Abstract:
Disclosed is a bump designed to certainly moderate a thermal stress applied between a semiconductor device and a printed circuit board and enhance the strength of a bonding portion without use of a sealing resin, and a method of manufacturing the bump. The bump includes a relatively elastic first ball bump formed on an electrode pad provided on a semiconductor device; and a second ball bump formed in such a manner as to be overlapped on the first ball bump at least in the direction perpendicular to the electrode pad. The second ball bump is different in material or composition from the first ball bump and is adapted to be in contact with an eutectic solder pre-coated on a connection land of a printed circuit board.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to bumps formed on connection terminals of a semiconductor substrate for mounting a semiconductor device such as a semiconductor IC, and a method of manufacturing the bumps. 
     In recent years, as portable electronic equipment such as a digital video camera, a digital portable telephone, or a notebook type personal computer has been widespread, there has been a strong demand toward miniaturization, thinning and lightweightness of the portable electronic equipment. 
     To realize the miniaturization, thinning and lightweightness of the portable electronic equipment, it is important to improve the mounting density of parts. 
     In particular, with respect to a semiconductor device such as a semiconductor IC, a high density mounting technique using a flip-chip type semiconductor device has been developed and practically used in place of a conventional package type semiconductor device. 
     For example, mounting of such a flip-chip type semiconductor device (flip-chip mounting) is carried out by forming solder ball bumps on A 1  electrode pads provided on a semiconductor IC, and bringing the solder ball bumps of the semiconductor IC in a chip state into contact with connection terminals of a printed circuit board, to thereby directly mount the IC chip on the printed circuit board. 
     The above solder ball bumps are manufactured by making use of an electroplating process. With this process, the thickness of a solder layer formed by electroplating is affected by the surface state of an underlying layer and a slight variation in electric resistance. This causes a problem that it is basically difficult to form solder ball bumps having uniform heights in one IC chip. 
     On the contrary, as a method of manufacturing solder ball bumps in such a manner as to suppress a variation in height of the solder ball bumps, there has been known a method of forming a pattern of solder ball bumps by making use of film formation due to vacuum vapor deposition and lift-off of a photoresist film. 
     The latter method will be described with reference to FIGS. 4A to  4 E. 
     Referring first to FIG. 4A, an electrode portion  1   a  of a flip-chip type semiconductor IC  1  is formed as follows: 
     An electrode pad  3  typically made from an Al—Cu alloy is formed on a semiconductor substrate  2  typically made from silicon by sputtering, etching and the like. A surface protective film  4  typically made from silicon nitride or polyimide is formed over the entire surface of the semiconductor substrate  2  in such a manner as to cover the electrode pad  3 . An opening  4   a  is formed in the surface protective film  4  at a region of the electrode pad  3 . A metal multi-layer film  5  typically made from Cr, Cu, Au and the like, which is called a BLM (Ball Limiting Metal) film, is formed by typically sputtering in such a manner as to cover the surface of the electrode pad  3  exposed to the side surface and the bottom of the opening  4   a.  The electrode portion la is composed of these electrode pad  3 , surface protective film  4 , opening  4   a,  and BLM film  5 . 
     To form a solder ball bump on the electrode portion  1   a  which is provided on the semiconductor IC  1  as described above, as shown in FIG. 4B, a resist film  6  having an opening  6   a  at a region of the BLM film  5  is formed. 
     Referring to FIG. 4C, a solder vapor deposition film  7  is formed on the entire surface of the semiconductor substrate  2  via the resist film  6 . 
     Referring to FIG. 4D, the resist film  6  is lifted-off, to remove an unnecessary portion of the solder vapor deposition film  7 , whereby the solder vapor deposition film having a specific pattern is formed. 
     Referring to FIG. 4E, the semiconductor substrate  2  is subjected to heat-treatment, to melt the solder of the solder vapor deposition film  7 , whereby an approximately spherical solder ball bump  7   a  is formed due to the surface tension of the solder. 
