U.S. patent application number 11/341991 was filed with the patent office on 2007-08-02 for warpage-reducing packaging design.
Invention is credited to David J. Dougherty, Betty H. Yeung.
Application Number | 20070175660 11/341991 |
Document ID | / |
Family ID | 38320907 |
Filed Date | 2007-08-02 |
United States Patent
Application |
20070175660 |
Kind Code |
A1 |
Yeung; Betty H. ; et
al. |
August 2, 2007 |
Warpage-reducing packaging design
Abstract
A flange and packaging assembly characterized by a structural
lip configured to impart resistance to thermal-induced deformation
(i.e. "stiffness") to the flange during elevated temperature die
attachment processes. The stiffness is achieved through appropriate
configuration of the lip (height, width and shape) and is
sufficient to at least partially counteract the stresses generated
from coefficient of thermal expansion differentials between the
flange and a die attached to it. The lip may be molded or otherwise
formed with the flange as a unitary lip-stiffened flange, or may be
a separate structure that is affixed to the flange. The flange has
a lip-stiffened region for die attachment. This region may be
raised above the first surface of the substrate base for enhanced
stiffening, and may be in the form of a single pedestal supporting
a single or multiple die, or may be a series of raised pedestals,
each supporting one or more die. The leads may be embedded in a
polymetric material that is attached to the flange that is of
sufficient thickess to provide the required stiffness. The embedded
leads may be shaped and formed so that the desired seating plane
height is achieved. The lead form placement can be either on the
interior or exterior of the embedded polymetric material. The
polymetric material can be supported along the edge of the flange
or be seated on a part of an extrusion from the flange.
Inventors: |
Yeung; Betty H.; (Chandler,
AZ) ; Dougherty; David J.; (Tempe, AZ) |
Correspondence
Address: |
INGRASSIA, FISHER & LORENZ, P.C.
7150 E. CAMELBACK ROAD
SUITE 325
SCOTTSDALE
AZ
85251
US
|
Family ID: |
38320907 |
Appl. No.: |
11/341991 |
Filed: |
January 27, 2006 |
Current U.S.
Class: |
174/521 ;
257/202; 257/E23.026; 257/E25.016 |
Current CPC
Class: |
H01L 2924/01033
20130101; H01L 2924/10329 20130101; H01L 2924/01031 20130101; H01L
2924/01042 20130101; H01L 2224/49 20130101; H01L 2924/01043
20130101; H01L 24/48 20130101; H01L 2224/48091 20130101; H01L
2924/01074 20130101; H01L 2924/014 20130101; H01L 2224/48137
20130101; H01L 2924/01029 20130101; H01L 2224/48091 20130101; H01L
2924/00014 20130101; H01L 2924/00014 20130101; H01L 2924/00014
20130101; H01L 2924/01013 20130101; H01L 24/49 20130101; H01L
25/072 20130101; H01L 2224/48247 20130101; H01L 2224/48472
20130101; H01L 2924/01082 20130101; H01L 23/492 20130101; H01L
2924/01005 20130101; H01L 2224/48472 20130101; H01L 2924/01027
20130101; H01L 2924/01014 20130101; H01L 2224/4813 20130101; H01L
2924/01006 20130101; H01L 2924/01058 20130101; H01L 2224/48472
20130101; H01L 2224/48091 20130101; H01L 2224/45099 20130101; H01L
2224/05599 20130101; H01L 2224/48247 20130101; H01L 2924/00014
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
174/521 ;
257/202 |
International
Class: |
H01L 23/34 20060101
H01L023/34 |
Claims
1. A packaging flange comprising: a flange having a first surface
and a second surface opposed to the first, the first surface of the
flange comprising a first region having a lip extending thereon,
the first region spaced from a second region adapted to receive at
least one die, the lip configured to impart stiffness to the
flange, the stiffness sufficient to at least partially counteract
thermally-induced stresses generated from coefficient of thermal
expansion differentials between the flange and a die during an
elevated temperature die attachment process.
2. The flange of claim 1, wherein the second region comprises a
pedestal comprising a surface sized and shaped to support at least
one die thereon.
3. The flange of claim 2, wherein the second region comprises a
plurality of pedestals.
4. The flange of claim 1, wherein the lip extends around a
peripheral region of the first surface of the flange to form a
frame surrounding the second region.
5. The flange of claim 4, wherein the second region comprises a
pedestal comprising a surface sized and shaped to support at least
one die thereon.
