Abstract:
A solderable injection-molded integrated circuit substrate provides a mounting and interconnect structure for integrated circuits. Circuit traces within channels on the substrate provide interconnects that are isolated by the channel sides and solderable mounting contacts for Ball Grid Array (BGA) or wire-bondable integrated circuit dies. The substrate is injection-molded and then electroplated or seed plated and an etchant-resistive material is applied. The substrate is exposed to an etchant, removing the plated material from undesired locations and leaving the plated material in contact areas and trace areas within the channels. An integrated circuit die is then wire-bonded or solder ball attached to the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor packaging, and more specifically, to a solderable injection-molded substrate for providing electrical and mechanical connection to integrated circuit dies. 
     BACKGROUND OF THE INVENTION 
     Semiconductors and other electronic and opto-electronic assemblies are fabricated in groups on a wafer. Known as “dies”, the individual devices are cut from the wafer and are then bonded to a carrier. The dies must be mechanically mounted and electrically connected to a circuit. For this purpose, many types of packaging have been developed, including “flip-chip”, ball grid array and leaded grid array among other mounting configurations. These configurations typically use a planar printed circuit etched on the substrate with bonding pads and the connections to the die are made by either wire bonding or direct solder connection to the die. 
     The resolution of the printed circuit is often the limiting factor controlling interconnect density. Photo-etch and other processes for developing a printed circuit on a substrate have resolution limitations and associated cost limitations that set the level of interconnect density at a level that is less than desirable for interfacing to present integrated circuit dies that may have hundreds of external connections. 
     As the density of circuit traces interfacing an integrated circuit die are increased, the inter-conductor spacing must typically be decreased. However, reducing inter-conductor spacing has a disadvantage that migration and shorting may occur more frequently for lowered inter-conductor spacings, thus setting another practical limit on the interconnect density. 
     Therefore, it would be desirable to provide a method and substrate having improved interconnect density with a low associated manufacturing cost. It would further be desirable to provide a method and substrate having reduced susceptibility to shorting and migration between conductors. 
     SUMMARY OF THE INVENTION 
     A solderable injection-molded substrate and a method for manufacturing an injection-molded substrate generate a circuit pattern within a substrate having solderable wire-bonding or solder ball attach pads. A substrate is injection molded using a tool formed in the shape of the desired circuit pattern and conductor is deposited on the substrate. An etchant resistive compound is applied to the conductor-coated substrate and conductor is removed with an etchant. The conductor that remains on the substrate is plated with a plating suitable for soldering or wire-bonding and finally, an integrated circuit die is attached via wire-bonding, solder-ball attach or a combination of attachment techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial diagram depicting a substrate and a tool for injection-molding in accordance with an embodiment of the invention; 
     FIGS. 2A-2F are pictorial diagrams depicting various stages of preparation of a substrate in accordance with an embodiment of the invention; 
     FIG. 3 is a pictorial diagram depicting an integrated circuit in accordance with an embodiment of the invention; and 
     FIGS. 4A and 4B are pictorial diagrams depicting integrated circuits in accordance with alternative embodiments of the invention. 
    
    
     The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout. 
     DETAILED DESCRIPTION 
     Referring now to the figures and in particular to FIG. 1, a mold  10  in accordance with an embodiment of the present invention is depicted. Mold  10  is used to form a substrate in a novel process that permits embedding circuits beneath the top surface of a substrate and isolating the circuits in channels. Mold  10  comprises a machine having a plate  11  for supporting a thin metal tool foil  12 . Tool foil  12  is stamped to form an outline that conforms to a reverse image of desired contour of the top of the substrate after processing. A bottom plate  14 , which may include a thin metal tool foil  13  having features defining bottom channels in the substrate, is placed adjacent to the top tool foil  12 , and fluid substrate material is injected to form a substrate. The mold is then opened by separating bottom plate  14  and top plate  11  and the molded substrate is removed. 
     Insulative plastic materials suitable for injection molding are introduced into mold  10  and permitted to cure until the surface is not tacky. Suitable materials include Plaskon SMT-B-IRC and Nitto HC100XJAA, which are mold compound materials presently used in injection molding processes. 
     The mold tool foils can be made with existing processes that are used in the formation of stamps for manufacturing compact discs (CDs). In the CD manufacturing process, metal foil is stamped using a master that is created for the production of multiple foils. The foils are then attached (embedded) in a polymer resin to support the foils. To support the process of the present invention, metal foils can be made in the same manner, but may be reused. 
     Referring now to FIG. 2A, the first stage in the preparation of molded substrate  20  in accordance with an embodiment of the present invention is depicted. Substrate  20  has been removed from mold  10  and metal tool foils  12  and  13  have generated voids and channels in accordance with the figure. Dashed lines show that the voids and channels are localized features and that substrate  20  extends into and out of the plane of the figure. 
