U.S. patent application number 15/633977 was filed with the patent office on 2018-12-27 for compliant heat sink.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Gerhard I. Meijer, Gerd Schlottig.
Application Number | 20180374771 15/633977 |
Document ID | / |
Family ID | 64604930 |
Filed Date | 2018-12-27 |
United States Patent
Application |
20180374771 |
Kind Code |
A1 |
Meijer; Gerhard I. ; et
al. |
December 27, 2018 |
COMPLIANT HEAT SINK
Abstract
A compliant heat sink for transporting heat away from at least
one electronic component, the heat sink includes a body, where the
body includes a flexible element thermally contacting at least one
electronic component. The heat sink further includes a cavity
located in the body, where the cavity is at least partially covered
by the flexible element. The heat sink further includes a raised
member of the body coupled to the flexible element, where a portion
of the raised member partially extends into the cavity. The heat
sink further includes a guiding structure of the body coupled in
the cavity of the body, wherein the guiding structure is adapted
for guiding the movement of the raised member in a moving
direction.
Inventors: |
Meijer; Gerhard I.; (Zurich,
CH) ; Schlottig; Gerd; (Uitkon Waldegg, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
64604930 |
Appl. No.: |
15/633977 |
Filed: |
June 27, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 23/3677 20130101;
H01L 2224/73253 20130101; H01L 23/367 20130101; H01L 23/427
20130101; H01L 23/3672 20130101; H01L 23/3736 20130101 |
International
Class: |
H01L 23/367 20060101
H01L023/367; H01L 23/427 20060101 H01L023/427 |
Claims
1. A compliant heat sink for transporting heat away from at least
one electronic component, the heat sink comprising: a body, wherein
the body includes a flexible element thermally contacting at least
one electronic component; a cavity located in the body, wherein the
cavity is at least partially covered by the flexible element; a
raised member of the body coupled to the flexible element, wherein
a portion of the raised member partially extends into the cavity;
and a guiding structure of the body coupled in the cavity of the
body, wherein the guiding structure is adapted for guiding the
movement of the raised member in a moving direction, wherein the
guiding structure includes a guiding wall protruding into the
cavity.
2. (canceled)
3. The heat sink of claim 1, wherein the guiding wall is bendable
in a bending direction perpendicular to the moving direction.
4. The heat sink of claim 3, further comprising: an expansion
layer, wherein the expansion layer is coupled to at least part of
the guiding wall and a surface of the body opposite to the guiding
wall, wherein the expansion layer includes a larger coefficient of
thermal expansion than the body.
5. The heat sink of claim 4, wherein the expansion layer extends
into a slot in the body opposite to the guiding wall.
6. The heat sink of claim 1, wherein at least part of the guiding
wall is elongated by a groove, the groove immediately coupled to
the guiding wall and extending into the body.
7. The heat sink of claim 1, wherein the guiding wall includes a
tapering towards the flexible element.
8. The heat sink of claim 1, wherein the cavity is filled with any
one of the following: air, nitrogen, methanol vapor, ethanol vapor,
and an arbitrary combination thereof.
9. The heat sink of claim 1, wherein a pressure within the cavity
exceeding an ambient air pressure of the heat sink.
10. The heat sink of claim 1, the guiding structure further
comprises: a friction lowering coating, wherein the friction
lowering coating interfaces with the guiding structure and the
raised member.
11. The heat sink of claim 1, the guiding structure further
comprises: a heat conducting coating, wherein the heat conducting
coating interfaces with the guiding structure and the raised
member.
12. The heat sink of claim 1, wherein the raised member being any
one of: a cone, a cylinder, a pin, a fin, a dome, a prism, and
combinations thereof.
13. The heat sink of claim 1, wherein a coefficient of thermal
expansion of the raised member exceeds a coefficient of thermal
expansion of the guiding wall.
14. (canceled)
15. The heat sink of claim 1, wherein the material of any one of
the raised member and the flexible element comprising any one of:
Cu and an alloy based on Mg, Zn, Al, Si, SiC, or W.
16. The heat sink of claim 1, further comprising: an air-cooling
component for exchanging heat with ambient air of the body, the
air-cooling component being thermally coupled to the body, and a
liquid-cooling component for exchanging heat with a heat transport
liquid, the liquid-cooling component being thermally coupled to the
body.
17. The heat sink of claim 1, wherein the cavity, the flexible
element, the raised member and the guiding structure form a heat
transfer unit.
18. The heat sink of claim 1, wherein the raised member and the
guiding structure form a heat transfer unit.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to cooling of electronic
components, and more specifically to cooling of microchips.
BACKGROUND
[0002] Electronic devices generate heat during operation. High
performance integrated circuits such as computer processors
containing nanometer scaled structures are among the electronic
devices that are most sensitive to heat. Subject to the available
cooling power, these components and devices are operated within
certain boundaries of operational parameters such as voltage,
clocking frequency, and idle time, which are known as the thermal
envelope of the electronic device.
[0003] Integrated circuits are commonly manufactured on thin, flat,
semiconductor dice mounted in a package. Heat generated in the die
is transported through the package into, for example, ambient air
or a liquid coolant. In practice, a semiconductor die is not
perfectly flat, but has a slightly curved or warped (e.g., convex)
surface.