6. The flange of claim 4, wherein the second region comprises a
plurality of pedestals.
7. The flange of claim 1, wherein the lip is molded in one piece
with the flange base.
8. The flange of claim 1, wherein the ratio of stiffening lip
height to flange thickness is from about 0.15 to about 0.25.
9. A die assembly comprising: a flange having a first surface and a
second surface opposed to the first, the first surface of the
flange comprising a first region having a lip extending thereon,
the first region spaced from a second region adapted to receive at
least one die, the lip configured to impart stiffness to the
flange, the stiffness sufficient to at least partially counteract
thermally-induced stresses generated from coefficient of thermal
expansion differentials between the substrate and a die to be
mounted thereon in an elevated temperature die attachment process;
at least one die mounted in the second region; and leads for
electrical contact with the die, the leads configured to preserve a
seating plane, the seating plane spaced from a plane of the second
surface of the flange.
10. The assembly of claim 9, wherein the leads are embedded in an
organic composition and the molded leads have end-portions embedded
in the lip.
11. The assembly of claim 9, wherein the leads are embedded in an
organic composition and the molded leads extend from raised inboard
end lead portions nearest the die to lower out board lead end
portions aligned substantially within the seating plane.
12. The assembly of claim 9, wherein the second region comprises a
pedestal comprising a surface adapted to support at least one die
thereon.
13. The assembly of claim 9, wherein the second region comprises a
plurality of pedestals.
14. A die assembly comprising: a packaging flange having a first
surface and a second surface opposed to the first, the first
surface of the flange comprising a first region having a lip
extending thereon, the first region spaced from a second region
adapted to receive at least one die, the flange having a body
configured to impart stiffness to the flange, the stiffness
sufficient to at least partially counteract thermally-induced
stresses generated from coefficient of thermal expansion
differentials between the flange and a die to be mounted thereon
during an elevated temperature die attachment process.
15. The assembly of claim 14, wherein the body of the flange has
thickness in the rage from about 65 to about 100 mils.
16. The assembly of claim 14, wherein the first region extends
along a side of the flange and the lip extends upward above the
upper surface of the flange.
17. The assembly of claim 16, further comprising leads, the leads
having inboard end portions embedded along a length of a lip wall
and extending to an upper surface of the lip to permit attachment
of connections to die on the flange.
18. The assembly of claim 14 wherein the first region extends along
a side of the flange, and the flange comprises a platform in the
first region, the lip resting on the platform.
19. The assembly of claim 18 further comprising leads, the leads
having inboard end portions embedded along a length of a lip wall
and extending to an upper surface of the lip to permit attachment
of connections to die on the flange.
20. The assembly of claim 14, further comprising at least one
pedestal in the second region.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to semiconductor
device fabrication. More particularly, the present invention
relates to semiconductor packaging for reduction of
thermal-mechanical deformation of the flange known as
"warpage."
BACKGROUND
[0002] It is well known that after semiconductor dies are
fabricated on a semiconductor wafer, such as but not limited to,
silicon or gallium arsenide, the wafer is sawn (also referred to as
"diced") to separate the individual semiconductor die (also known
as "dice" or "chips" or "device") on the wafer. These separated die
are then each packaged to facilitate safe die handling, and
attachment to circuit boards, heat sinks, and the like. During this
process, the die is attached or mounted to a packaging substrate
(also known as a "flange"). In the case of some devices, such as
power amplifiers, where heat generation is expected during
operation of the device, the die with packaging substrate backing
is then mounted or attached to a heat sink that acts to remove
generated heat by conduction away from the device in use.
[0003] In a typical packaging process, the die is attached to the
flange by any suitable method, for example by soldering or using an
adhesive. This process is carried out at elevated temperature, and
the die-flange combination is subsequently cooled to room
temperature.
[0004] For conventional materials used in packaging, such as those
described in U.S. patent publication 20050016750 A1, there is a
tendency for the flange to deform from planarity or "warp," upon
cooling after the die attachment process. The deformation leads to
further manufacturing problems in subsequent processes when the
window frame, leads and lid are attached. It also to leads to
inefficient transfer of heat generated by the die, while it is in
use, to the flange and heat sink. Consequently, the warpage
prevents the maximum potential heat transfer from die to flange and
from flange to heat sink. This leads to temperature increase at the
die, with undesirable consequences.
