U.S. patent application number 14/045076 was filed with the patent office on 2015-01-01 for method and system for thermomechanically decoupling heatsink.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Vijayeshwar D. Khanna, Gerard McVicker, Sri M. Sri-Jayantha.
Application Number | 20150000097 14/045076 |
Document ID | / |
Family ID | 52114195 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150000097 |
Kind Code |
A1 |
Sri-Jayantha; Sri M. ; et
al. |
January 1, 2015 |
METHOD AND SYSTEM FOR THERMOMECHANICALLY DECOUPLING HEATSINK
Abstract
A method of mounting a heat sink includes providing a heat sink
having a plurality of mounting points, and attaching a plurality of
mounting members to the heat sink at the plurality of mounting
points, respectively, at least one of a combination of a mounting
point of the mounting points and a mounting member of the mounting
members being configured so as to have a stiffness in a
thermally-induced expansion direction of the heat sink at the
respective mounting point which is less than a stiffness in an
other direction at the respective mounting point.
Inventors: |
Sri-Jayantha; Sri M.;
(Ossining, NY) ; McVicker; Gerard; (Stormville,
NY) ; Khanna; Vijayeshwar D.; (Millwood, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
52114195 |
Appl. No.: |
14/045076 |
Filed: |
October 3, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13931098 |
Jun 28, 2013 |
|
|
|
14045076 |
|
|
|
|
Current U.S.
Class: |
29/428 |
Current CPC
Class: |
H01L 2224/73253
20130101; H01L 2924/15311 20130101; H01L 2924/3511 20130101; H05K
7/2039 20130101; Y10T 29/49826 20150115; H01L 2224/16225 20130101;
B23P 15/26 20130101; H01L 2224/32225 20130101; H01L 23/3675
20130101; H01L 2924/0002 20130101; H01L 23/42 20130101; H01L
23/4093 20130101; H01L 23/36 20130101; H01L 23/4338 20130101; H01L
2224/73204 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101; H01L 2224/73204 20130101; H01L 2224/16225 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2924/15311
20130101; H01L 2224/73204 20130101; H01L 2224/16225 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
29/428 |
International
Class: |
B23P 15/26 20060101
B23P015/26 |
Claims
1. A method of mounting a heat sink, the method comprising:
providing a heat sink having a plurality of mounting points; and
attaching a plurality of mounting members to the heat sink at the
plurality of mounting points, respectively, at least one of a
combination of a mounting point of the mounting points and a
mounting member of the mounting members being configured so as to
have a stiffness in a thermally-induced expansion direction of the
heat sink at the respective mounting point which is less than a
stiffness in an other direction at the respective mounting
point.
2. The method according to claim 1, wherein the attaching of the
plurality of mounting members includes attaching at least an other
combination of a mounting point of the mounting points and a
mounting member of the mounting members so as to form a no-slip
connection.
3. The method according to claim 1, wherein the plurality of
mounting members have a soft bending axis, and wherein the
attaching of the plurality of mounting members further includes
orienting the soft bending axis of the plurality of mounting
members such that the soft bending axis is perpendicular to the
thermally-induced expansion direction at the plurality of mounting
points.
Description
[0001] The present Application is a Continuation Application of
U.S. patent application Ser. No. 13/931,098, filed on Jun. 28,
2013, the entirety of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to a system and
method for thermal management of electronic modules and chip scale
packages. More particularly the invention relates to providing heat
sink mounting methods that decouple the circuit board from a
thermomechanically driven deformation, induced by thermal or power
cycles, while maintaining required liner shock robustness.
[0004] 2. Description of the Related Art
[0005] An electronic product such as a computer system is built by
integrating a diverse set of components, each made from different
set of materials. FIG. 1 shows an assembled computer with a
microprocessor unit 1 which is attached to an organic substrate 2
which in turn is attached to a printed circuit board (PCB) 3. Lead
free solder interconnects 9 are used to electrically link the above
components.
