Abstract:
A printed circuit board comprises a circuit substrate, an electrically conductive cloth structure, and a shielding structure. The circuit substrate comprises a base layer, a grounded circuit layer, and a connecting pad formed on the grounded circuit layer. The cloth structure comprises an anisotropic conductive adhesive connected to the connecting pad, an insulating layer, and a metallic deposition layer arranged between the anisotropic conductive adhesive and the insulating layer. The shielding structure comprises a shielding metal layer, an adhesive matrix, and a number of electrically conductive particles electrically connected to the shielding metal layer. The insulating layer defines a number of through holes corresponding to the particles, the particles is arranged in the through holes respectively and electrically connected the metallic deposition layer and the shielding metal layer. A method for manufacturing the above PCB is also provided.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to printed circuit boards (PCBs) and particularly, relates to a printed circuit board having shielding structure and a method for manufacturing the same. 
     2. Description of Related Art 
     PCBs are widely used in most electronic devices for mounting electronic components and providing electrical transmission. In some conditions, a shielding structure is needed on a surface of a PCB to provide protection against electromagnetic interference (EMI) generated by peripheral electronic components or devices. 
     The PCB usually comprises a grounded circuit layer and a protective layer formed on a surface of the grounded circuit layer. The grounded circuit layer comprises a connecting pad. The protective layer defines an opening and the connecting pad is exposed in the opening. One kind of well-known shielding structure includes an anisotropic conductive adhesive filled in the opening, a metallic deposition layer formed on the adhesive, an insulating layer, and a release film. The static electricity existing in the PCB can be guided to the metallic deposition layer and eliminated. The metallic deposition layer can also shield against EMI. Another kind of well-known shielding structure includes an electrically conductive adhesive filled in the opening and solder covering the adhesive. The adhesive can be silver glue. The static electricity existing in the PCB can be guided to the adhesive and eliminated. 
     However, demand for smaller PCB is growing, and so connecting pads are becoming smaller and smaller, and shielding structures are also becoming smaller and smaller. Thus, the amount of anisotropic conductive adhesive or electrically conductive adhesive that can be used has become so little reducing effectiveness. 
     What is needed, therefore, is a PCB and a method for manufacturing the same to overcome the above-described problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present embodiment 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 embodiment. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a flow chart showing a method for manufacturing a PCB having a shielding structure in accordance with a first embodiment. 
         FIG. 2  is a perspective view of a circuit substrate in accordance with a first embodiment. 
         FIG. 3  is a perspective view of an electrically conductive cloth structure in accordance with a first embodiment. 
         FIG. 4  is similar to  FIG. 2 , but showing the electrically conductive cloth structure from  FIG. 3  laminated on a surface of the circuit substrate in accordance with a first embodiment. 
         FIG. 5  is a perspective view of a shielding structure in accordance with a first embodiment. 
         FIG. 6  is similar to  FIG. 4 , but showing the shielding structure from  FIG. 5  laminated on the electrically conductive cloth structure in accordance with a first embodiment. 
         FIG. 7  is a flow chart showing a method for manufacturing a PCB having a shielding structure according to a second embodiment. 
         FIG. 8  is a perspective view of a circuit substrate in accordance with the second embodiment. 
         FIG. 9  is similar to  FIG. 8 , but showing electrically conductive adhesive applied on a surface of the circuit substrate in accordance with the second embodiment. 
         FIG. 10  is similar to  FIG. 9 , but showing a solder mask formed on a surface of the electrically conductive adhesive in accordance with the second embodiment. 
         FIG. 11  is a perspective view of a shielding structure in accordance with the second embodiment. 
         FIG. 12  is similar to  FIG. 10 , but showing the shielding structure from  FIG. 11  laminated on the solder mask in accordance with the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will now be described in detail below and with reference to the drawings. 
     Referring to  FIG. 1 , a method for manufacturing a PCB having a shielding structure includes the following steps. 
     Referring to  FIGS. 1 and 2 , in step  101 , a circuit substrate  12  is provided. 
