Patent Publication Number: US-2016227648-A1

Title: Printed wiring board capable of suppressing mounting failure of surface mount device for flow soldering

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed wiring board capable of suppressing mounting failure, such as a short circuit, by means of adjacent lead terminals or lead-terminal connection pads of a surface mount device in a flow soldering step. 
     2. Description of the Related Art 
       FIGS. 7 to 9  show an example of the configuration of a conventional printed wiring board.  FIG. 7  illustrates a surface mount device  14  mounted on a printed wiring board  10 , lead terminals  13 , and lead-terminal connection pads  11 .  FIG. 7  shows a part of a package body of the surface mount device  14 .  FIG. 8  is a sectional view of one of the lead terminals  13  of  FIG. 7  taken in its extending direction. A top fillet  15  is formed on a front pad projection portion of the lead. A large back fillet  16  is formed on the rear portion of the lead. Further,  FIG. 9  is a sectional view of one of the lead terminals  13  of  FIG. 7  taken in a direction perpendicular to the extending direction. The lead-terminal connection pad  11  is made wider than the lead terminal  13  to form a side fillet  17 . 
     A process for mounting the surface mount device  14  on the printed wiring board  10  roughly comprises steps of reflow soldering and flow soldering. In the reflow soldering, the amount of solder can be adjusted by adjusting the supply of a solder paste. In the flow soldering, in contrast, the printed wiring board  10  is soldered by being brought into contact with jet solder in a molten solder bath, so that it is difficult to accurately adjust the solder supply to the surface mount device  14 . Therefore, the process for mounting the surface mount device  14  on the printed wiring board  10  by the flow soldering has a problem that a short circuit is caused between the adjoining lead terminal  13  and lead-terminal connection pad by the surface tension of the solder. 
     Conventionally, therefore, there are techniques in which a solder-free partition plate is formed in a gap in each lead-terminal connection pad  11  of the surface mount device  14  so as to suppress bridge failure between each lead terminal  13  and the pad  11  (Japanese Patent Applications Laid-Open Nos. 9-219487 and 5-259624). 
     Further, there are techniques in which bridge failure at the proximal portion of the lead terminal is suppressed by tapering the lead-terminal connection pad  11  for soldering the lead terminal  13  of the surface mount device  14  toward the proximal portion of the lead terminal (or toward the package of the surface mount device  14 ) so that the pad  11  is as wide as the lead terminal  13  (Japanese Patent Applications Laid-Open Nos. 2001-339146 and 3-229486). 
     SUMMARY OF THE INVENTION 
     The techniques disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 9-219487) and Patent Document 2 (Japanese Patent Application Laid-Open No. 5-259624) are disadvantageous in that the use of the partition plate results in an increase in cost and the need of additional processes. In the techniques disclosed in Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-339146) and Patent Document 4 (Japanese Patent Application Laid-Open No. 3-229486), bridge failure is suppressed by inhibiting the concentration of molten solder on curved portions by a capillary phenomenon in the reflow soldering step. In the flow soldering step, however, soldering is performed by bringing the printed wiring board into contact with the jet solder. The wider each lead-terminal connection pad, therefore, the shorter the spaces between the connection pads corresponding to the distal end portions of the lead terminals are. Thus, there is a possibility of the occurrence of bridge failure, so that a satisfactory effect cannot be expected. 
     Accordingly, in view of the above-described problems of the prior art, the object of the present invention is to provide a printed wiring board capable of suppressing mounting failure of a surface mount device in a flow soldering step. 
     A printed wiring board according to the present invention, on which a surface mount device with a plurality of lead terminals is mounted by flow soldering, comprises a plurality of lead-terminal connection pads for mounting the surface mount device, the distance between each two adjacent ones of the lead terminals being not greater than the distance between each two adjacent ones of the lead-terminal connection pads. 