[0006] High performance computers are usually built using a ceramic
based substrate with multiple processors and memory modules,
situated adjacent to the processors. Due to high heat dissipated in
the microprocessor unit 1, thermal performance becomes critical to
achieving a reliable computer system. To provide efficient heat
removal various types of heat sinks are used in practice. Most cost
effective heat sinks are air cooled, but for high heat dissipation,
liquid or vapor cooled heat sinks are utilized.
[0007] A heat sink (HS) 4 typically removes the heat generated from
a processor die (e.g., microprocessor unit 1) through a thin
thermal interface material (TIM) 8. A preload is imparted on the
TIM 8, for example with a preload mechanism 19, in order to keep
the thermal conduction thickness to the minimum, typically at about
25 .mu.m. FIG. 1, for example, shows an air cooled system. The
laminate polymer material (e.g., printed circuit board 3) and
metallic heat sinks 4 are known to have different coefficient of
thermal expansion.
[0008] Due to power or thermal cycling, discussed subsequently, a
computer system is subjected to differential expansion and
contraction between individual components. The heat sink 4 and
laminate primarily produce a thermomechanically induced strain in
critical components. Of particular interest is the strain induced
at the edge of the TIM 8 and within solder interconnects 9 that can
lead to tearing or fatigue failure respectively, thus negatively
impacting the reliability of a computer system.
[0009] FIG. 1 corresponds to a microprocessor unit 1 that is made
of a silicon die with a substantial foot print (25.times.25 mm) In
this case the heat sink 4 also has to have a substantial foot print
in the X-Y plane. Because of the wider foot print and relatively
large mass, the heat sink 4, in this case, is mounted rigidly on
four solid posts 5, one on each corner. It is noted that in order
to illustrate additional elements only one of the four solid posts
5 is illustrated in FIG. 1, but the solid posts (mounting posts)
are in fact located in each of the four corners (e.g., as
illustrated in FIG. 5 which will be discussed subsequently).
[0010] In another application, several chip scale packages (CSPs)
201, providing auxiliary functions such as regulating voltage, may
be arranged in-line as shown in FIG. 2. In this case, the thermal
management is achieved by a single heat sink 204 covering multiple
devices. In the in-line layout, the TIM 208 can extend over the CSP
201. Since the foot print of a CSP 201 in this case is relatively
small (6.times.4 mm), the heat sink 204 tends to be narrow and
long, primarily extending along one axis. Typically the heat sink
204 in the in-line mounting is held by two riveted joints 205
placed at the ends. Observe that a solid mounting post or a riveted
joint 205 does not allow substantial relative motion between the
heat sink 201 and the PCB 203. This condition will be referred to
as "no-slip" joint or boundary condition.
[0011] Based on the two real life electronic packages that have
been described, a heat sink mounting can be either in-plane (X-Y)
(e.g., see FIGS. 1 and 5) or in-line (along X or Y) (e.g., see FIG.
2).
[0012] Computer systems and data centers are turned on and off
regularly which gives rise to thermal cycles. Also, while under
power-on-state, an intermittent work load could drive a computer in
to and out of active mode from idle mode, generating a highly
transient temperature condition. In both cases, differential
thermal expansion of components is driven by the thermal and power
cycles, causing cyclic strain within critical components. In order
to guarantee reliability of parts that are used to build a computer
system, rigorous accelerated tests to accentuate field conditions
are applied at the development stage by power and thermal cycling
the system, as shown in FIG. 3. The ramp rate and dwell times
within, for example a 30 minute cycle, are important parameters in
determining the reliability of a package.
[0013] Once the thermal goals are met by suitable choice of a heat
sink, a computer system must be designed to withstand linear
shipping shock. Hence, the challenge is to support a heat sink,
typically 2 to 3 kg in mass for a high end module, arranged on top
of a microprocessor unit (also referred to as a die) without
substantially straining the TIM.
[0014] FIG. 4 shows a schematic side view of a heat sink 404
supporting a microprocessor 401 and a memory module 412. Any
relative motion between the heat sink 404 and the die (e.g.,
microprocessor 401 or memory module 412) could strain the TIM
material 408, and excess strain could lead to tearing of the TIM
408. In addition to linear shock, the products can also be
subjected to inadvertent rotational shock. Hence, the heat sink
mounting must provide robustness against shock induced damage of
TIM 408 or any other vulnerable component.
