U.S. patent application number 11/426166 was filed with the patent office on 2006-10-12 for semiconductor package having integrated metal parts for thermal enhancement.
Invention is credited to William D. Boyd, Anthony L. Coyle, Chris Haga, Leland S. Swanson.
Application Number | 20060226521 11/426166 |
Document ID | / |
Family ID | 35479780 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060226521 |
Kind Code |
A1 |
Coyle; Anthony L. ; et
al. |
October 12, 2006 |
Semiconductor Package Having Integrated Metal Parts for Thermal
Enhancement
Abstract
A semiconductor device comprising a metallic leadframe (103)
with a first surface (103a) and a second surface (103b). The
leadframe includes a chip pad (104) and a plurality of segments
(107); the chip pad is held by a plurality of straps (105), wherein
each strap has a groove (106). A chip (101) is mounted on the chip
pad and electrically connected to the segments. A heat spreader
(110) is disposed on the first surface of the leadframe; the heat
spreader has its central portion (110a) spaced above the chip
connections (108), and also has positioning members (110b)
extending outwardly from the edges of the central portion so that
they rest in the grooves of the straps. Encapsulation material
surrounds the chip, the electrical connections, and the spreader
positioning members, and fills the space between the spreader and
the chip, while leaving the second leadframe surface and the
central spreader portion exposed.
Inventors: |
Coyle; Anthony L.; (Plano,
TX) ; Boyd; William D.; (Plano, TX) ; Haga;
Chris; (McKinney, TX) ; Swanson; Leland S.;
(McKinney, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Family ID: |
35479780 |
Appl. No.: |
11/426166 |
Filed: |
June 23, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10871645 |
Jun 18, 2004 |
7084494 |
|
|
11426166 |
Jun 23, 2006 |
|
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|
Current U.S.
Class: |
257/666 ;
257/E23.092 |
Current CPC
Class: |
H01L 2224/48227
20130101; H01L 2924/181 20130101; H01L 2924/00014 20130101; H01L
2224/73265 20130101; H01L 2224/73265 20130101; H01L 2924/14
20130101; H01L 2924/01078 20130101; H01L 2924/14 20130101; H01L
2224/32225 20130101; H01L 2924/181 20130101; H01L 2924/16152
20130101; H01L 2224/45015 20130101; H01L 2224/48227 20130101; H01L
2924/00 20130101; H01L 2224/45099 20130101; H01L 2224/32225
20130101; H01L 2924/00 20130101; H01L 2924/00012 20130101; H01L
2924/207 20130101; H01L 23/4334 20130101; H01L 2924/00014 20130101;
H01L 2924/00014 20130101; H01L 24/48 20130101; H01L 2924/19041
20130101 |
Class at
Publication: |
257/666 |
International
Class: |
H01L 23/495 20060101
H01L023/495 |
Claims
1. A semiconductor device comprising: a metallic leadframe having a
first surface and a second surface, said leadframe including a chip
pad and a plurality of segments; a plurality of straps holding said
chip pad, each strap having a groove; a semiconductor chip mounted
on said chip pad and electrically connected to said segments; a
heat spreader disposed on said first surface of said leadframe,
said heat spreader comprising a central portion spaced above said
chip connections, and positioning members outwardly extending from
edges of said central portion to rest in said grooves of said
straps; and encapsulation material surrounding said chip,
electrical connections, and spreader members, filling said space
between said spreader and chip, while leaving said second leadframe
surface and said central spreader portion exposed.
2. The device according to claim 1 wherein said heat spreader
further comprises a three-dimensional bell shape to approximate,
together with said metallic chip pad, a closed, thermally
conductive shell surrounding said chip.
3. The device according to claim 1 wherein said heat spreader
further comprises at least one curving to enhance adhesion to said
encapsulation material.
4. The device according to claim 1 further including a first heat
sink in contact with said central spreader portion.
5. The device according to claim 1 further including a second heat
sink in contact with said second surface of said leadframe.