     The circuit substrate  12  comprises a base layer  121 , a grounded circuit layer  122 , a connecting pad  123 , a protective layer  124 , and an adhesive layer  125 . The grounded circuit layer  122  is formed on a surface of the base layer  121 . The connecting pad  123  is electrically connected to the grounded circuit layer  122 . In other words, the grounded circuit layer  122  is arranged between the base layer  121  and the connecting pad  123 . A cross-section area of the connecting pad  123  is much smaller than that of the grounded circuit layer  122 . 
     The adhesive layer  125  is used to attach the protective layer  124  to a surface of the base layer  121 . The adhesive layer  125  covers part of the grounded circuit layer  122  and the base layer  121  exposed from the grounded circuit layer  122 . The protective layer  124  is formed on a surface of the adhesive layer  125  and covers the adhesive layer  125 . The protective layer  124  and the adhesive layer  125  define an opening  130  to expose the connecting pad  123 . 
     Referring to  FIGS. 1 and 3 , in step  102 , an electrically conductive cloth structure  14  is provided. 
     The cloth structure  14  comprises an anisotropic conductive adhesive  142 , a metallic deposition layer  144 , an insulating layer  146 , and a release film  148 . The metallic deposition layer  144  is arranged between the adhesive  142  and the insulating layer  146 . The release film  148  is positioned on a surface of the insulating layer  146 , and is used to protect the metallic deposition layer  144  and the insulating layer  146  in a laminating process. In one embodiment, a thickness of the cloth structure  14  is from 0.5 micrometers (μm) to 100 μm. 
     Referring to  FIGS. 1 and 4 , in step  103 , the cloth structure  14  is laminated onto the circuit substrate  12 . 
     The cloth structure  14  is laminated onto the circuit substrate  12  by a laminating device (not shown in  FIG. 4 ). The adhesive  142  is pressed and filled in the opening  130 , and the adhesive  142  contacts a surface of the connecting pad  123 . After laminating the cloth structure  14 , the release film  148  is removed. 
     Referring to  FIGS. 1 and 5 , in step  104 , a shielding structure  16  is provided. 
     The shielding structure  16  comprises a shielding metal layer  162  and an electrically connecting layer  164  formed on a surface of the metal layer  162 . The connecting layer  164  comprises an adhesive matrix  1642  and a number of electrically conductive particles  1644  distributed in the matrix  1642 . A diameter of each of the particles  1644  is larger than a thickness of the insulating layer  146  of the cloth structure  14 . The material of the shielding metal layer  162  can be selected from the group consisting of Au, Ag, Cu, Al, Ni, and any combination thereof. The particles  1644  can be selected from the group consisting of Au particles, Ni particles, Ni particles coated with Au, plastic particles coated with Au, plastic particles coated with Ni, and any combination thereof. 
     In one embodiment, the shielding metal layer  162  includes a copper layer  1622  and a gold layer  1624 . The copper layer  1622  is arranged between the adhesive matrix  1642  and the gold layer  1624 . The thickness of the shielding structure  16  is from 0.5 μm to 50 μm. The diameter of each of the particles  1644  is from 0.1 μm to 40 μm. 
     Referring to  FIGS. 1 and 6 , the shielding structure  16  is laminated on a surface of the cloth structure  14 . 
     A thermal laminating process is applied to the shielding structure  16  and the cloth structure  14  by a laminating device (not shown in  FIG. 6 ). The adhesive matrix  1642  of the shielding structure  16  is adhered to the insulating layer  146 . The particles  1644  are laminated to be dispersed throughout the insulating layer  164  to electrically connect the metallic deposition layer  144  to the shielding metal layer  162 . In the laminating process, the laminating time is from about 100 seconds to 200 seconds to ensure that the particles  1644  are uniformly dispersed. After the laminating process, the particles  1644  are in electrical contact with the metallic deposition layer  144  and the shielding metal layer  162 . Moreover, some particles  1644  penetrate the metal deposition layer  144 . Therefore, the shielding metal layer  162  can electrically connect the metal deposition layer  144  to the adhesive  142  to lead residual static electricity existing in the adhesive  142  out. 
     As shown in  FIG. 6 , a PCB  10  is formed by the above method in the first embodiment. The PCB  10  comprises the circuit substrate  12 , the electrically conductive cloth structure  14 , and the shielding structure  16 . 
     Referring to  FIG. 7 , a method for manufacturing a PCB having a shielding structure in another embodiment comprises the steps below. 