[0015] FIG. 4 also shows a shock pulse in X-direction applied to
the PCB 403. To minimize shear strain in TIM 408 the heat sink 404
must be rigidly mounted using high stiffness posts 405 at four
corners of the HS 404 so that relative motion between PCB 403 and
HS 404 is minimized. FIG. 4 also includes a ceramic substrate 402
and a stiffener plate 414.
[0016] FIG. 5 shows a plan view and the location of the four
mounting posts 405 that support HS 404. A solid post tends to have
a large stiffness against bending, and it certainly prevents large
relative motion during a shock which is a positive attribute.
However, during power on-off cycle or during thermal cycling, the
heat sink 404 made of either aluminum or copper expands at a
different rate than the PCB 403. It is not only due to difference
in coefficient of thermal expansion (CTE) between HS 404 and PCB
403, but also due to difference in thermal conductivity and heat
capacity that determine the time constant of transient heat
spreading. During the power ramp up the HS 404 pushes the four
mounting posts 405 along the expansion vector (usually along the
diagonal) and forces the printed circuit board assembly to bend
through the lever arm provided by the mounting posts 405.
[0017] FIG. 6 shows an estimated thermal map in a computer assembly
corresponding to FIG. 4 ten seconds after maximum power was
released. When power is applied to the microprocessor and memory,
the temperature rises non-uniformly across various components as
illustrated by FIG. 6.
[0018] FIG. 7 shows an estimated thermal map for the corresponding
deformed state of the system due to thermo-mechanical interaction
between the heat sink and the adjoining structure. The deformation
creates distortion of TIM gap which can lead to TIM failure.
Similarly, the solder joints (not illustrated) are also subjected
to cyclic strain.
[0019] Heat sinks are widely used by the industry and various
methods of supporting them have been disclosed in the prior art.
One conventional device suggests plastically formed supports with
locking features so that heat sink could be easily removed to
rework a computer in the field. Another conventional device
envisages the use of flexible interposing elements between the
solder joints and solder pads that would reduce the strain in the
solder due to thermally induced deformation. Another conventional
device describes a method to reduce stress between the die surface
and heat sink surface by means of cantilever means embedded within
a TIM material. The cantilever beams do not support the weight of a
heat sink. Another conventional device introduces compliant support
of a heat sink in order to manage the preload applied to the TIM
material. However, in the prior art, no consideration is given to
the thermomechanical coupling effects or methods to mitigate it.
The majority of the prior art covering heat sink mounting focuses
on preload control and ease of removability.
SUMMARY
[0020] In view of the foregoing, and other, exemplary problems,
drawbacks, and disadvantages of the conventional systems, it is an
exemplary feature of the present invention to provide a device and
method that allows for a reliable operation of a heat sink.
[0021] An exemplary aspect of the invention may include a heat sink
including a body and mounting points configured so as to connect to
a mounting medium. At least one of the mounting points is
configured to allow movement in a thermally-induced expansion
direction.
[0022] Another exemplary aspect of the invention is embodied as a
method of mounting a heat sink. The method including providing a
heat sink having a plurality of mounting points, and attaching a
plurality of mounting members to the heat sink at the plurality of
mounting points, respectively, at least one of a combination of the
mounting points and the mounting members being configured so as to
have a stiffness in a thermally-induced expansion direction of the
heat sink at the respective mounting point which is less than a
stiffness in an other direction at the respective mounting
point.
[0023] Another exemplary aspect of the invention may include a heat
sink to be mounted on a printed circuit board. The heat sink
including a body, and flexural posts attached to the body so as to
decouple thermomechanically induced expansion.
