Abstract:
Printed circuit boards and methods for fabricating the same. A via in a printed circuit board electrically connects to trace lines of the PCB, such that only one plating line is required to electrically connect a plating bus and the plating through hole. Thus, in an electroplating step, current can flow to fingers in the trace lines to plate an anti-oxidation metal layer thereon. The via is separated into several sub-vias to electrically isolate the plating line from trace lines and fingers, each of which connects to the plating line or the trace lines. Finally, at least one plating line remains, thus avoiding negative impact on electrical performance of an electronic device that uses the printed circuit board.

Description:
BACKGROUND  
       [0001]     The invention relates to a fabrication method for a printed circuit board (PCB), and more particularly to a method of plating a metal layer of the PCB.  
         [0002]     PCBs, such as substrates for ball grid array (BGA) packages, generally have exposed pads or fingers for connection to an external device.  
         [0003]     In  FIG. 1 , a top view of an exemplary portion of a PCB for a BGA substrate is shown. Fingers  140  are disposed in a chip attachment area  120 . Trace lines  150  on a top surface respectively extend from fingers  140  to vias  160  and electrically connect to pads  130  on a bottom surface using trace lines on the bottom surface. A bus line  180 , disposed at an edge of the PCB, electrically connects to trace lines  150 , fingers  140 , and pads  130  using branch plating lines  181 . The pads  130 , fingers  140 , trace lines  150 , bus line  180 , and branch plating lines  181  are typically copper. The pads  130  and fingers  140  are typically exposed for electrical connection to external devices (not shown). The pads  130  and fingers  140  are usually plated with a Ni/Au layer (not shown) by electrical plating such that current flows to every pad  130  and finger  140  using bus line  180  and branch plating lines  181  to protect the exposed pads  130  and fingers  140  from oxidation.  
         [0004]     The connections between respective trace lines  150  and bus lines  180  are cut step to separate an encapsulated package from the PCB. The branch lines  181 , however, remain in the package.  
         [0005]     Due to the demand for small-aspect, light and powerful electronic products, PCB design rules demand layouts with increased density, resulting in increased overall density and reduced pitch in the remaining branch lines  181 , and increased distribution density of the vias  160 . The vias  160  suffer from the increased wiring density as follows:  
         [0006]     Via size decreases with increased wiring density, resulting in difficulty drilling, electroplating through holes to form the vias, with increased aspect ratio of the through holes negatively affecting the product reliability, and decreased durability of the vias.  
         [0007]     Moreover, crosstalk resulting from mutual inductance and capacitors between the branch lines  181  may not only negatively affect transmission of electrical signals and system stability, but also deviate character impedances of trace lines  150 , thereby further negatively affecting the electrical performance of an end product using the PCB.  
       SUMMARY  
       [0008]     Thus, embodiments of the invention provide PCBs and methods for fabricating the same, capable of reducing density of remaining plating lines to improve electrical performance of end products using the PCB, maintaining via size when increasing wiring density of the PCB to simplify drilling and electroplating for via formation and improve via reliability, and electrically isolating the plating line from the trace line and pads when completing electroplating to prevent the plating line negatively affecting electrical performance of other parts of the wiring.  
         [0009]     Embodiments of the invention provide a fabrication method for PCBs. First, a substrate, comprising a layout area and a periphery area on a surface, is provided. A patterned wiring layer, comprising a bus line in the periphery area, a via in the layout area, at least one pad in the layout area, a plating line electrically connecting the bus line and the via, and a trace line electrically connecting the via and the pad, is then formed overlying the substrate. A metal layer is further formed overlying the pad. Finally, the via is separated into a plurality of sub-vias electrically isolated from each other. The sub-vias connect to at least the plating line or the trace line.  
         [0010]     Embodiments of the invention further provide a printed circuit board (PCB). The PCB comprises a substrate and a patterned wiring layer. The substrate comprises a layout area and a periphery on a surface. The patterned wiring layer overlies the substrate. The wiring layer further comprises a bus line, at least one pad, a separated via, a plating line and at least one trace line. The bus line is disposed in the periphery area. The at least one pad comprises a metal layer thereon and is disposed in the layout area. The separated via comprises a plurality of sub-vias electrically isolated from each other and is disposed in the layout area. The plating line electrically connects the bus line and the via. The at least one trace line electrically connects the sub-vias and the at least one pad. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Embodiments of the invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
         [0012]      FIG. 1  is a top view of a conventional PCB.  