[0004] Semiconductor dice are often manufactured with standardized
sizes, for example, 20.times.20 mm. It is expected that dice for
future high-end applications such as servers in a data center will
be made with larger dimensions than are usual today. Another recent
development is the use of packages comprising vertically stacked
chips. Both developments increase the vertical amplitude of a
non-uniform cooling surface of an electronic device.
SUMMARY
[0005] One aspect of an embodiment of the present invention
discloses a compliant heat sink for transporting heat away from at
least one electronic component, the heat sink comprising, a body,
wherein the body includes a flexible element thermally contacting
at least one electronic component, a cavity located in the body,
wherein the cavity is at least partially covered by the flexible
element, a raised member of the body coupled to the flexible
element, wherein a portion of the raised member partially extends
into the cavity, and a guiding structure of the body coupled in the
cavity of the body, wherein the guiding structure is adapted for
guiding the movement of the raised member in a moving
direction.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0006] The following detailed description, given by way of example
and not intended to limit the disclosure solely thereto, will best
be appreciated in conjunction with the accompanying drawings, in
which:
[0007] FIG. 1 depicts a schematic cut through a compliant heat
sink, in accordance with an embodiment of the present
invention.
[0008] FIG. 2 depicts a schematic cut through a compliant heat
sink, the flexible element being actuated, in accordance with an
embodiment of the present invention.
[0009] FIG. 3 depicts a schematic cut through a compliant heat
sink, the guiding structure being part of an insert, in accordance
with an embodiment of the present invention.
[0010] FIG. 4 depicts a schematic cut through a compliant heat sink
with a tapered guiding wall, in accordance with an embodiment of
the present invention.
[0011] FIG. 5 depicts a schematic cut through a compliant heat sink
with an expansion layer being present in the cavity, in accordance
with an embodiment of the present invention.
[0012] FIG. 6 depicts a schematic cut through a compliant heat sink
with a groove adjoining the guiding wall, in accordance with an
embodiment of the present invention.
[0013] FIG. 7 depicts a schematic cut through a compliant heat
sink, the expansion layer extending into a slot in the body, in
accordance with an embodiment of the present invention.
[0014] FIG. 8A depicts a coating interfacing the guiding structure
and the raised member in cold state, in accordance with an
embodiment of the present invention.
[0015] FIG. 8B depicts a coating interfacing the guiding structure
and the raised member in heated-up state, in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION
[0016] It is an objective of the present invention to provide for
an improved heat sink for transporting heat away from at least one
electronic component and a cool-able electronics system as
specified in the independent claims. Embodiments of the invention
are given in the dependent claims. Embodiments of the present
invention can be freely combined with each other if they are not
mutually exclusive.
[0017] In one aspect, the invention relates to a compliant heat
sink for transporting heat away from at least one electronic
component. The heat sink comprises a body, a flexible element for
thermally contacting the at least one electronic component, and a
raised member. The body comprises a cavity, which is covered by the
flexible element. The raised member is fixed to the flexible
element and extends into the cavity. The body further comprises a
guiding structure located in the cavity and adapted for guiding the
movement of the raised member in a moving direction.
[0018] In a further aspect, the invention relates to a cool-able
electronics system, which comprises at least one electronic
component and the heat sink according to an embodiment of the
invention. The heat sink is mounted on the at least one electronic
component, which is thermally coupled to the heat sink by a
full-area contact of the at least one electronic component to the
flexible element.
[0019] The compliant heat sink 100 according to embodiments of the
present invention may provide a strong and stable thermal coupling
to sensitive electronic components with uneven, curved, warped or
other non-uniform cooling surfaces. Sensitive electronic components
require a cooling apparatus or heat sink to prevent overheating.
Thermal coupling to a heat sink is often provided for an electronic
component with a flat surface by means of a lidded package with a
flat, rigid lid and one or more layers of a thermal interface
material (TIM) to compensate unevenness on small scales. However,
electronic components with a large area cannot be treated as
approximately flat devices as they might exhibit unevenness on the
scale of the cooling area. A rigid lid might provide thermal
coupling to merely part of the cooling area of the component,
resulting in inefficient cooling due to long cooling paths for part
of the cooling area and unacceptable TIM thicknesses such that the
thermal insulation of the TIM compared to the heat sink material
outweighs the capability to increase heat conductivity by filling
microscopic gaps.
[0020] FIG. 1 depicts a schematic cut through a compliant heat
sink, in accordance with an embodiment of the present
invention.
[0021] The largest part of heat sink 100 is formed by heat sink
body 102, which is shown here with a basically rectangular contour.
Cavity 112 is formed on the side of body 102, which is intended to
face the electronic component to be cooled. Cavity 112 is closed
with flexible element 108, which is fixed to body 102 along the rim
of cavity 112. Cavity 112 is also shown with a basically
rectangular structure. Body 102 in the center of cavity 112 on a
cavity wall, which is facing flexible element 108, forms guiding
structure 106. Raised member 110, also with a rectangular
cross-section, is fixed to flexible element 108 in the center of
cavity 112. Height and width of raised member 110 are chosen such
that it extends sufficiently far into cavity 112 that guiding
structure 106 receives raised member 110 and part of the lateral
surfaces of raised member 110 are contacting guiding structure
106.
