Abstract:
A high density, non-bussed semiconductor package and a full body gold (FBG) method for manufacturing semiconductor packages are provided to improve electrical and mechanical connections with semiconductors and other electronic components and devices. The semiconductor package is fabricated by developing circuitry on the wire bond side of the semiconductor package prior to developing the ball attach side. The copper circuitry on the wire bond side is fully covered and protected from the environment. Solder masks are applied directly to the semiconductor substrate or copper layer to avoid contact with gold. The ball attach area is covered and protected by metallic layers, such as nickel and gold, or an organic solderable material to eliminate weak solder mask-gold connections.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to hybrid electronics, and more particularly, to improving electrical and mechanical connectivity to semiconductor packages. 
     2. Related Art 
     Semiconductor packages are known by a variety of generic names, such as ball grid arrays (BGA), plastic ball grid arrays (PBGA), multi chip module-laminates (MCM-L), and packaging substrates. Semiconductor packages can be composed of any one of a variety of materials, such as bismaldehyde trizaine resins, multifunctional epoxy resins, polyamide systems, and a range of other materials. The base materials used to manufacture the substrate, also referred to as a laminate, are usually a combination of resin systems and woven fiberglass. 
     A semiconductor package typically is manufactured by taking a substrate and depositing a metallization layer to develop circuitry for a wire bond area on one side and circuitry for a ball attach area on the opposite side of the substrate. Solder masks can be applied to provide mechanical and electrical support, and a via hole or through hole connects the wire bond circuitry to the ball attach circuitry. 
     Semiconductor packages serve as a mounting device for semiconductors. When a semiconductor is mounted on a semiconductor package, the semiconductor is usually a bare die. However, in certain instances, the bare die may be housed in some type of subassembly, which is in turn attached to the semiconductor package. 
     Semiconductor packages are actually interposers, which lie between the semiconductor and a larger printed circuit board. The function of the interposers is to serve as a “fan out” for the very high-density electrical outputs from the semiconductor. The interposer transitions the very high density electrical output from the semiconductor into a less dense output that is suitable for mounting an assembled packaging substrate (i.e., semiconductor package and semiconductor die) to the printed circuit board. 
     Today three major trends influence the electronics industry: an ever-increasing number of outputs from the semiconductor, miniaturization of all electronic components (especially semiconductor die and packaging substrates), and portability. As a result, these trends are forcing semiconductor packages to become smaller, which in turn requires circuit lines to become smaller, spaces between the circuit lines to become closer, and via holes connecting one side of the semiconductor package to the other to become smaller. 
     Depending on the number of output connections from the die, the final package size actually becomes larger because the routing of circuitry forces the semiconductor package to become denser. For instance, each input/output port (I/O) on the semiconductor die has to be connected to a circuit net on the wire bond area of the semiconductor package. Each circuit net on the semiconductor package is routed to a via hole to make the connection with the backside (ball attach side of the package). The ball attach area is attached to the printed circuit board. Thus, as circuit density increases, a limiting factor in package design is the size of the via hole and the size of the pad that surrounds the via hole. The area of the finished package is directly proportional to the number of I/Os on the semiconductor die and directly proportional to the area occupied on the printed circuit board. This high-density design makes it physically impossible to include an additional bus line for the electrical connection during electro plating between two bond fingers. 
     Due to the high-density design, semiconductor packages are generally manufactured by using a full body gold (FBG) process whereas, nickel and gold layers are the etch resist in the subsequent etching away of a copper layer. The circuitry is developed on both sides of the semiconductor package at the same time, resulting in the wire bond area having nickel and gold overhangs that expose the copper layer to the environment. As a result, the exposed copper can diffuse or oxidize, thus deteriorating the conductivity of the wire bond area. A gold layer is typically applied to both sides of the semiconductor package, and a solder mask is deposed directly onto the gold layer. However, solder mask-gold connections typically have poor adhesion. As can be seen, FBG manufacturing processes have several inherent weaknesses. 
     What is needed is a method for manufacturing semiconductor packages that overcomes the problems of FBG or other unreliable electrolysis processes. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to high density, non-bussed semiconductor packages and a full body gold (FBG) method for manufacturing the semiconductor packages. According to an embodiment of the present invention, the semiconductor package is fabricated by developing circuitry on the wire bond side of the semiconductor package prior to developing the ball attach side. The copper circuitry on the wire bond side is fully covered by nickel and gold layers. Solder masks are applied directly to the substrate or copper layer to avoid contact with gold. In one embodiment of the present invention, nickel and gold layers cover the ball attach area. In another embodiment, the ball attach area is protected by an organic solderable material. 
