Patent Publication Number: US-2009239339-A1

Title: Method of stacking dies for die stack package

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to die stacks, and more particularly, to a method of stacking dies for a die stack package. 
     2. Description of the Related Art 
     Before a die is packaged, a wafer is cut into a plurality of small dies and then the dies in the desired number are taken and adhered to a proper substrate and further processed with electric connection and packaging. 
     A general method of stacking dies for a die stack package includes the steps of adhering a first-layer die onto the substrate, electrically connect the first-layer die with the substrate by wire bonding, and disposing adhesive on a top side of the first-layer die for adhesion with a second-layer die. Because there are wires on the first-layer die, it must be very careful to dispose the adhesive to prevent it from staining contact pads of the first-layer die. Further, before a second-layer die is stacked upon a first-layer die, a predetermined position must be reserved to avoid the wires of the first-layer die, such that the second-layer die is usually smaller than the first-layer die. In this way, there are great limitations during production of the die stack package and it is very complicated to dispose the adhesive on the second-layer or above die and to adhere the two or more dies, such that improvement is needed. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a method of stacking dies for a die stack package, which simplifies the process of stacking the dies and decreases the production cost. 
     The foregoing objective of the present invention is attained by the method including the steps of providing a wafer having a first surface and a second surface, said first surface having a plurality of cut ways thereon, the second surface being coated with adhesive of a predetermined thickness at a predetermined position thereof; removing parts of the adhesive by photo-lithography, each of the parts of the adhesive corresponding to the cut way and being wider than the cut way; cutting the wafer along the cut ways to make a plurality of dies, each of the dies having a part of the adhesive thereon; and stacking each die, whose surface having the adhesive faces a lower-layer die, on the lowerlayer die. 
     In light of the above steps, each of the dies has the adhesive on the second surface thereof, and the adhesive has a predetermined thickness and is less wider than the die, such that a space is formed between the adjacent dies for receiving the wires of the next layer of the die, thus facilitating the stacking operation and saving the production cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of a preferred embodiment of the present invention. 
         FIG. 2(A)  is a schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(B)  is another schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(C)  is another schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(D)  is another schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(E)  is another schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(F)  is another schematic view of the preferred embodiment of the present invention in implementation. 
         FIG. 2(G)  is another schematic view of the preferred embodiment of the present invention in implementation. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  and  FIGS. 2(A)-2(G) , a method of stacking dies for a die stack package in accordance with a preferred embodiment of the present invention includes the following steps. 
     (A) Provide a wafer  11 . The wafer includes a first surface  12  and a second surface  13 . The first surface  11  has a plurality of cut ways  14  formed thereon. The second surface  13  is eccentrically coated with adhesive  15  at a predetermined position thereof for a predetermined thickness. 
     (B) Remove parts of the adhesive  15  by photolithography. The parts of the adhesive  15  correspond to the cut ways  14  respectively and each are wider than the cut way  14 . 
     (C) Cut the wafer  11  along the cut ways  14  to make a plurality of dies  16 . Each of the dies  16  has a part of the adhesive  15  on the surface thereof to become a die stack  10 . The adhesive  15  is C-stage adhesive, that is, hot and pressure adhesive. 
     (D) Stack the die stack  10 , whose surface having the adhesive  15  faces a lower-layer die  21 , on the lower-layer die  21 . 
     As shown in  FIG. 2(G) , the die stack  10  can be stacked straight on a die unit  20  having a substrate  22 , the lower-layer die  21 , and a wire  23 . The lower-layer die  21  is adhered to the substrate  22 . The wire  23  is bonded between the lower-layer die  21  and the substrate  22 . The die stack  10  is stacked on the lower-layer die  21 . After pressurization and heating, the adhesive  15  is disposed on the lower-layer die  21 . The thickness of the adhesive  15  enables the two dies  16  and  21  to avoid the wire  23 . After the die stack  10  is adhered to the lower-layer die  21 , the die stack  10  and the substrate  22  are electrically connected with each other by the wire  18 . 
     The key point of the present invention lies in that the adhesive of a predetermined thickness is coated to the bottom side of the die stack  10  and then adhered to the lower-layer die  21  by the pressurization and heating. In addition, the arrangement of stacking the dies is not limited to two layers of the dies but includes three and more layers of the dies. 
     In conclusion, the present invention facilitates the adhesion of the dies while stacked on the other and reserves space for wire bonding, such that the product cost is reduced. 
     Although the present invention has been described with respect to a specific preferred embodiment thereof, it is no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.