[0005] In another configuration, U.S. patent publication
20050012118 describes a package that can withstand high die-attach
temperatures and that can provide a hermetically sealed air cavity
for a die, without the use of adhesives. It discusses a circuit
package for housing semiconductor that has a metallic flange, one
or more high-copper leads and a liquid crystal polymer frame molded
to the flange and the leads. The flange includes a dovetail-shaped
groove that mechanically interlocks with the molded frame. During
molding, a portion of the frame forms a key that freezes in or
around the frame retention feature. There is no discussion of
heat-induced flange deformation.
[0006] The art recognizes that heat transfer from die to flange
poses a challenge in operations where the die is attached at high
temperature, and the combination is subsequently cooled to room
temperature and thermal-mechanical deformation is induced.
[0007] Accordingly, it is desirable to minimize the warpage of the
flange in the die attachment process. In addition, it is desirable
to develop a solution that retains the highest possible heat
transfer from die to flange, and from flange to heat sink.
Furthermore, other desirable features and characteristics of the
present invention will become apparent from the subsequent detailed
description and the appended claims, taken in conjunction with the
accompanying drawings and the foregoing technical field and
background.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A more complete understanding of the present invention may
be derived by referring to the detailed description and claims when
considered in conjunction with the following figures, which are not
represented as being to scale, wherein like reference numbers refer
to similar elements throughout the figures.
[0009] FIG. 1 is a schematic depiction of half of a prior art air
cavity package;
[0010] FIG. 2 is a schematic depiction of half of another prior art
air cavity package with a bonded ceramic insulator frame;
[0011] FIG. 3 is a schematic depiction of the half of the prior art
assembly product of FIG. 2 with attached leads;
[0012] FIG. 4 is a schematic perspective view of half of an
embodiment of the invention showing a flange with stiffening lip
and die on a die pedestal;
[0013] FIG. 5 is a schematic perspective view of half of an
embodiment of the invention showing a raised stiffening lip and
multiple die, each disposed on its own pedestal;
[0014] FIG. 6 is a schematic perspective view of half of an
embodiment of the invention with embedded leads extending through a
molded, stiffening lip;
[0015] FIG. 7 is a perspective view of half of an embodiment of the
invention with embedded leads configured to maintain a standard
seating plane and die on multiple pedestals;
[0016] FIG. 8 is a top view of the embodiment of the invention
showing a thicker flange with embedded leads, extending through a
lip in a peripheral region of the frame, while seating plane s is
maintained;
[0017] FIG. 8 is a schematic cross sectional view of the embodiment
of FIG. 8 with a single pedestal;
[0018] FIG. 10 is an alternative embodiment to FIG. 9 with multiple
pedestals; and
[0019] FIG. 11 is a graphical representation of the improvement in
flange stiffness over a control achieved in accordance with the
invention, for particular lip and pedestal configurations and under
specified conditions.
DETAILED DESCRIPTION
[0020] The following detailed description is merely illustrative in
nature and is not intended to limit the invention or the
application and uses of the invention. Furthermore, there is no
intention to be bound by any expressed or implied theory presented
in the preceding technical field, background, or the following
detailed description. In addition, for the sake of brevity,
conventional techniques related to finite element analysis, finite
difference analysis, and thermal modeling may not be described in
detail herein
[0021] In a package designed to remove heat from an operating
device, like a power amplifier, the main mechanism for heat
transfer from the die to the package substrate (also known as the
"flange") to which it is attached directly, is by conduction. Heat
transfer by conduction is dependent, among other parameters, upon
the surface area of the die that is in direct contact with the
flange. All other factors being equal, heat transfer increases as
contact area increases. Accordingly, complete physical contact over
the entire interfacial area between die and flange maximizes heat
transfer by conduction.
[0022] A predominant cause of loss of heat transfer efficiency from
a die to the flange is physical deformation or "warpage" of the
flange leading to separation between the die and the flange along
their interface. The warpage often arises during the die attachment
process, which is carried out at elevated temperatures. For some
packaging materials used in standard packaging the warpage can be
in excess of 5 mil. The extent of warpage varies, based on several
factors (die attachment temperature, flange thickness, number of
die attached, semiconductor and packaging material properties,
etc.) but can be significant. Excessive warpage has undesirable
effects in fabrication as well as subsequent use of the device.
