Abstract:
A die-attach method and assembly using film and epoxy bonds speeds manufacturing for large die assemblies while providing improved bond characteristics. An adhesive film defining an epoxy flow mask is attached to the die or substrate, epoxy is dispensed within the epoxy flow mask area and the die is then bonded to the substrate. The film controls the flow of the epoxy, preventing spillover. Additionally, the epoxy area can be made small with respect to the die size, reducing the time required to dispense the epoxy and reducing the amount of epoxy material required.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to semiconductor packaging, and more specifically, to An imprinting method and an imprinted 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 
   An imprinted substrate and a method for imprinting a substrate use imprinting to generate a circuit pattern within a substrate. A substrate is embossed using a tool formed in the shape of the desired circuit pattern and conductor is applied to the embossed pattern. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial diagram depicting a substrate and a tool for embossing in accordance with an embodiment of the invention; 
       FIGS. 2A–2D  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; 
       FIGS. 4A–4F  are pictorial diagrams depicting various stages of preparation of a substrate in accordance with an alternative embodiment of the invention; 
       FIG. 5  is a pictorial diagram depicting an integrated circuit in accordance with an alternative embodiment of the invention; 
       FIGS. 6A–6C  are pictorial diagrams depicting various stages of preparation of a substrate in accordance with alternative embodiments of the invention; and 
       FIGS. 7A–7C  are pictorial diagrams depicting various stages of preparation of a substrate in accordance with other 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 substrate  13  and an embossing tool  10  in accordance with an embodiment of the present invention are depicted. Embossing tool  10  is used to form substrate  13  in a novel process that permits embedding circuits beneath the top surface of substrate  13  and isolating the circuits in channels. Embossing tool  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 substrate  13  after processing. A force F is applied between substrate  13  and plate  11  and the substrate material flows to conform to the contour of tool foil  12 . Substrate  13  comprises a resin layer  14  that is deformable by the above-described embossing technique and a backing layer of copper  15 . While the embodiment of  FIG. 1  depicts single-sided embossing of a substrate having a backing layer, other embodiments of the invention extend to stand-alone substrates embossed from a resin material having no backing layer. Both single-sided and double-sided embossing processes may be used and the resulting circuits may form plated-through holes, embedded circuit traces, etched circuit traces and vias. All of the techniques illustrated in the embodiments of the invention may be applied to both sides of a substrate that has no backing layer, or has resin layers deposited on both sides of a metallic backing layer. Suitable materials for an embossable substrate are plastic resins such as PLASKON SMT-B-1RC, NITTO HC100XJAA or Liquid Crystal Polymers (LCPs) such as Rogers R/MAX 3700, R/MAX 3800, BIAC CC, or SUPEREX. 
   The embossing tool foil 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 substrate  13  in accordance with an embodiment of the present invention. Substrate  13 A has been deformed by tool foil  12  such that voids and indentations in accordance with the figure are generated in resin layer  14 . Referring now to figure  2 B, copper plating  16  is seed plated or electrolytically deposited on the surface of substrate  13 A to form plated substrate  13 B. Next, as depicted in  FIG. 2C , a permanent etchant resist material is applied to substrate  13 B and is planed to conform to the top of copper plating  16 . Then, as illustrated in  FIG. 2D , the copper plating  16  is etched and the permanent resist is removed, leaving a circuit channel  16 A and a conductive post mounting area  16 B for mounting a flip chip mechanical bonding post. Circuit channel  16 A can be used for electrically connecting terminals of a flip-chip package, or may be circuit traces extending out of the plane of the figures for routing circuit traces. 
   While the figures illustrate two conductive circuit channels  16 A, the figures are depicting only a portion of the total substrate. More than a hundred circuit channels  16 A will generally be used in an integrated circuit design and may be oriented in any direction within the surface of substrate  13 . 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 embossed 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 flip-chip die  31  having electrical terminal posts  32  and a mounting post  34  is attached to substrate  13 D by a solder ball  35 . The solder ball provides electrical and thermal connection from flip-chip die  31  to substrate  13 D via post mounting area  16 B formed from remaining copper conductor  16 . Channels  16 A contact electrical terminal posts  32 , providing contact to the electrical terminals. Since the circuits forming channels  16 A may extend in the plane of the figure to any point on substrate  13 D, routing of the electrical connections to terminal posts  32  may be made to other locations on substrate  13 D. Multiple dies may be mounted on substrate  13 D and the channels  16 A used to interconnect the various dies. 
