Abstract:
When package circuit boards are formed by dicing a circuit board sheet with a core substrate that conducts electricity, conductive material is not exposed from the outer side surfaces of the package circuit boards, thereby preventing electrical shorting of the package circuit boards. A method of dicing a circuit board sheet includes: a step of forming a circuit board sheet by forming a core portion that includes a conductive material and providing a wiring layer on the surface of the core portion; a step of forming concave channels in a thickness direction of the circuit board sheet from one surface of the circuit board sheet so as to pass through at least the core portion; a step of forming an insulating cover layer on a surface of the wiring layer and inside the concave channels; and a step of dicing the circuit board sheet within widths of the concave channels with positions of the concave channels as dicing positions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method of dicing a circuit board sheet and a package circuit board obtained by dicing a circuit board sheet, and in more detail to a favorable method of dicing a circuit board sheet where a wiring layer has been formed on a surface of a core portion that includes a conductive material and a package circuit board manufactured by such method. 
         [0003]    2. Related Art 
         [0004]    Among circuit boards used to mount semiconductor elements, circuit boards that use carbon fibers in the core portion have been provided to improve the strength of circuit boards and to make the thermal expansion coefficients of circuit boards match those of semiconductor elements. A circuit board that uses carbon fibers in the core portion has a low thermal expansion coefficient compared to a circuit board that uses a conventional glass epoxy substrate. By setting the thermal expansion coefficient of a circuit board so as to match the thermal expansion coefficient of a semiconductor element, it is possible to alleviate the thermal stress generated between the semiconductor element and the circuit board, which makes it possible to mount a large semiconductor element with a large number of pins. 
         [0005]    During a conventional manufacturing process for circuit boards, after wiring layers and insulating layers have been formed on a large substrate to form a circuit board sheet equipped with predetermined wiring layers, the large circuit board sheet is diced to obtain individual circuit boards. The dicing process needs to achieve high precision for the external dimensions of the package circuit boards after dicing and to avoid producing burrs on the cut surfaces of the package circuit boards.
   Patent Document 1   
 
         [0007]    Japanese Laid-Open Patent Publication No. 2003-218287
   Patent Document 2   
 
         [0009]    Japanese Domestic Republication No. 2004/064467 
       SUMMARY OF THE INVENTION 
       [0010]    When a large circuit board sheet is diced into individual circuit boards using a dicing saw, the end surfaces (i.e., side surfaces) of the circuit boards become exposed at the cut surfaces and fragments produced by the dicing saw are left on the cut surfaces of the circuit board. 
         [0011]    When a circuit board sheet that uses carbon fibers or metal fibers in the core portion (or prepregs) is used, the carbon fibers or metal fibers that conduct electricity will become exposed at the cut surfaces of the diced package circuit boards. That is, fragments of carbon fibers or metal fibers that are conductive will be left on the cut surfaces. This results in the problem of shorting occurring for the package circuit boards. 
         [0012]    Carbon fibers have a high mechanical strength but are brittle, and therefore when a circuit board sheet is cut, there is a further problem in that carbon fibers can fall off the side surfaces of an individual circuit board and such carbon fibers (or carbon powder) can pollute an electronic device. 
         [0013]    It is an object of the present invention to provide a method of dicing a circuit board sheet which, when forming package circuit boards by dicing a circuit board sheet including a conductive core substrate, does not expose conductive material from the outer surfaces of the package circuit boards and can therefore prevent electrical shorting by the package circuit boards, and to also provide a package circuit board. 
         [0014]    A method of dicing a circuit board sheet according to the present invention includes: a step of forming a circuit board sheet by forming a core portion that includes a conductive material and providing a wiring layer on the surface of the core portion; a step of forming concave channels in a thickness direction of the circuit board sheet from one surface of the circuit board sheet so as to pass through at least the core portion; a step of forming an insulating cover layer on a surface of the wiring layer and inside the concave channels; and a step of dicing the circuit board sheet within widths of the concave channels with positions of the concave channels as dicing positions. 
