Abstract:
A mold ( 1 ) for a semiconductor chip ( 9 ) has two mold halves ( 2, 3 ). One mold half ( 3 ) includes sealing means ( 10 ) adapted to exert a sealing pressure between a surface of the mold and a surface ( 18 ) of a substrate ( 8 ) located in the mold ( 1 ) during a molding operation.

Description:
SUMMARY OF THE INVENTION  
         [0001]    The invention relates to a mold, and especially, a mold for a semiconductor chip.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    One recently developed semiconductor package is the quad, flat, no-lead package sometimes known as a QFN package. QFN packages generally fall into one of two categories, either matrix QFN or block QFN. In both types of package the substrate to which the die (or semiconductor chip) is attached is normally a metal lead frame.  
           [0003]    A typical lead frame  7  for a matrix QFN is shown in FIG. 1. For the matrix QFN package a single die is attached to each die pad  8  and during molding, each die is located within its own mold cavity. Hence, each die is individually encapsulated on the lead frame  7 . After molding, the lead frame  7  is singulated by cutting the metal of the lead frame between each encapsulated die.  
           [0004]    A typical lead frame  90  for a block QFN is shown in FIG. 9. The lead frame  90  has four separate blocks  91  and each block  91  has a square array of die pads to which a die is attached. During molding, all the dies on the die pads  92  in the same block  91  are located in the same mold cavity. Hence, all the dies in the same block are encapsulated in the same block of molding and singulation is performed by cutting through the lead frame  90  and the molding material between the dies in a block  91 .  
           [0005]    In both matrix and block QFN packages the overall size of the package is close to the size of the semiconductor chip located within the package. In addition, QFN packages are lead frame based packages and have an exposed die paddle. Hence, QFN packages enable good electrical performance and thermal characteristics, as the exposed die paddle can be used as a ground plane and a heat sink.  
           [0006]    However, one problem with QFN packages, and indeed other semiconductor packages which are molded on only one side of the substrate, is the problem of epoxy flash seeping onto the other side of the lead frame during the molding operation.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with a first aspect of the present invention, there is provided a mold for a semiconductor chip comprising two mold halves, one mold half comprising sealing means adapted to exert a sealing pressure between a surface of the mold and a surface of a substrate located in the mold during a molding operation.  
           [0008]    In accordance with a second aspect of the present invention, there is provided a method of molding material around a semiconductor chip mounted on a substrate, the method comprising inserting a substrate having a semiconductor chip mounted on a mounting portion of the substrate into a mold, closing the mold halves and applying a sealing pressure between a surface of the mold and a surface of the substrate to substantially prevent molding material entering between the surfaces of the mold and the substrate; injecting a molding material into the mold cavity to be molded around the semiconductor chip; after the molding operation has been completed, separating the mold halves and removing the sealing pressure between the surfaces of the mold and the substrate; and removing the molded semiconductor chip and substrate from the mold.  
           [0009]    Preferably, the mold surface contacting the surface of the substrate is a compressible, and is typically elastically deformable. For example, the surface may comprise an elasticity deformable material, such as rubber or an elastomeric material.  
           [0010]    In one example of the invention, the sealing means may comprise a suction means formed in the mold surface contacting the surface of the substrate. Typically, this may be provided by holes in the mold surface which are coupled to a vacuum generating device which when activated, causes the surface of the substrate to be sucked against the mold surface.  
           [0011]    Preferably, the holes are arranged such that there is a substantially uniform sealing pressure across the surfaces.  
           [0012]    In addition, or alternatively, the surface of the mold and/or the substrate may be configured such that when the mold halves are closed, the surface of the substrate is forced against the surface of the mold to generate the sealing pressure.  
           [0013]    Preferably, the surface of the substrate to which the sealing pressure is applied is a surface of the mounting portion opposite the surface of the mounting portion on which the semiconductor chip is mounted.  
           [0014]    In accordance with a third aspect of the present invention, there is provided a substrate for mounting a semiconductor chip thereon, the substrate comprising a mounting portion adapted to have a semiconductor chip mounted on a first surface of the mounting portion, and a second surface of the mounting portion, opposite the first portion, including a recess extending continuously around the second surface, and the recess being adjacent to and within the edge of the second surface.  