[0022] Raised member 110 and guiding structure 106 form a heat path
from the center of flexible element 108 to body 102. Without raised
member 110 and guiding structure 106, heat generated by an
electronic component in thermal contact with flexible element 108
would be transported to body 102 through the thin flexible element
108 and possibly also by heat-conducting gas or vapor filled into
cavity 112. Preferably, flexible element 108, raised member 110,
and guiding structure 106 are made of materials with a high heat
conductivity. Therefore, the additional heat path formed by raised
member 110 and flexible element 108 in the center of cavity 112 may
strongly increase heat transport away from the center of the
electronic component.
[0023] All solid parts of compliant heat sink 100 according to
embodiments of the invention are preferably made of materials with
a high thermal conductivity. Although materials exist with a higher
thermal conductivity, a cost-effective choice for the solid
components of heat sink 100 are metals like aluminum or copper or
metal alloys with a high thermal conductivity. More specific
material choices for the single path will be discussed in the
following.
[0024] As used herein, body 102 of compliant heat sink 100
according to embodiments of the invention is a piece of a rigid,
heat conducting material which is large enough to host cavity 112
which is capable of covering a cooling area of a single electronic
component or a footprint area of an arrangement of more than one
electronic components to be cooled. Body 102 forms the mechanical
framework of heat sink 100. In the depictions of FIGS. 1-7, the
body consists of a thick plate for spreading and transporting the
heat collected from the electronic components to a heat exchanger,
and sidewalls of cavity 112 providing mechanical connectivity to a
structure hosting the electronic component (e.g., a substrate or a
circuit board).
[0025] Being the largest part of heat sink 100, it is preferably
made of a material which offers a sufficient amount of heat
conduction. Body 102 can be formed from a metal plate, the metal
being one of aluminum and copper. Body 102 may be implemented with
any shape which is suitable for the available space for mounting
and for connecting it to further cooling equipment downstream of
the heat transfer path, including, but not limited to, the
basically rectangular cross-section shown in FIGS. 1-7.
[0026] In an embodiment, body 102 is thermally coupled to an
air-cooling component for exchanging the heat with ambient air of
heat sink 100. In another embodiment, body 102 is thermally coupled
to a liquid cooling component for exchanging the heat with a heat
transport liquid. Both cooling components may establish a
continuous heat flow away from the one or more electronic
components to be cooled, enabling it to operate in an allowable
temperature range or thermal envelope.
[0027] On a side designed for facing the at least one electronic
component, body 102 comprises cavity 112. In FIGS. 1-7, cavity 112
is located at the bottom of body 102, which corresponds to an
application where the compliant heat sink 100 is installed above an
electronic component that is mounted on a substrate or circuit
board with a horizontal orientation. However, it is clear to those
of ordinary skill in the art that the electronic component to be
cooled may be likewise oriented in a vertical plane or in any other
tilted or slanted direction as required by the particular
installation. For the purpose of this disclosure, terms like
`horizontal`, `vertical`, `above`, `below`, `next to`, `left`,
`right` etc. which describe absolute or relative spatial
orientations or arrangements refer, unless otherwise noted, to the
embodiments depicted in FIGS. 1-7, where heat sink 100 is oriented
to face the at least one electronic component in a horizontal
plane.
[0028] A main purpose of cavity 112 is to provide space
perpendicular to a cooling surface of the electronic component,
i.e., in the vertical direction of FIG. 1. The horizontal
dimensions are chosen to slightly exceed the horizontal dimensions
or footprint area of a particular model, class, series, or
arrangement of electronic components. Vertically, cavity 112
dimensions should slightly exceed the largest expectable unevenness
of the electronic component such that it provides enough vertical
space for establishing a full area contact with the cooling surface
of the electronic component without wasting thermal path
length.
[0029] Cavity 112 may, for instance, be removed, e.g., by milling
or cutting, from the bulk material of body 102 from bottom upward,
such that guiding structure 106 can be formed as one piece with the
rest of body 102 and cavity 112 is closed or sealed afterwards with
flexible element 108, as shown e.g., in FIG. 1. Many other
manufacturing processes can be apparently used to create body 102
with cavity 112. In particular, body 102 can be formed with cavity
112 in a single manufacturing step. If, for instance, body 102 is
formed in a casting or molding process, designing the model
accordingly can provide cavity 112.
[0030] Alternatively, cavity 112 and most of the solid parts of
heat sink 100 may be formed by removing material from a precursor
of body 102 upside-down, such that flexible element 108 and raised
member 110 are formed as one part with body 102 and guiding
structure 106 is connected with body 102 to close cavity 112 as a
separate piece afterwards, as shown in FIG. 3. For the purpose of
description, cavity boundaries opposing flexible element 108 will
herein be referred to as the `ceiling` of cavity 112, whereas the
lateral boundaries, as depicted in FIGS. 1-7, will be referred to
as the `sidewalls`. Cavity 112 may be open or closed with respect
to ambient pressure.