[0024] The above aspects may provide a thermally decoupled heat
sink
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The foregoing and other purposes, aspects and advantages
will be better understood from the following detailed description
of exemplary non-limiting embodiments of the invention with
reference to the drawings, in which:
[0026] FIG. 1 illustrates a related art heat sink mounting
arrangement on an organic module carrying a microprocessor
unit;
[0027] FIG. 2 illustrates a related art heat sink mount for
multiple surface mounted modules;
[0028] FIGS. 3a and 3b show power cycle and thermal cycle profiles
and the corresponding impact of deformation on TIM and solder
joints;
[0029] FIG. 4 illustrates an example of in-plane heat sink assembly
set on top of a microprocessor and memory module using rigid
mounting posts;
[0030] FIG. 5 illustrates a plan view of the in-plane heat sink
(shown transparent) supported by four corner mounting posts having
a circular cross section;
[0031] FIG. 6 illustrates an estimate of the temperature
distribution after 10 seconds following a power on condition;
[0032] FIG. 7 shows the thermomechanically driven deformation
(exaggerated scale) of the system;
[0033] FIGS. 8a, 8b and 8c schematically show the coupling
mechanism as the temperature is increased or decreased from the
stress free room temperature state;
[0034] FIGS. 9a, 9b and 9c illustrate a modified mounting structure
where a flexure element is added to a rigid mount. The soft
"bending" axis is aligned with Y-axis, and the design is sensitive
to X-directional shock applied to PCB;
[0035] FIG. 10 illustrates an orientation of the soft bending axis
of an exemplary embodiment for maximum flexibility along the
thermal expansion vector while providing stiffness against X-Y
shock;
[0036] FIGS. 11a and 11b illustrate a cascaded heat sink with three
mounting locations. To decouple, one point is made of no-slip mount
and the remaining mounts are made with slip mounts;
[0037] FIG. 12 illustrates various exemplary design options for
mounting posts that could replace a standard mounting post to
provide thermomechanical decoupling;
[0038] FIG. 13 shows a simplified graph of cumulative strain energy
vs. thermal cycles with and without heat sink decoupling;
[0039] FIGS. 14a and 14b show an in-line heat sink providing
thermal management for two surface mounted modules. The mismatch in
coefficient of thermal expansion could drive cyclic strain of
solder joints;
[0040] FIG. 15 illustrates a slotted in-line heat sink that
provides compliance in X-direction;
[0041] FIGS. 16a and 16b illustrate the details of a slip-enabling
heat sink mount. The second end of the heat sink (not shown) is
secured by a no-slip design;
[0042] FIGS. 17a and 17b illustrate another slip-enabled mount
using split-post. The split-post provides compliance; and
[0043] FIGS. 18a, 18b and 18c illustrate a close up view of the
split-post. The split-post is attached to PCB similar to a surface
mounted component.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0044] The present invention addresses the limitations encountered
in the conventional method of mounting a heat sink to a PCB. The
conventional mounting posts essentially impose a "no-slip" boundary
condition which strongly couples a heat sink to a printed circuit
board detrimentally. Thermally driven expansion and contraction of
key components are coupled and the ensuing deformation strains
critical components unfavorably. The present invention provides
several design methods that can constructively decouple a heat sink
from PCB without compromising the shock robustness of the computer
system.
[0045] For example, in case of an in-plane heat sink, four rigid
mounting posts can be replaced by four flexural posts. The flexural
posts are oriented to present least resistance to thermal expansion
vector at mounting point while providing high stiffness against
linear shock along X or Y axis and rotational shock about Z-axis.
Optionally, to damp out any dynamic oscillatory movement between
heat sink and PCB, a damping material can be sandwiched between the
flexural elements.
[0046] In the case of an in-line heat sink, two mounting post or
rivets may be replaced by one mounting post and a flexural joint or
by one rivet and a slip joint.
[0047] In case of cascaded in-line heat sink, one joint can be made
of a mounting post and all others are replaced by flexured joints.
Alternatively, one joint is made of a rivet and all others by
slip-enabled joints. In general, one fixed boundary condition and
several slip boundary conditions are utilized.
[0048] In case of an in-line heat sink, a rigid heat sink may be
asymmetrically slotted to increase compliance along the X-axis
while conventional mounting posts or rivets are used on both
ends.
[0049] It is, therefore, an exemplary feature of the present
invention to provide a structure and method for a
thermomechanically decoupled heat sink.
[0050] Referring now to the drawings, and more particularly to
FIGS. 8a-18, there are shown exemplary, non-limiting, embodiments
of the method and structures according to the present
invention.
