Abstract:
An integrated circuit substrate having laser-exposed terminals provides a high-density and low cost mounting and interconnect structure for integrated circuits. The laser-exposed terminals can further provide a selective plating feature by using a dielectric layer of the substrate to prevent plating terminal conductors and subsequently exposing the terminals via laser ablation. A metal layer may be coated on one or both sides with a dielectric material, conductive material embedded within the dielectric to form conductive interconnects and then coating over the conductive material with a conformal protective coating. The protectant is then laser-ablated to expose the terminals. A dielectric film having a metal layer laminated on one side may be etched and plated. Terminals are then laser-exposed from the back side of the metal layer exposing unplated terminals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application entitled “INTEGRATED CIRCUIT SUBSTRATE HAVING LASER-EMBEDDED CONDUCTIVE PATTERNS AND METHOD THEREFOR”, Ser. No. 10/138,225 filed May 1, 2002, now U.S. Pat. No. 6,930,256, having at least one common inventor and assigned to the same assignee. 
     The present application is also a continuation-in-part of U.S. patent application entitled “INTEGRATED CIRCUIT SUBSTRATE HAVING EMBEDDED BACK-SIDE ACCESS CONDUCTORS AND VIAS”, Ser. No. 10/392,737 filed Mar. 19, 2003, now abandoned, by the same inventors and assigned to the same assignee. The specifications of the above-referenced patent applications are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor packaging, and more specifically, to a substrate having laser ablated terminals for conductors for providing an electrical interface between a die and external terminals in an integrated circuit package. 
     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. 
     The above-incorporated patent application “INTEGRATED CIRCUIT SUBSTRATE HAVING LASER-EMBEDDED CONDUCTIVE PATTERNS AND METHOD THEREFOR” provides increased conductor density and decreased inter-conductor spacing via laser-embedded circuit technologies, but still has a terminal pitch limitation dictated by traditional masking techniques (if any are used) for masking the wirebond points and ball/land grid array terminal mounting points of the exposed copper at the surfaces of the substrate. 
     The above-incorporated patent application “INTEGRATED CIRCUIT SUBSTRATE HAVING EMBEDDED BACK-SIDE ACCESS CONDUCTORS AND VIAS” provides increased conductor density and lower manufacturing cost by using a prefabricated substrate having metal plated, printed or etched on a dielectric. However, a solder mask is required to prevent plating of the ball/land grid array terminals during the plating process, adding an additional step to the overall manufacturing process. Additionally, other techniques commonly in use for selectively plating wire bonding areas with a Ni/Au or other plating material commonly use a resist process to mask off the ball/land grid array terminals that would otherwise be exposed after the substrate has been drilled or punched. 
     Therefore, it would be desirable to provide methods and substrates having improved interconnect density with a low associated manufacturing cost. It would further be desirable to provide a method and substrate wherein external terminals of a substrate may be selectively plated without requiring a masking or resist process. 
     SUMMARY OF THE INVENTION 
     The above objectives of providing improved interconnect density and a low associated manufacturing cost and providing improvements in selectively plating terminals without a masking or resist process are provided in a substrate and method for manufacturing a substrate. Laser-ablation is used to remove a coating or drill holes through a substrate to expose the terminals after plating and other manufacturing procedures have been completed. 
     Unplated terminals may be generated on a dielectric film having a laminated metal layer by imaging and etching the metal layer and then dip-plating without the use of any plating resistive material. The terminals are laser-exposed by ablating the film from the back-side of the metal layer to form terminal connection points. 
