Abstract:
A partially flexible circuit board and a method of manufacturing the same. The partially flexible circuit board of the present invention comprises: laminated alternating layers of prepreg and conductive printed circuitry with apertures in one or more of the prepreg layers in those areas that are required to undergo limited flexing. Fabrication is accomplished by removal of the prepreg in the aperture area of at least one of the prepreg layers; filling of the aperture with a suitable heat resistant and non-adhesive filler prior to lamination; laminating additional prepreg and conductive printed circuit layers prepared for revealing the aperture, subsequent to lamination; cutting slots at the edges of the circuit board that communicate with the apertures to permit flexing across the width thereof without interference with the encompassed circuitry; and removing the filler and any residual prepreg from the aperture to leave the aperture in the prepreg and conductive foil at the desired locations(s).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to circuit boards and their manufacture and more particularly to circuit boards that are neither rigid nor flexible in the conventionally accepted sense, but rather semi or partially flexible and thus can be used in applications that are unsuited to rigid circuit boards, but that do not require the flexibility of relatively high cost flexible circuit boards.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the prior art, there are basically two distinct classes of circuit boards, so-called rigid circuit boards and so-called flexible circuit boards. The former are those conventionally used in the manufacture of electronic devices such as cellular phones, televisions, radios etc. Flexible circuit boards are those used in applications where the “flat” circuitry must undergo relatively high levels of repetitive flexing. A common such application familiar to most users would be the flexible circuit boards used in computer printers where the printed circuit used to feed electrical pulses to the printer head must travel with the head resulting in a repetitive flexing of the board. Such boards are relatively expensive to manufacture due to the requirement that they be capable of undergoing somewhere on the order of several hundred thousand “flexes” without disruption of the contained circuitry. While rigid circuit boards use relatively inexpensive fiber glass prepreg materials as the prepreg material, flexible circuit boards use significantly more expensive prepreg materials.  
         [0003]     There are many applications for printed circuitry that require a circuit board to flex only a relatively limited number of times, perhaps on the order of one to as many as several times during, for example fabrication operations. An example of such an application would be in the case where it would be desirable, perhaps due to the size and/or shape of a device case, to have a circuit that requires a board of such a size that it must be comprised of a pair of circuit boards positioned at right angles to one another. Current manufacturing techniques would satisfy this need by producing two separate rigid boards that are joined by soldered or otherwise connected tines from each of the individual boards at the right angle bend. Such manufacturing operations are relatively expensive and labor intensive, but the application will not support nor does it require the capabilities of the even more expensive flexible circuit boards. The circuit board of the present invention allows for limited flexing for a limited number of times and permits the fabrication and installation of a single partially or semi flexible board in such an application.  
         [0004]     Thus, the availability of a relatively inexpensive hybrid rigid/flexible circuit board that is sufficiently flexible as to permit flexing once or twice, for example, during the fabrication operation, but never be flexed again, would be highly useful to the electronic components industry.  
       OBJECTS OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to provide a partially or semi-flexible printed circuit board that can undergo a relatively limited number of flexes without breaking.  
         [0006]     It is another object of the present invention to provide such a circuit board whose cost is relatively that of a conventional rigid circuit board, i.e. significantly less than that of a conventional flexible circuit board.  
       SUMMARY OF THE INVENTION  
       [0007]     According to the present invention, there are provided a partially flexible circuit board and a method of manufacturing the same. The partially flexible circuit board of the present invention comprises: laminated alternating layers of prepreg and conductive printed circuitry with apertures in one or more of the prepreg layers in those areas that are required to undergo limited flexing. Manufacture of such partially flexible circuit boards is accomplished by removal of the prepreg in the aperture area of at least one of the prepreg layers; filling of the aperture with a suitable removable filler prior to lamination; laminating additional prepreg and conductive foil layers prepared for revealing the aperture and subsequent to lamination removal of the filler to leave the apertures in the prepreg and conductive foil layers at the desired locations(s) as well as the cutting of slots at the edges of the circuit board that communicate with the apertures to permit flexing across the width thereof without interference with the encompassed circuitry. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic cross-sectional depiction of one embodiment of the circuit board of the present invention.  
         [0009]      FIG. 2  is a schematic cross-sectional depiction of another embodiment of the circuit board of the present invention.  
         [0010]      FIG. 3  is a schematic cross-sectional depiction of yet another embodiment of the circuit board of the present invention.  
         [0011]      FIG. 4  is a schematic exploded depiction showing the method of assembly of the circuit board of the present invention.  
         [0012]      FIG. 5  is a bottom plan view of a three part partially flexible circuit board in accordance with the present invention.  
         [0013]      FIG. 6  is a perspective, schematic, exploded view showing the method of fabrication of partially flexible circuit boards in accordance with the method of the present invention.  
