Patent Publication Number: US-2011056738-A1

Title: Package substrate and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices and methods of manufacturing the same, and more particularly, to a package substrate and a manufacturing method thereof. 
     2. Description of the Prior Art 
     In the early 1960s, IBM developed flip-chip package technology. Unlike wire-bonding techniques, flip-chip techniques not only entail electrically connecting a semiconductor chip and a substrate by solder bumps instead of bonding wires and thereby advantageously increase packaging density and downsize package components, but also dispense with long bonding wires and thereby advantageously shorten the distance of transmission of signals and enhance electrical performance. 
     Existing flip-chip techniques involve providing a plurality of electrode pads on a semiconductor chip having an integrated circuit (IC) therein, providing a plurality of conductive pads on a package substrate so as for the conductive pads to correspond in position to the electrode pads, providing solder bumps or other conductive materials between the semiconductor chip and the package substrate as appropriate, thereby allowing the semiconductor chip to be face-down mounted on the package substrate (that is, the electrode pad-disposed electrical contact surface of the semiconductor chip faces downward), wherein the solder bumps or conductive materials enable electrical input/output (I/O) and mechanical connection between the semiconductor chip and the package substrate. 
     Referring to  FIG. 1A  through  FIG. 1E , cross-sectional views of forming a conductive material on conductive pads of a package substrate by stencil printing in a known manner are shown. Referring to  FIG. 1A , a solder mask  11  is formed on a package substrate body  10  with a plurality of conductive pads  101  provided thereon, a plurality of openings  110  are formed in the solder mask  11  for exposing the conductive pads  101 ; After a patterning process is performed on the solder mask  11  and the openings  110  thereof, a solder mask foot  110   a  is formed on each of the conductive pads  101  exposed from each corresponding one of the openings  110 . Referring to  FIG. 1B , a stencil  12  with open areas  120  is positioned above the solder mask  11  on the package substrate body  10  so as for the open areas  120  to correspond in position to the conductive pads  101 . Referring to  FIG. 1C , a conductive material  13  is formed in the open areas  120  of the stencil  12  by printing. Referring to  FIG. 1D , the stencil  12  is removed to expose the conductive material  13 . Referring to  FIG. 1E , a conductive element  13 ′ is formed on the conductive pads  101  by a reflow process and configured for electrical connection of the conductive pads  101  and an external electronic device. Despite the presence of the solder mask foot  110   a  on each of the conductive pads  101  exposed from each corresponding one of the openings  110 , the openings  110  formed in the solder mask  11  by printing are large enough for a relatively large area of the conductive pads  101  to be exposed and thereby the bonding between the conductive element  13 ′ and the conductive pads  101  to continue unabated. 
     Nonetheless, the open areas  120  of the stencil  12  dwindle as the conductive pads  101  are provided at an increasingly high density, and in consequence the open areas  120  are unlikely to be filled with the conductive material  13 ; hence, it is difficult to form the conductive element  13 ′ on the conductive pads  101  of a high-density package substrate by printing. 
     Referring to  FIG. 2A  through  FIG. 2G , in an attempt to overcome the aforesaid drawbacks of known stencil printing techniques, manufacturers proposed electroplating a conductive material to a package substrate. Referring to  FIG. 2A , a package substrate body  10  that has undergone a wiring process and has at least a surface thereof having the conductive pads  101  formed thereon is provided. Referring to  FIG. 2B , the solder mask  11  is formed on the package substrate body  10 . Referring to  FIG. 2C , the openings  110  are formed in the solder mask  11  by a patterning process that entails exposure and development so as for the openings  110  thus formed to correspond in position to and expose the conductive pads  101 , respectively, and the solder mask foot  110   a  is left behind on each of the conductive pads  101  exposed from each corresponding one of the openings  110 . Referring to  FIG. 2D , a conductive layer  21  is formed on the solder mask  11  and the openings  110  thereof, and a resist layer  22  is formed on the conductive layer  21  to thereby allow a plurality of resist layer openings  220  to be formed in the resist layer  22  for exposing the openings  110 , the periphery of the openings  110 , and the conductive layer  21  covering the conductive pads  101 . Referring to  FIG. 2E , in an electroplating process wherein the conductive layer  21  functions as a path of electric current due to conductive characteristics of the conductive layer  21 , a conductive material  23  is electroplated to the conductive layer  21  exposed from the resist layer openings  220 . Referring to  FIG. 2F , the resist layer  22  and the conductive layer  21  thereunder are removed to further expose the conductive material  23 . Referring to  FIG. 2G , the conductive material  23  is turned into a conductive element  23 ′ by a reflow process to facilitate electrical connection with an external electronic device. 
