Patent Publication Number: US-2013228921-A1

Title: Substrate structure and fabrication method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to substrate structures and fabrication methods thereof, and more particularly, to a substrate structure having solder balls for external electrical connection and a fabrication method thereof. 
     2. Description of Related Art 
     Ball grid array (BGA) semiconductor package structures have been developed to meet the trend of lighter, thinner, shorter and smaller electronic products. In such a BGA semiconductor package structure, a semiconductor chip is disposed on a surface of a substrate and electrically connected to the substrate through a plurality of bonding wires, and a plurality of solder balls are mounted on conductive pads of the other surface of the substrate, respectively, so as to electrically connect another electronic device such as a circuit board or another package structure. 
       FIGS. 1A and 1B  are schematic cross-sectional views of a conductive pad in a conventional substrate structure (not shown). 
     Referring to  FIG. 1A , the conductive pad has a copper layer  11 , a nickel layer  12  and a gold layer  13  sequentially stacked on one another. 
     Referring to  FIG. 1B , a solder flux  14  is coated on the gold layer  13  for attaching a solder ball  15  to the gold layer  13 . Then, a reflow process is performed. Since the gold layer  13  is thin and diffuses quickly, the gold layer  13  is dissolved into the solder ball  15  during the reflow process. Further, a bonding layer  16  is formed between the solder ball  15  and the nickel layer  12  so as to bond the solder ball to the conductive pad. Therein, the bonding layer  16  contains nickel-tin alloy, thus leading to high thermal conductivity and low stress tolerance. Therefore, during a drop test, the solder ball may easily fall off from the conductive pad. 
       FIGS. 2A and 2B  are schematic cross-sectional views of a conductive pad in another conventional substrate structure (not shown). 
     Referring to  FIG. 2A , the conductive pad has a copper layer  21  and an OSP (Organic Solderability Preservative) layer  22  formed on the copper layer  21 . 
     Referring to  FIG. 2B , a solder flux  23  is coated on the OSP layer  22  for attaching a solder ball  24  to the OSP layer  22 . Then, a reflow process is performed, during which the OSP layer  22  and the solder flux  23  are volatilized. The solder flux  23  facilitates to clean the outer portion of the copper layer  21  so as to form a bonding layer  25  between the solder ball  24  and the copper layer  21 . Therein, the bonding layer  25  contains copper-tin alloy, thus leading to high stress tolerance and low thermal conductivity. Therefore, during a drop test, the solder ball is not easy to fall off. 
     However, compared with the gold layer  13 , the OSP layer  22  can be easily oxidized and absorb moisture, thereby resulting in a short duration period. Therefore, the bonding reliability of the solder ball  24  is reduced, which results in a low product reliability. 
     Therefore, there is a need to provide a substrate structure and a fabrication method thereof so as to overcome the above-described drawbacks. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a substrate structure, which comprises: a substrate body; and a plurality of conductive pads formed on the substrate body and each having a first copper layer, a nickel layer, a second copper layer and a gold layer sequentially stacked on one another, wherein the thickness of the second copper layer is less than the thickness of the first copper layer. 
     The present invention further provides another substrate structure, which comprises: a substrate body; a plurality of conductive pads formed on the substrate body and each having a copper layer and a nickel layer formed on the copper layer; a bonding layer formed on the conductive pads; and a plurality of solder balls disposed on the bonding layer of the conductive pads, respectively. 
     The present invention further provides a fabrication method of a substrate structure, which comprises the steps of: sequentially forming a first copper layer, a nickel layer, a second copper layer and a gold layer on a substrate body, wherein the thickness of the second copper layer is less than the thickness of the first copper layer. 
     The present invention further provides a substrate structure, which comprises: a substrate body; and a plurality of conductive pads formed on the substrate body and each having a copper layer, a nickel-copper mixed layer and a gold layer sequentially stacked on one another, wherein, in the nickel-copper mixed layer, the content of copper is less than the content of nickel. 
