Patent Publication Number: US-2007108603-A1

Title: Method of putting isolated metallic interconnections onto a metallic substrate

Description:
BACKGROUND OF THE INVENTION  
      This invention relates to a substrate and a method for putting isolated conductive (e.g., metallic) interconnections on the substrate and supporting electronic devices thereon for a predefined circuit design.  
      The completed package of the present invention is similar to a QFN (Quad Flat No-lead) package with the addition of a nonconductive layer and a layer of conductive interconnections. The QFN package is a leadless package with terminal pads on all sides, which provide an option for mechanical and thermal enhancement. The QFN package is either square or rectangular in shape. Typically, a QFN package supports and encases semiconductor dies or chips for protection against external elements.  
      In producing a conventional QFN package for a semiconductor die or chip, a paddle supports the die or chip above the inner ends of a plurality of terminals. The die attaches to an upper surface of the paddle using an adhesive. Then bond wires electrically couple or interconnect the die to terminals of the package to provide external package interconnections. The QFN package is enclosed by an encapsulant, typically a moldable resin material. The resin material extends upwardly above the semiconductor die and the top of the bond wire loops. The encapsulant protects the semiconductor die or chip and the bond wires from an external surrounding environment.  
      In general, conventional plastic and ceramic packages incorporate several common elements. The common elements include a sealed package enclosure, a die attachment area, bond wires for providing electrical communication with bond pads on the die, and leads or terminals for external connectivity through the package.  
      The present invention relates to the QFN package and is directed to a substrate that includes layers for which a predefined circuit design is implemented in lieu of a complete electronic system/device. The present invention is further directed to a method of fabricating a substrate with layers that include conductive traces applied onto a nonconductive layer. One layer comprises conductive traces instead of typical bond wires, in which the traces are generally planar. The advantage of using conductive traces on top of a non-conductive layer is the ability to put an entire working system and/or circuit in at least one of a plurality of paddle. Other advantages of the conductive traces over the bond wires are that the conductive traces are shorter in length with no loops created above the layer, which makes the completed package thinner.  
     BRIEF SUMMARY OF THE INVENTION  
      Briefly stated, a preferred embodiment of the present invention comprises a substrate for supporting one or more electronic devices. The substrate comprises a first layer that includes a plurality of interconnected metallic frames laid out in a predetermined pattern. Each frame has a frame member that surrounds at least a portion of the frame, one or more metal pads and a plurality of metal tabs. Each tab connects to at least one of the frame member and/or a metal pad. A second generally planar nonconductive layer is secured to and covers at least a portion of a first surface of one or more of the metal pads. The nonconductive layer insulates the covered portion of the first surface of at least one metal pad. A third generally planar layer comprises a plurality of conductive traces applied to at least a portion of the nonconductive layer. Each trace has a first surface secured to and covering at least a portion of the nonconductive layer and a second surface that receives and supports electronic devices.  
      The present invention provides a preferred method of fabricating a substrate that supports one or more electronic devices. The method includes providing a first layer that has a plurality of interconnected metallic frames arranged in a predetermined pattern. Each metallic frame includes a frame member that surrounds at least a portion of the frame, one or more metal pads, and a plurality of metal tabs. Each metal tab connects to the frame member and/or at least one or more metal pads. A second generally planar nonconductive layer is secured to and covers at least a portion of a first surface of at least one of the metal pads. The nonconductive layer insulates the covered portion of the first surface of at least one metal pad. A third generally planar layer that has a plurality of conductive traces is provided. Each trace has a first surface that is secured to and covers at least a portion of the nonconductive layer and a second surface for receiving an electronic device. The method further includes attaching the electronic device to at least one conductive trace then encasing the substrate and the supported electronic device with an encapsulant material. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
      In the drawings:  
       FIG. 1  is a perspective view of a substrate having a plurality of interconnected metallic frames in accordance with a preferred embodiment of the present invention;  
       FIG. 2  is a perspective view of the substrate of  FIG. 1  laid out in a matrix of 2-by-4 interconnected frames;  
       FIG. 3  is a top plan view of the substrate of  FIG. 1  arranged in a linear format of interconnected frames;  
       FIG. 4  is a top plan view of the substrate of  FIG. 1  arranged in a linear combination of 2-by-4 matrices;  
       FIG. 5  is a perspective view of the substrate of  FIG. 1  identifying a single frame arranged in a matrix of 2-by-4 interconnected frames;  
       FIG. 6  is an enlarged perspective view of the details of a single frame of the substrate of  FIG. 1 ;  
       FIG. 7  is a greatly enlarged perspective view of some of the of the metal pads, metal tabs and a portion of the frame member for a single frame of the substrate of  FIG. 1 ;  
       FIG. 8  is a perspective view of a single frame showing some of the elements;  
       FIG. 9  is a perspective view of a completed frame supporting a plurality of electronic devices; and  
       FIG. 10  is an exploded perspective view of the elements (e.g., substrate, layers, devices) of a completed frame supporting a plurality of electronic devices according to an embodiment of the present invention as illustrated in  FIG. 9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to the drawings in detail, wherein like reference numerals indicate like elements throughout, there is shown in  FIG. 1 a  preferred embodiment of a substrate  100  in accordance with the present invention having a plurality of interconnected metallic frames  120  laid out in a predetermined pattern or format. The predetermined format as shown in  FIG. 1  is laid out in a m-by-n matrix, where m is greater than one and n is greater than one. As illustrated, the matrix is 2-by-4, but it could be some other matrix.  FIG. 2  shows a plurality of eight interconnected frames supported and held together by an external frame member  150 .  