     In general, the formation of the solder ball bumps  7   a  are performed for the semiconductor ICs  1  left as a semiconductor wafer state, that is, in the state before the semiconductor wafer is cut into the individual semiconductor ICs  1 . 
     After the solder ball bumps  7   a  are formed on respective electrode portions  1   a  provided on the semiconductor ICs  1 , the wafer-like semiconductor substrate  2  is cut into the individual chip-like semiconductor ICs  1  typically by dicing. 
     Referring to FIG. 5, the solder ball bumps  7   a  of each semiconductor IC  1  are brought into contact with lands  8   a  as connection portions typically made from Cu formed on a printed circuit board  8  to which the semiconductor IC  1  is to be mounted. 
     The surface of the printed circuit board  8  is covered with a solder resist  8   b  excluding the lands  8   a,  and regions of the lands  8   a  are pre-coated with an eutectic solder film  8   c.    
     At a reflow step, the eutectic solder film  8   c  is melted. The melted eutectic solder permeates between the solder ball bumps  7   a  and the lands  8   a,  and cooled and hardened, whereby each ball bump  7   a  is soldered to the land  8   a,  that is, it is electrically connected thereto. 
     The above solder bonding between the solder ball bump  7   a  and the land  8   a  suffers thermal stress due to environmental temperature change because the thermal expansion coefficient of the semiconductor substrate  2  of the semiconductor IC  1  is different from that of the printed circuit board  8 . 
     To be more specific, the thermal expansion coefficient of silicon forming the semiconductor substrate  2  is as small as 3.4 ppm/° C., while the thermal expansion coefficient of a glass reinforced epoxy resin based substrate generally used as the printed circuit board  8  is as large as about 15 ppm/° C. Accordingly, when thermal stress is repeatedly applied to the solder bonding portion between the solder ball bump  7   a  and the land  8   a  due to a temperature difference caused by on/off operation of the semiconductor IC  1 , cracking may occur at the solder bonding portion, and at the worst case, the cracking leads to breakage, to cause electric disconnection at the solder bonding portion, thereby inducing a so-called breakage failure. That is to say, the above-described structure has a problem associated with the reliability of solder bonding. 
     To suppress breakage of the solder bonding portion due to thermal stress, there has been generally adopted a method in which a sealing resin  9  is injected between the semiconductor IC  1  and the printed circuit board  8  as shown in FIG.  5 . In this method, the entire sealing resin  9  sustains the above-described thermal stress, to moderate the thermal stress applied to the solder bonding portion, thereby enhancing the strength against the thermal stress. 
     In the above method using the sealing resin  9 , the semiconductor IC  1  is integrally fixed on the printed circuit board  8  by the sealing resin  9 . Accordingly if the semiconductor IC  1  fails, the printed circuit board  8  on which the failed semiconductor IC  1  is mounted is exchanged as a whole, then discarded as a defective or the failed semiconductor IC  1  is forcibly peeled from the printed circuit board  8  by applying an external chemical or mechanical force thereto. 
     The former exchange of the printed circuit board  8  as a whole causes a problem in raising the cost while the latter forced peeling of the semiconductor IC  1  causes a problem in damaging the printed circuit board  8 . 
     In this way, the method using the sealing resin  9  is disadvantageous in that if the semiconductor IC  1  fails, the exchange work, that is, the rework of the failed part cannot be easily performed. This is one reason why flip-chip mounting has been not widespread. 
     The method using the sealing resin  9  has another problem. With a tendency toward a so-called narrow pitch accompanied by miniaturization of a semiconductor device, the runabout of the sealing resin  9  having been injected between the semiconductor IC  1  and the printed circuit board  8  becomes poor, with a result that it is difficult to realize perfect injection of the sealing resin  9 . The sealing resin  9  thus imperfectly injected fails to sufficiently moderate the thermal stress applied to the solder bonding portion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a bump designed to certainly moderate a thermal stress applied between a semiconductor device and a printed circuit board and enhance the strength of a bonding portion without use of a sealing resin, and to provide a method of manufacturing the bump. 
