Abstract:
A method for mounting a semiconductor part on a circuit substrate is provided, which includes preparing the semiconductor part having a surface thereof provided with a plurality of stud-bumps, preparing a solder substrate having a surface thereof on which solid-solders corresponding to respective ones of the plurality of stud-bumps are arranged, preparing the circuit substrate having a surface thereof provided with connecting pads corresponding to respective ones of the plurality of stud-bumps, attaching the corresponding solid-solders on the solder substrate to respective tip ends of the plurality of stud bumps, separating the solid-solders attached to the tip ends of the stud-bumps from the solder substrate, contacting the solid-solder attached to respective ones of the tip ends of the stud-bumps with the corresponding connecting pads, and heating the solid-solders contacted with the corresponding connecting pads thereby establishing solder connection between respective ones of the stud-bumps and the corresponding connecting pads.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a method for mounting a semiconductor part or semiconductor parts on a circuit substrate. More particularly, the present invention relates to a method of connecting a semiconductor part having stud bumps with a circuit substrate having connection pads by using a solder. 
     BACKGROUND OF THE INVENTION 
     As a method of electrically connecting a semiconductor part or semiconductor parts to a circuit substrate, there is the flip-chip-bonding (FCB) method in which connection pads (electrode pads) of both are connected to one another by using a solder. As one of the FCB technology, there is a method referred to as (pre-coating method) in which a bump is provided for each of connection pads of semiconductor parts, and a solder layer is preliminarily provided on connecting pads of the circuit substrate.  FIGS. 1A to 1C  are schematic views for the explanatory illustration of the pre-coating method. Namely, a semiconductor part  1  provided with a bump  2  and a substrate  3  having thereon a connection pad  4  on which a solder layer  5  is provided are prepared ( FIG. 1A ). Positioning or aligning of the bumps  2  and the connection pads  4  is then performed and thereafter, both are brought into contact with one another ( FIG. 1B ). Heating is then applied to the solder layer  5  on the connection pad  4  to melt the solder thereby connecting bumps  2  and the connection pads  4  by the solder ( FIG. 1C ). 
     There are some problems in the conventional method illustrated in  FIG. 1  as described below. When the connection pas  4  is comprised of copper (Cu) and when the material of the solder is an alloy of tin and silver (Sn—Ag), an intermetallic compound (Sn—Cu) is formed between the tin (Sn) contained in the solder and the copper (Cu) constituting the connection pad  4  due to heating (pre-coating heat) which is applied during provision of the solder layer  5  on the connection pad  4 . As a result, the Sn atom in the solder is decreased while increasing the rate of the silver (Ag) therein, and therefore, the melting point of the solder goes up. Incidentally, even with the Sn—Pb eutectic solder made of tin (Sn) and lead (Pb), in the case where the ratio of the eutectic point of 63% Sn and 37% Pb changes, the melting point similarly goes up. In the case of the Sn—Ag system solder free of lead (Pb), this phenomena of increasing of the melting point is specifically noticeable. 
     When the melting point of the solder rises, it is necessary that a heating temperature for melting the solder shown in  FIG. 1C  must be increased. As a result, a difference in the extension (the thermal expansion) between a substrate and a semiconductor part, which is caused by a difference of the thermal expansion coefficient between the substrate and the semiconductor part increases. Therefore, a positional discrepancy between the stud bump  2  and the connection pad  4  as well as a flaw due to the curving of the substrate  3  occur. Further, the hardness of the crystal, i.e., the Ag 3 Sn crystal which is produced due to the twice melting of the solder upon pre-coating and upon soldering for the connection is very high and therefore, a crack is apt to occur in the hardened solder after melting thereof. 
     In a conventional method for avoiding the melting of the solder at the pre-coating, there has been provided a paste-transcription method.  FIGS. 2A to 2C  are schematic views illustrating the conventional paste-transcription method. Namely, a bump  2  of a semiconductor part  1  is immersed in a paste  7  ( FIG. 2A ). The solder paste  7  is a mixture of a solder particles and a flux having viscosity. Due to the viscosity of the flux, the solder paste  7  is attached to the surface of the bump  2  of the semiconductor part  1 . Then, the bump  2  to which the solder paste  8  is attached is positionally aligned on the connection pad  4  on the substrate  3  ( FIG. 2B ). Thereafter, the bump  2  is heated, to melt the solder paste  8  so that the bump  2  and the connection pad  4  are connected together by the hardened solder  9  ( FIG. 2C ). 
