Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority benefit of Taiwan application serial no. 92133698, filed Dec. 1, 2003. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a package structure. More particularly, the present invention relates to an anti-warp package and method of fabricating the same. 
   2. Description of the Related Art 
   The semiconductor packaging process can be roughly subdivided into a series of individual steps including wafer sawing, chip attaching, wire bonding, molding, printing and packaging. Molding is one of the processing steps whose purpose is to provide a barrier to the penetration of moisture, a medium for transferring internally generated heat away, a suitable shape for handing and insulate the internal electrical devices from external electrical devices. In general, the molding process comprises placing a substrate having semiconductor chips or electronic devices thereon inside a mold, heating some solid epoxy molding compound to a liquid form and injecting the liquid compound into the mold cavities inside the mold by applying a pressure through a plunger. Ultimately, the molding compound encapsulates the semiconductor chips or electronic devices on the substrate to form an airtight package. After the molding compound is cured, the package is released from the mold. 
   To produce packages en-mass, a plurality of semiconductor chips or electronic devices are disposed on a packaging substrate and encapsulated all at the same time. Thereafter, the packaging substrate is sawed to produce a plurality of independent package units. However, the material of the molding compound used for encapsulating and the material of the packaging substrate normally have different coefficient of thermal expansion. Thus, the degree of expansion or contraction between the packaging substrate and the molding compound as the temperature variation during the curing process may create stress that leads to a warping of the packaging substrate. As a rule, the higher the curing temperature and the longer the curing time, the greater will be the degree of warping in the packaging substrate so that the packaging substrate is more difficult to saw. Furthermore, because the semiconductor chips and the electronic devices are bonded to the packaging substrate, any warping stress within the package may be transmitted to the semiconductor chips or the electronic devices as a bending torque as well. If the bending torque is too large, the semiconductor chips or the electronic devices can be damaged. 
   However, for an optical device having the aforementioned package structure such as a land grid array (LGA) package structure, the degree of warping in the packaging substrate could be substantially increased because a transparent resin must be used as the molding compound. In general, the degree of warping in the packaging substrate is increased because the coefficient of thermal expansion of a transparent packaging resin is higher than most non-transparent packaging resin. Thus, it is necessary to provide a packaging substrate having a structure capable of minimizing warping after molding. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention provides an anti-warp package structure having a packaging substrate therein such that the problems and defects resulting from a warped packaging substrate are resolved. 
   The present invention provides a method of fabricating a package structure having a packaging substrate therein such that the problems and defects resulting from a warped packaging substrate are prevented. Hence, the yield and reliability of package production is improved. 
   As embodied and broadly described herein, the invention provides an anti-warp package structure. The package structure comprises a packaging substrate, a chip and a stiffening member. The chip is disposed on a top surface of the packaging substrate and the stiffening member is disposed on a bottom surface of the packaging substrate underneath an area surrounding the chip. Through the disposition of a stiffening member, warping stress on the packaging substrate when the chip is encapsulated by molding compound is counterbalanced. 
   The present invention also provides a method of fabricating a package comprising the following steps. First, a packaging substrate is provided. A chip is disposed on a top surface of the packaging substrate. Thereafter, a stiffening member is disposed on a bottom surface of the packaging substrate underneath the area surrounding the chip. Then, an encapsulant is formed to encapsulate the chip on the packaging substrate. After the encapsulant is formed, the packaging substrate is sawed to produce a plurality of individual package units to remove the stiffening member, wherein each package unit has the chip therein. In one embodiment of the present invention, the encapsulant is formed by the following steps, for example. First, an encapsulating mold having a top mold, a bottom mold and a gate is provided. The top mold has a mold cavity that corresponds to the chip and the bottom mold has a runner that corresponds to the stiffening member. The gate is located near the edge of the encapsulating mold connecting both the top mold cavity and the bottom mold cavity. The packaging substrate is placed between the top and the bottom of the encapsulating mold so that the chip are accommodated by the mold cavity and the stiffening member is accommodated by the runner mold cavity. Thereafter, a molding compound is injected through the gate to encapsulate the chip and the stiffening member on the packaging substrate. 
