U.S. patent application number 09/891627 was filed with the patent office on 2002-12-26 for thermal bond verification.
Invention is credited to Schonath, Peter, Weller, Steven A..
Application Number | 20020196835 09/891627 |
Document ID | / |
Family ID | 25398552 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020196835 |
Kind Code |
A1 |
Schonath, Peter ; et
al. |
December 26, 2002 |
THERMAL BOND VERIFICATION
Abstract
A system and method for evaluating the thermal bond between a
heat-producing device and a heat-absorbing apparatus. The
heat-producing device may be a CPU, such as an INTEL PENTIUM
microprocessor, and the heat-absorbing apparatus may be a heat
sink. The two may be joined with a heat-conducting substance such
as thermal grease or adhesive. In one exemplary embodiment, the
heat-producing device is operated at a first power level, a first
temperature measurement is then taken, the device is operated at a
second power level, and then a second temperature measurement is
then taken. The thermal resistance is then calculated, which may
involve subtracting the second temperature from the first, and may
involve dividing by the power level. The first power level may be
full power, and the second power level may be near zero. The first
temperature may be measured when equilibrium temperatures have been
reached, and the second temperature may be measured a predetermined
amount of time after the second power level is initiated, which may
be just enough time for the temperatures of the CPU and the heat
sink to equalize. The CPU may perform the calculations, and the
temperature may be measured with an on-board thermal sensor which
may be a thermal diode.
Inventors: |
Schonath, Peter; (Augsburg,
DE) ; Weller, Steven A.; (Milpatas, CA) |
Correspondence
Address: |
SNELL & WILMER
ONE ARIZONA CENTER
400 EAST VAN BUREN
PHOENIX
AZ
850040001
|
Family ID: |
25398552 |
Appl. No.: |
09/891627 |
Filed: |
June 25, 2001 |
Current U.S.
Class: |
374/45 ; 374/43;
73/150A |
Current CPC
Class: |
G01N 25/72 20130101;
G01N 25/18 20130101 |
Class at
Publication: |
374/45 ; 374/43;
73/150.00A |
International
Class: |
G01N 025/20; G01N
025/00 |
Claims
What is claimed is:
1. A system for evaluating a thermal bond between a heat-producing
device and a heat-absorbing apparatus, said system comprising: a
heat-producing device; a heat-absorbing apparatus; a thermal bond
between said device and said apparatus, said thermal bond having a
thermal resistance; a thermal sensor configured to measure the
temperature at or near said device; and a circuit configured to;
operate said device substantially at a first power level; measure a
first temperature with said thermal sensor; operate said device
substantially at a second power level; measure a second temperature
with said thermal sensor; and calculate the thermal resistance
using the first temperature and the second temperature.
2. The system according to claim 1, said circuit being configured
to calculate the thermal resistance by subtracting the second
temperature from the first temperature.
3. The system according to claim 1, said circuit being configured
to calculate the thermal resistance by subtracting the second
temperature from the first temperature and dividing the difference
by the first power level.
4. The system according to claim 1, said thermal bond comprising a
heat-conducting substance said heat-conducting substance being
selected from the group consisting of thermal grease, thermal
paste, thermal wax, glue, adhesive, and solder.
5. The system according to claim 1, said apparatus consisting of a
heat sink.
6. The system according to claim 1, said circuit being configured
so that the second temperature is measured a predetermined time
after the second power level is initiated.
7. The system according to claim 1: said circuit being configured
to calculate the thermal resistance by subtracting the second
temperature from the first temperature and dividing the difference
by a power level; further comprising a heat-conducting substance
thermally bonding said device to said apparatus, the
heat-conducting substance being selected from the group consisting
of thermal grease, thermal paste, thermal wax, and adhesive; said
apparatus consisting of a heat sink; and said circuit being
configured so that the second temperature is measured a
predetermined time after the second power level is initiated.
8. The system according to claim 1; said circuit comprising a CPU;
and said heat-producing device comprising said CPU.
9. The system according to claim 8, said CPU comprising said
thermal sensor.
10. The system according to claim 8, said CPU being configured to
calculate the thermal resistance by subtracting the second
temperature from the first temperature.
11. The system according to claim 8, said CPU being configured to
calculate the thermal resistance by subtracting the second
temperature from the first temperature and dividing the difference
by the first power level.
12. The system according to claim 8, said thermal bond comprising a
heat-conducting substance, said heat-conducting substance being
selected from the group consisting of thermal grease, thermal
paste, thermal wax, glue, adhesive, and solder.
13. The system according to claim 8, said apparatus consisting of a
heat sink.
14. The system according to claim 8, said CPU being configured so
that the second temperature is measured a predetermined time after
the second power level is initiated.
15. The system according to claim 8: said CPU comprising said
thermal sensor; said CPU being configured to calculate the thermal
resistance by subtracting the second temperature from the first
temperature and dividing the difference by a power level; further
comprising a heat-conducting substance thermally bonding said
device to said apparatus, said heat-conducting substance being
selected from the group consisting of thermal grease, thermal
paste, thermal wax, and adhesive; said apparatus consisting of a
heat sink; and said CPU being configured so that the second
temperature is measured a predetermined time after the second power
level is initiated.
16. A method of evaluating a thermal bond between a heat-producing
device and a heat-absorbing apparatus, said method comprising,
initiated in the following order, the steps of: operating the
device substantially at a first power level; measuring a first
temperature of or near the device; operating the device
substantially at a second power level; measuring a second
temperature of or near the device; and calculating the thermal
resistance of the thermal bond between the device and the
apparatus, said calculating comprising using the first temperature
and the second temperature.
