Abstract:
A semiconductor device package comprises a container including a base and sidewalls. The base is configured to support a semiconductor device chip, and a lead frame extends through at least one of the sidewalls. A portion of the lead frame within the sidewall has at least one aperture penetrating into the lead frame. The sidewall material extends into the aperture, thereby forming a strong interfacial bond that provides a low leakage, sidewall-lead-frame interface. The base has a reentrant feature that is positioned within the thickness of at least one of the sidewalls and engages the at least one sidewall, thereby forming a low leakage-base-sidewall interface. The top surface of the base has a groove that is positioned within the thickness of at least one of the sidewalls and engages the at least one sidewall, thereby enhancing the low leakage base-sidewall interface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to packages for semiconductor devices (e.g., integrated circuit chips, discrete device chips, or both) and, more particularly, to such packages designed to reduce leakage, for example, of encapsulant from the package or of harmful substances (e.g., vapors, liquids, particles) from the ambient into the package.  
         [0003]     1. Discussion of the Related Art  
         [0004]     In the semiconductor device industry it is common to fabricate device packages from a metallic base and plastic or other polymer sidewalls. One of the many potential package types is known as an open cavity package, which is commonly used in applications where high thermal loads must be handled including, but not limited to, power devices (e.g., radio frequency, laterally diffused MOSFETs or RFLDMOSFETs). The most common open cavity package includes a high thermal conductivity base, ceramic side walls and embedded leads. These packages are typically of the hermetic or semi-hermetic variety. In both varieties the semiconductor device or chip is connected to the base and the leads, and the chip is protected from the outside environment by a substantially leak-tight sealed lid. As such, there is no requirement for semiconductor device encapsulant for environmental protection. For lower cost applications, the ceramic side walls of the package can be replaced with plastic. Many of the open cavity plastic packages are non-hermetic by design. In this case the semiconductor device must be encapsulated so that unwanted environmental degradation does not occur. The best environmental protection is achieved when the entire cavity (including the semiconductor device, wire bonds, package leads, and package base) is filled with a protective encapsulant such as silicone.  
         [0005]     We have found that non-hermetic plastic packages fabricated in this or similar fashion have a number of problems associated with encapsulant leaking out of the cavity during the cavity fill process. Often the encapsulant leaking problem is not present in as- received open cavity packages. However, after die attach and wire bonding, when the packages are typically filled with encapsulant, we have observed that encapsulant does leak out at the plastic-to-metal interfaces of the package. This leakage results primarily from degradation of the interfaces between the different parts of the package, which in turn results from differences in the thermal expansion coefficients of the base, lead frame and sidewall materials as well as from less than ideal design. The difference in thermal expansion between the plastic side walls and the metal base and metal leads causes the plastic and metal to separate in one or more locations along the bond line. This separation provides leakage paths that allow the encapsulant to flow from the cavity to the outside surfaces of the package.  
         [0006]     Thus, a need remains in the art for a semiconductor device package that reduces the leakage of encapsulant from the filled cavity.  
         [0007]     In addition, these leaky interfaces also permit harmful substances (e.g., moisture, solvents, air-born particles) from the ambient to enter the package, where they can have a deleterious effect on device operation or package integrity.  
         [0008]     Thus, a need remains in the art for a semiconductor device package that reduces the leakage of such harmful substances from the ambient into the package.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In accordance with one aspect of our invention, a semiconductor device package comprises a container including a base and sidewalls of materials having different thermal expansion coefficients. The base is configured to support a semiconductor device chip, and a lead frame extends through at least one of the sidewalls. The package is characterized in that a portion of the lead frame within the sidewall has at least one aperture penetrating into the lead frame. The sidewall material extends into (e.g., molds around) the aperture, thereby forming a strong interfacial bond that provides a low leakage, sidewall-lead-frame interface.  
         [0010]     In accordance with another aspect of our invention, the base has a reentrant feature that is positioned within the thickness of at least one of the sidewalls. This feature acts to engage or capture the side wall in such a way as to provide a strong, low leakage base-sidewall interface.  