[0051] FIGS. 8a, 8b and 8c show the deformation process due to
heating and cooling of a heat sink of an in-plane system. FIG. 8a
shows a schematic representation of an assembled system represented
by FIG. 4 in which there is no substantial residual stress
imparted. FIG. 8b shows the power on heating cycle in which the
heat sink 404 temperature rises rapidly ahead of the PCB 403, and
forces the mounting posts 405 to move outwards. The bending moment
thus created causes the PCB 403 and associated laminate and die
into a convex shape. The power on process would cause a tensile
strain on the TIM. Once the dwell period is reached, after an
elapsed period of time, the expansion process would reach a steady
state and no further deformation of TIM is anticipated.
[0052] However, during the cool down or power down phase the heat
sink 404 would contract much more quickly compared to PCB 403 and a
reverse deformation occurs as shown by FIG. 8c. During the cooling
phase, for example, below the stress free assembly temperature, the
PCB 403, laminate and die could become concave and cause tearing of
TIM at the center rather than at the edge of a die.
[0053] An exemplary embodiment of the invention provides a design
that can decouple the heat sink expansion effect from the PCB
structure by utilizing flexured mounting posts.
[0054] FIGS. 9a, 9b and 9c illustrate a basic concept applied to an
in-plane heat sink 904. The flexured posts 905, for simplicity, are
oriented in such a way that the easy or soft bending axis is along
the Y-axis facilitating reduced compliance for X-directional
motion. The flexure section 920 is illustrated in FIGS. 9a, 9b and
9c. Thus, any relative expansion of the heat sink 904 along
X-direction will not force the PCB 903 to either bend or
stretch.
[0055] In addition, the Y-directional expansion of the heat sink
904 would still couple the PCB 903 because the bending stiffness of
the post 905 for Y-movement is still high. However, any shock
applied to PCB 903 in the X-direction will translate to substantial
relative motion between the heat sink 904 and PCB 903, causing TIM
908 to tear. Thus, additional design innovation is required to
mitigate these challenges.
[0056] In order to simultaneously reduce the bending stiffness to
thermal expansion while presenting substantial stiffness against
X-Y shock, the invention takes advantage of the nature of expansion
process. As the temperature rises, the heat sink 1004 mounting
points (e.g., A, B, C and D) displace along the diagonal vectors as
shown in FIG. 10. Therefore, the bending resistance needs to be
reduced only along the diagonal directions at the mounting points
(e.g., at A, B, C and D).
[0057] By orienting the easy bending axis S orthogonal to the
respective diagonals the thermally driven expansion process is
decoupled from the PCB. In an exemplary embodiment, the soft (easy)
bending axis S is orthogonal to the direction of expansion (e.g.,
Ea) at the mounting point (e.g., A). This technique can be done
with any number of mounting posts 1005 on any shape heat sink 1004.
In an exemplary embodiment the mounting posts 1005 support the heat
sink 1004 in both the positive Z (up) direction and the negative Z
(down) direction.
[0058] Since the specification of shock direction is either along
the X or Y axis, the four flexural mounting posts can be designed
to offer the required stiffness against shock induced motion. A HS
mounting post with a circular cross-section has a bending stiffness
that is proportional to its moment of area:
I.sub.circle=(1/4)*pi*R.sup.4
where R is the radius of a circular post. Observe that the
stiffness of this system is identical in all directions, and is
independent of the mounting orientation. Thus, with a mounting post
having a circular cross section it is impossible to develop a
decoupling design while ensuring robustness against shock.
[0059] However, if a flexured section 1020 as shown in FIG. 10 is
introduced into the mounting post 1005 two distinct stiffness
components are obtained. For a flexural section 1020 with a
rectangular geometry (b.times.t with b>t), as shown in FIG. 10,
the soft axis S and hard axis H (call S-H as the principal axes)
will have following moment of area:
I.sub.soft=(bt.sup.3)/12
I.sub.hard=(b.sup.3t)/12
[0060] Thus by maintaining b>>t (for example say by a factor
of 10) a stiffness ratio of 100 between soft (easy) axis S and hard
(stiff) axis H can be obtained.
