Patent Publication Number: US-2020305289-A1

Title: Flexible substrate and method for fabricating the same

Description:
BACKGROUND 
     1. Technical Field The present disclosure relates to packaging substrates, and, more particularly, to a flexible substrate and a method for fabricating the same. 
     2. Description of Related Art 
     Currently, rigid-flexible circuit boards are widely applied in smartphone boards, optoelectronic boards, CMOS, battery modules and wearable devices. In a conventional rigid-flexible circuit board, a soft board structure is encapsulated by a rigid board structure, a portion of the soft board structure that needs to be flexed is exposed from the rigid board structure, and interlayer connections are used to replace conventional surface mounting, thereby saving spaces, simplifying the assembly process and effectively reducing noises transmitted from the soft board structure to the rigid board structure. 
     Therefore, the application of rigid-flexible circuit boards in electronic products (especially in lighter, thinner, shorter and smaller mobile devices and wearable devices) is rapidly increasing due to their advantages of good reliability, easy assembly and noise suppression. 
       FIGS. 1A to 1D  are schematic cross-sectional views showing a method for fabricating a rigid-flexible packaging substrate  1  according to the prior art. 
     Referring to  FIGS. 1A to 1B , a soft board  10  having a flexible section A is provided and a rigid board  11  having a through hole  110  is attached to the two opposite sides of the soft board  10 . The through hole  110  corresponds in position to the flexible section A so as to expose the flexible section A. 
     The soft board  10  has a core layer  10   a,  first circuit layers  10   b  formed on the two opposite sides of the core layer  10   a,  adhesive layers  10   c  covering the first circuit layers  10   b,  and protection films  10   d  covering the adhesive layers  10   c.  For example, the core layer  10   a  and the first circuit layers  10   b  are made of a copper clad laminate (CCL). 
     The rigid board  11  had a rigid substrate  11   a,  a metal layer  11   b  formed on one side of the rigid substrate  11   a,  and a pure adhesive layer  11   c  formed on the other side of the rigid substrate  11   a . By performing a mechanical drilling process, the through hole  110   a  is formed in the rigid board  11  to penetrate the rigid substrate  11   a,  the metal layer  11   b  and the pure adhesive layer  11   c.  Further, the pure adhesive layer  11   c  is attached to the protection film  10   d  of the soft board  10 . 
     Referring to  FIG. 1C , by performing a mechanical drilling or laser ablation process, a plurality of through holes  100  are formed to penetrate the soft board  10  and the rigid boards  11 . 
     Referring to  FIG. 1D , by using the metal layers  11   b  as current conductive paths, an electroplating process is performed to electroplate a conductive material  12   a  on the metal layers  11   b  and walls of the through holes  100 . Then, the metal layers  11   b  and the conductive material  12   a  are patterned and etched to form second circuit layers  12  on the rigid substrates  11   a  and hollow conductive through holes  13  in the through holes  100 . The conductive through holes  13  electrically connect conductive pads  101  of the first circuit layers  10   b  and the second circuit layers  12 . Thereafter, solder mask layers  14 , such as ink or green paint, are formed on the rigid substrates  11   a  and the second circuit layers  12  and in the conductive through holes  13  and portions of the second circuit layers  12  are exposed from the solder mask layers  14 . But the solder mask layers  14  are not formed in the through holes  110 . 
     Therefore, the rigid-flexible packaging substrate  1  is flexible at the flexible section A exposed from the through holes  110 . 
     However, the soft board  10  and the rigid boards  11  cannot be aligned accurately due to great interlayer size variations of the soft board  10  and limitations of the attachment process. In particular, the pure adhesive layer  11   c  may cause a positional deviation of the rigid board  11 , resulting in an interlayer alignment accuracy of about +/−100 um between the soft board  10  and the rigid board  11 . As such, the area of the conductive pads  101  of the soft board  10  must be increased to prevent a positional deviation of the through holes and hence ensure the electrical connection between the conductive through holes  13  and the conductive pads  101 . As such, the available area for other functional circuits of the first circuit layers  10   b  is reduced and consequently, the width of the soft board  10  needs to be increased or the circuit function of the soft board  10  needs to be reduced. On the other hand, if the area of the conductive pads  101  is not increased, the distance t between the conductive pads  101  and surrounding circuits must be increased so as to prevent a short circuit from occurring due to the positional deviation of the through holes. But it cannot meet the fine pitch requirement. 