17. The method according to claim 16, the step of operating the
device at a first power level comprising operating the device at a
substantially constant power level until the device and the
apparatus substantially reach equilibrium temperatures.
18. The method according to claim 16, said step of measuring the
second temperature occurring at least 10 seconds after the
initialization of said step of operating the device at the second
power level.
19. The method according to claim 16 further comprising the step of
accepting the thermal bond if the thermal resistance is below a
threshold.
20. The method according to claim 16, said device being a an
integrated circuit chip.
21. The method according to claim 16, said calculating being
performed by the device.
22. The method according to claim 16, said calculating comprising
dividing by the power consumption of the device.
23. The method according to claim 16, the thermal bond comprising a
material selected from the group consisting of thermal grease,
thermal paste, thermal wax, and adhesive.
24. The method according to claim 16, the device comprising a flip
chip.
25. The product made according to the method of claim 16.
26. The method according to claim 16: the second power level being
less than the first power level; and said calculating comprising
subtracting the second temperature from the first temperature.
27. The method according to claim 26, said step of measuring the
second temperature occurring at least 10 seconds after the
initialization of said step of operating the device at the second
power level.
28. The method according to claim 26 further comprising the step of
accepting the thermal bond if the thermal resistance is below a
threshold.
29. The method according to claim 26, the device being a an
integrated circuit chip.
30. A method of evaluating a thermal bond between a heat-producing
device and a heat sink, said method comprising the steps of:
operating the device substantially at a first power level at least
until the device and the heat sink substantially reach equilibrium
temperature; measuring a first temperature of or near the device,
substantially at the equilibrium temperature; operating the device
substantially at a second power level, the second power level being
less than the first power level; after a period of time at the
second power level, measuring a second temperature of or near the
device; calculating the thermal resistance of the thermal bond
between the device and the heat sink, said calculating comprising
subtracting the second temperature from the first temperature; and
accepting the thermal bond if the thermal resistance is below a
threshold.
31. The method according to claim 30, the device comprising a
CPU.
32. The method according to claim 30, said measuring the first or
second temperatures comprising using a thermal diode.
33. The method according to claim 32 the thermal diode being
integral with the device.
34. The method according to claim 30, the period of time being a
predetermined amount of time between the beginning of said
operating the device at the second power level and said measuring a
second temperature.
35. The method according to claim 30, further comprising the step
of remounting the heat sink on the device if the thermal resistance
is above the threshold.
36. The method according to claim 30, said step of remounting
comprising the steps of: separating the heat sink and the device;
cleaning the heat sink; cleaning the device; applying or reapplying
a heat-conducting substance; and reattaching the heat sink and the
device.
37. The method according to claim 30, said calculating being
performed by the device.
38. The method according to claim 30, said calculating comprising
dividing by the power consumption of the device.
39. The method according to claim 30, the thermal bond comprising a
material selected from the group consisting of thermal grease,
thermal paste, thermal wax, and adhesive.
40. The method according to claim 30, the second power level being
less than 10% of the first power level.
41. The method according to claim 30, the device comprising a flip
chip.
42. The method according to claim 30 further comprising the step of
measuring a third temperature at or near the device, the third
temperature being: measured after said step of measuring the second
temperature; and measured after a second period of time while the
device is at the second power level.
43. The product made according to the method of claim 30.
44. The method according to claim 30: said measuring comprising
using a thermal sensor; the thermal sensor being integral with the
device; the period of time being a predetermined amount of time
between the beginning of said operating the device at the second
power level and said measuring a second temperature; the period of
time being greater than 2 seconds; the period of time being less
than 100 seconds; and the thermal bond comprising a material
selected from the group consisting of thermal grease, thermal
paste, thermal wax, and adhesive.
45. A method of evaluating the thermal bond between an integrated
circuit device and a heat sink, said method comprising the steps
of: operating the device substantially at a first power level until
the device and the heat sink substantially reach equilibrium
temperature, the device comprising an integrated circuit; measuring
a first temperature substantially of the device; operating the
device substantially at a second power level, the second power
level being less than the first power level; after a period of time
at the second power level, measuring a second temperature
substantially of the device; calculating the thermal resistance of
the thermal bond between the device and the heat sink, said
calculating comprising subtracting said second temperature from
said first temperature; and accepting the thermal bond if the
thermal resistance is below a threshold.
46. The method according to claim 45, the device comprising a
CPU.
47. The method according to claim 46, said measuring comprising
using a thermal diode.
48. The method according to claim 47 the thermal diode being
integral with the device.
49. The method according to claim 46: said measuring comprising
using a thermal sensor; the thermal sensor being integral with the
device; the period of time being a predetermined amount of time
between the beginning of said operating the device at the second
power level and said measuring a second temperature, the period of
time being greater than 2 seconds; the period of time being less
than 100 seconds; said calculating being performed by the device;
and the thermal bond comprising a material selected from the group
consisting of thermal grease, thermal paste, thermal wax, and
adhesive.
50. The method according to claim 46 further comprising the step of
measuring a third temperature substantially of the device, the
third temperature being: measured after said step of measuring the
second temperature; and measured while the device is being operated
at the second power level.
51. The method according to claim 45, the period of time being a
predetermined amount of time between the beginning of said
operating the device at the second power level and said measuring a
second temperature.
52. The method according to claim 45, further comprising the step
of remounting the heat sink on the device if the thermal resistance
is above the threshold.