     The present invention further provides another substrate structure, which comprises: a substrate body; a plurality of conductive pads formed on the substrate body and each having a copper layer and a nickel-copper mixed layer formed on the copper layer, wherein, in the nickel-copper mixed layer, the content of copper is less than the content of nickel; a bonding layer formed on the conductive pads; and a plurality of solder balls disposed on the bonding layer of the conductive pads, respectively. 
     The present invention further provides another fabrication method of a substrate structure, which comprises the steps of: forming a plurality of conductive pads on a substrate body, wherein each of the conductive pads has a copper layer; and sequentially forming a nickel-copper mixed layer and a gold layer on the copper layer, wherein, in the nickel-copper mixed layer, the content of copper is less than the content of nickel. 
     According to the present invention, each of the conductive pads merely contains little copper besides nickel and gold such that the bonding layer between the conductive pad and the corresponding solder balls is mainly comprised of Cu 6 Sn 5  instead of Ni 3 Sn 4  as in the prior art, thereby achieving a preferred bonding performance. Further, the gold layer on each of the conductive pads retards oxidation and moisture absorption so as to prolong the duration period of the substrate structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are schematic cross-sectional views of a conductive pad in a conventional substrate structure; 
         FIGS. 2A and 2B  are schematic cross-sectional views of a conductive pad in another conventional substrate structure; 
         FIGS. 3A to 3E  are schematic cross-sectional views showing a substrate structure and a fabrication method thereof according to a first embodiment of the present invention; and 
         FIGS. 4A to 4D  are schematic cross-sectional vies showing a substrate structure and a fabrication method thereof according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following illustrative embodiments are provided to illustrate the disclosure of the present invention, 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 invention. Various modification and variations can be made without departing from the spirit of the present invention. Further, terms such as “one”, “on”, “top” etc. are merely for illustrative purpose and should not be construed to limit the scope of the present invention. 
     First Embodiment 
       FIGS. 3A to 3E  are schematic cross-sectional views showing a substrate structure and a fabrication method thereof according to a first embodiment of the present invention. 
     Referring to  FIG. 3A , a substrate body  30  is provided and a plurality of conductive pads  31  (only one conductive pad is illustrated in the drawing) are formed on the substrate body  30 . Each of the conductive pads  31  has a first copper layer  311  and a nickel layer  312  formed on the first copper layer  311 . 
     Referring to  FIG. 3B , a second copper layer  313  and a gold layer  314  are sequentially formed on the nickel layer  312 . The second copper layer  313  has a thickness less than that of the first copper layer  311 . 
     Referring to  FIG. 3C , a solder flux  32  is formed on the gold layer  314  so as for a solder ball  33  to be mounted thereon. 
     Referring to  FIG. 3D , a reflow process is performed so as to volatilize the solder flux  32  and dissolve the gold layer  314  into the solder ball  33  and also dissolve the second copper layer  313 , thereby forming a bonding layer  34  between the solder ball  33  and the nickel layer  312 . The bonding layer  34  is comprised of Cu 6 Sn 5    341  and Ni 3 Sn 4    342 , and the content of Ni 3 Sn 4    342  is less than the content of Cu 6 Sn 5    341 . It should be noted that the bonding layer  34  is shown in an enlarged view for purpose of illustration and not intended to limit the present invention. 
     Referring to  FIG. 3E , a semiconductor chip  35  is disposed on the substrate body  30  opposite to the conductive pads  31  and electrically connected to the substrate body  30  through a plurality of bonding wires  36 . Further, an encapsulant  37  is formed to encapsulate the chip  35  and the bonding wires  36 . 
     The present invention further provides a substrate structure, which has: a substrate body  30 , and a plurality of conductive pads  31  formed on the substrate body  30  and each having a first copper layer  311 , a nickel layer  312 , a second copper layer  313  and a gold layer  314  sequentially stacked on one another. Therein, the second copper layer  313  has a thickness less than that of the first copper layer  311 . 
