Abstract:
Method for preventing solder from rising to a portion of an electric contact when the electric contact is being soldered to a copper foil so as to extend therefrom. The portion is plated with a noble metal and adapted to contact a mating object. Cooling means is brought into contact with at least the portion of the electric contact adapted to contact the mating object, and connection portion between the electric contact and the copper foil is heated by heating means. The rising of solder can be prevented and the electric contacts thus obtained are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a method for preventing solder from rising to a portion of an electric contact adapted to contact a mating object in producing the electric contact formed on a copper foil so as to extend therefrom by connecting the electric contact to the copper foil by soldering with enough bonding strength, and more particularly to an electric contact produced by the use of the method.  
         [0002]     The word “rising” or “rise” as used herein is to be understood to mean a phenomenon that in soldering process, melted solder climbs up with the aid of the capillary action to an area where existence of the solder is unfavorable.  
         [0003]     In order to bring a circuit board into contact with an electronic part, hitherto, if contact portions of the one part are flat, contact portions of the other part are frequently formed in extending shapes (for example, a hemisphere). As examples of such electric contacts, incorporated herein are Japanese Patent Application No. 2005-277,320 (Patent Literature 1) and Japanese Patent Application No. 2005-344,971 (Patent Literature 2) using metal balls, filed by the applicant of the present application.  
         [0004]     Patent Literature 1  
         [0005]     According to the abstract of the Japanese Patent Application No. 2005-277,320, the invention has an object to provide electric contacts and a method for producing the electric contacts having a predetermined height without causing defective connection between the electric contacts. Disclosed is an electric contact extending from a copper foil in which the copper foil is coated with a metal paste layer, and a metal ball is fixed to the copper foil by sintering the metal paste layer and is plated with gold over at least part adapted to contact a mating object. Further, disclosed is a method for producing an electric contact extending from a copper foil including steps of coating the copper foil with a metal paste layer over a predetermined area as a first step, arranging a metal ball on the metal paste layer and thereafter pushing the metal ball against the copper foil as a second step, fixing the metal ball to the copper foil by sintering the metal paste layer at a predetermined temperature as a third step, and plating the metal ball with gold over at least part adapted to contact a mating object as a fourth step.  
         [0006]     Patent Literature 2  
         [0007]     According to the abstract of the Japanese Patent Application No. 2005-344,971, this invention has an object to provide electric contact and a method for producing the electric contacts which have a predetermined height without any risk of defective or failed connection between the electric contacts. Disclosed is an electric contact extending from a copper foil, comprising a metal ball fixed to the copper foil by sintering a metal paste layer coated on the copper foil, the metal ball having a contact portion adapted to contact a mating object and plated with gold. Further disclosed is a method for producing an electric contact extending from a copper foil, comprising steps of coating a metal paste layer of a predetermined area on a copper foil as a first step, loading a metal ball on the metal paste layer and then pushing the metal ball to the copper foil as a second step, fixing the metal ball to the copper foil by sintering the metal paste layer at a predetermined temperature as a third step, and plating at least the contact portion of the metal ball adapted to contact a mating object with a noble metal as a fourth step.  
         [0008]     In recent years, with the miniaturization of electric and electronic appliances, electrical connectors have been miniaturized and their pitches have become narrower. It is desirable to produce electric contacts on a copper foil using the metal balls disclosed in the Patent Literatures 1 and 2, from the standpoint of the narrower pitches and reduced overall height. It is needed to fix a metal ball to a copper foil by the use of an electrically conductive adhesive in order to install the surface-treated metal ball as an electric contact on the copper foil. Copper paste, silver paste and solder paste are generally used as the electrically conductive adhesive, while the electric contact is required to have a bonding strength as a mechanical property. By comparison, when metal balls having a diameter of 300 μm are bonded to a copper foil, laterally pulling-out strengths are 80 to 130 grf for copper paste, 120 to 190 grf for silver paste, and 300 to 500 grf for solder (Sn3Ag0.5Cu) paste. The solder paste is the best as to the bonding strength.  
