Abstract:
A method of manufacturing a template having through-holes for attracting and supporting electrically conductive balls by vacuum suction is disclosed. The through-holes are formed by etching and the side walls of the through-holes are smoothed by irradiation, with laser beams, of the side walls of the through-holes. A template and metallic bumps can be formed using this method. Alternatively, the template can be formed in a two-layered structure.

Description:
This application is a division of prior application Ser. No. 09/015,265, filed Jan. 29, 1998 now U.S. Pat. No. 6,107,181. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming bumps used for forming conductive bumps in electronic parts. Also, the present invention relates to a template used in the bump forming method and a method of manufacturing the template. 
     2. Description of the Related Art 
     A method of forming metallic bumps such as solder bumps on an electronic part such as an LSI and joining the electronic part to a printed circuit board by the metallic bumps has come into wide use. In one method of forming the metallic bumps, metallic balls are previously formed and stored in a metallic ball container, and then the metallic balls are taken out from the metallic ball container and attached to the electrodes of the electronic part so that the metallic balls can be used as metallic bumps of the electronic part. For example, Japanese Unexamined Patent publication No. 8-112671 discloses the above-described metallic bump forming apparatus. 
     In the conventional metallic bump forming apparatus, a suction head is used to take out the metallic balls from the metallic ball container and attach them to the electrodes of electronic part. A template (suction pad) for mounting metallic balls is attached to the suction head, the template having through-holes arranged in the same arrangement as that of the electrodes of the electronic part. Suction grooves are provided in the surface of the sucking head and connected to a vacuum source, and the through-holes of the template for mounting metallic balls are connected to the suction grooves of the suction head, so that the metallic balls can be attracted by vacuum suction. 
     When the template for mounting metallic balls comes into contact with the metallic balls accommodated in the metallic ball container, the metallic balls are respectively sucked by and attracted to the through-holes in the template. When the suction head is then moved above the electronic part, the metallic balls attracted to the through-holes in the template are respectively located on the electrodes of the electronic part. Then, the metallic balls are joined to the electrodes of the electronic part under heat and pressure, the metallic balls becoming the metallic bumps. After that, the suction head is separated from the electronic part, with the result that the metallic balls are transferred from the suction head to the electronic part. 
     The template for mounting metallic balls is usually made of inorganic material such as glass, which is resistant to distortion, and its charging level is low. Plastics are not suitable for the material for the template for mounting metallic balls, because plastics are easily distorted, and their charging level is high. If the template for mounting metallic balls is made of a plate which is easily distorted, the template is distorted and the positions of the through-holes are shifted from the positions of the electrodes of the electronic parts, when the metallic balls are bonded to the electrodes of the electronic part under heat and pressure by means of thermo-compression. When the template for mounting metallic balls has an electrification property, the metallic balls are attracted to the template at positions other than those of the through-holes by static electricity when the template is moved above and then into the metallic ball container to suck the metallic balls to the through-holes. 
     Through-holes in the template for mounting metallic balls are formed by etching. Alternatively, the through-holes can be formed by machining such as drilling or electric discharge machining. However, when the template for mounting metallic balls is made of inorganic material such as glass, the side walls of the through-holes become unsmooth, and there is a possibility of generation of burrs which is an irregularity of the side walls of the through-holes. 
     If the side walls of the through-holes are not smooth, the following problems may be caused. The metallic balls bite into the end portions of the through-holes in the template for mounting metallic balls when the template is pressed against the electronic part to join the metallic balls to the electrodes of the electronic part, and the metallic balls adhere to the template and the metallic balls are torn off from the electrodes of the electronic part, when the suction head is separated from the electronic part. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a. method of forming bumps, a template for mounting metallic balls capable of reliably joining the metallic balls to the electrode pads of an electronic part and a method of manufacturing the template. 
     A method of forming bumps onto a chip having electrodes, according to the present invention, comprises the steps of using a template having through-holes with smoothed side walls formed corresponding to the electrodes of the chip, to attract conductive balls to be formed into bumps by vacuum suction, positioning the conductive balls attracted by the template to the electrodes, pressing the conductive balls against the electrodes by the template, and releasing vacuum suction. 
