Patent Publication Number: US-2005133241-A1

Title: Chip orientation and attachment method

Description:
FIELD OF THE INVENTION  
      The present invention relates to methods for attaching a semiconductor die or chip to a substrate. More particularly, the present invention relates to a novel chip orientation and attachment method which reduces stress applied to the corners of a chip during bonding of the chip to a substrate.  
     BACKGROUND OF THE INVENTION  
      A conventional method used by the semiconductor industry in the manufacturing of semiconductor integrated circuits includes the steps of fabrication, wafer sort, assembly and test, respectively. In the fabrication step, as many as several thousand dies (integrated circuits) are formed onto a semiconductor wafer. In the wafer sort step, each of the dies on the wafer is tested to determine its electrical characteristics and operability, and defective dies are distinguished from operable dies. The defective dies are often marked by an ink mark at the wafer sorting step. In the assembly step, the unmarked, operable dies are assembled into a package, and in the test step, the packaged integrated circuits are tested for operability and reliability.  
      After the semiconductor chips or die are fabricated on a semiconductor wafer, the die are separated from each other. This is carried out typically by cutting the die from the wafer using a diamond-blade dicing saw. The wafer is initially placed on an adhesive film provided on a rigid frame. During the cutting operation, the wafer is sprayed with deionized water to remove silicon slurry residue resulting from the cutting action. The separated die are held in place on the adhesive film. Final separation of the die is achieved using fully-automated equipment which includes alignment systems and integrated wafer cleaning.  
      One of the last processes in the production of semiconductor integrated circuits (IC) is multi-leveled packaging, which includes expanding the electrode pitch of the IC chips containing the circuits for subsequent levels of packaging; protecting the chip from mechanical and environmental stress; providing proper thermal paths for channeling heat dissipated by the chip; and forming electronic interconnections. The manner in which the IC chips are packaged dictates the overall cost, performance, and reliability of the packaged chips, as well as of the system in which the package is applied.  
      Package types for IC chips can be broadly classified into two groups: hermetic-ceramic packages and plastic packages. A chip packaged in a hermetic package is isolated from the ambient environment by a vacuum-tight enclosure. The package is typically ceramic and is utilized in high-performance applications. A chip packaged in a plastic package, on the other hand, is not completely isolated from the ambient environment because the package is composed of an epoxy-based resin. Consequently, ambient air is able to penetrate the package and adversely affect the chip over time. Recent advances in plastic packaging, however, has expanded their application and performance capability. Plastic packages are cost-effective due to the fact that the production process is typically facilitated by automated batch-handling.  
      Packaging of dies is begun after separation of the dies from each other in the wafer. A die attach operation is then carried out in which each functional die is individually removed from the adhesive film and attached to a leadframe or substrate. An automated die bonder uses a gripper, or collet, to grasp each die by the edges and place the die on the substrate for assembly. The die bonder distinguishes functional dies from non-functional ones based typically on the presence or absence of an ink mark on each die, with the non-functional dies having been previously marked with an ink spot in a separate die testing step.  
      There are three basic methods for bonding chips to a leadframe or substrate: the epoxy attach method, the eutectic method and the glass frit attach method. In the epoxy attach method, an epoxy is placed in the center of the leadframe or substrate and the back surface of the chip is placed on the epoxy. A thermal heat cycle is then used to cure the epoxy. In some applications, the epoxy is formulated with silver flakes to aid in dissipation of heat between the chip and the rest of the package.  
      In the eutectic method, a thin film of gold (Au) is provided on the backside of the wafer and alloyed to a metal leadframe or ceramic substrate. The substrate is heated to 420 degrees C. for approximately six seconds, during which a eutectic alloy interconnection is formed between the chip and the leadframe or substrate. The eutectic alloy interconnection enhances the thermal path and mechanical strength between a chip and a substrate or leadframe.  
      In the glass frit attach method, a mixture of silver and glass particles suspended in an organic medium is used to attach a chip directly to a ceramic substrate. The chip is attached to the ceramic substrate with a hermetic seal. The silver and glass in the organic medium form a bond to the substrate having good thermal conduction.  
      One of the most common methods of electrically connecting bond pads on a chip to inner lead terminals on a leadframe or substrate is the wirebonding method. The wirebonding method can be accomplished using high-speed operation tools, which forms a fine, typically gold or aluminum wire between the bond pads on the chip and the inner lead terminals on the substrate or leadframe. Wirebonding of a chip to a substrate is a high-speed process, with up to ten wire bonds per second capable of being formed.  