     An advantage of the present invention is the fabrication of a semiconductor device that eliminates solder mask-gold connections. Since the presence of gold tend to weaken solder joints, avoiding such connections strengthens the electrical and mechanical connections to the semiconductor package. 
     Another advantage of the present invention is having a fully encapsulated wire bond area. Since the copper circuitry is fully covered by one or more layer of other material, it is protected from the environment, which reduces diffusion and oxidation. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
     FIG. 1 a  illustrates a side-view of a semiconductor package. 
     FIG. 1 b  illustrates the top-view of the semiconductor package of FIG. 1 a , without solder masks. 
     FIG. 1 c  illustrates the top-view of the semiconductor package of FIG. 1 a , including solder masks. 
     FIG. 1 d  illustrates the bottom-view of the semiconductor package of FIG. 1 a , without solder masks. 
     FIG. 1 e  illustrates the bottom-view of the semiconductor package of FIG. 1 a , including solder masks. 
     FIG. 2 illustrates a block diagram of a method for manufacturing a semiconductor package. 
     FIG. 3 a  illustrates a side-view of a semiconductor package, formed in accordance with one embodiment of the present invention. 
     FIG. 3 b  illustrates the top-view of the semiconductor package of FIG. 3 a , without solder masks. 
     FIG. 3 c  illustrates the top-view of the semiconductor package of FIG. 3 a , including solder masks. 
     FIG. 3 d  illustrates the bottom-view of the semiconductor package of FIG. 3 a , without solder masks. 
     FIG. 3 e  illustrates the bottom-view of the semiconductor package of FIG. 3 a , including solder masks. 
     FIGS. 4 a - 4   b  illustrates a block diagram of a method for manufacturing a semiconductor package in accordance with one embodiment of the present invention. 
     FIG. 5 a  illustrates a side-view of a semiconductor package, formed in accordance with a second embodiment of the present invention. 
     FIG. 5 b  illustrates the top-view of the semiconductor package of FIG. 5 a , without solder masks. 
     FIG. 5 c  illustrates the top-view of the semiconductor package of FIG. 5 a , including solder masks. 
     FIG. 5 d  illustrates the bottom-view of the semiconductor package of FIG. 5 a , without solder masks. 
     FIG. 5 e  illustrates the bottom-view of the semiconductor package of FIG. 5 a , including solder masks. 
     FIG. 6 illustrates a block diagram of a method for manufacturing a semiconductor package in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Overview 
     The present invention relates to the manufacture of semiconductor packages. FIGS. 1 a - 1   e  illustrate five distinct views of a semiconductor package  100 . Referring to FIG. 1 a , semiconductor package  100  can be fabricated by depositing layers of copper  114 , nickel  116  and gold  118  onto a substrate  112  to develop circuitry for a wire bond area  122  (also referred to herein as the wire bond side) and a ball attach area  124  (also referred to herein as the ball attach side). As shown in FIG. 1 a , a solder mask  120  can also be applied to provide mechanical and electrical support to substrate  112 . Referring to FIG. 1 d , a via hole or through hole  126  connects the wire bond circuitry to the ball attach circuitry. 
     High density semiconductor packages can be manufactured by using a full body gold (FBG) process. Referring to FIG. 2, flowchart  200  represents the general operational flow of a process for manufacturing the semiconductor package of FIGS. 1 a - 1   e . FIG. 2 begins at step  201 . At step  202 , the copper layer on the substrate  112  is reduced by a controlled etching process. At step  204 , the through or via hole  126  is drilled through substrate  112 , and at step  206 , the via hole  126  and substrate  112  is copper plated to, for example, four to five microns. Referring to steps  208 - 218 , the circuitry is developed on both sides of semiconductor package  100  at the same time. As shown in FIG. 1 a , this technique results in the wire bond area  122  having nickel  116  and gold  118  overhangs that expose the copper layer  114  to the environment. As a result, the exposed copper  114  can diffuse or oxidize, thus deteriorating the conductivity of the wire bond area  122 . In steps  212 - 218 , the gold  118  is applied to both sides of the semiconductor package, and the solder mask  120  is deposed directly onto the gold  118  layer. Since solder mask and gold typically form weak connections and provide poor adhesion, these joints are very brittle and provides poor adhesion. 
