TECHNICAL FIELD

BACKGROUND

[0001] Graphite-metal complexes incorporating a graphite material are known as the metal-based complexes wherein graphite or graphite fiber is dispersed in metal matrix, or as the graphite-based complexes wherein the metal is diapered in the sintered graphite matrix molding that is the extruded molding, or pressed molding by applying the unidirectional pressure on an isostatic pressure molding die. (Japanese Patent Application Hl l-321828, Patent Application 2001135551)

[0002] On one hand, the complexes that incorporate graphite have the thermal diffusivity of 1.5-3 cm ² /sec. Compared with the material normally used as heat transfer media such as aluminum, copper or aluminum nitride having the thermal diffusivity of 0 7-1.0 cm ² /sec, the of the complexes' heat spreading performance thereof is superior.

[0003] On the other hand, graphite-metal complexes have the good electrical conductance. Therefore, it is required to form a kind of electrical insulating layer on one surface of the complex, on which electric circuit can be directly laid.

[0004] After the graphite-metal complex body is machined into the specific shape, there are many ways to form the insulating layer on its one surface, which include attaching insulating bodies on the complex surface by adhesives or solders etc. However, because the process diverges into many steps, it is costly. In addition, using adhesives or solders results in the thermal conductivity loss. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram of the impregnation process: having an electrically insulating plate (ceramic or diamond plate) contact to a graphite plate, adding the molten metal and applying high-pressure to impregnate.

1 is Mold; 2 is Die; 3 is Electrical Insulating Plate (Ceramic or Diamond Plate); 4 is Graphite Plate; 5 is Molten Metal; 6 is Supporting Jig.

Fig. 2. (a) and (b) reveal the typical cross-sectional SEM and EDS images of aluminum nitride/graphite-metal composite sample; (c) is diagram showing the interface jig-saw structure.

DETAILED DESCRIPTION

[0005] The invention relates to a heat radiator composed of an electrically insulating plate and graphite - metal or alloy (called metal bellow) composite used for LED package, high-load semiconductor, high-load capacitor or light- convergent solar cell board that assumes a short lifetime or breakdown at high temperatures.

[0006] The purpose of this invention is to take into account the aforementioned problems and provide a heat radiator that, while maintaining good thermal diffusivity of graphite-metal complex, tightly connects to an electrically insulating plate, and furthermore, the electrically insulating plate combination is complete in the same process as graphite-metal complex formation. [0007] The inventor reached an invention summarized as follows below that positively achieves the aforementioned purpose, by devising a combination of the material used.

[0008] Examples of insulating materials include oxide ceramics such as alumina, mullite, zirconia, beryllia, magnesia, magnesia-alumina spinel, thoria etc., and other materials having an electrical resistivity more than 10 Ω-cm such as nitrides containing boron nitride, aluminum nitride, silicon nitride etc., and insulating diamond such as non-doped chemical vapor deposition (CVD) diamond. The electrically insulating plate size depends on the practical application requirement. Although a thin thickness is preferred, it is needed to have a good combination of handing, electrical insulting and thermal diffusivity, preferably 0.01 mm to 1 mm.

[0009] Graphite matrix used can be a press formed plate or a cold isostatic molding body comprising at least one type of the materials selected from the group consisting of plates cut from the graphite electrode blocks, artificial graphite powders, natural graphite powders or chopped graphite fibers. The graphite matrix size depends on the practical application requirement. The thickness is expected to not cause the radiator thermal diffusivity decrease, preferably 0.5 - 10 mm. [0010] The invention enables the utilization of the good heat diffusivity of a graphite-metal complex that includes > 60% volume fraction graphite.

[0011] The metal component connecting to the electrically insulating plate and composing the graphite - metal composite may be the metal having a melting point lower than 1100 ⁰ C. Examples include aluminum, aluminum alloy, copper, copper alloy, magnesium, magnesium alloy, silver, silver alloy, zinc, zinc alloy, tin, tin alloy, gold, gold alloy etc..