      After a chip is wirebonded to a leadframe or substrate, the wirebonded chip structure may undergo plastic packaging, in which an epoxy polymer is used to completely encapsulate the chip and leadframe or substrate in a molding process. Plastic packaging is amenable to high-volume production techniques. Furthermore, the plastic package facilitates flexibility in the shape of the leads, including pin-in-hole (PIH) leads, which extend through openings in the circuit board, and surface mount technology (SMT) leads, which are attached to the surface of the circuit board. SMT leads enable higher-density packaging for both the chip and the circuit board.  
      A typical conventional plastic chip package structure  8  is shown in  FIG. 1A . The structure  8  includes a square-shaped leadframe or substrate  10  to which is attached a correspondingly square-shaped chip  12 , having chip corners  14   a - 14   d . A molding compound  16 , such as an epoxy, is applied to the chip  12  and cured to encapsulate the chip  12  in a cured molding compound  18  on the substrate  10 . An alternative plastic chip package  22  is shown in  FIG. 1B , wherein a rectangular-shaped chip  26  having chip corners  28   a - 28   b  is bonded to a rectangular substrate  24  and is encapsulated in a cured molding compound  18 , typically epoxy.  
      During the chip packaging process, the square chip  12  and the rectangular chip  26  are positioned in substantially the center of the correspondingly-shaped square substrate  10  and rectangular substrate  24 , respectively. The molding compound  16  is applied to a corner  14   a  of the square chip  12  and to a corner  28   a  of the rectangular chip  26 . This applies substantial thermal stress to the chips, causing damage to and delamination of the chip corners, particularly the corner  14   c  of the square chip  12  and the corner  28   c  of the rectangular chip  26 . In the chip package structure  8  of  FIG. 1A , the chip diagonal  13  of the chip  12  is aligned with the substrate diagonal  11  of the substrate  10 ; the chip edges  15  are parallel to the substrate edges  10   a ; and the center of the chip  12  overlaps the center of the substrate  10 . The same conditions exist with respect to the chip package structure  22  of  FIG. 1B .  
      It has been found that the corner-damaging and delaminating effects caused by thermal stress that is induced by molding compound application to a chip on a substrate can be substantially reduced by orientation of the chip on the substrate in such a manner that one or more of the following conditions exists: the chip diagonal and the substrate diagonal are in non-aligned relationship, the chip edges are non-parallel with respect to the substrate edges, or the chip center is in non-overlapping relationship with respect to the substrate center. Accordingly, a new and improved chip orientation and attachment method is needed for attaching a chip to a substrate in such a manner that damage to the chip induced by molding compound application is eliminated or substantially reduced.  
      An object of the present invention is to provide an improved chip orientation attachment method for attaching a chip to a substrate.  
      Another object of the present invention is to provide a new and improved chip orientation and attachment method which eliminates or substantially reduces chip damage induced by thermal stress caused by application of a molding compound to the chip.  
      Still another object of the present invention is to provide a new and improved chip orientation and attachment method in which at least one of the following conditions exists: the chip diagonal and the substrate diagonal are in non-aligned relationship, and/or the chip edges are non-parallel with respect to the substrate edges, and/or the chip center is in non-overlapping relationship with respect to the substrate center.  
      Yet another object of the present invention is to provide a new and improved chip orientation and attachment method which is applicable to attachment of a square-shaped or elongated rectangular chip on a square-shaped or elongated rectangular substrate, respectively.  
      A still further object of the present invention is to provide chip package structures in which a chip is attached to a substrate in such an orientation that chip damage induced by thermal stress during application of a molding compound is substantially reduced or eliminated.  
     SUMMARY OF THE INVENTION  
      In accordance with these and other objects and advantages, the present invention is generally directed to a new and improved chip orientation and attachment method which eliminates or substantially reduces chip damage caused by thermal stress induced by application of a molding compound to the chip and substrate. The chip is attached to the substrate in such a manner that at least one of the following conditions exists: the chip diagonal and the substrate diagonal are in non-aligned relationship, and/or the chip edges are non-parallel with respect to the substrate edges, and/or the chip center is in non-overlapping relationship with respect to the substrate center.  
      In one embodiment, the chip is attached to the substrate in such an orientation that the chip center is non-overlapping with respect to the substrate center. In another embodiment, the chip diagonal is in non-aligned relationship with respect to the substrate diagonal and the chip edges are non-parallel with respect to the substrate edges. In still another embodiment, the chip center is non-overlapping with respect to the substrate center, the chip diagonal is in non-aligned relationship with respect to the substrate diagonal and the chip edges are non-parallel with respect to the substrate edges. In each case, the chip and substrate are typically square-shaped or elongated rectangular.  