     In the paste transcription method of  FIG. 2 , the melting of the solder is permitted to occur only once and accordingly, it is possible to mitigate the problems caused by the generation of the Sn—Cu intermetallic compound or the Ag 3 Sn. Nevertheless, there are problems as described below with the paste transcription method. That is to say, when the viscosity of the flux in the solder paste is high, at the time of attaching the solder paste to the bumps, a continuity of the solder paste might occur while forming a bridging of the solder paste between neighboring bumps. On the contrary, when the viscosity of the flux in the solder paste is low, an amount of solder paste attaching to each bump is small. Therefore, in the former case, a short-circuiting might occur between the neighboring bumps after the melting of the solder, while in the latter case, a reduction in the strength of solder connection might take place causing electric disconnection. Consequently, both cases will eventually result in defective solder connection. 
     Japanese unexamined Patent Publications (Kokai) 06-188289 and 2000-232129 disclose such a method that electric connection between a semiconductor part and a circuit substrate is carried out without meting of solder upon doing the connection. In the disclosed prior art technique of these two publications, a contact pin on the substrate is permitted to pierce a ball-like solder bump on the semiconductor part to establish electric connection between both. However, since the contact pin is placed in a state of merely piercing the solder ball, strength of the connected portion is necessarily small, and an electric contact resistance in the connected portion is larger than the case where melting of the solder is carried out. Further, the prior art disclosed in the above-mentioned two publications takes the way of preliminarily providing a solder bump on an electrode pad of the semiconductor part, and accordingly is different from the conventional methods of  FIGS. 1 and 2  in which a solder layer is preliminarily provided on a circuit substrate side. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a method for connection of a stud-bump of a semiconductor part and a connecting pad of a circuit substrate by only one-time melting of a solder. 
     Another aspect of the present invention is to provide a method for connecting stud-bumps of a semiconductor part with connecting pads on a circuit substrate by solder under such a condition that all of the stud-bumps are beforehand supplied with an equal amount of solder. 
     A further aspect of the present invention is to provide a method for connecting stud-bumps of a semiconductor part with copper (Cu)-made connecting pads of a circuit substrate by solder, wherein the method is capable of alleviating generation of an intermetallic compound of tin and copper (Sn—Cu). 
     A still further aspect of the present invention is to provide a method for connecting stud-bumps of a semiconductor part with a circuit substrate by the employment of a lead (Pb)-free solder, wherein the method is capable of suppressing a rise in the reflow-temperature of the solder. 
     In accordance with the present invention, there is provided a method for mounting a semiconductor on a circuit substrate which comprises: 
     preparing a semiconductor part having a surface thereof provided with a plurality of stud-bumps; 
     preparing a solder substrate having a surface thereof on which solid-solders corresponding to respective ones of the plurality of stud-bumps are arranged; 
     preparing the circuit substrate having a surface thereof provided with connecting pads corresponding to respective ones of the plurality of stud-bumps; 
     attaching the corresponding solid-solders on the solder substrate to respective tip ends of the plurality of stud bumps; 
     separating the solid-solders attached to the tip ends of the stud-bumps from the solder substrate; 
     contacting the solid-solders attached to respective of the tip ends of the stud-bumps with the corresponding connecting pads; and 
     heating the solid-solders contacted with the corresponding connecting pads thereby establishing solder connection between respective ones of the stud-bumps and the corresponding connecting pads. 
     In accordance with the present invention, the stud-bumps of the semiconductor part and the connecting pads of the circuit substrate can be connected together by solder by one-time melting of the solder. 
     In accordance with the present invention, all of the stud-bumps of the semiconductor part are beforehand supplied with an equal amount of solder, respectively, and then the connecting pads of the circuit substrate and the stud-bumps can be connected together by using the solder. 
     In accordance with the present invention, when the stud-bumps of the semiconductor part are connected with the copper-made connecting pads of the circuit substrate by using the solder containing therein Sn, generation of the intermetallic compound of Sn—Cu is alleviated, and precipitation of crystals such as Ag 3 Sn may be suppressed. 
     Further, in accordance with the present invention, in the case of using the solder of which the major component is tin (Sn), in connection of the stud-bumps of the semiconductor part with the circuit substrate, a rise in the reflow temperature of the solder may be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the present invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings. 
         FIGS. 1A to 1C  are schematic views illustrating a pre-coat method according to the prior art; 
         FIGS. 2A to 2C  are schematic views illustrating a paste transcription method according to the prior art; 
         FIGS. 3A to 3E  are schematic views, in part cross-section, illustrating a method according to an embodiment of the present invention; 
         FIG. 4  is an obliquely viewed side view illustrating the relationship between a semiconductor part and a solder substrate; and 
         FIGS. 5A and 5B  are schematic views illustrating other embodiments of the solder substrate. 