         [0009]     Moreover, the electric contacts are required to have superior electrical conductivity and low contact resistance as electrical properties. Needless to say, the bonded portions of the electric contacts are also required to have electrical conductivity and low contact resistance as electrical properties. By comparison, resistances are 50 μΩcm for copper paste, 40 μΩcm for silver paste, and 11 μΩcm for solder paste. The solder paste is the best as to the resistance.  
         [0010]     Furthermore, as to resistance to environmental conditions, all the copper paste, silver paste and solder paste used as conductive adhesives are likely to be oxidized or sulfurized so that they are not suitable for contact portions adapted to contact mating objects, and stable connections with low contact resistance could not be obtained.  
         [0011]     Moreover, as to rising or spreading of these pastes due to capillary action when fixing a metal ball by heating, there are significant differences between the copper paste or silver paste from the solder paste. The copper paste or the silver paste is made by kneading fine metal powder into epoxy family resin and solidified by heating. When being solidified, they do not rise or spread beyond the coated or printed regions. In contrast herewith, the solder paste is made by kneading powder of tin (Sn)-based alloy into a flux as a reducing activator. Before the solder arrives at its melting temperature, the flux starts to melt and spread due to capillary action so that the metal surface to be bonded by soldering is activated, with the result that the metal surface arrives a temperature sufficient to melt the solder. The rise of the solder with the capillary action would spread to the range whose angle is less than the contact angle. In other words, with the solder paste, the solder will rise to the contact portion of an electric contact adapted to contact a mating object and the raised solder reacts with the ambient air so as to be oxidized, whereby a stable connection with low contact resistance could not be obtained.  
       SUMMARY OF THE INVENTION  
       [0012]     In view of the problems of the prior art described above, the invention has an object to provide a method for preventing solder from rising in producing electric contacts and to provide electric contacts produced by using the method, which are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0013]     The object of the invention is accomplished by the method for preventing solder from rising to a portion  47  of an electric contact  20  when said electric contact  20  is being soldered to a copper foil  40  so as to extend therefrom, said portion  47  being plated with a noble metal  46  and adapted to contact a mating object, wherein cooling means is brought into contact with at least said portion  47  of the electric contact  20  adapted to contact said mating object, and connection portion between said electric contact  20  and said copper foil  40  is heated by heating means.  
         [0014]     In the method for preventing solder from rising as defined in claim  2 , a shape of part of said cooling means to contact said portion  47  of the electric contact  20  adapted to contact said mating object is concave  52  to be commensurate with said portion  47  of the electric contact  20 .  
         [0015]     In the method for preventing solder from rising as defined in claim  3 , a U-shaped slit  22  is provided at position surrounding said electric contact  20 .  
         [0016]     In the method for preventing solder from rising as defined in claim  4 , part in continuity with said electric contact through said copper foil  40  is heated by said heating means to connect said copper foil  40  and said electric contact  20 .  
         [0017]     In the method for preventing solder from rising as defined in claim  5 , a heat sink  48  is used as said cooling means, and laser beam  50  is used as said heating means.  
         [0018]     In the method for preventing solder from rising as defined in claim  6 , said heat sink  48  is formed with a through-hole  54  at location corresponding to part to be heated, and said laser beam  50  or the object to be heated is moved so as to heat said part through said through-hole  54 .  
         [0019]     The electric contact  20  defined in claim  7  connected to a copper foil  40  by soldering so as to extend therefrom and having a portion  47  plated with a noble metal  46  and adapted to contact a mating object, in which said electric contact  20  is soldered to said copper foil  40  in a manner that said portion  47  of said electric contact  20  adapted to contact the mating object is cooled by cooling means in contact with at least said portion  47  of the electric contact  20 , while a connection portion between said electric contact  20  and said copper foil  40  is heated by heating means, and a U-shaped slit  22  is provided at position surrounding said electric contact  20 .  