   Accordingly, the present invention utilizes the disposition of a stiffening member below a packaging substrate and the subsequent molding process to counterbalance the warping stress on the chip-disposing region of the packaging substrate. Hence, the package structure is able to prevent all those problems and defects that result from a warped packaging substrate. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a top view of a package structure according to one embodiment of the present invention. 
       FIG. 2  is a bottom view of a package structure according to one embodiment of the present invention. 
       FIG. 3  is a cross-sectional view along line A-A of  FIG. 1 . 
       FIG. 4  is a perspective view showing the method of disposing a stiffening ring on a packaging substrate. 
       FIG. 5  is a cross-sectional view showing the packaging substrate shown in  FIG. 3  disposed inside an encapsulating mold. 
       FIG. 6  is a cross-sectional view showing the packaging substrate in  FIG. 5  after injecting a molding compound into the mold. 
       FIG. 7  is a top view showing the full package structure after molding and curing. 
       FIG. 8  is a bottom view showing the full package structure after molding and curing. 
       FIG. 9  is a top view showing the molded structure of a package having four chip disposing regions. 
       FIG. 10  is a bottom view showing the molded structure of a package having four chip disposing regions. 
       FIG. 11  is a top view showing the stiffening bars on a packaging substrate according to another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 1  is a top view of a package structure according to one embodiment of the present invention.  FIG. 2  is a bottom view of a package structure according to one embodiment of the present invention. FIG.  3  is a cross-sectional view along line A-A of  FIG. 1 . As shown in  FIGS. 1 ,  2  and  3 , the packaging substrate  10  has a top surface  12  and a bottom surface  14 . On the top surface  12  of the packaging substrate  10 , two chip-disposing regions  16  are defined, for example. Each chip-disposing region  16  can accommodate a chip  18 . The chip  18  is disposed within the chip-disposing region  16  and arranged in an array. A peripheral region  20  that encapsulates each chip-disposing region  16  is also defined on the packaging substrate  10 . Furthermore, a stiffening ring  22  is disposed on an area of the bottom surface  14  underneath each peripheral region  20  as shown in  FIG. 2 . 
     FIG. 4  is a perspective view showing the method of disposing a stiffening ring on a packaging substrate. As shown in  FIG. 4 , the bottom  24  of the stiffening ring  22  has a plurality of positioning pins  26  protruding down. Moreover, the positioning pins  26  are uniformly arranged on the bottom surface  24  of the stiffening ring  22 . In addition, the bottom surface  14  of the packaging substrate  10  has a plurality of positioning holes  28  whose locations correspond with the positioning pins  26 . The stiffening ring  22  is firmly attached to the bottom surface  14  of the packaging substrate  10  through the positioning pins  26  and the corresponding positioning holes  28 . Preferably, an adhesive is applied to the bottom surface  24  of the stiffening ring  22  so that the stiffening ring  22  is permanently bonded to the bottom surface  14  of the packaging substrate  10 . 
   According to one embodiment of the present invention, the stiffening ring  22  is fabricated using a heat-resistant plastic or a metal. Typically, the material of the stiffening ring  22  must be able to withstand a temperature at least as high as the melting point of the heat-resistant molding compound so that the stiffening ring  22  can prevent the packaging substrate from warping. 
     FIG. 5  is a cross-sectional view showing the packaging substrate shown in  FIG. 3  disposed inside an encapsulating mold. In the present embodiment, the bottom surface  14  of the packaging substrate  10  faces up while the top surface  12  faces down inside the mold  30 . The encapsulating mold  30  has a bottom mold  32 , a top mold  34  and a gate  36 . The bottom mold  32  faces the top surface  12  of the packaging substrate  10  and has two molding cavities  38  for accommodating the chip  18  within the chip-disposing regions  16  and providing space  40  for injecting molding compound over the chip  18 . The top mold  34  faces the bottom surface  14  of the packaging substrate  10  and has two ring type runners  42  for accommodating two stiffening rings  22  and providing space  44  for injecting molding compound over the stiffening rings  22 . Furthermore, the gate  34  is located on the top mold  34  of the encapsulating mold  30  and connected to the mold cavities  38  and the runners  42  through other runners (not shown). To mold the packaging substrate  20 , a molding compound  46  is injected into the gate  36  to fill up the space  40  and  44  defined by the mold cavities  38  and the runner  42 . Hence, the chip  18  and the stiffening rings  22  are encapsulated by the molding compound  46  as shown in  FIG. 6 . 