[0031] Cavity 112 may be filled with a gas or vapor to provide
additional thermal contact between body 102 and flexible element
108 besides the thermal path formed by the fixture of flexible
element 108 to body 102 and raised member 110 contacting guiding
structure 106. According to an embodiment, cavity 112 is filled
with air, nitrogen, methanol vapor, ethanol vapor, or an arbitrary
combination thereof. These materials may provide a means to achieve
mentioned additional thermal coupling with sufficient heat
transport performance. The cavity may be filled with any other gas
typically used in vapor chambers.
[0032] Moreover, a gas or vapor filling of cavity 112 may provide a
means for reacting to sudden temperature changes of the electronic
component to be cooled. This may yield a quicker thermal response
of the thermal expansion coefficient mismatched heat path formed by
flexible element 108 and guiding structure 106, or alternatively,
of an expansion layer 500 surrounding guiding structure 106, as
will be explained in further detail below.
[0033] According to an embodiment of the invention, the pressure
within cavity 112 exceeds an ambient air pressure of heat sink 100.
The ambient air pressure may be atmospheric pressure or a pressure
level of a pressurizing system, e.g., a cooling system, which is
typically used to provide positive air pressure for a computing
center. A pressurized cavity 112 may enable compliant heat sink 100
to form a full area of thermal contact with electronic components
having a concave cooling surface.
[0034] Flexible element 108, as used herein, is a thin piece of a
material with a high thermal conductivity. The dimensions are
similar to those of the cavity opening, such that it can be fixed
to body 102 to cover or seal cavity 112. Alternatively, flexible
element 108 is a thin, flat section of body 102, which has been
spared from forming cavity 112 in an upside-down process. In this
case, the horizontal dimensions of flexible element 108 are
identical to those of cavity 112. If implemented as a separate
piece, flexible element 108 may be formed from a different material
than body 102.
[0035] Flexible element 108 acts as a membrane, which is spanned
under cavity 112. The main purposes are to provide a full area
contact with a curved or other non-uniform cooling area of a
heat-generating electronic component and to spread this heat across
thickness into raised member 110 and cavity 112 and to transport
the heat along the area towards the bulk of body 102. It provides
the mechanical compliance, which is needed to collect the heat from
all regions of the non-uniform surface with a high coupling
efficiency.
[0036] According to an embodiment, flexible element 108 is made of
a metal or metal alloy with a high thermal conductivity, such as an
alloy based on Mg, Zn, or Al, copper, or alloys based on Si, SiC,
or W. The thickness is optimized for responding elastically to the
contact with the cooling surface of the electronic component. It
should be thick enough that it will not get ruptured when force is
applied to install heat sink 100 on the at least one electronic
component, but it should be as thin as possible in order to
minimize the force which is needed for installing in order to
prevent damage from the electronic component.
[0037] In addition to these requirements, flexible element 108
should also bear the shear stress occurring in the region between
the perimeter of the electronic component and the sidewalls of
cavity 112. In typical and projected application scenarios, large
electronic components like microchip dice have curvature amplitudes
between 100 .mu.m and 1 mm. Accordingly, suitable membrane
thicknesses are expected to be in the range between 300 .mu.m and
1.5 mm.
[0038] In order to maximize the thermal coupling efficiency between
flexible element 108 and body 102, a firm metal-to-metal fixture
should be used to fix flexible element 108, if implemented as a
separate part, to body 102. A soldered, brazed or welded connection
appears appropriate for this purpose.
[0039] FIG. 2 depicts a schematic cut through a compliant heat
sink, the flexible element being actuated, in accordance with an
embodiment of the present invention.
[0040] In this embodiment, compliant heat sink 100 of FIG. 1
includes flexible element 108 in an actuated state, i.e., during
application of heat sink 100 to an electronic component (not shown)
with a convexly curved cooling surface. The slightly pre-tensioned
flexible element 108 adapts to the curved geometry of the cooling
surface and gets bent into the cavity volume. Raised member 110
fixed to flexible element 108 at the center of cavity 112 is pushed
upward and further into guiding structure 106. The contact surface
between raised member 110 and guiding structure 106 thus becomes
larger, yielding increased heat conductivity of the heat path
formed by these two parts.
[0041] According to an embodiment, the coefficient of thermal
expansion of flexible element 108 exceeds the coefficient of
thermal expansion of body 102. During heat up of the at least one
electronic component, heat is stored in the large heat capacity of
body 102. The heat stored in body 102 causes body 102 to expand
thermally, the thermal expansion being greatest in the largest
dimension.
[0042] In the embodiments depicted in FIGS. 1-7, thermal expansion
of body 102 is greatest in horizontal directions. As a result, the
horizontal dimensions of cavity 112 increase slightly and the
fixture of flexible element 108 moves away from the center. Hence,
heating up heat sink 100 increases strain of flexible element 108,
which may eventually lead to destruction. This effect may be
compensated if flexible element 108 has a higher coefficient of
thermal expansion than body 102. However, care should be taken that
the difference in thermal expansion coefficients is not too large
as this may cause flexible element 108 to release from the cooling
surface of the electronic component.