      Referring to  FIG. 5 , there is shown an area  101  for the frame  120  in a predetermined pattern of a 2-by-4 matrix of the substrate  100 , in which the frame  120  is in a first position of the 2-by-4 matrix. A detailed description for a single frame  120  according to a preferred embodiment is set forth below and as shown in  FIG. 6 .  
       FIG. 3  shows another preferred embodiment of a substrate  300 , in which a plurality of interconnected metallic frames  120  is laid out in a predetermined pattern. The predetermined pattern resembles another m-by-n matrix, where m equals 1 and n equals 8 (e.g., 1-by-8). The substrate  300  has an outer perimeter that surrounds the interconnected frames  120  and represents the frame member  150 . The frame member  150  surrounds at least a portion of each interconnected frame  120  on the substrate  300 . Each frame  120  on substrate  300  is a preferred embodiment of the present invention and is described in detailed as shown in  FIG. 6 . The predetermined pattern of frames on a substrate is laid out and resembles an m-by-n matrix that m and n can have any value, in which the matrix creates other predetermined patterns of interconnected metallic frames  120  according to additional embodiments of the present invention.  
      In yet another preferred embodiment, a plurality of interconnected frames  120  is laid out in another predetermined pattern as shown in  FIG. 4  on a substrate  400 . The predetermined pattern resembles a linear combination of m-by-n matrices, where n and m are both greater than one. For example,  FIG. 4  shows a substrate  400  that has five individual groups of interconnected metallic frames  120  that are laid out in a linear combination of five 2-by-4 matrices. Each 2-by-4 matrix is separated from an adjacent matrix by the frame member  150  that surrounds at least a portion of each frame  120  for each group. The substrate  400  is one example of a preferred embodiment, it is not limiting in any way to other embodiments and therefore, the substrate can have other group arrangements or patterns of interconnected frames  120  with fewer or more groups. For example, substrate  400  can have more or less than five groups of matrices and the frames  120  in each matrix group, can be laid out in other predetermined patterns of m-by-n matrices. In addition, the predetermined pattern for each group need not be the same on the substrate. For example, a substrate can have three groups of matrices of interconnected frames  120 , with a first group as a 2-by-4 matrix, a second group as a 2-by-3 matrix and the third group as a 2-by-2 matrix. One skilled in the art would recognize that a substrate could have any number of groups and each group has a form of a m-by-n matrix.  
      Referring now to  FIG. 6 , this is a preferred embodiment of the single frame  120  and the following describes the details of the frame  120 . The frame  120  has four metal pads  167 ,  167   a ,  168  and  168   a , twenty-eight metal tabs  170 , and one frame member  150 . Frame  120  is an exemplary embodiment for all substrate embodiments of the present invention. Two of the metal pads  167 ,  167   a  generally have a uniform shape including four peripheral sides  163 ,  164 ,  165 ,  166 . Two of the peripheral sides  163 ,  164  are internal and adjacent to an open space  180  and the other two peripheral sides  165 ,  166  are external and connected to some of the metal tabs  170 . The other two metal pads  168 ,  168   a  have a generally rectangular shape including four peripheral sides. Three of the peripheral sides  163 ,  165 ,  166  are generally straight and parallel to at least one internal edge  151  of the frame member  150 . The fourth peripheral side  164  has a step-like shape that partially extends into an adjacent metal pad  168  or  168   a  without connecting the metal pads  168 ,  168   a  together. The step-like sides  164  of the metal pads  168  and  168   a  are separated by the open space  180  and are electrically isolated from the other metal pads  167 ,  167   a . Each external peripheral side  165 ,  166  of the metal pads  167 ,  167   a ,  168 ,  168   a  connects to at least one metal tab  170 , in which the metal tab  170  connects to the internal edge  151  of the frame member  150 .  