         [0011]     In accordance with yet another aspect of our invention, the top surface of the base has a groove that is positioned within the thickness of at least one of the sidewalls and engages the at least one sidewall, thereby forming a low leakage base-sidewall interface. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0012]     Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which:  
         [0013]      FIG. 1  is a schematic, cross sectional view of a semiconductor device package in accordance with one embodiment of our invention;  
         [0014]      FIG. 2  is a schematic, top view of a lead-frame-sidewall interface having horseshoe features in accordance with another embodiment of our invention;  
         [0015]      FIG. 3  is a cross-sectional view taken along line A-A of  FIG. 2  for the case where the feature extends entirely through the thickness of the lead frame;  
         [0016]      FIG. 4  is a cross-sectional view taken along line A-A of  FIG. 2  for case where the feature is a depression in the lead frame;  
         [0017]      FIG. 5  is a schematic, top view of a lead-frame-sidewall interface having oval features in accordance with another embodiment of our invention;  
         [0018]      FIG. 6  is a schematic, top view of a lead-frame-sidewall interface having rectangular features in accordance with one more embodiment of our invention;  
         [0019]      FIG. 7  is a schematic, top view of a lead-frame-sidewall interface having triangular features in accordance with another embodiment of our invention; and  
         [0020]      FIG. 8  is a cross-sectional view of a reentrant feature formed on the base in accordance with an illustrative embodiment of our invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     With reference now to  FIG. 1 , we show a semiconductor device package  10  for housing one or more semiconductor device chips  18 ; e.g., integrated circuit chips, discrete device chips, or both. The package  10  is formed by a container having a thermally conducting (e.g., metallic) base  12 , which often serves as a heat sink and an electrical connection, and an electrically insulating (e.g., plastic or other polymer) sidewalls  14 . An electrically conducting (e.g., metallic) lead frame  16  extends through at least one of the sidewalls  14  in order to facilitate making electrical contact to the chip  18 , which is mounted on the base  12 . To this end, electrical conductors  20  (typically wire bonds) are connected between terminals on the interior ends of the lead frame  16  and terminals on the chip  18 . The chip  18  and the conductors  20  are covered by a protective encapsulant  22 , which illustratively fills the container and is itself covered by a lid  24 . Illustratively, the encapsulant includes single or multiple layers of silicone and/or a hard material such as HYSOL® FP 4470 , which is commercially available from the electronics division of Henkel Loctite Corporation located in Industry, Calif.  
         [0022]     This type of open cavity package is typically formed in an insert mold cavity, where the base and lead frame are inserted into the mold cavity and plastic (or other polymer) is molded around these metallic components.  
         [0023]     The lead frame, as is well known in the art, is designed to accept wire bonds from the semiconductor chip and to carry electrical signals between the chip and an external electrical circuit.  
         [0024]     The metallic materials of the base and lead frame do not adhere well to the plastic material of the sidewalls and have thermal expansion coefficients different from that of the sidewall material, which in prior art designs tends to make leaky interfaces  30  between the lead frame  16  and the sidewalls  14  and leaky interfaces  40  between the base  12  and the sidewalls  14 . More specifically, during thermal excursions of the package (e.g., during high temperature processing steps, such as the common step of using eutectic AuSn solder, which melts at approximately 280° C., to bond the chip  18  to the base  12 ), the base and sidewall, and the lead frame and sidewall, both expand. Because the thermal expansion of the side wall is different from that of metallic base and lead, the metal exerts a force that deforms the plastic. Often this deformation is non-elastic. More specifically, whereas the metallic base and lead frame naturally return to their original shapes upon cooling, the plastic side wall does not, resulting in a gap between the metal and the plastic. Such gaps undesirably allow encapsulant to leak out. In addition, liquids, vapors and/or air-born particles can also enter the package cavity via these gaps. Both types of leakage can degrade the device reliability.  
         [0025]     In accordance with one aspect of our invention, as shown in  FIGS. 2-6 , the problem of a leaky lead-frame-sidewall interface  30  is addressed by forming fillable features 16 . 2 - 16 . 7  in the lead frame  16 ; that is, the features may be, for example, apertures  16 . 2 ,  16 . 3  ( FIG. 2 ,  FIG. 3 , respectively), which are etched through the entire thickness lead frame, or depressions  16 . 4  ( FIG. 4 ; only one is shown for simplicity), which are stamped into the lead frame. For purposes of illustration, the depression  16 . 4  is shown only in the top surface of the lead frame, but it could also be located in the bottom surface of the lead frame or in both surfaces. The features may take on myriad geometric shapes such as, for example, circular (e.g., a linear array of tandem, oppositely facing semicircles  16 . 2 - 16 . 4  as in  FIGS. 2-4 ; tandem stadium-like shapes or ovals  16 . 5  as in  FIG. 5 ), rectangular (e.g., parallel rows of offset rectangles  16 . 6  as in  FIG. 6 ), triangles (e.g., interlocking diamonds and bowties  16 . 7  as in  FIG. 7 ), or any suitable combination of such shapes. The number of such features utilized is a matter of design choice; only two or three are shown in  FIGS. 2 and 5 - 7  for simplicity. The features may be arranged head-to-tail, as the semicircles  16 . 2  of  FIG.2 . Alternatively, the semicircles may “interlock” to block line-of-sight leakage paths between adjacent horseshoes and across the sidewall. Line-of-sight leakage paths are blocked, for example, in the features illustrated by the rectangles and diamond-bowtie features of  FIGS. 6-7 .  
         [0026]     After the features are formed, the sidewall  14  is molded around the lead frame  16  so that material of the sidewall fills the features, thereby engaging the lead frame and forming a relatively leak-free interface  30 . The features  16 . 2 - 16 . 7  have an additional advantage; they inhibit unwanted movement of the lead frame within the package.  