     Further, since the rigid boards  11  are attached to the soft board  10  through the pure adhesive layer  11   c,  defects such as adhesive overflows e (at corners of the through holes  110  of  FIG. 1B ), shorts or edge sinks may occur at interfaces between the soft board  10  and the rigid boards  11 , thus adversely affecting the subsequent attachment process and the flexibility of the packaging substrate. 
     In addition, limited by the core layer  10   a  of the soft board  10 , the thickness of the rigid boards  11  (a certain thickness is needed for required rigidity), and the second circuit layers  12 , it is difficult to reduce the thickness H of the packaging substrate  1  to be less than 0.3 mm. Therefore, the packaging substrate  1  cannot meet the thinning requirement. 
       FIGS. 2A to 2D  are schematic cross-sectional views showing another method for fabricating a rigid-flexible packaging substrate  2  according to the prior art. 
     Referring to  FIGS. 2A to 2B , a soft board  20  having a flexible section A is provided and a dielectric layer  21  having a through hole  210  is laminated on each of the two opposite sides of the soft board  20 . The through hole  210  corresponds in position to the flexible section A so as to expose the flexible section A. Then, metal layers  21   b  made of such as copper are formed on the dielectric layers  21 , walls of the through holes  210  and the flexible section A. 
     The soft board  20  has a core layer  20   a,  first circuit layers  20   b  formed on the two opposite sides of the core layer  20   a,  adhesive layers  20   c  formed on the flexible section A, and protection films  20   d  covering the adhesive layers  20   c.  For example, the core layer  20   a  and the first circuit layers  20   b  are made of a copper clad laminate 
     The dielectric layers  21  are made of prepreg. By performing a mechanical drilling process, the through holes  210  are formed to penetrate the dielectric layers  21 . The dielectric layers  21  are bonded to the core layer  20   a,  the first circuit layers  20   b  and portions of the protection films  20   d  of the soft board  20 . 
     Referring to  FIG. 2C , by performing a mechanical drilling or laser ablation process, a plurality of through holes  200  are formed to penetrate the soft board  20 , the dielectric layers  21  and the metal layers  21 . 
     Referring to  FIG. 2D , by using the metal layers  21   b  as current conductive paths, an electroplating process is performed to electroplate a conductive material  22   a  on the metal layers  21   b  and walls of the through holes  200 . Then, the metal layers  21   b  and the conductive material  22   a  are patterned and etched to form second circuit layers  22  on the dielectric layers  22   a  and hollow conductive through holes  23  in the through holes  200 . The conductive through holes  23  electrically connect conductive pads  201  of the first circuit layers  20   b  and the second circuit layers  22 . Thereafter, solder mask layers  24  such as ink or green paint are formed on the dielectric layers  21  and the second circuit layers  22  and in the conductive through holes  23 , and portions of the second circuit layers  22  are exposed form the solder mask layers  24 . But the solder mask layers  24  are not formed in the through holes  210 . 
     Therefore, the rigid-flexible packaging substrate  2  is flexible at the flexible section A exposed from the through holes  210 . 
     However, since interlayer size variations of the soft board  20  are great and lamination of the dielectric layers  21  on the soft board  20  is performed under high temperature and high pressure, an irregular deformation may occur to the packaging substrate  2  and a poor interlayer alignment accuracy (about +/−100 um) may occur between the soft board  20  and the dielectric layers  21 . As such, the area of the conductive pads  201  of the soft board  20  must be increased to prevent a positional deviation of the through holes and hence ensure the electrical connection between the conductive through holes  23  and the conductive pads  201 . As such, the available area for other functional circuits of the first circuit layers  20   b  is reduced and consequently, the width of the soft board  20  needs to be increased or the circuit function of the soft board  20  needs to be reduced. On the other hand, if the area of the conductive pads  201  is not increased, the distance t between the conductive pads  201  and surrounding circuits must be increased so as to prevent a short circuit from occurring due to the positional deviation of the through holes. But it cannot meet the fine pitch requirement. 