     A method of manufacturing a template, according to the present invention comprises the steps of: forming through-holes in a plate for attracting and supporting conductive balls thereat, and smoothing the side walls of the through-holes formed in the through-hole forming step. 
     Preferably, the through-holes are formed by etching the template. Preferably, the side walls are smoothed by irradiating the template with laser beams. Also, preferably, the step of irradiating the template with laser beams comprising irradiating the side walls of the through-holes with laser beams via a mask having openings, the size of which is a little larger than the diameter of each through-hole. 
     A template, according to the present invention comprises through-holes for attracting and supporting conductive balls by vacuum suction, the through-holes having smoothed side walls. 
     In this structure, the side walls of the through-holes in the template are smoothed by irradiating the side walls with laser beams. Accordingly, the conductive balls do not bite into the end portions of the through-holes in the template, and, when the suction head is separated from the electronic part after the conductive balls have been joined to the electrode pads of the electronic part, the, conductive balls are surely separated from the template and the conductive balls are reliably joined to the electrode pads of the electronic part. 
     Further, the present invention provides a template comprising an inorganic material layer and an organic material layer laminated on each other, said template having through-holes formed through the inorganic and the organic material layer for attracting and supporting conductive balls. 
     In this structure, the inorganic material layer gives a sufficiently high mechanical strength to the template to resist distortion. The through-holes are provided through the inorganic and the organic layer, and the portion of the through-holes in the organic material layer can be more smoothly formed than the portion of the through-holes on the inorganic material layer. The conductive balls are sucked and attracted by the portion of the through-holes formed in the organic material layer. Accordingly, the conductive balls do not bite into the end portions of the through-holes in the template, and, when the suction head is separated from the electronic part after the conductive balls have been joined to the electrode pads of the electronic part, the conductive balls are surely separated from the template and the conductive balls are reliably joined to the electrode pads of the electronic part. 
     Further, the present invention provides a template comprising an air permeable material layer and a resin layer laminated on each other, said template having through-holes formed through the resin layer. 
     The air permeable material layer allows communication between the suction groove of the suction head and the through-holes in the resin layer, and the through-holes can suck and attract the conductive balls. In this case too, the conductive balls do not bite into the end portions of the through-holes in the template, and when the conductive balls are joined to the electrode pads of the electronic part and thereafter the suction head is separated from the electronic part, the conductive balls are surely separated from the template and the conductive balls can be reliably joined to the electrode pads of the electronic part. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more apparent from the following description of the preferred embodiments, with reference to the accompanying drawings, in which: 
     FIGS. 1A and 1B are views showing a process of forming the through-holes in the template, according to the first embodiment of the present invention, wherein FIG. 1A is a cross-sectional view showing the template for mounting metallic balls, and FIG. 1B is a plan view showing a through-hole including a resultant burr; 
     FIGS. 2A and 2B are views showing a process of irradiating the template shown in FIG. 1 with laser beams, wherein FIG. 2A is a cross-sectional view showing the template for mounting metallic balls, and FIG. 2B is a plan view showing a through-hole from which burr has been removed; 
     FIG. 3 is a schematic perspective view showing the periphery of the end portion of the through-hole which is smoothed by the irradiation of laser beams; 
     FIG. 4 is a cross-sectional view showing a variation of the template for mounting metallic balls; 
     FIG. 5 is a view showing the metallic bump forming apparatus including the template for mounting metallic balls of the present invention; 
     FIG. 6 is a perspective view showing the suction head shown in FIG. 5; 
     FIG. 7 is a perspective view showing the template shown in FIG. 5 for mounting metallic balls; 
     FIG. 8A to  8 E are views showing the operation of the metallic bump forming apparatus; 
     FIG. 9 is a cross-sectional view showing a portion of the template for mounting metallic balls, according to the second embodiment of the present invention; 
     FIG. 10 is a cross-sectional view showing a variation of the template for mounting metallic balls shown in FIG. 9; 
     FIG. 11 is a cross-sectional view showing a portion of the template for mounting metallic balls, according to the third embodiment of the present invention; 
     FIG. 12 is a plan view showing the air permeable material layer of the template for mounting metallic balls shown in FIG. 11; 
     FIG. 13 is a plan view showing the suction head used. with the template shown in FIGS. 11 and 12 for mounting metallic balls; 
     FIGS. 14A to  14 C are views showing an example of the manufacturing of the template for mounting metallic balls shown in FIG. 11; and 
     FIG. 15 is a view showing another example of the manufacturing of the template for mounting metallic balls shown in FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 shows a metallic bump forming apparatus  10  including a template of the present invention for mounting metallic balls (conductive balls). The metallic bump forming apparatus  10  includes a stage  14  to support an electronic part  12  such as an LSI, a metallic ball container  18  accommodating metallic balls  16  which are previously formed, and a suction head  22  having a template  20  for mounting metallic balls attached thereto. 
