Abstract:
A semiconductor package and method of producing the same has a semiconductor die having a first face and a second face. A coating material is coupled to the second face of the semiconductor die. A substrate having a cavity is provided wherein the semiconductor die is placed within the cavity. An encapsulant is used to encapsulate the second face of the semiconductor die placed in the cavity. Connection members are provided to couple the semiconductor die and the substrate in order to transfer signals between the semiconductor die and the substrate. Terminal members are couple to the substrate to connect the semiconductor package to an external device. In the semiconductor package, a thermal expansion coefficient of the coating material C and a thermal expansion coefficient of the encapsulant E should be approximately equal in value in order to limit the problems associated with warpage.

Description:
FIELD OF THE INVENTION 
     This invention relates to semiconductor devices and, more specifically, to a semiconductor package and method of manufacturing the same which reduces warpage during the cool down phase of the manufacturing process. 
     BACKGROUND OF THE INVENTION 
     In general, a semiconductor package is fabricated using a substrate such as lead frame, printed circuit board, circuit film, circuit tape, or the like. A semiconductor die is coupled to the substrate and packaged to be airtight to protected the semiconductor die from the outer environment as well as to enable the semiconductor package to be electrically connected to external devices. 
     Namely, each semiconductor package has the structure enabling the semiconductor package to protect the semiconductor die as well as to be mounted on an external device, for example a mother board, or the like. In order to complete such a structure, processes of various steps are required. 
     Generally speaking, there is Post Mold Cure (PMC) or reflow requiring thermal treatment in the processes of the various steps. On carrying out the PMC or reflow, a process temperature is increased up to a predetermined temperature, and then gradually decreased down to a room temperature. 
     In a general semiconductor package fabricating method, a face on which an integrated circuit of a semiconductor die is formed should be encapsulated with an encapsulant. In the PMC or reflow process, when the semiconductor package is cooled down after heating, a warpage occurs in the semiconductor package due to a heat expansion coefficient difference between the semiconductor die and the encapsulant. 
     Namely, the semiconductor die and the encapsulant have different coefficients of thermal expansion (CTE). The difference in the coefficients of thermal expansion brings about a warpage due to the difference of shrinkage on cooling-down. 
     Unfortunately, the warpage of the semiconductor package causes the following problems or disadvantages. A total height of the semiconductor package increases. Accordingly, a height of the semiconductor package may increases anywhere from 10 to 20% more than its original height due to warpage. Thus, a thin semiconductor package is no more. Moreover, when the semiconductor package having the warpage is mounted on an external device, it is difficult to mount the semiconductor package on the external device properly. 
     Therefore, a need existed to provide a device and method to overcome the above problem. 
     SUMMARY OF THE INVENTION 
     A semiconductor package and method of producing the same has a semiconductor die having a first face and a second face. A coating material is coupled to the second face of the semiconductor die. A substrate having a cavity is provided wherein the semiconductor die is placed within the cavity. An encapsulant is used to encapsulate the second face of the semiconductor die placed in the cavity. Connection members are provided to couple the semiconductor die and the substrate in order to transfer signals between the semiconductor die and the substrate. Terminal members are couple to the substrate to connect the semiconductor package to an external device. In the semiconductor package, a thermal expansion coefficient of the coating material C and a thermal expansion coefficient of the encapsulant E should be approximately equal in value in order to limit the problems associated with warpage. 
     The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of one embodiment of a semiconductor package according to the present invention; 
         FIG. 2  is a cross-sectional view of the semiconductor package according to one embodiment of the present invention for explaining a shrinkage difference using arrows when encapsulant, semiconductor die, and coating material are shrunk after a thermal expansion; 
         FIG. 3  is a flowchart of a method of fabricating one embodiment of a semiconductor package according to the present invention; 
         FIG. 4A  to  FIG. 4H  depicts a method of fabricating one embodiment of a semiconductor package according to the present invention; 
         FIG. 5  is a cross-sectional view of another embodiment of a semiconductor package according to the present invention; and 
         FIG. 6  is top plan view of  FIG. 5 , in which air vents are formed on a surface of a substrate. 
     
    
    
     Common reference numerals are used throughout the drawings and detailed descriptions to indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a cross-sectional view of one embodiment of a semiconductor package according to the present invention is illustrated. 
