Abstract:
According to an aspect of an embodiment, an electronic device comprises a bonding material, an electronic component providing a plurality of pads on a bottom surface thereof, and a printed circuit board providing a plurality of pads on a surface thereof, at least one of the pads of the printed circuit board being connected to at least one of the pads of the electronic component by the bonding material so as to connect the printed circuit board with the electronic component electrically, wherein either the electronic component or the printed circuit board provides a dummy pad on which the bonding material is formed, the bonding material on the dummy pad butting against the other of the surfaces of the electronic component or the printed circuit board.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a technique for mounting an electronic component(s) onto a printed circuit board by solder bonding. 
         [0003]    2. Description of the Related Art 
         [0004]    Electronic components (packages) each having at least one electrode (pad) on the bottom surface thereof and being mounted on a printed circuit board, are frequently used. Examples of the electronic components include a ball grid array (BGA), a chip scale package (CSP), a land grid array (LGA), a quad flat non-leaded package (QFN), a small outline non-leaded package (SON), and a leaded chip carrier (LCC). 
         [0005]    The mounting of such an electronic component onto the printed circuit board is performed by using the following procedures. First, solder paste for bonding is printed on pads on the bottom surface of the electronic component, or on pads on the printed circuit board corresponding to the pads on the bottom surface of the electronic component. The electronic component is placed on the printed circuit board and heated in a reflow oven, whereby the electronic component is bonded to the printed circuit board together by solder for mounting. 
         [0006]    Solder that bonds the pads on the printed circuit board and the pads of the electronic component together typically has a depressed shape like a drum. As a result, the distance (standoff) between the surface of the printed circuit board and the bottom surface of the electronic component decreases, and a short circuit between adjacent pads due to solder-solder contact is prone to occur. If a large standoff is ensured, short circuits due to solder-solder contact are reduced in the printed circuit board on which an electronic component is mounted, and thus, as well known, stresses are absorbed at joint portions of the solder to thereby prevent the solder from cracking or flaking-off, which leads to an improvement in product life. 
         [0007]    However, with the reduction in the size and weight of electronic products, the minimization of the pitch between pads and the pad area is getting an increase in speed. In order to realize the reduction in the size and weight of electronic products, the supply amount of solder paste for bonding is necessitated to be reduced. The current mounting situation, therefore, has a tendency toward adopting a process going against an improvement in the reliability. 
         [0008]    With such being the situation, the problem here is how to ensure a large standoff. 
         [0009]    Regarding techniques for bonding electronic component(s) and a printed circuit board together by solder, patent documents have been disclosed, such as Japanese Unexamined Patent Application Publication Nos. 8-46313, 2001-94244, 5-160563, 2000-307237, and 7-38225. 
       SUMMARY 
       [0010]    According to an aspect of an embodiment, an electronic device comprises a bonding material, an electronic component providing a plurality of pads on a bottom surface thereof, and a printed circuit board providing a plurality of pads on a surface thereof, at least one of the pads of the printed circuit board being connected to at least one of the pads of the electronic component by the bonding material so as to connect the printed circuit board with the electronic component electrically, wherein either the electronic component or the printed circuit board provides a dummy pad on which the bonding material is formed, the bonding material on the dummy pad butting against the other of the surfaces of the electronic component or the printed circuit board. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic view showing how electronic components are mounted onto a printed circuit board according to an embodiment of the present invention; 
           [0012]      FIG. 2  is a diagram showing characteristic portions of an electronic device according to an embodiment of the present invention; and 
           [0013]      FIG. 3  is a diagram showing a reflow process after solder paste has been supplied to pads according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The present invention is directed to an electronic device including an electronic component having at least one pad for solder bonding, provided on the bottom surface thereof, and a printed circuit board having at least one pad to be solder-bonded to the at least one pad on the bottom surface of the electronic component, the at least one pad of the printed circuit board being provided on the surface thereof; and an electronic component mounting method for mounting, by solder bonding, an electronic component having at least one pad for solder bonding on the bottom surface thereof, onto a printed circuit board on the surface of which there is provided at least one pad to be solder-bonded to the at least one pad on the bottom surface of the electronic component. 