     Referring to  FIG. 2C , with the openings  110  being formed in the solder mask  11  by a patterning process that entails exposure and development, the solder mask foot  110   a  is left behind on each of the conductive pads  101  exposed from each corresponding one of the openings  110 , and thus each of the openings  110  narrows toward the bottom thereof; subsequently, the contact area between the conductive element  23 ′ and a corresponding one of the conductive pads  101  is so small as to compromise electrical connection between the conductive element  23 ′ and the corresponding one of the conductive pads  101  or weaken the bonding between the conductive element  23 ′ and the corresponding one of the conductive pads  101  and thereby compromise electrical connection therebetween. 
     Accordingly, it is imperative to provide a package substrate having a conductive element to improve electrical connection between the conductive element and conductive pads. 
     SUMMARY OF THE INVENTION 
     In view of the drawbacks of the prior art, it is an objective of the present invention to provide a package substrate and a manufacturing method thereof so as to reinforce a conductive element and thereby augment bonding thereof and enhance the quality of electrical connection. 
     Another objective of the present invention is to provide a package substrate and a manufacturing method thereof so as to make fabrication of the conductive element simpler. 
     To achieve the above and other objectives, the present invention provides a package substrate, comprising: a package substrate body with at least a surface thereof having a plurality of conductive pads thereon; a solder mask provided on the package substrate body and the conductive pads and provided therein with a plurality of first-step openings and a plurality of second-step openings in communication with the first-step openings so as for the conductive pads to correspond in position to and be exposed from the first-step openings and the second-step openings, the second-step openings being above the first-step openings, respectively, and each having a bottom rim in contact with a corresponding one of the first-step openings and a top rim, the bottom rim being of a smaller diameter than the top rim and of the same diameter as the corresponding one of the first-step openings; and a conductive material provided in the first-step and second-step openings of the solder mask. 
     A sloped wall or a curved wall is defined by and provided between the top rim and the bottom rim of each of the second-step openings. 
     The package substrate of the present invention further comprises a conductive layer disposed between the conductive pads and the conductive material. The conductive material is solder, thereby allowing a conductive element to be made of the conductive material by a reflow process. Alternatively, the conductive material is copper, silver, nickel, gold, or platinum. 
     The present invention further provides a method of manufacturing a package substrate, comprising the steps of: providing a package substrate body having at least a surface, the at least a surface having a plurality of conductive pads formed thereon; forming a solder mask on the package substrate body and conductive pads; forming in the solder mask a plurality of first-step openings corresponding in position to and exposing a portion of the conductive pads; forming in the solder mask a plurality of second-step openings corresponding in position to the first-step openings, the second-step openings being above the first-step openings, respectively, and each having a bottom rim in contact with a corresponding one of the first-step openings and a top rim, the bottom rim being of a smaller diameter than the top rim and of the same diameter as the corresponding one of the first-step openings; and forming a conductive material in the first-step and second-step openings of the solder mask. 
     As regards the method, a sloped wall or a curved wall is defined by and provided between the top rim and the bottom rim of each of the second-step openings. 
     The method further comprises the step of forming the first-step openings by exposure and development and the step of forming the second-step openings by a laser-based or plasma-based drilling process. 
     The conductive material is solder, thereby allowing a conductive element to be made of the conductive material by a reflow process. Alternatively, the conductive material is copper, silver, nickel, gold, or platinum. 
     Regarding the method of manufacturing the package substrate of the present invention, the conductive material is formed by electroplating and by the steps of: forming a conductive layer on the solder mask and in the first-step and second-step openings; forming a resist layer on the conductive layer and forming a plurality of resist layer openings corresponding in position to the first-step and second-step openings in the resist layer for exposing the conductive layer on the conductive pads and in the first-step and second-step openings; electroplating the conductive material to the conductive layer exposed from the resist layer openings; and removing the resist layer and the conductive layer thereunder to expose the conductive material. 
     Also, the conductive material is formed by stencil printing and by the steps of: positioning above the solder mask a stencil having a plurality of open areas so as for the open areas of the stencil to correspond in position to the first-step and second-step openings, respectively; filling the open areas and the first-step and second-step openings with the conductive material; and removing the stencil to expose the conductive material. 
     Alternatively, the conductive material is provided in the form of solder balls received in the first-step and second-step openings, respectively. 
