Abstract:
Discrete semiconductor packages are described. The discrete package contains: a lead frame pad which has a first surface and a second surface, wherein the second surface which is the opposite surface of the first surface; leads connected to a side of the lead frame pad; a semiconductor chip attached to the first surface of the lead frame pad; a ceramic layer that directly contacts the second surface of the lead frame pad; and a molding material that entirely encapsulates the lead frame pad, the semiconductor chip, and a portion of the ceramic layer, except for a portion of the leads and the second surface of the ceramic layer. Methods for making such discrete packages are also described.

Description:
REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority of Korean Patent Application No. 2002-58857, filed Sep. 27, 2002, the entire disclosure of which is incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to semiconductor packages and methods for making the same. More particularly, the present invention relates to discrete packaging for semiconductor devices having an insulated ceramic heat sink and methods for making the same.  
         BACKGROUND OF THE INVENTION  
         [0003]    [0003]FIG. 1 illustrates a cross-sectional view of a conventional discrete package  100  for semiconductor devices (also referred to as a semiconductor package, discrete package, or package). Referring to FIG. 1, the discrete package  100  has a structure in which a ceramic layer  120 , a lead frame pad  130 , and a semiconductor chip  140  are sequentially formed on a heat sink  110 . The heat sink  110 , the ceramic layer  120 , the lead frame pad  130 , and the semiconductor chip  140  are encapsulated by any molding material  150 , such as an epoxy molding compound (EMC). To discharge heat to the outside of the package, the bottom of the heat sink  110  is usually not encapsulated and is therefore exposed to the outside of the package.  
           [0004]    A soldering process is often performed on the heat sink  110 , the ceramic layer  120 , the lead frame pad  130 , and the semiconductor chip  140  using a solder formed of PbSnSb. The soldering process attaches the ceramic layer  120  on the heat sink  110 , the lead frame pad  130  on the ceramic layer  120 , and the semiconductor chip  140  on the lead frame  130 . However, as well known in the art, ceramics are materials that can be difficult to be solder to other materials. Thus, in order to attach the ceramic layer  120  to the heat sink  110  and to the lead frame pad  130 , upper and lower surfaces  120   a  and  120   b  of the ceramic layer  120  are often coated with any conductive layer pattern. The conductive layer pattern may be formed of a conductive material like gold (Ag). Costs for fabricating a ceramic layer  120  coated with a conductive layer pattern are more expensive (by about three times) than a ceramic layer  120  that is not coated with a conductive layer pattern. Moreover, the soldering process must be carried out three times to successively attach the heat sink  110 , the ceramic layer  120 , the lead frame pad  130 , and the semiconductor chip  140 . This lengthy procedure can increase the costs for manufacturing the discrete package  100 .  
           [0005]    [0005]FIG. 2 is a cross-sectional view of another conventional discrete package  200 . As shown in FIG. 2, the discrete package  200  contains a direct bonding copper (DBC) substrate  210  that is used for thermal insulation and discharge. The DBC substrate  210  has a structure in which a lower copper layer  212  and an upper copper layer  216  are bonded to the lower and upper surfaces, respectively, of a ceramic layer  214 . A semiconductor chip  220  is attached to an upper surface of the upper copper layer  216  using a soldering process. Leads (not shown) are formed on the upper copper layer  216  of the DBC substrate  210 . The DBC substrate  210  and the semiconductor chip  220  are encapsulated by any molding material  230  so that the lower surface of the lower copper layer  212  and portions of the leads connected to the upper copper layer  216  are not encapsulated and are exposed to the outside of the molding material  230 .  
           [0006]    By using the DBC substrate  210 , the discrete package  200  improves its insulating characteristics and thermal transfer efficiency. To make the discrete package  200 , however, a two-step soldering process is performed between the DBC substrate  210  and the leads, as well as between the DBC substrate  210  and the semiconductor chip  220 . This two-step soldering process requires high manufacturing costs. Also, costs for manufacturing the DBC substrate  210  are more expensive (about eight times) than the costs for manufacturing a bare ceramic layer.  
           [0007]    [0007]FIG. 3 is a cross-sectional view of still another conventional discrete package  300 . As shown in FIG. 3, the discrete package  300  contains a lead frame pad  310  (which also acts as a heat sink) and a semiconductor chip  320  attached on an upper surface  310   a  of the lead frame pad  310  by a soldering process. The lead frame pad  310  and the semiconductor chip  320  are entirely encapsulated by a molding material  330 . Since the lower surface  310   b  of the lead frame pad  310  is encapsulated by the molding material  330 , the discrete package  300  can be insulated from the outside.  
           [0008]    Manufacturing the discrete package  300  only requires a one-step soldering process between the lead frame pad  310  and the semiconductor chip  320 , thereby reducing manufacturing costs. As well, using the molding material  330  enables the discrete package  300  to be insulated from the outside. Despite these advantages, however, the discrete package  300  is inconvenient to use because the thermal transfer efficiency of EMC (the material often used in the molding material  330 ) is more than ten times lower than those of ceramic materials.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides a discrete package having a high insulating and thermal transfer efficiency, yet which can be manufactured at a low cost.  