     Further, after lamination of the dielectric layer  21 , the rigid-flexible interface of the packaging substrate  2  may bulge due to the existence of two kinds of materials (the dielectric layer  21  of prepreg and the protection film  20   d ), thus adversely affecting the subsequent attachment process and the flexibility of the packaging substrate. 
     In addition, limited by the core layer  20   a  of the soft board  20 , the thickness of the protection films  20   d  (a certain thickness is needed to prevent cracking of the protection films  20   d  when being flexed), the thickness of the dielectric layers  21  (a certain thickness is needed for required rigidity), and the second circuit layers  22 , it is difficult to reduce the thickness h of the packaging substrate  2  to be less than 0.25 mm. Therefore, the packaging substrate  2  cannot meet the thinning requirement. 
       FIGS. 3A to 3E  are schematic cross-sectional views showing a further method for fabricating a rigid-flexible packaging substrate  3  according to the prior art. 
     Referring to  FIG. 3A , a rigid board  311  having an opening  311  is provided. The rigid board  31  has a rigid substrate  31   a  and internal circuit layers  31   c  formed on the two opposite sides of the rigid substrate  31   a.  For example, the rigid substrate  31   a  and the internal circuit layers  31   c  are made of a copper clad laminate, and the opening  311  is formed by mechanical drilling. 
     Referring to  FIG. 3B , a soft board  30  having a flexible section A is disposed in the opening  311 . The soft board  30  has a core layer  30   a,  first circuit layers  30   b  formed on the two opposite sides of the core layer  30   a,  adhesive layers  30   c  formed on the flexible section A, and protection films  30   d  covering the adhesive layers  30   c.    
     Referring to  FIG. 3C , dielectric layers  31   d  each having a through hole  310  are laminated on the two opposite sides of the rigid board  31  and the soft board  30 , and the through holes  310  correspond in position to the flexible section A so as to expose the flexible section A. Then, metal layers  31   b  made of such as copper are formed on the dielectric layers  31   d,  walls of the through holes  310  and the flexible section A. Thereafter, a plurality of through holes  300  are formed to penetrate the rigid board  31 , the dielectric layers  31   d  and the metal layers  31   b,  and a plurality of conductive vias  301  are formed to penetrate the dielectric layers  31   d  and the metal layers  31   b  to expose portions of the first circuit layers  30   b.    
     The dielectric layers  31   d  are made of prepreg. The dielectric layers  31  are bonded to the rigid board  31 , and portions of the first circuit layers  30   b  and portions of the protection films  30   d  of the soft board  30 . 
     Referring to  FIG. 3D , by using the metal layers  31   b  as current conductive paths, an electroplating process is performed to electroplate a conductive material  32   a  on the metal layers  31   b  and walls of the through holes  300  and in the conductive vias  300 . Then, the metal layers  31   b  and the conductive material  32   a  are patterned and etched to form second circuit layers  32  on the dielectric layers  31   d  and conductive through holes  33  in the through holes  300 . The conductive through holes  33  electrically connect the internal circuit layers  31   c  and the second circuit layers  32 . Thereafter, circuit structures  35  are formed on the dielectric layers  31   d  and electrically connected to the second circuit layers  32 . Then, solder mask layers  34  such as ink or green paint are formed on the circuit structures  35  and portions of the circuit structures  35  are exposed from the solder mask layers  34 . 
     Referring to  FIG. 3E , the portions over the through holes  310  (i.e., the circuit structures  35 , the conductive material  32   a  and the metal layers  31   b ) are removed by mechanical cutting to expose the flexible section A (i.e., expose the protection films  30   d ). 
     Therefore, the rigid-flexible packaging substrate  3  is flexible at the flexible section A. 
     However, since the second circuit layers  32  are formed by etching the metal layers  31   b  and the conductive material  32   a,  the line width/pitch of the second circuit layers  32  is limited by the fabrication process and cannot be less than 40/40 um. 