Thus, for example, warpage must be minimized to avoid deleterious
effects on subsequent assembly operations that must be performed on
the package. In addition, the physical separation between die and
flange results in a gap that, whether air-filled or not, disrupts
heat transfer by conduction from die to flange (which then transfer
heat to the heat sink). The gap reduces the area of the die in
direct contact with the flange and thereby reduces conductive heat
transfer.
[0023] Heat transfer rates from die to flange can be maintained by
preventing or minimizing the extent of flange warpage that causes
separation during die attachment processes. Warpage can be reduced
to some extent by appropriate selection of materials. For example,
flange materials may be selected from those that have a coefficient
of thermal expansion ("CTE") that closely approximates or more
closely matches the CTE of the die. This selection will avoid or
minimize the build up of thermally-induced mechanical stresses at
the die-flange interface that may cause physical separation of die
and flange along at least a part of the interface. However, those
materials that have a CTE that closely approximates the CTE of the
die tend to have poor thermal conductivity ("TC") properties. A
high TC is essential for high rates of heat transfer from die to
flange and then through the flange to the heat sink. This means
that a flange material with a poor TC may allow the die temperature
to rise (because of inability to conduct heat away) and the
temperature may reach a point that could result in adverse
performance, diminished reliability, or even die damage.
[0024] Accordingly, the invention provides modified flanges that
have physical features to reduce the tendency of the flange to warp
under thermally induced stresses, and that reduces the tendency for
separation at the interface between the flange and the die.
[0025] More particularly, in one embodiment, the invention provides
a packaging substrate base (also known as a "flange") that has a
structural lip configured to impart "stiffness" to the substrate
base. The term "stiffness" here means the degree of resistance to
thermally-induced warpage, where the warpage results from
attachment of die to a flange that has a different coefficient of
thermal expansion from the die.
[0026] The flange-stiffening lip may be made from a variety of
materials but commonly would be the same as the flange body. The
flange may be made of a conductive material such as
copper-tungsten, copper-molybdenum, copper, and high conductivity
ceramics. The stiffness of the raised lip is achieved through
appropriate configuration of the lip (height, width and shape) and
is sufficient to at least partially counteract the warpage-inducing
stresses generated from coefficient of thermal expansion
differentials between the flange and a die attached to it. The lip
may be molded or otherwise formed with the flange as a unitary
lip-stiffened flange, or may be a separate structure that is
affixed to the flange. In general, the packaging flange has a first
surface that has a first region having the stiffening lip. The
first surface also has a second region for die attachment. The
second region is a region that is stiffened by the lip, and may be
raised above the first surface of the flange. The second region may
be in the form of a single pedestal supporting a single or multiple
die, or may be a series of raised pedestals, each supporting one or
more die.
[0027] In another embodiment, the invention provides an assembly of
die on the lip-stiffened flange. In addition to the flange, the
assembly also includes at least one die in the second region of the
flange. To facilitate electrical communication, the assembly
includes leads for electrical contact with the die. The leads and
flange are insulated from each other by means of an intermediate
material, such as a (non-conductive) ceramic layer, interposed
between the flange and the leads. In one embodiment, the leads are
configured to preserve a seating plane at the desired height and
configuration. The leads may be embedded in an organic composition.
Further, the leads may have an end-portion embedded in and
extending along a length of the lip; or the lead end portions may
be attached along an upper surface of a length of the lip.
[0028] The stiffening lip may take any of a variety of suitable
shapes. In one embodiment, the lip is a raised wall that extends
around a peripheral area of the flange. Taking into account that
the flange is laterally divided in the accompanying drawings, the
lip is shown U-shaped, but should be understood to include its
mirror image to form a frame-shape. Other raised configurations
that also provide resistance to heat-induced stress deformation of
the flange are, of course, also useful and contemplated in the
invention. For example, the lip may be in the shape of a
rectangular array of a pair of spaced apart longitudinal and a pair
of spaced apart lateral rails, where the rails are not connected up
to form a frame. Or the stiffening features could be a series of
longitudinally extending rails, such as two or three or more
parallel rails.
[0029] It is useful to consider the prior art before turning to
details of embodiments of the invention depicted in the
accompanying figures. In referring to the attached figures showing
flanges of the invention and prior art flanges, it should be noted
that the flanges are symmetrical about a lateral axis at their mid
points. Each depicted flange has been cut in half laterally at the
midpoint, so that only one half is shown, for simplicity.