   Referring now to  FIG. 4A , an alternative embodiment of the present invention is depicted. Following the process of  FIGS. 2A–2C , substrate  13 C may be prepared in an alternative process to that illustrated above. Substrate  13 C of  FIG. 2C  is coated on both sides with a photo-sensitive etch resist material  41 . Next as illustrated in  FIG. 4B , portions of etch resist material  17  is removed by an imaging process, exposing copper plating  16  in areas for subsequent plating. Then, as illustrated by  FIG. 4C , the exposed areas of copper plating  16  are plated with a material resistant to chemical etchant such as nickel/gold to form wire bonding pads  42 . The remaining photo-etch resist material  17  is removed, yielding substrate  13 G of  FIG. 4D . Circuit material  16  is then etched to remove the portions uncovered by permanent etch resist, forming channels  16 B and interconnect  16 C of substrate  13 H of  FIG. 4E . Finally, the permanent etch resist material is removed, leaving the prepared substrate  13 J of  FIG. 4F . 
   Referring now to  FIG. 5 , an integrated circuit  50  is depicted in accordance with an alternative embodiment of the invention. Prepared substrate  13 J is connected to a die (not shown) by wires  51  that are bonded to wire bonding pads  42 , providing an electrical connection to the die. A ball grid array (BGA) solder ball is applied to the plated area  42 A on the backside of substrate  13 J and is electrically connected to wire bonding pads  42  via interconnect  16 C. Solder ball  35 A provides an electrical terminal for external connection to other circuits as in a typical BGA arrangement. Channels  16 B are used to route connections within substrate  13 J and may provide connection to flip-chip mounted dies in accordance with the earlier-described embodiment of the invention, forming a substrate that embodies both embodiments of the present invention. 
   Referring now to  FIG. 6A , an alternative double-sided substrate preparation is disclosed in accordance with an embodiment of the invention. Embossing tool  10 B comprises a machine having a top plate  11 A for supporting a thin metal tool foil  12 A for forming the top of substrate  13 K. Tool foil  12 A is stamped to form an outline that conforms to a reverse image of desired contour of the top of substrate  13 K after processing. Embossing tool  10 C comprises a machine having a bottom plate  11 B for supporting a thin metal tool foil  12 B for forming the bottom of substrate  13 K. Tool foil  12 B is stamped to form an outline that conforms to a reverse image of desired contour of the top of substrate  13 K after processing. A force F is applied between embossing tool  10 B and embossing tool  10 C, embossing substrate  13 K so that the substrate material flows to conform to the contour of tool foils  12 A and  12 B. Substrate  13 K as depicted comprises a top resin layer  14 B and a bottom resin layer  14 C deposited over a metal layer  15 A of copper. The metal layer is perforated by an resist-etch process or other means, so that plated through holes may be made through substrate  13 K, but a double-sided substrate may be embossed without metal layer  15 A or with multiple metal layers. Metal layer  15 A may be used to provide an electrical and thermal conductive path for devices mounted on substrate  13 K after the substrate is prepared. 
   Referring now to  FIG. 6B , substrate  13 L is depicted after embossing in accordance with  FIG. 6A . Depressions are made through substrate  13 L for generation of plated-through holes, through to metal layer  15 A for contact vias to metal layer  15 A, and within resin layer  14 B for circuit traces. Substrate  13 L is then plated by depositing metal, adding etch resist and then etching as described above for single-sided substrates. Referring now to  FIG. 6C , the final plated substrate is depicted. Plated-through hole  17 B provides insulation from the metal layer  15 A, since the embossing process removed an area of substrate  13 L that was smaller in diameter than the perforation in metal layer  15 A, but contact could be made with metal layer  15 A for other plated-through connections. Via  17 C provides contact to metal layer  15 A from one side of substrate  13 L and traces  17 C provide circuit paths. Many combinations of embossing and etching may be used to provide multi-layer substrates with or without incorporated metal planes. 
   Referring now to  FIG. 7A , an alternative double-sided substrate preparation without a metal layer is disclosed in accordance with yet another embodiment of the invention. Embossing tool  10 D comprises a machine having a top plate  11 C for supporting a thin metal tool foil  12 C for forming the top of substrate  13 M. Tool foil  12 C is stamped to form an outline that conforms to a reverse image of desired contour of the top of substrate  13 M after processing. Embossing tool  10 E comprises a machine having a bottom plate  11 D for supporting a thin metal tool foil  12 D for forming the bottom of substrate  13 M. Tool foil  12 D is stamped to form an outline that conforms to a reverse image of desired contour of the top of substrate  13 M after processing. A force F is applied between embossing tool  10 D and embossing tool  10 E, embossing substrate  13 M so that the substrate material flows to conform to the contour of tool foils  12 C and  12 D. Substrate  13 M as depicted comprises only a resin layer without metal layers. 
   Referring now to  FIG. 7B , substrate  13 N is depicted after embossing in accordance with  FIG. 7A . Depressions are made completely through substrate  13 N for generation of plated-through holes, and within substrate  13 N for circuit traces. Substrate  13 N is then plated by depositing metal, adding etch resist and then etching as described above for single-sided substrates. 
   Referring now to  FIG. 7C , the final plated substrate is depicted. Plated-through hole  17 D and circuit traces  17 E and  17 F have been added via the plating and selective etching processes described above for the single-sided embodiment of the invention. 
   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.