     
    
    
     The drawings are merely schematic representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict only typical embodiments of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbers represent like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of embodiments of the present invention will be provided hereinbelow with reference to the accompanying drawings.  FIGS. 3A to 3E  are schematic views illustrating a method according to an embodiment of the present invention. In the step (a) shown in  FIG. 3A , a semiconductor part  10  provided with a plurality of stud-bumps  12  on connecting pads  11  formed in a surface of the semiconductor part is prepared. Further, in the step (a), a solder substrate  13  on which solid-solders  14  are arranged to correspond to respective of the stud-bumps  12  is prepared. It is to be noted that the solid-solders  14  of  FIG. 3A  are shaped in a ball or a sphere, respectively. The spherical solder balls  14  may be formed with any arbitrary method such as the conventional printing or spray method and the like. The material of which the solder balls  14  are made may be selected from any arbitrary solder materials including a solitary system material such as a lead (Pb) contained Sn—Pb system material, a lead (Pb)-free Sn—Ag system material or binary or more system material. Each solder ball  14  may have the size thereof (i.e., the diameter thereof) that is optionally determined within a producible range thereof, depending on the size of the respective stud-bumps  12  and the spacing (the pitch) among the stud-bumps. Therefore, the size of the solder ball  14  may thus optionally range from on the order of a nanometer to on the order of a millimeter. 
     The semiconductor part  10  includes various kinds of electronic parts from large-scale IC chips to other relatively small-scale semiconductor chips such as a micro-processor unit, a system-on-a-chip (SOC) and the like. Each stud-bump  12  has a protrusion-like tip end  17 . The shape of the stud-bump  12  is not limited to that shown in  FIGS. 3A to 3E  and may have the shape of a circular cylinder, a circular cone, a rectangular parallelepiped and so on. The stud-bum  12  is made of either one metal or a combination of two or more metals selected from a group including, for example, gold (Au), silver (Ag), copper (Cu), aluminum (Al), and nickel (Ni). The solder substrate  13  is provided with openings  15  in which the solder balls  14  are received. The number of openings  15  provided in the solder substrate  13  corresponds to at least the number of associated stud-bumps  12 . It should be appreciated that each of the openings  15  has depth and width (inner diameter) thereof which permit at least a portion (a lower portion) of the solder ball  14  to be seated therein. Therefore, the respective openings  15  may be formed in through-holes, if they are able to hold the solder balls  14 . 
       FIG. 4  is an obliquely viewed side view illustrating the positional relationship between the semiconductor part  10  and the solder substrate  13 . Referring to  FIG. 4 , the openings  15  of the solder substrate  13  are disposed so that each opening  15  confronts one of the stud-bumps  12  of the semiconductor part  10  by one-to-one correspondence. Every opening  15  receives therein the solder ball  14  in a seated manner.  FIGS. 5A and 5B  are cross-sectional views illustrating other embodiments of the solder substrate  13  and another embodiment of a filled solder  18 . In the embodiment shown in  FIG. 5A , each of the openings  15  is formed in such a shape that the diameter of the opening  15  is gradually thinned from the surface of the substrate  13  toward the bottom of the opening  15  per se. In the case of this opening shape, the upper edge of the opening  15  is fully wide, and therefore the solder ball  14  can be easily taken out of the opening  15  by the associated stud-bump  12 . Alternatively,  FIG. 5B  indicates a different embodiment in which no solder ball  14  is used. That is to say, the bottomed openings  15  are filled with the solder  18 , respectively. In case of this embodiment, there is such a merit that no separate solder ball  14  is required for being prepared beforehand. To the contrary, there is such a demerit that the filled solder  18  may be somewhat difficult to be taken out of the openings  15  by the stud-bumps  12 . In order to alleviate this demerit, it is necessary to take such a measure that the bottom surface of each opening  15  is coated with flux before the solder is filled in the opening  15 , so that the filled solder  18  is apt to be easily pulled away from the opening  15 . 
