Method of making a ciccuit board

Abstract: A bonded ceramic-metal composite substrate oomprising a ceramic substrate (1) having opposite surfaces and a copper sheet (2) having a face directly bonded to one of the surfaces of the ceramic substrate, wherein the median surface roughness (R a ) of the outer surface of the copper sheet is not greater than 3 µm, and the maximum surface roughness (R max ) of the outer surface of the copper sheet is not greater than 18 µm. The invention improves the manufacturing reliability of various electronic devices such as semiconductor modules.

Description: This invention is concerned with a method of making a circuit board comprising a bonded ceramic metal composite substrate having a copper sheet directly bonded to a ceramic substrate. In recent years attempts have been made to develop ceramic-metal composite substrates for use in electronic devices. For example, a capacitor element bonded to a ceramic aluminum nitride substrate is disclosed in Scace et al., U.S. Patent No. 3,716 759. Direct bonding of metal sheets to ceramic substrates is also described in Hill, British Patent No. GB 2,099,742 and Jochym, U.S. Patent No. 4,409,278, whereby channels are provided in the metal sheet or ceramic substrate in order to avoid blistering by permitting gas escape during the bonding. Some of these attempts have involved disposing a metal sheet such as copper on a ceramic substrate and directly bonding the copper sheet to the substrate by heating to a temperature below the melting point of copper (1083 °C) and above the eutectic point of copper and oxygen (1065 °C). This kind of composite has several advantages. It provides a strong adhesion between the metal sheet and the ceramic substrate. It also provides a simpler structure which can be easily utilized to obtain a smaller circuit board. And it provides a potentially simpler method for producing a circuit board. However in attempting to use ceramic copper substrates prepared in the foregoing manner in circuit boards for transistor modules, the present inventors found that some of them failed to work correctly during their operation. Thus, there is a need for bonded ceramic metal substrates which will produce more reliable electronic devices. From "Galvanotechnik, vol. 76, no. 8, August 1985, pages 1063-1068" printed circuit boards are known which comprise copper members having an outer surface which has a maximum surface roughness in the range of 2.5 µm to 4.0 µm and an average roughness in the range of 0.4 µm to 0.7 µm. This invention was devised having regard to the foregoing problem. It is an object of the invention to provide a method of making a circuit board with a bonded ceramic metal composite substrate having a copper sheet directly bonded to a ceramic substrate. This invention is directed to a method as defined in claim 1. The method comprises the steps of producing a bonded ceramic-metal composite substrate by applying a copper member to a desired position on the surface of a ceramic substrate; heating the resulting assembly to a temperature below the melting point of copper but above the eutectic temperature of copper and oxygen to directly bond the copper member to the substrate, and polishing the surface of said copper member, said copper member having a free face comprising at least one mounting area and at least one electrode area, wherein after said polishing step, the median surface roughness (R a ) of said at least one mounting area is not greater than 3 µm, and the maximum surface roughness (R max ) of said at least one mounting area is not greater than 18 µm; mounting at least one electrical element on said at least one mounting area, and electrically connecting a bonding wire to said at least one electrical element and said at least one electrode area. It is preferred that the median surface roughness (R a ) of the copper circuit sheet be not greater than 1 µm, and that the maximum surface roughness (R max ) be not greater than 8 µm. Median surface roughness (R a ) and maximum surface roughness (R max ) are defined in Japanese Industrial Standard JIS B 0601. Median surface roughness (R a ) is determined by plotting a curve of measured roughness values, plotting an average line along a segment of the roughness curve where the sum of the squares deviation between the roughness curve and the average line is at a minimum, plotting a median line along the roughness curve parallel to the average line such that the median line divides the area between the roughness curve and the average line in halves, and extracting from the roughness curve a segment of length "l" along the median line. Median surface roughness (R a ) is then calculated according to the formula: wherein the median line defines the X axis of a coordinate system, the Y axis is defined by a perpendicular to the median line, and f(x) is the roughness curve. Maximum surface roughness (R max ) is determined by plotting a curve of measured roughness values, plotting an average line along a segment of the roughness curve where the sum of the squares deviation between the roughness curve and the average line is at a minimum, and enclosing the roughness curve between two lines parallel to the average line. The maximum surface roughness is then obtained by measuring the vertical distance between the two parallel lines. In order that the invention may be illustrated and readily carried into effect, a non-limiting embodiment thereof will now be described with reference to the accompanying drawing in which: Fig. 1 is a sectional view of a circuit board made according to the present invention showing how a semiconductor element is mounted on the copper sheet. Fig. 2 is a graph showing the relationships between the median roughness (R a ) of the copper sheet and the grain size of the copper and solder wettability. There are several steps to obtain a circuit board. As seen in Fig. 1, an electronic part 3 is mounted by soldering to a mounting area 2a of a copper sheet 2 which is bonded to a ceramic sheet 1. This electronic part 3 is electrically connected through an aluminum wire 4 etc. to a separate