     Further, after lamination of the dielectric layers  31   d,  an edge bulge or prepreg overflow may occur at the rigid-flexible interface of the packaging substrate  3  due to the existence of two kinds of materials (the dielectric layers  3   d   1  and the protection films  30   d ), thus adversely affecting the subsequent attachment process and the flexibility of the packaging substrate. 
     In addition, since the rigid board  31  corresponds in thickness to the soft board  30  (the soft board  30  needs to have a certain thickness to prevent cracking when being flexed), it is difficult to reduce the thickness H of the packaging substrate  3  to be less than 0.25 mm. Therefore, the packaging substrate  3  cannot meet the thinning requirement 
       FIG. 4  is a schematic cross-sectional view showing another method for fabricating a rigid-flexible packaging substrate  4  according to the prior art. 
     Referring to  FIG. 4 , circuit boards  40  each having a flexible section A are laminated on the two opposite sides of a core board  9 . Each of the circuit boards  40  has a circuit layer  40   b,  a soft portion  40   c  and a first dielectric layer  40   d.  The circuit layer  40   b  is laminated on the core board  9 . The first dielectric layer  40   d  has an opening  400  corresponding in position to the flexible section A for exposing the soft portion  40   c.  Then, second dielectric layers  41  each having a through hole  410  and protection films  90  are sequentially laminated on the circuit boards  40  according to the practical need. 
     As such, the packaging substrate  4  is flexible at the flexible section A and the line width/pitch of the circuit layers  40   b  are not limited as described above. 
     However, since two kinds of materials (the soft portions  40   c  and the first dielectric layers  40   d ) exist at the rigid-flexible interface of the packaging substrate  4 , after lamination of the second dielectric layers  41 , an edge bulge may occur in the openings  400  (or the through holes  410 ) due to an uneven stress distribution. As such, the subsequent attachment process and the flexibility of the packaging substrate are adversely affected. 
     Further, the core board  9  hinders thinning of the packaging substrate  4 . 
     Therefore, how to overcome the above-described drawbacks has become critical. 
     SUMMARY 
     In view of the above-described drawbacks, the present disclosure provides a flexible substrate, which comprises: a coreless substrate body having a flexible section and at least a dielectric layer, wherein the dielectric layer is made of a molding compound or a primer; and an additional element formed on the substrate body and having a through hole exposing the flexible section, wherein the through hole and the flexible section form a cavity. 
     In an embodiment, the substrate body further has a circuit structure bonded to the dielectric layer. 
     In an embodiment, the additional element further has an insulating layer and a conductive post embedded in the insulating layer. 
     In an embodiment, the insulating layer is made of a dielectric material, such as a molding compound or a primer. 
     In an embodiment, the additional element is a metal sheet. 
     The present disclosure further provides a method for fabricating a flexible substrate, which comprises: providing a coreless substrate body having a flexible section and at least a dielectric layer, wherein the dielectric layer is made of a molding compound or a primer; forming an additional element on the substrate body, wherein the additional element has an insulating layer, a conductive post embedded in the insulating layer, and a block penetrating the additional element; and removing the block to form in the additional element a through hole exposing the flexible section, wherein the through hole and the flexible section form a cavity. 
     In an embodiment, the substrate body further has a circuit structure bonded to the dielectric layer. 
     In an embodiment, the insulating layer is made of a dielectric material, such as a molding compound or a primer. 
     The present disclosure further provides another method for fabricating a flexible substrate, which comprises: forming a coreless substrate body on a carrier, wherein the substrate body has a flexible section and at least a dielectric layer, and the dielectric layer is made of a molding compound or a primer; and removing a portion of the carrier to form a through hole penetrating the carrier and exposing the flexible section, allowing the carrier having the through hole to serve as an additional element, wherein the through hole and the flexible section form a cavity. 
     In an embodiment, the substrate body further has a circuit structure bonded to the dielectric layer. 
     In an embodiment, the carrier is a metal sheet. 