     To achieve the above object, according to a first aspect of the present invention, there is provided a bump or solder bump including: a relatively elastic first ball bump or first solder ball bump formed on an electrode pad provided on a semiconductor device; and a second ball bump or second solder ball bump formed in such a manner as to be overlapped on the first ball bump or first solder ball bump at least in the direction perpendicular to the electrode pad, the second ball bump or second solder ball bump being different in material or composition from the first ball bump or first solder ball bump and being adapted to be in contact with a solder pre-coated on a connection land of a printed circuit board. 
     To achieve the above object, according to a second aspect of the present invention, there is provided a method of manufacturing a bump or solder bump, including: a first step of forming a relatively elastic first ball bump or first solder ball bump on an electrode pad provided on a semiconductor device; and a second step of forming a second ball bump or second solder ball bump on the first ball bump or first solder ball bump at least in the direction perpendicular to the electrode pad, the second ball bump or second solder ball bump being different in material or composition from the first ball bump or first solder ball bump and being adapted to be in contact with a solder pre-coated on a connection land of a printed circuit board. 
     In the bump or solder bump and the manufacturing method thereof according to the present invention, a core portion of the bump or solder bump is formed of the relatively elastic first ball bump or first solder ball bump, and accordingly even if there occurs a thermal stress due to a difference in thermal expansion coefficient between a silicon chip constituting the semiconductor substrate on which the semiconductor device is provided and the printed circuit board, the thermal stress is moderated by the elastic deformation of the bump or solder bump. 
     Further, a portion of the bump or solder bump, which is to be brought into contact with the solder pre-coated on the connecting land of the printed circuit board, is configured by the second ball bump or second solder ball bump different in material or composition from the first ball bump or first solder ball bump, and accordingly if the second ball bump or second solder ball bump is made from a solder having a good wettability to the solder pre-coated on the connecting land, the bump or solder bump can be certainly bonded to the connecting land. 
     Accordingly, upon mounting of the semiconductor device on the printed circuit board, the thermal stress is moderated and the bonding strength is enhanced. This makes it possible to significantly reduce a mounting failure and to improve the reliability of the bonding portion of the bump or solder bump. 
     The manufacturing method of the present invention, preferably, further includes a third step of forming a resin layer on the surface, on the electrode pad side, of the semiconductor device in such a manner as to cover the first ball bump or first solder ball bump. With this configuration, since a relatively weak root portion of the ball bump or solder ball bump, that is, the first ball bump or first solder ball bump and its neighborhood are reinforced by the resin, the thermal stress is moderated by the resin. 
     Since the formation of the resin layer is performed before mounting of the semiconductor device on the printed circuit board, it is not required to inject the resin between the semiconductor device and the printed circuit board after mounting of the semiconductor device, resulting in the improved productivity. Further, since the resin layer is not brought into contact with the printed circuit board, if the semiconductor device fails after mounting thereof, the failed semiconductor device can be easily removed from the printed circuit board, to facilitate the rework, thereby improving the reliability of the bump or solder bump. 
     Since the resin layer is formed on the semiconductor device before the semiconductor device is mounted on the printed circuit board, even if the semiconductor device has a narrow pitch structure for enhancing the level of integration, it is possible to certainly fill spaces between the bumps or solder bumps with the resin, and hence to certainly fixedly hold the bumps or solder bumps with the resin. 
     In the manufacturing method of the present invention, preferably, the thickness of the resin layer is in a range of the height of the first ball bump or first solder ball bump or less. With this configuration, the amount of the resin which inevitably remains on the top portion of the first ball bump or the first solder ball bump at the manufacturing step, can be minimized. As a result, it is possible to eliminate the inconvenience that the electrical conduction between the first ball bump or first solder ball bump and the second ball bump or second solder ball bump is obstructed by the remaining resin. 