         [0015]    In the step of forming the insulating cover layer, the insulating cover layer may be formed so that a surface height of the insulating cover layer is lower at the positions of the concave channels than at parts where the wiring layer is formed. This is favorable since it becomes possible to clearly know the dicing positions even after the insulating cover layer has been formed on the surface of the circuit board sheet. 
         [0016]    Alternatively, in the step of forming the insulating cover layer, the insulating cover layer may be formed so that a surface height of the insulating cover layer is uniform across the entire circuit board sheet including the positions of the concave channels at least above the core portion. This is favorable since it becomes possible to easily form the insulating cover layer on the wiring layer and the concave channels. 
         [0017]    Also, in the step of forming the circuit board sheet, the core portion may be formed by laminating prepregs that include one of carbon fibers and metal fibers. In this way, by producing the core portion with a laminated structure, it is possible to provide the core portion with anisotropic properties and thereby satisfy demand for higher mechanical strength. 
         [0018]    A package circuit board according to the present invention includes: a core portion including a material that conducts electricity; and a wiring layer that is formed on a surface of the core portion, wherein outer side surfaces of the package circuit board are covered by an insulating cover layer. 
         [0019]    Here, the insulating cover layer may be formed so as to cover entire outer side surfaces of the package circuit board in a thickness direction thereof. This is favorable since it becomes possible to produce a state where the outer side surfaces (i.e., dicing surfaces) of the package circuit board are reliably covered with an insulator. 
         [0020]    Alternatively, the insulating cover layer may be formed so as to cover at least side surfaces of the core portion out of the outer side surfaces of the package circuit board. To achieve this construction, it is necessary to form the insulating cover layer so that a surface height of the insulating cover layer is lower at positions where the circuit board sheet will be diced (i.e., the positions of the concave channels) than at other parts of the circuit board sheet. This is favorable since it is easy to check the dicing positions when dicing the circuit board sheet. 
         [0021]    One of a metal material and a carbon material may be used as the material of the core portion. By doing so, it is possible to provide a package circuit board with superior mechanical characteristics and/or a package circuit board with superior heat dissipation characteristics. 
         [0022]    According to the method of dicing a circuit board sheet according to the present invention and a package circuit board manufactured according to such method, no conductive fragments will be produced even when dicing a circuit board sheet that uses a conductive core portion. Accordingly, it is possible to provide a highly reliable package circuit board that is easy to dice from a circuit board sheet. Also, since the outer side surfaces of the package circuit board are covered by the insulating cover layer, the outer form of the package circuit board is determined by the insulating cover layer, and therefore it is possible to improve the precision of the outer dimensions of the package circuit board. In addition, since the outer form of the package circuit board is covered by the insulating cover layer, it is possible to provide a package circuit board that is highly reliable due to a conductive material included in the core portion, or a core portion that is itself conductive, not being exposed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIGS. 1A to 1D  are cross-sectional views showing the procedure of a dicing process for a circuit board sheet according to a first embodiment of the present invention; 
           [0024]      FIG. 2  is a cross-sectional view of a package circuit board produced by dicing the circuit board sheet shown in  FIG. 1 ; 
           [0025]      FIGS. 3A to 3D  are cross-sectional views showing the procedure of a dicing process for a circuit board sheet according to a second embodiment of the present invention; 
           [0026]      FIG. 4  is a cross-sectional view of a package circuit board produced by dicing the circuit board sheet shown in  FIG. 3 ; and 
           [0027]      FIGS. 5A and 5B  are cross-sectional views of examples of other embodiments of a package circuit board. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0028]    Preferred embodiments of a method of dicing a circuit board sheet according to the present invention will now be described with reference to the attached drawings.  FIGS. 1A to 1D  are cross-sectional views showing the procedure of a dicing process carried out on a circuit board sheet according to a first embodiment of the present invention.  FIG. 2  is a cross-sectional view of a package circuit board produced by dicing the circuit board sheet shown in  FIGS. 1A to 1D . Note that although entire wiring layers  20  and insulating layers  22  are shown with diagonal shading in the drawings referred to by the present specification for ease of understanding the present invention, wiring patterns or insulating films are not formed at all of the shaded parts. 
