Abstract:
An apparatus and method for packaging an electronic device that mechanically isolates the electronic device from its supporting substrate, eliminating transfer of mechanical stress from the substrate to the device. The apparatus includes a plurality of elongated members that extend from a frame support to a center opening, where the ends of the elongated members together support the electronic device. The shape, material and orientation of the elongated members combine to both support the electronic device and absorb mechanical force transmitted from the substrate to the support frame. In one example, the absorbing portion is substantially perpendicular to the direction of the transmitted force and the transmitted force is absorbed by mechanical displacement of one end of the perpendicular portion. The apparatus and method are particularly effective in eliminating the negative effects of thermal expansion mismatch between the electronic device and its supporting substrate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The materials from which semiconducting devices are made often have a coefficient of thermal expansion (CTE) significantly different from the CTE of their substrate material. For the case of a device bonded directly to a substrate, if the thermal expansion mismatch is ignored, stress can be imparted to the semiconductor device from the substrate. Sufficient stress can lead to broken electrical connections or damage to the semiconductor device. Through engineering, the CTE of substrate materials are often shifted to more closely match that of the semiconductor device. However achieving the CTE match can require engineering trade-offs. 
         [0002]    In a separate function, lead frames are used to support the semiconductor device during assembly and provide electrical connections to the inputs and outputs of the semiconductor device after assembly. The lead frame includes a die paddle, which supports the semiconductor device during assembly and leads that serve as the electrical connections. The semiconductor device is connected to the leads by either wire bonding or tape automated bonding. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention provides an apparatus and method for packaging an electronic device that mechanically isolates the device from its supporting substrate, eliminating transfer of mechanical stress from the substrate to the device. The apparatus includes a plurality of elongated members that extend from a frame support to a center opening, where the ends of the elongated members together support the electronic device. The shape, material and orientation of the elongated members combine to both support the electronic device and absorb mechanical force transmitted from the substrate to the support frame. In one example, the absorbing portion is substantially perpendicular to the direction of the transmitted force and the transmitted force is absorbed by mechanical displacement of one end of the perpendicular portion. The apparatus and method are particularly effective in eliminating the negative effects of thermal expansion mismatch between the electronic device and its supporting substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0005]      FIG. 1  shows a perspective view of an electronic semiconductor package assembly formed in accordance with the invention under assembly; 
           [0006]      FIG. 2  shows a top x-ray view of the electronic semiconductor package assembly of  FIG. 1 ; 
           [0007]      FIG. 3  shows a cross-sectional view of the completed electronic semiconductor package assembly in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]      FIG. 1  shows a perspective view of a semiconductor electronic package assembly  10  under assembly, including an electronic semiconductor device (die)  12 , a lead frame  14 , and a lead frame support  18 . The lead frame  14  includes a perimeter support  22  and a plurality of support fingers  26 . The lead frame support  18  surrounds a center opening  34 . 
         [0009]    The die  12  is a Micro Electro-Mechanical Systems (MEMS) sensor, but can also be any kind of semiconductor device. In this embodiment, the die  10  is a square MEMS sensor with a wide, flat aspect ratio. 
         [0010]    The support fingers  26  are elongated members made of metal or polymer that extend inward from the perimeter support  22  toward the center of the lead frame  14 . The support fingers  26  are stiff, but not inflexible. The support fingers  26  are sufficiently stiff to resist displacement under a rapid acceleration, but are sufficiently flexible to absorb a displacement under a gradually applied force, such as caused by an applied stress from a thermal expansion mismatch. 
         [0011]    The support fingers  26  connect to the perimeter support  22  at a top inside edge of the perimeter support  22 , however alternative connection points are possible. The support fingers  26  are typically narrow in width and thickness compared with their length, however wider and thicker fingers are possible. The perimeter support  22  is a thin, flat frame. The perimeter support  22  is thin compared with its width, but other aspect ratios are possible. 
         [0012]    The lead frame support  18  is a ring or frame. The lead frame support  18  can have a wide range of widths and thicknesses. The center opening  34  can also be a range of sizes, but the dimensions of the center opening  34  are typically greater than the width and thickness of the lead frame support  18 . 
         [0013]    In this embodiment, the die  12 , the lead frame support  18 , and the perimeter support  22  each lie one inside the next within a substantially horizontal plane: the die  12  inside the lead frame support  18 , and the lead frame support  18  inside the perimeter support  22 . The support fingers  26  of the lead frame  14  extend over the top face of the lead frame support  18 , and under the lower face of the die  12 . By the fact that the support fingers  26  pass over the lead frame support  18 , the lead frame support  18  carries the lead frame  14 . By the fact that the support fingers  26  of the lead frame  14  pass under the die  12 , the lead frame carries the die. 
         [0014]      FIG. 2  shows in an x-ray top view, through the die  12 , the position of the support fingers  26  underneath the die. The support fingers  26  contact the underside of the die  12 , possibly at bond pads  28  on the bottom face of the die. At least end portions of the support fingers  26  may be coated with an aluminum thin film deposition or an electrolytic gold deposition to facilitate attachment of bond pads  28  on the die  12  to the support fingers  26  by a solder, tape automated bonding (TAB), or conductive epoxy operation. In the view of  FIG. 2  the perimeter support  22  has been separated and removed from lead frame  14 . 