           [0015]    Preferably, the substrate of the third aspect is for use with the mold of the first aspect and the method of molding of the second aspect.  
           [0016]    Preferably, the substrate is a lead frame, and typically comprises a metal material.  
           [0017]    Typically, the substrate is for a quad, flat, no-lead (QFN) semiconductor package.  
           [0018]    Typically, there are two or more recesses extending around the second surface, one recess being closer to the edge of the second surface than the other recess. Preferably, the grooves are spaced apart from each other and typically, are substantially parallel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    A mold and a substrate in accordance with the invention will now be described with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a plan view of a portion of a conventional lead frame for a matrix QFN package;  
         [0021]    [0021]FIG. 2 is a cross-sectional view of a section of a first example of a mold for a matrix QFN package showing a lead frame and semiconductor chip located within the mold;  
         [0022]    [0022]FIG. 3 is a cross-sectional view of a second example of a mold for a matrix QFN package;  
         [0023]    [0023]FIG. 4 is a cross-sectional view of a third example of a mold for a matrix QFN package;  
         [0024]    [0024]FIG. 5 is a cross-sectional view of a fourth example of a mold for a matrix QFN package;  
         [0025]    [0025]FIG. 6 is a cross-sectional view of a fifth example of a mold for a matrix QFN package;  
         [0026]    [0026]FIG. 7 is a cross-sectional view of a sixth example of a mold for a matrix QFN package;  
         [0027]    [0027]FIG. 8 is a cross-sectional view of a seventh example of a mold for a matrix QFN package;  
         [0028]    [0028]FIG. 9 is a plan view of a conventional lead frame for a block QFN package;  
         [0029]    [0029]FIG. 10 is a plan view of part of a lower mold half for a block QFN package;  
         [0030]    [0030]FIG. 11 is a cross-sectional view along the line AA of FIG. 10;  
         [0031]    [0031]FIG. 12 is a cross-sectional view along the line BB of FIG. 10;  
         [0032]    [0032]FIG. 13 is a bottom view of one half of a lead frame in accordance with the invention for a matrix QFN package;  
         [0033]    [0033]FIG. 14 is an enlarged view of the region B of FIG. 13 showing a first example of a die paddle;  
         [0034]    [0034]FIG. 15 is a cross-sectional view of the enlarged region B shown in FIG. 14;  
         [0035]    [0035]FIG. 16 is an enlarged view of region C in FIG. 13;  
         [0036]    [0036]FIG. 17 is a view similar to FIG. 14 but showing a second example of a die paddle;  
         [0037]    [0037]FIG. 18 is a cross-sectional view of FIG. 17; and  
         [0038]    [0038]FIG. 19 is an enlarged view of the region D shown in FIG. 18. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    [0039]FIG. 1 shows a plan view of a portion of a conventional lead frame  7  for a matrix QFN package. The lead frame  7  has a number of die paddles  8  to which a die is attached, in use.  
         [0040]    [0040]FIG. 2 is a cross-sectional view through a section of a mold  1  which comprises an upper mold half  2  and a lower mold half  3  which define a number of mold cavities  4  (only one shown). The lower mold half  3  includes a recess  5  which is in communication with a hole  12  formed in the lower mold half  3 . The lower mold half  3  has a deformable coating on surface  6 . Typically, the deformable coating is an elastically deformable coating which is preferably heat resistant and may be, for example, a silicone or fluorine based rubber coating, which is typically laminated onto the surface  6  of the lower mold half  3 .  
         [0041]    In use, the lead frame  7  with a semiconductor chip  9  attached to each die paddle  8  is inserted into the mold  1  and the mold halves  2 ,  3  are closed so as to clamp onto lead fingers  17  of the lead frame  7 . When the lead frame  7  is located within the mold  1 , as shown in FIG. 2, one mold cavity  4  surrounds each chip  9  and a hole  12  aligns with each die paddle  8 . A vacuum is applied to the holes  12  in the lower mold half  3  to draw air through the holes  12  and out of the recess  5  below the die paddle  8 . This causes the die paddle  8  to be sucked onto the coating on the surface  6  and the suction generates a sealing pressure between the bottom surface  18  of the die paddle  8  and the surface  6 . Therefore, when molding material is injected into the mold cavity  4 , the sealing pressure between the surface  18  and the surface  6  prevents or minimises seepage of molding compound between the surface  18  and the surface  6 .  