6. The device according to claim 1 wherein said leadframe is made
of copper or copper alloy.
7. The device according to claim 1 where said leadframe material is
selected from a group consisting of brass, aluminum, iron-nickel
alloys, and invar.
8. The device according to claim 1 wherein said leadframe has a
thickness in the range from about 100 to 300 .mu.m.
9. The device according to claim 1 wherein said chip pad has four
straps.
10. The device according to claim 1 wherein said strap grooves have
a depth approximately one half of the thickness of the leadframe
material.
11. The device according to claim 1 wherein said heat spreader is
made of copper.
12. The device according to claim 1 wherein said heat spreader
material has a thickness in the range from about 100 to 300
.mu.m.
13. The device according to claim 1 wherein said heat spreader has
four positioning members.
14. The device according to claim 1 wherein said chip is mounted on
said chip pad by means of a thermally conductive adhesive.
15. The device according to claim 1 wherein said encapsulation
material is a molding compound having thermally conductive filler
material.
16. (canceled)
17. An apparatus comprising: a metallic leadframe having a first
surface and a second surface, said leadframe including a pad for
mounting a heat-generating object; a plurality of segments operable
to anchor connections to said object; a plurality of straps holding
said pad, each strap having a groove; and a heat spreader disposed
on said first surface of said leadframe, said heat spreader
comprising a portion spaced above said pad, and positioning members
outwardly extending from edges of said portion to rest in said
grooves of said straps.
Description
FIELD OF THE INVENTION
[0001] The present invention is related in general to the field of
electrical systems and semiconductor devices and more specifically
to thermally enhanced semiconductor devices having integrated
metallic chip support and heat spreader.
DESCRIPTION OF THE RELATED ART
[0002] Removing the thermal heat generated by active components
belongs to the most fundamental challenges in integrated circuit
technology. Coupled with the ever shrinking component feature sizes
and increasing density of device integration is an ever increasing
density of power and thermal energy generation. However, in order
to keep the active components at their low operating temperatures
and high speed, this heat must continuously be dissipated and
removed to outside heat sinks. This effort becomes increasingly
harder, the higher the energy density becomes.
[0003] In known technology, one approach to heat removal,
specifically for devices with metallic leads, focuses on thermal
transport through the thickness of the semiconductor chip from the
active surface to the passive surface. The passive surface, in
turn, is attached to the chip mount pad of a metallic leadframe so
that the thermal energy can flow into the chip mount pad of the
metallic leadframe. The layer of the typical polymer attach
material represents a thermal barrier. When properly formed, the
leadframe can act as a heat spreader to an outside heat sink. In
many semiconductor package designs, this implies a leadframe with a
portion formed such that this portion protrudes from the plastic
device encapsulation; it can thus be directly attached to the
outside heat sink. Examples are described in U.S. Pat. No.
5,594,234, issued on Jan. 14, 1997 (Carter et al., "Downset Exposed
Die Mount Pad Leadframe and Package") and U.S. Pat. No. 6,072,230,
issued on Jun. 6, 2000 (Carter et al., "Bending and Forming Method
of Fabricating Exposed Leadframes for Semiconductor Devices").
[0004] Another approach of known technology, specifically for
ball-grid array devices without leadframes, employs a heat spreader
spaced in proximity of the active surface of the semiconductor
chip, at a safe distance from the electrical connections of the
active surface. In this approach, the heat has to spread first
through the macroscopic thickness of the molding material
(typically an epoxy filled with inorganic particles, a mediocre
thermal conductor) and only then into a metallic heat spreader.
Frequently, the spreader is positioned on the surface of the molded
package; in other devices, it is embedded in the molded package, as
described in U.S. Pat. No. 6,552,428, issued on Apr. 22, 2003
(Huang et al., "Semiconductor Package having an Exposed Heat
Spreader").