     The present invention overcomes these problems by providing a manufacturing process that protects the copper circuitry and avoids the use of solder mask-gold connections. In particular, the multiple embodiments of the present invention develop the wire bond side of the semiconductor package prior to developing the ball attach side, and maintain electrical continuity with the copper surface of the ball attach side while forming the circuitry on the wire bond side. The present invention provides for the copper layer to be fully covered and protected by one or more layers of other material to improve the conductivity of the wire bond area. The present invention also applies gold only to the bond area as compared to the entire copper surface area representing a savings in precious metals. In the present invention, the solder masks are applied over copper, instead of gold, to strengthen the adhesive layer between the solder mask and the copper. 
     II. Nickel-Gold Surface Finish on Ball Attach Side 
     FIGS. 3 a - 3   e  illustrate multiple views of semiconductor package  300  manufactured by one embodiment of the present invention. Referring to FIG. 3 a , semiconductor package  300  includes several layers of metals, such as copper  114 , nickel  116  and gold  118 , on substrate  112 , with circuitry developed for wire bond area  302  (also referred to herein as the wire bond side) and ball attach area  124  (also referred to herein as the ball attach side). As shown in FIG. 3 a , solder mask  120  can also be applied to provide mechanical and electrical support to substrate  112 . Referring to FIG. 3 d , via hole or through hole  126  connects the wire bond circuitry to the ball attach circuitry. 
     Referring to FIGS. 4 a - 4   b , flowchart  400  represents the general operational flow of an embodiment of the present invention. More specifically, flowchart  400  shows an example of a process for manufacturing the semiconductor package of FIGS. 3 a - 3   e.    
     FIGS. 4 a - 4   b  begin at step  401 . At step  402 , the copper is reduced from 17 micro-meters (μm) to 5 μm by a controlled etching process. Next, at step  404 , via hole(s)  126  is mechanically drilled through substrate  112 . At step  406 , via hole(s)  126  and substrate  112  are copper plated to nominal thickness. Via hole(s)  126  is plugged with a thermo-curable ink at step  408 . Step  410  planarises the panels, removes the protruding ink and creates a uniform surface topography. Steps  408  and  410  differ significantly from the process shown in FIG. 2, where via hole  126  is filled by solder mask  120  at step  218  after the circuitry is etched on both sides of semiconductor package  100 . In the present invention, the via hole is filled before the circuitry is etched on either side. It is important to fill via hole  126  to prevent the dry film from breaking, while developing the circuitry in the subsequent process steps. 
     The present invention provides another significant improvement over FBG processes by developing the wire bond  302  and ball attach  124  sides separately. The wire bond  302  side is developed first, while maintaining electrical continuity with the copper surface on the ball attach  124  side. The exposed copper on the ball attach  124  side maintains the electrical continuity needed to deposit the nickel and gold. For instance, at step  412 , dry film is applied on the wire bond  302  side using a positive image, and the ball attach  124  side is covered with fully exposed dry film. At step  414 , the circuitry is etched on wire bond  302  side with an acidic cupric etchant or the like, as would be apparent to a person skilled in the relevant art(s). Using film as an etch resist on the wire bond  302  side allows finer lines and spaces and reads higher density. The process shown in FIG. 2 has a much stronger effect of underetch due to the presence of galvanic elements during the etching (i.e., Cu—Ni—Au interface). 
     At step  416 , the dry film is stripped on both sides of the panel. At step  418 , new dry film is applied on both sides. On the wire bond side, the gold finger or wire bond area  302  is developed, and on the ball side, the solder ball attach area  124  is developed. At step  420 , both sides are nickel and gold plated to nominal thickness. After this step, the top and sides of wire bond area  302  are fully covered with nickel  116  and gold  118  as compared to leaving a nickel and gold overhang as discussed above in regards to FIG. 1 a . In the present invention, on the wire bond side, the gold is only applied to the wire bond area  302  as compared to the entire copper surface as illustrated in FIG. 1 b  for semiconductor package  100 . This produces a substantial savings in precious metal cost. 
     At step  422 , the film is stripped on both sides, and at step  424 , dry film is applied on the wire bond  302  side completely exposing the entire dry film surface with UV light. At step  426 , the circuitry is etched on ball attach  124  side using an alkaline etching solution. At step  428 , the film is stripped on the wire bond  302  side. At step  430 , a photo imagible solder mask  120  is applied on both sides. At step  432 , the package strips are singulated by routing or stamping. Steps  434 - 440  provide for final cleaning, electrical testing, visual inspection and packing and shipping of the strips, or the like, as would be apparent to a person skilled in the relevant art(s). The control flow of flowchart  400  then ends as indicated by step  495 . 