53. The method according to claim 52, said step of remounting
comprising the steps of: separating the heat sink and the device;
cleaning the heat sink; cleaning the device; applying or reapplying
a heat-conducting substance; and reattaching the heat sink and the
device.
54. The method according to claim 45, said calculating being
performed by the device.
55. The method according to claim 54, said calculating comprising
dividing by the power consumption of the device.
56. The method according to claim 45, the thermal bond comprising a
material selected from the group consisting of thermal grease,
thermal paste, thermal wax, and adhesive.
57. The method according to claim 45, the second power level being
less than 10% of the first power level.
58. The method according to claim 45, the device comprising a flip
chip.
59. The product made according to the method of claim 45.
60. The method according to claim 45 further comprising the step of
classifying for lower power level applications devices having
thermal bonds above the threshold.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to systems and methods for
evaluating thermal bonds.
[0003] 2. Description of the Related Art
[0004] Many mechanical and electrical devices generate heat
internally which must be dissipated to the environment to keep the
devices within a range of desired operating temperatures. Such
heat-producing devices may include engines, bearings, motors, power
supplies, transformers, amplifiers, control modules, and computer
chips and components including graphics controllers, network
interfaces, and central processing units (CPUs). In some
applications it is desirable to transfer heat from a heat-producing
device to a heat-absorbing apparatus so that the heat may be used
for useful purposes. These useful purposes may include generating
electricity, providing heat for industrial processes, heating
water, heating air (e.g. to heat the occupied space in buildings),
or preventing freezing. Heat-absorbing apparatuses may comprise
fluids which move, change phase, or both, to transfer heat, and may
involve or approximate thermodynamic cycles such as a Rankine,
Carnot, or Brayton cycle. On the other hand, heat-absorbing
apparatuses may be simple solid devices such as heat sinks
(heatsinks), for instance, typically without macroscopic moving
parts.
[0005] A heat sink is one type of heat-absorbing apparatus. As an
example, heat sinks have been used to cool CPUs in computers
including general-purpose desk-top PCs. A heat sink is a device,
typically monolithic, that conducts and usually dissipates heat.
Heat sinks may be made of a metal such as aluminum or copper, or in
some applications may be made of other materials such as ceramic or
plastic. The heat typically travels from the heat-generating device
to the heat sink primarily through conduction, and then travels
through the heat sink, typically also via conduction. Heat sinks
may have a large surface area to dissipate heat (e.g. large
relative to the surface area of the heat-producing device),
generally through convection, e.g. to surrounding air. The high
surface area may be accomplished with fins, holes, hills and
valleys, or other geometric features. The air may be blown with a
fan to increase the Nusselt number and improve cooling, or the
system may rely on natural convection. Some heat may also be
transferred through radiation, particularly in high-temperature
applications, and heat sinks may be configured with a surface
having a high emissivity to facilitate radiant heat transfer. For
instance, heat sinks may have black coatings.
[0006] Heat sinks may comprise multiple parts, e.g. multiple fins
attached to the heat-producing device. In addition, e.g. in
applications where heat is produced transiently, heat sinks may not
have fins or other features to dissipate heat, but may rely on
their bulk to absorb and store the heat produced, which may then be
dissipated slowly over time. Heat sinks may also perform other
functions, including acting as a structure or enclosure, or may be
formed from components also used for other purposes. For instance,
the housing of a distributor in an automobile may serve as a heat
sink for an ignition control module (ICM) housed within the
distributor. In such an application, the heat produced by the ICM
may typically transfer by conduction into the distributor housing,
and then by convection to the air traveling through the engine
compartment of the automobile.
[0007] In order to conduct heat effectively, in many applications
it is desirable to have a good thermal bond (i.e. a low thermal
resistance) between the heat-producing device and the
heat-absorbing apparatus so that heat will transfer relatively
freely from the heat-producing device to the heat-absorbing
apparatus. The quality of the thermal bond may be more important
where the heat-producing device is small relative to the amount of
heat that is generated within, or where the heat-producing device
must be maintained at a temperature close to ambient. In typical
applications where the heat-producing device and heat-absorbing
apparatus are separate components and heat is transferred between
them by conduction, a heat-conducting substance such as a thermal
grease or thermal paste may be used between the heat-producing
device and the heat-absorbing apparatus. A heat-conducting
substance is typically a non-Newtonian fluid that may be tacky and
flexible, at least when installed, so that it fills most of the
microscopic gaps between the surfaces of the heat-producing device
and heat-absorbing apparatus. The heat-conducting substance may be
an adhesive or glue that holds the heat-producing device and
heat-absorbing apparatus together once assembled. The
heat-conducting substance may be a metal such as solder, or a
thermal wax, and may be melted during the joining of the
heat-producing device and heat-absorbing apparatus, but may remain
solid at the normal operating temperature of the heat-producing
device. A heat-conducting substance is preferably a good conductor
of heat, and may be an electrical conductor so that heat conduction
may occur via the movement of electrons. The surfaces of the
heat-producing device and heat-absorbing apparatus may be cleaned
prior to applying the heat-conducting substance in order to avoid
thermal resistance from foreign materials on the surfaces, such as
oxidation.
[0008] In one specific application, INTEL PENTIUM-based computers
may have a heat sink attached to the top of the CPU chip which are
designed to dissipate heat produced by the chip. The heat is
typically dissipated to air that may be moved by fans and may
ultimately be vented to the outside of the equipment case. It is
typically important that the CPU be kept below a temperature that
would shorten its life, cause mechanical damage, cause software to
malfunction, or destroy the device completely. Systems and methods
have been developed to test and evaluate the effectiveness of heat
sinks, including systems and methods that use thermal sensors such
as thermal diodes, which may be on-board components of the CPU
chips. A properly rated heat sink with sufficient airflow can
perform adequately if it is properly affixed to the CPU chip.
