Method of fabricating plastic mask for paste printing, plastic mask for paste printing, and paste printing method

Abstract: A method of fabricating a plastic mask for paste printing with a plastic sheet being irradiated with an excimer laser beam for forming at least one penetrating opening containing area including at least one through-hole in the plastic sheet is carried out by the steps of bringing a polymer film capable of absorbing at least part of the excimer laser beam into close contact with a back surface of the plastic sheet which is opposite to an excimer laser beam irradiation side thereof which is to be irradiated with the excimer laser beam; forming at least one penetrating opening containing area including at least one through-hole in the plastic sheet by irradiating the excimer laser beam irradiation side thereof with the excimer laser beam; and removing the polymer film from the plastic sheet after the formation of the penetrating opening containing area. By use of a plastic mask fabricated by the above method, a paste printing is also carried out.

Description: The present invention relates to a method of fabricating a plastic mask for paste printing for forming patterns by use of an ink, an adhesive agent, a solder paste, or a paste-like resin (hereinafter referred to as the paste) on a printing material, more particularly to a method of fabricating a plastic mask for such paste printing, which is carried out by forming a pattern of through-holes in a plastic sheet by excimer laser abrasion. For forming a printed pattern on a printing material with the above-mentioned paste by use of a printing mask, the following method is generally used: A printing mask with a pattern of through-holes is brought into close contact with a printing material, and a paste is spread on the surface of the printing mask by a squeegee so as to fill the through-holes with the paste. An excessive paste is then removed from the surface of the printing mask. The printing mask is then separated from the printing material in such a manner that the paste passes through the through-holes and remains in the same pattern as that of the through-holes formed in the printing mask on the printing material, whereby a paste pattern corresponding to the pattern of the through-holes is printed on the surface of the printing material. Such printing masks for use in practice can be roughly classified into screen masks and metal masks. As metal masks, there are known, for example, punch press mask, YAG laser mask, etching mask, additive mask and half-etching mask. The punch press mask was developed as a solder paste printing mask for fabricating electronic parts. The punch press mask can be prepared by forming round through-holes in a metal sheet with a punch. This printing mask can be easily and quickly prepared by utilizing NC data of a mounter used in a mounting process with high through-hole making speed. This printing mask has the advantages over other printing masks that the production method is simple and therefore the printing mask can be produced at extremely low cost. Furthermore, the apparatus for producing the printing mask is also inexpensive and small-sized and therefore can be brought into and used in a printing facility without difficulty. Despite such advantages, this printing mask has the shortcomings that upper edge portions of the through-holes tend to be rounded, and rough broken surfaces or burr tends to be formed in the lower edge portions of the through-holes, so that the paste filled in the through-holes cannot smoothly pass through the through-holes and therefore the printing quality is poor. A minimum diameter of the through-hole that can be formed is about 0.3 mm. For example, in the solder paste printing for fabricating electronic parts, a minimum pitch will be about 0.5 mm in QFP (quad flat package) at best. A YAG laser mask is prepared by fusing a metal sheet (usually SUS) with YAG laser and fused portions in the metal sheet are blown off by use of an assist gas to form through-holes in the metal sheet. In the case of the YAG laser mask, through-holes are made in accordance with the NC data or the like, so that the YAG laser mask has the advantage that the mask can be produced in a short time with a minimum dispersion in quality, but has the disadvantages that the inner walls of the through-holes are rough, and dross is deposited at the edge portions of the through-holes, so that the printing quality obtained by the YAG laser mask is slightly inferior to that obtained by other printing masks. Furthermore, convex portions are formed in the edge portions by the deposition of the dross thereon, and the thus formed convex portions tend to make imperfect the contact of the printing mask with a printing material during the printing process. If such imperfect contact takes place, it is necessary to remove the convex portions from the edge portions by grinding or polishing the surface of the printing mask. An etching mask is prepared as follows: A photo-sensitive resin is coated on both sides of a metal plate to form resist layers thereon. The resist layers are exposed to light through a photomask on which through-holes are depicted in a pattern (hereinafter referred to as the through-hole pattern depicted photomask) and are then developed by removing the portions corresponding to the through-hole pattern from the resist layers. This metal plate is then subjected to etching, whereby through-holes are made in the same pattern as the above-mentioned through-hole pattern in the metal plate. Thus, an etching mask for printing is prepared. In the thus prepared etching mask, etching is carried out on both sides of the metal plate for making the through-holes therein, so that a convex portion is formed at the central portion of the inner wall of each through-hole. Furthermore, the inner wall of the through-hole is rough, so that a paste cannot smoothly pass through each through-hole. The result is that the printing quality obtained by the thus prepared etching mask is poor. An additive mask is prepared as follows: A relatively thick photosensitive resin layer or dry film layer is provided on a plating substrate. The photo-sensitive resin layer is exposed to light through a through-hole pattern depicted photomask and developed by removing the portions in the photosensitive resin layer except the portions corresponding to the through-hole pattern. Thus, the photosensitive resin layer portions remain in the through-hole pattern on the plate substrate. A metal plate, for example, made of nickel, with an appropriate thickness, is formed by plating around the photosensitive resin layer portions which remain in the through-hole pattern. The metal plate is then removed from the plating substrate, and the photo-sensitive resin layer portions are dissolved away from the metal plate, whereby a metal plate with through-holes which correspond to the removed photosensitive resin layer portions is prepared as a printing mask. The inner walls of the through-holes formed in the above metal plate are the smoothest than any of the inner walls of through-holes formed in metal masks prepared by any other methods and accordingly, the printing quality obtained by the metal plate is relatively good. However, occasionally, the inner walls of the through-holes are roughened by the over-exposure of the dry film, which may be referred to as "attack phenomenon". It is considered that the "attack phenomenon" is caused as follows. It is necessary that the photo-sensitive resin layer or dry film for the formation of the through-holes in the printing mask be sufficiently thicker than the additive mask. Occasionally, the thickness of the photosensitive resin layer may be 200 µm or more. Since this thickness is much greater than that of a photosensitive resin layer for the masking of an etching liquid, which is in the range of 20 to 30 µm, a sufficient amount of light must be applied so as to have the light reach the bottom portion of the photosensitive resin layer on the side of the substrate. For this reason, light with high intensity is applied to the uppermost surface of the photosensitive resin layer on the side of the photomask. When the above-mentioned light applied to the uppermost side of the photosensitive resin layer causes halation within the photosensitive resin layer, the portions which should be shaded by the photomask are exposed to the light scattered from the adjacent portions within the photosensitive resin layer. As a result, the inner walls of the through-holes are roughened as mentioned above. Furthermore, stepped portions are formed in the inner walls of the through-holes with the attachment of the dry film thereto while in use, whereby