A METHOD OF MANUFACTURING A SUBSTRATE FOR PLACEMENT OF ELECTRICAL AND/OR ELECTRONIC COMPONENTS

Abstract: In a method for manufacturing of substrate for placement of electrical and/or electronical components a base of preferably metallic material is first sandblasted and thereafter by means of a thermal process coated with a bonding layer of for instance copper powder, whereafter the bonding layer are coated with a dielectric layer of ceramic material which also is applied by means of a thermal process, the ceramic material thereafter being impregnated with silicone oil. Thereafter the ceramic layer is provided with a dielectric conducting layer, also by means of a thermal process, the predetermined conducting pattern being formed by means of a template. Finally the finished substrate is cleaned by blasting with glass spheres.

Description: A method of manufacturing a substrate for placement of electrical and/or electronic components The invention relates to a method of manufacturing a substrate for placement of electrical and/or electronic components in accordance with the preamble of claim 1. Particularly the method according to the present invention concerns the manufacturing of substrates with good thermal and mechanical properties and especially suited for use in hybrid modules for power electronics. In practice a substrate of the above-mentioned type has the same function as a printed circuit board. Common printed circuit board for placement of electronic components are usually manufactured of an epoxy material on which is provided a predetermined electrical conducting pattern by means of electroplating. In regard of obtaining a good economy and reducing the requirement for space, the conducting pattern is usually provided on both sides of the board and the electrical conducting connection between the conducting patterns on each side is provided by through-plated holes in the epoxy sub¬ strate. Such circuit boards are however best suited in electronic equipment where low voltages and small currents are used and the equipment further is not subjected to mechanical and thermal loads in any degree worth mentioning. Substrates for use in electrical engineering or electronics where high voltages and large currents are used and where the equipment further may be subjected to large mechanical and thermal loads are often based on the use of a metallic substrate, for instance in form of a sheet of steel and aluminium in order to achieve good thermal conduction and sufficient mechanical strength. Electrical isolation between the conducting pattern and the metal substrate is provided by coating with an isolating layer, for instance in the form of a enamel coating, alumina coating or combination of oxides, nitrides of carbonitrides. The isolation layer may also be generated by applying an epoxy resin layer to the metal substrate. The eletrical isolating layer may be applied on both sides of the metal substrate or only on one side, something which is common if the substrate for instance will be used in modules of power electronics, as the metal substrate in that case usually is provided with or forms cooling ribs on the other sides in order to transport heat away. The electrical isolated layer are provided with an electrical conduction pattern or current leads by means of different processes, for instance by chemical methods or by vapour deposition. Examples of circuit boards of this type are for instance disclosed in DE-PS 3447520 and DE-OS 2556826. From GB-PS 2110475A there is further known the use of a substrate in the form of an alloy of Fe, Cr, Al and Yt where a ceramic surface layer is formed by the substrate being heated in an oxidizing atmosphere. The known circuit boards or substrates as well as the methods used in their manufature are however burdened with a number of disadvantages. Different processes are used in application of respectively the isolating and the conducting layer and those processes are often of such a nature that they in a negative way influence the properties of already applied coatings. It has proved to be difficult to achieve dielectric layers with satisfying dielectric, thermal and mechanical properties and when these properties then are influenced in a negative way by providing a conducting pattern or by soldering of components thereto the finished product appears as hardly suitable to meet the requirements that are demanded of e.g. hybrid modules in power electronics. It has for instance turned out to be difficult to achieve a satisfactory bond between the different layers or the isolating layer and the metal substrate which forms the core or base and it has similarly proved that the dielectric strength of the electrical isolating layer is not as good as desired. It might of course be a solution to increase the thickness of the electrical isolating layer, but this is an expensive solution and in the case of cermaic coatings it does in any case not lead to any improvement, as the coatings as a such usually are brittle and have a relatively low tensile and shear strength, although