Abstract:
A method of forming a plurality of holes of fine diameter d in a brittle material at fine intervals α includes the steps of forming at least two holes having a fine diameter d at intervals A in the brittle material through the use of punches. The intervals A are at least two times the fine interval α. Holes are then subsequently punched between the previously punched holes at intervals A without removing the die punches used to form the previous holes. The distances between the previously punched holes and the subsequently punched holes measure at least the distance corresponding to the fine interval α.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for punching a plurality of holes at fine pitches on brittle material such as compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates, or functional particles dispersed organic films without causing any cracks, and a die to be used therefor. 
     2. Description of Related Art 
     Conventionally, brittle materials such as compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates or functional particles dispersed in organic films are often used for fabricating circuit substrates, oscillating plates, or resilient plates. 
     For such brittle materials, a plurality of fine holes is formed at fine pitches according to the required configuration of target final products. 
     FIG. 4 is a cross-sectional view showing an example of the conventional punching die, wherein a die  30  comprises a plurality of punches  31 , a punch plate  32 , a die  34  having a plurality of fine holes  33 , and a stripper  35 . The reference numeral  36  denotes a cover portion and the numeral  37  denotes a brittle material such as ceramic green sheets. 
     However, since the demand for denser circuit boards of recent years requires lessening of the pitch of a plurality of fine holes, there is an increasing tendency to generate cracks between punched holes when the conventional die shown in FIG. 4 is used. In the case of ceramic green sheets, depending on the thickness or quality thereof, cracks occur with growing frequency during formation of a plurality of fine holes with pitches of 500 μm or less. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for forming a plurality of holes of fine diameters at fine pitches on a brittle material without generating any cracks thereon. 
     After an elaborate study and investigation to avoid occurrence of cracks during formation of fine holes, the inventor found that the problem described above may be overcome by punching the plurality of fine holes with a time shift, rather than all at one time, and thus attained the present invention. 
     According to the invention, the method for punching a plurality of holes having fine diameters d at fine intervals α on a brittle material comprising the steps of forming at least two holes including a hole of fine diameter d at intervals A, each of which is at least two times the fine interval α, by the use of a punching die, and punching holes between the previously punched holes at intervals A so that the distance from the punched holes of diameter d measures at least the distance corresponding to the fine interval α, without removing the punches used to form the holes in the previous step. 
     Brittle material in the present invention includes compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates, or functional particles dispersed organic films. The most preferable material is a ceramic green sheet. 
     The fine intervals α are preferably in the range between 30 and 500 μm, and more preferably between 30 and 300 μm. The fine diameter d for the hole is preferably in the range between 20 and 480 μm, and more preferably between 50 and 200 μm. 
     According to the present invention, the punching die comprising a plurality of punches having punching tips, a punch plate for holding the punch, a die having a plurality of fine holes of a prescribed diameter through which the punching tips are inserted and removed, and a stripper for grasping the tips of the punches and for peeling the punched brittle material off the punch when removing the punch from the die, wherein a plurality of punches consists of two groups of punches, so that the position of the apexes of the punching tips of one group differs from that of the other group. 
     While the die punch used in the invention is preferably formed in such a manner that the shortest apex-to-apex distance D out of the apex-to-apex distances between at least two groups is larger than the thickness of brittle material t, it is also possible to form so that the relation between the apex-to-apex distance D and the thickness brittle material t satisfies the relation; (2/3)×t&lt;D. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing an example of the punching die according to the present invention; 
     FIG. 2 is a cross sectional view showing another example of the punching die according to the present invention; 
     FIG. 3 is a cross sectional view showing still another example of the punching die according to the present invention; 
     FIG. 4 is a cross sectional view showing an example of the conventional punching die; and 
     FIG. 5 is an explanatory drawing illustrating the relation among the fine interval α, the fine diameter d, and the interval A for brittle material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is now described in detail. 
     According to the invention, as shown in FIG. 5, when a plurality of holes  2  of fine diameter d are punched on brittle material  1  such as a ceramic green sheet at fine intervals (pitches) α, the punching operation is initiated by the step of punching a plurality of holes including a hole  2  of fine diameter d at intervals A, each of which is at least two times the fine interval α (two times of α in FIG.  5 ), by the use of a die punch. 
     Then holes are punched between the previously punched holes at intervals A so that distances from the punched holes of fine diameter d measure at least the distance corresponding to the interval α before the die punch that has already started punching completes its punching operation or before such a die punch is removed. 
     Since, in the punching method of the present invention, a plurality of holes are formed first at prescribed intervals A, and then a plurality of holes are formed between the previously punched holes at intervals A so that the distances from the punched holes measure the distance corresponding to the fine interval α before removing the die punch, cracks that tend to occur when all the holes with fine intervals α are punched at the same time can be prevented. 
     Though ceramic green sheets are preferably used as brittle material to be punched according to the invention, compound sheets formed by particles and high polymer, glass epoxy substrates, or organic films dispersed with functional particles are also applicable. 