     In the manufacturing method of the present invention, preferably, the third step is performed after the first step and a fourth step of polishing a surface portion of the first solder ball bump projecting from the surface of the resin layer is performed after the third step. With this configuration, the resin component remaining on the top portion of the first solder ball bump and its neighborhood is removed by polishing, and also the surface of the top portion of the first solder ball bump is plarized by polishing to become a clean plane. 
     Accordingly, the bonding characteristic between the first solder ball bump and the second solder ball bump is improved, to enhance the electrical conduction therebetween, thereby reducing the electrical resistance of the bonding portion; and the heights of the finished solder bumps are equalized, to thereby reduce a connection failure between each solder bump and the associated connecting land, that is a mounting failure. 
     In the fabrication method of the present invention, preferably, after formation of the resin layer at the third step, the formation of the second solder ball bump at the second step is performed by a printing process, a plating process or a transfer process. With this configuration, after the first solder ball bumps are fixedly held by the resin, the second solder ball bumps having uniform heights can be easily formed on the first solder ball bumps. This makes it possible to further reduce a mounting failure and hence to improve the reliability of the solder bumps. 
     In the manufacturing method of the present invention, preferably, the first, second and third steps are performed for the semiconductor device formed on a semiconductor substrate in a wafer state. With this configuration, the formation of the first ball bump or first solder ball bump, formation of the resin layer, and formation of the second ball bump or second solder ball bump are performed for each semiconductor device before the semiconductor device is cut from the semiconductor wafer. In other words, these works are not required to be performed for the individual semiconductor devices after the semiconductor devices are cut from the semiconductor wafer. This makes it possible to handle a number of the semiconductor devices at a time and hence to further improve the productivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 E are schematic cross-sectional views illustrating sequential steps of manufacturing a bump according to a first embodiment of the present invention; 
     FIGS. 2A to  2 F are schematic cross-sectional views illustrating sequential steps of manufacturing a bump according to a second embodiment of the present invention; 
     FIG. 3 is a schematic side view showing one configuration of a polishing apparatus used for the manufacturing method shown in FIGS. 2A to  2 F; 
     FIGS. 4A to  4 E are schematic cross-sectional views illustrating sequential steps of manufacturing a solder bump in accordance with a related art manufacturing method; and 
     FIG. 5 is a schematic cross-sectional view showing a state in which a flip-chip type semiconductor IC including bumps manufactured by the manufacturing method shown in FIGS. 4A to  4 E. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. 1A to  3 . 
     FIGS. 1A to  1 E show a first embodiment of a method of manufacturing solder bumps according to the present invention. 
     Referring to FIG. 1A, there is shown one of a plurality of flip-chip type semiconductor ICs  11  provided on a semiconductor wafer  10 . 
     The flip-chip type semiconductor IC  11  has, as shown in FIG. 1A, two electrode portions  11   a  each of which is formed as follows: 
     An electrode pad  13  made from an Al—Cu alloy is formed on a semiconductor substrate  12  made from silicon by sputtering, etching and the like. A surface protective film  14  made from silicon nitride or polyimide is formed over the entire surface of the semiconductor substrate  12  in such a manner as to cover the electrode pad  13 . An opening  14   a  is formed in the surface protective film  14  at a region of the electrode pad  13 . A metal multi-layer film, that is, BLM film  15  made from Cr, Cu, Au and the like is formed by sputtering in such a manner as to cover the surface of the electrode pad  13  exposed to the side surface and the bottom of the opening  14   a.  A polyimide film  16  having an opening  16   a  at a region of the BLM film  15  is formed over the entire surface of the semiconductor substrate  12 . The electrode portion  11   a  is composed of these electrode pad  13 , surface protective film  14 , opening  14   a,  BLM film  15 , and the polyimide film  16 . 