they are hard materials and in theory have a high dielectric strength. The last however is reduced due to the formation of pores and impurities caused by migration. It has also turned out that ceramic layers applied by means of for instance plasma spraying do not attain better dielectrical properties if the thickness is increased beyond 0,3 mm. In addition the use of different processes for applying the separate layers to the metal substrate gives an impaired manufacturing economy. The object of the present invention is thus to surmount the above-mentioned and other disadvantages, as the present invention provides a method for the manufaturing of substrates charaterized by high mechanical and dielectrical strength and well suited to sustain great thermal loads which possibly the same process may be used for coating a base, preferably of a metalic material, with both the electrical isolating and con¬ ducting layers. The method according to the invention is distinguished by the features disclosed in the characterizing past of claim 1, whereas further features and advantages of the method are disclosed by the attached dependent claims. The method will be explained by a typical process example and in connection with the attached drawing. Fig. 1 shows a section of a substrate according to prior art. Fig. 2 shows schematically a plasma spray gun used with the method according to the present invention. Fig. 3 shows a section of a substrate manufactured according to the method of the present invention. Fig. 4 shows the application of an electrical conducting layer in the form of a conducting pattern. Fig. 5 shows in perspective and with partly exposed substrate a completed, mounted hybride module for power electronics. In Fig. 1 there are shown a substrate according to prior art and with a transistor contact deployed on the conducting layer by means of a conducting adhesive or soldering. Many different layers which make up the substrate are applied by means of different processes and the isolation layer is for instance glued to the base, while the heat sink or the cooling body is a separate component. Below the method according to the present invention described in connection with the manufacture of a substrate for one sided placement of components as shown in section in Fig. 3. The thermal coating process used in the following example is plasma spraying. The base, which may be a metal sheet, preferably of steel, aluminium or copper, is cleaned or degreased on that side of the surface which is to be coated with the isolating layer by being sprayed with the trichlorethen, aceton or the like. Then the surface is sandblasted in a first process step with alumina (AI2O3) , for instance "Metcolite" with grain size of 0,2-0,8um. The process parameters used in this step may for instance be: Blasting pressure 3-6 bar Nozzle diameter 4-8 mm Distance 100-200 mm Blasting angle 60-90° Surface roughness 20μm (mean) The sandblasted surface must immediately thereafter be cleaned of dust by blasting with completely dry and greasefree compressed air or nitrogen gas. In the next process step which takes place at the most 1 to 2 hours after the sandblasting, the surface is coated by a bonding layer by means of plasma spraying. The objective of the bonding layer is to function as a carrier for the ceramic layer which constitutes the dielectric coating. The bonding layer is preferably made of copper powder of the type PT2901, Metco 56 or the like. The cupper powder are sprayed in a plasma process with the use of plasma gun of the type that is shown schematically in Fig. 2. In this process step the substrate, i.e. the work piece, or the pistol is moved with a velocity of 1 m/s. At the same time the plasma spray gun are continuously moved in a direction at right angle to the substrate with a speed of 5 mm pro pass. The bonding layer are applied to a thickness of 0,05-0,15 mm, the applied thickness in each pass being 15 μ. The parameters used in this process step are: Current strength 300-500 A Voltage 50-65 V Distance (approximate) 150 mm Argon gas feed 50-70 1/min Hydrogen gas feed 6 1/min Powder feed 50 g/min A dielectric layer of ceramic material is now applied in a consecutive process step immediately after the application of the bonding layer. The cermaic layer consists of an alumina alternatively mixed with 25% zirconia. The grain size is typically 10-110 urn, for instance by use of Metco 105 SFP. If the ceramic layer are spray coated with a metal layer, a composition of 25% by weight zirconia (Zrθ2) is used. The parameters used in this process step are: Current strength 550-1000 A Voltage 50-75 V Distance 90-110 mm Argon gas feed 30-40 1/min Hydrogen gas feed 50-20 1/min Powder feed 20-1100 g/min Passage thickness 2-15 μm Layer thickness 0,3 mm During the application of the ceramic layer the temperature of the substrate must be between 50-150°C. This is easily achieved by the substrate being force-cooled with air. In the following process step the ceramic layer is impregnated in order to achieve the desired dielectrical