     Accordingly, the present invention provides a package substrate and a method of manufacturing the same. The method comprises the steps of: covering a package substrate body having a plurality of conductive pads thereon with a solder mask; forming a plurality of first-step openings in the solder mask by exposure and development for exposing the conductive pads; forming a plurality of second-step openings in the solder mask by a laser-based or plasma-based drilling process; and removing a solder mask foot from the bottom of each of the first-step openings so as to expose large surface areas of the conductive pads. Hence, the contact area between a conductive element and a corresponding one of the conductive pads is large enough to enhance bonding and electrical connection therebetween. Also, with the second-step openings outmatching the first-step openings in diameter, a conductive material can be formed in the first-step and second-step openings by printing or ball implantation so as to make fabrication of the conductive element simpler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To enable persons skilled in the art to gain insight into technical features and effects of the present invention, the present invention is hereunder illustrated with preferred embodiments in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  through  FIG. 1E  are cross-sectional views of forming a conductive material on a package substrate by printing in a known manner; 
         FIG. 2A  through  FIG. 2G  are cross-sectional views of forming a conductive material on a package substrate by electroplating in a known manner; 
         FIG. 3A  through  FIG. 3G  are cross-sectional views of a first embodiment of a method of manufacturing a package substrate according to the present invention; 
       FIG.  3 G′ is a cross-sectional view of another embodiment of the method illustrated by  FIG. 3G ; 
         FIG. 4A  through  FIG. 4D  are cross-sectional views of a second embodiment of a method of manufacturing a package substrate according to the present invention; and 
         FIG. 5A  and  FIG. 5B  are cross-sectional views of a third embodiment of a method of manufacturing a package substrate according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Referring to  FIG. 3A  through  FIG. 3G , cross-sectional views of a first embodiment of a package substrate and a method of manufacturing the package substrate according to the present invention are shown. 
     Referring to  FIG. 3A , a package substrate body  30  with at least a surface thereof having a plurality of conductive pads  301  formed thereon is provided. 
     Referring to  FIG. 3B , a solder mask  31  is formed on the package substrate body  30  and the conductive pads  301 . 
     Referring to  FIG. 3C , a plurality of first-step openings  311  corresponding in position to and exposing portions of the conductive pads  301 , respectively, are formed in the solder mask  31  by exposure and development, and yet a solder mask foot  311   a  is left behind in the bottom of each of the first-step openings  311 . 
     Referring to  FIG. 3D , a plurality of second-step openings  312  corresponding in position to the first-step openings  311  are formed in the solder mask  31  by a laser-based or plasma-based drilling process. Each of the second-step openings  312  has a bottom rim  312   b  in contact with a corresponding one of the first-step openings  311  and a top rim  312   a . The top rim  312   a  is of a larger diameter than the bottom rim  312   b . The bottom rim  312   b  is of the same diameter as the corresponding one of the first-step openings  311 . The first-step openings  311  and portions of the conductive pads  301  correspond in position to and are exposed from the second-step openings  312 , respectively, As shown in the drawing, the solder mask foot  311   a  is removed from the bottom of each of the first-step openings  311  by the laser-based or plasma-based drilling process. 
     A sloped wall or a curved wall is defined by and provided between the top rim  312   a  and the bottom rim  312   b  of each of the second-step openings  312 . Defined by and provided between the top rim  312   a  and the bottom rim  312   b  is preferably a curved wall, as shown in the drawing. 
     Refer to  FIG. 3E  for a better understanding of the following steps of manufacturing the package substrate according to the present invention as follows: a conductive layer  32  is formed on the solder mask  31  and the walls of the first-step and second-step openings  311 ,  312 ; a resist layer  33  is formed on the conductive layer  32 ; a plurality of resist layer openings  330  corresponding in position to the first-step and second-step openings  311 ,  312  of the solder mask  31 , respectively, are formed in the resist layer  33  for exposing portions of the conductive pads  301  and portions of the conductive layer  32  exposed from the first-step and second-step openings  311 ,  312 . 
     Referring to  FIG. 3F , a conductive material  34  is electroplated to the conductive layer  32  exposed from the resist layer openings  330 , by an electroplating process wherein the conductive layer  32  functions as a path of electric current due to conductive characteristics of the conductive layer  32 . 