     The stage  14  is provided with a suction hole  14   a , so that the electronic part  12  can be held on the stage  14  by the action of a vacuum. The electronic part  12  is provided with electrodes  24  arranged at a predetermined pattern. The suction head  22  is provided with a suction hole  22   a  and a suction groove  22   b.    
     FIG. 6 shows the suction head  22 . The suction hole  22   a  extends in the suction head  22 , and the suction groove  22   b  is formed in the surface of the suction head  22 . Also, plate suction holes  22   c  are open in the surface of the suction head  22 . 
     FIG. 7 shows the template  20  for mounting metallic balls. The template  20  for mounting metallic balls has through-holes  26 . The arrangement of the through-holes  26  is the same as the arrangement of the electrodes  24  of the electronic part  12 . According to the type of the electronic part  12 , a template  20  having differently arranged through-holes  26  may be prepared. The through-holes  26  are arranged in such a manner that they are located in the range of the suction groove  22   b  when the template  20  for mounting metallic balls is attracted to the suction head  22  by the plate suction holes  22   c . Accordingly, all the through-holes  26  are communicated with the suction groove  22   b.    
     Consequently, it is possible to attract the metallic balls  16  to the through-holes  26  in the template  20  by vacuum suction, as shown in FIG.  5 . The metallic bump forming apparatus  10  shown in FIG. 5 further includes a transfer unit  28 , so that the suction head  22  can be moved between a position above the metallic ball container  18  and a position above the electronic part  12 . 
     FIGS. 8A to  8 E show the operation of the metallic bump forming apparatus  10 . FIG. 8A shows a state in which the suction head  22  is moved above the metallic ball container  18  and the metallic balls  16  are attracted to the through-holes  26  in the template  20  by vacuum suction. FIG. 8B shows a state in which the suction head  22  is moved above the electronic part  12  arranged on the stage  14 . FIG. 8C shows a state in which the metallic balls  16  are fused to the electrodes  24  of the electronic part  12  under heat and pressure. FIG. 8D shows a state in which the suction head  22  is separated from the electronic part  12 . In this case, a vacuum supply to the plate suction holes  22   c  is continued, and a vacuum supply to the suction hole  22   a  is stopped. Due to the foregoing, the metallic balls  16  are joined to the electrodes  24  of the electronic part  12  and formed into metallic bumps. After that, it is possible to press the metallic balls  16  against a glass plate so that the metallic balls  16  are leveled, and to apply silver paste to the metallic balls  16 . FIG. 8E shows a state in which the electronic part  12  is joined to a printed circuit board  30  by the metallic bumps. In this case, it is possible to bond the electronic part  12  to the printed circuit board  30  with an insulating adhesive. 
     If the through-holes  26  of template  20  are not smooth, the metallic ball  16  may bite into burr  42  at the end portions of the through-holes  26  when the template  20  is pressed against the electronic part  12 , and the metallic ball  16  may be moved together with the template  20  when the suction head  22  is separated from the electronic part  12 , as shown in FIG. 8D, resulting in a problem that the metallic ball  16 , which had been joined to the electrodes  24  of the electronic part  12 , may be peeled off the electrode  24 . Therefore, it is preferable that metallic balls  16  do not bite into the end portions of the through-holes  26 . 
     FIGS. 1A to  2 B show the first embodiment of the present invention. The template  20  for mounting metallic balls is made of glass which is an inorganic material. In FIG. 1A, a template (glass plate)  20  for mounting metallic balls is subjected to etching to form through-holes  26 . In this case, etching is conducted in such a manner that a glass plate with a mask  40  is soaked in the etching liquid containing fluoride. When the through-holes  26  are formed in the glass plate by etching, the side wall of the through-holes  26  become irregular, as shown in FIG.  1 (B), that is, burr  42  is formed in the side walls of the through-holes  26 . 