     Therefore, by providing additional elements and substrate structures that are different from the prior art, the present disclosure greatly improves the circuit density of rigid-flexible board structures and reduces the overall thickness of substrates. The present disclosure is applicable to high-end mobile devices that require reduced thicknesses and complicated (or multi-functional) circuits. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1D  are schematic cross-sectional views showing a method for fabricating a rigid-flexible packaging substrate according to the prior art; 
         FIGS. 2A to 2D  are schematic cross-sectional views showing another method for fabricating a rigid-flexible packaging substrate according to the prior art; 
         FIGS. 3A to 3E  are schematic cross-sectional views showing a further method for fabricating a rigid-flexible packaging substrate according to the prior art; 
         FIG. 4  is a schematic cross-sectional view showing another method for fabricating a rigid-flexible packaging substrate according to the prior art; 
         FIGS. 5A to 5F  are schematic cross-sectional views showing a method for fabricating a flexible substrate according to a first embodiment of the present disclosure; 
         FIG. 5G  is a schematic top view of  FIG. 5F ; 
         FIGS. 6A to 6C  are schematic cross-sectional views showing a method for fabricating a flexible substrate according to a second embodiment of the present disclosure; and 
         FIGS. 7A to 7B  are schematic cross-sectional views showing a method for fabricating a flexible substrate according to a third embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following illustrative embodiments are provided to illustrate the disclosure of the present disclosure, these and other advantages and effects can be apparent to those in the art after reading this specification. It should be noted that all the drawings are not intended to limit the present disclosure. 
     Various modifications and variations can be made without departing from the spirit of the present disclosure. Further, terms such as “first”, “second”, “on”, “a” etc. are merely for illustrative purposes and should not be construed to limit the scope of the present disclosure. 
       FIGS. 5A to 5F  are schematic cross-sectional views showing a method for fabricating a flexible substrate  5  according to a first embodiment of the present disclosure. 
     Referring to  FIG. 5A , a plurality of first conductive posts  51  and a block  59   a  are disposed on a carrier  50  through a patterning process. 
     In an embodiment, the carrier  50  is metal, a semiconductor or an insulating substrate. 
     The first conductive posts  51  and the block  59   a  are made of metal, such as copper. The first conductive posts  51  and the block  59   a  can be made of the same or different materials. 
     Referring to  FIG. 5B , a first insulating layer  53  is formed on the carrier  50  to encapsulate the first conductive posts  51  and the block  59   a.  The surfaces of the first conductive posts  51  and the block  59   a  are flush with the surface of the first insulating layer  53  so as to be exposed from the first insulating layer  53 . The block  59   a  penetrates the first insulating layer  53 . 
     In an embodiment, the first insulating layer  53  serves as a rigid portion and is formed on the carrier  50  by molding, coating or lamination. The first insulating layer  53  is made of a dielectric material, such as an epoxy resin containing a molding compound or a primer, such as an epoxy molding compound. The epoxy molding compound contains 70 to 90 wt % of a filler. 
     Referring to  FIG. 5C , a substrate body  55  is disposed on the first insulating layer  53  and electrically connected to the first conductive posts  51 . 
     In an embodiment, the substrate body  55  is coreless and formed through a circuit built-up process. The substrate body  55  has at least a dielectric layer  550  formed by coating, a circuit layer  551  bonded to the dielectric layer  550 , and a plurality of conductors  552  formed in the dielectric layer  550  and electrically connected to the circuit layer  551 . 
     In another embodiment, the substrate body  55  is formed by molding. In yet another embodiment, a circuit layer  551  is formed on the first insulating layer  53  and then a plurality of conductive posts  551  are disposed on the circuit layer  551 . Thereafter, the dielectric layer  550  is formed by molding to encapsulate the circuit layer  551  and the conductive posts. The conductive posts serve as the conductors  552 . 
     The dielectric layer  550  serves as a soft portion. The dielectric layer  550  and the first insulating layer  53  can be made of the same or different materials. In an embodiment, the dielectric layer  550  and the first insulating layer  53  are made of the same or different kinds of epoxy resin. 