terminal electrode area 2b of the copper sheet. Finally, such a circuit board is usually covered with resin. Many factors were considered as the cause why circuit boards with ceramic-metal substrates develop defects during operation or do not work at all. The present inventors discovered what the cause was. Failure of the electronic components due to overheating was found to cause this problem. The present inventors discovered that gaps were produced between the copper circuit sheet and the electronic components, causing a lowering of heat transmissivity. This led to the serious problem of failure of the electronic components due to overheating. As a result of more experiments, the present inventors discovered that wettability with respect to solder of the copper circuit sheet caused the gaps between the copper circuit sheet and the electronic components. The present inventors also discovered that the etching treatments after heat-bonding the pattern of the copper circuit sheet are particularly subject to a lowering of solder wettability. A method of making a circuit board according to this invention is as follows. Specifically, first of all, a circuit pattern is formed by arranging a copper sheet worked to the shape of the required circuit or a copper sheet in the form of a flat sheet in contact with the ceramic substrate, bonding by heating to a temperature below the melting point of copper (1083°C) but above the eutectic temperature of copper and oxygen (1065°C), and carrying out etching of the copper sheet as required. If a copper sheet containing oxygen is used as the copper circuit sheet, the atmosphere during heat bonding is preferably an inert gas atmosphere. If a copper sheet not containing oxygen is used, an atmosphere containing 80 ppm to 3900 ppm of oxygen is preferable. In order to assure that the surface roughness of the copper circuit sheet will be within the aforementioned range, for example, copper members whose surface roughness is within the aforesaid range are used. Also, grain growth of the copper occurs due to the heat treatment when the copper circuit sheet is bonded the ceramic substrate. As shown for example by the graph of Fig. 2, the increase in surface roughness of the copper circuit sheet is substantially proportional to the increase in the grain size of the copper. Therefore, copper members of appropriate grain size are used, and the heat treatment conditions and cooling conditions are carefully controlled. By this means, the grain size can be adjusted, and a copper circuit sheet having the desired degree of surface roughness is obtained. Also, although a copper circuit sheet having the desired surface roughness can be obtained by appropriate selection of the copper member used and by control of the heat treatment conditions as described above, it is also possible to control the surface roughness of the copper circuit sheet by chemical polishing of the surface after heat bonding, or, if the circuit pattern is formed by etching treatment, after this treatment. In particular, when the circuit pattern is formed by an etching treatment, fine particles of photoresist are very likely to be left behind at the grain boundaries of the copper. This lowers the solder wettability. It is therefore beneficial to carry out chemical polishing to remove the surface layer to a thickness of, for example, 0.1 µm to 20 µm. It is also possible to achieve the desired surface roughness by mechanical polishing of the surface, instead of chemical polishing, but it is difficult to remove particles of resist etc. present at the grain boundaries of the copper by mechanical polishing, so chemical polishing is preferable in such cases. Such chemical polishing may be performed, for example, by immersion for about 1 to 5 minutes in a polishing liquid consisting of a mixture of sulfuric acid and hydrogen peroxide. Also, it is desirable that the copper should be rolled so that the oxygen constituting the adhesive at least one to the bonding face is contained in a proportion of 100 ppm to 3000 ppm with respect to the copper member that is used. A copper circuit sheet thickness in the range from 0.25 mm to 0.6 mm is suitable. Various types of ceramic substrates can be used, for example ceramic sintered bodies of the oxide type, such as alumina or beryllia, or ceramic sintered bodies of non-oxide type, such as aluminum nitride, silicon nitride, titanium nitride, and silicon carbide. When ceramic substrates of the non-oxide type are used, they are preferably employed after prior oxidizing treatment of the bonding surface. Further details of the invention will become apparent from a consideration of the following examples. Examples 1 to 3 Copper circuit sheets were initially obtained by preparing copper members having an oxygen content of 300 ppm and the grain sizes and surface roughnesses shown in the following table, and working these into the prescribed circuit shape. Next, the ceramic circuit substrates were obtained by placing these various copper circuit sheets in contact with both faces of ceramic substrates of which the main constituent is alumina, (containing 96 weight % alumina, and 4 weight % sintering adjuvant components), then heat treating in a nitrogen gas atmosphere under the respective heating conditions shown in the following table, to effect bonding between the respective ceramic substrates and copper circuit sheets. The surface roughnesses of the copper circuit sheets on the various ceramic circuit substrates obtained in this way were as shown in the following table. Next solder was melted by placing solder sheets (10 mm X 10 mm X 0.1 mm) consisting of Sn:Pb = 63:37 on the copper circuit sheets of the various ceramic circuit substrates, and heating to about 180 °C. The solder wettability was then evaluated from the ratio of the bonding area of the solder and the original size of the solder sheet. These results are also shown in the following Table. In the comparative examples shown in the Table, the solder wettability was evaluated in the same way as in the examples produced using a ceramic circuit substrate manufactured with a copper