness ratio of the metal sheets of constituting the material and without varying the surface area ratio of the through-holes. Accordingly, the heat-conductive materials thus produced according to the present invention may satisfy both the compatibility with materials to be applied thereto, such as metals, ceramics, semiconductors (e.g., Si) and plastics, with respect to the thermal expansion coefficients of them, and the good thermal conductivity of themselves. In addition, they may receive evenly the heat as applied thereto and have an improved heat-diffusing effect, and they are free from fine pores on the surfaces and therefore have excellent adhesiveness to thin films of plating materials or brazing materials. Accordingly, the heat-conductive materials of the present invention are the best ones as materials of heat-releasing substrates for mounting semiconductor chips thereon and as materials for lead frames. In particular, in producing the heat-conductive material according to the present invention, since the welding operation, which involves a factor of varying the thickness ratio of the constitutive metal sheets and the surface area ratio of the through-holes, is effected only once, the process is simple and may yield the intended heat-conductive material at a low cost. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof as defined by the claims.

Claims: A process of manufacturing a heat conductive laminated body which has oppositely facing major surfaces constituted by a metal having a high coefficient of thermal expansion, the body comprising a core sheet (1) of a metal having a high coefficient of thermal expansion having pressure welded on at least one of its sides, a perforated sheet (2) of a metal having a low coefficient of thermal expansion, the metal of the core sheet (1) occupying the holes in the perforated sheet (2); said process being characterized in that prior to its being perforated, said sheet (2) is welded to form a unitary structure with at least one foil (3) of a metal having a high coefficient of thermal expansion, which unitary structure is then perforated with a multiplicity of through holes (4) extending through the thickness thereof and distributed substantially evenly throughout the structure, and then said perforated unitary structure is united with said core sheet (1) to form the laminated body by said pressure welding, during which step, the core sheet material penetrates said holes (4) and integrates with each said foil (3). A process as defined in claim 1 and wherein said core sheet (1) is united on both of its sides with a said perforated sheet (2) which prior to perforation, has been previously united with a said foil (3); the sheet (2) and the foil (3) forming a unitary structure which is perforated with through holes (4) prior to being united with the core sheet (1) to form the laminated body, with the foils (3) forming the outside layers at the sides of said laminated body. A process as defined in either of claims 1 or 2 wherein prior to being united with said core sheet (1), the said sheet (2) or each said sheet (2) is united on both its sides with a said foil (3) of a metal having a high coefficient of thermal expansion, to form a three-layered unitary structure, which structure is subsequently perforated with through holes (4) before being united with the core sheet (1) to form the laminated body. A process as defined in any preceding claim in which the or each said sheet (2) is united with the or each said foil (3) by pressure welding after the sheet (2) has been annealed and wire brushed. A process as defined in claim 4 wherein said annealing is at 900°C. A process as defined in any preceding claim wherein said core sheet (1) is united on one or both sides with said perforated unitary structure by pressure welding at a temperature not lower than the recrystallization of the metal constituting the core sheet (1) whereby the material of said core sheet (1) flows into and occupies the holes constituting said perforations in the or each sheet (2). A process as defined in any one of the preceding claim 1 to 5 and wherein said core sheet (1) is united on one or both sides with said perforated unitary structure by pressure welding at room temperature whereby the material of said core sheet (1) flows into and occupies the holes constituting said perforations in the or each sheet (2). A process as defined in any preceding claim and wherein said core sheet (1) is composed of Cu, Cu alloy, Al, Al alloy, Ag and Ag alloy. A process as defined in any preceding claim and wherein the sheet (2) consists of a metal having a low coefficient of thermal expansion, being one selected from the group comprising Mo; Ni-Fe alloys having an Ni content of from 30 to 50% by weight; and Ni-Co-Fe alloys having an Ni content of from 25 to 35% by weight and a Co content of from 4 to 20% by weight, and W. A process as defined in any preceding claim and wherein the foil consists of a metal having a high coefficient of thermal expansion, being one selected from from the group Cu; Cu alloys; A1; A1 alloys; Ag; and Ag alloys. A process as defined in any preceding claim and including the further steps of subjecting the laminated body to annealing followed by cold rolling. A process as defined in claim 12 wherein said annealing is at 1000°C. A process as defined in any preceding claim and wherein the heating atmosphere or the welding atmosphere, or both, is a non-oxidizing atmosphere. A process as defined in any preceding claim and including the further step of applying to the laminated body a coating of one or more of Ni, Al, Cu or Sn at selected locations on the surface thereof. A process as defined in any preceding claim and including the further step of working the laminated body into a lead frame on which can be mounted a semi-conductor package.

Inventors: KAWAKAMI MAKOTO, NAKAMURA YASUYUKI, KAWAKAMI, MAKOTO, NAKAMURA, YASUYUKI
Applicants: SUMITOMO SPEC METALS, SUMITOMO SPECIAL METALS COMPANY LIMITED
Application year: 1992
Main classification: B23K20
IPC codes: H01L21, H01L23, B23K20