[0043] As used herein, raised member 110 is a small piece of metal,
which is fixed to the center of flexible element 108. The fixture
is preferably of the same kind as that used for connecting flexible
element 108 to body 102. Alternatively, raised member 110 and
flexible element 108 are machined from the same part such that no
connecting technology is needed. Flexible element 108 may be
selected from the list of materials given for flexible element 108.
Flexible element 108 may be formed from a different material than
body 102, if not implemented as a single part. Preferably, raised
member 110 is made of the same material as flexible element 108 to
prevent thermal expansion of flexible element 108 relative to
raised member 110.
[0044] Raised member 110 should match guiding structure 106 by
width and should be tall enough to be received by guiding structure
106 when the flexible element 108 is in the maximum deflection from
the ceiling of cavity 112. Upon actuation or relaxation of flexible
element 108, raised member 110 is pushed into or pulled out of
guiding structure 106 in moving direction 120 perpendicular to the
substrate or circuit board to which the at least one electronic
component is mounted. In FIGS. 1-7, moving direction 120 of raised
member 110 is vertical. Raised member 110 is depicted with a
rectangular cross-section in FIGS. 1-7, but it can be implemented
with numerous different shapes as will be understood by those of
ordinary skill in the art.
[0045] According to an embodiment raised member 110 is any one of a
cone, a cylinder, a pin, a fin, a dome, a prism, and combinations
thereof. The shape of raised member 110 may be suitably selected to
provide the best possible thermal coupling for the available space
and the targeted cooling performance. For instance, a pin or a
cylinder may be selected if cavity 112 has a regular horizontal
cross-section (e.g., circular or square), whereas a fin may be more
suitable for cavity 112 with a rectangular horizontal cross-section
to gain a larger contact area between raised member 110 and guiding
structure 106.
[0046] The mentioned shapes may also be advantageously combined
with each other. Examples include a cylinder with a dome-shaped
tip, which may facilitate insertion or re-insertion of raised
member 110 into guiding structure 106 during assembly or for
changing usage scenarios, or a slightly conical prism, which may
provide a robust mechanical and thermal contact to guiding
structure 106 as the pressure applied on heat sink 100 for mounting
is converted to horizontal component forces which press the flat
prism surfaces of raised member 110 and guiding structure 106
firmly against each other.
[0047] According to an embodiment, the coefficient of thermal
expansion of raised member 110 exceeds the coefficient of thermal
expansion of guiding wall 104. Heat sink 100 is usually installed
on the at least one electronic component in a cold state. When the
electronic component heats up, the heat will spread through
flexible element 108 into raised member 110, causing raised member
110 to grow vertically and horizontally. The vertical growth of
raised member 110 may increase the contacting surface with guiding
structure 106, while horizontal growth may cause raised member 110
to exert a vertical force on guiding structure 106, thus closing
microscopic gaps and providing a firmer mechanical and thermal
contact to body 102.
[0048] According to an embodiment, the material of any one of
raised member 110 and flexible element 108 comprises any one of: Cu
and an alloy based on Mg, Zn, Al, Si, SiC, or W. The mentioned
materials may ensure a high cooling performance of heat sink 100 by
providing a heat path with a high thermal conductivity.
[0049] Guiding structure 106, as used herein, is a part located at
the ceiling of cavity 112, opposing flexible element 108 and facing
raised member 110. Guiding structure 106 provides an opening
towards flexible element 108, as well as guiding wall 104 for
receiving and contacting raised member 110 as it moves in moving
direction 120. One guiding structure 106 always forms a pair with
one raised member 110. If only one pair of guiding structure 106
and raised member 110 is present, it is preferably horizontally
centered with cavity 112. Preferably, guiding structure 106 is
permanently receiving raised member 110 to provide an initial
contact length between raised member 110 and guiding structure 106
when flexible element 108 is unstrained or in a deflection away
from ceiling of the cavity 112.
[0050] Guiding wall 104 may be subdivided into more than one
segment. The top of guiding structure 106 may not necessarily be
aligned with the ceiling, but it should offer sufficient vertical
space such that raised member 110 does not touch the top for the
largest expectable unevenness of the cooling surface of the at
least one electronic component. The horizontal structure of guiding
wall 104 should fit the contour of raised member 110. Guiding
structure 106 may be formed as one part with body 102, or
alternatively, it may be part of a separate piece which is
connected to body 102 after cavity 112, raised member 110 and
flexible element 108 have been formed as one semi-finished
product.
[0051] Guiding structure 106 furthermore serves the purpose to
spread the heat transported from the at least one electronic
component through flexible element 108 and raised member 110 into
the heat spreading section or plate of body 102. The inner surfaces
of guiding wall 104 are preferably smooth as to provide a good
thermal coupling on a microscopic scale.
[0052] According to an embodiment, guiding wall 104 of guiding
structure 106 is protruding into cavity 112. This may allow for
designing body 102 with a thinner heat spreading section as no
additional heat path length is created, as would be the case if
guiding structure 106 were extending beyond the cavity ceiling into
body 102. In FIGS. 1-3, guiding wall 104 is shown with two
protruding rectangular segments whose width is comparable to the
width of raised member 110. Providing guiding wall 104 with a width
of at least half the width of raised member 110 may prevent a
bottleneck due to insufficient heat conduction in the central heat
path. Preferably, guiding wall 104 is dimensioned between 100 and
2000 .mu.m. Selecting too small of a width for guiding wall 104 may
diminish the thermal transport capability of guiding structure
106.