     A spherical first solder ball bump  20  is formed on each electrode portion  11   a  of each semiconductor IC  11  of the semiconductor wafer  10 . The ball bump  20  is desirable to have a high elasticity, and in this embodiment, the ball bump  20  is made from a high melting point solder. 
     The first solder ball bump  20  is formed on the BLM film  15  exposed from the opening  16   a  of the polyimide film  16 . 
     The high melting point solder contains about 97% of Pb and about 3% of Sn, and has a high melting point and a relatively high elasticity. 
     Referring to FIG. 1B, the entire surface of the semiconductor wafer  10  is covered with an epoxy region  21  by spin-coating. The semiconductor wafer  10  is then heat-treated at about 150° C. for about 5 hrs, whereby the resin  21  is cured. 
     The thickness of the resin layer  21  is selected to be nearly equal to or less than the height of the first solder ball bump  20 . Accordingly, the top portion of the first solder ball bump  20  projects from the surface of the resin layer  21 . 
     Referring to FIG. 1C, a patterned eutectic solder film is formed by screen printing, to form a second solder ball bump  22  on the top portion of the first solder ball bump  20 . 
     The eutectic solder contains about 40% of Pb and about 60% of Sn, and has a melting point of about 200° C. which is lower than that of the above-described high melting point solder. 
     The semiconductor wafer  10  is heated in a temperature range in which only the eutectic solder is melted but the high melting point solder is not melted, specifically, at a temperature in a range of 200° C. to 250° C., whereby the second solder ball bump  22  is melted so that the shape thereof becomes a ball-like shape by its surface tension, and it is also hardened to be bonded to the first solder ball bump  20 . In this way, a solder bump  23  having a stacked structure of the first and second solder ball bumps  20  and  22  is formed. 
     The semiconductor substrate  12  in the wafer state is then cut into the individual chip-like semiconductor ICs  11  by dicing. The semiconductor IC  11  is thus accomplished. 
     The semiconductor IC  11  thus manufactured is mounted on a printed circuit board  30  in the following procedure: 
     Referring to FIG. 1D, the flip-chip type semiconductor IC  11  is set such that each solder bump  23  faces to the associated one of lands  31  as connecting portions made from Cu formed on the printed circuit board  30 . 
     The surface of the printed circuit board  30  is covered with a solder resist  32  excluding the surfaces of the lands  31 , and the regions of the lands  31  are precoated with an eutectic solder film  33 . 
     The semiconductor IC  11  is moved close to the printed circuit board  30  until each solder bump  23  is brought into contact with the associated one of the lands  31 . Then, at a so-called reflow step, the eutectic solder film constituting the second solder ball bumps  22  and the eutectic solder film  33  pre-coated on the lands  31  formed on the printed circuit board  30  are melted, to be thus bonded to each other. 
     In this way, the flip-chip mounting of the flip-chip type semiconductor IC  11  to the printed circuit board  30  is terminated. 
     The embodiment having the above-described configuration has the following advantages: 
     Since the second solder ball bump  22  formed of the eutectic solder film has desirable wettability to the eutectic solder film  33  pre-coated on the lands  31 , it is desirably familiar to and strongly bonded to the eutectic solder film  33 . As a result, the second solder ball bump  22  is certainly solder-bonded to the eutectic solder film  33 . 
     Since the first solder ball bumps  20  are fixedly held by the resin layer  21 , even if a thermal stress is applied between the semiconductor substrate  12  and the printed circuit board  30  due to environmental temperature change after mounting, the resin layer  21  holding the solder bumps  23  sustains the thermal stress as a whole and the first solder ball bumps  20  having a relatively high elasticity are elastically deformed, whereby the thermal stress is moderated. As a result, it is possible to prevent breakage of the bonding portions of the solder bumps  23  due to the thermal stress and hence to improve the reliability of the solder bumps. 
