Patent Publication Number: US-9899748-B2

Title: Terminal connection structure and method for producing same

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application is a continuation application, under 35 U.S.C. § 111 (a) of international application No. PCT/JP2014/070260, filed Jul. 31, 2014. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a terminal for aluminum electric wire connection and a method for producing the same. 
     Description of Related Art 
     In recent years, there are increasing demands for an aluminum electric wire, which is light in weight and low in cost, for the purpose of achieving both an improvement of energy efficiency and a reduction of cost. In a case where this electric wire having the above characteristics is connected to an electric apparatus, means for connecting copper and aluminum is needed because a wire in the electric apparatus is copper in many cases, and a copper-alloy terminal is often used as such means. Various methods such as a method using soldering, a method using an ultrasonic wave, a method using fusing, and a method using laser have been proposed as a method for the terminal connection. Especially soldering is an excellent method that provides reliable joining because soldering makes it possible to firmly connect an entire connection part in a case where a large number of aluminum wires are connected. 
     However, in general, in a case where an aluminum electric wire is connected to a copper-alloy terminal, an aluminum wire in the terminal does not reach a predetermined necessary temperature due to low heat conductivity of the copper-alloy terminal even when the terminal is heated in soldering. It is therefore undesirably hard to introduce a flux or molten solder into the whole of the inside of the terminal. This results in formation of voids in the solder and parts that are not soldered, thereby causing a problem that the reliability of electrical joining becomes low. The voids can be a cause of discharge, occurrence of electric erosion, an increase of electric resistance, generation of heat during application of electricity, and the like and are therefore not desirable. Furthermore, the voids easily include air and moisture, and thermal conductivity decreases due to the presence of air and moisture. This leads to less certainty of joining. Meanwhile, in a case where the terminal and an end of the electric wire are compressed by swaging with great force in order to reduce voids and improve thermal conductivity, there is a risk of damaging of the electric wire and even breaking of the electric wire. 
     Techniques concerning joining between an electric wire and a terminal are, for example, disclosed in Patent Document 1 and Patent Document 2. Patent Document 1 discloses a technique concerning a connection terminal for ultrasonic wave joining between an aluminum electric wire and a terminal made of copper using an ultrasonic horn chip, and Patent Document 2 discloses a technique concerning a terminal metal fitting including a pipe-like conductor connection part that is integral with a plate member main body made of aluminum and into which a twisted-wire conductor is inserted, and a dissimilar-metal layer that is made of a metal that is not aluminum and is provided on a bottom surface of the plate member main body. 
     RELATED DOCUMENT 
     Patent Document 
     [Patent Document 1] JP Laid-open Patent Publication No. 2007-12329 
     [Patent Document 2] JP Laid-open Patent Publication No. H09-204947 
     SUMMARY OF THE INVENTION 
     In view of the circumstances, the applicant of the present application has conceived of a terminal joining structure for an electric wire end portion illustrated in  FIG. 11 . This structure is provided in Japanese patent application No. 2010-200802 by the applicant of the present application. 
     The terminal joining structure for an electric wire end portion illustrated in  FIG. 11  is described below. A terminal  11  is made up of a tubular terminal main part  11   a  and a terminal piece  11   b  having a flat-plate shape that is continuous with the terminal main part  11   a . The terminal piece  11   b  has a terminal hole  11   c . An electric wire  12  is formed of a large number of aluminum wires twisted together, and enamel insulation coating has been removed from an end portion  12   a  of the electric wire  12 . The electric wire end portion  12   a  is inserted into the terminal main part  11   a  and is exposed on the terminal piece  11   b  outside the terminal main part  11   a.    
     A plurality of linear heat transmitting members  13  are inserted into the electric wire end portion  12   a  so as to be apart from one another by an appropriate interval. Each of the heat transmitting members  13  forms a high-temperature passage for solder introduction in the electric wire end portion  12   a  during soldering. In this state, for soldering, first and second electrodes  14   a  and  14   b  are disposed on an outer side in a radial direction of the terminal main part  11   a  so as to face each other, and third and fourth electrodes  15   a  and  15   b  are disposed on an outer side in a radial direction of the terminal piece  11   b  so as to face each other. 
