Patent Application
20110042137
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.
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.
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.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