     As shown in  FIG. 1 , a semiconductor package includes a semiconductor die  1 . On a surface opposite to a integrated circuit forming face is a coating material  12 . A cavity  200  is formed in a substrate  20 . The cavity  200  is where the semiconductor die  1  is placed. An encapsulant  40  is used to encapsulate the integrated circuit forming face of the semiconductor die  1  lying in the cavity  200 . Electrically conductive wire  5  are used to connect electrically the semiconductor die  1  to the substrate  20  for signal transfer between the semiconductor die  1  and substrate  20 . 
     The semiconductor die  1  has bond pads  1   a  located so as to leave a predetermined interval from an inner face of the cavity  200  in the substrate  20 . 
     There are various forms for the substrate  20  used for the semiconductor package. The substrate  20  may be printed circuit board, circuit film, circuit tape, or the like. The materials for the substrate  20  are not specially restricted. The substrate  20  is generally constructed with at least two layers of electrical conductive patterns between which an insulating layer  21  is inserted. In the drawing, the double-layered electrical patterns  24  and  25  are shown exemplary. The electrical conductive patterns  24  and  25  are electrically connected to each other through via holes  26 . In the electrical conductive patterns  24  and  25 , most areas of the electrical conductive patterns  24  formed at the attaching face of conductive balls of the substrate  20  are covered with a solder resist  22 . Yet, some areas are exposed in part for input/output of electric signals. The exposed areas include areas of bond fingers  24   a  and ball lands  24   b . Meanwhile, the rest of the area of the electrical conductive patterns  24  and  25  formed on the surface of the substrate  20 , except the areas of the bond fingers  24   a  and ball lands  24   b , is covered with the resist  22 , thereby enabling to protect the electrical conductive patterns  24  and  25  as well as prevent the respective patterns from electrical short. 
     The semiconductor package according to the present invention, as shown in the drawing, uses electrically conductive wire  5  as contact means between the semiconductor die  1  and substrate  20 . The electrically conductive wire  5  is used for exchanging electric signals between the semiconductor die  1  and substrate  20 . One end of the electrically conductive wire  5  is bonded to bond pads  1   a  of the semiconductor die  1 . The other end of the electrically conductive wire  5  is bonded to bond fingers  24   a  of the substrate  20 . The electrically conductive wire  5  is made of a metal material having electro-conductivity such as one of Au, Al, Cu, and the like. It should be noted that the present invention is not limited to this scope of the metal materials. 
     Moreover, the ball lands  24   b , which are the exposed areas of the electrical conductive patterns of the substrate  20 , play a role in exchanging electrical signals inside the semiconductor package with an external device. 
     The conductive balls  30 , as shown in the drawing, are attached to the ball lands  24   b , respectively. The conductive balls  30  are made from a conductive metal material. The conductive balls  30  are generally solder balls. The conductive balls  30  may be made from electro-conductive materials such as Au, Cu, Al, and the like. It should be noted that the listing of the electro-conductive materials should not be seen as to limit the scope of the present invention. The conductive balls  30  are attached to the external device by welding when the completed semiconductor package is mounted on the external device or the like, thereby functioning as media connecting the semiconductor package and external device reciprocally. 
     The integrated circuit formed face of the semiconductor die  1  and the electrically conductive wires  5  are protected by the encapsulant  40 . The encapsulant  40  is mainly made of non-conductive paste or film, which is coated on the integrated circuit formed face of the semiconductor die  1  and then hardened by PMC. Besides, the method of fabricating the semiconductor package requires processes of thermal treatment such as reflow and the like as well as PMC. When the thermal treatment processes are carried out, the components of the semiconductor package have different CTE so as to differ in shrinkage as well as the degree of expansion. 
     The coating material  12  coated on the face opposite to the integrated circuit formed face of the semiconductor die  1  is made of a material of which the CTE is equal or similar to that of the encapsulant  40 . For instance, the coating material  12  can use the same material of the encapsulant  40  as well as an epoxy based resin. Moreover, if the CTE of the coating material  12  is C and the other CTE of the encapsulant  40  is E, a ratio C/E satisfies preferably the relation of 0.5≦C/E≦2. For instance, when EMC (epoxy mold compound) is used for the encapsulant  40 , a CTE of EMC is about 26.2 ppm. Therefore, a material satisfying the relation of 0.5≦C/E≦2 is selected for the coating material  12 . 
     Referring to  FIG. 2 , a cross-sectional view of the semiconductor package according to one embodiment of the present invention is illustrated.  FIG. 2  explains a shrinkage difference using arrows when encapsulant  40 , semiconductor die  1 , and coating material  12  are shrunk after heat expansion. 