         [0015]      FIG. 3  shows a cross-sectional view of the semiconductor electronic package assembly  10  after assembly, including the die  12 , the lead frame support  18 , the lead frame support fingers  26 , a substrate  38 , a lid  42  and a first and second sealing joints  48 ,  50 . The lead frame perimeter support  22  is absent due to its removal during a final assembly step. In this embodiment, the support fingers  26  are longer than in the previous embodiment, and extend under the lead frame support  18 . 
         [0016]    The substrate  38  is joined to the bottom face of the lead frame support  18  by the first sealing joint  48 . The substrate  38  provides a structure to support all the other elements of the semiconductor electronic package assembly  10 . The lid  42  is joined to the top face of the lead frame support  22  by the second sealing joint  50 . The lid  42  closes the die  12  off from the outside atmosphere. The lid  42  is optional, depending on if the die  12  needs to be hermetically sealed, sealed only against moisture, or not sealed at all. The first and second sealing joints  48 ,  50  use industry standard bonding techniques, for example solder, epoxy, adhesive, or glass frit. 
         [0017]    In this embodiment, the plurality of support fingers  26  extends beneath the lead frame support  18  to form a plurality of horizontal pads  52 . The horizontal pads  52  are a point where the plurality of fingers  26  could be attached to an outside electrical interface, if the support fingers  26  are also being used as electrical connections. In this embodiment, the support fingers  26  first extend from below the substrate  38  and upward along the outer face of the lead frame support  18 . Then they extend over the top face of the lead frame support  18  and underneath the die  12 , as described in the previous embodiment. 
         [0018]    The cross-sectional view shows a vertical segment  44  of the support fingers  26  that provides flexibility for the support fingers  26  to absorb displacement in the substrate  38  due to an applied shear stress, such as thermal expansion. For example, in a cooling environment (such as following a solder re-flow step during fabrication), due to CTE mismatch the substrate  38  may contract significantly more than the die  12 . If the die  12  is bonded directly to the substrate  38 , compressive stress from the contracting substrate  38  would be imparted into the less contracting die  12 . In accordance with this invention, compressive stress from the contracting substrate  38  is imparted to the lead frame support  18  to which it is bonded. But these stresses are not transferred to the die  12  due to flexibility in the vertical segment  44  of the support fingers  26  that suspend the die  12 . In this embodiment, the top of the vertical segment  44  may move inward due to the contracting lead frame support  18 , but the bottom of the vertical segment  44  would remain substantially fixed. Other shapes and orientations of the support fingers  26  are possible that take up the stress imparted by the expanding or contracting substrate. 
         [0019]    For purposes of automated manufacturing and ease of handling, the lead frame  14 , and especially the perimeter support  22 , may include holes, grooves, notches, and/or other means known to those of ordinary skill in the art. In one embodiment, the thickness of the lead frame  14  is between 0.25 and 1 mm and the width of the perimeter support  22  is between 2 and 20 mm. Any number of support fingers  26  is also possible, and there would likely be more support fingers  26  than the number shown in the embodiment of  FIG. 1 ,  2  or  3 . The support fingers  26  may also have varying width, pitch and distribution along the perimeter support  22  of the lead frame  14 . In one embodiment, the thickness of the support fingers  26  is the same as the perimeter support  22 , between 0.25 and 1 mm thick, and the width of the support fingers  26  is between 0.1 and 1 mm. 
         [0020]    The lead frame support  18  may also be curved, round, or have an irregular shape and may also have a curved, round, or irregularly-shaped cross-sectional profile. The lead frame support  18  may also be an incomplete ring. For example the lead frame support  18  may include two semi-circle sections positioned opposite one another around the die  12 , or four short independent straight segments on each side of the die  12 . The lead frame support  18  can be manufactured from a variety of materials, for example from ceramics for hermetic applications or polymers for low performance or cost-sensitive applications. 
         [0021]    The segment of the support fingers  26  outside and below the lead frame support  18  may also serve as a mechanical isolator, the same way the vertical segment  44  of the support fingers  26  do. In the example embodiment of  FIG. 3 , a “C” style shape is shown, although other shapes for the support fingers  26  outside and below the lead frame support  18  are also possible. 
         [0022]    The advantage of the semiconductor electronic package assembly  10  is that the negative impact of shear stresses due CTE mismatch or other sources between the electronic semiconductor device  12  and the substrate  38  is averted without requiring re-engineering of the substrate  38 . The lead frame support  18  also provides the benefit of a center opening  34  that provides a convenient support for the flat lid  42  to seal the die  12 . The semiconductor electronic package assembly  10  can also be manufactured relatively simply, lending itself to automated manufacturing techniques. 
         [0023]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. In particular, the support fingers  26  and lead frame support  18  may take on many shapes, orientations and material compositions. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.