     Next, as illustrated in  FIGS. 11 and 12 , the first and second electrodes  14   a  and  14   b  are moved in a direction indicated by arrow from the aforementioned locations so as to press the terminal main part  11   a  from both sides thereof. In this way, the terminal main part  11   a  is nipped by the first and second electrodes  14   a  and  14   b . Similarly, as illustrated in  FIGS. 11 and 12 , the third and fourth electrodes  15   a  and  15   b  are moved in a direction indicated by arrow from the aforementioned locations so as to press the terminal piece  11   b  and the electric wire end portion  12   a  from both ends thereof. In this way, the terminal piece  11   b  and the electric wire end portion  12   a  are nipped by the third and fourth electrodes  15   a  and  15   b.    
     In this state, the terminal main part  11   a , the terminal piece  11   b , and the electric wire end portion  12   a  are heated by applying an electric current between the first and second electrodes  14   a  and  14   b  and between the third and fourth electrodes  15   a  and  15   b . Heating of the electric wire end portion  12   a  by the third and fourth electrodes  15   a  and  15   b  is transmitted to the electric wire end portion  12   a  via the heat transmitting members  13  in the terminal main part  11   a . As a result, the electric wire end portion  12   a  in the terminal main part  11   a  is evenly heated. This allows a flux and solder to be efficiently introduced into the electric wire end portion  12   a  in the terminal main part  11   a , thereby accomplishing soldering with certainty. 
     However, in a case where the electric wire end portion  12   a  exposed on the terminal piece  11   b  is temporarily sectioned to a first end  12   a   1  that is close to the terminal main part  11   a  and a second end  12   a   2  that is far from the terminal main part  11   a , the first end  12   a   1  is not pressed against the inside of the terminal piece  11   b  by the third electrode  15   a , but the second end  12   a   2  is pressed against the inside of the terminal piece  11   b  by the third electrode  15   a.    
     This creates a sharp difference in height between the first end  12   a   1  and the second end  12   a   2 . Furthermore, since the electric wire end portion  12   a  is an aluminum wire and is therefore hard to heat, an electric current applied between the third and fourth electrodes  15   a  and  15   b  becomes excessively large. Accordingly, the end  12   a   2  is heated to an excessively high temperature, and stress acts between the end  12   a   1  and the end  12   a   2 . This undesirably increases the possibility of damaging of the electric wire end portion  12   a.    
     In view of this, a main purpose of the present invention is to provide a terminal connection structure in which the aforementioned sharp difference in height is not present and the shape of a third electrode is improved, and a method for producing such a terminal connection structure. 
     In order to attain the purpose, a terminal connection structure according to the present invention is a terminal connection structure for an electric wire end portion in which the electric wire end portion is soldered being exposed on a flat-plate terminal piece out from inside of a terminal main part, the terminal piece being continuous with a front end of the terminal main part, wherein the electric wire end portion exposed on the terminal piece from the terminal main part includes a first end section that is close to the terminal main part and a second end section that is far from the terminal main part, and the electric wire end portion has a shape, in side view, such that the first end section is compressed to have a gentle curved surface that approaches the terminal piece toward the second end section. 
     In the terminal connection structure according to the present invention, the electric wire end portion exposed on the terminal piece has a shape, in side view, such that the first end section is compressed to have a gentle curved surface that approaches the terminal piece toward the second end section. This allows stress acting between the first end section and the second end section to be dispersed, thereby preventing the electric wire end portion from being broken at a part where the electric wire end portion is exposed on the terminal piece from the terminal main part. It is therefore possible to provide a terminal connection structure that has high mechanical reliability. 
     In one embodiment of the present invention, an inclination angle θ of an intermediate part of the first end section in a longitudinal direction is 20° to 70° on an upper side that is a side far from a soldering surface on which the electric wire end portion on the terminal piece is soldered. 
     In another embodiment of the present invention, a recess having a gently inclined side wall surface is provided in a soldering surface on which the electric wire end portion on the terminal piece is soldered. It is preferable that a protrusion that promotes thermal conduction between the electric wire end portion and the terminal piece or a protrusion around which the electric wire end portion is wound is provided in the recess. 
     In a further embodiment of the present invention, a pair of restricting walls that regulate spread of the electric wire end portion on a soldering surface on which the electric wire end portion on the terminal piece is soldered are provided along respective side edges of the soldering surface. 
     A method for producing a terminal connection structure according to the present invention is a method in which an electric wire end portion is soldered being exposed on a flat-plate terminal piece from inside of a terminal main part, the terminal piece being continuous with a front end side of the terminal main part, the method including the steps of: heating the terminal piece and the electric wire end portion by applying an electric current between a pair of electrodes in a state where the terminal piece and the electric wire end portion exposed on the terminal piece are pressed from both sides thereof in a radial direction by the pair of electrodes; and introducing a flux and then solder into the inside of the terminal main part and onto the terminal piece in a state where the terminal piece and the electric wire end portion are heated, wherein one of the pair of electrodes that presses the electric wire end portion has a curved surface shape or a tapered surface shape at an edge part thereof that is on a radially inner side and on a terminal main part side. 
     In the method for producing a terminal connection structure according to the present invention, one of the pair of electrodes that presses the electric wire end portion has a curved surface shape or a tapered surface shape at an edge part thereof that is on a radially inner side and on a terminal main part side. Accordingly, when the pair of electrodes are pressed against the terminal piece and the electric wire end portion exposed on the terminal piece from both sides thereof in the radial direction, the electric wire end portion exposed on the terminal piece is pressed so that the first end section is compressed to have, in side view, a gentle curved surface that approaches the terminal piece toward the second end section. This allows stress acting between the first end section and the second end section to be dispersed, thereby preventing the electric wire end portion from being broken at a part where the electric wire end portion is exposed on the terminal piece from the terminal main part. 
     Furthermore, in a case where the terminal piece and the electric wire end portion are heated by applying an electric current between the electrodes, the electric current flows in a scattered manner throughout the electric wire end portion from the first end section side to the second end section side. Thus, a rise in temperature caused by heating of the electric wire end portion becomes gentle, leading to improvement in performance of a subsequent soldering process in which a flux and solder are introduced. Furthermore, since the electric wire end portion is heated by applying an electric current between the third electrode and the fourth electrode in a state where the electric wire end portion is nipped by the pair of third and fourth electrodes that face each other in the radial direction and the electric wire end portion is soldered by introducing solder and a flux into the terminal main part, the electric wire end portion and the terminal are soldered onto each other without creating voids in the terminal. As a result, it is possible to provide a terminal connection structure that is free from troubles caused by voids and that accomplishes a good solder connection state. 
     One embodiment of the method according to the present invention further includes the step of heating the terminal main part and the electric wire end portion inside the terminal main part by applying an electric current between a pair of first and second electrodes in a state where the terminal main part is pressed from both sides thereof in the radial direction by the pair of first and second electrodes. 
     Another embodiment of the method according to the present invention further includes the step of inserting, into the electric wire end portion, a plurality of heat transmitting members each having a wire shape and being made of a metal having higher thermal conductivity than the electric wire end portion. 
     A further embodiment of the method according to the present invention is arranged such that a recess having a gently inclined side wall surface is provided in a surface of the terminal piece on which the electric wire end portion is soldered; and the other one of the pair of electrodes that presses the terminal piece presses part of the terminal piece that is located out of the recess on a side opposite to the terminal main part. 
     A still further embodiment of the method according to the present invention is arranged such that a pair of restricting walls are provided along respective side edges of the terminal piece; and the steps are performed in a state where spread of the electric wire end portion exposed on the terminal piece is restricted by the pair of restricting walls. 
     Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawings should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
         FIG. 1  is a perspective view of a terminal used in a terminal connection structure for an electric wire end portion according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 ; 
         FIG. 3A  is an enlarged cross-sectional view of a substantial part of  FIG. 2 ; 
         FIG. 3B  is a plan view of the substantial part of  FIG. 2 ; 
         FIG. 3C  is a cross-sectional view taken along line III-III of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of a state where an electric wire is inserted into a terminal main part; 
         FIG. 5A  is a plan view illustrating a state where the electric wire end portion is restricted by restricting walls of a terminal piece; 
         FIG. 5B  is a plan view illustrating a state where the electric wire end portion is spread in a case where no restricting wall is provided on the terminal piece; 
         FIG. 6  is a cross-sectional view illustrating a state where the electric wire end portion is inserted into the terminal main part of the terminal and is exposed on the terminal piece and where electrodes are disposed on respective sides of the terminal main part and on respective sides of the terminal piece in a radial direction; 
         FIG. 7A  is a diagram illustrating an example of the shape of a third electrode of  FIG. 6 ; 
         FIG. 7B  is a diagram illustrating an example of the shape of the third electrode of  FIG. 6 ; 
         FIG. 7C  is a diagram illustrating an example of the shape of the third electrode of  FIG. 6 ; 
         FIG. 7D  is a diagram illustrating an example of the shape of the third electrode of  FIG. 6 ; 
         FIG. 8  is a cross-sectional view illustrating a state where the terminal main part, the terminal piece, and the like are pressed by the first to fourth electrodes; 
         FIG. 9  is a cross-sectional view of the terminal connection structure according to the embodiment from which the first to fourth electrodes have been removed in  FIG. 8 ; 
         FIG. 10A  is a plan view illustrating other embodiment; 
         FIG. 10B  is a longitudinal cross-sectional view of the other embodiment; 
         FIG. 10C  is a plan view illustrating a modification; 
         FIG. 11  is a cross-sectional view for explaining problems to be solved by the present invention and illustrates a state where first to fourth electrodes are not pressed against a terminal main part, a terminal piece, and the like; and 
         FIG. 12  is a cross-sectional view of a state where the first to fourth electrodes are pressed against the terminal main part, the terminal piece, and the like from the state of  FIG. 11 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A method for producing a terminal connection structure for an electric wire end portion and the terminal connection structure according to an embodiment of the present invention are described below. A terminal  11  in the terminal connection structure is described with reference to  FIGS. 1 to 3C . The terminal  11  includes a tubular terminal main part  11   a  and a terminal piece  11   b  having a flat-plate shape that is continuous with a front end side of the terminal main part  11   a . The terminal  11  is formed from a plate material made of copper, brass, a copper alloy, or a copper-based composite material. The terminal piece  11   b  has a terminal hole  11   c . The terminal  11  has a metal plating layer (not illustrated) made of tin or solder that is provided on at least a surface of inside of the terminal main part  11   a . A recess  11   e  is provided in a soldering surface  11   d , which is a top surface of the terminal piece  11   b , and a pair of restricting walls  11   f  are provided along both side edges of the soldering surface  11   d.    
     A side wall surface  11   g  of the recess  11   e  is a gentle slope, as illustrated in the enlarged view of  FIG. 3A . The slope of the side wall surface  11   g  may be gentler according to need. The recess  11   e  acts as a flux reservoir and a solder reservoir when the electric wire end portion  12   a  is soldered on the soldering surface  11   d  of the terminal piece  11   b , thereby allowing soldering to be accomplished well. The number of recesses  11   e  may be one or may be more than one. The width of the recess  11   e  in a direction orthogonal to a longitudinal direction C is approximately identical to an overall width of the electric wire end portion  12   a , and the length of the recess  11   e  is shorter than the width thereof. The depth of the recess  11   e  is approximately ½ of the thickness of the terminal piece  11   b.    
     A plurality of protrusions  110  are provided on a bottom surface of the recess  11   e . Each of the protrusions  110  has an elongated shape extending in the longitudinal direction C, which is an axial direction, as is clear from  FIG. 3B . A protruding end surface of each of the protrusions  110  is flush with the soldering surface  11   d , which is a top surface of the terminal piece  11   b , or is located slightly above or below the soldering surface  11   d . Part of the electric wire end portion  12   a  passes between adjacent protrusions  110 . The protrusions  110  increase a contact area between the electric wire end portion  12   a  and the terminal piece  11   b  and thereby promote thermal conduction between the electric wire end portion  12   a  and the terminal piece  11   b . Furthermore, the protrusions  110  make it possible to evenly heat the electric wire end portion  12   a  to the inside thereof, thereby allowing soldering that will be described later to be accomplished well. Since the end surfaces of the protrusions  110  are close to an electrode  15   a , heat transmission from the electrode  15   a  is improved, and therefore the protrusions  110  are effective especially in a case where an aluminum electric wire, which has lower thermal conductivity than copper, is used. The protrusions  110  may be omitted. 
     The restricting walls  11   f  rise in a direction of the soldering surface  11   d  and thus restrict spread of the overall width of the electric wire end portion  12   a  as illustrated in  FIGS. 5A and 5B . To make this restricting effective, the restricting walls  11   f  are preferably located lateral to the recess  11   e.    
     In the terminal  11  configured as above, an electric wire  12  made of aluminum, copper, or the like is mounted from an opening of the terminal main part  11   a  on one side, and an end  12   a , which is a front end portion of the electric wire  12 , is soldered inside the terminal main part  11   a  and on the soldering surface  11   d  which is a top surface of the terminal piece  11   b , as illustrated in  FIG. 4 . The electric wire  12  is formed of a large number of aluminum wires twisted together. Enamel insulation coating is removed from an end of the twisted wires. In the following description, enamel insulation coating is removed in advance from a part of the twisted wires from which the coating need be removed. The present invention is not limited to the twisted wires but encompasses collected wires obtained by merely collecting a plurality of wires. 
     A plurality of linear heat transmitting members  13  are inserted into the electric wire end portion  12   a  inside the terminal main part  11   a  so as to be disposed apart from one another by a predetermined interval in parallel with the longitudinal direction C of the electric wire end portion  12   a . The heat transmitting members  13  are inserted into the electric wire end portion  12   a  while keeping the linear shape so as to form high-temperature passages in the electric wire end portion  12   a  during soldering, and in this state, solder (not illustrated) is introduced to the inside of the terminal main part  11   a  through the high-temperature passages. The heat transmitting members  13  also have a function of promoting flow of the solder and flux in the electric wire end portion  12   a . The number of heat transmitting members  13  inserted into the electric wire end portion  12   a  may be more than one or may be one. 
     The heat transmitting members  13  are made of a metal (e.g., copper) having higher thermal conductivity than the electric wire end portion  12   a , and a metal plating layer having a thickness of approximately 6μ to 10μ and made of a material such as tin or solder is provided on a surface of each of the heat transmitting members  13 . The metal plating layer (not illustrated) on the surface of the terminal  11  and the metal plating layer on the surfaces of main body of the heat transmitting members  13  melt due to high heat applied to the terminal  11  during soldering. This makes it easier to form the high-temperature passages. 
     The electric wire end portion  12   a  is aligned along the pair of restricting walls  11   f  without spreading the overall width on the soldering surface  11   d  of the terminal piece  11   b  because of the pair of restricting walls  11   f , as illustrated in  FIG. 5A . Without the restricting walls  11   f , the overall width of the electric wire end portion  12   a  expands on the soldering surface  11   d  as illustrated in  FIG. 5B , and it is therefore impossible to accomplish soldering well. 
     Next, a soldering method according to the embodiment is described with reference to  FIGS. 6 to 8 . First, the electric wire end portion  12   a  is inserted into the terminal main part  11   a  and is exposed on the terminal piece  11   b  outside the terminal main part  11   a , as illustrated in  FIG. 6 . Next, a pair of first and second electrodes  14   a  and  14   b  are disposed on outer sides in a radial direction of the terminal main part  11   a  so as to face each other, and a pair of third and fourth electrodes  15   a  and  15   b  are disposed on outer sides in a radial direction of the terminal piece  11   b  outside the terminal main part  11   a  and the electric wire end portion  12   a  on the terminal piece  11   b  so as to face each other. 
     In this case, each of the first and second electrodes  14   a  and  14   b  and the fourth electrode  15   b  has a cubic shape, whereas the third electrode  15   a  has a shape such that an edge part thereof on the radially inner side and on the terminal main part  11   a  side has a shape of a predetermined curved surface bulging toward the terminal main part  11   a  as illustrated in  FIG. 7A . This shape of a curved surface is described below. As illustrated in  FIG. 7B , for example, the third electrode  15   a  has a curved surface S 3  between: a flat side surface S 1  extending in a direction orthogonal to the soldering surface  11   d  of the terminal piece  11   b  from the terminal main part  11   a , i.e., in a direction orthogonal to the longitudinal direction C; and a flat bottom surface S 2  that faces the electric wire end portion  12   a  on the terminal piece  11   b . The curved surface S 3  has a large radius of curvature and is gently curved. 
     The shape of the curved surface S 3  includes not only a partial cylindrical surface, but also a shape that changes in an exponential manner or in a parabolic manner. In this case, the third electrode  15   a  may have a curved surface S 4  obtained by curving the entire surface facing the electric wire end portion  12   a  while eliminating the side surface S 1 , as illustrated in  FIG. 7C . Alternatively, a flat tapered surface S 5  may be employed instead of a curved surface, as illustrated in  FIG. 7D . Even in a case where the flat tapered surface S 5  is employed, an electric wire part against which the tapered surface S 5  is pressed is made into a curved surface owing to springback. 
     Next, as illustrated in  FIG. 8 , the first and second electrodes  14   a  and  14   b  are pressed against the terminal main part  11   a  from both sides thereof in the radial direction, so that the terminal main part  11   a  is nipped by the first and second electrodes  14   a  and  14   b . In this state, the terminal main part  11   a  is heated by applying an electric current between the first and second electrodes  14   a  and  14   b.    
     Similarly, the third and fourth electrodes  15   a  and  15   b  are pressed against the terminal piece  11   b  outside the terminal main part  11   a  and against the electric wire end portion  12   a  on the terminal piece  11   b  from both sides thereof in the radial direction, so that the electric wire end portion  12   a  and the terminal piece  11   b  are nipped by the third and fourth electrodes  15   a  and  15   b . In this state, the terminal piece  11   b  and the electric wire end portion  12   a  are heated by applying an electric current between the third and fourth electrodes  15   a  and  15   b . A method for applying an electric current is not limited in particular. 
     In this case, an edge part  15   a   1  ( FIG. 7A ) of the third electrode  15   a  on the radially inner side and on the terminal main part  11   a  side is the curved surface S 3  or the tapered surface S 5 , and therefore in a case where the electric wire end portion  12   a  is temporarily sectioned to a first end section  12   a   1  close to the terminal main part  11   a  and a second end section  12   a   2  on the front end side far from the terminal main part  11   a , the first end section  12   a   1  is gently curved or tapered so as to approach the terminal piece  11   b  toward the second end section  12   a   2 , i.e., toward the front end side. Accordingly, in a case where the electric wire end portion  12   a  on the terminal piece  11   b  is soldered by pressing the third and fourth electrodes  15   a  and  15   b  against the electric wire end portion  12   a , the first end section  12   a   1  of the electric wire end portion  12   a  is not pressed against the soldering surface  11   d , which is the inner side of the terminal piece  11   b , by the third electrode  15   a , whereas the second end section  12   a   2  is pressed against the soldering surface  11   d  by the third electrode  15   a . This prevents formation of a steep difference in height between the first end section  12   a   1  and the second end section  12   a   2 . 
     That is, the electric wire end portion  12   a  is gently curved between a rear edge  121  which is the highest portion of the curved first end section  12   a   1  on an upper side (side far from the terminal piece  11 ) of the first end section  12   a   1 , and a rear edge  122  of the second end section  12   a   2  at which the curve ends and a straight part starts, and a ratio (L/H) of a dimension L in the longitudinal direction between the rear edges  121  and  122  to a height difference H between the rear edges  121  and  122  is 0.4 to 2.8, preferably 0.7 to 2.5, more preferably 1.0 to 2.0. The above three ranges of the ratio L/H are 20° to 70°, preferably 20° to 55°, more preferably 25° to 45° when expressed by an inclination angle θ of a longitudinal-direction intermediate part  12   m  of the first end section  12   a   1  with respect to the longitudinal direction C. In a case where the inclination angle θ is 70° or more, the first end section  12   a   1  is steeply curved, and the electric wire end portion  12   a  is more likely to be damaged. In a case where the inclination angle θ is less than 20°, the first end section  12   a   1  becomes too long, and the size of the terminal connection structure becomes large. 
     Furthermore, since a part of the terminal piece  11   b  in which the recess  11   e  is not provided is pressed by the third and fourth electrodes  15   a  and  15   b  with the electric wire end portion  12   a  interposed therebetween, a sufficient amount of electric current flows on the second end section  12   a   2  side of the electric wire end portion  12   a  so as to heat the second end section  12   a   2  side, whereas the temperature inside the recess  11   e  does not become high, and heat is gradually transmitted. This heat gradually evaporates the flux accumulated in the recess  11   e  and allows solder introduced from the second end section  12   a   2  to the first end section  12   a   1  to be sufficiently introduced into the electric wire end portion, whereby the electric wire end portion is soldered. In this case, because of the recess  11   e , gentle inclination of the side wall surface  11   g  of the recess  11   e , and taper of the edge part  15   a   1  of the third electrode  15   a , damage of the electric wire end portion  12   a  caused by heat is small, and since excessive stress does not act, mechanical damage of the electric wire end portion  12   a  is also small. 