     Referring to  FIG. 5D , a plurality of second conductive posts  52  and a block  59   b  are disposed on the substrate body  55  and a second insulating layer  54  is formed on the substrate body  55  to encapsulate the second conductive posts  52  and the block  59   b.  The surfaces of the second conductive posts  52  and the block  59   b  are flush with the surface of the second insulating layer  54  so as to be exposed from the second insulating layer  54 . The block  59   b  penetrates the second insulating layer  54 . 
     In an embodiment, the second conductive posts  52  and the block  59   b  are made of metal, such as copper. The second conductive posts  52  and the block  59   b  can be made of the same or different materials. 
     Further, the second insulating layer  54  serves as a rigid portion and is formed by molding, coating or lamination. The second insulating layer  54  is made of a dielectric material, such as an epoxy resin containing a molding compound or a primer, such as an epoxy molding compound. The epoxy molding compound contains 70 to 90 wt % of a filler. 
     Further, the second insulating layer  54  and the first insulating layer  53  can be made of the same or different materials, and the second insulating layer  54  and the dielectric layer  550  can be made of the same or different materials. 
     Referring to  FIGS. 5E and 5G , the carrier  50  is removed by stripping and the blocks  59   a,    59   b  are removed by etching. As such, a first through hole  530  is formed in the first insulating layer  53  and a second through hole  540  is formed in the second insulating layer  54 . The substrate body  55  is exposed from the first and second through holes  530 ,  540  to serve as a flexible section F, wherein the first through hole  530  and the flexible section F form a cavity, and the second through hole  540  and the flexible section F form another cavity. The first conductive posts  51  and the first insulating layer  53  can be regarded as an additional element  5   a,  and the second conductive posts  52  and the second insulating layer  54  can be regarded as another additional element  5   b.    
     In an embodiment, when the block  59   a  is removed, a portion of the substrate body  55  is also removed and hence the first through hole  530  extends into the substrate body  55 . Similarly, the second through hole  540  extends into the substrate body  55 . 
     Subsequently, a surface treatment layer  56  can be formed on the first and second conductive posts  51 ,  52  according to the practical need. 
     Therefore, the substrate body  55  is directly formed on the first insulating layer  53  (i.e., the dielectric layer  550  is coated on the first insulating layer  53 ) to replace the conventional lamination or attachment process. At the rigid-flexible interface of the flexible substrate  5 , two layers of epoxy resin are directly bonded so as to overcome the conventional drawbacks such as adhesive overflows, shorts and bulges and increase the interlayer alignment accuracy to +/−25 um. 
     Further, the coreless substrate body  55  greatly reduces the overall thickness of the flexible substrate  5 . Compared with a conventional four-layer board (generally having a thickness of a bout 0.25 mm), a four-layer board according to the present disclosure has a thickness D less than 0.2 mm. In an embodiment, the four-layer board has a thickness of 0.16 mm. 
     Furthermore, since the circuit layer  551  is formed by electroplating instead of metal etching, the edge of the circuit layer  551  is flat and straight, thereby overcoming the conventional foot problem caused by etching and facilitating impedance control. Further, the width/pitch of the circuit layer  551  can be reduced to be less than 20/20 um. 
     In addition, the flexible section F is fabricated by image transfer in combination with pattern electroplating copper (blocks  59   a,    59   b ) and etching of copper (blocks  59   a,    59   b ). Therefore, the shape, size and accuracy of the flexible section F are not limited by the conventional mechanical machining process, thereby improving the freedom of the structural design. In an embodiment, a plurality of flexible sections F having any shape can be fabricated at the same time. Further, the blocks  59   a,    59   b  and the first and second conductive posts  51 ,  52  can be fabricated together to reduce the fabrication cost. 
     Furthermore, the outermost conductive pads of the flexible substrate  5  are copper posts (i.e., the first and second conductive posts  51 ,  52 ) and the dielectric material (i.e., the first and second insulating layers  53 ,  54 ) is used to replace the conventional solder mask layers. As such, the present disclosure strengthens the bonding between the conductive pads and the dielectric material, increases the subsequent wire bonding strength, and improves the product reliability and packaging capability. 
