Patent Publication Number: US-2009239091-A1

Title: Method for printing a metal paste, metal mask, and bump forming method

Description:
This application is based on Japanese patent application NO. 2008069324, the content of which is incorporated hereinto by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a method for printing a metal paste, a metal mask, and a bump forming method. 
     2. Related Art 
     It is known that as electrode terminal formation in a flip chip BGA, a chip-size package, or the like, bump formation by a metal paste printing method using a metal mask is performed. The bump formation by the metal paste printing method uses a metal mask having a hole at a position corresponding to an electrode terminal formation position to fill the hole with a metal paste. More specifically, in order to fill the hole with the metal paste, a metal powder and a flux need to be mixed with each other to form a paste. 
       FIGS. 7A and 7B  shows a metal mask on a semiconductor wafer used when bump formation is performed by a metal paste printing method on a semiconductor wafer having a flat surface to be printed. 
     Japanese Laid-open patent publication NO. 2000-062136 describes that, as shown in  FIGS. 8A to 8E , when a solder paste is filled on an electrode formed on a bottom of a recessed portion, an opening size of a metal mask is made smaller than an opening size of the recessed portion to secure an interval between the metal mask and the substrate. It is described that in this manner air generated in filling of the solder paste is pushed out of the recessed portion from a gap near an edge of the recessed portion. 
     Techniques related to this include those described in Japanese Laid-open patent publication NO. 2002-118347, Japanese Laid-open patent publication NO. 2002-353263, Japanese Laid-open patent publication NO. 2006-148146, and Japanese Laid-open patent publication NO. 1999-040938. 
     SUMMARY 
     However, in the method described by using  FIGS. 7A  to  8 E, a pressure or the like generated in filling of a solder paste causes a flux to ooze from the inside of the filled solder paste, and the flux covers the solder paste. In  FIGS. 7A and 7B , since a surface to be printed is flat, a wafer upper surface and a metal mask lower surface are easily brought into tight contact with each other. For this reason, the flux remains in a through hole while the flux covers the surface of the solder paste. Therefore, in the method described with reference to  FIGS. 7A to 8E , when residual air is present in the filled solder paste, a deaeration path for the residual air is clogged with the flux, and the solder paste is incompletely filled disadvantageously. 
     In one embodiment, there is provided a method for printing a metal paste including: preparing a substrate having a surface on which an electrode is formed; arranging, to locate a through hole of a metal mask having the through hole on the electrode, the metal mask on the substrate and forming a gap portion communicating with the through hole in an interface between the substrate and the metal mask; and filling a metal paste containing a flux in the through hole, wherein the gap portion is formed by an upper surface of the substrate and a recessed portion formed in a lower surface of the metal mask, and, in the filling a metal paste, the flux is guided to the gap portion together with air in the through hole. 
     In another embodiment, there is provided a bump forming method using a method for printing a metal paste including: preparing a substrate having a surface on which an electrode is formed; arranging, to locate a through hole of a metal mask having the through hole on the electrode, the metal mask on the substrate and forming a gap portion communicating with the through hole in an interface between the substrate and the metal mask; and filling a metal paste containing a flux in the through hole, wherein the gap portion is formed by an upper surface of the substrate and a recessed portion formed in a lower surface of the metal mask, and, in the filling a metal paste, the flux is guided to the gap portion together with air in the through hole. 
     The method for printing a metal paste includes arranging, to locate a through hole of a metal mask having the through hole on the electrode, the metal mask on the substrate and forming a gap portion communicating with the through hole in an interface between the substrate and the metal mask, in the filling the metal paste, the flux is guided to the gap portion together with air in the through hole. According to this method, flux oozing on the surface of the metal paste can be moved to the gap portion in filling of the metal paste. In other words, a deaeration path for residual air in the metal paste clogged with the flux is secured by removing the flux. In this manner, air remaining in the metal paste in the through hole can be removed, and a filling rate of the metal paste can be increased. 
     In another embodiment, there is provided a metal mask used in a metal paste printing method which fills a metal paste containing a flux in a through hole of a metal mask arranged on an object to be printed to perform printing, wherein at least one surface of the metal mask has a recessed portion which forms a gap portion communicating with the through hole in an interface between the metal mask and the object to be printed, and the gap portion is configured to guide the flux together with air in the through hole. 