     Each of the lead-terminal connection pads is formed of a front pad projection portion located in front of the lead, a pad center portion located below the lead, and a rear pad projection portion located at the back of the lead, the front pad projection portion being longer than the rear pad projection portion. 
     Each of the lead-terminal connection pads is formed of a front pad projection portion located in front of the lead and a pad center portion located below the lead. 
     The printed wiring board and the reverse side of a package of the surface mount device are bonded together by adhesive means. 
     The printed wiring board and bottom surface ends and side surfaces of a package of a contour portion without a lead terminal in the surface mount device are bonded together by adhesive means. 
     According to the present invention, there can be provided a printed wiring board capable of suppressing mounting failure of a surface mount device in a flow soldering step. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will be obvious from the ensuing description of embodiments with reference to the accompanying drawings, in which: 
         FIGS. 1A, 1B and 1C  are views showing Embodiment 1 according to the present invention; 
         FIG. 2  is a view illustrating Embodiment 1 according to the present invention; 
         FIG. 3  is a view showing Embodiment 2 according to the present invention; 
         FIG. 4  is a view showing Embodiment 3 according to the present invention; 
         FIG. 5  is a view showing Embodiment 4 according to the present invention; 
         FIG. 6  is a view showing Embodiment 5 according to the present invention; 
         FIG. 7  is a view showing a conventional printed wiring board; 
         FIG. 8  is a sectional view of a lead terminal of a mounted component mounted on the conventional printed wiring board; and 
         FIG. 9  is a sectional view of the lead terminal of the mounted component mounted on the conventional printed wiring board. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
     Embodiment 1 
       FIGS. 1A, 1B and 1C  are schematic views showing a configuration of the present embodiment.  FIG. 2  is a front view of a lead terminal briefly showing the present embodiment. A surface mount device  14  is mounted on a printed wiring board  10  in a flow soldering step. The printed wiring board  10  is provided with lead-terminal connection pads  12  for soldering lead terminals  13  of the surface mount device  14 . 
     Each of the lead-terminal connection pads  12 , which is provided on the printed wiring board  10  so as to be electrically connected to a circuit pattern (not shown), has a rectangular shape with a width kept constant from an outer end portion  12   a  to an inner end portion  12   b.  The direction from the outer end portion  12   a  toward the inner end portion  12   b  is the direction in which the package of the surface mount device  14  is approached as the surface mount device  14  is mounted on the printed wiring board  10 . The width of each lead-terminal connection pad  12  is denoted by d 1 . The surface mount device  14  is provided with the lead terminals  13  and the width (denoted by d 2 ) of each lead terminal  13  is kept constant from a distal end portion  13   a  to a proximal portion  13   b.  The proximal portion  13   b  is a part of the lead terminal  13  projecting outward from the package of the surface mount device  14 . The width of that part of the lead terminal  13  which is connected to the lead-terminal connection pad  12  by soldering is at least equal to the width d 2 . 
     The width dl of the lead-terminal connection pad  12  is not larger than the width d 2  of the lead terminal  13  (d 1 ≦d 2 ). In order to suppress mounting failure in the flow soldering step, a gap in the lead-terminal connection pad or the lead terminal can be maximized by making the width of the lead-terminal connection pad not larger than that of the lead terminal. 
     In other words, there is a relation g 1 ≧g 2 , where g 1  is the distance between each two adjacent lead-terminal connection pads  12  of the printed wiring board  10  and g 2  is the distance between each two adjacent lead terminals  13  of the surface mount device  14 . Thus, the width of a gap between adjoining solder joints (respective joints of the lead-terminal connection pad  12  and the lead terminal  13 ) can be increased to suppress bridge failure. 
     Embodiment 2 
       FIG. 3  is a sectional view of a lead terminal briefly showing a lead length according to the present embodiment. In general, when a surface mount device  14  is mounted on a printed wiring board  10 , a lead terminal  13  rises at an angle α from its distal end portion  13   a,  further rises at an angle β (α&lt;β) from a lead curve portion  19 , and then reaches a package of the surface mount device  14 . The angles α and β are based on a surface of a lead-terminal connection pad  12  as a reference surface. 
