Abstract:
A method for manufacturing a multilayer FPCB which includes providing a first substrate, a second substrate and a binder layer; defining an opening on the binder layer; defining a first slit in the dielectric layer of the first substrate; laminating the first substrate, the binder layer and the second substrate; forming a second slit in the conductive layer of the first substrate, the second slit being created so as to align with the first slit, cutting the first substrate, the binder layer and the second substrate thereby forming a multilayer flexible printed circuit board having different numbers of layers in different areas.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is a divisional application of U.S. patent application Ser. No. 11/957,324 filed on Dec. 14, 2007, entitled “METHOD FOR MANUFACTURING MULTILAYER FLEXIBLE PRINTED CIRCUIT BOARD,” now U.S. Pat. No. 8,042,265, assigned to the same assignee, and disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a method for manufacturing a flexible printed circuit board, and particularly to a method for manufacturing a multilayer flexible printed circuit board having different thicknesses in different areas. 
     2. Discussion of Related Art 
     Flexible printed circuit boards (FPCB) have been widely used in electronic products such as mobile phones, printing heads and hard disks. In these electronic products, some parts may move relative to a main body. In such a situation, FPCBs are applied to provide electrical connections and transmit signals between such parts and the main body due to their flexibility. 
       FIG. 34  shows a multilayer FPCB structure, which has different numbers of layers in different areas. In other words, there are thick areas and thin areas within the same FPCB. The thick area can have a higher circuit density, whilst the thin area exhibits higher flexibility. 
       FIGS. 29-34  show a process for manufacturing such a type of FPCB. As shown in  FIGS. 29 and 30 , a first copper clad laminate (CCL)  41 , a binder layer  45  and a second CCL  42  are laminated. As is shown in  FIG. 31 , dry films  412 ,  422  are respectively applied on the first CCL  41  and the second CCL  42 , and then, the dry films  412 ,  422  are exposed and developed. Because there is a cliff-like thickness difference between the first CCL  41  and the second CCL  42 , a gap  46  is formed in the included angle at the base of the ‘cliff’. 
     As shown in  FIG. 32 , during an etching process, when the first CCL  41  and the second CCL  42  are immersed in an etching solution, the solution can seep into the gap  46  and react with the dielectric layers of the first CCL  41  and/or the second CCL  42 . As a result, the dielectric layers may become unstable and peel from the first CCL  41  and/or the second CCL  42 . 
     Referring to  FIG. 33 , a third CCL  43  and a fourth CCL  44  are respectively laminated with the first CCL  41  and the second CCL  42 , to make another multilayer FPCB. Referring to  FIG. 34 , in order to electrically connect the copper layers of the third CCL  43 , the first CCL  41 , the second CCL  42 , and the fourth CCL  44 , a via hole  47  is defined so as to penetrate all the four CCLs. The via hole  47  can be made by drilling or by laser ablation. After the via hole  47  is formed, a conductive layer, e.g., a copper layer, is formed on a sidewall of the via hole  47  by electroless plating or electroplating. In the plating process, the dielectric layer of the second CCL  42  is exposed in a plating solution, thereby forming a number of copper lumps  48  thereon. These copper lumps  48  can pierce dry film that is applied onto the second CCL  42  in the next pattern-forming process, and the etching solution used for developing the dry film can react with dielectric layer or copper layer of second CCL  42  and result in a poor quality product. 
     In the aforementioned process for manufacturing multilayer FPCB that has different number of layers in different areas, a step structure between different CCLs can causes a series of quality problems. Therefore, a new process for manufacturing multilayer FPCB is desired to overcome the aforementioned quality problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present method. 
         FIGS. 1 to 10  are schematic views, showing a process for manufacturing a multilayer FPCB having a different number of layers in different areas, in accordance with a first embodiment. 
         FIGS. 11 to 28  are schematic views, showing a process for manufacturing a multilayer FPCB having a different number of layers in different areas, in accordance with the second embodiment. 
         FIGS. 29 to 34  are schematic views, showing a process for manufacturing a multilayer FPCB having a different number of layers in different areas, in accordance with the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-10  show the successive stages of a process for manufacturing an FPCB that has a different number of layers in different areas, in accordance with a first embodiment. 
     Referring to  FIG. 1 , a first substrate  11  includes a dielectric layer  111  and a conductive layer  112  formed on the dielectric layer  111 . The second substrate  12  includes a dielectric layer  121  and a conductive layer  122  formed on the dielectric layer  121 . A binder layer  13  is sandwiched between the first substrate  11  and the second substrate  12 . A locating hole  104  is defined so as to penetrate through the first substrate  11 , the binder layer  13  and the second substrate  12 . 
     Referring to  FIG. 2 , the first substrate  11  includes a main portion  113  surrounded by an imaginary boundary line  116  (i.e. a functional portion prepared for making a circuit thereon according to need) and an excess portion  114  surrounded by an imaginary boundary line  117  (i.e. a sacrificing portion which will be removed in a later step). An imaginary boundary line  115  is defined between the main portion  113  and the excess portion  114 . The main portion  113  is encompassed by an imaginary boundary line  116  and the imaginary boundary line  115 . The excess portion  114  is encompassed by another imaginary boundary line  117  and the imaginary boundary line  115 . 