       FIGS. 6A to 6C  are schematic cross-sectional views showing a method for fabricating a flexible substrate  6  according to a second embodiment of the present disclosure. The second embodiment differs from the first embodiment in the number of the through holes. 
     Referring to  FIG. 6A , a substrate body  55  is formed on a carrier  50 . The dielectric layers  550  serve as a soft portion. They are formed on the carrier  50  by coating. 
     Referring to  FIG. 6B , a plurality of conductive posts  62  and a block  69  are formed on the substrate body  55 , and an insulating layer  64  is formed on the substrate body  55  to encapsulate the conductive posts  62  and the block  69 . The surfaces of the conductive posts  62  and the block  69  are flush with the surface of the insulating layer  64  so as to be exposed from the insulating layer  64 . 
     In an embodiment, the conductive posts  62  and the block  69  are made of metal, such as copper. Further, the conductive posts  62  and the block  69  can be made of the same or different materials. 
     Furthermore, the insulating layer  64  serves as a rigid portion and is formed by molding, coating or lamination. The insulating layer  64  is made of a dielectric material, such as an epoxy resin containing a molding compound or a primer, such as an epoxy molding compound. The epoxy molding compound contains 70 to 90 wt % of a filler. 
     In an embodiment, the dielectric layers  550  and the insulating layer  64  can be made of the same or different materials. In another embodiment, the dielectric layers  550  and the insulating layer  64  are made of the same or different kinds of epoxy resin. 
     Referring to  FIG. 6C , the block  69  is removed to form a through hole  640  in the insulating layer  64 . As such, a portion of the substrate body  55  is exposed from the through hole  640  to serve as a flexible section F, wherein the through hole  640  and the flexible section F form a cavity. Thereafter, the carrier  50  is removed. The conductive posts  62  and the insulating layer  64  can be regarded as an additional element  60 . 
     In an embodiment, when the block  69  is removed by etching, portions of the conductive posts  62  are also removed so as to have end surfaces lower than the surface of the insulating layer  64 . Further, a portion of the substrate body  55  can be removed to increase the depth of the through hole  640 . 
     Further, when the carrier  50  is removed, an exposed portion of the electroplated copper layer (a portion of the circuit layer  551 ) of the flexible substrate body  55  can serve as an electromagnetic shielding layer  651 , thereby eliminating the need to attach a silver adhesive conductive film as an electromagnetic shielding layer. 
     Subsequently, a surface treatment layer (not shown) can be formed on the conductive posts  62  according to the practical need. 
     Therefore, the substrate body  55  is directly formed on a single side of the carrier  50  and the number of the layers of the soft portion (the dielectric layer  550 ) or the rigid portion (the insulating layer  64 ) is optional and not limited by the symmetric additional layers on the two opposite sides of the core layer as in the prior art. 
     Further, the flexible substrate  6  has a rigid-flexible interface at a single side of the substrate body  55 . At the rigid-flexible interface, two layers of epoxy resin (i.e., the dielectric layer  550  and the insulating layer  64 ) are directly bonded together so as to overcome the conventional drawbacks such as adhesive overflows, shorts and bulges and reduce flexural variations. 
     Furthermore, the coreless substrate body  55  greatly reduces the overall thickness of the flexible substrate  6 . A four-layer board according to the present disclosure has a thickness d less than 0.2 mm, significantly less than the conventional four-layer board. 
     In addition, the outermost conductive pads of the flexible substrate  6  are copper posts (i.e., the conductive posts  62 ) and the dielectric material (i.e., the insulating layer  64 ) is used to replace the conventional solder mask layer. As such, the present disclosure strengthens the bonding between the conductive pads and the dielectric material, increases the subsequent wire bonding strength, and improves the product reliability and packaging capability. 
     Furthermore, the flexible section F is fabricated by image transfer in combination with pattern electroplating copper (block  69 ) and etching of copper (block  69 ). Therefore, the shape, size and accuracy of the flexible section F are not limited by the conventional mechanical machining process, thereby improving the freedom of the structural design. For example, a plurality of flexible sections F having any shape can be fabricated at the same time. Further, the block  69  and the conductive posts  62  can be fabricated together to reduce the fabrication cost. 