[0030] Referring now to FIG. 1, which is half of a symmetrical
device obtained by cutting it laterally across a plane of symmetry,
a flange 10 is a planar substantially rectangular block of
appropriate conductive material such as copper, to which are
attached die 12, which is but not limited to semiconductor device
like silicon. In this particular case there are six die arrayed in
two rows of three each (i.e. twelve in the entire package). Note
that upper surface 14 of the flange 10 is planar and devoid of
raised features.
[0031] Referring further to prior art depicted in FIG. 2, also a
half of a symmetrical device, the flange 20 is similar to that of
FIG. 1, but has an adhered non-conductive ceramic frame 22
extending at the periphery and around a central area of the flange
where the die 24 are attached.
[0032] Referring to FIG. 3, the prior art of FIG. 2 is depicted
with leads 26 attached to the upper surface 23 of the ceramic frame
22. The leads extend outboard away from the flange at a height and
in a plane that defines the "seating plane" for the leads to
facilitate subsequent processes involving the leads.
[0033] Turning now to embodiments of the invention, FIG. 4 is half
of a symmetrical device, cut along a lateral plane of symmetry.
FIG. 4 depicts half a flange 100 with an upper surface 102 and an
under surface (not shown) 104. A stiffening lip 200 extends along a
periphery of the upper surface 102. In this particular embodiment,
the lip has a U-shape when halved laterally (i.e. in full shape it
is a frame), although other shapes may also be useful and are
within the scope of the invention. Also, the lip has a width w and
a thickness or height h above surface 102. The height h and width w
are selected to enhance stiffness. These dimensions will vary
depending upon the size (thickness, length) of the flange and type
of flange material, for example. In general however, a greater
height h provides more stiffness than a lower height, in general,
all other factors being equal. Similarly, a greater width w
provides more stiffness to the flange than a lesser width, in
general, all other factors being equal. In general, the height h of
the lip would be in the range from about 10 to 25 mil for a copper
flange. The height h of the lip will vary with materials and
geometry of the flange and die, and materials with smaller CTE
differences from the die should require smaller h. Smaller h would
also be expected for comparatively thicker flanges or when using
comparatively thin die, all other factors being equal.
[0034] The stiffening lip may be molded with the flange as a one
piece unit, or may be subsequently attached to the flange. The
stiffening lip may be of the same or different material from the
flange. The lip is preferably of the same material as the flange,
or of a material with similar CTE. The location and size of the
area that is stiffened by the lip also depends upon the
configuration and location of the lip. The die may be attached in
the stiffened area for best effect.
[0035] In FIG. 4, a pedestal 300 is disposed in the lip-stiffened
area of the flange 100, and is shown in the form of a raised
platform. The raised pedestal 300 provides some enhancement of
flange stiffness as well, as shown in the Example provided here
below. Die (six die 400 are shown in this example, twelve in the
symmetrical device) are attached to the pedestal to take advantage
of the stiffened structure in that region that is at least
partially surrounded by the stiffening lip 200.
[0036] FIG. 5 depicts another embodiment of the invention, this one
also shown in symmetrical half view like that of FIG. 4. However,
in this case the pedestal on flange 100 is not a single pedestal
but a plurality of raised pedestals 330, each having an upper
surface adapted for die attachment. In the embodiment shown the
pedestals 330 are sized to conform to and register with the size of
the die, however, the pedestals may each be oversized relative to
the die. In addition, more than one die may be attached to each
pedestal 330, depending upon selected die and pedestal sizes.
[0037] FIG. 6 is a schematic depiction, according to an embodiment
of the invention, of half of a symmetrical product assembly. In
this embodiment, the half of the raised lip 200 is of a U-shape
(the complete lip is a frame) and disposed to surround die 400 that
are disposed on the upper surface 102 of flange 100. The leads 500
have inboard end portions 502 that are embedded into and extend
along the length of the wall 210 of the lip 200. Lead end portions
502 are isolated from flange 100 by surrounding non-conductive
materials, such as ceramic layers 540, shown. Further the leads 500
are configured so that when end portions 502 are embedded into the
lip 200, lead outboard end portions 504 are at a height s (the
seating plane) from a plane that is co-extensive with flange under
surface 104. Accordingly, very few if any tooling modifications are
necessary to accommodate the design for subsequent processing.