      The metal pads  167 ,  167   a ,  168 ,  168   a  can have any shape, size and number of peripheral sides, in which the shape does not have to be uniform, which all are selected according to a predefined design. In addition, the metal pads  167 ,  167   a ,  168 ,  168   a  are made of copper, but could be made of other conductive materials such as copper alloy, nickel and nickel alloys. The metal pads  167 ,  167   a ,  168 ,  168   a  are used to support and interconnect one or more electronic devices (not shown) according to a predetermined circuit design.  
      Referring to  FIG. 7 , the twenty-eight metal tabs  170  are generally rectangular and have four peripheral sides or ends  171 ,  172 ,  173 ,  174 . Two of the peripheral sides or ends  171 ,  173  are generally short and are perpendicular to the internal edge  151  of the frame member  150 , in which side  171  is opposite of side  173 . The short peripheral sides or ends  171 ,  173  of the metal tabs  170  do not connect to anything and are exposed to an open space  190  between adjacent metal tabs  170 . The open space  190  connects to the open space  180  at least once on each side of the frame  120 . The open space  190  is positioned between the peripheral sides  165  and  166  of the metal pads  167  and  168  and the internal edge  151  of the frame member  150 . The other two peripheral ends  172 ,  174  are generally long with one end  172  connecting to the internal edge  151  of the frame member  150 . The opposite end  174  of metal tabs  170  may or may not be connected to the metal pads  167 ,  168 , which is based on a predefined design. The opposite end  174  can also have a step-like shape end portion, in which the step-like end  174  partially connects to the metal pads  167 ,  168  with the remaining portion remaining unconnected. Referring again to  FIG. 6 , some metal tabs  170  have one end  172  connected to the frame member  150  and the opposite end  174  remains unconnected. Generally, all metal tabs  170  vary in length, but all can have the same length according to a predefined design or embodiment. All metal tabs  170  connect to the frame member  150  using the internal edge  151  while some metal tabs connect to the metal pads  167  and  168 .  
      The metal tabs  170  are made of copper, but could be made of other conductive materials such as copper alloys, nickel and nickel alloys. The metal tabs  170  are used to connect and support the metal pads  167  and  168  to the frame member  150 . In addition, the metal tabs  170  are used as dambars (not shown) when an encapsulant material (not shown) is used to encase the frame  120 , in which the encapsulant material securely holds the metal tabs  170 . When severed from the frame member  150 , at least one metal tab  170  is used to provide an electrical pathway (not shown) from an electronic device through an interconnect medium (e.g. wires, clips) to an external circuit (not shown).  
      Referring again to  FIG. 6 , the frame member  150  of the frame  120  has a uniform shape and defines an outer perimeter of the frame  120 . The outer perimeter forms a frame-like shape, having two sets of two perpendicular peripheral sides and can be either square or rectangular. All peripheral sides of the frame member, have an internal edge  151  and an external edge  152 . The internal edge  151  connects to all metal tabs  170  and the external edge  152  connects to adjacent frames  120  on the substrate.  
      The frame member  150  is made of copper, but could be made of other conductive materials such as copper alloys, nickel and nickel alloys. The frame member  150  functions to support the metal pads  167 ,  168  that are connected to the metal tabs  170  and interconnects the plurality of interconnected frames  120 . The frame  120  that is laid out in a predetermined pattern interconnects with an adjacent frame  120  by the external edge  152  of frame member  150 , which isolates the frame  120  from other interconnected frames  120 .  
       FIG. 7  is a greatly enlarged view of an area on the frame  120  showing the details of the metal pads  167 ,  168 , the metal tabs  170 , the frame member  150  and their interconnections. The details show that at least one of the sides  171 ,  172 ,  173  and  174  of the metal tab  170  has a lower step-like surface (e.g., a first surface)  179  and a second surface  178  above the first surface. At least one of the sides  163 ,  164 ,  165  and  166  of the metal pads  167  and  168  has a lower step-like surface (e.g., a first surface)  169  and a second surface  160  above the first surface. The peripheral side  172  of the metal tabs  170  connects to the internal edge  151  of the frame member  150 , in which the first surface  179  is coplanar or substantially coplanar with the first surface  169  of at least one of the metal pads  167 ,  168 . The peripheral sides  165 ,  166  of the metal pads  167 ,  168  connect to the peripheral side  174  of some metal tabs  170 , in which the first surface  169  is coplanar or substantially coplanar with the first surface  179 . The second surface  178  of the metal tabs  170  is coplanar or substantially coplanar with the second surface  160  of one or more metal pads  167 ,  168 .  