         [0013]      FIGS. 2A, 2B , and  2 D through  2 I are top views of fabrication methods of PCBs of exemplary embodiments of the invention.  
         [0014]      FIG. 2C  is a cross-section of a via of PCBs of exemplary embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     The following embodiments are intended to illustrate the invention more fully without limiting the scope of the claims, since numerous modifications and variations will be apparent to those skilled in this art.  
         [0016]      FIGS. 2A through 2I  show exemplary embodiments of PCBs of the invention and methods for fabricating the same.  
         [0017]     As shown in  FIG. 2A , a substrate  200 , such as a core substrate comprising fiber-reinforced or particle-reinforced materials such as epoxy resin, bismaleimide triazine-based (BT), Cyanate ester, or other materials, is provided. The substrate  200  may also be a core substrate plated with patterned wiring with an overlying dielectric layer. One surface of substrate  200 , such as that for connection to an external device, has a periphery  201  and layout area  202 .  
         [0018]      FIGS. 2B through 2I  illustrate an exemplary portion of the PCB of the invention. In practice, the quantity, shape, and size of the elements shown in the figures may be modified.  
         [0019]     In  FIG. 2B , a wiring layer  210  is formed overlying the surface of substrate  200 . A metal layer formed overlying substrate  200 , followed by patterning of the metal layer, forms wiring layer  210 . The metal layer is copper, tin, nickel, chrome, titanium, copper-chrome alloy, or tin-lead alloy. Alternatively, the wiring layer can be formed by physical vapor deposition such as sputtering or metal-organic chemical vapor deposition (MOCVD) directly in a predetermined pattern. In wiring layer  210 , a bus line  211  is disposed in the periphery  201  (shown in  FIG. 2A ) of substrate  200 , a via  212  is disposed in the layout area  203 , a conductive finger  215  is disposed in the layout area  203 , a plating line  213  electrically connects the bus line  211  and the via  212 , and a trace line  214  electrically connects the via  212  and the conductive finger  215 .  
         [0020]     An exemplary via  212  is shown in  FIG. 2C . Formation of via  212  includes a through hole  220  formed in the substrate  200  by a method such as laser drilling or mechanical drilling, followed by conformal formation of a copper seed layer  222  overlying the substrate  200  and the through hole  220  utilizing electroless plating. A mask layer such as a resist layer or dry film, comprising an opening exposing the seed layer  222  on sidewalls of the through hole  222 , is formed overlying the substrate  200  by a method such as stencil printing, spin coating, or laminating. Next, a copper layer  228  is formed overlying the seed layer  222  by electroplating, providing electrical connection of the subsequently formed wiring layer and the underlying circuitry, followed by removal of the mask layer. Finally, the via  212  is filled with a dielectric plug  230 . Subsequently, the metal layer  228  overlying the substrate  200  is patterned to form a wiring layer.  
         [0021]     In  FIG. 2B , an anti-oxidation layer is formed overlying the conductive finger  215  by electroplating utilizing current through electrical connection of the plating line  213 , via  212 , and trace line  214 . Two exemplary methods for forming the anti-oxidation layer follow.  
       EXAMPLE 1 OF FORMATION OF THE ANTI-OXIDATION LAYER  
       [0022]      FIGS. 2D through 2G  show exemplary methods for forming the anti-oxidation layer for PCBs, following that shown in  FIG. 2B . In  FIG. 2D , a patterned solder mask  230  is formed overlying the substrate  200  (shown in  FIG. 2A ). The solder mask  230  comprises an opening  231  exposing the bus line  211  and an opening  232  exposing the conductive finger  215 . The openings  231  and  232  are formed by a method such as photolithography or laser drilling.  