         [0020]     The electric contact  20  defined in claim  8  connected to a copper foil  40  by soldering so as to extend therefrom and having a portion  47  plated with a noble metal  46  and adapted to contact a mating object, in which said electric contact  20  is soldered to said copper foil  40  in a manner that said portion  47  of said electric contact  20  adapted to contact the mating object is cooled by cooling means in contact with at least said portion  47  of the electric contact  20 , while a connection portion between said electric contact  20  and said copper foil  40  is heated by heating means, a shape of part of said cooling means to contact said portion  47  of said electric contact  50  adapted to contact the mating object is concave  52  to be commensurate with said portion  47  of said electric contact  20 .  
         [0021]     As can be seen from the description described above, the method for preventing solder from rising in producing electric contacts and the electric contacts produced by the use of the method can bring about the following significant functions and effect.  
         [0022]     (1) The method for preventing solder from rising to a portion  47  of an electric contact  20  when said electric contact  20  is being soldered to a copper foil  40  so as to extend therefrom, said portion  47  being plated with a noble metal  46  and adapted to contact a mating object, according to the invention cooling means is brought into contact with at least said portion  47  of the electric contact  20  adapted to contact said mating object, and connection portion between said electric contact  20  and said copper foil  40  is heated by heating means. Consequently, the solder is prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object, and electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0023]     (2) In the method for preventing solder from rising as defined in claim  2 , a shape of part of said cooling means to contact said portion  47  of the electric contact  20  adapted to contact said mating object is concave  52  to be commensurate with said portion  47  of the electric contact  20 . Therefore, the solder is securely prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object, and electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0024]     (3) In the method for preventing solder from rising as defined in claim  3 , a U-shaped slit  22  is provided at position surrounding said electric contact  20 . Accordingly, even if there are differences in height of electric contacts  20 , all the electric contacts can be securely brought into contact with the cooling means, and the solder  421  is prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object so that electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0025]     (4) In the method for preventing solder from rising as defined in claim  4 , part in continuity with said electric contact through said copper foil  40  is heated by said heating means to connect said copper foil  40  and said electric contact  20 . Therefore, the solder is prevented from rising to the contact portion of the electric contact adapted to contact a mating object, and electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0026]     (5) In the method for preventing solder from rising as defined in claim  5 , a heat sink  48  is used as said cooling means, and laser beam  50  is used as said heating means. Accordingly, the solder  421  is prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object, and electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0027]     (6) In the method for preventing solder from rising as defined in claim  6 , said heat sink  48  is formed with a through-hole  54  al location corresponding to part to be heated, and said laser beam  50  or the object to be heated is moved so as to heat said part through said through-hole  54 . Therefore, the solder paste  42  at the bonding portion can be securely melted, and the metal ball  44  can be fixed to the copper foil. The solder  421  is prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object so that electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0028]     (7) The electric contact  20  defined in claim  7  connected to a copper foil  40  by soldering so as to extend therefrom and having a portion  47  plated with a noble metal  46  and adapted to contact a mating object, in which said electric contact  20  is soldered to said copper foil  40  in a manner that said portion  47  of said electric contact  20  adapted to contact the mating object is cooled by cooling means in contact with at least said portion  47  of the electric contact  20 , while a connection portion between said electric contact  20  and said copper foil  40  is heated by heating means, and a U-shaped slit  22  is provided at position surrounding said electric contact. Accordingly, even if there are differences in height of electric contacts  20 , all the electric contacts can be securely brought into contact with the cooling means, and the solder  421  is prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object so that electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0029]     (8) An electric contact  20  defined in claim  8  connected to a copper foil  40  by soldering so as to extend therefrom and having a portion  47  plated with a noble metal  46  and adapted to contact a mating object, in which said electric contact  20  is soldered to said copper foil  40  in a manner that said portion  47  of said electric contact  20  adapted to contact the mating object is cooled by cooling means in contact with at least said portion  47  of the electric contact  20 , while a connection portion between said electric contact  20  and said copper foil  40  is heated by heating means, and a shape of part of said cooling means to contact said portion  47  of said electric contact  50  adapted to contact the mating object is concave  52  to be commensurate with said portion  47  of said electric contact  20 . Accordingly, the solder  421  is securely prevented from rising to the contact portion  47  of the electric contact adapted to contact a mating object, and electric contacts obtained using the method are superior in mechanical property (sufficient bonding strength), electrical property (conductive property and low contact resistance), resistance to environmental conditions (impediment to oxidization), and physical property (limitation of rise of solder due to capillary action).  
