Patent Publication Number: US-2005142936-A1

Title: Multi-layered printed circuit board with a spacer post for preventing engagement between circuit paths of circuit layers

Description:
FIELD OF THE INVENTION  
      The invention relates to a multi-layered printed circuit board, more particularly to a spacer post for preventing engagement between circuit paths of upper and lower circuit layers in the multi-layered printed circuit board.  
     BACKGROUND OF THE INVENTION  
      A conventional printed circuit board includes a plurality of electronic components, and is generally formed with a plurality of circuit paths for establishing electrical connection among the electronic components. The number of circuit paths is restricted by and directly proportional to the entire surface area of the printed circuit board, i.e. the more the surface area, the more the circuit paths can be formed thereon. In order to provide multi-function purposes and high performance of the printed circuit board, more and more electronic components are required to be mounted on the printed circuit board. Hence, the number of circuit paths (the surface area) needs inevitably to be increased. However, it is against the trench of the manufacture wishes, since the present printed circuit boards are produced in compact size to facilitate transport or to minimize the occupying space thereof. To eliminate the size problem and under the given circumstances, the printed circuit boards of today are designed in multi-layered forms, thereby increasing the total surface area for forming the circuit paths and for assembling the electronic components thereon.  
       FIG. 1  shows a partly exploded view of a conventional printed circuit board used in an LCD device, and includes a base member  10  (generally an outer body of the LCD device), a lower circuit layer  20  disposed above the base member  10 , an upper circuit layer  30  disposed above the lower circuit layer  20 , a plurality of spacer posts  40  sandwiched between the upper and lower circuit layers  30 , 20  so as to prevent the circuit paths of the circuit layers  30 , 20  from engagement therebetween and so as to enhance heat-dissipation effect, and a plurality of fastening screws  50 .  
      Referring to  FIG. 2 , each spacer post  40  has an upper portion  41  and a threaded lower portion  42 . The threaded lower portion  42  of each of the spacer posts  40  extends through a hole  22  in the lower circuit layer  20  to fasten threadedly a threaded hole  12  in the base member  10  (not visible since the printed circuit board is exploded). The upper portion  41  of each of the spacer posts  40  has an upper end face  40   b  abutting against the upper circuit layer  30 , and is formed with an inner threaded hole  44  that extends inwardly and axially from the end face  40   b.  Each of the fastener screws  50  extends through a hole  32  in the upper circuit layer  30 , and engages the inner threaded hole  44  in the upper portion of a respective spacer post  40 . Note that in the past, in order to lower the manufacture cost and quicken the production thereof, the threaded lower portion  42  and the inner threaded hole  44  in the upper portion of each spacer post are arranged in such a manner to have the same screw pitch and the same pitch inclination (i.e. the threaded lower portion  42  can engage the inner threaded hole  44 ). Under such an arrangement (after assembly), a static force F is resulted due to engagement between the fastener screw  50  (or the threaded lower portion  42 ) with the wall A defining the inner threaded hole  44  in the upper portion  41  or the threaded hole  12  in the base member  10 , as best shown in  FIG. 3A . The strength of the static for F may vary according to the materials of the spacer post  40 , the fastener screw  50 , and the base member  10 .  
      When it is desired to remove the threaded lower portion  42  of the space post  40  from the base member  10  (or the fastener screw  50  from the spacer post  40 ), a torque (T) must be applied on the spacer post  40 . The strength of the applied torque T should be greater than that of the static force F so as to rotate the spacer post  40  relative to the base member  10 . Referring to  FIG. 3B , since the strength of torque T=F×r (radius of the threaded lower portion  42  or radius of the inner threaded hole  44 ), the torque T required to remove the threaded lower portion  42  of the spacer post  40  from the base member  10  is equivalent to that for removing the fastener screw  50  from the space post  40  by virtue of the same radius of the fastener screw  50  and the threaded lower portion  42 . Under such condition, removal of the fastener screw  50  from the upper circuit layer  30  by turning the former relative to the latter can accidentally result in simultaneous removal of the lower circuit layer  20  from the base member  10 , thereby untimely and unnecessarily damaging the electronic components mounted on the upper and lower circuit layers  30 ,  20 .  
     SUMMARY OF THE INVENTION  
      Therefore, it is the object of the present invention is to provide a multi-layered printed circuit board having a spacer post that is sandwiched between upper and lower circuit layers and that can prevent untimely removal of the lower circuit from the printed circuit board during removal of the upper circuit layer from the printed circuit board.  
