Abstract:
A package structure includes: a substrate having a chip-bearing area arranged thereon; an window type assistant element arranged on the substrate and surrounding the edge of the chip-bearing area; a plurality of chips arranged within the chip-bearing area; and a package encapsulation covering chips within the chip-bearing area. It can resist the deformation and reduce the damage from the warpage and simultaneously enhance the yield and stability of the package structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a package structure, particularly to a package structure to reduce warpage. 
         [0003]    2. Description of the Prior Art 
         [0004]    IC packaging process is a back-end process in the semiconductor industry and includes the following procedures: dicing, die attachment, wire bonding, encapsulation, printing, bumping, and singulation. The function of IC packaging is to provide an interface to transmit the internal IC signals to the external systems and enhance the strength of the IC chip and protect the IC chip from the corrosion and damage caused by water, moisture, chemical materials and external force. 
         [0005]    In the encapsulation process, a mold is placed on a substrate having semiconductor chips or electronic elements, and an encapsulant is filled into the cavity of the mold, and then the mold is stripped away. 
         [0006]    With the development of the thin package technology, the substrate becomes larger but thinner. The different kinds of packaging materials have different thermal expansion coefficients. The thermal stress, which is caused by the different extents of the dimensional variations occurring during temperature change, will induce the warpage of the package structure, and too great a warpage will crack the internal semiconductors or electronic elements. Therefore, how to solve the warpage problem of the encapsulation process is a focus in the field concerned. 
       SUMMARY OF THE INVENTION 
       [0007]    The primary objective of the present invention is to provide a package structure to reduce warpage and solve the abovementioned problems. 
         [0008]    Another objective of the present invention is to provide a package structure, which can prevent the damage caused by warpage and promote the yield and stability of the package structure. 
         [0009]    Further another objective of the present invention is to provide a package structure, which can reduce the warpage occurring during the molding process and PMC (Post Mold Cure) process. 
         [0010]    To achieve the abovementioned objectives, one embodiment of the present invention proposes a package structure, which includes: a substrate having a chip-bearing area arranged thereon, a window type assistant element arranged on the substrate and surrounding around the edge of the chip-bearing area, a plurality of chips arranged within the chip-bearing area, and an encapsulant covering the chips within the chip-bearing area. 
         [0011]    Another embodiment of the present invention proposes a package structure, which includes: a substrate having a plurality of chip-bearing areas arranged thereon, a plurality of window type assistant elements arranged on the substrate and respectively surrounding the edges of chip-bearing areas, a plurality of chips arranged within every chip-bearing area, and an encapsulant covering the chips on the chip-bearing areas. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a front view of the package structure according to one embodiment of the present invention; 
           [0013]      FIG. 2  is a cross-sectional view along Line A-A in  FIG. 1 ; 
           [0014]      FIG. 3  is a front view of the package structure according to another embodiment of the present invention; 
           [0015]      FIG. 4  is a cross-sectional view along Line B-B in  FIG. 3 ; 
           [0016]      FIG. 5  is a cross-sectional view of the package structure according to further another embodiment of the present invention; and 
           [0017]      FIG. 6  is a partial front view of the package structure according to further another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]      FIG. 1  is a front view of the package structure according to one embodiment of the present invention. In this embodiment, such as shown in  FIG. 1 , a plurality of chip-bearing areas  12  is arranged on a substrate  10 , and a window type assistant element  20  surrounds the edge of each chip-bearing area  12 ; a plurality of chips  30  is arranged within every chip-bearing area  12 , and the chips  30  are arranged in array; and an encapsulant  40  covers the chips  30  arranged on the chip-bearing areas  12 . 
         [0019]      FIG. 2  is a sectional view along Line A-A in  FIG. 1 . Referring to  FIG. 2  and following the foregoing description, in addition to the chips  30  on the chip-bearing areas, the encapsulant  40  also covers the window type assistant element  20  in this embodiment. In the current fabrication method of the package structure, it generally includes the following steps: firstly, a substrate  10  is provide; next, chips  30  are stuck onto every chip-bearing area on the substrate  10 ; next, a wire-bonding procedure is undertaken to electrically connect the chips  30  and the substrate  10 ; next, a molding procedure is undertaken to cover the chips  30  on the chip-bearing areas with an encapsulant  40 . The substrate  10  is made of at least one of the materials selected from the group consisting of polyimide, glass, alumina, epoxy, beryllium oxide and elastomer. The encapsulant  40  is mainly composed of EMC (Epoxy Molding Compound). The material used to stick the chips  30  (not shown in the drawings) onto the chip-bearing areas may be a silver paste, a chip-sticking film, or a nonconductive epoxy. After the molding procedure, a 0˜4 hour post mold cure procedure is usually needed to fully cure the encapsulant  40 . In this embodiment, the window type assistant element  20  should adopt a heat-resistant material, such as a heat-resistant plastic, ceramic or metal. The heat-resistant material should be able to endure the fusion temperature of the encapsulant  40  and the temperature of the post mold cure procedure so that the window type assistant element  20  can protect the package structure against warpage. Besides, the window type assistant element  20  should also have the properties of high rigidity and high strength and have a low thermal expansion coefficient. 