     Referring to  FIGS. 7 and 8 , in step  201 , a circuit substrate  22  is provided. 
     The circuit substrate  22  comprises a base layer  221 , a grounded circuit layer  222 , a connecting pad  223 , a protective layer  224 , and an adhesive layer  225 . The grounded circuit layer  222  is formed on a surface of the base layer  221 . The connecting pad  223  is electrically connected to the grounded circuit layer  222 . In other words, the grounded circuit layer  222  is arranged between the base layer  221  and the connecting pad  223 . A cross-section area of the connecting pad  223  is much smaller than that of the grounded circuit layer  222 . 
     The adhesive layer  225  is used to attach the protective layer  224  to a surface of the base layer  221 . The adhesive layer  225  covers part of the grounded circuit layer  222  and the base layer  221  exposed from the grounded circuit layer  222 . The protective layer  224  is formed on a surface of the adhesive layer  225  and covers the adhesive layer  225 . The protective layer  224  and the adhesive layer  225  define an opening  230  to expose the connecting pad  223 . 
     Referring to  FIGS. 7 and 9 , in step  202 , a conductive adhesive layer  24  is formed onto the circuit substrate  22 . 
     The conductive adhesive layer  24  can be formed on the circuit substrate  22  by printing or coating. The conductive adhesive layer  24  is filled in the opening  230  and electrically connected to the connecting pad  223 . In one embodiment, the thickness of the conductive adhesive layer  24  is from 0.5 μm to 100 μm. The conductive adhesive layer  24  is formed with silver glue or copper glue. The silver glue or copper glue is formed on the circuit substrate  22  by screen printing. 
     Referring to  FIGS. 7 and 10 , a solder mask layer  26  is formed on a surface of the conductive adhesive layer  24 . 
     The solder mask layer  26  can be formed on the surface of the conductive adhesive layer  24  by printing or coating. In one embodiment, a thickness of the solder mask layer  26  is from 0.5 μm to 100 μm. 
     Referring to  FIGS. 7 and 11 , a shielding structure  28  is provided. 
     The shielding structure  28  comprises a shielding metal layer  282  and an electrical connecting layer  284  formed on a surface of the shielding metal layer  282 . The connecting layer  284  comprises an adhesive matrix  2842  and a number of electrically conductive particles  2844  distributed in the matrix  2842 . A diameter of each of the particles  2844  is larger than a thickness of the solder mask layer  26 . The material of the shielding metal layer  282  can be selected from the group consisting of Au, Ag, Cu, Al, Ni, and any combination thereof. The particles  2844  can be selected from the group consisting of Au particles, Ni particles, Ni particles coated with Au, plastic particles coated with Au, plastic particles coated with Ni, and any combination thereof. 
     In one embodiment, the shielding metal layer  282  includes a copper layer  2822  and a gold layer  2824 . The copper layer  2822  is arranged between the adhesive matrix  2842  and the gold layer  2824 . The thickness of the shielding structure  28  is from 0.5 μm to 50 μm. The diameter of each of the particles  2844  is from 0.1 μm to 40 μm. 
     Referring to  FIGS. 7 and 12 , in step  205 , the shielding structure  28  is laminated onto the solder mask layer  26 . 
     A thermal laminating process is applied to the solder mask layer  26  and the shielding structure  28  by a laminating device (not shown in  FIG. 12 ). The adhesive matrix  2842  of the shielding structure  28  is adhered to the solder mask layer  26 . The particles  2844  are laminated to be dispersed throughout the solder mask layer  26  to electrically connect the conductive adhesive layer  24  to the shielding metal layer  282 . In the laminating process, the laminating time is from about 100 seconds to 200 seconds to ensure that the particles  2844  are uniformly distributed throughout solder mask layer  26 . After the laminating process, the particles  2844  are in electrical contact with the conductive adhesive layer  24  and the shielding metal layer  282 . Therefore, the shielding metal layer  282  can electrically connect to the conductive adhesive layer  24  to lead residual static electricity existing in the conductive adhesive layer  24  out. 
     As shown in  FIG. 12 , a PCB  20  is formed by the above method in another embodiment. The PCB  20  comprises the circuit substrate  22 , the conductive adhesive layer  24 , the solder mask layer  26 , and the shielding structure  28 . 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the disclosure.