         [0023]     In  FIG. 2E , the substrate  200  is immersed in an electro-bath, followed by supply of current to exposed conductive finger  215  from bus line  211  via plating line  213  and via  212  sequentially. Thus, an anti-oxidation layer  250 , of gold, nickel, palladium, silver, tin, nickel/palladium, chrome/titanium, nickel/gold, palladium/gold, or nickel/palladium/gold, is plated overlying the bus line  211  exposed by the opening  231  and the conductive finger  215  exposed by the opening  232 .  
         [0024]     In  FIG. 2F , the solder mask  230  is shown transparently, revealing the underlying wiring layer  210 . A mechanical drill or laser drill at the solder mask side cuts the via  212  along line a-a to separate the via  212  into two electrically isolated sub-vias  212   a  and form two isolation trenches  216  on the via  212 , exposing parts of sidewalls of the through hole and a bottom layer  234 . One sub-via  212   a  connects to the plating line  213 , and the other connects to the trace line  214 .  
         [0025]     In  FIG. 2G , the solder mask  230  is shown transparently, revealing the underlying wiring layer  210 . The isolation trenches  216  and via  212  are filled with an insulating material  236 .  
       EXAMPLE 2 FOR FORMATION OF THE ANTI-OXIDATION LAYER  
       [0026]      FIGS. 2D through 2G  show another exemplary methods for forming the anti-oxidation layer for PCBs, following that shown in  FIG. 2B . In  FIG. 2D , a patterned resist layer  230 ′ is formed overlying the substrate  200  (shown in  FIG. 2A ). The resist layer  230 ′ comprises an opening  231 ′ exposing the bus line  211  and an opening  232 ′ exposing the conductive finger  215 . The openings  231  and  232  are formed by a method such as photolithography or laser drilling.  
         [0027]     In  FIG. 2E , the substrate  200  is immersed in an electro-bath, followed by supply of current to exposed conductive finger  215  from bus line  211  via plating line  213  and via  212  sequentially. Thus, an anti-oxidation layer  250 , of gold, nickel, palladium, silver, tin, nickel/palladium, chrome/titanium, nickel/gold, palladium/gold, or nickel/palladium/gold, is plated overlying the bus line  211  exposed by the opening  231 ′ and the conductive finger  215  exposed by the opening  232 ′.  
         [0028]     In  FIG. 2F , the patterned resist layer  230 ′ is removed by a method such as etching, exposing the underlying wiring layer  210 . A mechanical drill or laser drill at the solder mask side cuts the via  212  along line a-a to separate the via  212  into two electrically isolated sub-vias  212   a  and form two isolation trenches  216  on the via  212 , exposing parts of sidewalls of the through hole and a bottom layer  234 . One sub-via  212   a  connects to the plating line  213 , and the other connects to the trace line  214 .  
         [0029]     In  FIG. 2G , the solder mask  230  is shown transparently, revealing the underlying wiring layer  210 . The isolation trenches  216  and via  212  are filled with an insulating material  236 , followed by forming a solder mask (not shown) overlying the substrate  200 , exposing the conductive finger  215 .  
         [0030]      FIGS. 2H and 2I  show an alternative embodiment, following that shown in  FIG. 2A , of PCBs of the invention and methods for fabricating the same.  
         [0031]     In  FIG. 2H , a wiring layer  310  is formed overlying the surface of substrate  200 , comprising a metal layer formed overlying substrate  200 , followed by patterning of the metal layer. The metal layer is copper, tin, nickel, chrome, titanium, copper-chrome alloy, or tin-lead alloy. Alternatively, the wiring layer  310  can be formed by physical vapor deposition such as sputtering or metal-organic chemical vapor deposition (MOCVD) directly in a predetermined pattern. In wiring  310 , a bus line  311  is disposed in the periphery area  201  (shown in  FIG. 2A ) of substrate  200 , a via  312  is disposed in the layout area  203 , three conductive fingers  315  are disposed in the layout area  203 , a plating line  313  electrically connects the bus line  311  and the via  312 , and three trace lines  314  electrically connect the via  212  and the respective conductive fingers  315 .  