properties. As the substrate temperature is below 50°C, but above the room temperature silicone oil, preferably of the type Baysilone 100 is applied with a suitable tool as a brush, spray gun etc. , to form a visible, shiny film which can be seen all over the surface. On the application the silicon oil is absorbed into the ceramic layer, while excess silicon oil are removed from the surface for instance by use of a moisture absorbent, porous paper. This paper is pressed two to five times against the surface, so that in the end there are no visible silicon oil to be found. In a subsequent process step the dieletctric layer now is coated with a conducting pattern or a current lead pattern of copper and this must take place withn one hour after the plasma spraying of the ceramic material. A desired conducting pattern is achieved by plasma spraying a copper powder through a template of stainless steel in the form of a sheet of 2 to 3 mm thickness, wherein the desired conducting pattern beforehand has been cut by means of a laser. It is to be understood that the conducting pattern is determined by the electrical function to be realized by use of a substrate. The distance between the template and the ceramic layer must be between 0,5 and 1 mm. The substrate with the ceramic coating are preheated by means of plasma to about 50°C by using the same plasma gun which was used for spraying. The same copper powder is used for the conducting pattern, i»e. the electrical conducting layer, as for the bonding layer, i.e. PT 2901, Metco 56 or the like. The parameters for the spraying velocity and the vertical feed are also the same as for the binding layer. The parameters used in this process step are: Current strength 300-500 A Voltage 50-65 V Distance 150 mm Argon gas feed 50-70 1/min Hydrogen gas feed 5 1/min Powder feed 50 g/min Passage thickness 0,05-0,15 mm Layer thickness 0, 1-1 mm The copper layer may be sprayed to the desired dimension or with an 0,1 mm excess. In the last case the layer are smoothed or made plane by grinding, milling or comparable processes. In the final process step the substrate now shall be cleaned, as possible copper dust is removed. This is done by blasting with small glas spheres in a separate chamber in order to avoid silicon pollution of the conducting pattern. Care must be taken in this process, where the parameters used are: Pressure 2-5 bar Nozzle diameter 1-6 mm Distance abt. 150 mm Sphere diameter 50-100 μm By the method disclosed in the above example a substrate that is very well suited for use in the production of for instance hybride modules for power electronics is provided, but which also may be employed generally in electronics where substrates of high mechanical strength and excellent thermal and di¬ electric properties are desired. Further such substrates may be used in non-traditional applications in eletrical engineering and in that connection possibly included as an integrated part of a more comprehensive electronic and mechanical equipment. The metallic base may then for instance be a part of the construction itself and coated with the dielectric layer by means of plasma spraying in situ for later placement of electrical or electronic components. A substrate manufactured as specified above is very well suited for use in an automatic production process for the manufacturing of miniaturized hybrid circuit modules in the form of ready made packages. The achieved reduction of costs in this connection amounts to about 20% relative to traditional mounting methods and results by the manufacturing of a circuit package in a volume reduction of up to 70%. The manufacturing method is also well suited in the production of custom specified circuits and offers good possibilities for a possible optimization of the circuit module, while the specific demands for mechanical and thermal properties may be achieved. It has thus been shown that the method is well suited to small production volumes with a frequent change of the circuit pattern, as the initial costs are low. The great economical and process engineering advantages are first and foremost achieved by the coating being based on the use of a thermal coating process, e.g. plasma spraying or jet coating. Besides it is well known to use thermal coating processes in the mechanical industry for instance for coating metals with wear resistant coatings of ceramic material. By integrating the ceramic coating with a base of metallic material good heat conducting properties are achieved and when the thermal coating process also is used for applying the electrical conducting coating in the form of predetermined conducting patterns, a substrate with excellent thermal conducting properties results. For the coating there may as stated be used different thermal processes. It has shown to be particularly advantageous to use plasma spraying for applying the ceramic coating. It is then formed a ceramic coating with 5% pore volume and 5% oxide (volume) . It is however desired that the conducting coating which cannot be impregnated, has greater