           [0010]    According to one aspect of the present invention there is provided a discrete package containing: a lead frame pad with a first surface and a second surface, wherein the second surface is opposite the first surface; leads connected to a side of the lead frame pad; a semiconductor chip attached to the first surface of the lead frame pad; a ceramic layer which is positioned to directly contact the second surface of the lead frame pad; and a molding material which entirely encapsulates the lead frame pad, the semiconductor chip, and a portion of the ceramic layer, except the leads and the second surface of the ceramic layer.  
           [0011]    According to another aspect of the present invention there is provided a discrete package containing: a lead frame pad which has a first surface and a second surface, the second surface being opposite the first surface; leads which are connected to a side of the lead frame pad; a semiconductor chip which is attached to the first surface of the lead frame pad; a ceramic layer which is attached with the second surface of the lead frame pad via an epoxy; and a molding material which entirely encapsulates the lead frame pad, the semiconductor chip, and a portion of the ceramic layer, except the leads and the second surface of the ceramic layer.  
           [0012]    In both aspects of the invention, the leads can be formed to have steps with respect to the lead frame pad. As well, the discrete package can further include wires which electrically connect the leads to the semiconductor chip. Also, the lead frame pad can be formed to a thickness of 0.5 mm. Further, the discrete package can further include an adhesive between the lead frame pad and the semiconductor chip. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The above and other aspects and advantages of the present invention will become more apparent by describing in detail the preferred aspects thereof with reference to the attached drawings in which:  
         [0014]    [0014]FIG. 1 is a cross-sectional view of a conventional discrete package;  
         [0015]    [0015]FIG. 2 is a cross-sectional view of another conventional discrete package;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of still another conventional discrete package;  
         [0017]    [0017]FIG. 4 is a plan view of an upper surface of a discrete package according to the present invention;  
         [0018]    [0018]FIG. 5 is a plan view of a lower surface of a discrete package according to one aspect of the present invention;  
         [0019]    [0019]FIG. 6 is a cross-sectional view of a discrete package according to one aspect of the present invention, taken along the line A-A′ of FIGS. 4 and 5;  
         [0020]    [0020]FIG. 7 is cross-sectional view of a discrete package according to another aspect of the present invention, taken along the line A-A′ of FIGS. 4 and 5;  
         [0021]    [0021]FIGS. 8 through 10 are views explaining a method of fabricating a discrete package according to an aspect of the present invention; and  
         [0022]    [0022]FIG. 11 is a cross-sectional view explaining a method of fabricating a discrete package according to another aspect of the present invention. 
     
    
       [0023]    FIGS.  1 - 11  illustrate specific aspects of the invention and are a part of the specification. Together with the following description, the Figures demonstrate and explain the principles of the invention. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. The same reference numerals in different drawings represent the same element, and thus their descriptions will not be repeated.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The present invention will now be described more fully with reference to the accompanying drawings, in which preferred aspects of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the aspects set forth herein. Rather, these aspects are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art.  
         [0025]    [0025]FIGS. 4 and 5 are plan views of upper and lower surfaces, respectively, of a discrete package according to one aspect of the present invention. As shown in FIG. 4, an upper surface of a molding material  450  in the discrete package is exposed. Leads  430  are formed on a side of the discrete package. There is no limit to the number of the leads  430  and the number may be determined according to the type of a semiconductor chip contained in the discrete package. In the aspect of the invention illustrated in FIGS. 4 and 5, the number of leads  430  is set to three for convenience. An upper surface of the discrete package contains a step  455 . As depicted in FIG. 5, a portion of the molding material  450  and a lower surface  410   b  of a ceramic layer  410  are exposed at a lower surface of the discrete package.  
         [0026]    [0026]FIG. 6 is a cross-sectional view of a discrete package  600  according to an aspect according to the present invention, taken along the line A-A′ of FIGS. 4 and 5. As illustrated in FIG. 6, the discrete package  600  contains a ceramic layer  410  (which operates as an insulating heat sink) having upper and lower surfaces  410   a  and  410   b . The discrete package also contains a lead frame pad  420  formed on the ceramic layer  410  and a semiconductor chip  440  formed on the lead frame pad  420 . The lead frame pad  420  has upper and lower surfaces  420   a  and  420   b . Leads  430  are connected to a side of the lead frame pad  420  via a bent portion  435   a . A portion of the ceramic layer  410 , the lead frame pad  420 , and the semiconductor chip  440  are entirely encapsulated by a molding material  450 . Only the lower surface  410   b  of the ceramic layer  410  and a portion of the lead  430  are not encapsulated and therefore exposed to the outside the molding material  450 . The discrete package also contains a groove  460  that is formed to pass through a portion of the molding material  450 . When a screw is inserted into the groove  460 , the discrete package  600  can be engaged with an outer heat sink (not shown).  
