Abstract:
A molded plastic package for semiconductor devices incorporating a heat sink, controlled impedance leads and separate power and ground rings is described. The lead frame of the package, separated by a dielectric layer, is attached to a metal heat sink. It has more than one ring for power and ground connections. The die itself is attached directly onto the heat sink through a window on the dielectric and provides high power dissipation. The package is molded using conventional materials and equipment.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the design of a semiconductor package; and, in particular, the present invention relates to a semiconductor package designed for high electrical and thermal dissipation performances.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Semiconductor devices are becoming larger, integrating a larger number of circuits, and operating at increasingly higher clock frequencies. As a result, semiconductor devices are requiring, without compromising reliability, packages of increasingly higher lead count, and higher electrical and thermal performances.  
           [0005]    In the prior art, conventional plastic molded packages can dissipate up to 2 watts of power. With some improvements in the lead frame, and by adding a heat spreader or heat sink, a plastic molded package can dissipate up to 4 watts. A further improvement in power dissipation can be achieved by attaching the semiconductor device, also called the semiconductor “die”, onto an integral heat sink. Such a heat sink typically has a surface exposed to the ambient to conduct heat away from the package. An example of such a package, also called a “thermally enhanced” package, is shown in FIG. 1.  
           [0006]    [0006]FIG. 1 shows a thermally enhanced package comprising a semiconductor die  105  attached by a layer of thermally conductive epoxy to a metallic heat sink  101 . The input and output terminals of semiconductor die  105  are electrically coupled to connection terminals (“leads”) of a lead frame  103  by wire bonds  104 , which connect the bonding pads of semiconductor die  105  to individual leads in lead frame  103 . Lead frame  103  attaches to heat sink  101  by a layer of dielectric adhesive  107 . The thermally enhanced package is encapsulated in a plastic molding  102 . In package  100 , high thermal dissipation is achieved by attaching semiconductor die  105  directly onto the lower surface of heat sink  101  using a thermally conductive epoxy layer  106 .  
           [0007]    Although plastic molded packages are typically of high reliability, the incorporation of a heat sink in a thermally enhance plastic molded package, such as package  100  of FIG. 1, leads to failures which are directly related to the design and the material used in the heat sink. For example, heat sink  101  is often made of aluminum. The large difference between the thermal expansion coefficients (TCE) of the silicon die, at 3ppm/° C., and of aluminum, at 25 ppm/° C. induces significant strain on semiconductor die  105 . Such strain causes die-cracking and thus a package failure. For this reason, in the prior art, semiconductor die sizes are kept well below 10×10 mm to minimize the induced stress. Alternatively, a heat sink material with lower TCE can be chosen to minimize the large mismatch in the heat sink&#39;s and the semiconductor die&#39;s coefficients of thermal expansion.  
           [0008]    A similar mismatch in TCEs exists between heat sink  101  and the plastic molding  102 . Typically, a plastic molding compound has a TCE of about 17 ppm/° C. The thermal cycle package  100  experiences during assembly and normal operations induces high stress at the metal-to--molding interface (i.e. between heat sink  101  and plastic molding  102 ) which can lead to delamination, cracking of the molding, and die failures. For this reason, a close matching of the TCEs of heat sink  101  to plastic molding  102  is very desirable.  
           [0009]    During the assembly of package  100 , plastic molding  102  shrinks significantly after the molding operation and during post-mold curing, which is typically carried out at or about 175° C. The shrinking molding causes significant stress at the metal-to-molding interface, which can lead to delamination. Delamination is very undesirable and usually causes long-term reliability failures. Delamination can be minimized by including on the heat sink “locking” features, which strengthen mold adhesion, and by choosing a heat sink material with a TCE closer to that of the molding compound.  
           [0010]    In the prior art, frequency performance is limited by the electrical parasitic impedances of the lead frame to 50 MHz or less. The lead frame usually consists of a single metal layer without the ability to provide controlled impedance connections. U.S. Pat. No. 4,891,687, entitled “Multilayer Molded Plastic IC Package”, to Mallik et al, filed on Jan. 27, and issued on Jan. 2, 1990, discloses a package achieving a high electrical performance. However, the package disclosed in U.S. Pat. No. 4,891,687 requires two lead frames, and hence, such package is significantly more costly than a conventional plastic molded package.  
           [0011]    Furthermore, the prior art&#39;s use of long wire bonds between the semiconductor die and the lead frame increases the impedances of ground connections. A high impedance to a ground connection results in “ground bounce” and other electrical noises which further restrict the overall electrical performance of the conventional plastic molded package. In logic semiconductor devices, which usually require high lead counts, about 25% of the leads in each package are used for power and ground connections. The large number of leads devoted to power and ground connections significantly reduces the number of pins available for signal connections, which usually determine the level of available performance.  