         [0030]     The invention will be more fully understood by referring to the following detailed specification and claims taken in connection with the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1A  is a sectional view of an electric contact soldered to a copper foil;  
         [0032]      FIG. 1B  is a sectional view of the electric contact in contact with a heat sink as cooling means and laser beam irradiating the part to be heated;  
         [0033]      FIG. 1C  is a sectional view of an electric contact in contact with another heat sink and laser beam irradiating the part to be heated;  
         [0034]      FIG. 2  includes a plan and a longitudinal-sectional view of an electrical connector;  
         [0035]      FIG. 3  includes a partly plan and a partly longitudinal-sectional view of the electrical connector;  
         [0036]      FIGS. 4A, 4B ,  4 C and  4 D are views for explaining the method for producing the electric contact according to the invention;  
         [0037]      FIGS. 5A, 5B ,  5 C and  5 D are views for explaining another method for producing an electric contact according to the invention with a heat sink commensurate in shape with the electric contact and laser beam irradiating through a through-hole of the heat sink; and  
         [0038]      FIG. 6  is a view for explaining a temperature gradient in an electric contact according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     The solder rise preventing method and the electric contact  20  produced by using the method according to the invention will be explained with reference to  FIGS. 1A  to  5 D. In the illustrated embodiment, an electrical connector  10  using said electric contacts  20  will be explained.  
         [0040]      FIG. 1A  is a sectional view of an electric contact after being soldered, and  FIG. 1B  is a sectional view illustrating that a heat sink as cooling means is in contact with an electric contact and its part to be heated is irradiated with laser beam, while  FIG. 1C  is a sectional view illustrating that another sink is in contact with an electric contact and its part to be heated is irradiated with laser beam.  FIG. 2  includes a plan view and a longitudinal-sectional view of an electrical connector.  FIG. 3  includes a partly enlarged plan view and a partly enlarged longitudinal-sectional view of the electrical connector.  FIGS. 4A  to  4 D are views for explaining the method for producing an electric contact according to the invention.  FIGS. 5A  to  5 D are views for explaining another method for producing an electric contact according to the invention in which a heat sink commensurate in shape with the electric contact is used and laser beam irradiates through a through-hole of the heat sink.  FIG. 6  is a view for explaining a temperature gradient of an electric contact according to the invention.  
         [0041]     The electrical connector  10  of one embodiment of the invention at least comprises an elastomer, fine conductors  12 , and flexible printed circuit boards  14 .  
         [0042]     First of all, the construction of an electric contact  20  after being soldered will be explained. Said electric contact  20  constructed in a manner that a metal ball  44  plated with a noble metal  46  is arranged on solder paste  42  on a copper foil  40 , and the soldering paste  42  is then melted to cause said metal ball  44  and said copper foil  40  to be in electrical continuity with each other. When the solder paste  42  is melted, the soldering is effected so as not to allow the melted solder  421  to rise due to the capillary action to the contact portion  47  of said metal ball  44  adapted to contact a mating object. The material of said metal ball  44  is suitably selected in consideration of electric conductivity and surface treatment, and copper alloys are preferable for the metal ball. The size of said metal ball  44  may be suitably designed in consideration of a peripheral space and a required electric contact height.  
         [0043]     The method will be explained for preventing melted solder  421  from rising due to the capillary action to the contact portion  47  of the metal ball adapted to contact a mating object when said solder paste is melting. As shown in  FIG. 1B , cooling means (for example, a heat sink  48 ) is brought into contact with the contact portion  47  of said metal ball  44  adapted to contact the mating object, and under such a condition, said copper foil  40  is heated by heating means (for example, laser beam  50 ) without directly heating the solder paste  421  by means of said heating means, thereby melting said solder paste  42  and hence soldering said metal ball  44  onto said copper foil  40 .  