     Materials of the dielectric layers  111 ,  121  and the binder layer  13  can be selected from the group consisting of polyimide, polytetrafluoroethylene, polythiamine, polymethacrylic acid, polycarbonate, polycarbonate ester, polyester, copolymer of imide, ethylene and dimethyl terephthalate. The conductive layers  112 ,  122  can be a film made of copper, silver or aluminum. 
     Referring to  FIGS. 3 and 4 , the binder layer  13  includes two opposite surfaces  131 ,  132 . An opening  133  is formed in the binder layer  13  thereby an inner side wall surface  134  of the binder layer  13  is defined. The opening  133  can be formed by cutting, stamping, laser ablation or etching. In this embodiment, the opening  133  has a rectangular shape, but the opening  133  can also be of other shapes, for example, trapezium, triangle etc. 
     Referring to  FIG. 5 , a first slit  110  is formed in the dielectric layer  111  of the first substrate  11  along the boundary line  115 . The first slit  110  can be formed by laser ablation or etching. E-beam etching or plasma etching can also be used to form the first slit  110 . 
     Referring to  FIG. 6 , the first substrate  11  and the second substrate  12  are respectively laminated on the two opposite surfaces  131 ,  132  of the binder layer  13 , thereby a semi-finished FPCB  14  is obtained. The excess portion  114  is exposed to and suspended above the opening  133 . The boundary line  115  is aligned with the inner side wall surface  134  of the binder layer  13 . 
     Referring to  FIG. 7 , conductive patterns are formed in the conductive layers  112 ,  122 . In this embodiment, the conductive patterns are formed using a DES (Developing, Etching and Stripping) process. Alternatively, the conductive patterns can also be formed using laser. A second slit  120  is formed in the conductive layer  112  along the boundary line  115 . The second slit  120  can be formed with the conductive patterns in the conductive layer  112  simultaneously, that is, the second slit  120  is a portion of the conductive patterns in the conductive layer  112 . Alternatively, the second silt  120  can also be formed after the making of the conductive patterns in the conductive layer  112 . For example, the second slit  120  can be formed using laser ablation after the conductive patterns is formed. 
     Referring to  FIGS. 8 and 9 , the semi-finished FPCB  14  is cut along the boundary  116  and  117 . The excess portion  114  is not conglutinated with the binder layer  13  and is therefore very easy to remove. In this embodiment, the semi-finished FPCB  14  is cut using a stamper, and the excess portion  114  can be removed together with the stamper. 
     Referring to  FIG. 10 , a FPCB  140  with a different number of layers in different areas is obtained. 
       FIGS. 11-17  show the successive stages of a process for manufacturing an FPCB that has a different number of layers in different areas, in accordance with a second embodiment. 
     Referring to  FIG. 11 , a first substrate  21  includes a dielectric layer  211 , a conductive layer  212  and an outer conductive layer  213 . The conductive layer  212  and the outer conductive layer  213  are respectively formed on two opposite surfaces of the dielectric layer  211 . The conductive layer  212  has conductive patterns formed therein, i.e., the conductive layer  212  is made into a conductive pattern. The first substrate  21  includes a main portion  201  (i.e. a remaining portion which is designed in a particular fashion) and an excess portion  202  (i.e. a sacrificing portion which will be removed in a later step). A boundary  203  is sandwiched between the main portion  201  and the excess portion  202 . The main portion  201  has a boundary  204 . The excess portion has a boundary  205 . 
     The second substrate  22  includes two dielectric layers  221  and  223 , two conductive layers  222  and  224 , and a binder layer  225 . The conductive layer  222  is formed on the dielectric layer  221 . The conductive layer  224  is formed on the dielectric layer  223 . The binder layer  225  is in contact with the conductive layer  222  and the dielectric layer  223 . 
     Referring to  FIG. 12 , the binder layer  23  includes two opposite surfaces  231  and  232 . An opening  233  is formed in the binder layer  23  in such a way that an inner sidewall surface  234  of the binder layer  23  is formed. The opening  233  can be formed by cutting, stamping, laser ablation or etching. In this preferred embodiment, the opening  233  has a rectangular shape, but the opening  233  can also be of other shapes, for example, trapezium, triangle etc. 
     Referring to  FIG. 13 , a first slit  210  is formed in the dielectric layer  211  and the conductive layer  212  along the boundary  203 , that is, the first slit  210  is formed in all the layers in the first substrate  21  except the outer conductive layer  213 . 
     Referring to  FIG. 14 , the first substrate  21  and the second substrate  22  are respectively laminated on two opposite surfaces  231  and  232  of the binder layer  23 . The conductive layer  212  is in contact with the surface  231 . The dielectric layer  221  is in contact with the surface  232 . The boundary  203  is aligned with the inner sidewall surface  234 . The excess portion  202  of the first substrate  21  is exposed to and suspended above the opening  233 . 