     The side walls of the through-holes  26  are then smoothed, by irradiating the side walls of the through-holes with laser beams LB, as shown in FIG.  2 A. FIG. 2B diagrammatically shows the through-hole  26  in which the burr  42  is removed from the side wall of the through-hole  26  shown in FIG.  1 B and the side wall is smoothed. 
     It is preferable that only a restricted region in the periphery of the through-hole  26  is irradiated with laser beams LB. If a portion between two adjacent through-holes  26  in the template  20  is excessively irradiated with laser beams LB, a problem occurs that the surface of the template  20  may be deformed. It is preferable to use the excimer laser. An intensity of energy emitted by the YAG laser is so high that it is not appropriate when fine perforation is to be conducted. The CO 2  laser is not appropriate for processing a glass plate. 
     In this example, the template  20  is irradiated with laser beams LB after the mask  44  has been set on it. In the case where the diameter of the through-hole  26  is  24  μm, the diameter of the opening of the mask  44  is 26 μm. In other words, it is preferable that laser beams LB is irradiated at a region larger than the diameter of the through-hole  26  by 2 μm. In this example, the output condition of the laser was as follows: when the thickness of the template  20  is 300 μm, the output is 300 mJ/200 Hz, and the number of pulses is  1000 . Also, krypton gas and fluorine gas are used. In this case, He gas or Ne gas is added as an assist gas. As a pretreatment, carbon (C) is vapor-deposited to the glass plate. 
     FIG. 3 shows the end portion (a portion of the through-hole  26  close to the surface of the template  20  for mounting metallic balls) of the through-hole  26  which has been made smooth by the irradiation of laser beams LB. Burr  42  shown in FIG.  1 (B) in the through-hole  26  formed when etching is conducted-is melted away and the surface of the through-hole  26  becomes smoother. In this connection, a state in which the burr  42  is formed in the through-hole  26  and also a state in which the burr  42  is removed from the through-hole  26 , as shown in FIG. 3, can be seen by the microscopic observation and. microphotograph. 
     In FIG. 1A, the diameter of the through-hole  26  in the template  20  is substantially uniform, but FIG. 4 shows an example in which the through-hole  26  in the template  20  is tapered. When the through-hole  26  is formed in the template  20  by etching, the through-hole  26  is frequently tapered, as shown in FIG.  5 . Burrs are caused also in this case too, so the side wall of the through-hole  26  is irradiated with laser beams LB to smooth the side wall of the through-hole. In this case, one end portion of the through-hole  26 , the diameter of which is smaller than that of the other end portion, is irradiated with laser beams LB, and in use, this end portion irradiated with laser beams LB is directed toward the metallic ball  16 . 
     In the above example, the through-holes  26  in the template  20  for mounting metallic balls are formed by etching, but it is possible to form the through-holes  26  by machining such as electric discharge drilling, and after the formation of the through-holes  26 , they are irradiated with laser beams LB. 
     FIG. 9 shows the second embodiment of the present invention. In this embodiment, the template  20  for mounting metallic balls comprises an inorganic material layer  50  and an organic material layer  52  laminated on each other, and the through-holes (only one shown in FIG. 9)  26  ( 50   a ,  52   a  ) to attract and support the metallic balls  16  are formed through the inorganic material layer  50  and the organic material layer  52 . The organic material layer  52  is bonded to the inorganic material layer  50  by prepreg or adhesive. 
     The inorganic material layer  50  comprises a glass plate, and the through-holes  50   a  are formed by etching. The inorganic material layer  50  provides a necessary high mechanical strength to the template  20  for mounting metallic balls. The organic material layer  52  is made of heat-resistant plastics such as polyimide, fluorine resin of PTFE, and nylon. The smooth, through-holes  50   a  can be easily formed in the organic material layer  52  by laser beam processing, compared with the case in which the through-holes  50   a  are formed in the inorganic material layer  50 . In use, the through-holes  52   a  are directed to the metallic balls  16 . 