       FIGS. 7A and 7B  are schematic cross-sectional views showing a method for fabricating a flexible substrate  7  according to a third embodiment of the present disclosure. The third embodiment differs from the second embodiment in the fabrication of the through holes. 
     Referring to  FIG. 7A , following  FIG. 6A , an insulating layer  77  is formed on the substrate body  55 . The insulating layer  77  has a plurality of openings  771  exposing the circuit layer  551  and at least an open area  770  exposing the dielectric layer  550 . In an embodiment, the carrier  50  is a metal sheet. 
     Referring to  FIG. 7B , a portion of the carrier  50  corresponding in position to the open area  770  is removed to form a through hole  700  penetrating the carrier  50  and exposing a portion of the substrate body  55 . The carrier  50  having the through hole  700  serves as an additional element  70  (a rigid portion), and the exposed portion of the substrate body  55  serves as a flexible section F, wherein the through hole  700  and the flexible section F form a cavity. 
     Therefore, a metal sheet is used as the carrier  50  so as for a built-up process to be performed thereon. Further, a portion of the carrier  50  is retained and not removed to serve as an additional element  70  for supporting the rigid area and facilitating heat dissipation. 
     Further, the flexible substrate  7  has a rigid-flexible interface at a single side of the substrate body  55 . At the rigid-flexible interface, the dielectric layer  550  and the carrier  50  made of metal are directly bonded together, and a portion of the carrier  50  is removed to form the through hole  700  penetrating the carrier  50 . As such, the present disclosure overcomes the conventional drawbacks such as adhesive overflows, shorts and bulges at the rigid-flexible interface. 
     Furthermore, the flexible section F is fabricated by metal etching (removing a portion of the carrier  50 ). Therefore, the shape, size and accuracy of the flexible section F are not limited by the conventional mechanical machining process, thereby improving the freedom of the structural design. For example, a plurality of flexible sections F having any shape can be fabricated at the same time. 
     The coreless substrate body  55  greatly reduces the overall thickness of the flexible substrate  7 . A four-layer board according to the present disclosure has a thickness R less than 0.2 mm, significantly less than the conventional four-layer board. 
     The present disclosure further provides a flexible substrate  5 ,  6 ,  7 , which has a coreless substrate body  55  having a flexible section F and an additional element  5   a,    5   b,    60 ,  70  formed on the substrate body  55  and having a through hole  640 ,  700  (first and second through holes  530 ,  540 ) exposing the flexible section F. 
     In an embodiment, the substrate body  55  has at least a dielectric layer  550  and a circuit structure (for example, a circuit layer  551  and/or conductors  552 ) bonded to the dielectric layer  550 . 
     In an embodiment, the additional element  5   a,    5   b,    60  has an insulating layer  64  (first and second insulating layers  53 ,  54 ) and a plurality of conductive posts  62  (first and second conductive posts  51 ,  52 ) embedded in the insulating layer  64 . For example, the insulating layer  64  (the first and second insulating layers  53 ,  54 ) is made of a molding compound or a primer. 
     In an embodiment, the additional element  70  is a metal sheet. 
     According to the present disclosure, the coreless substrate body facilitates to reduce the overall thickness of the flexible substrate so as to meet the thinning requirement. 
     Further, at the rigid-flexible interface, the dielectric material is directly bonded to the additional element so as to overcome the conventional drawbacks such as adhesive overflows, shorts and bulges and improve the interlayer alignment accuracy. 
     Furthermore, since the circuit layer is formed through a semi-additive process without metal etching, the present disclosure facilitates impedance control and reduces the line width/pitch to meet the fine pitch/circuit requirement. 
     In addition, since the flexible section is defined by metal etching, the shape, size and accuracy of the flexible section F are not limited by the conventional mechanical machining process, thereby improving the freedom of the structural design. 
     The above-described descriptions of the detailed embodiments are only to illustrate the implementation according to the present disclosure, and it is not to limit the scope of the present disclosure. Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present disclosure defined by the appended claims.