[0061] The orientation of flexures 1020 shown in FIG. 10 also
enhances shock robustness. When the principal axis H of the
hard-soft flexure 1020 is rotated with respect to the global X-Y
axes by an angle "a" , then the effective stiffness along global X
and Y axes are now given by the following equations:
I.sub.x=(bt/12)*(b.sup.2 Cos.sup.2a+t.sup.2 Sin.sup.2a)
I.sub.y=(bt/12)*(b.sup.2 Sin.sup.2a +t.sup.2 Cos.sup.2a)
[0062] For simplicity, for a system with a square heat sink, angle
"a" will be 45 degrees, and the corresponding stiffness will
be:
I.sub.x=I.sub.y=(b.sup.3t/24)
Thus, the stiffness against shock vs. thermomechanical coupling can
be differentiated by a factor given by:
I.sub.soft/I.sub.x=2*(t/b).sup.2
For a case where (t/b)=(1/10), the stiffness ratio can be as much
as 1:50.
[0063] Thus, it is possible to present two drastically different
stiffnesses to the thermomechanical system and shock-response
system using soft flexural posts 1020 whose orientations are made
coincidental with the respective thermal expansion vector (e.g.,
Ea) of the mounting point (e.g., A).
[0064] In an exemplary embodiment of the invention an in-plane heat
sink can be mounted on a flexured mounting post. In an exemplary
embodiment one or more components (e.g., microprocessors, memory,
etc.) can be used.
[0065] FIG. 12 shows an array of mounting post designs. Compared to
a standard post 1251 with circular cross section, the same post can
be thinned at the center zone to provide flexure action, which is
referred to as a split post 1252. In another design, a flexure 1261
made of appropriate material with higher fatigue life can be
embedded between two sections of a conventional circular post 1253.
Finally, two parallel flexures 1262 with damping material 1271
sandwiched in between circular posts 1254, as shown in FIG. 12, can
be considered.
[0066] FIG. 13 graphically illustrates the relationship between
cumulative strain energy and number of strain cycles applied to a
material. In one design, where the heat sink is coupled to the PCB,
the expected life time is not met. By decoupling the heat sink, the
strain energy dissipated per cycle is reduced and the desired life
time is met.
[0067] In FIG. 2, a representative 4-module in-line assembly on an
organic printed circuit board 203 with a heat sink (HS) 204 was
described. During a thermal cycle, the strain within the solder 209
of a module is generated as a result of external and internal
thermal expansion/contract process. The strain generation mechanism
for in-line as well as in-plane system is very similar, but TIM 208
failure in the in-line case is less severe because conductive tape
instead of a cured thermal interface material can be inserted at
the interfaces, and tearing is therefore not encountered. However,
solder joint 209 fatigue failure can be a problem in the in-line
heat sink systems.
[0068] FIGS. 14a and 14b show two stages of an in-line CSP 1401
structure. Prior to the HS 1404 attachment at room temperature, the
solder is assumed to be stress free. Once the HS 1404 is loaded and
riveted at its ends (in order to retain the preload on the
thermally conductive tape) substantial stress in the solder joint
is generated. The stress due to preload is not strictly cyclic, and
does not contribute to fatigue life of the material. However, once
the assembly is completed, thermal cycling produces cyclic stress
similar to that of in-plane heat sink system.
[0069] Use of rivets 1405 reduces the cost of assembly. The rivets
1405, however, produce a nearly slip free joint which undesirably
couples the heat sink 1404 to the PCB 1403. Observe that the
mounting posts discussed under in-plane design can be
interchangeably used for the in-line design. Only two mounting
posts 1405 are required for the in-line assembly.
[0070] A finite element model of an in-line heat sink with four
surface mount modules was built to analyze and compare the effect
heat sink mounting with and without slip boundary condition. It was
estimated that a slip-enabled mounting could reduce the strain
energy density in solder joints due to thermal cycling by as much
as 25%.