[0012] The electrically insulating heat radiator can be obtained by the following way (see Figure 1): clipping an electrically insulating plate (4) and a graphite plate (3) together with a jig (6) that can be made of steel; putting them into mold (1) and pouring the molten metal into mold; applying the pressure on molten metal by pressing die (2); and after the metal coagulation, cutting off the formed complex body.

EFFECT OF THE INVENTION

[0013] The invention enables the utilization of the good thermal diffusivity of formed body consisting of an electrically insulating plate and graphite-metal complex. Fabrication process is extremely simple and economic. The invention can offer the useful substrates for LED package, high-load semiconductor, high- load capacitor or light-convergent solar cell device.

OPTIMIZED METHOD

[0014] The electrically insulating heat radiator can be obtained by the following way: having an electrically insulating plate and a graphite plate closely contact together; applying a pressure more than 10 MPa on molten metal; and coagulating the molten metal. If the pressure is lower than 10 MPa, metal cannot be tightly fixed on the electrically insulating plate surface, and the goal cannot be accomplished. It is advised to pre -treat insulating plate surface by thermally spraying metal on it. [0015] In the case that no pre-treatment is performed on electrically insulating plate surface, an ideal connection can also be obtained by applying a pressure more than 50 MPa.

[0016] The graphite plate used for the present invention may be ones that have good thermal diffusivity along plate thickness direction. Therefore, the graphite plates are preferred to be ones made by extruding electrode-graphite material, preferably having a thermal diffusivity more than 2 cm /sec. A part of the graphite contents can be the needle-like graphite, chopped graphite fibers or natural graphite etc., and then a high thermal diffusivity should be obtained.

[0017] The electrically insulating plate used for the present invention is preferred to have a high thermal diffusivity. However, even if an electrically insulating plate having a low thermal diffusivity is used, the bad affects on the diffusivity of the final formed body can be suppressed by reducing electrically insulating plate thickness. Vice versa, when an insulating plate having a low thermal diffusivity is used, by increasing the thickness of graphite-metal part, electrically isolating heat radiator that has good thermal properties can be obtained as well.

[0018] For electrically insulating plates used for the present invention, oxide ceramic general has a low thermal diffusivity and is not preferred. Oxide ceramic has the excellent eclectically isolating properties that can be used by slicing it into a thin thickness. But when the plate having the thickness less than 0.01 mm, it is hard to handle, and it is not preferred.

Nitride ceramic has a better thermal diffusivity than oxide ceramic, even if the nitrate plate used is as thick as about lmm, an excellent heat radiator can still be produced. Non-doped diamond plate that can be fabricated by chemical vapor deposition (CVD) method has excellent thermal diffusivity and electrically insulating property. It can be used in a very lager thickness range: 0.01 - 10 mm.

[0019] The metals used for the present invention are preferred to be the ones having a melting point lower than 1100 C, because the mold is made of iron. Economical low-cost and low meting point materials are preferred. Aluminum, and aluminum alloy are preferred because they are easy to handle during casting process.

EXAMPLES

[0020] Hereunder, the present invention shall be specifically described by

Examples. The present invention is, however, not to be limited in any way by these Examples etc.

[0021] Example 1 [0022] A 120 mm x 60 mm x 2 mm plate was obtained from an electrode graphite extrusion material containing > vol 85 % graphite content; the thickness plane was cut normal to extrusion direction. The plate had a thermal diffusivity of 2.3cm /sec. On the other hand, electrically insulating plate was the aluminum nitride in the size of the 120 mm x 60 mm x 0.4 mm. Referring Figure 1, two plates were tightly clipped together by an iron-made jig. The thermal diffusivity of aluminum nitrate plate was 0.8cm /sec.

[0023] The ceramic attached radiator was obtained by the following process; putting the ceramic plate -graphite plate -jig set into the mold (as shown in Figure 1); pouring the molten JIS-AC3A alloy into the mold; applying 80 MPa pressure on die and coagulating the molten alloy. The molten AC3A alloy was pressed into the pores of graphite plate and the gap between graphite and ceramic plates, making the two plates closely connect together.

[0024] The electrically insulating radiator made by the method mentioned above has a thermal diffusivity of 2.1 cm /sec. The adhesion strength between the ceramic plate and graphite-metal composite was 45 MPa. The heat radiator had sufficient thermal conduction and electrical insulation abilities for 55 W high-load semiconductor substrate application.