[0009] In many applications, it is desirable that a good thermal
bond be produced between a heat-producing device and a
heat-absorbing apparatus. However, as with any manufacturing
process, it is difficult or expensive to verify that all items
manufactured have a good thermal bond. This is particularly true
where the heat-producing device has a small surface area for heat
conduction, such as a flip-chip (also written flip chip or
flipchip) CPU. Therefore, it would be desirable to have a
convenient system and process or method to test or evaluate the
thermal bond between a heat-producing device and a heatabsorbing
apparatus.
[0010] For instance, it is desirable to have a convenient system
and method to test the thermal bond between a computer CPU and its
heat sink. During the manufacture of a computer, a heat sink may be
affixed to the CPU and a path of low thermal conductivity
established with a combination of thermal paste or adhesive and
pressure. There is typically a relatively small area on the top of
the chip that may need to have a low resistance path to the heat
sink and modifications to this area (e.g. drilling to insert a
temperature probe) may dramatically affect its operation. Due to
the mechanical arrangement of the two parts (CPU and heat sink) as
well as the complexities encountered when trying to remove heat
from CPUs in confined spaces, in the past it has been difficult to
verify during manufacture that the thermal bond between the CPU and
the heat sink has been correctly made. Therefore, as a specific
example, it would be desirable to have a convenient system and
process or method to test or evaluate the thermal bond between a
CPU and a heat sink.
SUMMARY OF THE INVENTION
[0011] This invention provides a system and method for evaluating
the thermal bond between a heat-producing device and a
heat-absorbing apparatus. In an exemplary embodiment, it provides a
convenient system and process or method to test or evaluate the
thermal bond between a CPU and a heat sink. Features of this
invention include that it is easy and convenient to use, that in
some applications it requires few or no additional parts, and that
costs are therefore minimal.
[0012] In furtherance of these features, this invention provides a
system for evaluating the thermal bond between a heat-producing
device and a heat-absorbing apparatus which may be a heat sink. A
thermal bond with a thermal resistance generally exists between the
heat-producing device and the heat-absorbing apparatus. In some
embodiments, a thermal sensor measures the temperature at or near
the device, and a circuit or CPU is configured to perform the
functions of operating the device substantially at a first power
level, measuring a first temperature with the thermal sensor,
operating the device substantially at a second power level,
measuring a second temperature with the thermal sensor, and
calculating the thermal resistance using the first temperature and
the second temperature. The heat-producing device may be the
circuit, and the thermal sensor may be located within the circuit.
The circuit may be configured to calculate the thermal resistance
by subtracting the second temperature from the first temperature,
and in some embodiments, by dividing the difference by the first
power level. The thermal bond may utilize a heat-conducting
substance which may be thermal grease, thermal paste, thermal wax,
glue, adhesive, or solder. Furthermore, the second temperature may
be measured a predetermined time after the second power level is
initiated.
[0013] The present invention also provides a method of evaluating
the thermal bond between a heat-producing device and a
heat-absorbing apparatus. In one embodiment the method includes,
initiated in the following order, the steps of operating the device
substantially at a first power level, measuring a first temperature
of or near the device, operating the device substantially at a
second power level, measuring a second temperature of or near the
device, and calculating the thermal resistance of the thermal bond
between the device and the apparatus. The calculating procedure may
include use of at least the first temperature and the second
temperature. In fact, the second power level may be less than the
first power level, and the calculating procedure may involve
subtracting the second temperature from the first temperature. The
step of operating the device substantially at a first power level
may involve operating the device until the heat-producing device
and the heat-absorbing apparatus substantially reach equilibrium
temperature. In addition, the step of measuring the second
temperature may occur at least 10 seconds after the initialization
of the step of operating the device at the second power level. In
addition, the method according to the present invention may also
include the step of accepting the thermal bond if the thermal
resistance is below a threshold.
[0014] The present invention further provides a method of
evaluating the thermal bond between a heat-producing device and a
heat sink. In one embodiment, the method includes the steps of
operating the device substantially at a first power level, at least
until the device and the heat sink substantially reach equilibrium
temperature, measuring a first temperature substantially of the
device, operating the device substantially at a second power level,
typically less than the first power level, and after a period of
time at the second power level, measuring a second temperature,
also substantially of the device. The method also usually includes
calculating the thermal resistance of the thermal bond between the
device and the heat sink, which may involve subtracting the second
temperature from the first temperature. The method may also include
the step of accepting the thermal bond if the thermal resistance is
below a threshold. The device may be an integrated circuit chip,
such as, for example, a CPU, and the measuring may involve using a
thermal diode, which may be integral with the device.
[0015] In addition, the period of time before the second
measurement is made may be a predetermined amount of time between
the beginning of the operating the device at the second power level
and the measuring of the second temperature. The method may further
include the step of remounting the heat sink on the device if the
thermal resistance is above the threshold. Remounting may involve
the steps of separating the heat sink and the device, cleaning the
heat sink and the device, applying or reapplying a heat-conducting
substance, and reattaching the heat sink and the device. The
calculating procedure may be performed by the device, and may
involve dividing by the power consumption of the device. The
thermal bond may utilize thermal grease, thermal paste, thermal
wax, or adhesive. The second power level may be less than 10% of
the first power level, and the CPU may be a flip-chip CPU.