[0071] FIG. 16a shows a schematic of a HS 1604 assembly with a slip
joint used on a module 1601. The second end (not shown) is either
riveted or mounted on a stiff post. The invention provides solution
to the following two problems simultaneously. Clearance between the
mounting post 1605 and HS 1604 allows the heat sink to move along
X-direction without substantial resistance. Thus, the differential
expansion between the organic board 1603 and the HS 1604 is
decoupled.
[0072] This can be accomplished by providing an elongated hole 1618
which can optionally be filled with a compliant material 1619. Note
that the second mounting point with a no-slip design provides the
required shock robustness along the X-direction. The clearance
along the Y-direction is minimized so that Y-directional linear
shock induced force is transferred to the heat sink as a balanced
force through both mounting points. In an exemplary embodiment, the
elongated hole 1618 allows relative movement of the heat sink in
the X-direction but not in the Y-direction.
[0073] The second feature is that the preloaded spring 1617 in the
Z-direction presents a constant force on the thermal interface
material 1608. Preloading through spring action is commonly used in
the industry, but the shock robustness is largely overlooked by a
loosely tolerant spring loaded mounting system.
[0074] In an exemplary aspect, for shock robustness of an in-line
system, the shock vector should pass through the center of gravity
of the heat sink and the mounting point with one fixed boundary
condition. Otherwise there can be a torque that will force relative
movement between the heat sink and the PCB. In the event that there
is a slip boundary condition along the X-axis and a fixed boundary
condition along the Y axis (e.g., see FIG. 16b) using a design, for
example as denoted by elongated hole 1618 the shock vector along
the Y-axis is balanced by forces generated by mounting posts on
both sides. In this case, the shock vector is not passing through
the center of gravity of the fixed boundary condition. When using
flexures the same general principle may apply. The soft direction
of displacement should be along the X-axis while the hard or stiff
direction should be along the Y-axis.
[0075] FIG. 17a shows a snap-on method. This solution facilitates
easy removal of HS 1704 for re-workability. An exemplary aspect of
this configuration is that the split-post 1705 is supported by a
platform 1717 that is surface mounted to the organic board 1703
like any other electronic component. It distributes local stress on
the PCB 1703 that may result from other forms of mechanical
mounting operation. FIG. 17b is a schematic representation of a
snap-on fixture 1705 subsequently used in other illustrations.
[0076] Also illustrated is a spring or wave washer 1716 to impart a
preload. Even though the two mounting posts (1705) appear
identical, in order to survive shock, one post should be stiffer
than the other to bear shock induced force while the flexible
second post accommodates the thermally-induced expansion. The
flexible post can be designed to have higher flexibility in
X-direction (for expansion) and higher stiffness in Y-direction to
bear part of the Y-direction shock and rotational shock load about
Z-axis.
[0077] FIG. 18 shows more details of the snap-on method. By
adjusting the relative stiffness of the snap post, for example by
adjusting the thickness or width of each leg 1720, it is possible
to have more or less stiffness in any desired direction. In
addition, the four posts 1720 can be rotated or manufactured in
different directions (e.g., coincidental with the X and Y
direction, or 45 degrees from the X and Y direction) when
[0078] FIGS. 11a and 11b show a cascade of in-line heat sinks to
support a larger number of surface mounted modules. The examples
shown support eight surface mounted modules. As illustrated in FIG.
11b, the center mounting point is retained with a no-slip joint,
and the remaining mounting points are slip-enabled. In the
illustrated embodiment of FIG. 11b, there would be clearance in the
X-direction on both end through holes 1118.
[0079] FIG. 15 shows a modified in-line heat sink 1504 with
multiple slots 1523 cut near one mounting location. Slots 1523 are
cut in the Y-direction and they provide compliance in the
X-direction without impeding the thermal performance of the heat
sink 1504. By providing compliance in the X-direction, the
thermo-mechanical coupling is minimized while allowing conventional
mounting methods, such as low cost rivets 1521, to be employed.
[0080] While the invention has been described in terms of exemplary
embodiments, those skilled in the art will recognize that the
invention can be practiced with modification within the spirit and
scope of the appended claims.
[0081] Further, it is noted that Applicant's intent is to encompass
equivalents of all claim elements, even if amended later during
prosecution.
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