[0053] According to an embodiment, guiding wall 104 is bendable in
bending direction 122 perpendicular to moving direction 120 of
raised member 110. This may allow for guiding wall 104 to adapt to
the thermal expansion of raised member 110. Furthermore, bendable
guiding wall 104 can be pressed against raised member 110 to ensure
a robust thermal contact. As described below, the external force
necessary to bend guiding wall 104 against raised member 110 may be
generated by thermal expansion of a secondary material present in
cavity 112.
[0054] Another advantage of bendable guiding wall 104 may be that
sticking of guiding structure 106 and raised member 110 may be
avoided when heat sink 100 is removed from the electronic
component. The bendability of guiding wall 104 is preferably of an
elastic nature. This may allow for repeated usage of compliant heat
sink 100 with different electronic components, where heat sink 100
may undergo more than one cycle of installing and uninstalling.
Bendability of guiding wall 104 is dependent on a selection of a
width within a specified range. A maximum width of 2000 .mu.m is
deemed feasible to prevent usage limitations due to unnecessary
rigidity of guiding wall 104.
[0055] FIGS. 1-3 show different scenarios of creating a strong
thermal link between raised member 110 and guiding structure 106 by
means of producing them from materials with mismatched thermal
expansion coefficients. In FIGS. 1 and 2, raised member 110 is made
of a material with a higher thermal expansion coefficient than body
102. During usage, heat will spread into raised member 110, causing
it to expand thermally and exert a horizontal force on guiding
structure 106, thus strengthening the thermal coupling in the
central heat path.
[0056] FIG. 3 depicts a schematic cut through a compliant heat
sink, the guiding structure being part of an insert, in accordance
with an embodiment of the present invention.
[0057] In FIG. 3, however, body 102, flexible element 108, and
raised member 110 are made, as one piece from the same material and
guiding structure 106 is part of insert 300, which is connected to
the sidewalls of cavity 112. For example, insert 300 may be
soldered or welded with body 102, but other connections like a
threaded joint are also possible. In FIG. 3, insert 300 comprising
guiding structure 106 is made of a material with a higher thermal
expansion coefficient than body 102. As heat spreads through heat
sink 100 during usage, the dimensions of insert 300 will grow
relative to the framework of body 102 and also raised member 110.
This may likewise improve the thermal coupling between raised
member 110 and guiding structure 106, but care should be taken that
the thermal expansion of insert 300 does not adversely affect
structure of the body 102. This may, for instance, be achieved by
designing body 102 with horizontal interconnections out of the
image plane, which provide a sufficient rigidity against thermal
expansion of insert 300, or by using a threaded joint with
sufficient tolerance for connecting insert 300 to body 102.
[0058] In general, raised member 110 should be manufactured with a
width between 10% and 40% of the cavity width. As mentioned before
for guiding structure 106, a too narrow raised member 110 might
yield insufficient heat transport capability to provide a necessary
cooling power, which is expected to range up to 1 kW, while too
large a raised member 110 may deteriorate the elasticity of
flexible element 108.
[0059] FIG. 4 depicts a schematic cut through a compliant heat sink
with a tapered guiding wall, in accordance with an embodiment of
the present invention.
[0060] According to an embodiment, the guiding wall 104 comprises a
tapering towards the flexible element 108. FIG. 4 shows a schematic
cut through an exemplary heat sink 100, where raised member 110 is
made of a material with a different coefficient of thermal
expansion than guiding structure 106 and guiding wall 104 comprises
a tapering towards flexible element 108. The tapering direction
towards flexible element 108 means that the thickness of guiding
wall 104 is smallest at the tip, which is facing flexible element
108.
[0061] As a consequence, sections of guiding wall 104 which are in
contact with raised member 110 may be more flexible in bending
direction 122 perpendicular to moving direction 120 of raised
member 110, while the rigidity and the thermal coupling of guiding
structure 106 with the bulk of body 102 increases towards the
ceiling of cavity 112 due to the larger thickness in this region.
Tapered guiding wall 104 may therefore provide an improved
adaptability to thermal expansion of raised member 110 and high
heat conductivity towards body 102 at the same time. If heat sink
100 is implemented with expansion layer 500 as described further
below, the thin part of guiding wall 104 may analogously respond
more flexibly to thermal expansion of expansion layer 500.
[0062] According to an embodiment, heat sink 100 further comprises
expansion layer 500, where expansion layer 500 adjoins at least
part of guiding wall 104 and a surface of body 102 opposite to
guiding wall 104, and where expansion layer 500 further has a
larger coefficient of thermal expansion than body 102.
[0063] FIG. 5 depicts a schematic cut through a compliant heat sink
with an expansion layer being present in the cavity, in accordance
with an embodiment of the present invention.