         [0027]     In accordance with another aspect of our invention, as shown in  FIG. 1 , the problem of a leaky base-sidewall interface  40  is addressed by forming reentrant features  12 . 1  (e.g. hook-like flanges) on the outer edges of the base  12 . After the reentrant feature  12 . 1  is formed, the sidewall is molded around the base (at the same time that it is molded around the lead frame  16 ). The molding process, of course, means that the base and sidewalls are at an elevated temperature. During the cooling cycle, the reentrant feature  12 . 1  of the metallic base  12  engages the plastic sidewall  14 , thereby pulling the sidewall back into place and forming a relatively leak-free interface  40 .  
         [0028]     In general, the reentrant feature  12 . 1  is formed on at least one edge of the base  12 , but preferably on all such edges to ensure that a relatively leak-free interface  40  is formed around the entire package. Moreover, the reentrant feature may be continuous along each edge, or it may be segmented. The latter design has the advantage that it forms “teeth” that provide better locking of the base to the sidewalls. Although the reentrant feature  12 . 1  is depicted as being located at the top surface of the base, it could be positioned lower.  
         [0029]     In addition, in accordance with yet another aspect of our invention, the problem of a leaky base-sidewall interface  40  is addressed by forming grooves  12 . 2  in the top surface of the base  12  and under the sidewalls  14 . The groove should be formed with sharp (rather than rounded) top edges, since the former inhibit the encapsulant  22 , before it is cured (i.e., when it is still in low-viscosity gel form) from leaking out through interface  40 . In general, the groove  12 . 2  is formed along at least one edge of the base  12 , but preferably on all such edges to ensure that a relatively leak-free interface  40  is formed around the entire package. The shape of the groove is otherwise not critical; for example, it can be V-shaped (as shown in  FIG. 1 ) or it can be rectangular (not shown).  
         [0030]     It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments that can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, each of the three design features discussed above may be used separately or in any combination thereof, but preferably all three are used together to lock the polymer to the metal, thereby maximizing the reduction in leakage of encapsulant from the package and/or the leakage of harmful substances from the ambient into the package.  
       EXAMPLE  
       [0031]     This example describes the fabrication of a package for RFLDMOS (i.e., radio frequency, laterally diffused, metal-oxide-semiconductor) ICs, in accordance with one embodiment of our invention. Various materials, dimensions and operating conditions are provided by way of illustration only and, unless otherwise expressly stated, are not intended to limit the scope of the invention. This type of package illustratively contains six chips: two chips that function as active ICs (i.e., high power RFLDMOS transistors) and four that function as passive ICs (i.e., capacitors).  
         [0032]     The container  12  had overall dimensions of 810 mils in length, 385 mils in width, and 99 mils in height. The sidewalls  14  and lid  24  were made of a liquid crystal polymer (LCP) material (e.g., Ticona Vectra S-135). The sidewalls were 72.5 mils thick in the region where the lead frame extended through the sidewalls, whereas the lid  24  had overall dimensions of 810 mils in length, 384 mils in width, and 43 mils in thickness. The base was made of copper (e.g., CDA 194) and had overall dimensions of 810 mils in length, 325 mils in width, and 50 mils in height/thickness.  
         [0033]     The lead frame  16  was made of copper (e.g., CDA 151) and the portion extending through each of two opposite sidewalls had overall dimensions of about 600 mils in length and 5 mils in thickness. The width, which is measured in the same dimension as the sidewall width, is unspecified and not critical since the lead frame extends on both sides of the sidewalls. Five stadium-like or oval features were formed in tandem in each of the two lead frames  16 . Thus, each stadium feature included two semicircular features that faced one another. The stadium features were about 55 mils in length and were spaced on 92.5 mil centers. Each semicircular feature had about a 10 mil radius.  
         [0034]     The reentrant feature  12 . 1  was formed along the entire length and width of the base  12 . The feature  12 . 1  had the reentrant shape shown in  FIG. 8  and had the following approximate dimensions: d 1 =20 mils, d 2 =73 mils, d 3 =35 mils, d 4 =8 mils, d 5 =100 mils, θ 1 =28°and θ 2 =18°.  
         [0035]     The groove  12 . 2  was also formed along the entire length and width of the base  12 . It was stamped into the base, had a V-shape with a rounded bottom in cross-section as shown in  FIG. 1 , and had the following approximate dimensions: 15 mils in maximum width and 10 mils in depth, with the sides of the V making a 60° angle with one another.  
         [0036]     The encapsulant  22  was a single layer of cured silicone gel and was about 40 mils thick. The gel, which was purchased from Dow Coming, Midland, Mich. and was identified as HIPEC® Q3-6646, was oven cured at a temperature of about 150° C. for about 120 min.