         [0042]    [0042]FIG. 3 shows a second example of a mold  20  which includes an upper mold half  2  and a lower mold half  21 . In this example, the lower mold half  21  does not have the vacuum holes  12  or the pocket  5 . Instead, the lower mold half  21  has a raised surface  22  corresponding to the die paddle  8  of the lead frame  7 . The raised surface  22  also has a deformable coating similar to the coating on the surface  6  in FIG. 2. When the mold halves  2 ,  3  are closed, the raised surface  22  forces the die paddle  8  upwards relative to the lead fingers  17 . This force exerted by the raised surface  22  on the under surface  18  of the die paddle  8  generates sealing pressure between the surface  18  and the raised surface  22  to minimise seepage of molding material between the surface  18  and the raised surface  22 .  
         [0043]    [0043]FIG. 4 shows a third example of a mold  25  which combines the raised surface feature of the lower mold half  21  in FIG. 3 with the vacuum hole feature of the lower mold half  3  in FIG. 2 on a lower mold half  26 . The lower mold half  26  includes a raised surface  27  with a coating similar to the coatings on the surfaces  6  and  22 , and also including holes  12  below the die paddle  8  so that the sealing pressure between the lower surface  18  of the die paddle  8  and the surface  27  is generated by a combination of the raised surface  27  and the vacuum applied to the holes  12 .  
         [0044]    A fourth example of a mold  30  is shown in FIG. 5. The mold  30  includes an upper mold half  31  and a lower mold half  33  which is similar to the lower mold half  21 . However, the lower mold half  33  has a lowered central surface  34  surrounded by a raised edge  35  which cooperates with a lowered edge  36  on the upper mold half  31 . Located between the mold halves  31 ,  33  is the lead frame  7  with the die paddle  8  on which the semiconductor chip  9  is attached. The lower surface  18  of the die paddle  8  is forced against a coating on the surface  34  by the lowered edge  36  pushing the central section of the lead frame  7 , including the die paddle  8  against the lower mold half  33  so that the lower surface  18  is forced against the surface  34  of the lower mold half  33  to generate a sealing pressure to minimise seepage of molding material between the surfaces  18 ,  34  when molding material is injected into the cavity  4 .  
         [0045]    [0045]FIG. 6 shows a fourth example of a mold  40  that includes an upper mold half  31  and a lower mold half  42 . The lower mold half  42  is similar to the lower mold half  33  and also include a raised edge  35 . However, the lower mold half  42  also includes vacuum holes  12 . Hence, the sealing pressure between the lower surface  18  of the die paddle  8  and surface  43  of the insert  42  is generated by a combination of the lowered edge  36  pushing the central section of the lead frame  7  downwards towards the surface  43  and suction applied to the suction holes  12  sucking the surface  18  of a die paddle  8  onto the coating of the surface  43 .  
         [0046]    [0046]FIG. 7 shows a sixth example of a mold  45  which includes the upper mold half  31  and a lower mold half  46 . The lower mold half  46  is similar to the lower mold half  33  except that the lower mold half  46  also includes the raised surface feature of the lower mold half  21 . Hence, the lower mold half  46  includes a raised edge  35  surrounding an inner surface  47  having a central raised surface  48 . Therefore, the combination of the lower edge  36  pushing the inner section of a lead frame  7  downwards with the surface  48  pushing the die paddle upwards generates a sealing pressure between the surface  48  and surface  18  of the die paddle  8  to minimise seepage of molding material between the surfaces  18 ,  48  during molding.  
         [0047]    [0047]FIG. 8 shows a seventh example of a mold  50  which includes the upper mold half  31  and a lower mold half  51 . The lower mold half  51  is similar to the lower mold half  46 , except that the lower mold half  51  also includes vacuum holes  12 . Hence, the sealing pressure between the lower surface  18  of the die paddle  8  and surface  52  of the lower mold half  51  is generated in the same way as the sealing pressure generated in the mold  45 , except that there is the additional use of suction applied to the holes  12  to suck the die paddle  8  towards the lower mold half  51 , so that the under surface  18  of the die paddle  8  is sucked onto the coating material on the surface  52 .  