         [0032]     Formation of the via  312  is substantially the same as the description in  FIG. 2C , and thus, is omitted herefrom. Further, an anti-oxidation layer  350  is formed respectively overlying the conductive fingers by electroplating utilizing electrical connection of the plating line  313 , via  312 , and trace lines  314 . Exemplary methods for forming the anti-oxidation layer  350  are substantially the same as the descriptions for  FIGS. 2D through 2G , and thus, are omitted herefrom.  
         [0033]      FIG. 2I  shows separation of the via  312  of this embodiment. A mechanical drill or laser drill cuts the via  312  along lines b-b and c-c to separate the via  312  into four electrically isolated sub-vias  312   a  and form four isolation trenches  316  on the via  312 , exposing parts of sidewalls of the through hole and a bottom layer  334 . One sub-via  312   a  connects to the plating line  313 , and the others respectively connect to the trace lines  314 , followed by filling an insulating material  336  in the trenches  316  and the via  312 .  
         [0034]     As described, the invention discloses the via  212  electrically connecting to the trace line  214  and the via  312  electrically connecting to the trace lines  314 . Thus, at least one plating line  213  is required to connect the via  212  and bus line  211 , and at least one plating line  313  is required to connect the via  312  and bus line  311 . Current may flow to finger  250  in the trace line  214  via the plating line  213  and the via  212 , and to fingers  350  in the corresponding trace lines  314  via the plating line  313  and the via  312  to respectively electroplate the anti-oxidation layers  250  and  350  overlying the conductive fingers  215  and  315 . Finally, only one plating line  213 / 313  remains, which does not negatively affect the electrical performance of end products utilizing the PCBs of the invention.  
         [0035]     Further, the invention discloses the via  212  separated into two sub-vias  212   a  and the via  312  separated into four sub-vias  312   a  to replace the reduced via of the known art, increasing the wiring density of the PCBs, and electrically isolating the plating line  213  from the trace line  214  and the pad  215 , and the plating line  313  from the trace lines  314  and the pads  315 . The separation of the vias  212  and  312  simplifies the drilling and electroplating of the vias  212  and  312 , improving via reliability and simplifying the electrical isolation process for the plating line.  
         [0036]     As shown in  FIG. 2I , the PCB of an exemplary embodiment of the invention comprises a substrate  200  and a patterned wiring layer  310  overlying a surface of the substrate  200 . The substrate  200  comprises a layout area  203  and a periphery  201  on the surface. The wiring layer  310  is copper, tin, nickel, chrome, titanium, copper-chrome alloys, or tin-lead alloys.  
         [0037]     The wiring layer  310  further comprises a bus line  311 , a plurality of conductive fingers  315 , a separated via  312 , a plating line  313  and at least one trace line  314 . The bus line  311  is disposed in the periphery area  203 . The conductive fingers  315  respectively comprise an anti-oxidation layer  350  thereon and are disposed in the layout area  203 . The anti-oxidation layer  350  is gold, nickel, palladium, silver, tin, nickel/palladium, chrome/titanium, nickel/gold, palladium/gold, or nickel/palladium/gold. The separated via  312  comprises a plurality of electrically isolated sub-vias  312   a  and is disposed in the layout area  203 . The plating line  313  connects the bus line  311  and the sub-vias  312   a . One sub-via  312   a  connects to the plating line  313 , and the others respectively connect to different trace lines  314 . The at least one trace line  314  electrically connects the sub-vias  312   a  and the conductive fingers  315 .  
         [0038]     The via  312  comprises isolation trenches  316  on either side of the sub-vias  312   a . The sub-vias  312   a  connect to at least the plating line  313  or the at least one trace line  314 . The plating line  313  preferably connects the bus line  311  and one of the sub-vias  312   a . Further, the sub-vias  312   a  and the isolation trenches  316  are filled with an isolating material  336 .  
         [0039]     Thus, the invention discloses separation of the via into a plurality of sub-vias to replace the reduced via of the known art, increasing the wiring density of the PCBs and simplifying the drilling and electroplating of the vias to improve via reliability. Moreover, only one plating line remains, with no negative affect on the electrical performance of end products utilizing the PCBs of the invention. The invention improves via reliability with increased wiring density and electrical performance.  
         [0040]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. It is therefore intended that the following claims be interpreted as covering all such alteration and modifications as fall within the true spirit and scope of the invention.