      For another preferred embodiment (not shown), at least one first surface  179  of the metal tab  170  is not coplanar with a lower step-like surface (e.g., a first surface)  169  of the metal pads  167 ,  168 . The second surface  178  of the metal tabs  170  is not coplanar with the second surface  160  of the metal pads  167 ,  168 . The step-like surfaces  169  and  179  provide the encapsulant material (not shown) a place to securely hold the metal pads  167 ,  167   a ,  168  and  168   a  and the metal tabs  170  when encasing the frame  120  in a completed electronic package (not shown).  
      The frame member  150  is generally coplanar with the first surface  179  of the metal tabs  170  and the first surface  169  of the metal pads  167 ,  168 . However, the frame member  150  is not coplanar with the second surfaces  160 ,  178  of the metal pads  167 ,  168  and the metal tabs  170 . In another embodiment (not shown), frame member  150  is co-planar with the second surface  160 ,  178  of metal pads  167 ,  168  and metal tabs  170 . The internal edge  151  of the frame member  150  connects to the peripheral side or end  172  of all metal tabs  170  that are adjacent to the first surface  179 . An upper portion of the peripheral side  172  that is adjacent to the second surface  178  does not connect to the internal edge  151  of the frame member  150 . The peripheral side  174  that is adjacent to the step-like surface  179  of some metal tabs  170  connects to the peripheral sides  165  and  166 , which are adjacent to the first surface  169  of metal pads  167 ,  168 . The peripheral side  174  does not connect to the peripheral sides  165 ,  166  of the metal pads  167 ,  168 .  
      Referring now to  FIG. 8 , the frame  120  is shown with the second nonconductive layer  130  and the third layer having a plurality of conductive traces  140  applied. According to a preferred embodiment, the nonconductive layer  130  is made with a polyimide film, such as Kapton® tape, but could be made of other nonconductive materials with a high thermal capability.  
      The Kapton® tape  130  has two main surfaces, a first or top main surface and a second or bottom main surface with one or both surfaces having an adhesive pre-applied. For this embodiment, the pre-applied adhesive is used to secure the Kapton® tape  130  to and cover at least a portion of the first surface of three metal pads  167 ,  167   a ,  168 ,  168   a  and insulate the covered portions. For another embodiment, a separate adhesive may be used to secure the nonconductive layer  130  (e.g., Kapton® tape or other nonconductive material) to the metal pads  167 ,  167   a ,  168 ,  168   a  instead of having the adhesive pre-applied.  
      Before the Kapton® tape  130  is applied to the frame  120  or the metal pads  167 ,  167   a ,  168 ,  168   a , it is formed by die cutting or some other forming method, to a specific shape and size according to a predefined design. One skilled in the art would recognize that other materials and other methods could be used for forming the nonconductive layer  130  before applying it to the frame  120  or to the metal pads  167 ,  167 ,  168 ,  168   a  or that it could be applied in some other manner such as spraying on.  
      Referring again to  FIG. 8 , the Kapton® tape layer  130  applied to the metal pads,  167 ,  167   a ,  168  and  168   a  has a specific shape and size, in which it covers a portion of the metal pads  167 ,  167   a ,  168  and  168   a . The Kapton® tape  130  also covers a portion of the open space  180  by forming a bridge over the open space  180 . There exist at least two open voids  161  and  162  that are formed in the Kapton® tape  130  thus allowing portions of the metal pads  167  and  167   a  to remain uncovered and un-insulated. The open voids  161  and  162  allow electronic devices (not shown) to be connected directly to the metal pads  167 ,  167   a , which provide electrical interconnections between the electronic devices (not shown) and the metal pads  167  and  167   a.    
      The conductive traces  140  of the third layer are applied to the top or first main surface of the Kapton® tape  130 . The conductive traces  140  are made with copper, but could be made with other conductive materials.  
      The conductive traces  140  are generally planar and have a first surface that is secured to and covers at least a portion of the top surface of the Kapton® tape  130 . The conductive traces  140  have a second surface for receiving, supporting and interconnecting electronic devices as shown in  FIG. 9 . However, the size, shape and length of the conductive traces  140  may or may not be unique and are defined according to a predefined circuit design. Referring again to the embodiment shown in  FIG. 8 , the frame  120  has conductive traces  140  that have sizes, shapes and lengths for supporting electronic devices (not shown) that are attached thereon according to a predefined circuit design. Portions of some conductive traces  140  are generally straight, rectangular or square, while others are irregularly shaped with multiple bends and have varying lengths. Other conductive traces  140  are generally straight, but still have different sizes, shapes and lengths.  