[0005] For leadless devices with small outlines (both relative to
area consumption and device height), the application of any one of
these thermal approaches is aggravated by the need for robustness
during the encapsulation process, especially in transfer molding or
injection molding methods. Any thermal advancement has to be
low-cost, since cost-adding technical proposals are contrary to the
strong market emphasis on total semiconductor device package cost
reduction.
SUMMARY OF THE INVENTION
[0006] A need has therefore arisen to for a concept of a low-cost,
thermally improved and electrically high performance leadless
structure. In addition, a general semiconductor package structure
is needed which based on fundamental physics and design concepts
flexible enough to be applied for different semiconductor product
families and a wide spectrum of design and assembly variations. It
should not only meet high thermal and electrical performance
requirements, but should also achieve improvements towards the
goals of enhanced process yields and device reliability.
[0007] The present invention provides improved thermal performance
of integrated circuits, especially of the SON, SOIC, SOP, and PDIP
families, solving one of the most intractable limitations of
semiconductor technology. One embodiment of the invention is a
semiconductor device comprising a metallic leadframe with a first
surface and a second surface. The leadframe includes a chip pad and
a plurality of segments; the chip pad is held by a plurality of
straps, wherein each strap has a groove. A chip is mounted on the
chip pad and electrically connected to the segments. A heat
spreader is disposed on the first surface of the leadframe; the
heat spreader has its central portion spaced above the chip
connections, and also has positioning members extending outwardly
from the edges of the central portion so that they rest in the
grooves of the straps. Encapsulation material surrounds the chip,
the electrical connections, and the spreader positioning members,
and fills the space between the spreader and the chip, while
leaving the second leadframe surface and the central spreader
portion exposed.
[0008] The base metal for both the leadframe and the heat spreader
is preferably copper. The preferred thickness range for leadframe
as well as spreader is between about 300 and 150 .mu.m. The depth
of the grooves in the chip pad straps are preferably about half the
leadframe thickness.
[0009] Preferably, the heat spreader has a three-dimensional
dome-like shape to approximate, together with the metallic chip
pad, an almost closed thermally conductive shell surrounding the
chip. Further, the heat spreader has preferably at least one groove
formed so that it enhances the adhesion to the encapsulation
material.
[0010] In another embodiment of the invention, a heat sink is in
contact with the central spreader portion. In yet another
embodiment of the invention, another heat sink is in contact with
the second surface of the leadframe.
[0011] It is a technical advantage of the invention that it offers
low-cost design and structure options for semiconductor packages to
create short paths of steepened temperature gradient in order to
dissipate the heat flux away from high-temperature IC portions to
(outside) heat sinks.
[0012] It is another technical advantage that the innovation of the
invention is accomplished using the installed equipment base so
that no investment in new manufacturing machines is needed.
[0013] The technical advances represented by the invention, as well
as the objects thereof, will become apparent from the following
description of the preferred embodiments of the invention, when
considered in conjunction with the accompanying drawings and the
novel features set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic X-ray view of a packaged semiconductor
device illustrating an embodiment of the invention. The X-ray view
of FIG. 1 includes a cross section along line A-A' in FIG. 2.
[0015] FIG. 2 is a schematic top view of the semiconductor device
shown in FIG. 1, illustrating an embodiment of the invention.
[0016] FIG. 3 is a schematic X-ray view and cross section of a
semiconductor device illustrating another embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] The present invention is related to U.S. Pat. No. 6,597,065,
issued on Jul. 22, 2003 (Efland, "Thermally Enhanced Semiconductor
Chip having Integrated Bonds over Active Circuits").
[0018] FIG. 1 is a schematic X-ray view of a packaged semiconductor
device, generally designated 100, illustrating an embodiment of the
invention. Chip 101 is mounted by an adhesive chip attach material
102 onto the first surface 103a of a metallic leadframe 103. Attach
adhesive 102 is thermally conductive; preferably, it is a
silver-filled epoxy. The semiconductor material of chip 101 is
preferably silicon; alternatively, chip 101 may comprise silicon
germanium, gallium arsenide, or any other semiconductor material
used for device fabrication.