     In this embodiment of the present invention, solder mask  120  is applied to the copper layer  114  on the wire bond  302  side. This solder over copper technique provides better adhesion than applying the solder mask  120  directly to gold, which produces a more brittle connection. The solder mask over copper technique has the potential of passing the Joint Electron Device Engineering Council (JEDEC) level II requirements. However, on the ball attach side, solder mask  120  is deposited on gold layer  118 . 
     III. Organic Solderable Protection Finish on the Ball Attach Side 
     In another embodiment, the present invention relates to manufacturing a semiconductor package by applying solder masks over copper, in lieu of gold, on both sides of the semiconductor package. The copper is protected from environmental exposure by depositing a layer of an organic solderable protection (OSP) material over the copper on the ball attach side. FIGS. 5 a - 5   e  illustrate multiple views of semiconductor package  500  manufactured by one embodiment of the present invention. 
     Referring to FIG. 5 a , semiconductor package  500  includes several layers of copper  114 , nickel  116  and gold  118  on substrate  112 , with circuitry developed for wire bond area  302  (also referred to herein as the wire bond side) and ball attach area  504  (also referred to herein as the ball attach side). As shown in FIG. 5 a , solder mask  120  can also be applied to provide mechanical and electrical support to substrate  112 . Referring to FIG. 5 d , via hole or through hole  126  connects the wire bond circuitry to the ball attach circuitry. FIG. 5 e  shows a layer of an organic solderable protectant (OSP)  506  can be applied to the ball attach  504  side to protect the copper layer  114  from the environment. 
     Referring to FIG. 6, flowchart  600  represents the general operational flow of an embodiment of the present invention. More specifically, flowchart  600  shows an example of a control flow for manufacturing the semiconductor package of FIGS. 5 a - 5   e.    
     FIG. 6 begins at step  601 . Steps  402 - 416  are identical to the process steps in control flow  400 . After step  416 , control flow  600  begins to differ significantly from the embodiment described in control flow  400 . At this point, control flow  600 , passes to step  602 . At step  602 , new dry film is applied on both sides of the substrate  112 . Unlike step  418  in control flow  400 , at step  602 , a nickel and gold layer is only deposited on the wire bond  302  side; therefore, in step  602 , only the gold finger or wire bond area  302  is developed. On the ball attach  504  side, the entire area is covered with exposed film. At step  604 , the wire bond  302  side is nickel and gold plated to nominal thickness. After step  604 , control flow  600  passes to step  422  as described in control flow  400 . 
     After step  422 , control flow  600  passes to step  606 . At step  606 , dry film is applied on both sides. The entire surface of the wire bond  302  side is completely exposed; however, on the ball attach side, only the ball attach area  504  is exposed. Control flow  600  then passes to step  426 , where the circuitry is etched on the ball attach  504  side, as described in control flow  400 . After step  426 , control flow  600  passes to step  608 . At step  608 , the film is stripped on both sides. 
     Control flow  600  then passes to steps  430 - 436  as described in control flow  400 . After electrical testing at step  436 , the flow passes to step  610 . At step  610 , a monolayer of an organic solderable protectant (OSP)  506 , such as Entek™ 56 which is available from Enthone-OMI, is applied to the ball attach  504  side to protect the exposed copper layer  114  from the environment. The OSP  506  is a transparent material making the copper layer  114  visible. The control flow then passes to steps  438 - 440 , as described in reference to control flow  400 . Afterwards, the control flow of flowchart  600  ends as indicated by step  695 . 
     In this embodiment of the present invention, solder mask  120  is applied to the copper layer  114  on both sides of semiconductor package  500 . This solder over copper technique provides better adhesion than applying the solder mask  120  directly to gold, which produces a more brittle connection. The solder mask over copper technique has the potential of passing the Joint Electron Device Engineering Council (JEDEC) level II requirements. On the ball attach  504  side, the copper layer  114  is covered and protected from the environment by OSP layer  506 , and on the wire bond  302  side, the copper layer is covered and protected from the environment by the nickel  116  and gold  118  layers. As a result, the copper layer  114  is protected from diffusion and oxidation, which in turn improves the conductivity of the wire bond  302  and ball attach  504  areas. 
     IV. Conclusion 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. For example, semiconductor packages  300  and  500 , and process flows  400  and  600  depict only two examples of a semiconductor package of the present invention. The semiconductor package can have multiple wire bond areas or ball attach areas. The metallization layers can include other metals or materials, or multiple layers of the same metals or materials. Thus, the present invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.