         [0044]     In order to ensure the cooling effect of the cooling means and to achieve the reliable contact with the cooling means, said cooling means (for example, heat sink  48 ) is formed to be commensurate in shape with the contact portion  47  of said metal ball  44  as shown in  FIG. 1C . For example, the heat sink  48  is formed with a recess  52 .  
         [0045]     A temperature gradient in the electric contact  20  when soldering will be explained herein with reference to  FIG. 6 . In order to melt the solder paste  42 , the temperature of the part to be soldered must be 220° C. to 230° C. However, since said electric contact  20  is 0.3 mm in diameter which is very small, when the temperature of the solder paste  42  arrives at 220° C. to 230° C., the temperature of the entire electric contact  20  will be also 220° C. to 230° C. so that the solder  421  rises due to the capillary action to the contact portion  47  adapted to contact the mating object. In order to prevent the solder  421  from rising to the contact portion  47 , therefore, a temperature gradient (temperature difference) is caused in the electric contact  20  according to the invention. In other words, the contact portion  47  of said electric contact  20  adapted to contact the mating object is cooled by bringing the heat sink  48  as cooling means into contact with the contact portion  47 , and said copper foil  40  is heated at a location spaced apart from said solder paste  42  by the laser beam  50  as the heating means without directly heating said solder paste  42 . As a result, in the case that a point E  64  of the copper foil  40  is heated at 250° C., a temperature gradient (temperature difference) is obtained in a manner that temperatures at points D  62 , C  60 , B  58  and A  56  are 230° C., 220° C., 210° C. and 200° C., respectively. The boundary temperature of solder melting lies between the temperatures at points B  58  and C  60 .  
         [0046]     The flexible printed circuit board  14  will then be explained. Said flexible printed circuit board  14  is provided with a plurality of electric contact elements  18  at locations corresponding to contacts of a mating connector. The electric contact elements  18  are each provided with a hemispherical electric contact  20  so as to be most commensurate with the mating contact to facilitate contacting the mating contact. Said flexible printed circuit board  14  is provided at its center portion with a recess or a through-groove (through-hole) at a position corresponding to capacitors, integrated circuit chips, resistors and the like extending higher than the contacts of the mating connector in order to avoid the printed circuit board from contacting these capacitors, integrated circuit chips, resistors and the like. The size of the recess or through-groove need only serve to prevent the circuit board from contacting the capacitors, integrated circuit chips, resistors and the like extending higher than the contacts of the mating connector and may be suitably designed in consideration of the miniaturization of the connector and positional accuracy and the like.  
         [0047]     The flexible printed circuit board  14  is formed with substantially U-shaped slits  22  each around the electric contact element  18 . By providing the U-shaped slit  22  at position surrounding the electric control element  18 , the electric contact element  18  is elastically supported by a cantilever surrounded by the U-shaped slit  22  so that upon contacting a mating contact of the connector, the electric contact element  18  is deformed by the mating contact, with the result that the mating contact slides on the electric contact element  18 . The size of the slits  22  may be suitably designed in consideration of such a function, the miniaturization of the connector  10 , and the like. Said electric contact element  18  is connected through its conductive portion  24  to the through-hole  26  which is in turn connected to a fine conductor  12  as shown in  FIG. 3 . The size of said through-holes  26  need only receive said fine conductors  12  and be able to connect the fine conductors  12  by soldering, and may be suitably designed in consideration of the miniaturization of the connector  10  and strength and connectivity of the fine conductors  12 .  
         [0048]     The fine conductors  12  will then be explained. The fine conductors  12  are substantially circular cylindrical and have a larger diameter at their center portion and a smaller diameter at both end portions to have two shoulders. Said fine conductors are made of a metal, and a metal superior in conductive characteristics, for example, a brass bar is cut into a predetermined size and further worked at both the end portions to a smaller diameter. Both the end portions are inserted in the through-holes  26  of the flexible printed circuit boards. The diameters of both the end portions of the fine conductors  12  may be suitably designed so as to be inserted in said through-holes  26  and to be connected thereto by soldering. The center portions of the fine conductors  12  are embedded in the elastomer  16 . The diameter of the center portions may be suitably designed in consideration of the miniaturization and narrower pitches of the connector  10  and conductivity of the fine conductors. The lengths of the respective parts of the fine conductors may be suitably designed in consideration of thicknesses of the flexible printed circuit boards  14  and the elastomer  16 .  