     As shown in  FIG. 2 , lengths of the arrows indicate the sizes of the shrinkage forces when the coating material  12 , semiconductor die  1 , and encapsulant  40  are shrunk after thermal expansion. 
     Being relatively lower than CTE of the coating material  12  or encapsulant  40 , the semiconductor die  1  has the degree of shrinkage smaller than that of the coating material  12  or encapsulant  40  when being cooled after thermal expansion. 
     The coating material  12  and encapsulant  40  wrapping both faces of the semiconductor die  1  have CTEs bigger than that of the semiconductor die  1  so as to have the shrinkage size bigger than that of the semiconductor die  1  as well. 
     In this case, the coating material  12  and encapsulant  40  wrapping the two faces of the semiconductor die  1  have the CTE which are almost identical to each other so as to have the similar shrinkage forces. Thus, the shrinkage forces of the coating material  12  and encapsulant  40  cancelled each other so as to meet the reciprocal balance. Therefore, there occurs little to no warpage of the semiconductor package. 
     Namely, as explained in the above description of the present invention, materials having the same or similar CTE are laid on both faces of the semiconductor die  1  so as to make the shrinkage quantity similar or identical on the cooling step of the thermal treatment process such as PMC or the like. Thus, the warpage of the semiconductor package is prevented, thereby enabling the semiconductor package to maintain the original designed height. Moreover, being made of the metal material having excellent property of thermal-conductivity, the coating material  12  plays a role in helping heat dissipation of the semiconductor die  1  as a heat sink. 
     Moreover, when the semiconductor die  1  is detached individually from the wafer in order to attach the semiconductor die  1  to the substrate  20  after sawing the thin wafer, an ejector pin (not shown in the drawing) pushes the coating material  12  instead of the semiconductor die  1  so as to prevent the damage on the semiconductor die  1 . Namely, the coating material  12  prevents die scratch or die crack which may be caused by a direct contact between the ejector pin and semiconductor die  1 . 
     Referring to  FIG. 3 , a flowchart of a method of fabricating a semiconductor package according to the present invention is illustrated. 
     Referring to  FIG. 3 , in a first step  100 , back-grinding is carried out on the face opposite to the integrated circuit formed face of the wafer  10 . The back-grinded face is then coated with the coating material  12 . 
     Since the coating material  12  makes the handling of the wafer  10  easier, the wafer  10  is preferably back-grinded thinner than the related art. 
     In a second step  200 , sawing is carried out on the coated wafer  10  so as to divide the coated wafer  10  into units of individual semiconductor dies  1 . 
     In a third step  300 , a cover tape  60  is attached to a backside of the substrate  20  having the cavity  200  at a center thereof into which the semiconductor die  1  is inserted. 
     In a fourth step  400 , the sawed semiconductor die  1  is attached to the substrate  20  having the cavity  200 . 
     In a fifth step  500 , the semiconductor die  1  is electrically connected to the substrate  20 . The electrical connection is preferably achieved using conductive wires or bumps. 
     In a sixth step  600 , the integrated circuit formed area of the semiconductor die  1  and the areas of the bond fingers  24   a  of the substrate  20  are encapsulated with the encapsulant  40 . 
     In a seventh step  700 , the conductive balls  30  are attached to the ball lands  24   b  of the substrate  20 , respectively. 
     In an eighth step  800 , the cover tape  60  attached to the backside of the substrate  20  is removed so as to complete the package. 
     Thereafter, a process of marking on the coating material  12  may be carried out in addition. 
     Referring to  FIG. 4A  to  FIG. 4H , views of a method of fabricating one embodiment of a semiconductor package according to the present invention are illustrated. 
     As shown in  FIG. 4A , the wafer  10  is through the back-grinding process. And, illustrated schematically is a state that the coating material  12  is being coated on the back-grinded face. For reference, in order to fabricate the semiconductor die  1 , integrated circuit is firstly formed through a fabrication process on the round wafer  20  made of SiO 2  crystals. 
     The wafer  10  maintains a predetermined thickness when the integrated circuit is formed. The predetermined thickness should be minimized so as to be applied to the respective products, for which grinding is carried out on the face of the wafer  10  opposite to the integrated circuit formed face. 
     The coating material  12  is preferably coated using spin coating, which is not limited in the present invention. One of stencil coating, sputtering, tape attach, plating, and the like can be selectively applied thereto. For the selection of the coating material  12 , a physical property of the encapsulant  40  should be considered. As mentioned in the foregoing description, if the CTE of the coating material  12  is C and the other CTE of the encapsulant  40  is E, a ratio C/E satisfies preferably the relation of 0.5≦C/E≦2. 