[0038] FIG. 7 depicts another (lateral symmetrical half of)
embodiment of a product assembly of the invention. Here, flange 100
has a stiffening lip 200 and die 400 are each attached to one of a
plurality of pedestals 330. Of course, a single raised pedestal
with one or a plurality of die attached may also be used. The leads
500 are shaped such that when inboard end portions 502 are attached
to the assembly, outboard end portions 504 are in the seating plane
s. The inboard end portions 502 of leads 500 are embedded in the
proximity of the upper surface of the lip 200 on ceramic
inter-layer 540. Alternatively, the lead end portions may be
embedded into and extend along the lip wall itself 510, while
ensuring electrical isolation from the flange. The leads 500 are
shaped so that outboard end portions 504 are at an elevation below
the inboard end portions 502 when the leads are attached. End
portions 504 will lie in a plane that is a height s (the desired
seating plane) above flange undersurface 104, as also shown in FIG.
6.
[0039] FIGS. 8, 9, and 10 represent top and alternative cross
sectional views, respectively, of embodiments of the invention.
Referring first to FIG. 8, a common top view, and FIG. 9, the
flange 100 has a raised lip (shown here as a pair of walls 200
extending longitudinally on each side of the flange 100) that
stiffens the area between the lip walls 200. A plurality of die 400
is attached in the stiffened area, on a raised platform 300, as
seen from FIG. 9. The need for raised platform 300 is discretionary
since the flange thickness t could be increased to provide
identical stiffness as provided by the coupled system of flange 100
and raised platform 300. From FIGS. 8 and 9, inboard end portions
502 of leads 500 are embedded in opposing lip wall sides 210, and
extend upward through the lip walls 210 to be exposed on the upper
surface of the lip walls for attachment of connecting leads 550 to
die 400. Walls 200 may be of a nonconductive material, such as
ceramic or a polymeric material, to ensure isolation of leads 500
from flange 100. In another embodiment, the leads may be isolated
electrically from the lip walls 200 by a coating or intermediate
layer of a non conductive of material. Leads 500 are configured so
that outboard end portions 504 are in the desired seating plane s.
Clearly, the seating plane s can be adjusted by controlling the
shape of leads 500 taking into account the shape of the device and
the seating plane necessary.
[0040] As shown in FIG. 9, the flange 100 is of greater thickness t
than prior art flanges. The thickness t is sufficient to provide
resistance to thermally-induced warpage, and is typically in the
range from about 65 to 100 mil. The flange 100 has a pair of raised
walls 200 that extend around the sides of the flange 100 to form
outer side walls 210. The raised walls 200 are penetrated by lead
end portions 502 that extend through the walls 200 (all along the
length of the walls) to be exposed, for electrical connection,
above walls 200 to lead wires 550. These lead wires 550 also
connect to the die 400 that are all attached to a single raised
pedestal 300. Clearly, each die 400 might also be attached to one
of a multiplicity of raised pedestals 330, as shown in FIG. 10. The
need for raised pedestal 330 is discretionary since the flange
thickness t could be increased to provide identical stiffness as
provided by the coupled system of flange 100 and raised platform
330.
[0041] The embodiment of FIG. 10 is similar to the embodiment of
FIG. 9 except that the raised walls 200 rest upon a platform 120 on
the perimeter of the flange 100. Thus, the sides of the device are
made up of: (1) upper sidewalls that are the sidewalls 210 of the
walls 200; and (2) and lower sidewalls that are the flange
sidewalls 110, 112. Leads penetrate the walls 200 as in FIG. 9.
Above comments, with reference to FIG. 9, regarding lead electrical
isolation from the flange apply here as well.
[0042] In general, in one aspect, the invention provides a
packaging flange that has a first surface and a second surface
opposed to the first. The first surface of the flange includes a
first region that has a raised lip extending thereon. The first
region is spaced from a second region that is adapted to receive at
least one die. The lip is configured to impart stiffness to the
flange, the stiffness sufficient to at least partially counteract
thermally-induced stresses generated from coefficient of thermal
expansion differentials between the flange and a die during an
elevated temperature die attachment process. The second region may
include a pedestal that has a surface sized and shaped to support
at least one die thereon. Or, the second region may include a
plurality of pedestals. The lip may extend around a peripheral
region of the first surface of the flange to form a frame
surrounding the second region. The flange and the lip may be molded
in one piece. The ratio of stiffening lip height h to flange
thickness t for a configuration like that shown in FIG. 8, using
copper flanges, would be in the range from about 0.15 to about 0.25
for a 100 mil thickness flange. The ratio will vary based on
several considerations, including flange configuration, materials,
and the like.