         [0029]    In the present specification, a large laminated circuit board before dicing is referred to as a “circuit board sheet” and the laminated circuit boards produced by dicing such circuit board sheet are referred to as “package circuit boards”. 
         [0030]    Note that the present specification describes a circuit board sheet with a core portion  10  formed by laminating a plurality of prepregs  12 , which have been formed by impregnating carbon fibers with resin, through the application of heat and pressure. 
         [0031]    As shown in  FIG. 1A , the core portion  10  is constructed by stacking prepregs  12 , which have been produced by impregnating carbon fibers with a composite resin such as epoxy resin, in the thickness direction and applying heat and pressure to laminate the prepregs  12 . 
         [0032]    After the core portion  10  has been formed, the wiring layers  20  are formed on both surfaces of the core portion  10 . The wiring layers  20  are formed by an arbitrary method such as laminating wiring pattern films via the prepregs  12  or a build-up method.  FIG. 1A  shows a state where a large circuit board sheet  100  has been formed by forming wiring layers  20  on both surfaces of the core portion  10 . On the illustrated circuit board sheet  100 , the surfaces of the wiring layers  20  form the insulating layers  22 . 
         [0033]    After the circuit board sheet  100  has been formed, as shown in  FIG. 1B , concaves or concave channels  30  are formed in the thickness direction of the circuit board sheet  100  from the insulating layer  22  on the upper surface side using a dicing blade at dicing positions where the circuit board sheet  100  will be diced into package circuit boards  200 . Although not shown, it is preferable to provide a dicing blade protecting member such as dicing tape on the lower surface side of the circuit board sheet  100  when forming the concave channels  30 . The concave channels  30  are formed from the insulating layer  22  on the upper surface side so as to reach a position that is deeper than (i.e., beyond) at least the core portion  10  (i.e., up to the height of a lower surface of the lowest prepreg  12 ). When doing so, care is taken so that the circuit board sheet  100  is not diced by the concave channels  30 . 
         [0034]    When forming the concave channels  30 , it should be obvious that the depth of the concave channels  30  should be adjusted so that the circuit board sheet  100  does not become diced, and in addition sufficient thickness should be provided from the inner base surfaces of the concave channels  30  to the lower surface of the circuit board sheet  100  so that the circuit board sheet  100  does not break up at the positions of the concave channels  30  due to shock applied when the circuit board sheet  100  is conveyed following the formation of the concave channels  30 . 
         [0035]    After the concave channels  30  have been formed in the upper surface of the circuit board sheet  100 , as shown in  FIG. 1C , an insulating cover layer  40  composed of an insulating protective material such as a solder resist is formed on the upper surface and the side surfaces of the circuit board sheet  100  and inside the concave channels  30 . The insulating cover layer  40  can be formed so as to cover the circuit board sheet  100  by a method that bonds a B-stage insulating film by thermal compression bonding or by a method that prints insulating paste. 
         [0036]    In the present embodiment, the upper surface of the insulating cover layer  40  is formed flat across the entire surface of the circuit board sheet  100  regardless of the presence of the concave channels  30 . The insulating cover layer  40  is also formed on the outermost end surfaces of the circuit board sheet  100 . 
         [0037]    After the insulating cover layer  40  has been formed, a dicing protective material  50  such as a die attach film is stuck onto the lower surface of the circuit board sheet  100 . Next, a dicing blade  60  with a cutting width that is narrower than the gap between the inner walls (i.e., the channel width) B of the concave channels  30  is used to dice the circuit board sheet  100  with the concave channels  30  formed in the circuit board sheet  100  as the dicing positions. By dicing the circuit board sheet  100 , package circuit boards  200  such as that shown in  FIG. 2  are obtained. 