         SUMMARY OF THE INVENTION  
         [0012]    In accordance with the present invention, a plastic molded package is provided comprising (i) a heat sink having an upper surface and a lower surface, (ii) a ceramic or dielectric ring attached by an adhesive film to the lower surface of the heat sink; (iii) a semiconductor die attached using a thermally conductive epoxy adhesive to the lower surface of the heat sink through an aperture in the dielectric ring; (iv) a lead frame, which is attached to a surface of the dielectric ring, having a number of leads extending outside of the plastic molded package; and (v) a plastic molding enclosing the ceramic ring, the lead frame, except at the exposed portion of the leads and the semiconductor die.  
           [0013]    In accordance with one aspect of the invention, the heat sink comprises a base portion enclosed in the encapsulation and a raised portion protruding above the base portion having a surface exposed to the ambient. In one embodiment, the exposed surface of the raised portion is free of corners (e.g. in the shape of a circle). The base portion of the heat sink includes a number of conical protrusions enclosed in the molding, and a number of through holes filled by the molding. The exposed portion of the raised surface of the heat sink is coated with nickel to provide a good conductive surface for attaching an external heat sink. Suitable materials for the heat sink includes oxygen free high conductivity copper, copper/molybdenum/copper laminate, copper/tungsten/copper laminate and beryllium composites.  
           [0014]    In accordance with another aspect of the present invention, the lead frame of the plastic molded package further comprises an interposer ring downset and attached to the heat sink. The interposer ring comprises either a single loop (360°), or a number of electrically isolated sections for independent connections to power and ground terminals. Such electrically isolated sections of the interposer ring can be supported in the encapsulation by tie bars of the lead frame. For an electrically isolated section of the interposer ring, an electrical short to the heat sink allows the heat sink to be used as a ground plane for the semiconductor die. That electrical short can be accomplished by a drop of electrically conductive adhesive. The leads of the lead frame allow the internal power and ground planes in the interposer ring to be connected to power and ground supplies outside of the plastic molded package.  
           [0015]    In accordance with another aspect of the present invention, the dielectric ring comprises a material selected from the group consisting ceramic materials, epoxy materials including Ablefilm 564AKHM, and a dielectric sheet material sold under the trade name of Neoflex. A ceramic dielectric ring provides higher thermal conductivity than the other materials. Thus, the heat from the semiconductor die can be conducted through the wire bonds to the heat sink, rather than through the leads to the ambient, which is a path of much higher thermal impedance. Consequently, the package of the present invention provides higher performance in power dissipation.  
           [0016]    In a package of present invention, an 8-watt thermal performance is achieved by adding the combination of the ceramic dielectric ring and an integral heat sink made out of oxygen-free high-conductivity copper (OFHC). Since copper&#39;s TCE is 17 ppm/° C., which is significantly less than aluminum&#39;s TCE, the package of the present invention can attach a larger die than the prior art (e.g. up to 14×14mm) without the risk of a die-cracking failure. Furthermore, in the present invention, the combined effects of the unique locking features on the integral heat sink, and the close matching of TCEs between copper and the molding compound, eliminate delamination and cracking failures modes observed in aluminum heat sinks of the prior art.  
           [0017]    Further, by providing controlled impedance traces and separate power and ground rings on the lead frame, electrical performance in the packages of the present invention is significantly enhanced over the prior art. Controlled impedance is achieved by connecting the heat sink to electrical ground and using the heat sink as an electrical ground plane. By attaching a ceramic ring of an appropriate thickness as a dielectric layer between the heat sink and the lead frame, an impedance in the range of 40-60 ohms is achieved. Such controlled impedance provides high frequency performance in the range of 100 MHz.  
           [0018]    The power and ground rings in a package of the present invention are provided separately to allow low impedance connections between the semiconductor die and the leads. Such low impedance connections are achieved by retaining only a peripheral part of a conventional die attach pad either as an entire ring or divided into two or more sections. In one embodiment, the entire ground ring is electrically shorted to the heat sink. Alternatively, in a second configuration, the two or more sections of the retained peripheral part of the die attach pad are each shorted to either a ground plane (e.g. the heat sink) or one or more power terminals. Such a configuration has lower inductance than the prior art leads because the wire bonds to the rings are shorter, and because the rings have a larger width than the leads. Furthermore, since most of the power and the ground connections are internal to the package, the available lead count for signal transmission is significantly increased. The higher lead count allows higher performance and achieves a cost which compares favorably with a conventional package of comparable performance.  
           [0019]    The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a cross sectional view of thermally enhanced package  100  of the prior art.  
         [0021]    [0021]FIG. 2 is a cross sectional view of a package  200 , in accordance with the present invention.  
         [0022]    [0022]FIG. 3 a  is a top view of heat sink  201  of FIG. 2.  
         [0023]    [0023]FIG. 3 b  is a cross sectional view of heat sink along the dotted labelled A-A&#39;in FIG. 3 a.    
         [0024]    [0024]FIG. 4 is a bottom view of package  200  with plastic  204  molding removed to show lead frame  205 .  
         [0025]    [0025]FIG. 5 is a schematic view of the lower side of the lead frame with the interposer ring sections.  
         [0026]    [0026]FIG. 6 is a top view of ceramic ring  206 .  
         [0027]    [0027]FIG. 7 shows lead frame  700  including an interposer ring  701  and leads  702 .  
         [0028]    [0028]FIG. 8 shows a plastic molded package  800  having a lead frame including interposer rings  801  and  802 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The present invention relates to a package for encasing a semiconductor device. Such a package facilitates electrical connections between a semiconductor device and an external printed circuit board (PCB). The package of the present invention provides higher thermal dissipation and higher electrical performance than conventional plastic molded packages. The package of the present invention uses materials and design features that significantly improve the performance of the package without compromising reliability.  
         [0030]    [0030]FIG. 2 is a cross sectional view of a package  200 , in accordance with the present invention. As shown in FIG. 2, a semiconductor die  211  is attached by a film of thermally conductive epoxy  210  to a thick copper heat sink  201 . An annular ceramic ring  206  is attached by dielectric adhesive  213  onto heat sink  201  on one surface of ceramic ring  206  and onto a lead frame  205  on an opposite surface of ceramic ring  206 . Package  200  forms a transmission line for each lead in lead frame  205 , with heat sink  201  acting as a ground plane. In addition, lead frame  205  includes an interposer ring  208  which surrounds semiconductor die  211  inside the window  212  of ceramic ring  206 . In this embodiment, interposer ring  208  is severed into four sections  208   a - 208   d  to allow independent connections to the power and ground terminals.  
         [0031]    In this embodiment, lead frame  205  is a copper lead frame having leads which are each  6  mils wide and  5  mils thick. Lead frame  205  can also be constructed from other conductive materials, e.g. alloy  42 . Ceramic ring  206  is  10  mils thick, and the dielectric layers attaching heat sink  201  and lead frame  205  to the surfaces of ceramic ring  206  are each about  1  mil thick. Dielectric ring can be constructed from a ceramic material, an epoxy such as Ablefilm 564AKHM, or a dielectric sheet material sold under the trade name of Neoflex. Under this configuration, each lead can be considered a 40-60 ohm transmission line capable of applications requiring a clock frequency of up to 100 MHz.  
         [0032]    Heat sink  201  is made out of oxygen-free high-conductivity (OFHC) copper. Other suitable materials for heat sink  201  include copper/molybdenum/copper laminate, copper/tungsten/copper laminate and beryllium composites. The back surface of semiconductor die  211  is attached to heat sink  201  via thermally conductive epoxy  210 . Although the thermal coefficient of expansion (TCE) of copper is significantly higher than the TCE of silicon, the flexible nature of epoxy  211  provides the compliance necessary to prevent die cracking from differential thermal expansion for semiconductor dies up to an area about 14×14 mm. Furthermore, since copper has the high thermal conductivity of 0.934 cal-cm/cm 2 -sec-° C., heat sink  201  provides high power dissipation. Further, since copper&#39;s TCE is approximately 17 ppm/° C., the TCE of heat sink  201  is well matched to the TCEs of most molding compound materials, which are typically in the range of 16-17 ppm/° C. The well-matched TCEs at the heat sink-molding interface minimize stress, thereby causing no delamination even during thermal cycling or thermal shock tests. The top surface of heat sink  201 , which is exposed to the ambient, is plated with a film  202  of nickel to provide a clean surface (i.e. free of copper oxides) for attaching an external heat sink, if needed.  
         [0033]    In the present embodiment, adhesion of heat sink  201  to molding  204  is enhanced by a thin layer of copper oxide at the interface between heat sink  201  and plastic molding  204 . The copper oxide at the heat sink-molding interface, which is formed by annealing copper at 300°C. for one hour, has good adhesion to molding compounds.  
         [0034]    [0034]FIG. 3 a  is a top view (i.e. viewed above the surface coated by nickel film  202 ) of heat sink  201 . In the embodiment shown in FIG. 3 a,  heat sink  201  has a number of mold-locking features to maximize the adhesion of heat sink  201  to plastic molding  204 . First, heat sink  201  which, as shown, is suitable for use with a  208 -lead package. In this embodiment, heat sink  201  is a 1 inch by 1 inch square (i.e. measures 1 inch at side  216  of FIG. 3 a ) with a central raised portion  220  which provides a 0.8 inch diameter circular surface. This circular surface is the surface plated with nickel film  202 . A cross sectional view along the dotted line A-A′ through central raised portion  220  is shown in FIG. 3 b.  Referring to FIG. 3 b,  central raised portion  220  rises a distance a, which is 0.060 inch in this embodiment, above a base  221  of heat sink  201 . Base  221  of heat sink  201  has a thickness b, which is 0.03 inch in this embodiment. The circular contour of central raised portion  220  is free of corners to avoid creating stress concentration points.  