the printing quality is lowered. The plated surface of the printing metal plate which comes into contact with a printing material tends to be roughened. In particular, when finely-divided particles are contained in a paste for printing, such as in a solder paste, and such a paste spreads, the finely-divided particles in the paste enter the roughened portions at the plated surface of the mask and are fixed thereon, so that the roughness of the surface is further intensified. The result is that the close contact with the printing metal plate with the printing material is hindered, and the spreading of the paste is further increased. On the other hand, one of the most representative materials used as plating materials in the additive mask is nickel. Nickel is softer than stainless steel (usually SUS 304) which is frequently used as a material for an etching mask, so that the portions near minute through-holes are easily deformed by the convex portions in the roughened surface or by some foreign materials, whereby the printing quality tends to be easily degraded. As a countermeasure to avoid this problem, the hardness of the additive mask is increased, for example, with the addition of cobalt thereto at the time of plating. This countermeasure, however, tends to increase the roughness of the plated surface and to promote the above-mentioned fixing of the finely-divided particles in the paste, thereby increasing the spreading of the paste. A half-etching mask is used for printing, particularly a solder paste in print-circuit boards for electronic parts. In this field, it is required that a mixture of large and micro electronic parts be mounted in accordance with the recent tendency that electronic parts are small-sized. When solder paste printing is performed for micro electronic parts, it is required that the printing mask be made thin and that micro through-holes be formed in the printing mask and filled with the solder paste; while when solder paste printing is performed for relatively large parts, it is required that the printing mask be made thick and that relatively large through-holes be formed in the printing mask and filled with the solder paste. When solder paste printing is performed on a print-circuit board on which large parts and micro parts are to be mixedly mounted, by use of a thin printing mask for solder printing for micro parts, the solder paste for the large parts quickly runs out; while when solder paste printing is performed on the print-circuit board by use of a thick printing mask for solder printing for large parts, the solder paste does not smoothly pass through the micro through-holes for the micro parts, and even if the solder paste passes through the micro through-holes, excellent paste printing cannot be performed due to the presence of excessive solder paste. It is the half-etching mask that can solve the above-mentioned problems since it is partially made thin by etching and paste printing for large parts and that for micro parts can be performed by use of one and the same printing mask. The half-etching mask is prepared as follows: First of all, in accordance with the previously mentioned method for preparing the additive mask, there is prepared a printing mask with a necessary thickness for large parts, micro through-holes for micro parts, and relatively large through-holes for large parts. The micro through-holes within a half-etching area are filled with a resin for protecting inner walls of the micro through-holes. The area except the half etching area, for which etching is unnecessary, is masked by use of a resist agent, and then etching is performed until the thickness of the etched area reaches a suitable thickness for the micro parts, whereby a thin printing mask is prepared only in the area where micro through-holes for micro parts are formed. Such a half-etching mask has made it possible to perform solder paste printing for large, normal, and micro electronic parts by use of one and the same printing mask in the field of electronic part mounting. On the other hand, even if the micro through-holes are filled with the resin for protecting the inner walls thereof, the spreading of the etching liquid cannot be completely prevented, so that the smooth inner walls made by the additive method are roughened by the spread etching liquid. The result is that the half-etching mask tends to have the shortcomings that the micro through-holes for most precise printing are most roughened and accordingly the printing quality is lowered. Furthermore, the above-mentioned process for preparing the half-etching mask includes the step of carrying out the additive method, so that the previously mentioned shortcomings of the additive method may also appear in the above-mentioned process. Namely, the additive method has the shortcomings that the inner walls of the through-holes are roughened by the over-exposure of the dry film, or stepped portions are formed in the inner walls of the through-holes with the attachment of the dry film thereto while in use, whereby the printing quality is lowered. Furthermore, the above-mentioned metal masks, such as punch press mask, YAG laser mask and etching mask, lack flexibility, so that when a printing material has convex portions on the surface thereof, even if it is tried to bring such a printing mask into close contact with the printing material, there are formed gaps between the printing mask and the printing material. Therefore, when the through-holes in the printing mask are filled with a paste, the paste enters the gaps between the printing mask and the printing material, so that the printed paste pattern improperly spreads. Furthermore, when a paste containing solid particles is employed, the solid particles adhere to the back side of the mask, and during the repeated printing by use of the mask, the adhering solid particles make a gap between the printing mask and the printing material, so that the improper spreading of the printed paste pattern is further increased. Metal sheets (mainly a nickel sheet), such as the additive mask and the half-etching mask, which are prepared by plating, are flexible, so that when such a metal sheet is brought into close contact with a printing material, even if there are foreign materials or convex portions on the surface of the printing material, the mask can be easily deformed so as to be in close contact therewith. Therefore, when a portion near the through-holes of the mask is deformed and the mask is repeatedly used for printing, the paste enters behind the mask from the deformed portion thereof. The result is that the printed paste patterns spread and the printing quality is lowered. Furthermore, the thicker the plated layer, the rougher the surface of the plated layer. In particular, when the paste contains solid particles which can be easily deformed, such as solder particles, such solid particles enter between the mask and the printing material and are depressed therebetween. The depressed solid particles adhere to the rough surface of the mask. When such a printing mask is repeatedly used for printing, the adhering solid particles form gaps between the printing mask and the printing material. When the through-holes of the mask are filled with a paste, the paste enters the gaps between the printing mask and the printing material, so that the printed paste pattern spreads. When the adhering solid particles overflow out of the through-holes, the paste does not easily pass through the through-holes, so that the printing quality of the paste is degraded. In the case where a paste enters the gap between a printing mask and a printing material, and the paste or solid particles contained in the paste adhere to the back side of the printing mask, it is required to wipe off the paste which adheres to the back side of the printing mask or to clean the back side of the printing mask quite often, especially when the printing mask is used repeatedly. In accordance with the recent trend that electronic appliances and parts are made smaller and smaller in size, demands for highly precise paste printing of micro patterns with high quality free from non-printing portions or chips and spreading, are increasing in the field of the application of paste printing to electronic appliances and the like. In order to solve the previously mentioned various problems of the conventional metal masks and to perform precise paste printing with high quality, thereby meeting the above-mentioned demands, the inventor of the present invention has prepared a plastic mask for paste printing by forming through-holes in a predetermined pattern in a plastic sheet by excimer laser abrasion, and has conducted paste printing by use of the plastic