     The point of intersection of a thickness center line  18   a  of a front portion of the lead terminal  13 , ranging from the distal end portion  13   a  to the lead curve portion  19 , and a thickness center line  18   b  of a rear portion behind the lead curve portion  19  corresponds to the lead curve portion  19 . Numeral  25  denotes the point of intersection of the lead-terminal connection pad  12  and a perpendicular line  24  drawn down from the distal end portion  13   a  to the lead-terminal connection pad  12 . 
     Further, numeral  27  denotes the point of intersection of the lead-terminal connection pad  12  and a perpendicular line  26  drawn down from the lead curve portion  19  to the lead-terminal connection pad  12 . 
     The lead-terminal connection pad  12  can be divided between a front pad projection portion  12   c,  pad center portion  12   d,  and rear pad projection portion  12   e.  The front pad projection portion  12   c  is located in front of the lead and covers a section from the outer end portion  12   a  to the intersection point  25 . The pad center portion  12   d  is located below the lead and covers a section from the intersection point  25  to the intersection point  27 . The rear pad projection portion  12   e  is located at the back of the lead and covers a section from the intersection point  27  to the inner end portion  12   b.  The length of the pad center portion  12   d  below the lead is called a lead length  20 . The length of projection of the lead-terminal connection pad  12  from the distal end portion  13   a  (or the length of the front pad projection portion  12   c  in front of the lead) is characterized in being longer than the length of the rear pad projection portion  12   e  at the back of the lead. 
     Further, the length of projection of the lead-terminal connection pad  12  at the proximal portion of the lead terminal (or the length of the rear pad projection portion  12   e  at the back of the lead) is shorter than that of the lead-terminal connection pad  11  (see  FIGS. 7 to 9 ) on the conventional printed wiring board. Thus, in the printed wiring board  10  of Embodiment  1 , a solder pool at the proximal portion of the lead terminal can be reduced to suppress bridge failure. 
     Embodiment 3 
     The present embodiment shown in  FIG. 4  is characterized in that the length of a projection portion  12   e  of a lead-terminal connection pad  12  from an end of its portion at the back of a lead terminal is 0 and that an inner end portion  12   b  of a lead-terminal connection pad  12  is coincident with a point  27  of intersection of the lead-terminal connection pad  12  and a perpendicular line  26  drawn down from the lead curve portion  19  of  FIG. 3  to the lead-terminal connection pad  12 . Specifically, the lead-terminal connection pad  12  is formed of a front pad projection portion  12   c,  which is located in front of the lead and covers a section from an outer end portion  12   a  to an intersection point  25 , and a pad center portion  12   d,  which is located below the lead and covers a section from the intersection point  25  to the intersection point  27 . Thus, in a printed wiring board  10  of Embodiment 3, a solder pool at the proximal portion of the lead terminal can be reduced to suppress bridge failure. 
     Embodiment 4 
       FIG. 5  is a general top view briefly showing the present embodiment.  FIG. 5  shows how an adhesive is applied between a printed wiring board  10  and the bottom surface of a package of a surface mount device  14  for flow soldering. This configuration is characterized in that good mechanical strength can be achieved even in case the amount of solder used for soldering is small. A double-sided tape may be used in place of the adhesive. A member such as the adhesive or the double-sided tape used to bond two members is called an adhesive member. 
     Embodiment 5 
       FIG. 6  is a general top view briefly showing the present embodiment. In  FIG. 6 , the entire body of surface mount device  21  is illustrated. An adhesive is applied between a printed wiring board and the bottom surface ends and side surfaces of a package of a contour portion (adhesive application area  22 ) without a lead terminal in a surface mount device for flow soldering. Thus, good mechanical strength can be achieved even in case the amount of solder used for soldering is small.