     Alternatively, a metal layer may be coated with a dielectric material on one or both sides and laser-embedded conductors placed in the dielectric layers and covered with a conformal coating. Wirebond and/or ball/land grid array terminals are laser-exposed by ablating the conformal coating only above the terminals, providing a protected surface having only the terminal locations exposed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A–1G  are pictorial diagrams depicting a cross-sectional view of stages of preparation of an integrated circuit substrate in accordance with an embodiment of the present invention; 
         FIGS. 2A and 2B  are pictorial diagrams depicting integrated circuits in accordance with embodiments of the present invention; 
         FIGS. 3A–3G  are a pictorial diagrams depicting a cross-sectional view of stages of preparation of an integrated circuit substrate in accordance with another embodiment of the present invention; and 
         FIGS. 4A and 4B  are pictorial diagrams depicting integrated circuits in accordance with other embodiments of the present 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  FIGS. 1A through 1G , a cross-sectional view of stages of preparation of a substrate in accordance with an embodiment of the present invention is shown. A first substrate stage  10 A, having a dielectric layer  12  and an adhesive layer  14  disposed thereon is punched to add sprocket holes  16  forming substrate stage  10 B of  FIG. 1B . Sprocket holes  16  are added for machine feeding the substrate stages, which are generally formed on a continuous tape, through processing machines for performing the method that prepares the substrate of the present embodiment. Substrate stage  10 A may be fabricated from a dielectric film tape such as a polymide film. Alternatively, substrate stage  10 A may be fabricated from a rigid or semi-rigid dielectric material such as polyimide resin having, in accordance with another embodiment of the present invention. Suitable tape materials are KAPTON, APICAL, UPILEX and various liquid crystal polymers (LCPs) may also be used to form the core of the substrate of the present invention. Rigid substrate layers may be cured epoxy resin, FR4, or other substrate materials commonly used to form integrated circuit substrates. 
     Next, a metal conductive layer  18  (generally a copper film) is laminated to substrate stage  10 B, using adhesive layer  14  to secure the lamination forming substrate stage  10 C of  FIG. 1C . Substrate  10 C is imaged and etched for form a conductive pattern  18 A yielding substrate stage  10 D of  FIG. 1D . Substrate state  1 D is then immersion plated to deposit a plating forming plated conductive pattern  18 B (generally Ni/Au) on conductive pattern  18 A yielding substrate stage  10 E of  FIG. 1E . Unique to the method and structure of substrate stage  10 E is that no masking is required to selectively plate conductive pattern, which is done for environmental protection and to yield wire-bondable contact surfaces on conductive pattern  18 B. It is desirable not to plate solder terminal locations, as eventual diffusion of gold into the solder bond weakens the solder bond, causing early failures due to fracture. Because the solder ball terminals for the external contacts of substrate stage  10 D have not been exposed, substrate stage  10 D can be plated without masking to yield substrate stage  10 E without plating the external contact terminals. 
     Next, substrate stage  10 E is laser-ablated from the back side to expose terminals for solder ball attach through holes  20  yielding substrate stage  10 F. Finally, substrate stage  10 F is dipped in an organic solderable protectant (OSP) that coats the terminal locations  22 , yielding final substrate  10 G. 
     The above-described process yields an advantage over prior solutions that typically punch all voids in dielectric layer  12  when sprocket holes  16  are punched. Then, in order to selectively plate the conductive pattern (without plating the solder ball terminals) a plating mask is applied through the solder ball terminal holes. The method of the present invention and the consequent resulting structure, provides an advantage of eliminating the masking step in order to produce substrate  1 G without gold present in the terminal/solder interfaces. 
     Referring now to  FIG. 2A , an integrated circuit  21 A, in accordance with an embodiment of the invention is shown. A die  26  is mounted on substrate  10 G via a plurality of solder balls or posts  24  in a flip-chip configuration. External terminal solder balls  22  are added to the external terminal locations forming a complete integrated circuit package that may be subsequently encapsulated. 
     Referring now to  FIG. 2B , another integrated circuit  21 B is shown exemplifying a wire bonded configuration. A die  26 A is mounted to substrate  10 H with an adhesive (generally epoxy) and wires  24 A are bonded between plated lands on the conductive pattern side of substrate  10 H. Solder balls  22  are added to the external terminal locations. Substrate  10 H is manufactured according to the same steps as substrate  10 G, but has a different circuit pattern and die mounting area for accommodating wire-attach type die  26 A. 