     As shown in  FIG. 4B , the wafer  10  in  FIG. 4A  is sawed so as to divide the semiconductor dies into individual units. 
     Scribing lines are already formed on the wafer  10 . In a sawing process, a rotating saw wheel  80  is moved along the scribing lines so that blades of the saw wheel  80  divide the respective semiconductor dies  1  of the wafer  10  into the individual units. 
     The integrated circuit is formed at one face of the semiconductor die  1  divided into each individual unit, and the coating material  12  is formed at the other face. 
     As shown in  FIG. 4C , the cover tape  60  is attached to the backside of the substrate  20 . The cavity  200  at the central part of the substrate  20  provides a space in which the semiconductor die  1  is inserted, and the cover tape  60  provides an attachment face for the semiconductor die  1 . 
     As shown in  FIG. 4D , the semiconductor die  1  is attached to the substrate  20 . The semiconductor die  1  fabricated through the first and second steps is inserted in the cavity  200  of the substrate  20 , to which the cover tape  60  is attached in the third step, so as to be attached to the cover tape  60 . 
     As shown in  FIG. 4E , the bond pads  1   a  of the semiconductor die  1  and the bond fingers  24   a  of the substrate  20  are connected each other through the conductive wires  5 . 
     As shown in  FIG. 4F , encapsulated are the integrated circuit formed area of the semiconductor die  1  and the areas of the bond fingers  24   a  of the substrate  20  at which the conductive wires  5  are installed. 
     The encapsulant  40 , as mentioned already, is preferably made of a material of which the CTE is similar to that of the coating material of the semiconductor die  1  within a 10% error limit. 
     As shown in  FIG. 4G , the conductive balls  30  are attached to the ball lands  24   a  of the substrate  20 , respectively. 
     As shown in  FIG. 4H , the cover tape  60  attached to the backside of the substrate  20  is removed so as to complete the semiconductor package according to the present invention. 
     Thereafter, after the cover tape  60  has been removed, a process (not shown in the drawing) of marking product information on a surface of the coating material  12  using ink or laser may be carried out in addition. 
     Referring to  FIG. 5  and  FIG. 6 , a cross-sectional view of a semiconductor package according to another embodiment of the present invention and a layout in which air vents are formed on a surface of a substrate are illustrated respectively. 
     As shown in  FIG. 5  and  FIG. 6 , another embodiment of the present invention is shown. This embodiment is similar to the previous embodiment in aspect of using the semiconductor die  1  coated with a coating material of which heat expansion coefficient is similar to that of the encapsulant  40 . However, this embodiment is different from the previous embodiment in that air vents  29  are formed additionally at a surface of the substrate  20 . The air vents  29 , as shown in the  FIG. 6 , are formed by removing portions of the solder resist  22  coated on the backside of the substrate  20  like slots. 
     Arrows in  FIG. 5  indicate a flow path of the encapsulant  40  in an encapsulating process. Namely, while the semiconductor die  1  wire-bonded to the substrate  20  is placed in a metal mold, the encapsulant  40  is injected in a direction opposite to that of the integrated circuit formed face of the semiconductor die  1  so as to surround a circumference of the semiconductor die  1 . Likewise, when encapsulation is carried out, air contained in the encapsulant  40  is discharged outside through air vents  29 . 
     Accordingly, if the air vents  29  are formed at the semiconductor package itself, there is no chance for the encapsulant  40  to penetrate the ball lands  24   b . Besides, a flow characteristic of the encapsulant  40  is improved so as to prevent previously poor encapsulation caused by generation of voids and the like. 
     As shown in  FIG. 6 , a plurality of air vents  29  are formed on a top of the substrate  20  so as to extend long from a die hole  25  of the substrate  20  to the circumference like slot figures. 
     Each of the air vents  29  forms the slot figure connecting one corner of the die hole  25  to a closest corner of the substrate  20 . In the embodiment of the present invention, four air vents  29  are formed. The number of the air vents  29  is not limited to four, but can be adjusted in accordance with a quantity of encapsulation. 
     Moreover, when the encapsulating process is carried out using EMC and the like, the air vents  29  are formed at the surface of the substrate  20  so as to provide smooth flow of EMC in the metal mold. Therefore, the present invention enables one to provide uniform encapsulation surface as well as prevent the encapsulant  40  from penetrating the ball lands  24   b.    
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.