[0025] Example 2 [0026] The extrusion body was formed including vol 30% pitch graphitization fiber material which was chopped to 1 mm in average size. This body was sliced into 100 mm x 100 mm x 3mm plates; the slicing plane was normal to the body extrusion direction. The extrusion body had a thermal diffusivity of 2.6 cm ² /sec. [0027] On the other hand, 100 mm x 100 mm x 0.2 mm silicon nitride plate had a thermal diffusivity of 0.5 cm ² /sec. An electrically insulating heat radiator was obtained by using the same casting way as described in Example 1. This radiator had a thermal diffusivity of 2.1 cm ² /sec.

[0028] Example 3 [0029] A heat radiator was prepared in the same way as Example 2 except that a 0.05 mm thick alumina plate was used as an insulating plate. The alumina plate had a thermal diffusivity of 0.08 cm ² /sec. The fabricated electrically insulating heat radiator had a thermal diffusivity of 2.1 cm /sec. [0030] The heat radiator substrates obtained by the Examples 2 and 3 methods are used in the light-convergent solar cells, and then the silicon chip temperature could be kept under 80 ⁰ C.

[0031] Comparative Example 1 [0032] A heat radiator was obtained by the following process: plating 5 um-thick Ni layer on the graphite-metal composite plate that was same as that used in Example 1 ; cementing the aluminum nitride plate that was also the same one used in Example 1 on one side by copper - silver - zinc solder. The adhesion strength was 28 Mpa, and thermal diffusivity was 1.4 cm /sec.

[0033] Comparing with Example 1 , this heat radiator was inferior on both strength and the thermal diffusivity. Moreover, the process that included plating, soldering, and cementing was costly, and economically disadvantageous.

[0034] Embodiment of Diamond / Graphite-Metal Composites

[0035] The embodiment of present invention can successfully connect the electrically insulating diamond plate on graphite-metal composites. The diamond plate can be the non-doped CVD diamond. Since the diamond plate is thermally isotropic and has the better thermal diffusivity (e.g. 10 cm ² /sec) than graphite- metal composites, the combination of diamond plate on composite plate can greatly increase in-plane (along the plate surface) thermal conduction ability without causing thermal diffusivity loss along the plate thickness direction. This kind of heat radiators can be used as the substrates for the extremely high-load laser or semiconductor devices. INTERFACE STRUCTURE

[0036] The interface structure plays the critical role on the enhancement of the adhesion strength between the electrically insulating plate and graphite-metal composite. Figure 2 (a) and (b) reveal the typical cross-sectional SEM and EDS images of aluminum nitride/graphite-metal composite sample. From Fig. 2, one can clearly see the ceramic plate is connected to composite by the intermediate metal (Al) layer. Both the insulating plate/metal and composite/metal interfaces are not flat, presenting the jig-saw-like structure. The jig-saw-like interfacial structure is sketched in Figure 2 (c). The jig-saw-like connection strongly hooks the insulating plate to the metal layer and then to the composite block, leading to the high adhesion strength.

[0037] In order to form the ideal jig-saw-like structure, it is require the original electrically insulating plate surface to have certain roughness. The surface roughness is expected to be on the order of sub-um - 10 um. Here, we define that "H" is the average height of surface protuberances, "W" is that average separation between the two adjacent protuberances, and "R" is H/W ratio. H ranges 0.1 - 10 um, preferably, 0.5 - 3 um. R ranges 0.1 - 100, preferably 1 - 10. IfH and R are too small it is hard to generate enough hooking force between electrically insulating plate and metal layer. On the other hand, if the H and R are too large, it may result the formation of interfacial voids. INDUSTRIAL APPLIC ABLITY

[0038] The electrically insulating heat radiator composed of a combination of an electrically insulating plate and a graphite-metal complex in the invention has superior heat diffusivity, and the process is extremely simple and economically advantageous, and therefore is an effective substrate for an LED package, high-load semiconductor, high-load capacitor or light-convergent solar cell, and is useful in a wide range of industrial fields.