         [0014]      FIG. 7  is a schematic depiction of the circuit board of  FIG. 5  in a flexed or bent position. 
     
    
     DETAILED DESCRIPTION  
       [0015]     The partially or semi flexible circuit boards of the present invention are manufactured using many of the conventional and essential process steps and materials used for the manufacture of rigid circuit boards, all of which are well known to those skilled in the circuit board fabrication/manufacture arts. Because of this fact, many of the process steps, details of the finished board and materials used in the processes and products of the present invention are described only in general terms in this application, it being well within the skill of the art to prepare the products of the present invention in accordance with the methods of the present invention using the prior art methods and materials once the process variation techniques described herein are known. Such is the case, for example, where plated through holes are not represented in the many of the attached schematic Figures. The presence of these features and their introduction into the product are well known to the skilled artisan and their incorporation into the novel partially flexible circuit boards of the present invention is well within the skill of the art. Similarly, lamination and photoetching techniques and the like conventionally used in the fabrication of circuit boards are not described in, any detail as these are common in the fabrication of rigid circuit boards. It should also be noted that in the various Figures attached hereto, the relative thickness of the metallic foil layers and patterns is exaggerated for purposes of clarity, it being understood that the foil layers are significantly thinner than the prepreg layers as is the case in the conventional manufacture of printed circuit boards.  
         [0016]     Referring now to  FIG. 1 , the simplest embodiment of the novel circuit boards of the present invention is a single sided board  10  comprising a prepreg layer  12  having a single discontinuous conductive foil layer, a pattern, (represented by areas  14  and  22  in  FIG. 1 ) thereon formed using conventional fabrication techniques, i.e. through the lamination of conductive foil to prepreg layer  12  and photo etching the conductive foil to provide areas  14  and  22 , the pattern, in the appropriate locations. Cut out area  16 , formed in second prepreg layer  13  as described below, allows essentially rigid circuit board portions  18  and  20  to limitedly flex relative to one another. In this configuration, conductive foil pattern  22  and section  24  of prepreg layer  12  can flex allowing rigid portions  18  and  20  to flex relative to one another.  
         [0017]      FIG. 2  depicts a schematic cross-section of a second embodiment of the partially flexible circuit board of the present invention. The embodiment depicted in  FIG. 2  shows a double sided partially flexible circuit board  26  comprising a prepreg layer  12  to which are laminated foil patterns  14  and  22  and apertured prepreg  13  to which a second array of foil patterns  28  have been applied by conventional lamination and photo-etching techniques. Aperture  30  in prepreg layer  13  is provided as described below. In this double sided configuration, partially flexible circuit board  26  can flex about portion  24  as described above in connection with the single sided configuration depicted in  FIG. 1 .  
         [0018]     Referring now to  FIG. 3 , in this figure, there is schematically depicted a multi-layer partially flexible circuit board  27  of the present invention. As shown in this Figure, the multi-layer partially flexible circuit board  27  comprises a first prepreg layer  12  having conductive foil patterns  14  and  22  laminated thereon and second prepreg layer  13  laminated to the “underside” (that opposite the side to which foil patterns  14  and  22  are laminated) thereof which is the structure of the double sided circuit board depicted in  FIG. 2 . Additional layers are then laminated to the structure of  FIG. 2  as follows: over the top of foil patterns  14  and  22  is laminated another prepreg layer  32 ; to prepreg layer  13  are laminated foil patterns  34  formed in the conventional fashion by lamination and photo-etching; another prepreg layer  36  is laminated over conductive foil patterns  34 ; additional conductive foil patterns  38  are laminated and formed on prepreg layer  36 ; yet another prepreg layer  40  is applied over conductive foil patterns  38 ; and finally, conductive foil patterns  42  are applied over prepreg layer  40  to complete the structure. As will be apparent to the skilled artisan, this type of construction/fabrication can continue virtually indefinitely. Aperture  44  in the bottom of circuit board  27  provides the point about which partial flexing can occur. Application of the conductive foil layers/areal portions described in this configuration is, of course, accomplished using conventional lamination and photo-etching techniques as are well known in the circuit board manufacturing arts.  