         [0038]    Note that to expose connection pads and the like for connecting a semiconductor chip on the surface of the individual package circuit boards  200 , it is possible to attach a photosensitive insulating film that will become the insulating cover layer  40  to the surface of the circuit board sheet  100  and then expose and develop the insulating cover layer  40  to remove the insulating material at predetermined positions. 
         [0039]    On the package circuit board  200  shown in  FIG. 2 , at the outer surfaces where the concave channels  30  were formed, cut surfaces produced by the dicing blade  60  are exposed across the entire thickness of the package circuit board  200 . With the package circuit board  200 , surfaces where the insulating cover layer  40  was cut during dicing become exposed at the outer surfaces, so that the side surfaces of the core portion  10  are completely sealed by the insulating cover layer  40 . 
         [0040]    Accordingly, it is possible to reliably prevent the carbon fibers included in the core portion  10  from becoming exposed at the side surfaces of the package circuit board  200  and to reliably avoid the carbon fibers coming off to the outside. Also, when the circuit board sheet  100  is diced, since the dicing blade does not touch the core portion  10 , no carbon fibers from the core portion  10  become mixed in the fragments produced by dicing. For this reason also, electrical shorting of the package circuit board  200  can be avoided. 
       Second Embodiment 
       [0041]      FIGS. 3A to 3D  are cross-sectional views showing the procedure of a dicing process for a circuit board sheet according to a second embodiment.  FIG. 4  is a cross-sectional view of a package circuit board produced by dicing the circuit board sheet shown in  FIGS. 3A to 3D . 
         [0042]    Since the constructions of the prepregs  12 , the wiring layers  20 , and the insulating layers  22  in this second embodiment are the same as the constructions in the first embodiment, detailed description thereof is omitted here. 
         [0043]    First, as shown in  FIG. 3A , the core portion  10  is constructed by laminating the prepregs  12  that include carbon fibers. By carrying out a build-up method or the like on both surfaces of the core portion  10  to form the wiring layers  20 , the circuit board sheet  100  is formed. After the circuit board sheet  100  has been formed, as shown in  FIG. 3B , the concave channels  30  are formed by a dicing blade at positions that will be used as the dicing positions of the circuit board sheet  100 . The processes up to this point are the same as in the first embodiment. 
         [0044]    Next, as shown in  FIG. 3C , the insulating cover layer  40  that is composed of an insulating protective material such as a solder resist is formed on the side surfaces of the circuit board sheet  100  and inside the concave channels  30 . The circuit board sheet  100  according to the present embodiment is characterized by the height of the insulating cover layer  40  at the concave channel  30  parts being formed lower than the height of the surface of the insulating cover layer  40  at positions where the wiring layer  20  is formed. 
         [0045]    In more detail, as shown in  FIG. 3D , the surface height of the insulating cover layer  40  in the present embodiment is set so that the height is uniform across the entire circuit board sheet  100  at positions above the core portion  10 . On the other hand, in each range SB with the width of the concave channels  30 , the surface height of the insulating cover layer  40  is lower than the height of the insulating cover layer  40  above the core portion  10  in keeping with the form of the concave channels  30 . In the insulating cover layer  40 , the parts (i.e., the parts with the width SB) where the surface height is lower than the other parts should preferably be wider than the cutting width CB of the dicing blade  60  used when dicing the circuit board sheet  100 . That is, the channel width B of the concave channels  30 , the width SB of the parts where the surface height of the insulating cover layer  40  is lower, and the cutting width CB of the dicing blade  60  are set so that CB&lt;SB&lt;B. 
         [0046]    In any case, it is favorable for the surface of the insulating cover layer  40  to be depressed at the positions of the concave channels  30  where the circuit board sheet  100  will be diced since this makes it possible to clearly know the dicing positions even after the concave channels  30  have been filled with the insulating cover layer  40 . Note that before the circuit board sheet  100  is diced at the positions of the concave channels  30 , a dicing protective material  50  such as a die attach film is stuck onto the lower surface of the circuit board sheet  100 . 