     In the metal mask, at least one of the surfaces of the metal mask has the recessed portion which forms the gap portion communicating with the through hole on the interface between the metal mask and the object to be printed, and the gap portion is configured to guide the flux together with the air in the through hole. According to the metal mask having the structure, in filling of the metal paste, the flux oozing on the surface of the metal paste can be moved to the gap portion. In other words, a deaeration path for residual air in the metal paste clogged with the flux is secured by removing the flux. In this manner, air remaining in the metal paste in the through hole can be removed, and a filling rate of the metal paste can be increased. 
     According to the present invention, a method for printing a metal paste and a metal mask which are better suited for improving a filling rate of a meal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  are a cross-sectional view and a plan view showing a first embodiment of a metal mask according to the present invention; 
         FIGS. 2A and 2B  are a cross-sectional view and a plan view for explaining a metal mask and a solder paste printing method according to the present embodiment; 
         FIGS. 3A and 3B  are a cross-sectional view and a plan view for explaining a metal mask and a solder paste printing method according to the present embodiment; 
         FIGS. 4A and 4B  are a cross-sectional view and a plan view for explaining a metal mask and a solder paste printing method according to the present embodiment; 
         FIGS. 5A and 5B  are a cross-sectional view and a plan view showing a second embodiment of a metal mask according to the present invention; 
         FIGS. 6A and 6B  are a cross-sectional view and a plan view showing a third embodiment of a metal mask according to the present invention; 
         FIGS. 7A and 7B  are a sectional view and a plan view showing a conventional metal mask; and 
         FIGS. 8A to 8E  are cross-sectional views for explaining a conventional metal mask and steps in a conventional metal paste printing method. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     Preferred embodiments of a method for printing a metal paste, a bump forming method, and a metal mask according to the present invention will be described below with reference to the accompanying drawings. The same reference numerals as in the description of the drawings denote the same parts in the description, and the description will not be repeated. 
     First Embodiment 
       FIGS. 1A and 1B  are a cross-sectional view and a plan view showing a first embodiment of a metal mask according to the present invention. The cross-sectional view shown in  FIG. 1A  shows a section along an I-I line shown in the plan view in  FIG. 1B . 
     A metal mask  13  has, on at least one surface of the metal mask  13 , a recessed portion  113  which forms a gap portion  13   a  communicating with a through hole  10  between the metal mask  13  and a substrate  11 . The gap portion  13   a  is configured to guide a flux together with air in the through hole  10 . 
     As shown in  FIGS. 2A and 2B , the metal mask  13  is used in a method for printing a metal paste which fills a solder paste  14  containing a flux in the through hole  10  of the metal mask  13  arranged on the substrate  11 . 
     The substrate  11  is an object to be printed and has an upper surface which is a surface to be printed. An electrode  12  is formed on the substrate  11 . 
     The through hole  10  is an opening reaching from an upper surface (printing surface) of the metal mask  13  to a lower surface (surface of the object to be printed) of the metal mask  13 . The solder paste  14  is filled in the through hole  10 . The through hole  10  may have a tapered shape in a cross-sectional view. In this manner, the solder paste  14  is printed on the electrode  12 . Thereafter, a bump  19  shown in  FIGS. 4A and 4B  can be formed. 
     As shown in  FIG. 1A , the gap portion  13   a  is configured to guide a flux together with air in the through hole  10 . More specifically, the gap portion  13   a  is formed in an interface between the substrate  11  and the lower surface of the metal mask  13 . More specifically, the gap portion  13   a  is formed by the upper surface of the substrate  11  and a recessed portion  133  formed on the lower surface of the metal mask  13 , and a gap is secured in the interface between the upper surface of the substrate  11  and the recessed portion  133 . In other words, the gap portion  13   a  can be secured in the interface between the substrate  11  and the metal mask  13  by the recessed portion  133  of the metal mask  13 . Furthermore, two through holes  10  communicate with each other through the gap portion  13   a.  In other words, the recessed portion  113  (step) communicating with the through hole  10  is formed in a lower surface (surface of the object to be printed) of the metal mask  13 . When the solder paste  14  is filled from an upper surface of the metal mask  13 , a flux oozing from the solder paste  14  due to its weight or pressure can be moved to the gap portion  13   a.  Residual air  16  (see  FIG. 4A ) remaining in the solder paste  14  can also be moved to the gap portion  13   a.    