Furthermore, some embodiments include a step of measuring a third
temperature at or near the device, typically measured after the
second temperature, but while the device is still being operated at
the second power level.
[0016] The present invention even further provides products made
according to the above methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention is illustrated by way of example and
not limitation in the accompanying figures, in which like reference
numerals indicate similar elements and in which:
[0018] FIG. 1 is a side view schematically illustrating an
exemplary bond between a heat-producing device and a heat-absorbing
apparatus;
[0019] FIG. 2 is a side view schematically illustrating, as an
example, the bond between a heat sink and a CPU mounted on a
circuit board;
[0020] FIG. 3 is a block diagram illustrating an exemplary thermal
sensor on a CPU;
[0021] FIG. 4 is a flow chart illustrating the steps in an
exemplary method according to the present invention of evaluating
the thermal bond between a heat-producing device and a
heat-absorbing apparatus;
[0022] FIG. 5 is a flow chart illustrating the steps in an
exemplary method according to the present invention of evaluating
the thermal bond between a heat-producing device and a heat sink,
and remounting the heat sink if the thermal resistance of the bond
is below a threshold of acceptability; and
[0023] FIG. 6 is a graph illustrating as an example, how the
temperature of a 850 MHz INTEL PENTIUM III microprocessor may
change over time after the power level is reduced from
approximately full power to idle.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0024] This invention provides a system and method for evaluating
the thermal bond between a heat-producing device and a
heat-absorbing apparatus. The heat-producing device may be an
electrical device such as an electrical circuit or chip, e.g. a
CPU, and the heat-absorbing apparatus may be a heat sink. The
heat-producing device may be attached to the heat-absorbing
apparatus with a heat-conducting substance such as a thermal
grease.
[0025] In an exemplary embodiment, this invention solves the
problem of verifying adequate CPU-heat sink thermal conductivity
for a computer that has a thermal diode mounted close to the CPU,
mounted in the CPU's packaging, or integrated onto or into the
chip's die. In many embodiments, no additional tools or equipment
are required by the system and method described, nor is it
typically necessary to remove or modify the heat sink.
[0026] The system and method typically involves operating the
heat-producing device at two different power levels. The
heat-producing device and heat-absorbing apparatus may be allowed
to reach equilibrium temperature at the first, typically higher,
power level, after which the temperature, e.g. of the
heat-producing device, is measured. Next the power level of the
heat-producing device is changed to a second power level, typically
lower than the first, which may be at or near zero. The
temperature, e.g. of the heat-producing device, is then measured
again, in some embodiments after the second power level has been
maintained for a predetermined amount of time. Then, the thermal
resistance of the thermal bond between the heat-producing device
and the heat-absorbing apparatus is calculated, for example, by
subtracting the two temperatures and dividing by the first power
level.
[0027] Referring now to FIGS. 1 through 3, various systems
according to the present invention and products made in accordance
with methods described herein, will now be described in detail.
Accordingly, FIG. 1 is a side view schematically illustrating the
bond between a heat-producing device 111 and a heat-absorbing
apparatus 103 in exemplary embodiment 100 of the present invention.
In the exemplary embodiment illustrated, heat-absorbing apparatus
103 is a heat sink, having fins 104. In the embodiment wherein
heat-absorbing apparatus 103 is a heat sink, heat-absorbing
apparatus 103 may be a heat sink as described above or as known in
the art, and may have a different shape than shown, including being
a structural component or enclosure. Heat-absorbing apparatus 103
may be another type heat-absorbing apparatus, including those
described above. Heat-producing device 111 may be a mechanical or
electrical device as described above, such as a computer chip or
CPU. The thermal bond between heat-producing device 111 and
heat-absorbing apparatus 103 may be enhanced by a heat-conducting
substance 107 as shown and described above. For example,
heat-conducting substance 107 may be a thermal grease, thermal
paste, thermal wax, or a metal such as solder. For instance, a
thermal wax with a 0.12 mm cold thickness, that melts at 60.degree.
C. may be used for a typical heat-producing device 111 that is a
CPU. Heat-conducting substance 107 may be a glue, adhesive, or
adhesive heat glue and may physically attach heat-producing device
111 to heat-absorbing apparatus 103. Other embodiments may not have
a heat-conducting substance 107, and for the required heat
transfer, may rely on physical contact between a surface of
heat-producing device 111 and a surface of heat-absorbing apparatus
103. In such embodiments there may be some isolating gaps between
heat-producing device 111 and heat-absorbing apparatus 103, which
may contain air.
[0028] In various embodiments according to the present invention,
there may be more than one heat-producing device used with one
heat-absorbing apparatus, or there may be more than one
heat-absorbing apparatus used with one heat-producing device. For
instance, a plurality of integrated circuit chips may be cooled by
one heat sink, e.g. in a multichip module or microchip module
(MCM).
[0029] FIG. 2 is a side view schematically illustrating the bond
between a heat-absorbing apparatus 103 and a CPU 216 mounted on a
circuit board such as a printed circuit board (PCB) 218 in
exemplary embodiment 200 of the present invention. As described
above with reference to FIG. 1, the heat-absorbing apparatus 103
illustrated is shown as a heat sink with fins 104, but may be
another type heat sink including those described above or known in
the art, or may be another type heat-absorbing apparatus. CPU 216
may be a flip-chip CPU as shown, and may be a flip-chip pin grid
array (FC-PGA) or a ball grid array (BGA). Thermal solutions may be
attached directly to the back of the processor core package without
the use of a thermal paste or heat spreader. CPU 216 may be
comprised of CPU silicon die 212 and die carrier 215 as shown,
which may be attached in ways known in the art. Silicon die 212 may
be flipped (top down) on a substrate containing the solder balls or
pins, and directly soldered, glued, or both, to it, typically by
the chip manufacturer. In other words, silicon die 212 may be a
flip chip. Silicon die 212 and die carrier 215 may form flip-chip
CPU 216. CPU 216 may be mounted on PCB 218. CPU 216 is generally
one type of heat-producing device (e.g. heat-producing device 111
shown in FIG. 1).