      The present invention is further directed to chip package structures in which a chip is attached to a substrate in such a manner that chip damage induced by thermal stress at the molding compound-application method is eliminated, or at least, substantially reduced. In a first embodiment, a square-shaped chip is attached to a square-shaped substrate with the chip center in non-overlapping with respect to the substrate center. In a second embodiment, the chip diagonal is in non-aligned relationship with respect to the substrate diagonal and the chip edges are non-parallel with respect to the substrate edges. In a third embodiment, the chip center is non-overlapping with respect to the substrate center, the chip diagonal is in non-aligned relationship with respect to the substrate diagonal and the chip edges are non-parallel with respect to the substrate edges. In fourth, fifth and sixth embodiments, the same conditions exist with respect to the first, second and third chip package structures, respectively, except both the chip and the substrate have an elongated rectangular shape.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1A  is a top, partially schematic, view of a conventional chip package structure in which a square-shaped chip is oriented on a square-shaped substrate with the chip center and the substrate center overlapping each other, the chip edges and substrate edges parallel with each other, and the chip diagonal and substrate diagonal aligned with each other;  
       FIG. 1B  is a top, partially schematic, view of a conventional chip package structure in which an elongated rectangular chip is oriented on an elongated rectangular substrate with the chip center and the substrate center overlapping each other, the chip edges and substrate edges parallel with each other, and the chip diagonal and substrate diagonal aligned with each other;  
       FIG. 2A  is a top, partially schematic, view of a chip package structure according to the present invention, with a square-shaped chip attached to a square-shaped substrate in such a manner that the chip center is in non-overlapping relationship with respect to the substrate center and the chip diagonal is in non-alignment with respect to the substrate diagonal;  
       FIG. 2B  is a top, partially schematic, view of a chip package structure according to the present invention, with a square-shaped chip attached to a square-shaped substrate in such a manner that the chip edges are non-parallel with respect to the substrate edges and the chip diagonal is disposed in non-alignment with respect to the substrate diagonal;  
       FIG. 2C  is a top, partially schematic, view of a chip package structure according to the present invention, with a square-shaped chip attached to a square-shaped substrate in such a manner that the chip center is non-overlapping with respect to the substrate center, the chip edges are non-parallel with respect to the substrate edges and the chip diagonal is disposed in non-alignment with respect to the substrate diagonal;  
       FIG. 3A  is a top, partially schematic, view of a chip package structure according to the present invention, with an elongated rectangular chip attached to an elongated rectangular substrate in such a manner that the chip center is in non-overlapping relationship with respect to the substrate center and the chip diagonal is in non-alignment with respect to the substrate diagonal;  
       FIG. 3B  is a top, partially schematic, view of a chip package structure according to the present invention, with an elongated rectangular chip attached to an elongated rectangular substrate in such a manner that the chip edges are non-parallel with respect to the substrate edges and the chip diagonal is disposed in non-alignment with respect to the substrate diagonal; and  
       FIG. 3C  is a top, partially schematic, view of a chip package structure according to the present invention, with an elongated rectangular chip attached to an elongated rectangular substrate in such a manner that the chip center is non-overlapping with respect to the substrate center, the chip edges are non-parallel with respect to the substrate edges and the chip diagonal is disposed in non-alignment with respect to the substrate diagonal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention contemplates a new and improved chip orientation and attachment method which eliminates or substantially reduces chip damage caused by thermal stress induced by application of a molding compound to the chip and substrate in the plastic encapsulation packaging of the chip. The chip is initially attached to the substrate in such a manner that the chip diagonal and the substrate diagonal are in non-aligned relationship; the chip edges are non-parallel with respect to the substrate edges; or the chip center is in non-overlapping relationship with respect to the substrate center, or two or all of the above conditions exist. After the chip is attached to the substrate, the molding compound, which may be an epoxy resin, for example, is applied to the chip to encapsulate the chip on the substrate. Accordingly, the corners of the chip are positioned out of the direct flow path of the molding compound, substantially reducing thermal stress which otherwise causes damage to and delamination of a corner or corners of the chip.  
      The present invention further contemplates chip package structures in which a square-shaped or elongated rectangular chip is attached to a correspondingly-shaped substrate. In the respective embodiments, the chip is oriented on the substrate in such a manner that the chip diagonal and the substrate diagonal are in non-aligned relationship, and/or the chip edges are non-parallel with respect to the substrate edges, and/or the chip center is in non-overlapping relationship with respect to the substrate center. A molding compound, which may be epoxy, encapsulates the chip on the substrate.  