     Referring now to  FIGS. 3A–3G , stages of preparation of a substrate in accordance with another embodiment of the invention are shown. A first stage of preparation of substrate  30 A is shown as a metal layer  32 , which is etched to provide holes  34 . Metal layer  32  is generally a copper core that may be etched or die-cut, but other suitable metal layers may be used for form the core of the substrate of the present invention, such as a copper-INVAR-copper laminate. The ratio of copper to Invar can be varied to provide adjustment of the coefficient of thermal expansion (CTE) of the substrate. Holes  34  are generated in metal layer  32  to permit the passage of circuit paths through metal layer  320 , while avoiding electrical contact with metal layer  32 . Referring now to  FIG. 3B , the second stage of preparation forming substrate stage  30 B is shown. A dielectric outer layer  36  has been added to the top and bottom surface of metal layer  32  and can be provided by injection molding a plastic material around metal layer  32  or by laminating a dielectric such as KAPTON film or PTFE on each side of metal layer  32 . 
     Referring now to  FIG. 3C , the next stage is depicted. Substrate  30 B is laser-ablated to form substrate  30 C having vias  38 , blind vias  40  and conductor channels  42 . Blind vias  40  show a conical shape, which is preferred for addition of conductive material and can be generated by varying the laser angle or beam diameter as dielectric material  36  is ablated. 
     Next, referring to  FIG. 3D , the next step in the preparation of substrate  30 C providing a substrate  30 D having conductive circuit paths. Conductive material is added within channels  42 , blind vias  40  and through vias  38  to provide conductive paths  42 A conductive blind vias  40 A and conductive through vias  38 A. The conductive material may be a silver or copper paste that is screen printed into channels  42 , blind vias  40  and through vias  38 , and planarized to remove conductive material on the surface of outer dielectric layer  36  after printing. Alternatively, an electroplating process (generally copper electroplate) can be used to add conductive material within channels  42 , blind vias  40  and through vias  38  and a planarization process or chemical etching process can be used to remove excess conductive material on the surface of dielectric layer  36 . 
     Next, a conformal coating  44  is applied to both sides of substrate state  30 D and cured, yielding coated substrate stage  30 E. Then, only terminal areas (wire bond lands and solder ball lands) are laser-ablated in conformal coating  44  to expose the terminal connection areas for solder ball and wire-bond lands  46 . Next, plating  48  is applied to the exposed terminals (generally Ni/Au) to provide wire-bondable and solderable surfaces for attachment of wires and/or solder balls forming final substrate  30 G. 
     Nickel-Gold is generally used to provide a barrier migration layer and to provide electrical contact for wire or chip bonding in subsequent manufacturing steps. In general, silver-nickel is an appropriate electroplating material and if a silver paste was used to form conductive channels  42 A, blind vias  40 A and through vias  38 A, plating may not be needed to provide solderable conductive connections, but may be added to eliminate oxidation. 
     Referring now to  FIG. 4A , an integrated circuit  21 A, in accordance with an embodiment of the invention is shown. A die  26  is mounted on substrate  30 G via a plurality of solder balls or posts  24  in a flip-chip configuration. External terminal solder balls  22  are added to the external terminal locations forming a complete integrated circuit package that may be subsequently encapsulated. 
     Referring now to  FIG. 4B , another integrated circuit  41 B is shown exemplifying a wire bonded configuration. Die  26 A is mounted to substrate  30 H with an adhesive (generally epoxy) and wires  24 A are bonded between plated lands on the conductive pattern side of substrate  30 H. Solder balls  22  are added to the external terminal locations. Substrate  30 H is manufactured according to the same steps as substrate  30 G, but has a different circuit pattern and die mounting area for accommodating wire-attach type die  26 A. 
     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.