     Referring to  FIG. 3G  and FIG.  3 G′, the resist layer  33  and the conductive layer  32  thereunder are removed to expose the conductive material  34 , wherein the conductive material  34  is copper, silver, nickel, gold, or platinum, as shown in  FIG. 3G . Alternatively, the conductive material  34  is solder such that a conductive element  34 ′ can be made of the conductive material  34  by a reflow process to thereby allow the conductive pads  301  to be electrically connected with an external electronic device as shown in FIG.  3 G′. 
     The present invention further provides a package substrate, comprising: a package substrate body  30  with at least a surface thereof having a plurality of conductive pads  301  formed thereon; a solder mask  31  provided on the package substrate body  30  and the conductive pads  301  and provided therein with a plurality of first-step openings  311  and a plurality of second-step openings  312  in communication with the first-step openings  311  so as for the conductive pads  301  to correspond in position to and be exposed from the first-step openings  311  and the second-step openings  312 , the second-step openings  312  being above the first-step openings  311 , respectively, and each having a bottom rim  312   b  in contact with a corresponding one of the first-step openings  311  and a top rim  312   a , the bottom rim  312   b  being of a smaller diameter than the top rim  312   a  and of the same diameter as the corresponding one of the first-step openings  311 ; and a conductive material  34  provided in the first-step and second-step openings  311 ,  312  of the solder mask  31 . 
     The package substrate of the present invention further comprises a conductive layer  32  disposed between the conductive pads  301  and the conductive material  34 . The conductive material  34  comprises one selected from the group consisting of copper, silver, nickel, gold, and platinum. Alternatively, the conductive material  34  is solder, thereby allowing a conductive element  34 ′ to be made of the conductive material  34  by a reflow process and configured to electrically connect the conductive pads  301  and an external electronic device. 
     Second Embodiment 
     Referring to  FIG. 4A  through  FIG. 4D , cross-sectional views of a second embodiment of a method of manufacturing a package substrate according to the present invention are shown. The second embodiment differs from the first embodiment in that, in the second embodiment, the conductive material is formed by stencil printing. 
     Referring to  FIG. 4A , a structure shown in  FIG. 3D  is provided, a stencil  41  with a plurality of open areas  410  is positioned over the solder mask  31  so as for the open areas  410  of the stencil  41  to correspond in position to the first-step and second-step openings  311 ,  312  of the solder mask  31 . 
     Referring to  FIG. 4B , the open areas  410  and the first-step and second-step openings  311 ,  312  are filled with the conductive material  34 , using a roller or a spray, so as to form the conductive material  34  on each of the conductive pads  301  exposed from each corresponding one of the first-step and second-step openings  311 ,  312 . 
     Referring to  FIG. 4C , the stencil  41  is removed so as to expose the conductive material  34 . 
     Referring to  FIG. 4D , the conductive material  34  is turned into a conductive element  34 ′ by a reflow process for electrical connection of the conductive pads  301  and an external electronic device. 
     Third Embodiment 
     Referring to  FIG. 5A  and  FIG. 5B , cross-sectional views of a third embodiment of a method of manufacturing a package substrate according to the present invention are shown. The third embodiment differs from the first and second embodiments in that, in the third embodiment, the conductive material is provided in the form of balls. 
     Referring to  FIG. 5A , the structure shown in  FIG. 3D  is provided, and a plurality of solder balls  51  are received in the first-step and second-step openings  311 ,  312  of the solder mask  31 . 
     Referring to  FIG. 5B , the solder balls  51  are turned into a conductive element  51 ′ by a reflow process for electrical connection of the conductive pads and an external electronic device. 
     In conclusion, the present invention provides a package substrate and a method of manufacturing the same. The method comprises the steps of: forming a solder mask on a package substrate body and a plurality of conductive pads thereon; forming a plurality of first-step openings in the solder mask by exposure and development; forming a plurality of second-step openings in the solder mask by a laser-based or plasma-based drilling process; removing a solder mask foot from the bottom of each of the first-step openings so as to expose large surface areas of the conductive pads. Hence, the contact area between a conductive element and a corresponding one of the conductive pads is large enough to enhance the bonding and electrical connection therebetween. Also, with the second-step openings outmatching the first-step openings in diameter, a conductive material can be formed in the first-step and second-step openings by printing or ball implantation so as to make fabrication of the conductive element simpler. 
     The foregoing specific embodiments are only illustrative of the features and functions of the present invention but are not intended to restrict the scope of the present invention. It is apparent to those skilled in the art that all equivalent modifications and variations made in the foregoing embodiments according to the spirit and principle in the disclosure of the present invention should fall within the scope of the appended claims.