         [0015]      FIG. 1  is a schematic view showing how electronic components are mounted onto a printed circuit board. 
         [0016]    In  FIG. 1 , a state is shown in which a BGA package electronic component  20  and an LCC package electronic component  30  are mounted onto the surface  11  of a printed circuit board (mother board)  10 . 
         [0017]    On the surface  11  of the printed circuit board  10 , there are provided three pads  101 ,  102 , and  103  for solder bonding to the BGA package electronic component  20 , and one pad  104  for solder bonding to the LCC package electronic component  30 . On the other hand, regarding the BGA package electronic component  20 , three pads  201 ,  202 , and  203  for solder bonding to the printed circuit board  10  are provided on the bottom surface thereof, and regarding the LCC package electronic component  30 , one pad (as far as  FIG. 1  shows)  301  for solder bonding to the printed circuit board  10  is provided on the bottom surface thereof. 
         [0018]    The three pads  201 ,  202 , and  203  on the bottom surface  21  of the electronic component  20  are, during mounting, respectively disposed at positions opposed to the three pads  101 ,  102 , and  103  on the surface  11  of the printed circuit board  10 . Likewise, the pad  301  on the bottom surface  31  of the electronic component  30  is, during mounting, disposed at a position opposed to the pad  104  on the surface  11  of the printed circuit board  10 . Here, the electronic component  30  is the LCC package. The pad  301  extends as far as a side surface of the LCC package. However, here, it suffices for the pad only to exist on the bottom surface. It does not matter whether or not the pad extends up to the side surface. 
         [0019]    Here, mutually opposing pads, i.e., the pads  101 ,  102 ,  103 ; and  104  on the printed circuit board  10 , and the pads  201 ,  202 ,  203 ; and  301  of the electronic components  20  and  30  are bonded together, respectively, by a solder  401 . In mounting the electronic components  20  and  30  onto the printed circuit board  10 , solder paste is printed on the pad(s) of one side or both sides of the printed circuit board  10  and the electronic components  20  and  30 , and the electronic components  20  and  30  are placed on the printed circuit board  10 . Then, the electronic components  20  and  30  on the printed circuit board  10  are heated in the reflow oven, and the solder is solidified after having been melted. Thereby, the electronic components  20  and  30  are mounted onto the printed circuit board  10  by solder bonding. 
         [0020]    Here, the solder that bonds pads  101 ,  102 ,  103 ; and  104  on the printed circuit board  10  and pads  201 ,  202 ,  203 ; and  301  of the electronic components  20  and  30  together, typically has a depressed shape like a drum, as shown in  FIG. 1 . As a result, the distances (standoff S) between the surface  11  of the printed circuit board  10  and the bottom surfaces  21  and  31  of the electronic components  20  and  30  decrease, short circuits between adjacent pads due to solder-solder contact are prone to occur, resulting in a significantly detrimental effect on the long term reliability of solder joint portions. On the other hand, if a large standoff is ensured, then, as is well known, short circuits due to solder-solder contact are reduced, and also stresses at the solder joint portions are absorbed, which prevents the solder from cracking or flaking-off, leading to improvement in product life. 
         [0021]    To solve the above-described problem, a technique is proposed for ensuring a large standoff by arranging a plurality of dummy bumps made of resin or the like at four corners of the bottom surface of the electronic component to thereby causing the dummy bumps to function as spacers. 
         [0022]    However, the adoption of the above-described method necessitates a process of arranging spacers on the bottom surface besides routine processes, thereby causing an increase in cost. 
         [0023]    Hereinafter, an embodiment according to the present invention will be described with reference to the appended drawings. 
         [0024]      FIG. 1  shows how electronic components are mounted onto a printed circuit board, and  FIG. 2  shows characteristic portions of an electronic device according to the present invention. 
         [0025]    As in the case of  FIG. 1 ,  FIG. 2  shows how a BGA package electronic component  60  and an LCC package electronic component  70  are mounted onto a printed circuit board (mother board)  50 . 