         [0035]    A mold-locking feature is provided by a number of raised conical protrusions (collectively labeled by reference numeral  203  in FIG. 3 a ) around the central raised portion  220 . In addition, a through hole is provided in each corner of base  221 . These through holes are collectively labeled by reference numeral  214  in FIG. 3 a.  Raised conical protrusions  203  and through holes  204  provide additional surface areas for locking plastic molding  204  onto heat sink  201 . Heat sink  201 &#39;s mold-locking features have resulted in excellent adhesion of the plastic molding  204  to heat sink  201 , allowing package  200  to pass all the conventional tests including the thermal cycling, thermal shock, pressure pot, ink penetration and high humidity tests.  
         [0036]    [0036]FIG. 4 is a bottom view of package  200  with plastic molding  204  removed so as to clearly show lead frame  205  and interposer ring  208 . FIG. 4 shows the interposer ring sections  208   a - 208   d  downset and attached to the heat sink  201  with dielectric adhesive  213 . Interposer ring sections  208   a - 208   d  are further supported by tie bars  241   a - 241   d,  which are imbedded in plastic molding  204 . Lead frame  205  is severed to provide electrically isolated leads  250 . Each of interposer ring sections  208   a - 208   d  is wire bonded to one of leads  250 . Further, interposer ring sections  241   b  and  241   d,  which are dedicated for connections to a ground terminal, are electrically shorted to heat sink  201  via electrically conductive epoxy  240 . Alternatively, spot welding or other suitable mechanism can be used to electrically short interposer ring sections  208   b  and  208   d  to heat sink  201 . Interposer ring  208  is designed to surround semiconductor die  211  in close proximity without being in contact with semiconductor die  211 . Consequently, very short wire bonds to both semiconductor die  211  and leads  250  are possible. Such wire bonds have low inductance, which, in turn, reduces the parasitic impedances of package  200 , thereby enhancing package  200 &#39;s electrical performance. Interposer ring  208  provides an additional advantage in that each interposer ring section can be assigned for power or ground connection to any of leads  250  within the interposer ring section&#39;s proximity. As mentioned above, power and ground connections usually take about 25% of the total lead count in a conventional package. However, because the interposer ring sections are internal to package  200  and are accessed readily for connections, the number of leads on lead frame  205  required for power and ground connections is reduced, thereby effectively increasing the available lead count of package  200 .  
         [0037]    [0037]FIG. 5 shows the lower side of lead frame  205 . Unlike conventional lead frames, lead frame  205  retains only the peripheral section of the conventional die attach pad to form interposer ring  208 . When interposer ring  208  is divided into at two or more electrically isolated sections, these sections can be shorted at the designer&#39;s choice either to the heat sink as a ground connection, or to the power terminals.  
         [0038]    [0038]FIG. 6 shows a top view of ceramic ring  206 , showing the ceramic window  212  in which semiconductor die  211  and interposer ring sections  208   a - 208   d  are placed.  
         [0039]    [0039]FIG. 7 shows a lead frame  700  including an interposer ring  701  and leads  702 . As shown in FIG. 7, lead frame  700  includes interposer ring  701  and leads  702  formed integrally in the manner described above for lead frame  205 . Interposer ring  701  is supported tie bars  704   a - 704   d.  Unlike interposer ring  208  above, interposer ring  701  is not severed into sections. Lead frame  700  is designed for a plastic molded package with a footprint indicated by the dotted outline  703 . Although lead frame  700  is shown with a single interposer ring. Multiple annular interposer rings can be provided. FIG. 8 shows a plastic molded package  800  having a lead frame including interposer rings  801  and  802 . In package  800 , power and ground supply voltages can be provided on different interposer rings.  
         [0040]    As shown in FIG. 8, semiconductor  801  is attached by conventional die attach epoxy  809  onto heat sink  806 . A lead frame, including leads  802  and interposer rings  804  and  805  (shown as cross sections  804   a,    804   b,    805   a  and  806   b ), is attached to heat sink  806  using a conventional non-conductive adhesive  808 . As shown interposer rings  804  and  805 , and leads  802  have been severed from each other. Conventional wire bonds  803  are provided between bonding pads of semiconductor die  801 , interposer rings  804  and  805  and leads  802 . A conventional plastic encapsulation  807  is then provided. The lower surface of heat sink  807  is exposed.  
         [0041]    The above detailed description is provided to illustrate the specific embodiments present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is defined by the following claims.