      Before applying and securing the conductive traces  140  to the Kapton® tape  130 , the Kapton® tape  130  is first prepared to receive the conductive traces  140 . To prepare the top surface of the Kapton® tape, an adhesive, an epoxy or an etching material is applied for securing and forming the conductive traces  140 . In a preferred embodiment, the conductive traces  140  are applied to the Kapton® tape  130  by using typical etching methods. In other preferred embodiments, the conductive traces  140  are formed by using an epoxy or Kapton® tape with an adhesive pre-applied to the top main surface, and then attaching the conductive traces  140  to the epoxy or adhesive. Once the conductive traces  140  are applied to the Kapton® tape  130 , the traces  140  are ready to receive and support the electronic devices (not shown). In another preferred embodiment, the third layer  140  has two or more generally planar layers of conductive traces, thereby forming multilayer circuits (not shown). The multilayer circuits electrically connect via a node extending through the Kapton® tape (e.g., a nonconductive layer) according to a predefined design.  
      In a preferred embodiment of the present invention,  FIG. 9  is a perspective view of a frame  120  supporting electronic devices  220  and  230 , with the electronic devices  220  on the conductive traces  140  and electronic devices  230  connected directly on to the metal pads  167 ,  167   a ,  168  and  168   a . The electronic devices or components  220  are applied to the conductive traces  140  and the metal pads  167 ,  167   a ,  168  or  168   a  using direct soldering or soldering reflowing methods. One skilled in the art would recognize that other methods may be used to attach the electronic components  220 ,  230  to the conductive traces  140  and the metal pads  167 ,  167   a ,  168  and  168   a . The techniques listed here are exemplary methods that are used for the preferred embodiments and do not impose any limitations for using other methods. The electronic components  220 ,  230  are applied to the conductive traces  140  and metal pads  167 ,  167   a ,  168 ,  168   a  according to a predefined circuit design. According to a preferred embodiment, the electronic devices  220 ,  230  are electrically connected to at least one conductive trace  140  each and/or at least one metal pad  167 ,  167   a ,  168 ,  168   a . At least two electronic devices  220  use the plurality of conductive traces  140  as interconnections for a predetermined circuit design.  
       FIG. 10  is an exploded view showing details for the frame  120  shown in  FIG. 9 .  FIG. 10  illustrates an order for the elements that form the completed frame  120  and how the elements are positioned on the frame.  
       FIG. 10  shows the frame  120  surrounded by the frame member  150  that includes the plurality of metal tabs  170  and four metal pads  167 ,  167   a ,  168 ,  168   a . All metal tabs connect to the internal edge  151  of the frame member  150  while some connect to the metal pads  167 ,  167 ,  168 ,  168   a . Above the frame is the nonconductive layer  130  (e.g., Kapton® tape). The Kapton® tape is formed before it is applied to the metal pads  167 ,  167   a ,  168 ,  168   a  and has at least two open voids  161 ,  162  according to a predefined design. When applied, the nonconductive layer  130  is secured to portions of the first surface  169  of some of the metal pads  167 ,  167   a ,  168 ,  168   a . In addition, the nonconductive layer  130  also insulates the portions that cover the metal pads  167 ,  167   a ,  168 ,  168   a . The open voids  161 ,  162  allow portions of the metal tabs  167 ,  167   a ,  168 ,  168   a  to remain uncovered and un-insulated for receiving and securing electronic devices  230  directly. Above the second layer is the third layer that has a plurality of conductive traces  140 . The conductive traces  140  are formed onto the Kapton® tape  130  layer. Each trace covers at least a portion of the Kapton® tape in preparation for receiving, supporting and interconnecting an electronic device  220 . Above the second layer, the electronic devices  220 ,  230  are applied and attached to the conductive traces  140  and the metal pads  167 ,  167   a ,  168 ,  168   a . By connecting the electronic device  230  directly to the metal pad  167 ,  167   a ,  168  or  168   a , the interconnection provides electrical conductivity between the electronic device and the metal pad  167 ,  167   a ,  168 ,  168   a . In addition, the metal pads  167 ,  167   a ,  168 ,  168   a  also provide thermal conductivity by heat sinking the temperature away from the electronic device  220 ,  230  according to a preferred embodiment. Afterwards, the substrate with at least one electronic device is encased with an encapsulant material for forming a completed package.  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.