     Now, returning to  FIG. 3B , in the step (b), the semiconductor part  10  is permitted to approach the solder substrate  13  so that respective stud-bumps  12  are brought into in alignment with the corresponding solder balls  14 . Thereafter, the tip ends  17  of the respective stud-bumps  12  are stuck into the corresponding solder balls  14  under such a condition that a force is applied from either the side of the semiconductor part  10  or the solder substrate  13  to promote the sticking of the tip ends  17  into the solder balls  14 . The amount of sticking, i.e., the length of entering of the respective tip ends  17  into the corresponding solder balls  14  is adjustably regulated by the strength of the above-mentioned applied force. Incidentally, the application of the force is made to the semiconductor part  10  or the solder substrate  13  either directly by a suitable pressurizing member or indirectly via a pneumatic medium such as the air or a hydraulic medium. Upon sticking of the stud-bumps  12  into the solder balls  14 , the solder balls  14  may be heated to a temperature in a range that does not exceed the melting point of the solder. Further, in a case where the tip ends  17  of the stud-bumps  12  fail to stick into the solder balls  14  due to slippage thereof on the surface of the respective solder balls  14 , the upper portion of each solder ball  14  may be flattened. 
     In the step (c) of  FIG. 3C , the solder balls  14  attached to the tip ends  17  of the stud-bumps  12  are separated from the solder substrate  13 . At this step, the above-mentioned separation may be effected by any one of the movements selected from moving the semiconductor part  10  upwards and moving the solder substrate  13  downwards. In addition, at the moment of the above-mentioned separation of the solder balls  14  form the solder substrate  13 , a blow of a gas such as, for example, the air or the nitrogen gas, may be applied from the under side of the solder substrate  13  to thereby assist the solder balls  14  to be easily pulled away from the openings  15 . Alternatively, there may be produced a vacuum pressure prevailing between the solder balls  14  and the solder substrate  13 , so that an appropriate force caused by a pressure differential is applied to the under portion of the respective solder balls  14  to thereby urge the solder balls  14  to be separated away from the solder substrate  13 . 
     In the step (d) of  FIG. 3D , the semiconductor part  10  is moved toward a position above the circuit substrate  20 , which is separately prepared beforehand. The circuit substrate  20  is provided, on its surface, with connecting pads  21  disposed to confront each of the plurality of stud-bumps  12  in a one-to-one correspondence. At this moment, the respective connecting pads  21  and the respective stud-bumps  12  with the solder balls  14  attaching thereto are brought into a position such that the latter are in a slight contact with the former. It should be appreciated that the circuit substrate  20  may include substrates of various kinds of materials such as a glass-epoxy substrate, a flexible substrate, e.g., a polyimide substrate, a ceramic substrate, or a silicon substrate. The circuit substrate  20  may further include various circuit substrates such as an ordinary printed circuit board, a multilayer wiring substrate in which an insulating layer and a wiring layer are alternately laminated to form a multilayer form. The connecting pads  21  are connected to the other wiring layer (not shown) on the surface of the substrate. The connecting pads  21  may have the shape of recesses formed, respectively, in the surface of a circuit substrate. Further, the connecting pads  21  are made of either one metal or a combination of two or more metals selected from the group of copper (Cu), gold (Au), silver (Ag), aluminum (Al), tin (Sn) and nickel (Ni). Furthermore, the connecting pads  21  may have a multilayer structure configured with the above-mentioned metallic material or materials. 
     In the step (e) of  FIG. 3E , the solder balls  14  are heated (e.g. melted) so as to provide mechanical and electric connection between the respective stud-bumps  12  and the corresponding connecting pads  21 . Heating of the solder balls  14  is performed by using either a reflow oven or a flip-chip-bonder. However, another heating means may alternatively be employed. Heating temperature is set at such a temperature that the solder material may be successfully melted to be able to achieve a solder connection between the stud-bumps  12  and the connecting pads  21 . Thus, the heating temperature is practically determined depending on the used solder material. The solder balls  14  produce solder-connections  22  after being melted, and the produced solder connections  22  cover the whole surface of the respective connecting pads  21 . Nevertheless, it should be appreciated that the solder connections  22  do not extend beyond the connecting pads  21  toward the exterior portions for the reason that an amount of each solder ball  14  is restricted to such an amount that is required only for achieving connection by the solder. Also, since the respective solder balls  14  are all formed to be equivalent, formation of solder bridging between the connecting pads  21 , which might cause a short-circuiting can be prevented. When the connecting pads  21  are made of copper (Cu), and when the solder material is made of a material of which the main part is tin (Sn), the melting of the solder occurs only one time, and therefore a production of an intermetallic compound of tin and copper (Sn—Cu) can be alleviated. 
     The present invention has been described with reference to the accompanying drawings. However, it should be understood that the present invention is not intended to be limited to the described and illustrated embodiments, and that many variations and modifications will occur to a person with an ordinary skill in the art without departing from the scope and gist of the invention as claimed in the claims.