     Accordingly, the mechanical strength of the template  20  comprising the inorganic material layer  50  and the organic material layer  52  is sufficiently high, the metallic balls  16  do not bite into the through-holes  26 , and it is possible to reliably form metallic bumps in the electronic part, using this template  20  for mounting metallic balls. In this connection, the inorganic material layer  50  and the organic material layer  52  may be bonded together after the through-holes  50   a ,  52   b  have been respectively formed, or alternatively the through-hole  26  may be formed after both layers have been bonded together. The surface of the organic material layer  52  may be coated with a film for preventing the generation of static electricity. 
     FIG. 10 shows a variation of the template  20  shown in FIG.  9 . The through-holes  26  in this template  20  are tapered. Except for this point, the template  20  shown in FIG. 10 is the same as that shown in FIG.  9 . 
     FIGS. 11 to  13  show the third embodiment of the present invention. As shown in FIG. 11, the template  20  comprises an air permeable material layer  60  and a resin layer  62  laminated on each other, and the through-holes (only one shown in FIG. 11)  62   a  is formed in the resin layer  62 . The air permeable material layer  60  and the resin layer  62  are bonded to each other. The air permeable material layer  60  is made of a porous material having a sufficient mechanical strength, or a fibrous material such as glass fiber, so that, air can pass through the air permeable material layer  60 . In the structure shown in FIG. 11, the air permeable material layer  60  comprises a plate of a porous material, and fine holes  61  are formed in the plate. 
     FIG. 12 shows the air permeable material layer  60  of the template  20  for mounting metallic balls. An unpermeable holding member  64  is arranged on the surface of the air permeable material layer  60  at the center thereof on the opposite side to the resin layer  62 . 
     FIG. 13 shows the suction head  22 . The suction head  22  includes suction holes  22   a , a suction groove  22   b , and a plate suction hole  22   c . The plate suction hole  22   c  is arranged at the center of the suction head  22  to attract the holding member  64  provided on the air permeable material layer  60 , to thereby attract the entire template  20 . The suction groove  22   b  is formed in the annular shape around the plate suction hole  22   c , and the suction holes  22   a  are open to the inside of the suction groove  22   b . The through-holes  62   a  in the resin layer  62  are arranged in the region of the suction groove  22   b.    
     Accordingly, the mechanical strength of the template  20  comprising the air permeable material layer  60  and the resin layer  62  is sufficiently high, and the metallic balls  16  do not bite into the through-holes  62   a , so that it is possible to reliably form metallic bumps on the electronic part, using this template  20  for mounting metallic balls. In this connection, the air permeable material layer  60  and the resin layer  62  may be bonded to each other after the through-holes  62   a  have been formed, or alternatively the through-hole  62   a  may be formed after both layers have been bonded to each other. 
     FIGS. 14A to  14 C show an example in which the through-holes  62   a  are formed after the air permeable material layer  60  and the resin layer  62  are bonded onto each other. FIG. 14A shows a state in which the surface of the air permeable material layer  60  is coated or impregnated with an ultraviolet-ray curable resin  62   p . FIG. 14B shows a state in which a mask  66  is formed on the ultraviolet-ray curable resin  62   p  at positions where the through-holes  62   a  are to be formed and the ultraviolet-ray curable resin  62   p  is irradiated with ultraviolet rays (UV). FIG. 14C shows a state in which an unmasked portion of the ultraviolet-ray curable resin  62   p  exposed to ultraviolet-rays is hardened, and a masked portion of the ultraviolet-ray curable resin  62   p  not exposed to ultraviolet-rays is not hardened, the unhardened portion being removed by solvent, so that the resin layer  62  having the through-holes  62   a  is formed. 
     FIG. 15 shows an example in which the air permeable material layer  60  and the resin layer  62  are bonded onto each other after the through-holes  62   a  have been formed in the present layer  62 . 
     According to the present invention, it is possible to provide a template for mountion metallic balls, having through-holes, the surfaces of which are smooth and no burr is caused on the surface. Therefore, metallic bumps can be easily and reliably formed on the electronic part. Further, the reliability of the connecting sections can be enhanced, and the structure of the metallic bump forming apparatus and also the structure of the electronic part joining machine can be simplified. Furthermore, quality and productivity of the metallic bumps can be enhanced by the present invention.