[0030] A thermal sensor is typically located integral with, in
contact with, or near, heat-producing device 111 or CPU 216.
Accordingly, FIG. 3 is a block diagram illustrating a thermal
sensor 305 on a CPU 216 in exemplary embodiment 300 of one aspect
of the present invention. Although in the example shown, CPU 216 is
a CPU, it may be another heat-producing device, e.g. heat-producing
device 111 in FIG. 1. Thermal sensor 305 may be a thermal diode, a
thermocouple, a thermistor, a resistance temperature detector
(RTD), and infrared temperature measuring device, a thermometer, or
any other device or system suitable for measuring temperature. CPU
216 may also have an analog to digital converter (ADC) 308
connected to thermal sensor 305 that may be read by CPU 216 and
used to provide temperature readings, e.g. with a resolution of
1.degree. C. and accuracy of +/-2.degree. C. Although thermal
sensor 305 and ADC 308 are shown being on-board or integral with
CPU 216, either or both may be external to CPU 216 or to the
heat-producing device (111). Thermal sensor 305 is generally
configured and located near enough to substantially measure the
temperature of heat-producing device 111. Thermal sensor 305 may be
located at or near CPU 216 (or heat-producing device 111), or may
be at some distance away, and measure the temperature of CPU 216,
e.g. by sensing infrared radiation emitted by CPU 216. For
instance, thermal sensor 305 may be mounted close to the CPU,
mounted in the CPU's packaging, or integrated onto the chip's die,
e.g. into CPU silicon die 212. Thus, in various exemplary
embodiments, thermal sensor 305 may be on or near the surface of
CPU 216, or referring to FIGS. 1 and 2, may be in, on, or near
(e.g. close enough to adequately detect the temperature of)
heat-absorbing apparatus 103, heat-conducting substance 107,
heat-producing device 111, CPU 216, CPU silicon die 212, die
carrier 215, or PCB 218. ADC 308 may, as an example, be mounted on
PCB 218.
[0031] Referring now to FIGS. 4 and 5, various methods according to
the present invention for evaluating the thermal bond between a
heat-producing device (e.g. 111) and a heat-absorbing apparatus
(e.g. 103) will now be described, with frequent reference to the
systems, products, and components illustrated in the previous
figures and described above. The steps according to the methods
described herein may generally be performed, initiated, or
controlled by various components including electrical circuits,
microprocessors, CPUs, computers and the like, and may be performed
automatically, in whole or in part. Accordingly, FIG. 4 is a flow
chart illustrating the steps in exemplary method 400 according to
the present invention of evaluating the thermal bond between a
heat-producing device (e.g. 111 in FIG. 1) and a heat-absorbing
apparatus (e.g. 103 in FIG. 1).
[0032] Typically initiated first, the heat-producing device (e.g.
111) is operated at a first power level (step 421). The first power
level may at or near the maximum sustainable power level of the
heat-producing device (e.g. 111), or may be a typical operating
power level, e.g. of heat-producing device 111. For instance, in
embodiments where heat-producing device 111 is a CPU (e.g. 216),
the first power level may involve CPU 216 doing some arithmetic
operation, e.g. repeatedly, or may involve just waiting for
keyboard input. Thus, the first power level may be a substantially
constant power level, generally resulting in a substantially
constant equilibrium temperature.
[0033] Heat-producing device (e.g. 111) may be operated at the
first power level until equilibrium temperatures are reached, i.e.
the temperatures of heat-producing device 111, heat-absorbing
apparatus 103, or both, level off or stop changing. For instance,
equilibrium may be recognized by monitoring thermal sensor 305 over
time and noting when the temperature indicated stops increasing. As
an example, CPU 216 may be operated at full speed for long enough
that CPU 216 and heat sink or heat-absorbing apparatus 103 reach
their equilibrium temperature, which may be established by the
leveling off of the reading produced by thermal sensor 305. In
other embodiments, it may not be necessary or desirable to operate
the heat-producing device (e.g. 111) at the first power level (step
421) long enough to reach equilibrium temperature.
[0034] Typically initiated next, the first temperature is measured
(step 424). The first temperature may be measured while the
heat-producing device (e.g. 111) is still operating at the first
power level (i.e. during step 421). Alternatively, the first
temperature measurement may be concurrent with or after, e.g.
immediately after, the termination of the operating the
heat-producing device (e.g. 111) at the first power level (e.g.
step 421). Measurement of the first temperature may be triggered by
the reaching of equilibrium temperature (e.g. at thermal sensor
305), passage of a predetermined amount of time, by the reaching of
a particular temperature (e.g. at thermal sensor 305), or by some
combination of these. Since the heat sink or heat-absorbing
apparatus 103 is dissipating the heat, the die 212 or
heat-producing device 111 temperature will typically be slightly
above the heat sink or heat-absorbing apparatus 103 temperature.
The thermal resistivity, e.g. of the CPU-heat sink bond, may
determine this temperature difference. The first temperature may be
recorded, for instance in a computer memory, which may be in CPU
216.