         [0049]     The elastomer  16  will then be explained. Said elastomer  16  is formed with inserting holes  28  for inserting said fine conductors  12 , respectively. The size of the inserting holes  28  need only be able to receive said fine conductors  12  and may be suitably designed in consideration of holding force for the fine conductors and the like. In the illustrated embodiment, the diameter of the inserting holes  28  is approximately 20 μm smaller than the diameter of the center portions of the fine conductors  12 . The elastomer  16  is preferably formed with a recess  32  at each of the ends of the inserting holes  28  for preventing warp of part of the elastomer  16  onto a shoulder of the fine conductor  12 . The elastomer  16  is formed from silicon rubber or fluororubber.  
         [0050]     Finally, the methods for producing the electric contacts  20  according to the invention will be explained with reference to  FIGS. 4A  to  4 D and  FIGS. 5A  to  5 D.  
         [0051]     The method shown in  FIGS. 4A  to  4 D will be explained. As a first step, a copper foil  40  is coated with a solder paste layer  42  of a predetermined area as shown in  FIG. 4A .  
         [0052]     As a second step, a metal ball  44  plated with a noble metal  46  is loaded on said solder paste layer  42  in the direction shown by an arrow A in  FIG. 4B .  
         [0053]     As a third step, a heat sink  48  as cooling means is brought into contact with the contact portion  47  of said metal ball  44  as shown in  FIG. 4C . Although the heat sink having the flat surface to contact the metal ball is used in  FIG. 4C , a heat sink having a recess  52  commensurate with the shape of the contact portion  47  of the metal ball  44  may be used as shown in  FIG. 1C .  
         [0054]     As a fourth step, under the condition that the heat sink  48  is in contact with the metal ball  44 , said copper foil  40  is heated by laser beam  50  as heating means to melt said solder paste  42  as shown in  FIG. 4C  so that said metal ball  44  is soldered to the copper foil  40 . Since there is the temperature gradient (temperature difference) in the electric contact  20  as described above, the molten solder  421  does not rise due to capillary action to the contact portion  47  of said metal ball  44 .  
         [0055]     The method shown in  FIGS. 5A  to  5 D will then be explained. As a first step, a copper foil  40  is coated with a solder paste layer  42  of a predetermined area as shown in  FIG. 5A .  
         [0056]     As a second step, a metal ball  44  plated with a noble metal  46  is loaded on said solder paste layer  42  in the direction shown by an arrow A in  FIG. 5B .  
         [0057]     As a third step, a heat sink  48  (having a recess  52  commensurate with the shape of the contact portion  47  of the metal ball  44 ) as cooling means is brought into contact with the contact portion  47  of said metal ball  44  as shown in  FIG. 5C .  
         [0058]     As a fourth step, under the condition that the heat sink  48  is in contact with the metal ball  44 , a land about the through-hole  26  is heated by laser beam  50  as heating means through a through-hole  54  formed in the heat sink  48  to melt the solder paste  42  as shown in  FIG. 5C  so that said metal ball  44  is soldered to the copper foil  40 . Since there is the temperature gradient (temperature difference) in the electric contact  20  as described above, the molten solder  421  does not rise due to capillary action to the contact portion  47  of said metal ball  44 , and simultaneously the fine conductors  12  are soldered to the flexible printed circuit board  14 .  
         [0059]     Examples of applications of the invention are electrical connectors fitted between circuit boards and electronic parts, and particularly in producing a number of electric contacts  20  formed on a copper foil in a manner extending therefrom, the solder rising-preventing method for preventing the solder from rising due to capillary action to the contact portions  47  of the electric contacts  20  while maintaining sufficient bonding strength of the electric contacts to the copper foil.  
         [0060]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention.