Abstract:
A connector mountable to a printed circuit board is disclosed including a nonconductive housing with a plurality of slots, each slot having an opposed first and second end walls with grooves in each end wall and a plurality of terminals ( 41 ). Each terminal has a longitudinal axis, a retaining base ( 42 ), a solder portion ( 44 ), a resilient contact arm ( 43 ) and an alignment tab ( 45 ). The retaining base ( 42 ) has opposed edges with each edge fitting into a respective groove in the end walls holding each terminal to the non-conductive housing. The solder portion ( 44 ) extends from the retaining base ( 42 ) adjacent to the first end wall of the slot for soldering to the printed circuit board. The resilient contact arm ( 43 ) has opposed first and second sides extending from the retaining base ( 42 ), the first side of the arm is located adjacent the second end wall of the slot. The contact arm ( 43 ) is adapted to engage with a terminal ( 21 ) from a mating connector. The resilient contact arm and the solder portion ( 44 ) are offset from each other in a direction parallel to the longitudinal axis of the terminal. A locating tab ( 45 ) is stamped from the retaining base ( 42 ) with one end adjacent the second side of the resilient contact arm so that the second side of the resilient contact arm will engage the one edge of the locating tab if a side force is placed on the resilient contact arm ( 43 ) causing it to move away from the first side wall of the slot.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a board-to-board connector. 
   DESCRIPTION OF THE RELATED ART 
   Conventionally, a board-to-board connector is used to electrically connect two parallel circuit boards together (see, for example, Japanese Patent Application Laid-Open (kokai) No. H10-125420). Such a board-to-board connector includes two connector sections which are respectively attached to mutually facing surfaces of two circuit boards and projects therefrom. The two connector sections are mated and connected with each other so as to establish electrical connection between the two circuit boards. In this case, each of the connector sections has a plurality of terminals whose tail portions are connected, through soldering, to wiring traces formed on the surface of the corresponding circuit board. When the connector sections are mated together, the terminals of one connector section come into contact with the corresponding terminals of the other connector section, whereby the two circuit boards are electrically connected to each other. 
   However, the above-mentioned conventional board-to-board connector encounters difficulty in sufficiently reducing the size and mounting area on a circuit board. In recent years, with advancement of miniaturization and densification of electronic apparatuses, a larger number of electronic components are mounted on a circuit board, so that area for mounting a connector is limited. In addition, the number and density of wiring traces formed on a circuit board increases, and therefore, when two circuit boards are connected together, a large number of wiring traces on one circuit board must be connected to a large number of wiring traces on the other circuit board. Therefore, a connector is required to have a large number of terminals, reduced size, and a reduced mounting area. However, the conventional board-to-board connector cannot sufficiently meet these requirements. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to solve the above-mentioned problems in the conventional board-to-board connector and to provide a board-to-board connector which includes a plurality of pairs of rows of terminals which are disposed in an integrally formed single housing and each of which has a surface-mount-type solder tail portion such that the solder tail portion does not project to the outside of the housing, which has a large number of terminals, reduced size, and a reduced mounting area, which is be easily manufactured and mounted to a board, and which has high reliability. 
   To achieve the above object, a board-to-board connector according to the present invention comprises an integrally formed housing, and a plurality of terminals attached to the housing and forming a plurality of pairs of terminal rows, each terminal having a surface-mount-type solder tail portion, wherein the solder tail portion does not project to the outside of the housing. 
   Preferably, each of the terminals has a contact portion which comes into contact with a counterpart terminal, and a barrier portion formed between the solder tail portion and the contact portion and formed of a film to which solder hardly adheres. 
   Preferably, at least some of the terminals have a lean-preventing portion formed on at least one side of the contact portion to be located near the contact portion. 
   Preferably, the terminals are generally arranged in a grid pattern, and at least terminals located at the outermost positions in the grid pattern are electrically grounded to a board. 
   Preferably, the terminals are generally arranged in a grid pattern, a signal trace is connected to at least one terminal, and terminals located at positions surrounding the one terminal are electrically grounded to the board. 
   The board-to-board connector according to the present invention comprises an integrally formed housing, and a plurality of terminals attached to the housing and forming a plurality of pairs of terminal rows, each terminal having a surface-mount-type solder tail portion, wherein the solder tail portion does not project to the outside of the housing. Therefore, the board-to-board connector according to the present invention can have a large number of terminals and reduce size, reduce the mounting area, facilitate manufacture and mounting to a board, and enhance reliability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a first connector according to an embodiment of the present invention; 
       FIG. 2  is an enlarged perspective view showing a main portion of the first connector according to the embodiment; 
       FIG. 3  is an enlarge a plain view snowing the main portion of the first connector according to the embodiment; 
       FIG. 4  is a perspective view of the first terminal according to the embodiment; 
       FIG. 5  is a view of the first terminal as viewed from three sides according to the embodiment; 
       FIG. 6  is a lateral cross-sectional view of the first connector according to the embodiment; 
       FIG. 7  is an enlarged lateral cross-sectional view showing a main portion of the first connector and an enlarged view of portion A of  FIG. 6  according to the embodiment; 
       FIG. 8  is a perspective view of a second connector according to the embodiment; 
       FIG. 9  is an enlarged perspective view showing a main portion of the second connector according to the embodiment; 
       FIG. 10  is an enlarged plan view showing the main portion of the second connector according to the embodiment; 
       FIG. 11  is a perspective view of a second terminal according to the embodiment; 
       FIG. 12  is a view of the second terminal as viewed from three sides according to the embodiment; 
       FIG. 13  is a lateral cross-sectional view of the second connector according to the embodiment; 
       FIG. 14  is an enlarged lateral cross-sectional view showing a main portion of the second connector and an enlarged view of portion B of  FIG. 13  according to the embodiment; 
       FIG. 15  is a sectional view showing a state before the first and second connectors of the embodiment are mated together; and 
       FIG. 16  is a sectional view showing a state after the first and second connectors of the embodiment are mated together. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of the present invention will next be described in detail with reference to the drawings. 
     FIG. 1  is a perspective view of a first connector according to an embodiment of the present invention;  FIG. 2  is an enlarged perspective view showing a main portion of the first connector according to the embodiment; and  FIG. 3  is an enlarged plan view showing the main portion of the first connector according to the embodiment. 
   In these drawings, reference numeral  10  denotes a first connector, which is one of paired board-to-board connectors according to the present embodiment and which is a surface-mount-type connector to be mounted on a first circuit board  20  to be described later. The first connector  10  is inserted into a second connector  30 , which is a counterpart connector and which will be described later. The second connector  30  is a surface-mount-type connector to be mounted on a second circuit board  40  to be described later. The board-to-board connectors according to the present embodiment include the first connector  10  and the second connector  30  and electrically connect a pair of circuit boards; i.e., the circuit boards  20  and  40 . Although the circuit boards  20  and  40  are printed circuit boards, the circuit boards  20  and  40  can be of any type. 
   In the present embodiment, terms for expressing direction, such as up, down, left, right, front, and rear, are used for explaining the structure and action of respective portions of the board-to-board connectors; however, these terms represent respective directions for the case where the board-to-board connectors are used in an orientation shown in the drawings, and must be construed to represent corresponding different directions when the orientation of the board-to-board connectors is changed. 
   The first connector  10  includes a first housing  11  integrally formed from an insulative material such as a synthetic resin. As shown in  FIGS. 1 to 3 , the first housing  11  has a shape of a generally rectangular thick plate, and a generally rectangular concave portion  12  is formed on an upper surface of the first housing  11 . The first connector  10  has a size of about 15 mm (length)×about 7 mm (width)×about 1.3 mm (thickness); however, the size can be changed freely. In the concave portion  12 , a plurality of ridge portions  13  are formed integrally with the first housing  11 . The ridge portions  13  project upward from the bottom surface of the concave portion  12  and extend along the longitudinal direction of the first housing  11 . Thus, an elongated groove portion  15  extending along the longitudinal direction of the first housing  11  is formed on either side of each ridge portion  13 . In the illustrated example, the number of the ridge portions  13  is three; however, the number is arbitrary insofar as the number is not less than 2. Although each of the ridge portions  13  has a width of about 0.8 mm, the width may be changed freely. 
   First-terminal accommodation cavities  14  for accommodating first terminals  21  are formed on each of opposite side walls of each ridge portion  13 . For example, ten first-terminal accommodation cavities  14  are formed on each side wall of each ridge portion  13  at a pitch of about 1 mm. Therefore, ten first terminals  21 , which are accommodated in the first-terminal accommodation cavities  14 , are disposed on each side wall of each ridge portion  13  at a pitch of about 1 mm. The first terminals  21  are disposed in a staggered manner such that the first terminals  21  on one side wall are positionally shifted from those on the other side wall by half a pitch. That is, each of the first terminals  21  on one side wall of each ridge portion  13  is centrally located between the first terminals  21  on the other side wall thereof with respect to the longitudinal direction of the first housing  11 . 
   As shown in  FIG. 3 , when the first connector  10  is viewed from the upper side, the first terminals  21  which constitute a pair of terminal rows are arranged such that a single first terminal  21  disposed on a first side wall of the corresponding ridge portion  13  and two corresponding first terminals  21  disposed on a second side wall of the ridge portion  13  form a first triplet in the form of an isosceles triangle wherein the single first terminal  21  is located at the apex between the two equal sides and the two first terminals  21  are located on the remaining apexes, and such that a single first terminal  21  disposed on the second side wall of the corresponding ridge portion  13  and two corresponding first terminals  21  disposed on the first side wall of the ridge portion  13  form a second triplet in the form of an isosceles triangle which has an orientation opposite that of the first triplet. The first and second triplets are alternately arranged along the longitudinal direction of the ridge portion  13 . The ground conductor of a differential signal line is connected to one first terminal  21  of each triplet, which terminal is located at the apex between the two equal sides of the isosceles triangle, and the paired signal conductors of the differential signal line are connected to the remaining first terminals  21  of each triplet, whereby generation of crosstalk can be prevented. 
   The pitches and numbers of the first-terminal accommodation cavities  14  and the first terminals  2  can be changed freely. Since the numbers of the first-terminal accommodation cavities  14  and the first terminals  21  are considerably large, in  FIGS. 1 to 3 , only those located in the vicinity of the opposite longitudinal end portions of each ridge portion  13  are illustrated. 
     FIGS. 1 to 3  show only portions of the first terminals  21  which serve as contact portions  23 , which will be described later.  FIGS. 1 to 3  show only portions of the first-terminal accommodation cavities  14 , which portions accommodate the contact portions  23 . However, in actuality, each first-terminal accommodation cavity  14  is formed such that it penetrates the corresponding ridge portion  13  from its upper surface to the lower surface of the first housing  11 , and is expanded in the interior of the ridge portion  13 , etc., to a size sufficient to accommodate the entirety of the first terminal  21 . 
   Next, the structure of the first terminal  21  will be described. 
     FIG. 4  is a perspective view of the first terminal according to the embodiment; and  FIG. 5  is a view of the first terminal as viewed from three sides according to the embodiment. Specifically,  FIG. 4A  is a front perspective view as viewed from the upper right;  FIG. 4B  is a rear perspective view as viewed from the upper left;  FIG. 4C  is a front perspective view as viewed from the upper left;  FIG. 4D  is a rear perspective view as viewed from the upper right;  FIG. 5A  is a front view;  FIG. 5B  is a side view; and  FIG. 5C  is a top view. 
   As shown in  FIGS. 4 to 5 , each first terminal  21  has a body portion  22 , the contact portion  23 , and a solder tail portion  24 , and is integrally formed from an electrically conductive metal sheet through punching and forming (bending). The contact portion  23  is formed from an elongated plate-shaped portion extending from the upper end of the body portion  22 , the plate-shaped portion being bent at a curved portion  23   a  by about 180°, so that the contact portion  23  is generally parallel to the body portion  22 . A tip-end-side portion of the contact portion  23  is formed to serve as a contact flat portion  23   b , whose front surface (left side surface in  FIG. 5B ) comes into contact with a contact portion  43  of a second terminal  41  (which will be described later) of the second connector  30 . The contact portion  23 , the curved portion  23   a , the contact flat portion  23   b , and the body portion  22  form a generally inverted-U-shaped side profile, as shown in  FIG. 5B . Therefore, when the first connector  10  is mated with the second connector  30  and the front surface of the contact flat portion  23   b  is pushed toward the body portion  22  by the contact portion  43  of the second terminal  41 , the contact flat portion  23   b  is pressed against the contact portion  43  of the second terminal  41 , which is a counterpart terminal. Therefore, contact with the contact portion  43  of the second terminal  41  can be maintained reliably. 
   The solder tail portion  24  is formed from an elongated plate-shaped portion extending from the lower end of the body portion  22 , the plate-shaped portion being bent at a bent portion  24   a  by about 90°, so that the solder tail portion  24  is generally perpendicular to the body portion  22 . The solder tail portion  24 , the bent portion  24   a , and the body portion  22  form a generally L-shaped side profile, as shown in  FIG. 5B . The solder tail portion  24  is soldered to a wiring land in a state in which the lower surface of the solder tail portion  24  faces the upper surface of the wiring land. 
   The first terminal  21  is formed such that the body portion  22  has a width of about 0.8 mm and a height of about 1 mm as measured from the lower end thereof to the upper surface of the curved portion  23   a . However, the size of the first terminal  21  can be changed freely. 
   A laterally extending, strip-shaped solder barrier portion  25 , serving as a barrier portion, is formed on the front surface (the left-hand surface in  FIG. 5B ) and rear surface (the right-hand surface in  FIG. 5B ) of the body portion  22 . The solder barrier portion  25  is formed of, for example, a nickel (Ni) film formed through plating. However, film of any type may be used, insofar as solder hardly adheres to the formed film. Further, no limitation is imposed on the method of forming the solder barrier portion  25 . The solder barrier portion  25  prevents so called “solder rising phenomenon” in which solder rises along the surface of the body portion  22  and adheres to the contact flat portion  23   b  when the solder tail portion  24  is soldered to a wiring land of the circuit board  20 . 
   Further, in the first terminal  21 , whereas the solder tail portion  24  extends from the lower end of the body portion  22 , the contact portion  23  extends from the upper end of the body portion  22  and is bent by about 180°. Therefore, the distance between the solder tail portion  24  and the contact flat portion  23   b  is long, so that solder is less likely to rise along the surface of the body portion  22  and reach the contact flat portion  23   b . Moreover, as shown in  FIG. 5A , whereas the solder tail portion  24  extends from the vicinity of the right end of the body portion  22  as viewed from the front side of the body portion  22 , the contact portion  23  extends from the vicinity of the left end of the body portion  22  as viewed from the front side of the body portion  22 . Therefore, not only in the vertical direction, but also in the lateral direction, the distance between the solder tail portion  24  and the contact flat portion  23   b  of the first terminal  21  is long, so that solder is less likely to rise along the surface of the body portion  22  and reach the contact flat portion  23   b . Thus, by virtue of its shape, the first terminal  21  can prevent the solder rising phenomenon. 
   Notably, gold (Au) film is preferably formed on the solder tail portion  24  through plating in order to improve adherence of solder to the solder tail portion  24 . Further, gold (Au) film is preferably formed on at least the front surface of the contact flat portion  23   b  in order to reduce electrical contact resistance. 
   Next, the first terminals  21  in a state in which they are mounted to the first housing  11  will be described. 
     FIG. 6  is a lateral cross-sectional view of the first connector according to the embodiment; and  FIG. 7  is an enlarged lateral cross-sectional view showing a main portion of the first connector and an enlarged view of portion A of  FIG. 6  according to the embodiment. 
   As shown in  FIGS. 6 and 7 , each first-terminal accommodation cavity  14  is formed such that it penetrates the corresponding ridge portion  13  from its upper surface to the lower surface of the first housing  11 . In the state in which the first terminal  21  is accommodated in the first-terminal accommodation cavity  14 , the solder tail portion  24  projects downward (downward in  FIG. 6 ) from the wall under the concave portion  12 ; i.e., from the lower surface of the first housing  11 . Notably, the upper end of the first terminal  21 ; i.e., the upper end of the curved portion  23   a , does not project from the upper surface of the ridge portion  13 . 
   The solder tail portion  24  does not project to the outside of the first housing  11  with respect to the longitudinal direction and lateral direction of the first housing  11 ; i.e., with respect to directions parallel to the lower surface of the first housing  11  or the surface of the circuit board  20 . Therefore, even when the first connector  10  is surface-mounted to the circuit board  20  by means of soldering the solder tail portion  24  to a corresponding wiring land of the circuit board  20 , the solder tail portion  24  does not project to the outside of the first housing  11 , whereby the mounting area can be reduced. 
   As shown in  FIGS. 1 to 3  as well, a portion of each first-terminal accommodation cavity  14  corresponding to the contact portion  23  is opened to the upper surface and the corresponding side wall surface of the ridge portion  13 . Meanwhile, a portion of the first-terminal accommodation cavity  14  corresponding to the body portion  22  assumes the form of a thin slit and extends within the ridge portion  13 . The first terminal  21  is inserted and fitted into the first-terminal accommodation cavity  14  from the lower side of the first housing  11 , so that the first terminal  21  is accommodated in the first-terminal accommodation cavity  14 . At this time, the front surface and rear surface of the body portion  22  are nipped between opposite wall surfaces of the thin-slit-shaped portion of the first-terminal accommodation cavity  14 , whereby the first terminal  21  is fixed. Further, a projection is formed at least one side edge (left side edge in  FIG. 5A ) of the body portion  22 . Since the projection bites into the corresponding wall surface of the first-terminal accommodation cavity  14 , the first terminal  21  is prevented from coming off the first-terminal accommodation cavity  14 . 
   Next, the second connector  30  will be described. 
     FIG. 8  is a perspective view of the second connector according to the embodiment;  FIG. 9  is an enlarged perspective view showing a main portion of the second connector according to the embodiment; and  FIG. 10  is an enlarged plan view showing the main portion of the second connector according to the embodiment. 
   The second connector  30  includes a second housing  31  integrally formed from an insulative material such as a synthetic resin. As shown in  FIGS. 8 to 10 , the second housing  31  has a shape of a generally rectangular thick plate, and a generally rectangular upper surface. The second connector  30  has a size of about 14 mm (length)×about 6 mm (width)×about 1.1 mm (thickness); however, the size can be changed freely. A plurality of trench portions  32  are formed on the upper surface. The trench portions  32  extend along the longitudinal direction of the second housing  31 . Thus, an elongated ridge portion  33  extending along the longitudinal direction of the second housing  31  is formed on each of opposite sides of each trench portion  32 . In the illustrated example, the number of the trench portions  32  is three; however, the number is arbitrary, insofar as the number is not less than 2. Although each of the trench portions  32  has a width of about 0.8 mm, the width may be changed freely. 
   Second-terminal accommodation cavities  34  for accommodating second terminals  41  are formed on each of opposite side walls of each trench portion  32 . For example, ten second-terminal accommodation cavities  34  are formed on each side wall of each trench portion  32  at a pitch of about 1 mm. Therefore, ten second terminals  41 , which are accommodated in the second-terminal accommodation cavities  34 , are disposed on each side wall of each trench portion  32  at a pitch of about 1 mm. The second terminals  41  are disposed in a staggered manner such that the second terminals  41  on one side wall are positionally shifted from those on the other side wall by half a pitch. That is, each of the second terminals  41  on one side wall of each trench portion  32  is centrally located between the second terminals  41  on the other side wall thereof with respect to the longitudinal direction of the second housing  31 . 
   As shown in  FIG. 10 , when the second connector  30  is viewed from the upper side, the second terminals  41  which constitute a pair of terminal rows are arranged such that a single second terminal  41  disposed on a first side wall of the corresponding trench portion  32  and two corresponding second terminal  41  disposed on a second side wall of the trench portion  32  form a first triplet in the form of an isosceles triangle wherein the single second terminal  41  is located at the apex between the two equal sides and the two second terminals  41  are located on the remaining apexes, and such that a single second terminal  41  disposed on the second side wall of the corresponding trench portion  32  and two corresponding second terminal  41  disposed on the first side wall of the trench portion  32  form a second triplet in the form of an isosceles triangle which has an orientation opposite that of the first triplet. The first and second triplets are alternately arranged along the longitudinal direction of the trench portion  32 . The ground conductor of a differential signal line is connected to one second terminal  41  of each triplet, which terminal is located at the apex between the two equal sides of the isosceles triangle, and the paired signal conductors of the differential signal line are connected to the remaining second terminals  41  of each triplet, whereby generation of crosstalk can be prevented. 
   The pitches and numbers of the second-terminal accommodation cavities  34  and the second terminals  41  can be changed freely. Since the numbers of the second-terminal accommodation cavities  34  and the second terminals  41  are considerably large, in  FIGS. 8 to 10 , only those located in the vicinity of the opposite longitudinal end portions of each trench portion  32  are illustrated. 
     FIGS. 8 to 10  show only portions of each second terminal  41  which respectively serve as a contact portion  43  and a lean preventing portion or locating tab  45 , which will be described later.  FIGS. 8 to 10  show only portions of each second-terminal accommodation cavity  34 , which portions accommodate the contact portion  43  and the lean preventing portion  45 , respectively. However, in actuality, each second-terminal accommodation cavity  34  is formed such that it penetrates the corresponding ridge portion  33  from its upper surface to the lower surface of the second housing  31 , and has a size sufficient to accommodate the entirety of the second terminal  41 . 
   Next, the structure of the second terminal  41  will be described. 
     FIG. 11  is a perspective view of the second terminal according to the embodiment; and  FIG. 12  is a view of the second terminal as viewed from three sides according to the embodiment. Specifically,  FIG. 11A  is a front perspective view as viewed from the upper right;  FIG. 11B  is a rear perspective view as viewed from the upper left;  FIG. 11C  is a front perspective view as viewed from the upper left;  FIG. 11D  is a rear perspective view as viewed from the upper right;  FIG. 12A  is a front view;  FIG. 12B  is a side view; and  FIG. 12C  is a top view. 
   As shown in  FIGS. 11 and 12 , each second terminal  41  has a body portion  42 , the contact portion  43 , a solder tail portion  44 , and the lean preventing portion or locating portion  45 , and is integrally formed from an electrically conductive metal sheet through punching and forming (bending). The contact portion  43  is formed from an elongated plate-shaped portion extending from the lower end of the body portion  42 , the plate-shaped portion being bent at a curved portion  43   a  by about 180°, so that the contact portion  43  is generally parallel to the body portion  42 . A tip-end-side portion of the contact portion  43  is bent in a generally S-like shape to thereby form a contact convex surface portion  43   b , which comes into contact with the front surface of the contact portion  23  of the first terminal  21 , which is a counterpart terminal. The contact portion  43 , the curved portion  43   a , and the body portion  42  form a generally U-shaped side profile, as shown in  FIG. 12B . Further, the contact convex surface portion  43   b  has a generally S-shaped side profile. That is, a lower portion of the contact portion  43  has a U-shaped side profile, and an upper portion of the contact portion  43  has an S-shaped side profile. The contact portion  43  has a spring property realized mainly by means of elastic deformation of the curved portion  43   a  and the contact convex surface portion  43   b.    
   Therefore, when the second connector  30  is mated with the first connector  10  and the front surface (left-side surface in  FIG. 12B ) of the contact convex surface portion  43   b  is pushed toward the body portion  42  by the contact portion  23  of the first terminal  21  of the first connector  10 , the contact convex surface portion  43   b  is caused to react by means of the spring property of the contact portion  43 , so that the contact convex surface portion  43   b  is pressed against the contact portion  23  of the first terminal  21 , which is a counterpart terminal. Therefore, contact with the contact portion  23  of the second terminal  21  can be maintained reliably. 
   The solder tail portion  44  is formed from an elongated plate-shaped portion extending from the lower end of the body portion  42 , the plate-shaped portion being bent at a bent portion  44   a  by about 90°, so that the solder tail portion  44  is generally perpendicular to the body portion or retaining base  42 . The solder tail portion  44 , the bent portion  44   a , and the body portion  42  form a generally L-shaped side profile, as shown in  FIG. 12B . The lower surface of the solder tail portion  44  is soldered to a wiring land formed on the surface of the circuit board  40 , whereby the second connector  30  is mounted to the circuit board  40 . 
   The second terminal  41  is formed such that the body portion  42  has a width of about 0.8 mm and a height of about 1 mm as measured from the upper end thereof to the lower surface of the curved portion  43   a . However, the size of the second terminal  41  can be changed freely. 
   A laterally extending, strip-shaped solder barrier portion  46 , serving as a barrier portion, is formed on the front surface (the left-hand surface in  FIG. 12B ) and rear surface (the right-hand surface in  FIG. 12B ) of the body portion  42 . The solder barrier portion  46  is a nickel (Ni) film formed through plating. However, film of any type may be used, insofar as solder hardly adheres to the formed film. Further, no limitation is imposed on the method of forming the solder barrier portion  46 . The solder barrier portion  46  prevents so called “solder rising phenomenon” in which solder rises along the surface of the body portion  42  and adheres to the contact convex surface portion  43   b  when the solder tail portion  44  is soldered to a wiring land of the circuit board  40 . 
   Further, in the second terminal  41 , as shown in  FIG. 12A , whereas the solder tail portion  44  extends from the vicinity of the right end of the body portion  42  as viewed from the front side of the body portion  42 , the contact portion  43  extends from the vicinity of the left end of the body portion  42  as viewed from the front side of the body portion  42 . Therefore, the lateral distance between the solder tail portion  44  and the contact convex surface portion  43   b  is long, so that solder is less likely to rise along the surface of the body portion  42  and reach the contact convex surface portion  43   b . Moreover, as shown in  FIG. 12B , the contact convex surface portion  43   b  is separated away from the front surface of the body portion  42 . Therefore, not only in the lateral direction, but also in the front-rear direction (lateral direction in  FIG. 12B ), the distance between the solder tail portion  44  and the contact convex surface portion  43   b  is long, so that solder is less likely to rise along the surface of the body portion  42  and reach the contact convex surface portion  43   b . Thus, by virtue of its shape, the second terminal  41  can prevent the solder rising phenomenon. 
   Notably, gold film is preferably formed on the solder tail portion  44  through plating in order to improve adherence of solder to the solder tail portion  44 . Further, gold film is preferably formed on at least the front surface of the contact convex surface portion  43   b  in order to lower electrical contact resistance. 
   The above-mentioned lean preventing portion  45  is extends from the vicinity of the upper end of the body portion  42  such that the lean preventing portion  45  projects forward (leftward in  FIG. 12B ) to a point near the side edge (right-hand edge in  FIG. 12A ) of the contact portion  43 . As shown in  FIG. 12B , the tip end of the lean preventing portion  45  is located at substantially the same position as the tip end of the contact portion  43  with respect to the front-rear direction. By virtue of this configuration, when the contact portion  43  receives a force in the rightward direction in  FIG. 12A , the contact portion  43  comes into contact with and is supported by the lean preventing portion  45 . Therefore, the contact portion  43  does not lean toward the right in  FIG. 12A . As shown in  FIGS. 9 and 10 , in a state in which the second terminal  41  is accommodated in the second-terminal accommodation cavity  34 , one side wall of the second-terminal accommodation cavity  34  is located near the side edge (left-hand edge in  FIG. 12A ) of the contact portion  43  opposite the lean preventing portion  45 . Therefore, when the contact portion  43  receives a force in the leftward direction in  FIG. 12A , the contact portion  43  comes into contact with and is supported by the side wall of the second-terminal accommodation cavity  34 . Therefore, the contact portion  43  does not lean toward the left in  FIG. 12A . In the present embodiment, the curved portion  43   a  and the contact convex surface portion  43   b  of the second terminal  41  elastically deform, and the curved portion  23   a  of the first terminal  21  does not elastically deform. However, this relation may be reversed. Further, the first and second terminals  21  and  41  may be configured such that both the curved portion  23   a  of the first terminal  21  and the curved portion  43   a  of the second terminal  41  elastically deform so as to establish electrical contact between the contact portion  23  of the first terminal  21  and the contact portion  43  of the second terminal  41 . 
   Next, the second terminals  41  in a state in which they are mounted to the second housing  31  will be described. 
     FIG. 13  is a lateral cross-sectional view of the second connector according to the embodiment; and  FIG. 14  is an enlarged lateral cross-sectional view showing a main portion of the second connector and an enlarged view of portion B of  FIG. 13  according to the embodiment. 
   As shown in  FIGS. 13 and 14 , each second-terminal accommodation cavity  34  is formed such that it penetrates the corresponding ridge portion  33  from its upper surface to the lower surface of the second housing  31 . In the state in which the second terminal  41  is accommodated in the second-terminal accommodation cavity  34 , the solder tail portion  44  projects downward (downward in  FIG. 13 ) from the wall under the trench portion  32 ; i.e., from the lower surface of the second housing  31 . Notably, the upper end of the second terminal  41  does not project from the upper surface of the ridge portion  33 . 
   The solder tail portion  44  does not project to the outside of the second housing  31  with respect to the longitudinal direction and lateral direction of the second housing  31 ; i.e., with respect to directions parallel to the lower surface of the second housing  31  or the surface of the circuit board  40 . Therefore, even when the second connector  30  is surface-mounted to the circuit board  40  by means of soldering the solder tail portion  44  to a corresponding wiring land of the circuit board  40 , the solder tail portion  44  does not project to the outside of the second housing  31 , whereby the mounting area can be reduced. 
   As shown in  FIGS. 8 to 10  as well, a portion of each second-terminal accommodation cavity  34  corresponding to the contact portion  43  is opened to the upper surface of the ridge portion  33  and the corresponding side wall surface of the trench portion  32 . Meanwhile, at that portion, each second-terminal accommodation cavity  34  has a width (with respect to the lateral direction of the second terminal  41 ) corresponding to the distance between the left-hand edge of the contact portion  43  and the right-hand edge of the solder tail portion  44  in  FIG. 12A . Further, at the opposite lateral edges of the body portion  42 , corresponding portions of the second-terminal accommodation cavity  34  each assume the form of a thin slit and extends within the ridge portion  33 . The second terminal  41  is inserted and fitted into the second-terminal accommodation cavity  34  from the upper side of the second housing  31 , so that the second terminal  41  is accommodated in the second-terminal accommodation cavity  34 . At this time, at the opposite lateral edges of the body portion  42 , the front surface and rear surface of the body portion  42  are nipped between opposite wall surfaces of the thin-slit-shaped portion of the second-terminal accommodation cavity  34 , whereby the second terminal  41  is fixed. Further, a projection is formed at least one side edge (left side edge in  FIG. 12A ) of the body portion  42 . Since the projection bites into the corresponding wall surface of the second-terminal accommodation cavity  34 , the second terminal  41  is prevented from coming off the second-terminal accommodation cavity  34 . 
   Next, action of mating the first connector  10  with the second connector  30  will be described. 
     FIG. 15  is a sectional view showing a state before the first and second connectors of the embodiment are mated together; and  FIG. 16  is a sectional view showing a state after the first and second connectors of the embodiment are mated together. 
   As shown in  FIGS. 15 and 16 , the first connector  10  has already been surface-mounted to the circuit board  20  through soldering of the solder tail portions  24  of the first terminals  21  to the corresponding wiring lands of the circuit board  20 . Similarly, the second connector  30  has already been surface-mounted to the circuit board  40  through soldering of the solder tail portions  44  of the second terminals  41  to the corresponding wiring lands of the circuit board  40 . 
   As shown in  FIG. 15 , the circuit board  20  is placed on the circuit board  40  such that the upper surface (lower surface in  FIG. 15 ) of the first connector  10  faces the upper surface of the second connector  30 . In this state, the upper surface of the first connector  10  and the upper surface of the second connector  30  are parallel to each other, and the circuit board  20  and the circuit board  40  are parallel to each other. 
   Subsequently, the first connector  10  or the second connector  30  is moved to the counterpart connector and is mated therewith as shown in  FIG. 16 . Since the peripheral wall of the first housing  11  of the first connector  10  is tapered at the inner edge thereof, even when the first connector  10  is misaligned with the second connector  30 , the first connector  10  can be smoothly mated with the second connector  30 . In a state in which the first connector  10  is mated with the second connector  30 , the ridge portions  13  of the first connector  10  are received within the corresponding trench portions  32  of the second connector  30 , and the contact flat portion  23   b  of each first terminal  21  comes into contact with the contact convex surface portion  43   b  of the corresponding second terminal  41 , whereby electrical connection is established between each first terminal  21  and the corresponding second terminal  41 . 
   As described above, in the present embodiment, the first connector  10  and the second connector  30 , which serve as a pair of board-to-board connectors, include the integrally formed first and second housings  11  and  31 , respectively; and the first terminals  21  and the second terminals  41  are attached to the first and second housing  11  and  31 , respectively, such that a plurality of (e.g., three) pairs of terminal rows are formed on each housing. Therefore, the first connector  10  and the second connector  30  can support a large number of (e.g., 60) first terminals  21  and second terminals  41 , even though they are small in size. Moreover, by mating the first connector  10  and the second connector  30  together, reliable connection can be easily established between a large number of signal traces formed on the circuit board  20  to which the first connector  10  is mounted and a large number of signal traces formed on the circuit board  40  to which the second connector  30  is mounted. Since rows of the first terminals  21  and rows of the second terminals  41  are respectively disposed to form pairs, even in the case where the above-mentioned signal traces include traces for differential signals, the signal traces can be connected to the corresponding terminals such that no crosstalk is generated. 
   The first and second terminals  21  and  41  have the surface-mount-type solder tail portions  24  and  44  which do not project to the outside of the first housing  11  and the outside of the second housing  31 , respectively. That is, in a state in which the first connector  10  is mounted to the circuit board  20 , the solder tail portions  24  are located between the first connector  10  and the circuit board  20  and have no portion which projects outward from the outer circumference of the first connector  10 . Similarly, in a state in which the second connector  30  is mounted to the circuit board  40 , the solder tail portions  44  are located between the second connector  30  and the circuit board  40  and have no portion which projects outward from the outer circumference of the second connector  30 . Therefore, areas on the circuit boards  20  and  40  required to mount the first and second connectors  10  and  30 , respectively; that is, their mount areas, can be reduced, whereby the circuit boards  20  and  40  can be densified. 
   The first and second terminals  21  and  41  have the solder barrier portions  25  and  46 , respectively, which are formed of film to which solder hardly adheres. Therefore, it becomes possible to prevent a so-called solder rising phenomenon which would otherwise occur when the solder tail portions  24  and  44  are soldered to wiring lands of the circuit boards  20  and  40 . Moreover, since the first and second terminals  21  and  41  are each shaped such that a large distance is provided between the solder tail portion and the contact portion, the solder rising phenomenon can be prevented more reliably. 
   Moreover, the second terminals  41  each have the lean preventing portion  45 . Therefore, even if the opening of the second-terminal accommodation cavity  34  is wide, the contact portion  43  does not lean laterally, because the contact portion  43  is supported by the lean preventing portion  45 . By virtue of this configuration, even when the contact portion  43  of each second terminal  41  comes into contact with the contact portion  23  of the corresponding first terminal  21  and receives a lateral force as a result of mating of the first connector  10  with the second connector  30 , the contact portion  43  does not lean in the lateral direction. Therefore, the contact portion  23  of each first terminal  21  and the contact portion  43  of the corresponding second terminal  41  come into contact without fail, whereby electrical continuity is established between each first terminal  21  and the corresponding second terminal  41 . 
   Since the first terminals  21  and the second terminals  41  are arranged in a grid pattern, the degree of freedom in allocation of wire conductors to the first terminals  21  and the second terminals  41 ; i.e., the degree of freedom of pin assignment, is high. Therefore, the arrangement of the first and second terminals  21  and  41  assigned to signal conductors and the arrangement of the first and second terminals  21  and  41  assigned to ground conductors can be determined freely. For example, when a ground conductor is connected to at least terminals  21  or  41  located at the outermost positions in the grid pattern so as to ground them to the circuit board  20  or  40 , an effect similar to that attained through provision of an electromagnetic shield can be attained. Further, when a ground conductor is connected to terminals  21  or  41  located at positions surrounding one or more terminals  21  or  41  to which a signal conductor(s) is connected, whereby the former terminals are grounded to the circuit board  20  or  40 , a function similar to that of a coaxial connector to which a coaxial cable is connected can be attained. The present invention is not limited to the above-described embodiment. Numerous modifications and variations of the present invention are possible in light of the spirit of the present invention, and they are not excluded from the scope of the present invention.