         [0047]    The package circuit boards  200  obtained in this way have the construction shown in  FIG. 4 . On such package circuit boards  200  also, cut surfaces of the insulating cover layer  40  that was cut during dicing become exposed at the outer side surfaces, so that the side surfaces of the core portion  10  are completely sealed by the insulating cover layer  40 . Accordingly, it is possible to reliably prevent the carbon fibers included in the core portion  10  from becoming exposed at the side surfaces of the package circuit board  200  and to reliably avoid the carbon fibers coming off to the outside. Also, when the circuit board sheet  100  is diced, since the dicing blade does not touch the core portion  10 , no carbon fibers from the core portion  10  become mixed in the fragments produced by dicing. For this reason also, in the same way as in the first embodiment, electrical shorting of the package circuit board  200  can be avoided. 
         [0048]    Although the circuit board sheet  100  and the package circuit boards  200  produced by dicing the circuit board sheet  100  according to embodiments of the invention have been described in detail, it should be obvious that the present invention is not limited to the embodiments described above. For example, although the dicing blade  60  is used when forming the concave channels  30  in the circuit board sheet  100  or when carrying out dicing on the circuit board sheet  100  according to the above embodiments, it is also possible to use a drill such as a router, a laser beam, or the like in place of the dicing blade  60 . 
         [0049]    Although an aspect where the cross-sectional form of the concave channels  30  is rectangular has been described in the embodiments described above, it is also possible to form the concave channels  30  so as to be substantially U-shaped or inversely trapezoidal in cross section.  FIGS. 5A and 5B  show package circuit boards  200  that are obtained by forming the concave channels  30  with such shapes, forming the insulating cover layer  40  on the upper surface and side surfaces of the circuit board sheet  100 , and then dicing the circuit board sheet  100  inside the concave parts (i.e., at the positions of the concave channels  30 ) in the upper surface of the insulating cover layer  40 . 
         [0050]      FIG. 5A  shows a package circuit board  200  obtained by dicing the circuit board sheet  100  where the concave channels  30  have been formed so as to be substantially U-shaped in cross section, while  FIG. 5B  shows a package circuit board  200  obtained by dicing the circuit board sheet  100  where the concave channels  30  have been formed so as to be inversely trapezoidal in cross section. 
         [0051]    When dicing the circuit board sheet  100  according to the second embodiment, although the circuit board sheet  100  is cut using the dicing blade  60  with a cutting width CB that is narrower than the upper surface width SB of the insulating cover layer  40  formed at a lower position inside the concave channels  30  than a part where the lower wiring layer  20  is formed, it is also possible for the dicing blade  60  to not satisfy such condition regarding the cutting width CB. However, the cutting width CB of the dicing blade  60  needs to be narrower than the width B between the inner walls of the concave channels  30 . 
         [0052]    Also, although the above embodiments describe an example where the circuit board sheet  100  is formed using the core portion  10  made by laminating a plurality of prepregs  12  where carbon fibers are impregnated with resin such as epoxy resin, it is also possible to form the core portion  10  using prepregs  12  where metal fibers are mixed into carbon fibers and to mix filler such as glass filler into the prepregs  12 . In addition, it should be obvious that it is also possible to apply the constructions described in the above embodiments to a circuit board sheet  100  formed using a metal plate as one of the prepregs  12 . 
         [0053]    When a metal plate is used as part of the core portion  10 , there is the advantage that it is possible to improve the heat dissipation performance of the package circuit board  200 . 
         [0054]    Although the package circuit board  200  described above is formed so that part of the cut surface of the lower wiring layer  20  is not covered by the insulating cover layer  40  and is exposed, it should be obvious that when at least one of the thickness and strength of an insulating layer  22  is sufficient, it is possible to set the base positions of the concave channels  30  inside such insulating layer  22 . Such construction is favorable in that it is possible to eliminate exposed parts of the lower wiring layer  20  (i.e., parts not covered with the insulating cover layer  40 ) of the package circuit board  200 .