     The height of the gap portion  13   a  is preferably smaller than the diameter of a solder particle included in the solder paste  14 . In this manner, at least one of the air in the flux of the filled solder paste  14  and the air in the through hole  10  is removed to make it possible to increase a filling rate. Since the number of filled solder particles is stable, accuracy of the outside dimension of the bump  19  formed thereafter can be improved (see  FIGS. 4A and 4B ). 
     The diameter of the solder particle can be set to various values depending on sizes and pitches of the electrodes  12  on the object to be printed. For example, when a pad pitch of inner bumps on a flip chip is about 200 μm, the diameter of the solder particle can be set to 5 to 15 μm. When the pitch is relatively large, the diameter can be set to 15 to 25 μm. 
     A soldering material is not limited to a specific material. However, for example, a Pb-free solder, an eutectic solder, a high-temperature solder, or the like can be used. As the Pb-free solder, an SnAgCu-based alloy can be used. As the eutectic alloy or the high-temperature solder, a PbSn-based solder having variable Pb/Sn ratios can be used. 
     In the present embodiment, the SnAgCu-based solder was used, and the diameter of a solder particle was set to 5 to 15 μm. 
     As shown in  FIG. 1B , on a lower surface of the metal mask  13 , the through hole  10  is formed, and the recessed portion  113  (step) is formed throughout the circumference of the opening of the through hole  10 . Furthermore, the recessed portion  113  radially extending from the circumference of the opening of the through hole  10  is formed to cause the through holes  10  to communicate with each other. By the recessed portion  113 , the gap portion  13   a  is formed in an interface between the substrate  11  and the metal mask  13 . More specifically, the gap portion  13   a  is formed to cause the plurality of through holes  10  to communicate with each other. 
     In this manner, a large-volume interval can be secured in an interface between the substrate  11  and the surface to be printed of the metal mask  13 . 
     The solder paste  14  contains a flux and solder particles. The flux makes it possible to print and fill the solder paste  14 . The type of the flux is not limited. The flux oozes from the inside of the solder paste  14  onto the surface of the solder paste  14  in printing or filling of the solder paste  14  due to the pressure of the printing or the filling to cover the surface of the filled solder paste  14 . In this manner, a deaeration path of air remaining in the solder paste  14  is clogged. 
     A method for printing a metal paste which forms the bump  19  on an upper surface of the electrode  12  on the substrate  11  will be described below with reference to  FIGS. 1A to 4B  are cross-sectional views and plan views for explaining a metal mask and a solder paste printing method according to the present embodiment. The cross-sectional views in  FIGS. 2A ,  3 A and  4 A show sections along a II-II line to a IV-IV line shown in the plan views in  FIGS. 2B ,  3 B and  4 B. 
     The solder paste printing method according to the present embodiment includes: the step of preparing the substrate  11  having a surface on which the electrode  12  is formed; the step of arranging, to locate the through hole  10  of the metal mask  13  having the through hole  10  on the electrode  12 , the metal mask  13  on the substrate  11  and forming the gap portion  13   a  communicating with the through hole  10  in an interface between the substrate  11  and the metal mask  13 ; and the step of filling the solder paste  14  containing a flux in the through hole  10 . The gap portion  13   a  is formed by the upper surface of the substrate  11  and the recessed portion  133  formed in the lower surface of the metal mask  13 . In the step of filling the solder paste  14 , the flux is guided to the gap portion  13   a  together with air in the through hole  10 . 
     The air in the through hole  10  includes air in the through hole  10  generated by the filled solder paste  14  and the residual air  16  remaining in the solder paste  14  filled in the through hole  10 . A bump forming method according to the present embodiment is realized by using the solder paste printing method according to the present embodiment. 
     Details will be described below. 
     As shown in  FIGS. 1A and 1B , the substrate  11  having a surface on which the electrode  12  is formed is prepared. Subsequently, the metal mask  13  is arranged on the substrate  11  to locate the through hole  10  of the metal mask  13  having the through hole  10  on the electrode  12 , and the gap portion  13   a  communicating with the through hole  10  is formed in the interface between the substrate  11  and the metal mask  13 . 