     The above-described substrate structure further has a solder flux  32  applied on the gold layer  314  and a plurality of solder balls  33  disposed on the solder flux  32 . 
     The present invention further provides another substrate structure, which has: a substrate body  30 ; a plurality of conductive pads  31  formed on the substrate body  30  and each having a first copper layer  311  and a nickel layer  312  formed on the first copper layer  311 ; a bonding layer  34  formed on the conductive pads  31 ; and a plurality of solder balls  33  disposed on the bonding layer  34  of the conductive pads  31 , respectively. 
     The bonding layer is comprised of Cu 6 Sn 5    34  land Ni 3 Sn 4    342 , and the content of Ni 3 Sn 4    342  is less than the content of Cu 6 Sn 5    341 . 
     Second Embodiment 
       FIGS. 4A to 4D  are schematic cross-sectional views showing a substrate structure and a fabrication method thereof according to a second embodiment of the present invention. 
     Referring to  FIG. 4A , a substrate body  40  is provided and a plurality of conductive pads  41  are formed on the substrate body  40 . Each of the conductive pads  41  has a copper layer  411 . 
     Referring to  FIG. 4B , a nickel-copper mixed layer  412  and a gold layer  413  are sequentially formed on the copper layer  411 . In the nickel-copper mixed layer  412 , the content of copper is less than the content of nickel. 
     Referring to  FIG. 4C , a solder flux  42  is formed on the gold layer  413  so as for a plurality of solder balls  43  to be mounted thereon. Referring to  FIG. 4D , a reflow process is performed to volatilize the solder flux  42  and dissolve the gold layer  413  into the solder ball  43 , thus forming a bonding layer  44  between the solder ball  43  and the nickel-copper mixed layer  412 . The bonding layer  44  is comprised of Cu 6 Sn 5    441  and Ni 3 Sn 4    442 , and the content of Ni 3 Sn 4    442  is less than the content of Cu 6 Sn 5    441 . It should be noted that the bonding layer  44  is shown in an enlarged view for purpose of illustration and not intended to limit the present invention. 
     In the present embodiment, a die attaching process and a packaging process similar to the first embodiment can further be performed. Since the processes are well known in the art, detailed description thereof is omitted herein. 
     The present invention further provides a substrate structure, which has: a substrate body  40 , and a plurality of conductive pads  41  formed on the substrate body  40  and each having a copper layer  411 , a nickel-copper mixed layer  412  and a gold layer  413  sequentially stacked on one another. In the nickel-copper mixed layer  412 , the content of copper is less than the content of nickel. 
     The substrate structure can further have a solder flux  42  applied on the gold layer  413  and a plurality of solder balls  43  disposed on the solder flux  42 . 
     The present invention further provides another substrate structure, which has: a substrate body  40 ; a plurality of conductive pads  41  formed on the substrate body  40  and each having a copper layer  411  and a nickel-copper mixed layer  412  formed on the copper layer  411 , wherein in the nickel-copper mixed layer  412 , the content of copper is less than the content of nickel; a bonding layer  44  formed on the conductive pads  41 ; and a plurality of solder balls  43  disposed on the bonding layer  44  of the conductive pads  41 , respectively. 
     The bonding layer  44  is comprised of Cu 6 Sn 5    441  and Ni 3 Sn 4    442  and the content of Ni 3 Sn 4    442  is less than the content of Cu 6 Sn 5    441 . 
     According to the present invention, each of the conductive pads merely contains little copper besides nickel and gold such that the bonding layer between the conductive pad and the corresponding solder ball is mainly comprised of Cu 6 Sn 5  instead of Ni 3 Sn 4  as in the prior art, thereby achieving a preferred bonding performance. Further, the gold layer on each of the conductive pads retards oxidation and moisture absorption so as to prolong the duration period of the substrate structure. 
     The above-described descriptions of the detailed embodiments are only to illustrate the preferred implementation according to the present invention, and it is not to limit the scope of the present invention. Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.