         [0020]    In the abovementioned embodiment, the substrate  10  has a plurality of opening trenches  14  for releasing the thermal stress induced by temperature change, as shown in  FIG. 1 . The window-type assistant element  20  is fixed to the substrate  10  with an adhesive (not shown in the drawings), and the window-type assistant element  20  is covered by the encapsulant  40  so that it can be more securely fixed to the substrate  10 , as shown in  FIG. 2 . In the present invention, the window-type assistant element  20  may be installed to the substrate  10  before the chips  30  are stuck onto the substrate  10 , or after the chips  30  have been stuck onto the substrate  10 , or after the wire-bonding procedure has been undertaken. Thereby, the window-type assistant element  20  can be used to protect the package structure from the warpage occurring in various conditions, e.g. to prevent the warpage induced by the baking procedure after the chips  30  have been stuck onto the substrate  10 , or by the fusion temperature of the encapsulant  40  in the molding procedure, or by the post mold cure procedure. 
         [0021]    Refer to  FIG. 3  a front view of the package structure according to another embodiment of the present invention. In this embodiment, a chip-bearing area  12  is arranged on a substrate  10 , and a window type assistant element  20  surrounds the edge of the chip-bearing area  12 ; a plurality of chips  30  is arranged within the chip-bearing area  12 ; and an encapsulant  40  covers the chips  30  arranged within the chip-bearing area  12 . The fabrication method of the package structure of the present invention has been described above and will not repeat here. Refer to  FIG. 4  a sectional view along Line B-B in  FIG. 3 . As shown in  FIG. 4 , the window type assistant element  20 , which surrounds the edge of the chip-bearing area  12 , can effectively protect the package structure against the warpage induced by temperature change in the succeeding procedures. As all the chips  30  are arranged in array within a single chip-bearing area  12 , the space of the substrate  10  can be fully utilized. Therefore, the substrate  10  can achieve the highest usage efficiency in this embodiment. Thus, the yield is promoted, and the cost is reduced. 
         [0022]    Refer to  FIG. 5  and  FIG. 6  respectively a sectional view and a partial front view of the package structure according to further another embodiment of the present invention. In this embodiment, the encapsulant  40  only covers the chips  30  arranged on the chip-bearing area  12 . However, the window type assistant element, which is not covered by the encapsulant  40 , can still protect the package structure against warpage. The window type assistant element should be made of a heat-resistant plastic, ceramic or metal. In this embodiment, the window type assistant element is formed of a plurality of strip-like elements  20 ′, such as plastic strips, metallic strips or ceramic strips. 
         [0023]    The present invention is characterized in that at least one window type assistant element is installed around the edge of the chip-bearing area to enhance the strength of the substrate and prevent the package structure from being damaged by the warpage induced by temperature change. The window type assistant element may apply to any one of the package structures wherein chips are installed onto a substrate, such as the BGA (Ball Grid Array) package structure, the FBGA (Fine pitch Ball Grid Array) package structure, the VFBGA (Very Fine pitch Ball Grid Array) package structure, the BGA (micro Ball Grid Array) package structure and the wBGA (window Ball Grid Array) package structure. The window type assistant element may be either covered by the encapsulant or not covered by the encapsulant but surrounds the edge in the exterior of the encapsulant. The window type assistant element can effectively prevent the package structure from the warpage or deformation induced by temperature change during packaging process. Therefore, the window type assistant element can promote the yield and reduce the cost. In the succeeding singulation procedure, the window type assistant element will be cut off and abandoned. Thus, none extra structure exists in the final package structure. 
         [0024]    In summary, the present invention discloses a package structure to reduce warpage, which can prevent a package structure from being damaged by the warpage occurring during packaging process and promote the yield and stability of the package structure. 
         [0025]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that other modifications and variations can be made without departing the spirit and scope of the invention as hereafter claimed.