     As shown in  FIGS. 2A and 2B , the solder paste  14  is moved from an upper surface of the metal mask  13  by using a squeegee  15  in the direction indicated by an arrow  17  (printing direction), so that the solder paste  14  containing a flux is filled in the through hole  10 . 
     At this time, the solder paste  14  is filled in the through hole  10  in the direction opposite to the direction of the arrow  17  (printing direction). For this reason, air in the through hole  10  is moved by the filled solder paste  14  in the direction indicated by an arrow  18  and pushed into the gap portion  13   a  communicating with the through hole  10  formed in the interface between the substrate  11  and the metal mask  13 . Furthermore, since a pressure acts on the inside of the through hole  10  due to a pressure generated in printing, a flux contained in the solder paste  14  oozes out of the solder paste  14  and is pushed from the through hole  10  into the gap portion  13   a.  In this manner, the residual air  16  in the solder paste  14  is pushed into the gap portion  13   a  without clogging the deaeration path for the residual air  16 . 
     As shown in  FIGS. 3A and 3B , the metal mask  13  is separated from the substrate  11  on which the solder paste  14  is printed. In this manner, the solder paste  14  can be printed on the substrate  11 . 
     Subsequently, by using the solder paste printing method according to the present embodiment, as shown in  FIGS. 4A and 4B , the solder paste  14  is heated and melted to form the bump  19 . 
     An effect of the present embodiment will be described below. 
     The solder paste printing method includes the step of arranging the metal mask  13  on the substrate  11  to locate the through hole  10  of the metal mask  13  having the through hole  10  on the electrode  12  and forming the gap portion  13   a  communicating with the through hole  10  in the interface between the substrate  11  and the metal mask  13 . In the step of filling the solder paste  14 , the flux is guided to the gap portion  13   a  together with the air in the through hole  10 . 
     In the metal mask  13  used in the method for printing a metal paste which fills the solder paste  14  in the through hole  10  and prints the solder paste  14  on the substrate  11 , at least one surface of the metal mask  13  has a recessed portion which forms the gap portion  13   a  communicating with the through hole  10  between the metal mask  13  and the substrate  11 . The gap portion  13   a  is configured to guide the flux together with the air in the through hole  10 . 
     According to the solder paste printing method and the metal mask  13  having the above configurations, in filling of the solder paste  14 , the flux oozing on the surface of the solder paste  14  can be moved to the gap portion  13   a.  In other words, the deaeration path for the residual air  16  in the solder paste  14  clogged with the flux is secured by removing the flux. In this manner, air remaining in the solder paste  14  in the through hole  10  can be removed, and a filling rate of the solder paste  14  can be increased. Air which is present in the through hole  10  in filling of the solder paste  14  can also be moved to the gap portion  13   a  by the filled solder paste  14 . 
     By using the solder paste printing method described above, a bump forming method which is suitable for formation of a bump having high connection reliability and good outer dimension accuracy is realized. 
     Japanese Laid-open patent publication NO. 2000-062136 discloses that, when a solder paste is filled on an electrode formed on a bottom of a recessed portion, an opening size of the metal mask is made smaller than an opening size of the recessed portion to secure an interval between the metal mask and the substrate. It is described that in this manner air in filling of the solder paste can be pushed out from a gap above a portion near an edge of the recessed portion. However, in order to form a deaeration path for air, a mask having a small opening size is disadvantageously further required above the opening of the through hole. In contrast to this, in the present embodiment, since the metal mask  13  having the recessed portion  113  communicating with the through hole  10  is used, a deaeration path for air is secured in the interface between the metal mask  13  and the substrate  11 . In the present embodiment, deaeration is not performed from an upper side of the through hole  10 , and the gap portion  13   a  communicating with the through hole  10  is formed between the metal mask  13  and the substrate  11  to secure a deaeration path above the through hole  10 . For this reason, even when a pressure acts from the upper side in printing, the deaeration path can be secured. 
     Since the solder paste printing method is not limited to a specific method, a filling rate of a metal can be improved without increasing the number of steps of forming a bump. Design of an apparatus for forming a bump is not required to be changed. 