     Since the resin layer  21  is formed on the surface, on the electrode pad  13  side, of the semiconductor IC  11  before the semiconductor IC  11  is mounted to the printed circuit board  30 , the resin layer  21  is not brought into contact with the surface of the printed circuit board  30 ; and since unlike the related art method, it is not required to inject the resin  21  between the semiconductor IC  11  and the printed circuit board  30 , even if the semiconductor IC  11  has a narrow pitch structure, the entire surface of the semiconductor IC  11  is certainly covered with the resin layer  21 . This makes it possible to certainly moderate a thermal stress, and hence to improve the durability against the thermal stress. 
     As described above, according to this embodiment, the reliability and durability of the flip-chip mounting of the semiconductor IC  11  to the printed circuit board  30  is significantly improved. 
     While the first and second ball bumps made from the lead-containing solders different in composition are used in the above-described embodiment, they may be made from lead-free solders different in composition. Even in this case, the same effect can be obtained. 
     To be more specific, the first ball bump may be made from a solder of a metal other than lead, and the second ball may be made from a solder of an alloy not containing lead. For example, the first ball bump may be made from a solder of Cu, and the second ball bump may be made from a solder of an alloy containing 99.3% of Sn and 0.7% of Cu. The Cu ball is preferably subjected to Au plating for improving the electric contact characteristic. 
     Further, the first ball bump may be made from a solder of Ni, and the second ball bump may be made from a solder of an alloy containing 96.5% of Sn and 3.5% of Ag. Even in the case, the Ni ball is preferably subjected to Au plating. 
     FIGS. 2A to  2 F show a second embodiment of the method of manufacturing solder bumps according to the present invention. 
     The manufacturing method in this embodiment is the same as that in the first embodiment except that after first solder ball bumps are formed and the entire surface of a semiconductor wafer is coated with a resin layer, the top portions of the first solder ball bumps are planarized by polishing (see FIG.  2 C). 
     Referring first to FIG. 2A, an approximately spherical first solder ball bump  20  made from a high melting point solder is formed on each electrode portion  11   a  of each semiconductor IC  11  of a semiconductor wafer  10 . 
     The first solder ball bump  20  is formed on a BLM film  15  exposed from an opening  16   a  of a polyimide film  16 . 
     Referring to FIG. 2B, the entire surface of the semiconductor wafer  10  is coated with an epoxy resin  21  by spin coating. The semiconductor wafer  10  is then heat-treated at about 150° C. for about 5 hrs, whereby the resin  21  is cured. 
     Referring to FIG. 2C, the top portion of the first solder ball bump  20  is polished by a polishing apparatus, to be planarized. The planarized top portion of the first solder ball bump  20  is designated by reference numeral  20   a.    
     The polishing apparatus will be described with reference to FIG.  3 . 
     Referring to FIG. 3, the polishing apparatus designated by reference numeral  40  includes a table  42  rotated by a rotating shaft  41 , a polishing cloth  43  stuck on the upper surface of the table  42 , a wafer carrier  44  which supports the semiconductor wafer  10  and is rotated, and a nozzle  45  for dripping a polishing solution on the table  42 . 
     The wafer carrier  44  is rotated while pressing the semiconductor wafer  10  on the polishing cloth  43  on the table  42 , to polish the top portions of the first solder ball bumps  20 , which are the highest portions of the semiconductor wafer  10 . 
     Referring back to FIG. 2D, a second solder ball bump  22  made from an eutectic solder is formed on the planarized top portion of the first solder ball bump  20  by a ball transfer process. At this time, the second solder ball bump  22  is bonded to the first solder ball bump  20 . As a result, a solder bump  23  having a stacked structure of the first and second solder ball bumps  20  and  22  is formed. 
     The semiconductor substrate  12  in the wafer state is then cut into the individual chip-like semiconductor ICs  11  by dicing. The semiconductor IC  11  is thus accomplished. 
     The semiconductor IC  11  thus manufactured is mounted on a printed circuit board  30  in the following procedure: 
     Referring to FIG. 2E, the flip-chip type semiconductor IC  11  is set such that each solder bump  23  faces to the associated one of lands  31  as connecting portions made from Cu formed on the printed circuit board  30 . 