         [0048]    [0048]FIG. 9 is a plan view of a conventional lead frame  90  for a block QFN package. The lead frame  90  has a four separate blocks  91 , and each block  91  has an array of 9×9 die paddles  92 . In use, a semiconductor chip (not shown) is attached to each die paddle  92 .  
         [0049]    A lower mold half  83  for molding a molding material around chips attached to the lead frame  90  is shown in FIGS.  10  to  12 . The lower mold half  83  includes a number of inserts  10  located in recesses in the lower mold half  83 . Each insert  10  has a deformable coating  11  on its surface and corresponds to one block  91  of die paddles  92 . Typically, the deformable coating  11  is an elastically deformable coating which is preferably heat resistant and may be, for example, a silicone or fluorine based rubber coating, which is typically laminated onto the surface of the cavity insert  10 . Each cavity insert  10  has a number of holes  112  extending through the cavity insert  10  and the coating  11 . The holes  112  are connected to passageways  13  within the lower mold half  83  which are in communication with a suction port  14 . The suction port  14  can be coupled to a vacuum device (not shown) to suck air through the holes  112  via the passageways  13  and out of the port  14 . The holes  112  are arranged as a set of inner holes  15  and a set of outer holes  16  so that the set of outer holes  16  are adjacent to the perimeter of a block  91  and the set of inner holes  15  are adjacent to the centre of the a block  91 . This helps maintain a substantially uniform suction pressure across each block  91 .  
         [0050]    In use, the lead frame  90  is inserted into a mold including the lower mold half  83  with the attached dies on the opposite side of the lead frame to the inserts  1   0 . The mold halves are then closed and suction is applied to the port  14  to suck the blocks  91  on to the coating  1   1  of the respective insert  1   0 . When the molding material is injected into the mold, the suction force between each block  91  and the respective insert  10  helps minimise seepage of molding material onto the underside of the blocks  91  of the lead frame  90 .  
         [0051]    In all the above examples, the lead frames  7 ,  90  are standard QFN lead frames. However, flashing can be further minimised by using a modified QFN lead frame, as described below.  
         [0052]    [0052]FIG. 13 is a plan view of a modified lead frame  60  for a matrix QFN package, which includes a number of die paddles  61 . An enlarged view of one die paddle  61  is shown in FIG. 14 and a cross-sectional view shown in FIG. 15. The die paddle  61  has an outer edge  62  and located within the outer edge  62  (but adjacent to the outer edge  62 ) is a continuous groove  63  which is typically formed by etching of the lower surface of the die paddle  61 . FIG. 16 shows an enlarged view of the section C in FIG. 15 which shows a cross-section of the groove  63  in more detail.  
         [0053]    The inventors have found that the use of a groove  63  on the underside of the die paddle  61  acts to reduce flashing of molding material on the underside of the die paddle  61 , especially when used in conjunction with the molds  1 ,  20 ,  25 ,  30 ,  40 ,  45 ,  50  shown in FIGS.  1  to  8  or the lower mold half  83  shown in FIGS.  9  to  12 .  
         [0054]    A second example of a die paddle  71 , which can be used instead of the die paddle  61 , is shown in FIGS.  17  to  19 . The die paddle  71  is similar to the die paddle  61  except that two grooves  72 ,  73  are provided within the edge of the die paddle  74 . As with the groove  61 , the grooves  72 ,  73  are typically etched into the die paddle  71 . Typically, the double grooves  72 ,  73  will have the advantage of further reducing flashing of the die paddle  71  compared to flashing of the die paddle  61 .  
         [0055]    As with the die paddle  61 , the die paddle  71  is preferably used on a lead frame which is molded using one of the molds described above and shown in FIGS.  1  to  8  or a mold including the lower mold half shown in FIGS.  9  to  12 .  
         [0056]    The invention has the advantage that by generating a sealing pressure between the underside of the die paddle and a surface of the mold, and by providing a continuous recess on the underside of the die paddle, flashing of the underside of a die paddle can be reduced.