      According to the present invention, a multi-layered printed circuit board for an LCD device is provided to include: base member; a lower circuit layer disposed above the base member and formed with a plurality of through holes; an upper circuit layer disposed above the lower circuit layer and formed with a plurality of through holes; a plurality of vertical spacer posts disposed between the lower and upper circuit layers so as to space apart the circuit paths of the lower and upper circuit layers from each other, each of the spacer posts including an upper portion having an end face abutting against the upper circuit layer and being formed with an inner threaded hole extending inwardly and axially from the end face, and a threaded lower portion extending through a respective one of the through holes in the lower circuit layer to fasten threadedly the base member; and a plurality of fastener screws. Each of the fastener screws extends through a respective one of the through holes in the upper circuit layer to fasten threadedly the inner threaded hole in the upper portion of a respective one of the spacer posts. The threaded lower portion of each of the spacer posts defines an outer diameter that is greater an inner diameter defined by the inner threaded hole in the upper portion of a respective spacer post. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other features and advantages of this invention will become more apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which:  
       FIG. 1  is a perspective partly exploded view of a conventional printed circuit board;  
       FIG. 2  is a cross-sectional view of the spacer post used to prevent engagement between circuit paths of upper and lower circuit layers in the conventional printed circuit board;  
       FIG. 3A  illustrates how the spacer post of  FIG. 2  engages a threaded hole in a base member of the conventional printed circuit board;  
       FIG. 3B  is a top planar view, illustrating how the spacer post of  FIG. 2  engages the threaded hole in a base member of the conventional printed circuit board;  
       FIG. 4  is a perspective partly exploded view of the preferred embodiment of a multi-layered printed circuit board according to the present invention;  
       FIG. 5  is a perspective partly exploded view of the modified preferred embodiment of the multi-layered printed circuit board according to the present invention;  
       FIG. 6  is an enlarged cross-sectional view of a spacer post for fastening circuit layers of the preferred embodiments shown in  FIGS. 4 and 5 ;  
       FIG. 7  illustrates how a modified spacer post fastens a circuit layer onto a base member of the printed circuit board according to the present invention;  
       FIG. 8  illustrates how another modified spacer post fastens the circuit layer onto the base member of the printed circuit board according to the present invention; and  
       FIG. 9  is graph, illustrating the applied torque relative to loosening condition of the spacer post used in the printed circuit board of the present invention. 
    
    
     DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 2 , the preferred embodiment of a multi-layered printed circuit board  20  of the present invention is used in an LCD device, and includes a base member  110 , a lower circuit layer  120  disposed above the base member  110  and formed with a plurality of through holes  122 , an upper circuit layer  130  disposed above the lower circuit layer  120  and formed with a plurality of through holes  132 , a plurality of vertical spacer posts  40  disposed between the upper and lower circuit layers  130 ,  120  so as to space apart the circuit paths of the upper and lower circuit layers  130 ,  120  from one another and so as to prevent the electronic components (not visible) mounted on the upper and lower circuit layers  130 ,  120  from colliding one another, and a plurality of fastener screws  150 .  
      Referring to  FIGS. 4 and 6 , each of the spacer posts  140  includes an upper portion  141 , a constricted threaded lower portion  142 , and an abutment shoulder  140   a  between the upper and lower portions  141 ,  142 . The upper portion  141  of each of the spacer posts  140  has an upper end face  140   b  abutting against the upper circuit layer  120  (not visible since the printed circuit board is shown in exploded view), and is formed with an inner threaded hole  44  extending inwardly and axially from the upper end face  140   b.  The abutment shoulder  140   a  of each of the spacer posts  140  abuts against the lower circuit layer  120  (not visible since the printed circuit board is shown in exploded view) to permit extension of the threaded lower portion  142  through the respective one of the through holes  122  in the lower circuit layer  120  to fasten threadedly the base member  110 , thereby securing the lower circuit layer  120  on the base member  110 .  
      Each of the fastener screws  150  extends through a respective one of the through holes  132  in the upper circuit layer  130  to fasten threadedly the inner threaded hole  144  in the upper portion of a respective one of the spacer posts  140 , thereby securing the upper circuit layer  130  above the lower circuit layer  120  with a gap between the upper and lower circuit layers  130 , 120 . Note that the base member  110  can be an outer casing of the LCD device or the outer casing of a CPU of a computer system. In order to enhance heat dissipation effect, the spacer posts  140  can be made from copper metal. Preferably, each of the spacer posts is constructed in such a manner to have multi-sided outer surface to facilitate turning of the same by an operating tool, such as a spanner or wrench.  