[0064] FIG. 5 shows the schematic cut of FIG. 4 with the difference
that raised member 110 and flexible element 108 are made of the
same material and that expansion layer 500 is deposited at the
ceiling of cavity 112 in a manner that it fills the space between
tapered guiding wall 104 and sidewalls of cavity 112, while guiding
structure 106 encompassed by guiding wall 104 is free from the
expansion layer material to provide vertical space for guiding
movement of raised member 110 in moving direction 120. Due to a
larger coefficient of thermal expansion compared to body 102,
expansion layer 500 may exert a horizontal force on guiding wall
104, pressing it tighter onto raised member 110. Expansion layer
500 receives the heat causing the expansion primarily from body
102. A gas or vapor atmosphere in the chamber assist this process,
which provides a shorter thermal path between flexible element 108
and expansion layer 500.
[0065] Generating external pressure on guiding wall 104 may reduce
the size of microscopic gaps between raised member 110 and guiding
wall 104 caused by surface unevenness. Hence, expansion layer 500
may improve a poor initial thermal contact caused by surface
roughness of the parts in the central heat path in a cold state of
heat sink 100. As can be seen in FIGS. 5-7, expansion layer 500
preferably covers the cavity ceiling with a thickness, which is
comparable to the length or height of guiding wall 104 or the
different sections. It is formed from a suitable solid material
with a larger coefficient of thermal expansion than body 102,
including metals and metal alloys, but also non-metallic materials
such as epoxy compounds.
[0066] According to an embodiment, at least part of guiding wall
104 is elongated by a groove 600, which immediately adjoins guiding
wall 104 and extends into body 102. Groove 600 is understood here
as a pit extending along guiding wall 104 or at least one of the
segments.
[0067] FIG. 6 depicts a schematic cut through a compliant heat sink
with a groove adjoining the guiding wall, in accordance with an
embodiment of the present invention.
[0068] FIGS. 6 and 7 show a cut through an exemplary heat sink
100,where guiding wall 104 comprises one section with a tapering
towards flexible element 108 and another thin section with a
rectangular cross-section, body 102, guiding structure 106,
flexible element 108 and raised member 110 are formed from the same
material, expansion layer 500 is present at the ceiling of cavity
112, and the thin section of guiding wall 104 is prolonged outside
of the gap receiving raised member 110 by groove 600 immediately
adjoining the thin section of guiding wall 104 and extending into
the ceiling. Such gap may increase the bendability of the thin
guiding wall section by reducing surface strain when exerted to
vertical force generated by expansion layer 500.
[0069] Dividing guiding wall 104 into a thin and a tapered part as
shown in FIGS. 6 and 7 may enable the formation of a tighter
thermal-mechanical contact between raised member 110 and guiding
structure 106 and thus further increase the heat transport
performance of heat sink 100 through the central heat path.
[0070] FIG. 7 depicts a schematic cut through a compliant heat
sink, the expansion layer extending into a slot in the body, in
accordance with an embodiment of the present invention.
[0071] According to an embodiment, expansion layer 500 extends into
slot 700 in body 102 opposite to guiding wall 104. FIG. 7 shows a
cut through a similar heat sink 100 as that shown in FIG. 6, the
difference being that slot 700 is formed along the ceiling into one
of the sidewalls of cavity 112 and expansion layer 500 extends into
slot 700 such that it is completely filled by the expansion layer
material. Expansion layer 500 enlarged this way may provide for a
sufficiently large thermal expansion of expansion layer 500 to form
a tight thermal-mechanical connection between guiding structure 106
and raised member 110 even when the temperature difference between
the at least one electronic component to be cooled and a coolant
outside of heat sink 100 (e.g., ambient air or a heat transfer
liquid) is small.
[0072] As an example, the highest operating temperature is
80.degree. C. for many electronic components and a cooling air
temperature is typically 30.degree. C. In this example, an
operating state with a low temperature difference would feature an
electronic component temperature of, e.g., 50.degree. C. Heat sink
100 providing a high cooling power also in this state could enable
a high-performance operation of the electronic component over a
longer time until it reaches temperature maximum. Moreover, a
prolonged expansion layer 500 may provide support for rapid
temperature changes of the electronic component because the heat
causing thermal expansion of expansion layer 500 reaches the
expansion layer material over a shorter path and through an
increased surface area, and therefore high cooling performance may
be provided also for electronic components which are frequently
subject to strong variations in workload or performance.
[0073] According to an embodiment, guiding structure 106 further
comprises a friction lowering coating interfacing guiding structure
106 and raised member 110. According to an embodiment, guiding
structure 106 further comprising a heat conducting coating
interfacing guiding structure 106 and raised member 110.
[0074] A friction lowering coating may be any liquid, viscous or
solid material (e.g., a grease, oil or powder), which is capable of
compensating the surface roughness of raised member 110 and guiding
structure 106. A heat conducting coating may be any liquid,
viscous, or solid material, which is capable of increasing thermal
conduction between guiding structure 106 and raised member 110 by
providing an interface with a high thermal conductivity, compared
to dry metal-metal contact.