[0035] The temperature measurements (e.g. in steps 424 and 430) may
be made with thermal sensor 305, which may be via ADC 308. In some
embodiments, thermal sensor 305 may be non-linear, and there may be
a translation step from the number read from ADC 308 (or chip
register, e.g. of CPU 216) to get an actual temperature (or
relative temperature). This may be carried out using a look-up
table or a polynomial equation, and may be performed by CPU
216.
[0036] Typically initiated next, the power level of the
heat-producing device (e.g. 111) is changed, and the heat-producing
device is operated at a second power level (step 427). The second
power level may be below the first power level (of step 421) and
may be less than 10% of the first power level, or even at or near
zero (e.g. approximately 1% of the first power level). In other
words, heat-producing device may be turned off (i.e. not operating
at all) or reduced to an idle mode for the second power level (step
427). For instance, using software, CPU 216 may be essentially
stopped so that the heat production within falls close to zero. The
operation of the heat-producing device at the second power level
(step 427) typically starts after the operation of the device at
the first power level (step 421) and may start after or
concurrently with the measurement of the first temperature (step
424).
[0037] Typically initiated next, the second temperature is measured
(step 430). The second temperature may also be measured with
thermal sensor 305, and may be measured while the heat-producing
device (e.g. 111) is still operating at the second power level
(i.e. during step 427). Alternatively, the second temperature
measurement may be concurrent with or after, e.g. immediately
after, the termination of the operating the heat-producing device
(e.g. 111) at the second power level (e.g. step 427). Measurement
of the second temperature may be triggered by the reaching of
equilibrium temperature (e.g. at thermal sensor 305), passage of a
predetermined amount of time, by the reaching of a particular
temperature (e.g. at thermal sensor 305), by the equalization or
near equalization of the temperatures of heat-producing device 111
and heat-absorbing apparatus 103, by a decreased rate of change of
the temperature, or by some combination of these. The second
temperature may be recorded, for instance in a computer memory,
e.g. in CPU 216.
[0038] As an example, FIG. 6 illustrates how the temperature of
heat-producing device 111 or CPU 216 may change over time after the
initiation of a lower second power level (step 427). For the
exemplary embodiment which is the subject of FIG. 6, heat-producing
device 111 or CPU 216 was a 850 MHz INTEL PENTIUM III
microprocessor operated under the six different conditions
indicated. Its temperature was measured with an on-board thermal
diode and read with an on-board ADC. The heat-absorbing apparatus
103 was a heat sink. In this example, the first power level (step
421) was approximately the full power of the CPU (around 16 W when
operated at 850 MHz), and the second power level was near zero
(idle). As can be seen, the temperature of the CPU changed
(dropped) quickly for the first 2 to 10 seconds, and then almost
leveled off, but still changed (dropped) gradually after 100
seconds, e.g. asymptotically approaching equilibrium with the
environment under the second power level. During the initial period
of rapid temperature change, the heat was still transferring from
the CPU to the heat sink. However, during the later period of
gradual or asymptotic temperature change, the CPU and heat sink
reached equilibrium temperature with respect to each other (in this
case probably the same temperature, collectively a Biot body), but
continued to cool together asymptotically as heat dissipated to the
environment.
[0039] Various embodiments of the present invention involve waiting
a period of time from the beginning of the period of operation at
the second power level (the beginning of step 427) and the
measuring of the second temperature (step 430). A predetermined
amount of time may be chosen for the period of time between the
beginning of the period of operation at the second power level (the
beginning of step 427) and the measuring of the second temperature
(step 430). For instance, as can be seen in FIG. 6, a predetermined
amount of time, such as 20 seconds, may be selected that does not
allow the entire assembly to cool too much, but gets past the
initial period of rapid drop in temperature. A shorter
predetermined amount of time, such as 10 or 12 seconds may work,
particularly if the ADC has a short response time, e.g. giving
results every second rather than every four seconds. Thus, as shown
in FIG. 6, the first temperature may be the temperature at time
zero (e.g. the reference temperature of zero degrees shown on FIG.
6). The second temperature may be, for example, the temperature at
the time of 20 seconds. Other periods of time, which may be
predetermined amounts of time, may be desirable for other
heat-producing devices and heat-absorbing apparatuses.
[0040] To verify that a sufficient period of time has passed for
the heat-producing device 111 and heat-absorbing apparatus 103 to
reach the same temperature, or to verify that the initial period of
rapid temperature change (e.g. as shown on FIG. 6) has ended, a
third temperature may be measured which may be similar to the
second temperature measurement and may also be measured with
thermal sensor 305. The third temperature measurement may take
place while the heat-producing device 111 is still being operated
at the second power level (step 427), or may be after, e.g.
immediately after, the termination of the operating the
heat-producing device (e.g. 111) at the second power level (e.g.
step 427). Measurement of the third temperature may, inter alia,
also be triggered by the passage of a (second) period of time (e.g.
after measurement of the second temperature), which may be
predetermined, or measurement of the third temperature may be
triggered by the reaching of a particular temperature (e.g. at
thermal sensor 305). The third temperature may also be recorded,
for instance in a computer memory, e.g. in CPU 216. Additional
temperature measurements may be taken in a similar manner, e.g. at
regular intervals of time. The taking of third or additional
temperature measurements may provide confirmation that the
temperature change is asymptotic or not too great at the second
temperature measurement, e.g. due to different system
characteristics such as a really bad thermal joint. Thus, the
taking of third or additional temperature measurements may improve
the accuracy of the present invention, particularly where it is
desirable to evaluate thermal bonds over a wide or unpredictable
range of thermal resistances.