[0019] Chip 101 has an active surface 101a, which comprises
components such as integrated circuits, discrete transistors and
diodes, and also passive parts including capacitors and resistors.
Active surface 101a generates the thermal energy/heat during device
operation, which needs to be transported away in order to maintain
the preferred operation temperatures and to prevent overheating of
the active components.
[0020] The most effective way of removing thermal energy is by
conduction; less effective, but nevertheless welcome mechanisms
include thermal transport by convection and by radiation. In
quantitative terms, the energy transport by thermal conductance is
expressed by a differential equation, which expresses, following
FOURIER's approach, the thermal flux Q per unit of time as being
the product of thermal conductivity .lamda. multiplied by the
gradient of temperature T, in the direction of decreasing
temperature, and by the area q perpendicular to the temperature
gradient: dQ/dt=-.lamda.(grad T)q. In this equation, Q is the
vector (in magnitude and direction) of thermal flux, and .lamda. is
the thermal conductivity, a materials characteristic. The thermal
flux is in the direction of the temperature difference and is
proportional to the magnitude of that difference.
[0021] When, over the length 1, the temperature drop is steady and
uniform from the high temperature T2 to the low temperature T1,
then (grad T) reduces to (T2-T1)/l: dQ/dt=-.lamda.(q/l)(T2-T1).
.lamda.(q/l) is called the thermal conductance, and the inverse
value l/(.lamda.q) is called thermal resistance (in analogy to
OHM's law).
[0022] In the present invention, improvements of both .lamda.q and
(grad T) are simultaneously provided to enhance the thermal flux
vertically away from the heat-generating active components on the
active surface 101a of the semiconductor chip 101.
[0023] In addition to this enhanced thermal flux vertically away
from the active chip surface, there is the traditional possibility
of conducting thermal energy in the opposite direction through the
semiconductor material chip 101 to its passive surface 101b and
beyond into leadframe 103. Through the second surface 103b of
leadframe 103, the thermal flux can enter the ambient or an
attached substrate.
[0024] Leadframe 103 is made of a base metal, preferably copper or
a copper alloy. Alternative base metals include brass, aluminum,
iron-nickel alloys (such as "Alloy 42"), and invar. Frequently, the
base metal is fully covered with a plated layer; as an example, the
copper base metal may be plated with a nickel layer.
[0025] [As defined herein, the starting material of the leadframe
is called the "base metal", indicating the type of metal.
Consequently, the term "base metal" is not to be construed in an
electrochemical sense (as in opposition to `noble metal`) or in a
structural sense.]
[0026] The base metal of leadframe 103 originates with a metal
sheet in the preferred thickness range from 100 to 300 .mu.m;
thinner sheets are possible. The ductility in this thickness range
provides the 5 to 15% elongation that facilitates the segment
bending and forming operation. The leadframe is stamped or etched
from the starting metal sheet.
[0027] As stated above, the base metal of leadframe 103 may often
be plated, for instance with a nickel layer. The plated layer is
preferably rough, non-reflective nickel having a thickness between
0.2 and 1.0 .mu.m, preferably 0.5.+-.25 .mu.m. Nickel is the
preferred metal because, positioned under the tin-based solder of
contemporary devices, it reduces the propensity for tin whiskers.
The plated nickel layer is ductile for the leadframe bending and
forming process.
[0028] The X-ray view of FIG. 1 includes a cross section along line
A-A' of the device depicted in FIG. 2. Consequently, the material
of leadframe 103 appears continuous in FIG. 1, but actually
includes the chip pad 104 and a plurality of straps 105, which hold
the chip pad 104. Each strap 105 has a groove 106 (see FIG. 2) of
depth 106a and width 106b. Preferably, the depth 106a of the strap
groove is approximately one half of the thickness of the leadframe
material, resulting in a depth of about 50 to 150 .mu.m. Not shown
in FIG. 1 is the plurality of leadframe segments 107 (they are,
however, clearly represented in FIG. 2). The semiconductor chip 101
is electrically connected the leadframe segments; as an example,
FIG. 1 depicts a couple of wire bonds 108 serving as chip
connections between the chip contact pads and the leadframe
segments (not shown).