     Though the fine intervals α described above may vary depending on the type of brittle material, in case where ceramic green sheets are used, the intervals α are preferably in the range between 30 and 500 μm, and more preferably between 30 and 300 μm. The fine diameter d of the holes may vary depending on the type of brittle material as well, in the case where ceramic green sheets are used, the fine diameter d is preferably in the range between 20 and 480 μm, and more preferably in the range between 30 and 200 μm. The fine diameters d for the plurality of holes does not have to be necessarily identical. 
     The die to be used for the punching method of the invention will now be described in detail according to attached figures. 
     FIG. 1 is a cross-sectional view showing an example of the punching die according to the invention. The die  10  comprises a plurality of punches  11 , a punch plate  12 , a die  14  including a plurality of fine holes  13 , and a stripper  15 . 
     The plurality of punches  11  respectively comprise a punching tip portion  16 , a columnar portion  17  and a base portion  18 . The columnar portions  17  of the punches are respectively inserted into a plurality of holes formed on the punch plate  12 , and a cover portion  20  is placed on the base portions  18  so that the cover portion  20  prevents the base portions  18  that are larger than the columnar portions  17  in diameter from being removed. 
     The die  14  comprises a plurality of fine holes  13  of prescribed diameter for insertion and extraction (insertion and removal) of the punching tips  16  of the punches  11 , and brittle material to be punched  21  is placed on the die  14 . 
     The stripper disposed between the punch plate  12  and the die  14  serves to position the punches  11  by gripping the punching tips  16  of the punches  11  and to peel brittle material  21  off the punches  11  so that brittle material  21  to be punched is prevented from being adhered to punches  11  and from being moved together with the punches  11 . 
     The reference numeral  22  denotes a stopper disposed on the die  14  on the peripheral side of brittle material  21  to prevent brittle material  21  from being crushed by the stripper  15 . Without stopper  22 , the stripper  15  may crush brittle material  21 , thereby deforming brittle material  21  and thus resulting in the reduced positional accuracy of the holes after being punched. 
     In the die  10  of the invention, as shown in FIG. 1, a plurality of punches  11  consists of two groups of punches that are different in the position of the apexes of the punching tips  16  from each other. 
     By constituting a plurality of punches  11  of two groups that are different in the portion of the apexes of the punching tips  16  from each other as described above, when a plurality of fine holes are punched on brittle material  21 , those fine holes are punched with a time shift, but not all at once. 
     In the first embodiment, two groups of punches; one group  11   a  having the apexes of the punching tips  16  at the higher position and the other group  11   b  having the apexes of punching tips  16  at the lower position are shown. In FIG. 1, the height of the apexes is differentiated not by changing the length of the columnar portion  17 , but by changing the length of the punching tips  16 . 
     In this case, the apex-to-apex distance D between the apexes of punches belonging to the group  11   a  and the apexes of punches belonging to the group  11   b  is preferably larger than the thickness t of brittle material, but the relation satisfying (2/3)×t&lt;D is also acceptable. 
     FIG. 2 is a cross sectional view showing another example of the punching die according to the invention, showing three groups of punches; a group of punches  11 A of which the apexes of the punching tips  16  are in the highest position, a group of punches  11 B of which the apexes of the punching tips  16  are in the middle position, and a group of punches  11 C of which the apexes of the punching tips  16  are in the lowest position. 
     In such a case, it is preferable that the apex-to-apex distance D 1  between the apexes of the punches that belong to the group  11 A and those belong to the group  11 B, and the apex-to-apex distance D 2  between the apexes of the punches that belong to the group  11 B and those belong to the group  11 C are respectively larger than the thickness t of brittle material, but the relation satisfying (2/3)×t&lt;D 1  (and D 2 ) is also acceptable. 
     Though the three groups of punches in the embodiment shown in FIG. 2 are arranged in the order of decreasing height of the apex, that is, of group  11 A, group  11 B, and group  11 C consecutively from the left to right, they may be arranged in the order of, for example, group  11 A, group  11 C, and group  11 B from the left to right. 
     FIG. 3 is a cross sectional view showing still another example of the punching die according to the invention, in which the height of the apexes are differentiated by changing the length of the columnar portions  17 , not by changing the length of the punching tips  16 . 
     Embodiments 
     A ceramic green sheet of 0.2 mm in thickness as brittle material was punched by the use of the punching die shown in FIG.  1 . When punching operation was carried out under the conditions where the interval A is 400 μm, the fine interval (pitch) α is 200 μm, and the fine diameter d is 150 μm, for both cases where the apex-to-apex distance D, that is, the distance between the apexes of the punches belonging to the group  11   a  and the apexes of the punches belonging to the group  11   b  is 250 μm, and where it is 150 μm, occurrence of cracks could be prevented. 
     Though the invention has been described so far according to the embodiments shown in the figures, it is obvious that the invention is not limited to these embodiments. 
     As is described thus far, according to the punching method and the die of the invention, when a plurality of fine holes are punched at fine pitches, occurrence of cracks can be prevented by punching a plurality of fine holes with a time shift, rather than by punching all the holes at the same time.