   It should be noted that the bottom surface  14  of the packaging substrate  10  faces up inside the mold  30  to prevent the stiffening ring  22  from falling off from the bottom surface  14  of the packaging substrate  10  during mold injection. However, if the stiffening ring  22  is engaged to the bottom surface  14  of the packaging substrate  10  through any type of fastening mechanism such as a latch or positioning pins  26  with adhesive, the bottom surface  14  of the packaging substrate  10  may face down during mold injection. 
   When the molding process and a subsequent curing process are complete, the top and bottom view of the packaging substrate is as shown in  FIGS. 7 and 8 . The cured molding compound  46  encapsulates the chip  18  on the top surface  12  of the packaging substrate  10  to form a chip encapsulant  50 . Similarly, the cured molding compound  46  encapsulates the stiffening ring  22  on the bottom surface  14  of the packaging substrate  10  to form a stiffener encapsulant  48 . In  FIG. 8 , the position of the chip  18  on the top surface  12  is also shown in this drawing. Finally, the packaging substrate  10  is sawed to form individual package units in a singulation process, wherein each package unit encapsulates the chip  18 . The stiffening rings  22  are removed by sawing along the lines C-C, D-D, C 1 -C 1  and D 1 -D 1  in the singulation process. 
   The embodiment of the present invention is best applied to the fabrication of an optical land grid array (LGA) package. If the present invention is applied to fabricate an optical LGA package, the chip  18  in  FIG. 7  includes optical devices and the molding compound  46  for encapsulating the chip  18  is a transparent molding resin. It should be noted that the packaging substrate of the present invention has a structure capable of counterbalancing any warping stress on the substrate resulting form a difference in coefficient of thermal expansion between the transparent molding resin and the packaging substrate. 
     FIG. 9  is a top view showing the molded structure of a package having four chip disposing regions.  FIG. 10  is a bottom view showing the molded structure of a package having four chip disposing regions. In the present embodiment, the top surface  12  of the packaging substrate  10  has four chip-disposing regions  16  altogether. Each chip-disposing region  16  accommodates a chip  18 . Furthermore, a stiffening ring  22  is disposed on the bottom surface  14  of the packaging substrate  10  underneath the peripheral areas of the chip-disposing regions  16  to strengthen the packaging substrate  10 . Then, an encapsulant is formed to encapsulate the chip on the packaging substrate. In the present embodiment, a molding compound is injected to form a chip encapsulant encapsulating the chip  18 . Moreover, a stiffener encapsulant is further formed by the molding process to encapsulate each stiffening ring  22 , for example. Finally, the stiffening rings  22  are removed in the singulation process for sawing the packaging substrate into individual package units each encapsulating the chip  18 . 
   According to the aforementioned embodiment, the present invention can be applied to a packaging substrate having a plurality of chip-disposing regions. By disposing a stiffening ring on the bottom surface of a packaging substrate to correspond with each chip-disposing region, the warping stress on the packaging substrate resulting from mold injection is counterbalanced. 
   According to another embodiment of the present invention, the stiffening rings  22  in  FIG. 10  can be replaced by a plurality of stiffening bars  22   a  as shown in  FIG. 11  to prevent the packaging substrate from warping. Furthermore, it should be noted that the number of chip-disposing areas encapsulated by the stiffening ring or stiffening bars on the bottom surface  14  of the packaging substrate is not limited to one or two. In general, a plurality of stiffening rings or bars can be used to support a multiple of chip-disposing regions. 
   In summary, one major aspect of the present invention is the set up of a stiffening member such as a stiffening ring or a series of stiffening bars on the bottom surface of a packaging substrate to reinforce the structural strength of the packaging substrate. Furthermore, in the process of encapsulating the chip and the stiffening members with a molding compound, the stiffening members can also counterbalance the warping stress resulting from a difference in coefficient of thermal expansion between the top and bottom surface of the packaging substrate. Ultimately, the amount of warping in the packaging substrate is substantially minimized. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

Technology Category: 5