         [0019]      FIG. 5  depicts a bottom plan view of a partially flexible circuit board  50  fabricated in accordance with the present invention. As shown in this Figure, partially flexible circuit board  50  comprises three portions  52 ,  54  and  56 , each of which comprises essentially a rigid, multi-layer circuit board fabricated as described above. The apertures or recesses  58  in circuit board  50  are as shown for aperture or recess  44  in  FIG. 3 . Apertures or recesses  58  allow the three portions to flex about these recesses as shown in  FIG. 7 . As will be apparent to the skilled artisan, although apertures  16 ,  34  and  44  depicted in  FIGS. 1-3  as well as apertures  58  depicted in  FIG. 5  will permit flexing in those areas of circuit boards  16 ,  30 , 27  and  50  in the regions where they are located, there are areas along the edges such as  60  and  62  in  FIG. 5  where allowance must be made to permit flexing in areas outside the boundaries of apertures  58 . To provide such allowance, slots  64  and  66  are routed through the total thickness of those portions of partially flexible circuit board  50  outside of the area of apertures  58 , but adjoining apertures  58  during fabrication. With the presence of these slots, partially flexible circuit board  50  possesses limited flexibility at each of the “joints” formed by apertures  58  and slots  64  and  66 . Electrical connection between portions  52 ,  54  and  56  is accomplished by the presence of conductive foil patterns  22  “embedded” or laminated with or between one or more prepreg layers  12  and/or  32  as depicted in  FIGS. 1 and 3 . Plated through holes  70  are provided in the conventional fashion as are conventional contacts  71 , both of which allow for connection/installation of electronic components in the manner well known in the electronic fabrication arts.  
         [0020]     A better understanding of the structure and fabrication of the partially flexible circuit boards of the present invention can be gathered from an understanding of the fabrication method used to produce them. This is best illustrated by reference to  FIG. 6  that shows a partial, schematic, inverted, exploded cross-sectional view of a partially flexible circuit board  62  in accordance with the present invention. As shown in this Figure, partially flexible circuit board  62  comprises a core of a prepreg layer  64  having two conductive foil layers  66  and  68  laminated thereto. Another prepreg layer  70  is laminated over the top of conductive foil layer  66 . Prior to lamination of prepreg layer  70  to conductive foil layer  66  an aperture  72  is cut in prepreg layer  70  at the appropriate location and of the appropriate size to permit flexing of partially flexible circuit board  62  about the area of aperture  72  after lamination into the circuit board structure. Also prior to lamination of prepreg layer  70  to conductive foil layer  66  a filler block  74  of silicone or some similar high temperature resistant and non-adhesive material is inserted into aperture  72  to support the overlaid conductive foil layer  73  during the lamination and photo-etching processes as well as subsequently applied additional prepreg layers. Subsequently applied additional prepreg and conductive foil patterns are added in similar fashion with those portions of the additional foil layers that lie within the area of aperture  72  being removed during photoetching and the edges  76  and  78  of areas  80  and  82  of subsequently applied prepreg layers  84  and  86  perforated or cut along lines  82  and  83  such that in a subsequent routing operation to provide, for example, slot  66 , the unscored, perforated or cut ends  85  and  87  are cut and areas  80  and  82  easily removed from the structure along with filler  74  to provide an aperture as shown at  58  in  FIG. 5 .  
         [0021]      FIG. 6  depicts an exploded perspective view of a circuit board assembly similar to that depicted in  FIG. 4 , except that a pair of apertures  72  are provided therein.  
         [0022]     In the final routing step of the fabrication process of the present invention, slots  64  and  66  are routed from the outside edges ( 60  and  62  in  FIG. 5 ) of the partially flexible circuit board. Such routing removes the full thickness of circuit board  50 , provides the slots  64  and  66  necessary to permit flexing of circuit board  50  and also allows for removal of areas of prepreg layers such as  80  and  82  as depicted in  FIGS. 4 and 6  as well as the previously inserted filler. To provide proper flexing, it is important that those portions of slots  64  and  66  at right angles to edges  60  and  62  be coaxial.  
         [0023]     Thus what have been described are: 1) a partially flexible circuit board comprising at least two rigid circuit board portions comprising a core of at least one connective conductive foil pattern laminated to one or more prepreg members and optionally additional alternating layers of prepreg and conductive foil on one or both surfaces of the core, the rigid circuit board portions being joined at a joint defined by the core and an aperture in the rigid circuit board portions on one or both surfaces in the area of the joint; and 2) a method for the fabrication of a partially flexible circuit board comprising: a) forming a circuit board core having longitudinal edges and comprising at least one conductive foil layer laminated to at least one prepreg layer, b) laminating to the at least one conductive foil layer a first prepreg layer having an aperture therein in the area of an intended joint, c) inserting a heat resistant, non-adhesive filler member into the aperture, d) laminating to the first prepreg layer a first layer of conductive foil, e) forming a conductive pattern in the first layer of conductive foil, at least a portion of the pattern including a vacant area that registers with the aperture; f) further laminating alternating layers of: A) prepreg having score lines that define opposing edges of areas that register with the aperture upon lamination and are perpendicular to the longitudinal edges, and B) conductive foil; g) forming a conductive pattern in each of the alternating layers of conductive foil which pattern includes a vacant area that registers with the aperture when laminated to the core until a desired number of layers is achieved; routing slots from the longitudinal edges to and along opposing extremities of the aperture and removing the heat resistant, non-adhesive filler and any areas of prepreg from the aperture.  
         [0024]     As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.