     Referring now to FIG. 2B, top copper plating  21  is seed plated or electrolytically deposited on the surface of substrate to form plated substrate  20 B. Bottom copper plating  24  may also be applied depending on whether or not a double-sided substrate is being produced. Top channel  22  is plated to form a conductive circuit path, a via  23  is plated through to connect to a bottom channel  25 . The sides of via  23  are angled to promote the growth of the plating material through the void in substrate  20 , yielding a more reliable plating process. 
     Next, as depicted in FIG. 2C, a permanent etchant resist material is applied to substrate  20 B to produce coated substrate  20 C. Then, as illustrated in FIG. 2D, the etchant resist is planarized to remove the etchant resist from areas in which the copper plating is to be removed, and leaving the etchant resist  27 A in areas in which the copper plating is to remain. 
     Next, as depicted in FIG. 2E, the copper plating is etched and the permanent resist is removed, leaving circuit channels  25 A and  22 A, via  23 A and a solder ball mounting area  28 . Circuit channel  22 A can be used for wire-bonding connections from a die mounted on the substrate, or may be circuit traces extending out of the plane of the figures for routing circuit traces. 
     Finally, referring to FIG. 2F, substrate  20 E is electroplated with a material resistant to oxidation such as gold or nickel  29 , preparing the substrate for solder ball attachment or wire-bonding of dies or external circuits by producing plated channels  22 B and  25 B. 
     While the figures illustrate three conductive circuit channels, the figures are depicting only a portion of the total substrate. More than a hundred circuit channels  22 A and  25 A will generally be used in an integrated circuit design and may be oriented in any direction within the surface of substrate  20 E. Additionally, materials other than copper may be used, depending on the process used. For example if etching is not necessary for a particular circuit, gold foil may be applied to the channels formed in an injection-molded substrate. The present invention provides a process for forming circuits within channels in a substrate that are below the top surface of the substrate. This an improvement over the present state of the art, which generally provides only surface conductors. The channels formed by embossing place the conductors below the surface and the conductors are thereby insulated from adjacent conductors by the substrate. 
     Referring now to FIG. 3, an integrated circuit  30  in accordance with an embodiment of the invention is depicted. A die  31  having electrical contact  32  is attached to substrate  20 F by a solder ball  35 . The solder ball provides electrical and thermal connection from die  31  to substrate  20 F. A wire  34  is bonded to a wire-bond area  33  on die and to channel  22 B providing a combined wire-bond (for top side connections) and solder ball attach (for bottom side connections) for die  31 , but exclusively wire-bond or solder ball techniques may be used. Additionally, die may be mounted in a recess molded into substrate, rather than surface mounting as depicted in FIG.  3 . Ball grid array (BGA) connections for the integrated circuit package are provided by solder balls  36  attached to the bottom channels (or pads) formed in substrate  20 F. 
     While channels  22 B and  25 B are shown as contact points for integrated circuit die  31  connections, channels  22 B and  25 B may extend in the plane of the figure to any point on substrate  20 F providing routing of the electrical connections to die  31  and possibly other components mounted on substrate  20 F, including multiple integrated circuit dies. Channels may be formed purely for interconnect or for solder ball attachment or a combination. 
     Referring now to FIG. 4A, an integrated circuit in accordance with an alternative embodiment of the invention is depicted. Substrate  20 G is injection molded and plated by the above-described process, but includes a recess for mounting a die  31 A and wire-bond pads for connection of die  31 A via wires  34 A. Solder ball  36 A is attached to circuit material having a via through to a conductive surface  41  attached to the bottom of substrate  20 G, which may be electrically conductive, thermally conductive or both. Solder ball  36 B provides a connection to circuit material forming traces, which may be connected to the wire-bond pads. The purpose of FIG. 4A is to illustrate examples of alternative structures that may be generated by the techniques of the present invention, such as vias to a conductive plane and die mounting within the plane of the substrate. 
     Referring now to FIG. 4B, an integrated circuit in accordance with yet another alternative embodiment of the invention is depicted. Substrate  20 H is injection molded and plated by the above-described process, but includes a through-hole for attaching a solder ball  36 B to a mounting post from die  31 B and flip chip post terminals for connection of die  31 B via the integral flip-chip posts. Solder ball  36 B secures die  31 B mechanically while the posts  42  provide electrical connections. Circuit material within the mounting post area also provides electrical connection to the mounting post. 
     The above description of embodiments of the invention is intended to be illustrative and not limiting. Other embodiments of this invention will be obvious to those skilled in the art in view of the above disclosure and fall within the scope of the present invention.