Abstract:
A DRAM module package, which uses a board on chip (BOC) manner to form multiple windows on the module printed circuit board (PCB). A chip is directly adhered to a backside of the module PCB and the bonding pads of the chip are arranged to be located at a center of the windows. The conductive wire penetrates through the window and connects crossing between a mounting pad in front of the module PCB and the bonding pad of the chip. Then, an encapsulation process is performed to protect the conductive wires, the mounting pads on the module PCB, the chip and the bonding pads.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims the priority benefit of Taiwan application serial no. 90122965, filed Sep. 19, 2001  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates in general to a DRAM module package, and more particularly, to a dynamic random access memory (DRAM) module package using a direct chip attach (DCA) method to attach a chip on a module printed circuit board (PCB) directly with a type of board on chip (BOC).  
           [0004]    2. Description of the Related Art  
           [0005]    In semiconductor circuit products that do not require very high pin counts, wire bonding is often used to electrically connect the bonding pads on a chip to leads of a lead frame. Such products include the small outline package (SOP) and the quad flat package 4  (QFP). The distribution of the pads on the chip includes the peripheral pads and the central pads  
           [0006]    The DRAM chip requires a lower pin count and has simple internal circuits, so that the pads can be designed as the central pads to share a part of the interconnection. The conventional DRAM module normally adapts the surface mount technology (SMT) to fix the packaged chip on a module PCB.  
           [0007]    To simplify the process, the direct chip attach method is used before encapsulation to attach the rear surface of the chip onto the surface of the module PCB. The wire bonding method is further used to form conductive wires connecting the pads on the active surface of the chip with the terminals on the surface of the module PCB. The encapsulation is then performed to complete the package of the DRAM module with a type of chip on board (COB).  
           [0008]    Referring to FIGS. 1 and 2, FIG. 1 shows a conventional DRAM module, and FIG. 2 shows an enlarged view of the local area  100  in FIG. 1 In FIG. 1, several DRAM devices  102  are arranged on the module PCB  104  For the convenience of description, FIG. 2 does not illustrate the molding compound applied on the exterior of the DRAM  102 . The bonding pads  106  are distributed on a center of the chip  108 . Via the conductive wires  110 , the bonding pads  106  are electrically connected to the mounting pads  112  on the module PCB  104  at two sides of the chip  108 .  
           [0009]    Referring to FIG. 3, a local cross-sectional view of FIG. 1 is shown. The chip has an active surface  108   a  and a rear surface  108   b  opposite the active surface  108   a  Using a tape  102 , the rear surface  108   b  of the chip  108  is attached to a top surface  104   a  of the module PCB  104 . Using the wire bonding technique, the conductive wires  110  are connected between the bonding pads  106  of the chip  108  and the mounting pads  112  on the module PCB  104 . A molding compound  114  is then applied to enclose the chip  108 , the conductive wires  110 , the mounting pads  112  and a part of the module PCB  104 . In this manner of chip on board, COB, the chip  108  can thus be assembled on the module PCB  104  directly.  
           [0010]    As mentioned above, as the bonding pads  106  of the chip  108  is the central distribution type, the longer distance for the bonding wires is longer than the peripheral bonding pads. Problems caused by overlong conductive wires  110  thus occur. In addition, the collapsed long conductive wires easily contact the edge of the chip  108 , so that a short circuit problem (short to chip edge) is caused.  
