Abstract:
A printed wiring board having a wiring layer formed by a wiring pattern for wiring an electronic circuit comprised of an electric part mounted thereon and including a connecting terminal, a first conductor layer formed by a first conductor pattern that is maintained at either a power potential or a ground potential when the electronic circuit is in operation, a second conductor layer formed by a second conductor pattern that is maintained at either the power potential or the ground potential when the electronic circuit is in operation; and a through hole having the connecting terminal of the electric part inserted therein and including, formed on an internal wall thereof, a conductor film that is directly connected to the first conductor pattern and not directly connected to the second conductor pattern.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field 
         [0002]    The present invention relates to a printed wiring board with electric parts mounted thereon and an electric apparatus which includes the printed wiring board and the electric parts mounted on the printed wiring board. 
         [0003]    2. Description of the Related Art 
         [0004]    A conventional way to construct a desired electronic circuit is to mount electric parts on a printed wiring board, and this method is widely used. 
         [0005]    Among such electric parts, there are many parts (IMT: Insert Mounting Technology parts) equipped with lead wires. The lead wires are wire-form connecting terminals (connecting pins) for inputting and outputting signals to and from internal electronic circuits, and for receiving power supplied to the electronic circuit. 
         [0006]    A mounting method is employed when mounting an IMT part on a printed wiring board in which through holes are provided in the printed wiring board for inserting lead wires. The lead wires are passed through the through holes, and the portions of the lead wires which protrude from a reverse-side surface are soldered. 
         [0007]    Then, a conductor film such as a copper film or the like is formed on the internal walls of the through holes, and the conductor film is connected to wiring extending over the surface of or inside the printed wiring board. 
         [0008]    Hence, inserting the lead wires into the through holes and soldering the lead wires in place electrically connects the electronic circuit inside the electric part to the surface or internal-plane wiring of the printed wiring board via the lead wiring, soldering, and conductor film. 
         [0009]    Thus, the plurality of electric parts mounted on the printed wiring board is connected by the wiring on the printed wiring board to form an electronic circuit for performing the desired operations. 
         [0010]    Here, it is preferable that sufficient solder rise occurs when the lead wires of an electric part are inserted into the through holes provided on the printed wiring board and soldered. 
         [0011]    Solder rise is a phenomenon by which melted solder applied to the protruding lead wires on a reverse side of the printed wiring board intrudes into the through holes and rises to a front surface of the printed wiring board. 
         [0012]    When the solder rise is insufficient, the amount of solder adhering to the lead wire is insufficient and faults may result. 
         [0013]      FIG. 1  is a view of solder rise. 
         [0014]      FIG. 1  shows a state in which the lead wire  21  of an electric part  20  has been inserted into a through hole  11  of the printed wiring board  10  and soldered using solder  30 . 
         [0015]    The printed wiring board  10  includes the through hole  11  which has a conductor film  111  formed on internal walls thereof. 
         [0016]    The printed wiring board  10  is a multilayer printed wiring board. In the example shown here, four conductor pattern layers  101 , 102 , 103 , and  104 , which spread across the printed wiring board  10 , are connected to the conductor film  111  on the internal wall of the through hole  11 . 
         [0017]    Here, as an example, the lead wire  21  inserted into the through hole  11  is a lead which connects the ground of the electronic circuit within the electric part  20  to the conductor patterns  101 , 102 , 103 , and  104 . The conductor patterns  101 , 102 , 103 , and  104  form a ground pattern and spread across the printed wiring board  10 . 
         [0018]    These conductor patterns  101 , 102 , 103 , and  104 , which form the ground pattern, are all held at the same ground level when the circuit is in operation. 
         [0019]    Besides the conductor patterns  101 , 102 , 103 , and  104 , the printed wiring board  10  includes a conductor pattern  131  for use as a power line and a plurality of wiring patterns such as  141 , 142 , 143  and  144  for transmitting signals. 
         [0020]    Here, as described above, it is desirable that the solder  30  of the soldering melts and rises towards a surface of the printed wiring board  10  when the lead wire  21  is inserted into the through hole  11  and soldering is performed on the reverse side of the printed wiring board  10 . In other words, it is desirable that sufficient solder rise occurs. 
         [0021]    There is a risk that solder faults will develop under long-term use when the solder rise is insufficient, even if the conductivity between the lead wire  21  and the conductor patterns  101 ,  102 , 103 , and  104  may be sufficient directly after manufacture. 
         [0022]    A thickness of approximately ½ the thickness of the printed wiring board  10  is often used as a lower limit for solder rise. In  FIG. 1 , the solder rise is approximately ⅓ of the thickness of the printed wiring board, a level which is insufficient. 
         [0023]    It may not be possible to obtain sufficient solder rise for the following reasons. 