      Note that in the present preferred embodiment, the threaded lower portion  142  of each of the spacer posts  140  defines an outer diameter R that is greater an inner diameter r defined by the inner threaded hole  144  in the upper portion  141  of said each of the spacer posts  140 , i.e. the radius R of the threaded lower portion  142 &gt;the radius of the inner threaded hole  144 . However, the threaded lower portion  142  and the inner threaded hole  144  in the upper portion  141  of each of the spacer posts  140  can be arranged in such a manner to have the same screw pitch and the same pitch inclination. Referring again to  FIGS. 3A and 3B , since the radius of the fastener screw  50  is equivalent the radius r of the inner threaded hole  144 , and the radius of the threaded hole  112  in the base member  110  is equivalent the radius R of the threaded lower portion  142 , the torque applied (T=F×r) onto the spacer post  140  for removal of the lower circuit layer  120  from the base member  110  is generally greater than that applied onto the fastener screw  150  for removal of the upper circuit layer  130  from the lower circuit layer  120 . Under such a condition, during detaching the upper and lower circuit layers  130 ,  120  from the base member  110 , the amount of torque applied onto the fastener screw  150  can result in timely removal of the upper circuit layer  130  from the lower circuit layer  120  prior to reaching the extent for removal of the lower circuit layer  120  from the base member  110 .  
      Referring to  FIG. 5 , in case the printed circuit board includes only one circuit layer  220  having electronic components mounted on top and bottom surfaces thereof. The spacer posts  240  are sandwiched between the base member  210  and the circuit layer  220  in such a manner that the abutment shoulder  240   a  of each of the spacer posts  240  abuts against the base member  210  (not visible since the printed circuit board is shown in exploded view) to engage the threaded hole  212  in base member  210 , thereby securing the spacer post  240  uprightly on the base member  210 . Each of the fastener screws  250  extends through a hole in the circuit layer  220  to fasten threadedly the inner threaded hole  244  in the upper portion  241  of a respective one of the spacer posts  140  so as to secure the circuit layer  220  above the base member  210  with a gap between the circuit layer  220  and the base member  210 , thereby preventing the electronic components (not visible) from colliding against the base member  210  if mounted on the bottom surface of the circuit layer  220 .  
      Referring to  FIG. 7 , a modified printed circuit board of the present invention includes the spacer post  340  in the form of a post body disposed between the base member  310  and a circuit layer  320  so as to support the circuit layer  320  above the base member  310  in cooperation with first and second fastening members  350 ,  312 . The first fastening member  350  can be a nut while the second fastening member  312  is a threaded hole. As illustrated, the post body includes an upper portion  340   b  and a lower portion  340   a.  The lower portion  340   a  of the post body is formed with a second threaded member  342  that engages the threaded hole  312  in the base member  310 . The upper portion  340   b  of the post body has an end face abutting against the circuit layer  320 , and is formed with a first threaded member  344  extending through a hole in the circuit layer  320  to engage the nut  350  so as to secure the circuit layer  320  on the upper portion of the post body. The second threaded member  342  of the lower portion  340   a  of the post body defines a first diameter that is greater a second diameter defined by the first threaded member  344  of the upper portion of the post body such that a torque required to remove the second threaded member  342  is greater than that for removing the nut  350  from the first threaded member  344 .  
      Referring to  FIG. 8 , another modified preferred embodiment of the present invention is shown to have the structure similar that of  FIG. 7 . The only difference resides in that each of the first and second threaded members  444 ,  442  of the post body  440  is a threaded hole while each of the first and second fastening members  450 , 412  is a screw rod that is adapted to engage a respective one of the threaded holes in the post body  440 . The second threaded member  442  of the lower portion  440   a  of the post body  440  defines a first diameter that is greater a second diameter defined by the first threaded member  444  of the upper portion  440   b  of the post body  440 .  
       FIG. 9  illustrates a graph representing the changes of the torque applied to the spacer post during loosening states of the spacer post. Assuming that, T 1  represents the torque required for removal of the threaded lower portion  142  of the spacer post  140  from the base member  110 , T 2  represents the torque required for removal of the fastener screw  150  from the spacer post  140  (see  FIG. 4 ), wherein T 1 &gt;T 2 . It is noted that T 2  represents the torque to overcome the static friction between the fastener screw and the spacer post so as to startup the rotation of the fastener screw. As the rotation of the fastener screw is started, the torque applied to the spacer post is reduced to T 3 , which represents the torque to overcome the kinetic friction between the fastener screw and the spacer post. During the removing operation, even with a continuously increasing external toque applied on the fastener screw  150 , the torque applied to the spacer post will never reaches the extent of T 1  so as to permit removal of the upper circuit layer  130  from the lower circuit layer  120 .  
      Some advantages provided by the multi-layered printed circuit board of the present invention as follows: 
          (i) By simply altering the structures of the threaded lower portion and the inner threaded hole in the upper portion of the spacer post  140 , when removing the circuit layers from the printed circuit, damage caused to the electronic components mounted on the circuit layers can be avoided.     (ii) Since the circuit layers can be removed one after another in succession, time-waste problem encountered during removal of the circuit layers from the conventional printed circuit board can be eliminated.        

      While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.