[0075] A friction lowering coating and a heat conducting coating
may be different materials used at the same time. For instance, the
two coatings may be two immiscible gels or a suspension like
thermal grease, which comprises the friction lowering coating and
the heat conducting coating. However, a single coating material 800
may provide the functions of both coatings as well. Such
dual-purpose coating material 800 is preferably selected from the
group of carbon or carbon fiber-based materials, two non-exhaustive
examples being graphite or carbon nanotubes (CNTs).
[0076] Another advantage of using a friction lowering coating may
be the capability to facilitate vertical relaxation of flexible
element 108 and raised member 110 upon relief of heat sink 100 from
the electronic component. For this purpose, a friction lowering
coating with a high thermal stability is preferably used.
[0077] FIG. 8A depicts a coating interfacing the guiding structure
and the raised member in cold state, in accordance with an
embodiment of the present invention.
[0078] FIG. 8A and 8B shows a detail from FIGS. 6 and 7, where the
surface roughness of guiding structure 106 and raised member 110 is
visible. The microscopic gap between raised member 110 and guiding
structure 106 is partially filled with coating material 800
depicted as a black area. Coating material 800 extends into the
irregularities of the two adjoining surfaces. In FIG. 8A, heat sink
100 is in a cold state, e.g., after installation of heat sink 100
on the electronic component, but before the electronic component is
switched on. FIG. 8B depicts a coating interfacing the guiding
structure and the raised member in heated-up state, in accordance
with an embodiment of the present invention.
[0079] In FIG. 8B, heat sink 100 is heated up and expansion layer
500 is extended thermally, pressing guiding wall 104 against raised
member 110. It is seen from FIGS. 8A and 8B that merely parts of
the adjoining surfaces form a metal-to-metal contact with a high
thermal conductivity. By spreading into the microscopic surface
structure of the two interfacing parts, coating 800 provides a
thermal coupling between surface regions, which are not contactable
by purely mechanical means. Coating 800 may therefore increase
thermal conductivity of the central heat path by increasing the
active heat transfer surface of the mechanical contact between
raised member 110 and guiding structure 106. In FIGS. 8A and 8B,
coating 800 is shown as an elastic material which does not
significantly spread along the two adjacent surfaces upon actuation
of guiding wall 104. It is understood however that the coating 800
may be inelastic and move vertically if the gap between raised
member 110 and guiding structure 106 is deformed horizontally.
[0080] The compliant heat sink 100 may advantageously be used to
cool more than one electronic component in parallel. The two
embodiments described in the following are envisioned to provide
this capability. According to an embodiment, cavity 112, flexible
element 108, raised member 110 and guiding structure 106 form a
heat transfer unit which comprises a single one of body 102 and
multiple ones of the heat transfer unit. According to an
embodiment, raised member 110 and guiding structure 106 form a heat
transfer unit which comprises a single one of body 102, a single
one of cavity 112, a single one of the flexible element 108, and
multiple ones of the heat transfer unit.
[0081] Cooling more than one electronic component in parallel may
be accomplished by using multiple heat transfer units. In the first
case, body 102 comprises multiple cavities, wherein each cavity 112
is equipped with a flexible element 108, a raised member 110, and a
guiding structure 106 and is adapted for providing high performance
heat transfer for one of the multiple electronic components. In the
other case, only one cavity 112 covered by a flexible element 108
is present in body 102, but it comprises multiple heat transfer
paths, each comprising a raised member 110 and guiding structure
106.
[0082] Embodiments under the first case may be more advantageous if
the plurality of electronic components spreads over a large
footprint area, while embodiments under the second case may be more
advantageous if the plurality of electronic components has merely
small differences in form factor. Embodiments under both cases may
be advantageous if the amount of available space is insufficient to
use multiple heat sinks 100 comprising merely one heat transfer
unit in parallel.
[0083] The compliant heat sink 100 may form a cool-able electronic
system together with at least one electronic component to be
cooled. According to an embodiment of the cool-able electronics
system, the at least one electronic component comprises a
semiconductor die with a curved surface and the thermal coupling is
a thermal coupling of the die to heat sink 100.
[0084] Advantages of this system shall be demonstrated with the
following example. A large-area semiconductor die has a thermal
envelope which allows for a maximum operational temperature of
85.degree. C. and exhibits a convex curvature to the heat sink 100.
Using a package with a flat lid, a sufficiently strong thermal
coupling can only be achieved in a central area of the bulge. Heat
generated at the boundaries of the semiconductor die spreads
towards the heat sink 100 through a second bond line of thermal
interface material and through the semiconductor itself into a
central region, where higher cooling power is available. In order
to maintain all regions of the die below maximum operational
temperature, the ambient air of the system must be maintained at
30.degree. C. to keep the die's essential region at 80.degree. C.,
allowing for a temperature gradient within the die of 5.degree.
C.
[0085] If the semiconductor die, however, is part of the mentioned
cool-able electronic components, the flexible element 108 may adapt
to the convex surface structure of the die, providing a more
uniform heat transport for all regions of the die. An ambient air
temperature of 35.degree. C. may be used to keep the die
temperature at 80.degree. C. throughout, which may beneficially
reduce the power requirement for air conditioning. Furthermore, a
larger heat tolerance may be created for operating the
semiconductor die, which may substantially reduce the risk of
overheating.
* * * * *