density and lower oxide content. This is advantageously achieved by the con¬ ducting layer being applied by means of jet coating, for instance of the type Metco "Diamondjet" which produces a conducting layer with only 2% pore volume and 2% oxide volume. Moreover it will be obvious that by jet coating, where high speed jet combustion gases from the combustion of for instance propane are used, the parameters of current strength, voltage and gas feed given in the relevant process example do not apply. Although a substrate made according to the above-mentioned example only can be used for one sided mounting of components, there has been shown that it has a lower cost per area unit than conventional double-sided circuit boards of epoxy resin, when the price of the metal base used is exempted, as the metal base initially possibly could be part of a more comprehensive structure where the electrical or the electronic components are to be used. By means of the method according to the invention there are thus provided a substrate with a thermal resistance of 0,6°C/W (computed) , when a 10 mm base of aluminiumsheet is used, a volume resistivity of 25.10 12 ohm/m 2 with a thickness of the ceramic layer of 0,3 mm, a dielectric strength of 3000 V for the same thickness of the ceramic layer and an electrical conductivity of the plasma sprayed copper layer of 40 to 50% to that of pure copper, but which is 1000 times greater than the conductivity of a thickfilm paste. The use of a thermal process in the application of the ceramic coating has shown to be very well compatible with different base materials and when a thermal coating process also are used for applying conducting patterns, it may easily be integrated with various technologies and methods for the mounting of components. It must be remarked that even though the method of manufacturing the dielectric layer in the form of ceramic coating according to the invention is used in connection with substrates for placement of electrical and/or electronic components, persons skilled in the art may easily realize that such coatings also may be used in quite different technical and industrial contexts.

Claims: CLAIMS 1. A method for manufacturing of a substrate for placement of electrical and/or electronical components, the substrate at least comprising a base preferably formed of a metalic material which possible may constitute a part of or being integrated in a more comprehensive structure, a dielectric layer and an electrical conducting layer which forms a predetermined pattern, characterized in that the method comprises the following successive steps: a) sandblasting the surface of the base with grains of AI2O3 (alumina) , b) coating the sandblasted surface of the base with a bonding layer by means of thermal coating process, c) coating the bonding layer with a dielectric layer by means of a thermal coating process, the dielectric layer being formed of a ceramic material, d) impregnating the dielectric layer formed of ceramic material with silicone oil, e) coating the dielectric layer with the electrical conducting layer by means of a thermal coating process, a template being used to form the predetermined pattern, and f) cleaning the surface of the electrical layer and the electrical conducting layer by blasting with spheres of glass. 2. A method according to claim 1, characterized in that the thermal coating process is plasma spraying. 3. A method according to claim 1, characterized in that the thermal coating process is jet coating. 4. A method according to one of the preceeding claims, characterized in that plasma spraying are used in step c) and jet coating in step d) . 5. A method according to claim 1, characterized in that the impregnation in step d) takes place by applying silicone oil on the surface of the dielectric layer, so that there on the surface a visible film of silicone oil is formed, whereafter the silicone oil immediately after the application is removed from the surface such that the visible film of silicone oil no longer is present. 6. A method according to claim 1, characterized in that the coating of the bonding layer in step b) takes place at most one to two hours after step a) . 7. A method according to claim 1-4 or 6, characterized in that the bonding layer are formed of a copper powder. 8. A method according to claim 1-4, characterized in that the dielectric layer are formed of alumina powder (AI2O3 powder) . 9. A method according to claim 1-4, characterized in that the dielectric layer is formed of a composition of powder of alumina (AI2O3) and zirkonia Zrθ2) , preferably with 25%-by-weight zirconia. 10. A method according to any of the claims 1, 8 or 9, characterized in that the dielectric layer preceding step e) is preheated to about 50°C, preferably by means of plasma.

Inventors: HESTEVIK SVEIN, STORFOSSENE TORE, HESTEVIK, SVEIN, STORFOSSENE, TORE
Applicants: HESTEVIK SVEIN, STORFOSSENE TORE, HESTEVIK, SVEIN, STORFOSSENE, TORE
Application year: 1990
Main classification: H05K1
IPC codes: C04B37, C04B41, C23C4, H01L23, H05K1, H05K3