         [0024]    (1) The number of layers of ground and power wiring patterns formed on the surface and internal-planes of the printed wiring board increase as the number of layers in the printed wiring board increases. Thus, the number of layers of wiring patterns to be connected to the internal wall  111  of the through hole  11  also increases. Consequently, the heat generated by soldering is dissipated more easily by the wiring patterns. 
         [0025]    The melted solder will begin to harden earlier as a result of the heat dissipation, inhibiting the solder rise. 
         [0026]    (2) The thickness of the printed wiring board and thus the height of the through holes increases as the number of layers in the printed wiring board increases. Hence, even with the same amount of solder rise, there will be cases where the reference level of, for instance, ½ of the thickness of the printed circuit board is not reached. 
         [0027]    (3) There is a trend towards use of lead-free solder to protect against the damaging effects of lead (element symbol: Pb). Surface treatments are implemented on the printed wiring boards to allow use of the lead-free solder. The surface treatments cause a reduction in the amount of solder rise. 
         [0028]    (4) The lead-free solder material itself causes deterioration in the solder rise characteristics. 
       SUMMARY 
       [0029]    The present invention was conceived in consideration of the above described conditions, and provides a printed wiring board with a structure that allows sufficient solder rise and an electric apparatus using the printed wiring board. 
         [0030]    The above-described embodiments of the present invention are intended as examples, and all embodiments of the present invention are not limited to including the features described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0031]      FIG. 1  is a diagram illustrating solder rise; 
           [0032]      FIG. 2  shows a portion of a through hole for connecting a connecting terminal of the printed wiring board according to the first embodiment of the present invention; 
           [0033]      FIG. 3  is a schematic view of a printed wiring board of a second embodiment of the present invention; 
           [0034]      FIG. 4  is a schematic view of a printed wiring board of a third embodiment of the present invention.; and 
           [0035]      FIG. 5  is a plan view from a top side of a printed wiring board of a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    Reference may now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0037]    The following describes embodiments of the present invention. 
         [0038]      FIG. 2  shows a portion of a through hole (corresponding to a first through hole of the present invention) for joining a connecting terminal of the printed wiring board of the first embodiment of the present invention. 
         [0039]    Here, components which are the same as elements in the above-described  FIG. 1  are allocated the same symbols, and only the differences between the two printed wiring boards are described below. 
         [0040]    In the printed wiring board  10 A shown in  FIG. 2 , the conductor film  111  of an internal wall  18  of the through hole  11  directly connects only to the conductor pattern  101 . The conductor pattern  101  is the layer nearest a front surface of the printed wiring board  10 A among the conductor patterns  101 ,  102 ,  103 , and  104 . The conductor patterns  101 ,  102 ,  103 , and  104  are held at ground level when the circuit is in operation. 
         [0041]    As a result, the heat is hard to be dissipated when soldering the lead wire  21 , and sufficient solder rise can be anticipated. 
         [0042]      FIG. 2  shows an example of the solder  30  in a state of having reached the front surface of the printed wiring board as a result of the solder rise. 
         [0043]    Note that, of the four layers constituting conductor patterns  101 ,  102 ,  103  and  104 , the layers constituting the conductor patterns  102 ,  103 , and  104  (i.e. all layers other than the conductor pattern  101  nearest the front surface of the printed wiring board) may connect to the conductor pattern  101  at one or more other locations not shown in the drawings. 
         [0044]    In the embodiment shown in  FIG. 2 , a dummy through hole  12  is formed in the printed wiring board  10 A in a position adjacent to the through hole  11 . The dummy through hole  12  does not receive a lead wire of an electric part. As in the through hole  11 , a conductor film  121  is formed on an internal wall of the dummy through hole  12 . 
         [0045]    The conductor film  121  on the internal wall of the dummy through hole  12  is connected to the conductor film  111  on the internal wall of the through hole  11  by wiring  105  of the surface layer of the printed wiring board  10 A. 
         [0046]    Thus, only the conductor pattern  101 , which is one layer of the four layers of conductor patterns  101 ,  102 ,  103  and  104  and kept to ground level, is directly connected to the conductor film  111  on the internal wall of the through hole  11 . However, the conductor patterns  101 ,  102 ,  103  and  104  connect to one another via the conductor film  121  of the internal wall of the dummy through hole  12 , and are therefore held at the same potential when the circuit is in operation. 
         [0047]      FIG. 3  is a schematic view of the printed wiring board of the second embodiment of the present invention. 
         [0048]    The following describes differences between the second embodiment shown in  FIG. 3  and the first embodiment shown in  FIG. 2 . 
         [0049]    In the printed wiring board  10 B shown in  FIG. 3 , the conductor film  111  of the internal wall of the through hole  111  is connected directly to the conductor patterns  101  and  102 . The conductor patterns  101  and  102  are the two surface-side layers of the four layers constituting the conductor patterns  101 , 102 , 103 , and  104 , which are held at ground level when the circuit is in operation. 