[0041] Referring once again to FIG. 4, typically initiated next,
the thermal resistance (e.g. of the thermal bond between
heat-producing device 111 and heat-absorbing apparatus 103) is
calculated (step 433). The thermal resistance may be calculated,
for example, by subtracting the second temperature (measured in
step 430) from the first temperature (measured in step 421), and
dividing by the power level of the heat-producing device (e.g.
111), which may be the first power level (e.g. of step 421) or an
average power level of the heat-producing device, e.g. from the
manufacturer's specifications. In general, the poorer the bond
between the heat-producing device and the heat-absorbing apparatus,
the greater the difference between the first temperature and the
second temperature. Subtracting the two temperatures also
eliminates inaccuracy by subtracting out any consistent error. In
fact, the first and second temperature measurements need not be
absolute or conventional temperature measurements. Rather they may
be relative to some reference temperature, e.g. relative to the
first temperature as illustrated in FIG. 6. In addition, the
greater the power level (e.g. the first power level) the greater
the temperature difference as well. Therefore, the above
calculation provides a good indication of the thermal resistance of
the thermal bond between the heat-producing device (e.g. 111 ) and
the heat-absorbing apparatus (e.g. 103). Other formulas may be
desirable in particular circumstances, which may use some or all of
the calculations described herein. The calculation of the thermal
resistance (step 433) may be performed by an electrical circuit,
such as a microprocessor, which may be CPU 216.
[0042] In embodiments where a third temperature measurement is
taken, the difference between the third temperature and the second
temperature may be calculated. If the difference is below a second
threshold, as an example, then the thermal resistance may be
calculated as described above using the first and second
temperatures. If the difference is above the second threshold,
then, as examples, the third temperature measurement may be used in
the calculation described above (step 433) instead of the second
temperature measurement. In embodiments having more than three
temperature measurements, subsequent temperature measurements may
be used similarly.
[0043] FIG. 5 is a flow chart illustrating the steps in an
exemplary method according to the present invention for evaluating
the thermal bond between a heat-producing device (e.g. 111) and a
heat sink (e.g. one embodiment of heat-absorbing apparatus 103),
including remounting the heat sink if the thermal resistance of the
bond is below a threshold of acceptability. In addition to steps
421, 424, 427, 430, and 433 described above with reference to FIG.
4, FIG. 5 includes the step of evaluating the acceptability of the
thermal bond (step 536). In one embodiment, which is illustrated in
FIG. 5, the thermal bond is considered to be acceptable if the
thermal resistance of the bond is less than a certain threshold
value. The threshold value may be calculated, determined
empirically, or some combination thereof. As an example, where a
thermal wax is used between a CPU and a heat sink (i.e. as
described in FIG. 6), the calculated thermal resistance may be
0.86.degree. K./W. However, based on experience, the acceptable
threshold thermal resistance may be selected to be 1.2.degree. K./N
to limit the maximum temperature difference to acceptable
parameters (e.g. 20.degree. K.) while allowing as many CPU's as
possible to be accepted. If the thermal resistance is below the
threshold, then the bond may be accepted (step 542). If the thermal
resistance is above the threshold, then the bond may be rejected,
and the heat sink (or heat-absorbing apparatus 103) may be
remounted (step 539).
[0044] Remounting a heat sink (step 539) may involve separating the
heat sink and the device, cleaning the mounting surfaces of the
heat sink and the device (e.g. of any heat-conducting substance,
oxidation, or other foreign substances) applying (or reapplying) a
heat-conducting substance (e.g. as described herein), and
reattaching the heat sink to the device. Other embodiment
heat-absorbing apparatuses may be remounted in a similar manner.
Once the heat sink is remounted, the method of the present
invention may be performed again to verify that the new thermal
bond is acceptable. In alternative (or in addition) to remounting,
devices with rejected thermal bonds may be discarded, recycled, or
classified or used for a purpose for which they are suitable, such
as lower power level applications (e.g. lower speeds), or operation
in cooler or better ventilated environments. In addition, mounting
methods, cleaning methods, and heat-conducting substances may be
adjusted or changed, and the present invention may be used to
evaluate whether the changes are effective or desirable.
[0045] The methods according to the present invention may be
performed once after heat-producing device 111 is joined with
heat-absorbing apparatus 103, after final assembly, before
shipping, or at other times. For instance, the thermal bond may be
evaluated once at the customer site, or periodically, to monitor
the integrity of the thermal bond between heat-producing device 111
and heat-absorbing apparatus 103. For instance, in applications
where the heat-producing device is a CPU (e.g. 216) that contains
the thermal sensor (e.g. 305), the method according to the present
invention may be performed periodically, and the operator may be
alerted if the thermal bond has deteriorated, e.g. below a
threshold. For instance, a method of the present invention may be
performed when the computer is restarted after being improperly
shut down. In addition, or alternatively, it may be performed
whenever the device happens to be operated at a high power level
until equilibrium temperatures are substantially obtained, and then
it happens to be operated in an idle mode for the predetermined
amount of time. In other circumstances, it may be desirable to
perform the methods according to the present invention only on a
sample of products.
[0046] Although the subject application has been described herein
with reference to the appended drawing figures, it will be
appreciated that the scope of the invention is not so limited.
Various modifications in the design and implementation of various
components and method steps discussed herein may be made without
departing from the spirit and scope of the invention, as set forth
in the appended claims.
* * * * *