circuit sheet having a different surface roughness of the copper circuit sheet, but the other conditions were the same as in the examples made according to the invention. As is clear from the results listed in the following Table, in all of the embodiments in which the surface roughness of the copper circuit sheet was kept within the range of this invention, solder wettability such as to give no practical problems was obtained. In contrast thereto, in the case of the ceramic circuit substrates of the comparative examples, the solder wettability was poor. If electronic components, such as semiconductor elements, were then directly mounted, solder cavities were formed, thereby increasing the thermal resistance and risking failure of the electronic components. Fig.2 is a graph showing the relationship between the median roughness (R a ) of the copper circuit sheet in the above embodiments and in the comparative examples with solder wettability. As is clear from this graph, by making the median roughness (Ra) not greater than 3 µm, solder wettability such as to give rise to no practical problems is obtained. In particular, if the median roughness (R a ) is made not greater than 1 µm, much better solder wettability is obtained, and the solder wettability is stable, approaching a constant value. Example 4 A flat copper sheet having a thickness of 0.3 mm and an oxygen content of 300 ppm was placed on top of a ceramic substrate as described in Example 1. Bonding was effected by heating for 30 minutes at a temperature of 1070 °C under a nitrogen gas atmosphere. Next, a resist to produce the required circuit pattern was printed onto the bonded copper sheet, and the desired circuit pattern was formed by means of an etching treatment using ferric chloride solution. After this chemical polishing was performed (amount removed = about 5 µm) of the surface by immersing for about 1 minute in a mixed aqueous solution of sulphuric acid and hydrogen peroxide (H 2 SO 4 = 5 to 10 %; H 2 O 2 = 3 % approximately). The median surface roughness (R a ) of the resulting copper circuit sheet was 1.9 µm, and the maximum surface roughness (R max ) was 8 µm. Next, this copper circuit sheet was nickel plated to a thickness from about 0.5 µm to 2.0 µm, and the solder wetability was then measured under the same conditions as in Example 1. A value of 98 % was obtained. For comparison, a ceramic circuit substrate was manufactured under the same conditions as Example 4, except that chemical polishing with a mixed aqueous solution of sulphuric acid and hydrogen peroxide solution was omitted. when the solder wettability was determined in the same manner as in Example 4, a value of only 90% was obtained. Also, when an electron microscope was used to observe the surface of the copper circuit sheet before bonding of the solder, it was found that fine particles of resist had been left behind at the grain boundaries. As described above, with the method of the invention, the surface roughness of the copper circuit sheet is controlled so that excellent solder wettability is obtained. Consequently, reliability in manufacture of various electronic devices, such as semiconductor modules, can be improved. The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the scope of the invention should be limited solely with reference to the appended claims.

Claims: A method of making a circuit board comprising a ceramic composite substrate, the method comprising: (a) applying one face of a copper member having opposite faces and a thickness of 0.25mm-0.6mm to one surface of a ceramic substrate having opposite surfaces to form a pre-bonded assembly, (b) heating said assembly to a temperature above 1065°C but below 1083°C thereby forming a copper-oxygen eutectic direct bond between said member and substrate, the copper member including oxygen in which case the heat bonding is effected in an inert gas atmosphere, otherwise the bonding is effected in an atmosphere containing oxygen, and characterised in that (c) the copper member has an initial, pre-bonded surface roughness such that, if necessary when adjusted by polishing after heat bonding,the median surface roughness (R a ) of the heat-bonded copper member is not greater than 3µm, and its maximum surface roughness (R max ) is no greater than 18µm, and (d) said copper member has a free surface comprising at least one mounting area and at least one electrode area and in that the method further comprises mounting at least one electrical element on said at least one mounting area by soldering and electrically connecting a bonding wire to said at least one electrical element and said at least one electrode area. A method as claimed in claim 1 wherein the maximum crystal grain size of the heat-bonded copper member is 400µm. A method as claimed in claim 2, wherein said maximum crystal grain size is 300µm. A method as claimed in claim 3 wherein said maximum crystal grain size is 200µm. A method as claimed in any preceding claim, wherein the median surface roughness (R a ) of the heat bonded copper member is not greater than 1µm and its maximum surface roughness (R max ) is not greater than 8µm. A method as claimed in any preceding claim, wherein the copper member has an initial, pre-bonded oxygen content of 100 to 3000 ppm. A method as claimed in claim 6 wherein the initial pre-bonded median surface roughness (R a ) is not greater than 0.3µm, and its initial pre-bonded maximum surface roughness (R max ) is not greater than 2µm. A method as claimed in any preceding claim wherein the copper member is chemically polished after heat bonding using a mixture of sulphuric acid and hydrogen peroxide to achieve said values of surface roughness.

Inventors: KOMORITA HIROSHI, MATSUMURA KAZUO, MIZUNOYA NOBUYUKI, TANAKA TADASHI, KOMORITA, HIROSHI, MATSUMURA, KAZUO, MIZUNOYA, NOBUYUKI, TANAKA, TADASHI, KOMORITA, HIROSHI INTELLECTUAL PROPERTY DIVISION, MATSUMURA, KAZUO INTELLECTUAL PROPERTY DIVISION, MIZUNOYA, NOBUYUKI INTELLECTUAL PROPERTY DIVISION, TANAKA, TADASHI INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H05K3
IPC codes: H05K1, H01L23, H01B5, H01L21, H05K3