         SUMMARY OF THE INVENTION  
         [0011]    The invention shortens the conductive wires of the DRAM module, so that the resistance of the conductive wires is effectively reduced, and the operation performance of the DRAM is very much enhanced. Further, the short to chip edge problem caused by overlong conductive wires in the prior art can also be resolved  
           [0012]    Accordingly, the invention provides DRAM module package including a module PCB. The module PCB having a top surface and an opposing bottom surface comprises several windows penetrating through the module PCB. A module circuit is located in the module PCB. The module circuit includes several internal mounting pads and several external mounting pads. The internal mounting pads are located on the top surface of the module PCB, and the external mounting pads are located on a side of the module PCB. The internal mounting pads are electrically connected to each, and also to the external mounting pads. Each of a plurality of chips comprises an active surface and several bonding pads, which are located at approximately the center of the corresponding active surface. One chip corresponds to one window and has the active surface thereof attached to the bottom surface of the module PCB, and the bonding pads are substantially positioned at the center of the window. In addition, several conductive wires are connected to the bonding pads and the internal mounting pads through the window. Several molding compounds are used to cover the corresponding die, internal mounting pads, bonding pads, conductive wires and a part of the module PCB.  
           [0013]    In the invention, the internal mounting pads of the module PCB is closer to the bonding pads at the center of the active surface, so that the conductive wires are greatly reduced. The resistance is greatly reduced to enhance the performance of the DRAM module.  
           [0014]    The invention reduces the length of the conductive wires connecting between the bonding pads of the chip and the mounting pads of the module PCB, so that the problem of short to chip edge is resolved  
           [0015]    Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 shows the conventional DRAM module;  
         [0017]    [0017]FIG. 2 shows an enlarged view of a local area  100  of FIG. 1;  
         [0018]    [0018]FIG. 3 shows a local cross-sectional view of FIG. 1;  
         [0019]    [0019]FIG. 4 shows an embodiment of a DRAM module in the invention;  
         [0020]    [0020]FIG. 5 shows an enlarged cross-sectional view of a local area  200  in FIG. 4;  
         [0021]    [0021]FIG. 6 shows a local cross-sectional view of FIG. 4; and  
         [0022]    [0022]FIGS. 7A to  7 D are cross-sectional views showing the packaging process of the DRAM module according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Referring to FIGS. 4 and 5, FIG. 4 shows an embodiment of a DRAM module and FIG. 5 shows an enlarged view of the local area  200  in FIG. 4. In FIG. 4, a plurality of DRAM devices  202  is arranged on the module printed circuit board (PCB)  204  For the convenience of description, the molding compounding is not illustrated in FIG. 5. The module PCB  204  has a plurality of windows  216  penetrating through the module PCB  204 . A chip  208 , for example, a DRAM chip, has bonding pads  206  distributed on the center of an active surface  208   a  thereof and the center of the windows  216 . Conductive wires  210  are used to connect between the bonding pads  206  of the chip  208  and the mounting pads  204  on the module PCB. The electric connection between the bonding pads  206  and the corresponding mounting pads  212  is thus established  
         [0024]    In FIG. 6, the local cross-sectional view is illustrated. The module PCB  204  has a top surface  204   a  and a bottom surface  208   b.  The chip  208  has the active surface  208   a  and an opposing rear surface  208   b.  In addition, the bonding pads  206  are distributed on the center of the active surface  208   a.  The inside of the module PCB  204  has a module circuit  211  to electrically connect each chip  208 . The module circuit  211  has a plurality of internal mounting pads  212  and a plurality of external mounting pads  213  (as shown in FIG. 4). The internal mounting pads  212  are located on the top surface  204   a  of the module PCB in response to the bonding pads  206 . The internal mounting pads  212  are electrically connected to each other. The external mounting pads  213  are located at one side of the module PCB  204  and electrically connected to the internal mounting pads  212 . A tape  222  is used to attach the active surface  208   a  of the chip  208  to the bottom surface  204   b  of the module PCB  204 . In addition, conductive wires  210  through the windows  216  are formed to connect between the bonding pads  206  of the chip  208  and the internal mounting pads  212  of the module PCB  204 . A top molding compound  214 a and a bottom molding compound  214   b  are applied to cover the chip  208 , the conductive wires  210 , the internal mounting pads  212  and a part of the module PCB  204 . Thus, the chip  208  is directly assembled to the module PCB  204  to complete the packaging process on the DRAM module under a type of board on chip (BOC).  