     The above configuration prevents formation of a sharp difference in height between the first end section  12   a   1  and the second end section  12   a   2  of the electric wire end portion  12   a  outside the terminal main part  11   a . Thus, during soldering, the electric wire end portion  12   a  is hard to be broken, and owing to the recess  11   e , a flux is less apt to evaporate and is maintained for a long period. As a result, the electric wire end portion  12   a  can be soldered onto the soldering surface  11   d  of the terminal piece  11   b  with high reliability. 
     In the electric wire end portion  12   a , the heat transmitting members  13  are almost parallel with the twisted wires of the electric wire end portion  12   a . Upon application of heat to the terminal main part  11   a  from outside, high heat is transmitted to the heat transmitting members  13 . This melts the metal plating layer provided on each of the heat transmitting members  13 , thereby forming a high-temperature passage around each of the heat transmitting members  13 . In this state, a flux and molten solder such as aluminum solder are introduced into the terminal main part  11   a  from above. The flux introduced into the terminal main part  11   a  flows into the electric wire end portion  12   a  along the heat transmitting parts  13  having a high temperature and expands to the inside of the terminal main part  11   a.    
     Furthermore, the flux is absorbed into the electric wire end portion  12   a  due to a capillary action caused by gaps around each of the heat transmitting members  13 . As a result, the flux acts on the surface of the electric wire end portion  12   a  and removes an oxide film formed on the surface of the electric wire end portion  12   a . Next, the introduced molten solder flows into the electric wire end portion  12   a  while being guided by the heat transmitting members  13 , as in the flux. As a result, the solder spreads inside the terminal main part  11   a  and connects the twisted wires of the electric wire end portion  12   a  to one another and connects the electric wire end portion  12   a  and the terminal main part  11   a . In this way, inside the terminal main part  11   a , the flux acts on the surface of the electric wire end portion  12   a , and the molten solder makes contact with the electric wire end portion  12   a  from which the oxide film has been removed by the flux, thus forming a good soldered part having no void inside the terminal main part  11   a.    
       FIG. 9  illustrates a terminal joining structure for an electric wire end portion after soldering in which no electrode is present.  FIG. 9  illustrates a terminal connection structure from which electrodes have been removed after soldering in  FIG. 8 . 
       FIGS. 10A to 10C  illustrate another embodiment. As illustrated in  FIGS. 10A and 10B , a protrusion  112  is provided at the center of the recess  11   e , and the second end section  12   a   2  of the electric wire end portion  12   a  is wound around the protrusion  112 . The upper-side third electrode  15   a  has, on a bottom surface thereof, a recessed part  130  into which a top end of the protrusion  112  enters, and the second end section  12   a   2  is pressed into the recess  11   e  by the third electrode  15   a  and is firmly joined to the terminal  11  by solder accumulated in the recess  11   e . The number of protrusions  112  may be more than one. 
     The electric wire end portion  12   a  may be divided into two parts in a width direction orthogonal to the longitudinal direction C, and the divided parts may be wound around the protrusion  112 , as illustrated in  FIG. 10C . Thus, the electric wire end portion  12   a  is evenly heated to the inside thereof. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein. 
     REFERENCE NUMERALS 
     
         
         
           
               11  . . . terminal 
               11   a  . . . terminal main part 
               11   b  . . . terminal piece 
               11   d  . . . soldering surface 
               11   e  . . . recess 
               11   g  . . . side wall surface 
               11   f  . . . restricting wall 
               12  . . . electric wire 
               12   a  . . . electric wire end portion 
               12   a   1  . . . first end section 
               12   a   2  . . . second end section 
               13  . . . heat transmitting member 
               14   a  . . . first electrode 
               14   b  . . . second electrode 
               15   a  . . . third electrode (electrode that presses electric wire end portion) 
               15   b  . . . fourth electrode (electrode that presses electric wire end portion) 
               110  . . . protrusion for promoting thermal conduction 
               112  . . . protrusion for winding 
             S 3 , S 4  . . . curved surface 
             S 5  . . . tapered surface 
             θ . . . inclination angle