mask prepared by excimer laser abrasion. As a result, it has been discovered that the previously mentioned various problems of the conventional metal masks can be solved by the plastic mask. Through-holes can be formed in a plastic sheet by excimer laser abrasion, so that the through-holes formed in the plastic sheet are free from burr in the edges and deposition of dross as in the through-holes in the conventional metal masks, and also free from non-clear-cut edges as in the through-holes formed in the conventional punch press mask. Furthermore, the inner walls of the through-holes formed in the plastic mask are extremely smooth and the formed through-holes themselves are precise and clean. In a conventional metal mask made of stainless steel (usually SUS304), the surface of the metal mask is subjected to hair-line finishing or crape finishing. Further, in the case where the metal mask is made of an electro-forming nickel, the surface of the metal mask is rough. In such a case, it may occur that fine particles, for example, solder particles, contained in a solder paste which is spread onto the back side of the mask, are depressed between the mask and a printing material, so that the depressed particles are fixed to the rough surface of the metal mask. This is apt to occur at the convex portions in the printing surface of the mask where the paste tends to spread. When this takes place, the contact of the mask with the printing material become imperfect, so that the spreading of the paste is increased. In contrast to this, in the case of the plastic mask, the surface thereof can be easily made extremely smooth, so that the fine particles contained in the paste are hardly fixed to the surface of the plastic mask. This significant feature of the plastic mask makes it possible to attain (a) close contact of the mask with a printing material, (b) significant reduction of the spreading of the paste, and (c) improvement of continuous printing performance. Further, by use of the plastic mask, a so-called "half-etching mask" can be easily prepared, in which a portion of the plastic sheet on the side with which a squeegee comes into contact is partially made thin, and a predetermined pattern of through-holes for micro parts is formed in the partially thinned portion. The plastic mask is softer and more flexible than the metal masks and therefore can flexibly follow up any convex and concave portions on a printing surface to attain close contact therewith without any gap or with a minimum gap between the mask and the printing surface. Furthermore, since the plastic mask is flexible, when it is peeled away from the printing surface, the plastic mask can immediately return to its original flat shape. In other words, in the plastic mask, there occurs no deformation as in a metal mask such as a nickel sheet formed by plating, when the mask is brought into close contact with a printing surface with convex and concave portions thereon. Therefore, by performing paste printing by use of the plastic mask, the through-holes in the plastic mask can be completely filled with the paste, and when the plastic mask is removed from the printing material, the filled paste can pass through each through-hole in a complete shape, and the formation of gaps between the plastic mask and the printing surface can be minimized, so that on the printing surface, there can be formed a paste pattern which accurately corresponds to the pattern of the through-holes formed in the mask and is free from any spreading of the paste, whereby highly precise, high quality paste printing can be performed. However, there are the following problems in the production of the plastic mask and also in the paste printing by use of the plastic mask: (1) The plastic mask is usually prepared by placing a plastic sheet on a working table, then bringing the plastic sheet into close contact with the surface of the working table, and forming through-holes in the plastic sheet with the irradiation with an excimer laser beam from above the plastic sheet. In this method, however, in a portion of the plastic sheet with which the working table is not sufficiently in close contact, occasionally, there occurs that a plastic thin film remains at the bottom of through-holes made in the plastic sheet, which is hereinafter referred to as the "thin-film remaining phenomenon"; and in a portion of the plastic sheet with which the working table is completely in close contact, there occurs that the lower edges of the formed through-holes are rounded, which is hereinafter referred to as the "edge-rounding phenomenon". When a through-hole is made in the plastic sheet by excimer laser, the through-hole is usually tapered with the opening of the through-hole becoming narrower in the direction from the upper opening to the lower opening thereof. When paste printing is performed in practice by use of a plastic mask with such through-holes, the plastic mask is reversed upside down in such a manner that the narrower opening of the through-hole comes to the upper side and the larger opening of the through-hole comes to the lower side. When the "thin-film remaining phenomenon" takes place, the presence of the remaining thin film at the upper end portion of the hole hinders the filling of the hole with the paste and the passing of the paste through the hole, thereby making it impossible to perform paste printing with high quality and high precision. Furthermore, when the "edge-rounding phenomenon" takes place, the rounded edge of the upper opening of the through-hole hinders the filling of the through-hole with the paste and the passing of the paste through the through-hole, thereby making it impossible to perform paste printing with high quality and high precision. WO-A-90/13389 discloses a method of fabricating a plastic mask by using a CO 2 laser for irradiating a plastic sheet (a polyester) to form a through-hole, wherein an adhesive layer (preferably comprising an acrylic polymer) is provided under neath the plastic sheet and covered with a backing strip. After the application of the CO 2 laser the backing strip is removed while the adhesive layer is not removed. It is therefore an object of the present invention to provide a method of fabricating a plastic mask which is capable of performing paste printing with high quality and high precision by forming precise and fine through-holes in a plastic sheet, which are free from the "thin-film remaining phenomenon", the "edge-rounding phenomenon" and burr in the through-holes, or in which the "thin-film remaining phenomenon", the "edge-rounding phenomenon" and burr in the through-holes are minimized, and therefore have excellent paste passing performance. A method of fabricating a plastic mask for paste printing of the present invention is a plastic mask fabricating method with a plastic sheet being irradiated with an excimer laser beam for forming at least one penetrating opening containing area comprising at least one through-hole in the plastic sheet, comprising the steps of (1) bringing a polymer film capable of absorbing at least part of the excimer laser beam into close contact with a back surface of the plastic sheet which is opposite to an excimer laser beam irradiation side thereof which is to be irradiated with the excimer laser beam; (2) forming at least one penetrating opening containing area comprising at least one through-hole in the plastic sheet by irradiating the excimer laser beam irradiation side thereof with the excimer laser beam; and (3) removing the polymer film from the plastic sheet after the formation of the penetrating opening containing area. In the above-mentioned plastic mask fabrication method, the polymer film may be releasably stuck on the back surface of the plastic sheet opposite to the excimer laser beam irradiation side thereof. A plastic mask for paste printing can be produced by this plastic mask fabrication method, which comprises a plastic sheet comprising at least one penetrating opening containing area which comprises at least one through-hole therein, the penetrating opening containing area being formed by the plastic sheet being irradiated with an excimer laser beam, with the depth of each of the through-holes in the penetrating opening containing area measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes in the penetrating opening containing area being 10 µm or less. The above-mentioned plastic mask may have a thickness of 10 to 500 µm and a modulus of bending elasticity of 20 to 500 Kgf•mm -2 . The plastic masks produced by the plastic mask fabrication method of the present invention can be used in a paste printing method which comprises the steps of (1) bringing a paste printing mask comprising at least one penetrating opening containing area which comprises at least one through-hole into close contact with a printing material; (2) filling each of the through-holes in the penetrating opening containing area with a paste comprising solid particles by spreading and pressing the paste on the paste printing mask; and (3) removing the paste printing mask from the printing material, thereby forming on the printing material a paste pattern in the same pattern as that of the through-holes in the penetrating opening containing area, wherein as the paste printing mask, there is employed a plastic mask for paste printing provided with a penetrating opening containing area, the penetrating opening containing area being formed in a plastic sheet so as to include at least one through-hole by excimer laser beam abrasion, with the depth of each of the through-holes of the penetrating opening containing area measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes in the penetrating opening containing area being 1/2 or less the particle size of the solid particles contained in the paste. In the above-mentioned paste printing method, the plastic mask for paste printing may have a thickness of 10 to 500 µm and a modulus of bending elasticity of 196.1 to 4903.3 MPa (20 to 500 Kgf•mm -2 ). BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Fig. 1 is a schematic cross-sectional view of a plastic sheet for use in a plastic mask fabrication method of the present invention. Fig. 2 is a schematic cross-sectional view of an example of a plastic sheet body mounted before the excimer laser abrasion thereof. Fig. 3 is a schematic cross-sectional view of a plastic sheet which is abraded by an excimer laser abrasion apparatus. Figs. 4(a) to 4(c) are schematic cross-sectional views of a through-hole during the formation thereof in a plastic sheet, showing a mechanism for the formation of a remaining thin film in the through-hole during the fabrication of a plastic mask. Fig. 5 is a schematic cross-sectional view of a bottom portion of a through-hole which is about to be completely formed in a plastic sheet during the fabrication of a plastic mask. Figs. 6(a) to 6(c) are schematic cross-sectional views of a through-hole during the formation thereof in a plastic sheet, showing a mechanism for the formation of a non-clear-cut edge in the through-hole during the fabrication of a plastic mask. Fig. 7 is a schematic cross-sectional view of a through-hole which passes through both a plastic sheet and a polymer film by excimer laser abrasion during the fabrication of a plastic mask. Fig. 8(a) is a schematic cross-sectional view of a through-hole free from a non-clear-cut edge. Fig. 8(b) is a schematic cross-sectional view of a through-hole free with a non-clear-cut edge. Figs. 9(a) to 9(c) schematically show the steps of the filling of a through-hole free from a non-clear-cut edge in a fine pattern of through-holes with a paste in a plastic fabrication method of the present invention. Fig. 10 is a schematic cross-sectional view of a through-hole through which a paste passes to form a through-hole pattern on a printing material. Fig. 11 schematically shows the mechanism of insufficient filling of a paste in a through-hole with a non-clear-cut edge in a plastic mask. Fig. 12 schematically shows the formation of a paste pattern with a non-printed portion on a printing material by a through-hole with a non-clear-cut edge, with the paste being caught by the upper end portion of the through-hole, and with poor paste-passing performance thereof. DESCRIPTION OF THE PREFERRED EMBODIMENTS As mentioned previously, a method of fabricating a plastic mask for paste printing of the present invention is a plastic mask fabricating method with a plastic sheet being irradiated with an excimer laser beam for forming at least one penetrating opening containing area comprising at least one through-hole in the plastic sheet, comprising the steps of (1) bringing a polymer film capable of absorbing at least part of the excimer laser beam into close contact with a back surface of the plastic sheet which is opposite to an excimer laser beam irradiation side thereof which is to be irradiated with the excimer laser beam; (2) forming at least one penetrating opening containing area comprising at least one through-hole in the plastic sheet by irradiating the excimer laser beam irradiation side thereof with the excimer laser beam; and (3) removing the polymer film from the plastic sheet after the formation of the penetrating opening containing area. According to the above plastic mask fabrication method, since the polymer film is in close contact with the back surface of the plastic sheet which is opposite to the excimer laser beam irradiation side thereof, a thin film which may remain at the bottom of each through-hole to be made by excimer laser cannot come to the back side of the plastic sheet, so that the thin film, if any, is irradiated with excimer laser beams, until the through-hole is completely made, and eventually no thin film remains. Even if a thin film remains at the bottom of the through-hole for some reason, the thin film is removed when the polymer film is peeled away from the plastic sheet. Furthermore, after the formation of the through-holes, the excimer laser beam passes through the polymer film which absorbs part of the laser beam, and therefore is attenuated, but reaches the working table. Part of the laser beam is reflected by the surface of the working table. The reflected excimer laser beam is again absorbed by the polymer film and attenuated, so that the lower edge portions of the through-holes near the working table are no longer abraded by the reflected excimer laser beam and therefore, the formation of the through-holes with non-clear-cut edges can be effectively prevented. As mentioned previously, the presence of the remaining thin film at the bottom of the through-holes and the through-holes with non-clear-cut edges cause improper filling of the paste and unsmooth passing of the paste through the through-holes, when paste printing is performed by use of a squeegee which comes into contact with the plastic mask. However, by disposing the polymer film in close contact with the back surface of the plastic sheet, the formation of the through-holes, and the paste-filling performance and paste-passing performance thereof can be significantly improved. In the above-mentioned plastic mask fabrication method, the polymer film may be releasably stuck on the back surface of the plastic sheet opposite to the excimer laser beam irradiation side thereof. When the polymer film is releasably stuck on the back side of the plastic sheet, the close contact of the polymer film with the plastic sheet, and the removal of the polymer film from the plastic sheet can be easily performed. Another plastic mask for paste printing comprises a plastic sheet comprising at least one penetrating opening containing area which comprises at least one through-hole therein, the penetrating opening containing area being formed by the plastic sheet being irradiated with an excimer laser beam, with the depth of each of the through-holes of the penetrating opening containing area measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes in the penetrating opening containing area being 10 µm or less. No remaining thin films are formed during the fabrication of this plastic mask, and since the depth of each of the through-holes measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes is 10 µm or less, when paste printing is performed by use of this plastic mask with a paste which contains solder solid particles, for example, solder solid particles mostly with a minimum diameter of 20 µm in a solder paste, the paste spread on the surface of the plastic mask can be sharply scraped off the uppermost level of each through-hole, and the through-holes are completely filled with the paste, so that high quality paste printing with high precision can be performed without improper spreading or non-printing portions in the same through-hole pattern in the penetrating opening containing area of the plastic mask. The above-mentioned plastic mask may have a thickness of 10 to 500 µm and a modulus of bending elasticity of 196.1 to 4903.3 MPa (20 to 500 Kgf•mm -2 ). When paste printing is performed by use of this plastic mask, in particular, when the paste is spread on the plastic mask with the plastic mask being brought into close contact with a printing material by a squeegee which applies pressure to the upper surface of the plastic mask downward, even if the printing material has convex portions on the surface thereof, the gaps between the plastic mask and the printing material can be eliminated or minimized, whereby the paste can be prevented from entering the gaps between the plastic mask and the printing material when the through-holes are filled with the paste. As a result, improper spreading of the printed paste pattern can be prevented or minimized. Furthermore, the smearing of the back side of the plastic mask with the spread paste can be minimized and the paste printing can be repeated a greater number of times than by the conventional metal masks, without wiping the paste off the back side of the plastic mask or cleaning the back side of the plastic mask. A paste printing method comprises the steps of (1) bringing a paste printing mask comprising at least one penetrating opening containing area which comprises at least one through-hole into close contact with a printing material; (2) filling each of the through-holes in the penetrating opening containing area with a paste comprising solid particles by spreading and pressing the paste on the paste printing mask; and (3) removing the paste printing mask from the printing material, thereby forming on the printing material a paste pattern in the same pattern as that of the through-holes in the penetrating opening containing area, wherein as the paste printing mask, there is employed a plastic mask for paste printing provided with a penetrating opening containing area, the penetrating opening containing area being formed in a plastic sheet so as to include at least one through-hole by excimer laser beam abrasion, with the depth of each of the through-holes of the penetrating opening containing area measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes in the penetrating opening containing area being 1/2 or less the particle size of the solid particles contained in the paste. According to this paste printing method the depth of each of the through-holes measured from the top surface of the plastic mask to the narrowest portion of each of the through-holes being 1/2 or less the particle size of the solid particles contained in the paste, so that the paste can be smoothly spread onto the surface of the plastic mask and sharply scraped off the uppermost level of each through-hole, and the lower portion of the spread paste is easily scraped off by the upper end portions of the through-holes in the front and the through-holes can be completely filled with the scraped paste and the filled paste can smoothly pass through the through-holes, so that high quality paste printing with high precision can be performed in the same through-hole pattern in the penetrating opening containing area of the plastic mask. In the above-mentioned paste printing method, the plastic mask for paste printing may have a thickness of 10 to 500 µm and a modulus of bending elasticity of 196.1 to 4903.3 MPa (20 to 500 Kgf•mm -2 ). Because of the above-mentioned combined conditions for the thickness and modulus of bending elasticity of the plastic mask, when paste printing is performed by use of this plastic mask, in particular, when the paste is spread on the plastic mask with the plastic mask being brought into close contact with a printing material by a squeegee which applies pressure to the upper surface of the plastic mask downward, even if the printing material has convex portions on the surface thereof, the gaps between the plastic mask and the printing material can be eliminated or minimized, whereby the paste can be prevented from entering the gaps between the plastic mask and the printing material when the through-holes are filled with the paste. As a result, improper spreading of the printed paste pattern can be prevented or minimized. Furthermore, the smearing of the back side of the plastic mask with the spread paste can be minimized and the paste printing can be repeated a greater number of times than by the conventional metal masks, without wiping the paste off the back side of the plastic mask or cleaning the back side of the plastic mask. Other features of this invention will become apparent in the course of the following description of exemplary embodiments, which are given for illustration of the invention and are not intended to be limiting thereof. Example [Method of Fabricating Plastic Mask] As illustrated in Fig. 1, a polymer film 2 capable of absorbing part of an excimer laser beam was releasably stuck on a plastic sheet 1 through an adhesive agent 2a. A polyimide film with a thickness of 125 µm was employed as the plastic sheet 1; and a polyester film with a thickness of 25 µm was employed as the polymer film 2 capable of absorbing part of an excimer laser beam. To this polyester film, a silicone adhesive agent was applied as the adhesive agent 2a. Thus, a layered material composed of the polyester film and the polyimide film was prepared. As illustrated in Fig. 2, a polyester mesh screen 4 stuck on a mask frame 3, there was stuck by use of an adhesive agent 5 the thus prepared layered material cut with an appropriate size in such a posture that the polyester film came downward and the polyimide film serving as the plastic sheet 1 came upward. After the adhesive agent 5 was cured, a plastic sheet body 6 to be subjected to excimer laser abrasion was prepared by cutting off an inside portion of the polyester mesh screen with respect to the applied adhesive agent 5. As illustrated in Fig. 3, the plastic sheet body 6 was set on a working XY table 8 in such a manner that the polyester film of the plastic sheet body 6 was brought into close contact with the surface of the working XY table 8 by fixing the mask frame 3 to the working XY table 8 by a clump 7, whereby accurate registration of the plastic sheet body 6 at an excimer laser beam irradiation position was carried out by appropriately moving the working XY table 8. The polyimide film of the plastic sheet 1 was irradiated with an excimer laser beam 10 which was output from an excimer laser beam generator 9 situated above the plastic sheet body 6, through a light-shielding aperture mask 11 with a pattern of through-holes of a plastic mask, a condenser lens 12 and then a lens 13. The above excimer laser beam was generated with a wavelength of 248 nm by krypton-fluorine gas and at a pulse frequency of 200 Hz/sec, and the surface of the polyimide film was irradiated with this excimer laser beam at an energy density of 1.7 Joule/cm 2 through the condenser lens 12 and the lens 13, whereby the abrasion was carried out at an abrasion speed of about 0.5 µm/pulse for 1.25 seconds. The plastic sheet body 6 was then detached from the working XY table 8, and the polyester film which was applied to the polyimide film was removed together with the silicone adhesive agent, whereby a plastic mask with a pattern of through-holes was obtained. The thus obtained plastic mask was free from remaining films and non-clear-cut edges in the through-holes. A minimum pitch of the pattern obtained by this method was 0.1 mm. The thus fabricated plastic mask with a pattern with a pitch of 0.4 mm was brought into close contact with a print-circuit board made of glass epoxy. A solder paste containing spherical solid particles with a particle size of 20 µm to 44 µm was spread on the plastic mask by use of an urethane squeegee, and the pattern-shaped through-holes were filled with the solder paste. The plastic mask was then removed from the print-circuit board, whereby a paste pattern of the through-holes was formed on the print-circuit board. The paste-filling performance and paste-passing performance of the through-holes of the plastic mask were excellent, so that high quality paste printing with high precision of the pattern free from non-printing portions and improper spreading was carried out on the print-circuit board. The back side of the plastic mask was not substantially smeared with the paste, so that it was possible to repeat the paste printing with high quality and high precision more than 100 times without wiping the paste and also without cleaning the back side of the plastic mask. In contrast, even in the case of an additive mask, which is considered to be best of the metal masks in terms of the printing quality, the paste must be wiped from the back side of the mask at every several printings, and the back side of the mask must be cleaned at several ten times. The excimer laser which is employed in the fabrication of the plastic mask of the present invention can perform the abrasion working, or non-heating or photo-chemical working which cannot be performed by other lasers such as CO 2 or YAG laser. The excimer laser is particularly suitable for precise working of organic polymers. The CO 2 or YAG laser working is thermal working which mainly utilizes laser beams with wavelengths in infrared region. In contrast to this, the abrasion working or non-heating working conducted by use of the excimer laser is carried out by utilizing the laser beams in ultra-violet region. When an organic polymer is irradiated with the excimer laser beam, the molecules of the organic polymer absorb the energy of the excimer laser beam and are excited to the extent that the bonds of the molecules are cleaved. As a result, the organic polymer is decomposed to the constituents molecules and scattered in the form of a gas. This decomposition process proceeds within several 10 ns, so that although the molecules are heated to about 6000°K at the time of the cleavage and scattered at an initial speed of as great as 5000 to 10000 m/sec, there is not enough time for the heated molecules to heat any materials adjacent to the heated molecules. Therefore in the abrasion working by the excimer laser, problems such as denaturation or modification which may be caused by the application of heat are never caused in the portions adjacent to the portion worked by the excimer laser beam, and the worked surface is extremely smooth and clean. Furthermore, in the case of CO 2 or YAG laser working, a laser beam which is narrowed down is applied to a working subject. In sharp contrast to this, the excimer laser beam has a larger beam projection cross-section. For instance, some excimer laser beam has a beam projection cross-section with a major axis of about 20 mm and a minor axis of 8 to 9 mm. Thus, a larger area can be worked at a time by the excimer laser abrasion than by the CO 2 or YAG laser working. When an excimer laser beam is used in practice, there may be the case where the output energy thereof is insufficient for performing actual abrasion. In such a case, the excimer laser beam is condensed. When condensed, the working area may be reduced to less than 1/2, but the excimer laser abrasion still has the advantage over other working methods that planar working is possible. By utilizing this feature of the excimer laser abrasion, when a plastic sheet is irradiated with an excimer laser beam through a light-shielding aperture mask with a pattern of through-holes through which the excimer laser beam can pass, a thin concave portion or through-holes can be formed in a similar shape or pattern to that of the pattern in the light-shielding aperture mask can be formed in the plastic sheet. Thus, there can be prepared a printing plastic mask with excellent smoothness of the inner walls of the through-holes. By this excimer laser abrasion, a plastic mask with a pattern of through-holes with a pitch of as small as 0.1 mm can be fabricated. Furthermore, according to the plastic fabrication method of the present invention, a polymer film which is capable of absorbing at least part of the excimer laser beam is disposed in close contact with the back surface of the plastic sheet which is opposite to the excimer laser beam irradiation side thereof; the plastic sheet is irradiated with the excimer laser beam to form through-holes in the plastic sheet; and the polymer film is removed from the plastic sheet, whereby there can be formed a plastic mask which is free from thin films which may remain at the bottom of each through-hole, the so-called "remaining thin films" and free from non-clear-cut edges in the through-hole, the so-called "non-clear-cut edges", or there can be formed a plastic mask with minimized formation of such "remaining thin film" and "non-clear-cut edges". In particular, a plastic mask with through-holes with a minimum pitch of 0.1 mm to about 0.01 mm, free from the "remaining thin film" and "non-clear-cut edges" or with minimized formation thereof, can be fabricated according to the present invention. The above-mentioned minimum pitch obtained by the plastic mask of the present invention is in the range of 1/4 to 1/3 the minimum pitch obtained by the conventional metal mask, which is in the range of 0.4 mm to 0.3 mm. With respect to the non-formation of the "remaining thin film" in the plastic mask fabrication method of the present invention, the following mechanism can be considered: Figs. 4(a) to 4(c) show a mechanism for the formation of the "remaining thin film" in a plastic mask fabrication method. Fig. 4(a) shows the formation of part of a through-hole in the plastic sheet. Fig. 4(b) is an enlarged partial view of a gouged portion in the bored through-hole. Fig. 4(c) shows a remaining thin film. As shown in Fig. 4(a), an excimer laser beam irradiation surface 1a of a plastic sheet 1 is irradiated with an excimer layer beam 10, part of a through-hole 14 with a tapered inner wall to the bottom as illustrated in Fig. 4(a) is formed. In the course of the formation of the through-hole 14, a laser beam reflected by the tapered inner wall 1c, which is hereinafter referred to as the reflected laser beam 10a, proceeds the abrasion of a corner portion 1d to form a gouged portion therein. When the abrasion of the corner portion 1d does not proceed uniformly, a thin film 1f is formed and turned to the back side 1b of the plastic sheet 1 on a portion which is slightly connected to the plastic sheet 1 and serves as a pivot for the turning of the thin film 1f, in a portion of the plastic sheet 1 which is not in close contact with a working table 8, by the repulsion of the abrasion. The thin film 1f is then no longer irradiated with the excimer laser beam 10, so that the thin film 1f remains at the bottom of the through-hole 14. In sharp contrast to this, according to the printing plastic mask fabrication method of the present invention, a polymer film is in close contact with the back side of the plastic sheet which is opposite to the excimer laser beam irradiation surface thereof, so that the thin film at the bottom of the through-hole which is about to be completely formed by the excimer laser beam abrasion is not turned to the back side of the plastic sheet and remains until the through-hole is completely formed, and such a thin film does not remain. Even if some thin film remains on the polymer film which is in close contact with the plastic sheet, since the polymer film is removed after the formation of the through-hole, no thin film remains on the printing plastic mask. Fig. 5 schematically shows the state of the bottom of a through-hole which is about to be completely formed by the excimer laser beam by the plastic mask fabrication method of the present invention. As shown in Fig. 5, a gouged portion is formed at a corner portion 1d between a bottom surface 1g and an inner wall 1c of the through-hole since the abrasion is significantly promoted because of an increased energy density due to the overlapping of a reflection laser beam 1a reflected by the tapered surface of an inner wall 4 and a direct laser beam 10 which is directly projected from right above. On the other hand, at the other corner 1e, an inner wall 1c is cut off the bottom surface 1g. When the number of the cut-off corners is increased and the number of connected portions is decreased, the thin film at the bottom of the through-hole receives such a force that makes the thin film turn pivotally on a singly connected portion at the corner portion 1d toward the back side 1b of the plastic sheet by the repulsion force which is generated by the molecules which are scattered in the state of a plasma at an initial speed of 5000 to 10,000 m/sec by the excimer laser beam abrasion. However, the thin film cannot move to the back side 1b of the plastic sheet because it is stopped by the polymer film 2 disposed at the lower side of the bottom portion 1g through an adhesive agent 2a, which polymer film 2 is capable of absorbing at least part of the excimer laser beam. As a result, the thin film is irradiated by the excimer laser beam until the through-hole is completely made, so that the thin film does not remain. According to the plastic mask fabrication method of the present invention, there are formed no "non-clear-cut edges" in the through-holes or the formation of the "non-clear-cut edges" is minimized. It is considered that this is accomplished by the following mechanism: Figs. 6(a) to 6(c) show the mechanism for the formation of the "non-clear-cut edges" in the through-holes. Fig. 6(a) shows the state of the formation of part of a through-hole. Fig. 6(b) is an enlarged view of part of a through-hole immediately after the formation thereof. Fig. 6(c) is an enlarged view of a non-clear-cut edge formed in the through-hole. As shown in Fig. 6(a), a through-hole 14 is being formed as the plastic sheet 1 in close contact with the working table 8 is irradiated with the excimer laser beam 10. As shown in Fig. 6(b), after the formation of the through-hole 14, the applied excimer laser beam 10 is scattered by the surface of the working table 8 as shown by 10b. As shown in Fig. 6(c), the lower end portion of the through-hole 14 formed in the plastic sheet 1 is abraded by the excimer laser beam 10b scattered near the lower end portion of the through-hole 14, so that the edge of the through-hole 14 is rounded and the "non-clear-cut edge" 1h is formed at the lower end portion of the through-hole 14. In contrast to this, according to the plastic mask fabrication method of the present invention, the polymer film is applied to the back side of the plastic sheet opposite to the excimer laser beam irradiation surface thereof, so that after the formation of the through-hole, the excimer laser beam is attenuated, but reaches the working table. Part of the excimer laser beam is reflected by the surface of the working table and absorbed by the polymer film, so that the lower end portion of the through-hole is no longer abraded by the reflected excimer laser beam. Thus, the formation of the "non-clear-cut edge" in the through-hole can be prevented. When a through-hole which passes through the plastic sheet and the polymer film is formed, the excimer laser beam is not attenuated until it reaches the working table. However, the excimer laser beam is reflected by the surface of the working table and then absorbed by the polymer film which is in close contact with the plastic sheet at the edge portion of the through-hole, so that the reflected excimer laser beam is attenuated. Thus, the lower end portion of the through-hole is no longer abraded by the reflected excimer laser beam, and therefore the formation of the "non-clear-cut edge" in the through-hole can be prevented. In this case, it may occur that the polymer film which is in close contact with the plastic sheet is abraded at the lower portion of the through-hole and a non-clear-cut edge is formed at the polymer film. However, since the polymer film is removed from the plastic sheet after the formation of the through-hole, such abrasion in the polymer film has no adverse effects on the paste printing by use of the plastic mask thus prepared. Furthermore, in the case where there is formed a through-hole which passes not only through the plastic sheet, but also through the polymer film, the excimer laser beam reflected by the surface of the working table reaches the inner wall of the through-hole, but the inner wall is rarely abraded by the reflected excimer laser beam. It is considered that this may be because the intensity of the received excimer laser beam per unit area on the inner wall of the through-hole is significantly decreased by (1) the attenuation of the excimer laser beam reflected by the surface of the working table, (2) the decrease of the energy density thereof by the scattering of the laser beam by the reflection, and (3) the slanting projection of the laser beam from the lower position onto the inner wall of the through-hole. Fig. 7 schematically shows a through-hole which passes not only through the plastic sheet, but also through the polymer film, formed by the plastic mask fabrication method of the present invention. When the excimer laser beam abrasion proceeds, the plastic sheet 1, the adhesive agent 2a and the polymer film 2 in the respective abraded portions are scattered and disappear. The excimer laser beam 10 reaches the working table 8, and is partly absorbed by the working table 8 and reflected by the working table 8, whereby scattered beams 10c, 10d, 10e and the like are formed. The scattered beams 10d and 10e strike at the inner wall 1c of the through-hole formed in the plastic sheet 1. However, the energy density of the scattered beam 10d per unit area of the inner wall 1c is significantly lowered because of the attenuation of the energy density thereof by the scattering of the beam, and the slanted projecting direction with respect to the inner wall 1c. With respect to the scattered beam 10e, the slanted projection angle with respect to the inner wall 1c is smaller than that of the scattered beam 10d, but the travelling distance up to the inner wall 1c is larger than that of the scattered beam 10d, so that the attenuation of the energy density of the scattered beam 10e is much larger than that of the scattered beam 10d. Therefore, it is not considered that the inner wall 1c of the through-hole 14 is abraded by any of the scattered beams 10d and 10e. The scattered beam 10c reflected immediately near the inner wall lc of the through-hole 14 is small in the scattering of the energy thereof, so that it is possible that the lower tip portion of the polymer film 2 is abraded by the scattered beam 10c. However, the polymer film 2 is removed together with the adhesive agent 2a after the formation of the through-hole, so that the abrasion of the polymer film 2 does not have any adverse effect on the paste printing by use of this plastic mask. When paste printing is performed by use of the plastic mask prepared by the plastic mask fabrication method of the present invention, the plastic mask is used upside down. Therefore, when no "non-clear-cut edges" are formed in the through-hole formed in the plastic sheet, the narrowest opening portion of the through-hole comes to the uppermost surface of the plastic mask when used in practice for paste printing, and even if some "non-clear-cut edges" are formed in the through-hole, the narrowest opening portion of the through-hole comes near the uppermost surface of the plastic mask when used in practice. Fig. 8(a) is a schematic cross sectional view of a through-hole free from a non-clear-cut edge formed in a plastic mask 50. As shown in Fig. 8(a), the narrowest opening portion W is formed at the uppermost surface of the plastic mask 50. Fig. 8(b) is a schematic cross sectional view of a through-hole with a non-clear-cut edge formed in a plastic mask 50. As shown in Fig. 8(a), the narrowest opening portion W is formed very close to the uppermost surface of the plastic mask 50 at a depth H therefrom. According to the plastic mask fabrication method of the present invention, there can be fabricated a plastic mask with through-holes free from the "remaining thin films" and "non-clear-cut edges" or with minimized formation thereof, with the depth of the through-holes from the uppermost surface of the plastic mask to the narrowest portion of the through-holes being 25 µm or less, 20 µm or less, 15 µm or less, 10 µm or less, and 5 µm or less, if necessary. Such a plastic mask for printing can be selectively used in accordance with the fabrication efficiency and the kind of a paste to be used in printing. Plastic masks free from the "remaining thin films" and "non-clear-cut edges", and plastic mask free from the "remaining thin films" and a minimum depth from the uppermost surface of the mask to the narrowest portion of the through-holes are preferable for use since such plastic masks can be used in any kinds of paste printing for obtaining fine patterns free from non-printed portions or improper spreading. The inventor of the present invention has made investigations about a paste printing method by use of the thus fabricated plastic mask and discovered that in a paste printing for printing a pattern of a paste containing solid particles on a printing material by bringing the plastic mask into close contact with the printing material, spreading the paste on the plastic mask to fill a pattern of through-holes formed in the mask with the paste, removing an excessive paste from the surface of the mask, and removing the mask from the printing material, high quality paste printing can be carried out, with excellent paste-filling performance and paste-passing performance with respect to the through-holes, when the depth from the uppermost surface of the mask to the narrowest portion of each through-hole is not more than 1/2 the particle size of the solid particles contained in the paste. It is considered that the above-mentioned excellent paste-filling performance and paste-passing performance can be attained by the following mechanism in this printing method. Figs. 9(a) to 9(c) schematically show the steps of the filling of the paste in a through-hole free from "non-clear-cut edge" in a pattern of fine through-holes formed in a plastic mask fabricated by the plastic mask fabrication method of the present invention. Fig. 9(a) shows the step of the initiation of the spreading of the paste. Fig. 9(b) shows the step of scraping the spread paste. Fig. (c) shows the step of filling the through-hole with the paste. As shown in Fig. 9(a), when a paste 16 is spread by a squeegee 17 on the surface of a plastic mask 50 which is set in close contact with a printing material 19, the paste 16 is moved along the surface of the plastic mask 50 as the paste 16 rolls and slips on the plastic mask 50. The paste 16 then passes over the upper portion of a through-hole 14, and the lower portion of the paste 16 is then scraped by the front upper end 14a of the through-hole 14 as shown in Fig. 9(b). The scraped paste 16 is then moved toward the squeegee 17 and the through-hole 14 is then filled with the successively scraped paste 16. When the plastic mask with the through-holes being thus filled with the paste 16 is removed from the printing material 15, the paste 16 is caused to smoothly pass through the through-hole 14, so that a pattern of fine through-holes free from non-printed portions and improper spreading is printed on the printing material 15. Even if there is a slight non-clear-cut edge in the through-hole 14, when the depth from the uppermost surface of the plastic mask to the narrowest portion of the through-hole 14 is not more than 1/2 the particle size of the solid particles contained in the paste, a pattern of fine through-holes free from non-printed portions and improper spreading can be printed on the printing material 15, with excellent paste-filling performance and paste-passing performance. In contrast to this, when the depth from the uppermost surface of the plastic mask to the narrowest portion o f the through-hole 14 exceeds 1/2 the particle size of the solid particles contained in the paste, it is difficult for the lower portion of the paste 16 to be scraped by the front upper end 14a of the through-hole 14 as shown in Fig. 11, so that the filling of the through-hole 14 with the paste becomes insufficient. Further-more, as shown in Fig. 12, when the plastic mask 50 is removed from the printing material 15, part of the paste 16 is caught by the non-clear-cut edge of the upper portion of the through-hole, so that the paste-passing performance is degraded and a paste pattern with a non-printed portion is printed on the printing material 15. As the material for the plastic sheet for use in the present invention, any organic polymers which permit excimer laser beam abrasion can be employed. Even an organic polymer for which excimer laser beam abrasion is difficult or impossible to conduct, can also be subjected to excimer laser beam abrasion by mixing therewith a compound capable of absorbing excimer laser beam such as finely-divided carbon particles and an ultraviolet absorbing agent, or an organic polymer which can be subjected to the excimer laser beam abrasion. It is preferable that the plastic sheet for use in the present invention have the properties that are required for the plastic mask, namely, the following properties: (1) chemical resistance, for example, against the paste, solvents contained in the paste, and cleaning agents employed after paste printing; (2) sufficient abrasion resistance for use as a printing sheet; (3) sufficient mechanical strength for attaining dimensional stability when tension is applied when it is mounted on a frame; and (4) low friction, and water repellency suitable for attaining the previously mentioned excellent paste-passing performance. Specific examples of the plastic sheet for use in the present invention are organic polymers for which the excimer laser beam abrasion can be easily conducted, such as polyimide, polyester, epoxy resin and polycarbonate; and organic polymers for which the excimer laser beam abrasion is difficult or impossible to conduct, such as polyethylene, polypropylene, polyacetal and teflon. The latter organic polymers can be used by mixing therewith a compound capable of absorbing excimer laser beam, or an organic polymer which can be subjected to the excimer laser beam abrasion as mentioned previously. It is preferable that the plastic sheet, or the plastic mask of the present invention, have a thickness in the range of about 10 µm to 500 µm; and that a plastic mask for paste printing by use of a solder paste have a thickness in the range of about 100 µm to 200 µm. It is also preferable that the plastic mask have a modulus of bending elasticity of about 196.1 to 4903.3 MPa (20 to 500 Kgf•mm -2 ). A plastic mask with a thickness in the above-mentioned range and the above-mentioned modulus of bending elasticity is capable of eliminating or minimizing the gaps between the plastic mask and a printing material when the plastic mask is brought into close contact with the printing material even if the printing material has some convex portions on the surface thereof, whereby the entering of the paste in the gaps between the plastic mask and the printing material can be prevented. As a result, a paste pattern free from improper spread can be sharply formed. Furthermore, since the back side of the plastic mask is not smeared or hardly smeared with the plate, the plastic mask can be repeatedly used for an extended period of time without cleaning the back side of the plastic mask. As the material for the polymer film capable of absorbing excimer laser beam for use in the plastic mask fabrication method of the present invention, the same material as that for the above-mentioned plastic sheet for use in the present invention can be employed. It is preferable that as the material for the polymer film, a tenacious material that can be smoothly peeled away from the plastic sheet, without being broken, not easily pulled apart or torn be employed. In practice, it is preferable that the same material as that for the plastic sheet be employed for the polymer film for the excimer laser beam abrasion. It is preferable that the polymer film have a thickness of about 5 µm to 50 µm. In the plastic mask fabrication method of the present invention, the polymer film can be disposed in close contact with the back side of the plastic sheet opposite to the excimer laser beam irradiation side thereof, for example, by any of the following methods: (a) the polymer film is brought into pressure contact with the plastic sheet; (b) the polymer film is covered with a plastic sheet which is larger than the polymer film and the polymer film is attracted to the plastic sheet by air suction; and (c) a polymer film coated with an adhesive agent is releasably applied to the plastic sheet. As the adhesive agent for the above-mentioned purpose, it is preferable to use an adhesive agent which is capable of absorbing excimer laser beams, or which contains a material capable of absorbing excimer laser beams, or which allows the passage of the excimer laser beams or is transparent thereto. It is also preferable that the adhesive agent be such an adhesive agent that allows the peeling of the polymer film away from the plastic sheet without remaining thereon. When the polymer film is stuck on the plastic sheet through the adhesive agent, for example, a plastic sheet is unrolled from a roll of the plastic sheet, and a polymer film coated with an adhesive layer thereon is also unrolled from a roll of the polymer film, and the adhesive layer of the polymer film is brought into close contact with the plastic sheet unrolled from the roller thereof by causing the polymer film and the plastic sheet to pass between pressure application rollers, and the polymer-film-applied plastic sheet is wound around a take-up roller. This method permits continuous preparation of the polymer-film-applied plastic sheet by using rollers. It is preferable that no bubbles and wrinkles be formed during the sticking of the polymer film on the plastic sheet.

Claims: A method of fabricating a plastic mask for paste printing with a plastic sheet being irradiated with an excimer laser beam for forming at least one penetrating opening containing area comprising at least one through-hole in said plastic sheet, comprising the steps of: bringing a polymer film capable of absorbing at least part of said excimer laser beam into close contact with a back surface of said plastic sheet which is opposite to an excimer laser beam irradiation side thereof which is to be irradiated with said excimer laser beam; forming at least one penetrating opening containing area comprising at least one through-hole in said plastic sheet by irradiating said excimer laser beam irradiation side thereof with said excimer laser beam; and removing said polymer film from said plastic sheet after the formation of said penetrating opening containing area. The method of fabricating a plastic mask for paste printing as claimed in Claim 1, wherein said polymer film is releasably stuck on the back surface of said plastic sheet opposite to said excimer laser beam irradiation side thereof.

Inventors: KINOSHITA MAKOTO, KINOSHITA, MAKOTO
Applicants: RICOH MICROELECTRONICS CO LTD, RICOH MICROELECTRONICS CO., LTD.
Application year: 1995
Main classification: B41C1
IPC codes: B23K26, B41M1, B41N1, B41C1, H05K3