     Second Embodiment 
       FIG. 5A  is a sectional view showing a second embodiment of a metal mask according to the present invention. In the first embodiment, the gap portion  13   a  is formed to cause the through holes  10  to communicate with each other. In the present embodiment, the through holes  10  do not communicate with each other. 
     As shown in  FIG. 5A , gap portions  13   b  are formed for two through holes  10 , respectively, and do not communicate with each other. More specifically, the through holes  10  do not communicate with each other. As shown in  FIG. 5B , on a lower surface of a metal mask  23 , the through hole  10  and a recessed portion  123  (step) extending throughout the circumference of an opening of the through hole  10  are formed. By the recessed portion  123 , the gap portion  13   b  is formed in an interface between the substrate  11  and the metal mask  23 . 
     The solder paste printing method using the metal mask  23  with the above configuration can be performed by the same manner as described in the first embodiment. The bump  19  using the metal mask  23  with the configuration can be formed in the same manner as in the case in which the metal mask  13  is used. 
     In the present embodiment, the gap portion  13   b  (recessed portion  123 ) does not communicate with the gap portion  13   b  formed to communicate with another through hole  10 . For this reason, movement of the flux generated from one through hole  10  can be limited to the inside of the gap portion  13   b.  Another effect of the present embodiment is the same as that of the above-described embodiment. 
     Third Embodiment 
       FIGS. 6A and 6B  are cross-sectional view and plan view showing a third embodiment of a metal mask according to the present invention. In the second embodiment, the gap portion  13   b  is formed throughout the circumference of the opening of the through hole  10 . In the present embodiment, the gap portion  13   c  is formed in a part of the circumference of the opening of the through hole  10 . 
     As shown in  FIG. 6A , the gap portion  13   c  is formed in a part of the circumference of the opening of the through hole  10 . As shown in  FIG. 6B , the through hole  10  and the recessed portion  133  (step) in a part of the circumference of the opening of the through hole  10  are formed on a lower surface of a metal mask  33 . More specifically, the gap portion  13   c  is formed in a part of the circumference of the through hole  10  on a side (direction indicated an arrow  18 ) opposite to the arrow  17  indicating a printing direction. The gap portion  13   c  only has to be a part of the circumference of the through hole  10 , or ¼, a half, or ¾ of the circumference. By the recessed portion  133 , the gap portion  13   c  is formed in the interface between the substrate  11  and the metal mask  33 . 
     The solder paste printing method using the metal mask  33  with the above configuration can be performed by the same manner as that described in the first embodiment. The bump  19  using the metal mask  33  with the above configuration can be formed in the same manner as that in the case in which the metal mask  13  is used. 
     In the present embodiment, the gap portion  13   c  is formed in a part of the circumference of the through hole  10  on a side opposite to the printing direction. For this reason, moving directions of the flux and the air can be specified by a forming position of the gap portion  13   c  (recessed portion  133 ). Other effects of the present embodiments are the same as those in the above-described embodiments. 
     The solder paste printing method and the metal mask according to the present invention are not limited to the above-described embodiments, and can be variously modified. 
     For example, in the above-described embodiments, in a cross-sectional view, the gap portion  13   a  extends in a horizontal direction. However, the gap portion  13   a  may be formed in a vertical direction. In this case, the gap portion  13   a  is preferably a longitudinally long slit shape, and a slit width is preferably smaller than a solder particle diameter. 
     For example, in the descriptions of the above-described embodiments, the gap portion  13   a  communicating with the through hole  10  is formed between the substrate  11  and the metal mask  13  as a recessed portion formed in the lower surface of the metal mask  13 . A gap portion may be formed by forming a projecting portion on the lower surface of the metal mask  13 . In this case, a portion on which the projecting portion is not formed corresponds to the gap portion  13   a.    
     In the descriptions of the above-described embodiments, the printing method uses a solder paste. However, the paste is not limited to the solder. The number of the through holes  10 , arrangement positions of the through holes  10 , and the shapes of the through holes  10  are not limited to specific ones. The shapes of the gap portions  13   a,    13   b,  and  13   c  are riot limited to the shapes described above. 
     It is apparent that the present invention is not limited to the above-described embodiment, and may be modified and changed without departing from the scope and spirit of the invention.