      According to the chip orientation and attachment method of the present invention, the chip may be attached to a leadframe or substrate using any of a variety of methods known by those skilled in the art. For example, the chip may be attached to the substrate using an epoxy attachment method, a eutectic attachment method or a glass frit method. Typically, the chip is wire-bonded to the substrate. The liquid molding compound is applied to the attached chip typically using equipment and methods known by those skilled in the art, to encapsulate the chip. After application, the liquid molding compound is cured, or hardened, according to thermal curing techniques and process parameters which are known by those skilled in the art.  
      Referring to  FIG. 2A , a chip package structure  32  fabricated according to a first embodiment of the chip orientation and attachment method of the present invention is shown. The chip package structure  32  includes a square-shaped chip  36  which is attached to a correspondingly square-shaped substrate  34 . The chip center  38  is disposed in non-overlapping relationship to the substrate center  35   a , and the chip diagonal  37  is disposed in non-aligned relationship with respect to the substrate diagonal  35 . The chip edges  39 , however, are disposed in substantially parallel relationship to the respective substrate edges  34   a . As the liquid molding compound  40  is applied to the chip package structure  32 , the compound  40  obliquely contacts an edge  39 , rather than directly contacting a corner, of the chip  36 . This substantially reduces the application of thermal stress to the chip  36  which would otherwise tend to damage and/or delaminate one or more corners of the chip  36 . The cured molding compound  41  encapsulates the chip  36  on the substrate  34 .  
      Referring next to  FIG. 2B , a chip package structure  42  fabricated according to a second embodiment of the chip orientation and attachment method of the present invention is shown. Although the chip center  38  is disposed overlapping relationship to the substrate center  35   a , the chip diagonal  37  is disposed in non-aligned relationship with respect to the substrate diagonal  35 . Moreover, the chip edges  39  are disposed in non-parallel relationship to the respective substrate edges  34   a . As the liquid molding compound  40  is applied to the chip package structure  32 , the compound  40  directly contacts an edge  39 , rather than a corner, of the chip  36  to reduce the application of thermal stress to the chip  36 .  
      Referring next to  FIG. 2C , a chip package structure  44  fabricated according to a third embodiment of the chip orientation and attachment method of the present invention is shown. The chip center  38  is disposed in non-overlapping relationship to the substrate center  35   a , the chip diagonal  37  is disposed in non-aligned relationship with respect to the substrate diagonal  35  and the chip edges  39  are disposed in non-parallel relationship to the respective substrate edges  34   a . The liquid molding compound  40  directly contacts an edge  39 , rather than a corner, of the chip  36  to substantially reduce the application of thermal stress to the chip  36 .  
      Referring next to  FIG. 3A , a chip package structure  48  fabricated according to a fourth embodiment of the chip orientation and attachment method of the present invention is shown. The chip package structure  48  includes an elongated rectangular chip  52  which is attached to a correspondingly elongated rectangular substrate  50 . The chip center  54  is disposed in non-overlapping relationship to the substrate center  51   a , and the chip diagonal  53  is disposed in non-aligned relationship with respect to the substrate diagonal  51 . The chip edges  55  are disposed in substantially parallel relationship to the respective substrate edges  50   a . Liquid molding compound  40  applied to the chip package structure  48  obliquely contacts an edge  55 , rather than directly contacting a corner, of the chip  52  to substantially reduce the application of thermal stress to the chip  52 . The cured molding compound  41  encapsulates the chip  52  on the substrate  50 .  
      Referring next to  FIG. 3B , a chip package structure  62  fabricated according to a fifth embodiment of the chip orientation and attachment method of the present invention is shown. The chip center  54  is disposed overlapping relationship to the substrate center  51   a . The chip diagonal  53  is disposed in non-aligned relationship with respect to the substrate diagonal  51 , and the chip edges  55  are disposed in non-parallel relationship to the respective substrate edges  50   a . The liquid molding compound  40  directly contacts an edge  55 , rather than a corner, of the chip  36 .  
      Referring next to  FIG. 3C , in a chip package structure  64  fabricated according to a sixth embodiment of the chip orientation and attachment method of the present invention, the chip center  54  is disposed in non-overlapping relationship to the substrate center  51   a , the chip diagonal  53  is disposed in non-aligned relationship with respect to the substrate diagonal  51  and the chip edges  55  are disposed in non-parallel relationship to the respective substrate edges  50   a . The liquid molding compound  40  directly contacts an edge  55 , rather than a corner, of the chip  52  to substantially reduce the application of thermal stress to the chip  52 .  
      While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.