[0043] In another aspect, the invention also provides a die
assembly that has a flange having a first surface and a second
surface opposed to the first. The first surface of the flange
includes a first region having a lip extending thereon. The first
region is spaced from a second region that is adapted to receive at
least one die. The lip is configured to impart stiffness to the
flange, the stiffness sufficient to at least partially counteract
thermally-induced stresses generated from coefficient of thermal
expansion differentials between the substrate and a die to be
mounted thereon in an elevated temperature die attachment process.
The flange has at least one die mounted in the second region. The
assembly includes leads for electrical contact with the die. The
leads may be configured to preserve a seating plane, which is
spaced from a plane of the second surface of the flange. In
addition, the leads may be embedded in an organic composition, with
inboard end-portions embedded in the lip. The leads may be embedded
in an organic composition so that the molded leads extend from
raised inboard end portions nearest the die to lower outboard end
portions that are aligned substantially within the seating plane.
The inboard end portions of the leads may be exposed for electrical
communication with the die. Further, the second region may include
a pedestal having a surface adapted to support at least one die
thereon. Or, the second region may have a plurality of pedestals.
Each of the pedestals has a surface that may be adapted to support
at least one die thereon.
[0044] In yet another aspect, the die assembly includes: a
packaging flange having a first surface and a second surface
opposed to the first, the first surface of the flange comprising a
first region having a lip extending thereon, the first region
spaced from a second region adapted to receive at least one die,
the flange having a body configured to impart stiffness to the
flange, the stiffness sufficient to at least partially counteract
thermally-induced stresses generated from coefficient of thermal
expansion differentials between the flange and a die to be attached
thereon during an elevated temperature die attachment process. The
assembly of claim 14, wherein the body of the flange has thickness
in the range from about 65 to about 100 mils. The first region may
extend along a side of the flange and the lip may extend upward as
a wall above the upper surface of the flange. The first region may
alternatively extend along a side of the flange, where the flange
has a platform so that the lip rests on the platform. There may be
at least one pedestal in the second region, or none. The assembly
may have leads with inboard end portions embedded along a length of
a lip wall and extending to an upper surface of the lip to permit
attachment of connections to die on the flange.
[0045] The following example and test results are intended to
illustrate some of the many improvements that the invention
provides, but does not limit either the scope of the invention, or
the benefits thereof.
EXAMPLE
[0046] Tests were conducted to quantify the improvement in
stiffness (resistance to warpage from heat-induced stress) of
flanges made according to the invention as compared to a control
without any stiffening structure. The flanges were all made of
copper and 100 mil thick. In these tests the following were
compared:
[0047] A. A 100 mil thick flange as control
[0048] B. A 100 mil thick flange with a 21 mil height frame
[0049] C. A 100 mil thick flange with one 21 mil raised pedestal
(no frame)
[0050] D. A 100 mil thick flange with 21 mil height frame and one
21 mil thick pedestal
[0051] E. A 100 mil thick flange with 21 mil height frame and six
21 mil thick die stands
[0052] Each flange was measured for planarity to eliminate any that
had inherent warpage. Testing was then conducted according to the
following: the flanges were each heated to 380 degrees Centigrade
for die attachment. Twelve die were attached to each flange, and
each was then cooled to room temperature at approximately the same
rate. After cooling, each flange was measured from center to edge
for extent of warpage. The results are graphically illustrated in
FIG. 11, with reference to the alphabetic designations, above, for
each flange. As can be seen, each of the frame-bearing stiffened
flanges (B, D, and E) had less warpage than the control flange A.
Further, the flanges that had both frame and raised pedestal (D, E)
had less warpage than those with only a frame (B). Finally, the
flange with frame and multiple pedestals E had the best result
(lowest warpage).
[0053] While at least one example embodiment has been presented in
the foregoing detailed description, it should be appreciated that a
vast number of variations exist. It should also be appreciated that
the example embodiment or embodiments described herein are not
intended to limit the scope, applicability, or configuration of the
invention in any way. Rather, the foregoing detailed description
will provide those skilled in the art with a convenient road map
for implementing the described embodiment or embodiments. It should
be understood that various changes can be made in the function and
arrangement of elements without departing from the scope of the
invention as set forth in the appended claims and the legal
equivalents thereof.
* * * * *