     The surface of the printed circuit board  30  is covered with a solder resist  32  excluding the surfaces of the lands  31 , and the regions of the lands  31  are pre-coated with an eutectic solder film  33 . 
     As shown in FIG. 2E, the semiconductor IC  11  is moved close to the printed circuit board  30  until each solder bump  23  is brought into contact with the associated one of the lands  31 . Then, at the so-called reflow step, the eutectic solder film constituting the second solder ball bumps  22  and the eutectic solder film  33  pre-coated on the lands  31  formed on the printed circuit board  30  are melted, to be thus bonded to each other. 
     In this way, the flip-chip mounting of the flip-chip type semiconductor IC  11  to the printed circuit board  30  is terminated. 
     The embodiment having the above-described configuration has the following advantages: 
     Like the solder bump  23  in the first embodiment shown in FIGS. 1A to  1 E, the second solder ball bump  22  formed of the eutectic solder film is strongly bonded to the eutectic solder film  33  pre-coated on the lands  31 , so that the second solder ball bump  22  is certainly solder-bonded to the eutectic solder film  33 . 
     Even if a thermal stress is applied between the semiconductor substrate  12  and the printed circuit board  30 , the resin layer  21  fixedly holding the solder bumps  23  sustains the thermal stress as a whole and the first solder ball bumps  20  having a relatively high elasticity are elastically deformed, whereby the thermal stress is moderated. As a result, it is possible to prevent breakage of the bonding portions of the solder bumps  23  due to the thermal stress and hence to improve the reliability of the solder bumps. 
     Since the resin layer  21  is formed on the surface, on the electrode pad  13  side, of the semiconductor IC  11  before the semiconductor IC  11  is mounted to the printed circuit board  30 , even if the semiconductor IC  11  has a narrow pitch structure, the entire surface of the semiconductor IC  11  is certainly covered with the resin layer  21 . This makes it possible to certainly moderate a thermal stress, and hence to improve the durability against the thermal stress. 
     Since the top portion of the first solder ball bump  20  is polished, the resin  21  remaining on the top portion of the first solder ball bump  20  is perfectly removed. As a result, the top surface of the first solder ball bump  20  becomes a clean plane. The second solder ball bump  22  is formed on the clean plane of the first solder ball bump  20 , to thereby reduce a connection resistance at the boundary between the first solder ball bump  20  and the second solder ball bump  22 . This makes it possible to obtain the solder bump  23  having a lower resistance and a higher performance. Further, since the first solder ball bumps  20  are planarized by polishing, the heights of the solder bumps  23  are further equalized. This makes it possible to reduce occurrence of a mounting failure. 
     In this case, since the second solder ball bumps  22  are formed by the ball transfer process, the transfer amounts thereof are equalized, so that the heights of the solder bumps  23  are more accurately equalized. 
     As described above, according to the second embodiment, the reliability and durability of the flip-chip mounting of the semiconductor IC  11  to the printed circuit board  30  can be improved more than those in the first embodiment. 
     The second solder ball bumps  22  are formed by the screen printing or ball transfer process in the above-described embodiments; however, the present invention is not limited thereto, and it is to be understood that they may be formed by another printing or transfer process, or a plating process. 
     The solder bumps  23  are formed on the electrode pads of the semiconductor IC  11  in the above-described embodiments; however, the present invention is not limited thereto but is applicable to the case where solder bumps are formed on electrode pads of another semiconductor device. 
     The second solder ball bump is overlapped to the first solder ball bump in the above-described embodiments; however, one or a plurality of solder bumps may be overlapped thereto or interposed therebetween. 
     The first and second ball bumps made from the lead-containing solders different in composition are used in the above-described embodiment; however, they may be made from lead-free solders different in composition. Even in this case, the same effect can be obtained. 
     While the preferred embodiments of the present invention have been described using the specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.