         [0026]    On the surface  51  of the printed circuit board  50 , three pads pad  501 ,  502 , and  503  are provided at positions opposed to the BGA package electronic components  60 . Of these pads, two of the pads  502  and  503  are pads used for electric connection between the printed circuit board  50  and the electronic component  60 . The pad  501  is not used for electric connection between the printed circuit board  50  and the electronic component  60 . That is, the pad  501  is a dummy pad. In other words, on the bottom surface of the electronic component  60 , the electronic component  60  has no pad at a position opposed to the pad  501 . Also, the printed circuit board  50  has a pad  504 . On the surface of the printed circuit board  50 , the pad  504  is disposed at a position opposed to the LCC package electronic component  70 . The pad  504  is one used for electric connection between the printed circuit board  50  and the electronic component  70 . 
         [0027]    On the bottom surface  61  of the electronic component  60 , there are provided two pads  601  and  602 . On the bottom surface  61  of the electronic component  60 , the above-described two pads  601  and  602 , respectively, are disposed at positions opposed to the pads  502  and  503  formed on the surface of the printed circuit board  50 . Each of the sets of the pad  601  and pad  502 ; and the pad  602  and the pad  503  is bonded using a solder  402 . In contrast, as described above, on the bottom surface of the electronic component  60 , no pad is provided at a position opposed to the pad  501 . A solder particle  403  for bonding the printed circuit board  50  and the electronic component  60  together makes a direct contact with the bottom surface  61  of the electronic component  60 . As a result, the solder particle  403  supplied to the pad  501  formed on the printed circuit board  50  takes a round shape on the side of the printed circuit board  50 . The solder particle  403  pushes up the electronic component  60  by the round tip thereof, to thereby form a large standoff S between the printed circuit board  50  and the electronic component  60 . Here, the area (here, shown as a size A 1 ) of the pad  501  serving as a dummy pad is made smaller than the area (here, shown as a size a 1 ) of the other pads  502  and  503  on the printed circuit board  50  (i.e., A 1 &lt;a 1 ). During the mounting of the electronic component  60 , the pads  501 ,  502 , and  503  are supplied with a mutually equal amount of solder paste. Since the area of the pad  501  serving as a dummy pad is smaller than the area of the other pads  502  and  503  for electrically connecting the printed circuit board  50  and the electronic component  60 , on the pad  501 , the solder particle  403  having a larger height than that of the solder particles  402  formed on the pads  502  and  503  is formed by the melting and solidification of the solder, thereby more pushing up the standoff S. 
         [0028]    Also, on the bottom surface  71  of another electronic component  70  of LCC package, there are provided two pads  701  and  702 . On the bottom surface  71  of the electronic component  70 , the pad  701  is disposed at a position opposed to the pad  504  formed on the surface of the printed circuit board  50 . The pad  701  and the pad  504  are electrically connected by a solder particle  404  formed by the supply, melting, and solidification of solder paste. In contrast, the pad  702  is a dummy pad. On the surface  51  of the printed circuit board  50 , no pad is formed at a position opposed to the pad  702 . Therefore, solder paste is supplied to the pad  702  and after having been melted, it is solidified, to thereby form a solder particle  405  having a shape round on the side of the printed circuit board  50 . The solder particle  405  pushes down the printed circuit board  50  by the round tip thereof to thereby form a large standoff S between the printed circuit board  50  and the electronic component  70 . Here, the area (here, shown as a size A 2 ) of the pad  702  is made smaller than the area (here, shown as a size a 2 ) of the other pad  701  (i.e., A 2 &lt;a 2 ). During the mounting of the electronic component  70 , all pads regarding the electronic component  70  are supplied with a mutually equal amount of solder paste. Since the area of the pad  702  is smaller than the area of the other pad  701  for electrically connecting the printed circuit board  50  and the electronic component  70 , on the pad  702 , a solder particle  405  having a larger height than that of the solder particles  404  formed on the pad  701  is formed by the melting and solidification of solder, thereby more pushing up the standoff S. 
         [0029]      FIG. 2  shows an example in which a dummy pad is formed on each of the printed circuit board and the electronic component. Dummy pads may be provided only on the printed circuit board or may be provided only on the electronic component(s). Also, it is preferable that, for example, one dummy pad is provided at each of the four corners of the bottom surface of the electronic component, i.e., a total of four dummy pads are provided thereat, or that more than four dummy pads be provided. Providing at least three dummy pads on one electronic component allows the electronic component to be mounted in parallel to the printed circuit board without inclining. Furthermore, the more the number of electronic components, the more strongly acts the force for pushing up the standoff S between the printed circuit board and the electronic component. 