         [0050]    A plurality of conductor pattern layers may be connected to the conductor film of the internal wall of the through hole  11  in the above-described manner when sufficient solder rise is desired. 
         [0051]    In the printed wiring board  10 B shown in  FIG. 3 , the conductor film  121  of the internal wall of the dummy through hole  12  is connected only to the conductor patterns  103  and  104 . The conductor patterns  103  and  104  are the two reverse-side layers that are not directly connected to the conductor film  111  of the through hole  11  of the four layers constituting the conductor patterns  101 , 102 , 103 , and  104 . 
         [0052]    In this way, the direct connections of the conductor patterns are shared between the conductor film  111  of the through hole  11  and the conductor film  121  of the dummy through hole  12 . 
         [0053]      FIG. 4  is a schematic view of the printed wiring board of the third embodiment of the present invention. 
         [0054]    The following describes differences between the third embodiment in  FIG. 4  and the first and second embodiments shown in  FIGS. 2 and 3  respectively. 
         [0055]    The conductor film  111  on the internal surface of the through hole  11  of the printed wiring board  10 C is not directly connected to any of the four layers constituting the conductor patterns  101 , 102 , 103 , and  104 , which are held at ground level when the circuit is in operation. 
         [0056]    Instead, the four layers constituting the conductor patterns  101 , 102 , 103  and  104  are all directly connected to the conductor film  121  of the internal wall of the dummy through hole  12  provided in a position adjacent to the through hole  11 . 
         [0057]    In this manner, the functions can be entirely split between the through hole  11  for inserting and soldering the lead wire  21  of the electric part  12  and the dummy through hole for connecting to the conductor patterns  101 , 102 , 103 , and  104 . 
         [0058]      FIG. 5  is a plan view from a top side of the printed wiring board of the fourth embodiment of the present invention. 
         [0059]    The following describes differences between the fourth embodiment shown in  FIG. 5  and the third embodiment shown in  FIG. 4 . 
         [0060]      FIG. 4  shows only a single dummy through hole  12  adjacent to the through hole  11 .  FIG. 5 , on the other hand, shows a plurality (four here) of dummy through holes  12   a ,  12   b ,  12   c , and  12   d  surrounding the through hole  11 . 
         [0061]    In the same way as in  FIG. 4 , the conductor film  111  of the internal wall of the through hole  11  in  FIG. 5  is not directly connected to any of the four layers constituting the conductor patterns  101 , 102 , 103  and  104 . 
         [0062]    Conductor films  121   a ,  121   b ,  121   c , and  121   d  are formed on internal walls of the four corresponding through holes  12   a ,  12   b ,  12   c , and  12   d  surrounding the through hole  11  shown in  FIG. 5 . The conductor film  111  of the through hole  11  is connected to the conductor films  121   a ,  121   b ,  121   c , and  121   d  of the four through holes  12   a ,  12   b ,  12   c , and  12   d  via wiring pattern  106  on the surface of the printed wiring board  10 D. 
         [0063]    Further, the conductor films  121   a ,  121   b ,  121   c , and  121   d  of the four through holes  12   a ,  12   b ,  12   c , and  12   d  each connect to a different layer of the four layers constituting the conductor patterns  101 , 102 , 103 , and  104 . 
         [0064]    Thus, a plurality of dummy through holes may be provided and the connections for a multi-layered conductor pattern can be shared among those dummy through holes. 
         [0065]    Alternatively, a plurality of dummy through holes may be provided and each of those dummy through holes may be directly connected to all layers of the plurality of conductor patterns. 
         [0066]    An arrangement somewhere between the above-described arrangements is also possible. For instance, using the case of  FIG. 5  as an example, the dummy though holes  12   a  and  12   b  of the four dummy through holes  12   a ,  12   b ,  12   c , and  12   d  may be directly connected to the two conductor patterns  101  and  102  while the other two dummy through holes  12   c  and  12   d  are directly connected to the other two conductor patterns  103  and  104 . 
         [0067]    Here, the connection of ground patterns of the first to fourth embodiments have been described, but power patterns can be connected in substantially the same way. 
         [0068]    Moreover, it is possible, while taking into account differences between the ground pattern and the power pattern, to apply one of the first to fourth embodiments in one of the power and ground arrangements and apply another of the embodiments in the other of the power and ground arrangements. 
         [0069]    Furthermore, although a printed wiring board was described in the embodiments above these embodiments could be seen as embodiments for an electric apparatus including the printed wiring board  10 A (or the printed wiring board  10 B, or the printed wiring board  10 C) and the electric part constructed by soldering a lead wire of the electric part on the printed wiring board. 
         [0070]    Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.