     Referring to  FIG. 15 , conductive patterns are formed in the outer conductive layer  213  and the conductive layer  224 , thereby a semi-finished FPCB  24  is obtained. A second slit  220  is also formed in the outer conductive layer  213  along the boundary  203 . In this embodiment, the conductive patterns and the second slit are formed at a same time using a DES process. 
     Referring to  FIGS. 16 and 17 , the semi-finished FPCB  24  is cut along the boundary  204  and  205  so as to remove the excess portion  202 , thereby a FPCB  240  with a different number of layers in different areas is obtained. 
       FIGS. 18-26  show the successive stages of a process for manufacturing an FPCB that has different number of layers in different areas, in accordance with a third preferred embodiment. 
     Referring to  FIG. 18 , the first substrate  31  includes a dielectric layer  311  and a conductive layer  312  formed on the dielectric layer  311 . Referring to  FIG. 19 , the first substrate  31  includes a main portion  301  and an excess portion  302 . The main portion  31  has a boundary  304 . The excess portion  302  has a boundary  305 . A boundary  303  is provided between the main portion  301  and the excess portion  302 . A first slit  310  is formed in the first substrate  31  along the boundary  303 . 
     Referring to  FIGS. 20 and 21 , an inner binder layer  35  has two opposite surfaces  351 ,  352 . An opening  353  is formed in the inner binder layer  35 , thereby an inner sidewall surface  354  is formed in the inner binder layer  35 . 
     Referring to  FIG. 22 , a first substrate  31  and a second substrate  32  are respectively laminated on two opposite surfaces  351 ,  352 . The second substrate  32  includes a dielectric layer  321  and a conductive layer  322  formed on the dielectric layer  321 . The dielectric layer  311  contacts the surface  351 . The dielectric layer  321  contacts the surface  352 . The boundary  303  is aligned with the inner sidewall surface  354 . 
     Referring to  FIG. 23 , conductive patterns are formed in the conductive layer  312  and  322  thereby an inner laminated structure  330  is obtained. A second slit  320  is formed in the conductive layer  312  along the boundary  303 . In the present embodiment, the conductive patterns are formed using a DES process. The second slit  320  is formed at a same time with conductive patterns. 
     Referring to  FIG. 24 , a first outer binder layer  36  includes a third slit  362  formed therein, in such a way that an inner sidewall surface  363  is formed in the first outer binder layer  36 . Referring to  FIG. 25 , the first outer binder layer  36  is applied on the conductive layer  312 , an second outer binder layer  37  is applied on the conductive layer  322 . Another first substrate  31  is applied on the first outer binder layer  36  and another second substrate  32  is applied on the second outer binder layer  37 . Then, the first substrate  31 , the first outer binder layer  36 , the inner laminated structure  330 , the second outer binder layer  37  and the second substrate  32  are laminated using a laminating machine. The dielectric layer  311  is in contact with the first outer binder layer  36 . The dielectric layer  321  is in contact with the second outer binder layer  37 . 
     Referring to  FIG. 26 , conductive patterns are formed in the conductive layer  312  and  322  thus a semi-finished FPCB  350  is obtained. A second slit  320  is formed in the conductive layer  312 . In this embodiment, the second slit  320  is formed together with the conductive patterns. The two first slits  310 , the two second slits  320  and the third slit  362  are configured to be aligned and in communication with the opening  353 . 
     Referring to  FIG. 27 , the semi-finished FPCB  350  is cut along the boundary of the main portion  301  and the excess portion  302  of the first substrate  31 , so as to remove the excess portion  302  of the first substrate  31 . Referring to  FIG. 28 , all the excess portion  302  of the first substrate  31  is removed, thereby a FPCB  360  with different number of layers in different areas is obtained. 
     In this embodiment, FPCBs are manufactured with the first substrate  31  and the second substrate  32 . The inner binder layer  35  separates the FPCB  360  into a first side and a second side. Two first substrates  31  and corresponding binder outer layer  36  constitute the first side. Two second substrate  32  and corresponding second outer binder layer  37  constitute the second side. Because the first substrate  31  has a first slit  310  preformed in the dielectric layer, thus when a second slit  320  aligned with the first slit  310  is formed in the conductive layer  312  of the first substrate  31 , the first substrate  31  is cut off at the first slit  310 . Furthermore, a third slit  362  aligned with first slit  310  is preformed in the first outer binder layer  36 . As a result, after the FPCB  360  is cut along the boundaries of the main portion  301  and the excess portion  302  of the first substrate  31 , the excess portion  302  exposed to the opening  353  of the inner binder layer  35  can be easily removed. Thus, a FPCB  360  with a different number of layers in different areas is obtained. In the present embodiment, the FPCB  360  is a four-layer structure. However, more first substrates  31  can be built up on the first side, until the predetermined number of layers is obtained. 
     In all of these preferred embodiments of manufacturing a FPCB has a different number of layers in different areas, there is no cliff-like structure created in the process, therefore all the aforementioned disadvantages are overcome. 
     Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than to limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.