         [0027]    An adhesive, such as a solder, may be positioned on the upper surface  420   a  of the lead frame pad  420  to adhere the semiconductor chip  440  to the lead frame pad  420 . However, the lower surface  420   b  of the lead frame pad  420  is directly bonded to the upper surface  410   a  of the ceramic layer  410  without an adhesive. In other words, the lead frame pad  420  is bonded to the ceramic layer  410  by using the molding material  450 . When manufacturing the discrete package  600 , a soldering process is not performed between the ceramic layer  410  and the lead frame pad  420 . Thus, there is no need to form a conductive layer pattern on the upper surface  410   a  of the ceramic layer  410  for the soldering process. Since the discrete package  600  uses the bare ceramic layer  410  (which can be fabricated at a cost of about three times less than a ceramic layer coated with a conductive layer pattern) as an insulating heat sink, the present invention is able to reduce the manufacturing costs. The ceramic layer  410  is also cheaper than using a DBC substrate, which is itself more expensive than the ceramic layer coated with the conductive layer pattern. Also, the thermal transfer efficiency of the discrete package  600  is higher than that of the discrete package  300  which is insulated using a portion of a molding material. In general, a semiconductor package containing EMC with a filler of 80 wt % (which is used as a molding material) has a thermal transfer efficiency of 2.09 W/m° C. at a temperature of about 25° C. A semiconductor package contains a ceramic layer (made of Al 2 O 3  with 96 degree of purity) has a thermal transfer efficiency of 27 W/m° C. at a temperature of about 25° C.  
         [0028]    [0028]FIG. 7 is a cross-sectional view of a discrete package  700  according to another aspect of the present invention and is taken along the line A-A′ of FIGS. 4 and 5. In this aspect of the invention, elements that are the same as in FIG. 6 are indicated with the same reference numerals and their descriptions will not be repeated.  
         [0029]    As shown in FIG. 7, the discrete package  700  is different from the discrete package  600  in that an epoxy  470  is used to bond the lower surface  420   b  of a lead frame pad  420  with the upper surface  410   a  of a ceramic layer  410  (which functions as an insulating heat sink). The epoxy  470  is formed to a thickness of about 20 μm and has a thermal transfer efficiency of 4 W/m° C. at a temperature of about 25° C. The discrete package  700  compensates for the disadvantages of the conventional discrete packages and yet has the same advantages as the discrete package  600 .  
         [0030]    Our experiments revealed that the conventional discrete package of FIG. 1 has a thermal resistance of 2.10° C./W while the discrete package  700  of FIG. 7 has a thermal resistance of 0.66° C./W. In other words, the thermal resistance of a discrete package according to the present invention is much lower than that of a comparable conventional package. In these experiments, the respective discrete packages shown in FIGS. 1 and 7 contained a lead frame pad having a thickness of 1.3 mm; an adhesive having a thickness of 20 μm, through which a semiconductor chip was bonded with the lead frame pad; a silicon semiconductor chip having a cross-sectional area of 5.8×4.9 mm 2  and a thickness of 0.3 mm; and an EMC encapsulant having a thickness of 0.4 mm. The discrete package of FIG. 7 contained a ceramic layer having a cross-sectional area of 8.8×72 mm 2  and a thickness of 0.5 mm, and an epoxy having a thickness of 20 μm through which the lead frame pad was bonded with the ceramic layer.  
         [0031]    [0031]FIGS. 8 through 10 illustrate a method of fabricating a discrete package according to one aspect of the present invention. In particular, FIG. 9 is a cross-sectional view of a discrete package according to the present invention and is taken along the line B-B′ of FIG. 8.  
         [0032]    As shown in FIGS. 8 and 9, a semiconductor chip  440  is attached to a chip bonding region of a lead frame pad  420 . A side of the lead frame pad  420  is attached to leads  430 . Although not shown in the drawings, the semiconductor chip  440  may be attached to the lead frame pad  420  using an adhesive, such as a solder. Next, as shown in FIG. 10, wire bonding is performed to electrically connect the semiconductor chip  440  to the leads  430  using wires  480 . Thereafter, as shown in FIG. 6, the structure of FIG. 10 and a ceramic layer  410  are placed in molding equipment and a molding process as known in the art is performed using EMC as the molding material. Then, a general trimming process as known in the art is performed on the resulting structure to obtain a discrete package according to one aspect of the present invention.  
         [0033]    [0033]FIG. 11 is a cross-sectional view illustrating a method of fabricating a discrete package according to another aspect of the present invention. First, a method similar to that explained above (with reference to FIGS. 8 through 10) is carried out. Next, as shown in FIG. 11, a bare ceramic layer  410  is attached to a surface of a lead frame pad  420  using epoxy  470 . The other surface of the lead frame pad  420  is then attached to a semiconductor chip  440 . Thereafter, as shown in FIG. 7, general molding and trimming processes are performed on the resulting structure as known in the art to obtain a discrete package.  
         [0034]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.  
         [0035]    As described above, a discrete package according to the present invention uses a ceramic layer as an insulating heat sink, thereby increasing the thermal transfer efficiency of the discrete package. When manufacturing the discrete package, a soldering process is not performed to bond a lead frame pad with the ceramic layer. Therefore, the ceramic layer does not need to be coated with a conductive layer pattern. Consequently, the discrete package according to the present invention contains a bare ceramic layer that is cheaper than a ceramic layer coated with a conductive layer pattern, thereby reducing manufacturing costs.