[0029] FIG. 1 further shows a heat spreader 110, which is disposed
on the first surface 103a of the leadframe. Heat spreader 110 has a
central portion 110a spaced above chip 101 and the chip connections
107. Further, heat spreader 110 has a plurality of positioning
members 110b, which extend outwardly from the edges of the central
portion 110a and rest in the grooves 106 of the leadframe straps
105. The heat spreader is preferably made of copper in a thickness
range comparable to the metal of the leadframe 100 to 300
.mu.m).
[0030] Typically, the heat spreader is stamped and formed form
sheet metal; in this process, the positioning members 110b obtain a
configuration, which provides strong anchoring of the encapsulation
material onto the heat spreader. An example of suitably formed
members with at least one curving for encapsulation adhesion is
schematically illustrated in FIG. 1; this FIG. further shows the
manner, in which the tips 111 of the members are positioned in the
strap grooves 106. Using the depth of the grooves, this positioning
secures a strong enough anchoring of the heat spreader to withstand
any pressure of the molding compound during the transfer molding
process.
[0031] In its final form, the heat spreader 110 comprises a
three-dimensional bell-shaped metal part. Together with the
metallic chip pad 104 of the leadframe, the bell-shaped heat
spreader approximates for the device 100 an integrated, thermally
strong conductive shell surrounding chip 101.
[0032] Device 100 is packaged by encapsulation material 120, which
surrounds chip 101, the electrical connections 108, and the
spreader positioning members 110b. The encapsulation material fills
the space between spreader 110 and chip 101, but leaves the second
leadframe surface 103b and the central spreader portion 110
exposed. The preferred encapsulation material is a molding compound
containing thermally conductive fillers.
[0033] The cross section of FIG. 1 is taken along line A-A' of FIG.
2; FIG. 2 itself is a schematic top view of an embodiment of the
present invention, wherein dashed lines indicate device features
inside the plastic encapsulation. 110a outlines the central portion
of the heat spreader. The four positioning members 110b extend
outwardly from the edges of the central portion to rest in the
grooves 106 of the straps 105 of the leadframe chip pad. The chip
101, mounted on the pad 103, has a plurality of contact pads 201,
of which only four pads are shown in FIG. 2 for clarity. Bonding
wires 108 connect the chip pads 201 to the leadframe segments 107;
for clarity, only four bonding wire connections out of the
plurality of segments are shown in FIG. 2.
[0034] In the schematic cross section of FIG. 3, the device 100 of
FIG. 1 is shown with heat sinks attached, actually, FIG. 3 depicts
two heat sinks, but for some applications, one sink is sufficient.
The first heat sink 301 is mounted, by means of attach material
302, on the device surface having the exposed central portion 110a
of heat spreader 110. The second heat sink 303 is mounted, by means
of attach material 304, on the leadframe surface having the exposed
surface 103b of chip pad 104. Heat sinks 301 and/or 303 provide a
steep temperature gradient (grad T) for enhancing the thermal flux
away from the heat-generating active components on the active chip
surface 101a. At the same time, the combination of metallic
leadframe and metallic heat spreader with their high thermal
conductivity ? and large area q perpendicular to the temperature
gradient, improves the product ?q.
[0035] While this invention has been described in reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description.
[0036] As an example, the invention covers integrated circuits made
in substrates of silicon, silicon germanium, gallium arsenide, or
any other semiconductor material used in integrated circuit
manufacture.
[0037] As another example, the invention covers generally a
heat-generating semiconductor unit. This concept thus includes
single-chip as well as multi-chip devices. Further, the concept
includes devices employing wire-bonded assembly as well as
flip-chip assembly.
[0038] It is therefore intended that the appended claims encompass
any such modifications or embodiments.
* * * * *