         [0025]    To further describe the DRA module in details, please refer to FIGS. 7A to  7 D which are cross-sectional views of the DRAM module package in FIG. 6 In FIG. 7A, a module PCB  204  is formed. For example, the module PCB  204  is made of glass epoxy such as FR-4, FR-5 substrates, or BT substrate made of bismaleimide-triazine (BT) resin. The module PCB  204  has a top surface  204   a  and a bottom surface  204   b.  A window  216  is formed to penetrate through the module PCB  204 .  
         [0026]    A module circuit  211  that has several internal mounting pads  212  and external mounting pads  213  (as shown in FIG. 4) is designed in the module PCB 204 . The internal mounting pads  212  are located on the top surface  204   a  and electrically connected to each other. The external mounting pads  213  are located on a side of the module PCB  204  to electrically connect to the internal mounting pads  212 . A tape  222  is attached on the bottom surface  204   a  of the module PCB  204  with respect to two sides of the window  216 , so as to attach the active surface  208   a  of a chip  208  subsequently. The tape  222  can be replaced with a thermal glue.  
         [0027]    In FIG. 7B, the chip  208  has the active surface  208   a  and a rear surface  208   b  opposing the active surface  208   a  A plurality of bonding pads  206  is distributed on the center of the active surface  208   a.  The internal mounting pads  212  on the module PCB  204  are in response to the bonding pads  206 . Using the tape  222 , the active surface  208   a  of the chip  208  is adhered to the bottom surface  204   b  of the module PCB  204  with the bonding pads  206  located at the center of the window  216 . An operation temperature is then controlled to between about 150° C. and about 200° C. to cure the tape  222 . Preferably, the operation temperature is about 170° C.  
         [0028]    In FIG. 7C, a high frequency bonder is used to form conductive wires  210  by wire bonding technique. The conductive wires  210  include gold wires, for example. The conductive wires  210  penetrate through the window  216  to connect the bonding pads  206  of the chip  208  and the internal mounting pads  212  of the module PCB  204 . The operation temperature is controlled to between about 100° C. and about 150° C., and preferably is about 120° C.  
         [0029]    In FIG. 7D, a molding or coating process is used to encapsulate the active surface  208   a  and the rear surface  208   b  using a top molding compound  214   a  and a bottom molding compound  214   b,  respectively. A curing step is then performed on the top and bottom molding compound  214   a  and  214   b . The top molding compound  214   a  encapsulates the conductive wires  210 , the internal mounting pads  212 , the active surface  208   a  of the chip  208  and a part of the module PCB  204 . The bottom molding compound  214   b  encapsulates the rear surface  208   b  and a part of the module PCB  204  The material for forming the top and bottom molding compounds  214   a  and  214   b  includes epoxy. Thus, the chip  208  is directly attached on the module PCB  204  to complete a packaging process on the DRAM module by the board on chip, BOC, manner.  
         [0030]    Accordingly, the invention shortens the distance between the internal mounting pads of the module PCB and the bonding pads of the die, so that the conductive wires connected therebetween are effectively shortened The performance of the DRAM module is greatly enhanced since the resistance of the conductive wires is reduced.  
         [0031]    As the conductive wires across the bonding pads of the chip and the internal mounting pads of the module PCB are shortened, the short to chip edge problem caused by overlong conductive wires is resolved.  
         [0032]    The invention has at least the following advantages:  
         [0033]    (1) With the design of the window perforating through the module PCB, the internal mounting pads of the module PCB are closer to the bonding pads of the module PCB to reduce the length of the conductive wires. The resistance is reduced, and the performance of the DRAM module is enhanced.  
         [0034]    (2) The DRAM module package provided in the invention attaches the DRAM chip with centrally distributed bonding pads on the PCB directly, and shortens the length of the conductive wires connecting the bonding pads of the chip and the internal mounting pads of the PCB. The short to chip edge caused by overlong conductive wire is resolved.  
         [0035]    Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.