         [0030]    The standoff between the printed circuit board and the electronic component depends on the weight of a target component, the number of pads, the pad area, the amount or kind of supply solder. However, the height of the standoff can be controlled by adjusting the diameter of dummy pads, the number thereof, and the paste amount to be supplied. Specifically, in a 72-pin QFN component with a size of 10 mm square, ten dummy pads with a diameter of 0.3 mm are installed on the mother board side. On each of the installation portions of the dummy pads, a metal mask (thickness: 0.12 mm) that is about 20 percent larger than the dummy pads is perforated. When no dummy pad is provided, the standoff between the printed circuit board and the electronic component is 30 to 50 μm, but it has been ascertained that the installation of dummy pads under the above-described conditions allows the standoff between the printed circuit board and the electronic component to be increased up to 70 to 90 μm. Also, it has been demonstrated that the installation of still more dummy pads or the control of solder amount allows the standoff between the printed circuit board and the electronic component to be even more increased. 
         [0031]    In an electronic component with bottom electrodes, when there is little handoff for solder bonding (0 to 20 μm), a concentrated stress applied to the terminal electrode of the component is about 150 to 200 MPa. According to long-term reliability tests, it has been found that the time period until the solder junctions in the above-described component with bottom electrodes arrive at crack failure is very short. On the other hand, in the present invention, installing dummy pad(s) on either of the printed circuit board and the electronic component or both of them and thereupon making solder bonding to ensure a solder bonding standoff of about 150 μm, relaxes the concentrated stress down to 50 to 70 MPa. In the long-term reliability tests, it has been ascertained that the time period until the solder junctions arrive at crack failure is 50 to 60 times longer than the above-described former case. 
         [0032]      FIG. 3  is a diagram showing a reflow process after solder paste has been supplied to pads. 
         [0033]    As shown in  FIG. 3 , the electronic component  60  and the electronic component  70  to be mounted onto the printed circuit board  50  shown in  FIG. 2 , are put on an entrance-side conveyor  80 , and conveyed into a reflow oven  80  to arrive at a main heating zone  80   a . Solder paste has already been supplied between the printed circuit board  50  and the electronic components  60  and  70 . The solder paste is melted by its arrival at the main heating zone  80   a . In the main heating zone  80   a , a substrate reversing mechanism  81  lifts the printed circuit board  50 , then slowly reverses it, and the substrate reversing mechanism  81  puts it on an exit-side conveyor  802 . The exit-side conveyor  802  supports only the printed circuit board  50  in the reversed state so that the self-weights of the electronic components  60  and  70  are applied to the electronic components  60  and  70  on the printed circuit board  50 . The melting temperature of the solder at this time is restrained to a temperature level so as to maintain the viscosity of the solder to such an extent that the solder does not drop off the printed circuit board  50  with the electronic components  60  an  70  reversed. The exit side conveyor  802  puts the printed circuit board  50  thereon and conveys outside the reflow oven  80 . The molten solder for bonding the printed circuit board  50  and the electronic components  60  and  70  become cold and solidified outside the reflow oven  80 . Here, since the printed circuit board  50  is reversed, and the self-weights of the electronic components  60  and  70  are applied to the electronic component  60  and the electronic component  70  that are put on the printed circuit board  50 , the molten solder is extended, thereby forming a higher standoff S. 
         [0034]    The electronic component and the printed circuit board according to the present invention has at least one dummy pad for forming the above-described standoff, and the dummy pad connects with solder particle, and therefore, although the solder particle spreads on the dummy pad side, its tip side apart from the dummy pad has a round shape due to a surface tension, the standoff is enhanced correspondingly. 
         [0035]    As described above, according to the electronic device of the present invention, either one of the bottom surface of the electronic component and the surface of the printed circuit board has at least one dummy pad that is absent at an opposed position on the other of the surfaces, and a solder particle is connected to the at least one dummy pad, whereby the standoff can be effectively enhanced without increasing the supply amount of solder paste for bonding the electronic component and the printed circuit board together.