As you may already know, TIMs – thermal interface materials – are not especially easy to remove from heat sinks. But the needs to do so are common: repositioning the sink, switching to a better performing TIM, replacing components, etc.
Most of today’s TIMs are bonded to
heat sink surfaces with a pressure-sensitive adhesive layer. The rest of the
TIM may be an elastomer filled with thermally conductive particles. Bonding
pressure, such as from clips and screws increase the thermal performance, but
the result can be a TIM that’s hard to remove from the heat sink surface.
Another kind of TIM is thermal grease,
which is an excellent heat conductor but is inherently messy. This must be
accounted for when removing thermal grease from a heat sink surface.
With the right tools and some
patience, a heat sink can have its TIM removed completely. The approaches are
similar for removing different kinds of TIMs.
Removing
Thermal Tapes
Double-sided thermal interface tapes
provide exceptional bonding properties between components and heat sinks. For
many applications they remove the need for mechanical fasteners to secure the
sink to the component.
Here we are showing the removal of
Parker Chomerics Thermattach T412 tape from a heat sink. The T412 tape has
aluminum mesh carrier which helps improve heat transfer. The aluminum also
helps keep the TIM together when it’s removed.
Use a razor blade, but don’t gouge the
aluminum because if it makes the surface uneven that will negatively impact
heat transfer. Start at one corner, try to lift the TIM slightly the razor
blade, be careful not to cut yourself.
Next, put the heat sink (upside down)
on a paper towel. This is mainly to protect the fins of the heat sink from getting
scraped. Then, use a putty knife with a flexible blade, or a plastic scraper or
something similar with a non-gouging edge to help remove the rest of the TIM.
Push the scraping edge carefully
forward under the TIM corner, while pulling the TIM slightly up. While you are
starting to lift the original TIM corner from the surface, you can start doing
the same on another corner. And if continue to work at it from different angles
eventually you can get the TIM off.
If there is TIM residue left over on
the heat sink surface, you can use a lint-free cloth and a solvent to wipe it
clean. The solvent should be something that won’t damage the finish on the heat
sink. Here we used isopropyl alcohol. You may have to repeat this several times
to get everything off and you have a clean surface.
Removing Phase-Change Materials
Phase-change materials (PCM) can be removed with similar steps as with thermal tapes. These images show the removal process using a maxiFLOW™ push pin heat sink from Advanced Thermal Solutions, Inc. (ATS).
Start with sharp, clean razor blade
because a plastic scraper’s edge isn’t fine enough to penetrate cleanly under
the TIM. Be sure the razor blade is straight to minimize the risks of nicks to
the heat sink surface. The TIM manufacturer’s data sheet recommends using a
razor blade to remove the phase change TIM. In this demo, the TIM is Chomerics
T766 PCM. Slowly work the razor blade edge under the TIM, be careful of your
fingers.
After a lot of use phase change
materials can be hard to remove compared to new pieces. Use the razor under different
corners. Go slowly until you get all the material off. You will be left with some
PCM residue on the heat sink surface.
You can put a small amount of
isopropyl alcohol on the surface and use this as a lubricant and go back with
the razor blade to get a close shave on that surface to remove much of the remaining
TIM residue.
When your sink’s surface is nearly
clean, get a lint free cloth or a wipe. With some alcohol and with a bit of
rubbing you should be able to remove the rest of the phase change material.
Removing
Thermal Grease
Here we start with a heat sink with
thermal grease on its mounting surface.
Start by removing as much grease as possible
using a dry cloth or paper towel. You should be able to get most of the grease
off this way.
Then, for the leftover grease residue,
use a lint-free cloth or rag with some alcohol or another type of solvent that
won’t eat away at the heat sink’s anodized surface. With a little elbow grease,
you should be able to get the surface clean.
Note that If you’re going to be doing a lot of handling of thermal greases and solvents, it’s advisable to wear protective gloves.
(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications.
This Technology Review will focus on recent developments in Thermal Interface Materials. (Wiklmedia Commons)
We are specifically focusing on patented technologies to show the breadth of development in thermal management product sectors. Please note that there are many patents within these areas. Limited by article space, we are presenting a small number to offer a representation of the entire field. You are encouraged to do your own patent investigation.
Further, if you have been awarded a patent and would like to have it included in these reviews, please send us your patent number or patent application.
In this issue our spotlight is on thermal interface materials.
There are many U.S. patents in this area of technology, and those presented here are some recent. These patents show some of the salient features that are the focus of different inventors.
A thermal interface material includes
a macromolecular material, and a plurality of carbon nanotubes embedded in the
macromolecular material uniformly. The thermal interface material includes a
first surface and an opposite second surface. Each carbon nanotube is open at
both ends thereof, and extends from the first surface to the second surface of
the thermal interface material. A method for manufacturing the thermal
interface material includes the steps of: (a) forming an array of carbon
nanotubes on a substrate; (b) submerging the carbon nanotubes in a liquid
macromolecular material; (c) solidifying the liquid macromolecular material;
and (d) cutting the solidified liquid macromolecular material to obtain the
thermal interface material with the carbon nanotubes secured therein.
An object of the present invention is
to provide a thermal interface material having a reduced thickness, small
thermal interface resistance, good flexibility and excellent heat conduction. To
achieve the above-mentioned object, the present invention provides a thermal
interface material comprising macromolecular material and a plurality of carbon
nanotubes embedded in the macromolecular material uniformly. The thermal
interface material also comprises a first surface and an opposite second
surface. Each carbon nanotube is open at two ends thereof, and extends from the
first surface to the second surface of the thermal interface material.
Unlike in a conventional thermal interface material, the carbon nanotubes of the thermal interface material of the present invention are disposed in the macromolecular material uniformly and directionally. Thus, each carbon nanotube of the thermal interface material can provide a heat conduction path in a direction perpendicular to a main heat absorbing surface of the thermal interface material. This ensures that the thermal interface material has a high heat conduction coefficient. Furthermore, the thickness of the thermal interface material of the present invention can be controlled by cutting the macromolecular material. This further enhances the heat conducting efficiency of the thermal interface material and reduces the volume and weight of the thermal interface material.
Moreover, each carbon nanotube is open at two ends thereof, and extends from the first surface to the second surface of the thermal interface material. This ensures the carbon nanotubes can contact an electronic device and a heat sink directly. Thus, the thermal interface resistance between the carbon nanotubes and the electronic device is reduced, and the thermal interface resistance between the carbon nanotubes and the heat sink is reduced. Therefore, the heat conducting efficiency of the thermal interface material is further enhanced.
In one aspect of the invention there
is provided a fibrous interface material sandwiched between two layers of a
removable paper or release liner. The interface comprises flocked, e.g.
electroflocked, mechanically flocked, pneumatically flocked, etc., thermally
conductive fibers embedded in an adhesive or tacky substance in substantially
vertical orientation with portions of the fibers extending out of the adhesive.
An encapsulant is disposed to fill spaces between portions of the fibers that
extend out of the adhesive, leaving a free fiber structure at the fiber tips.
Another aspect of the invention is a method of making a fibrous interface. In the method, thermally conductive fibers of desired length are provided and, if necessary, cleaned. A release liner is coated with an adhesive or tacky substance, and the fibers are flocked to that release liner so as to embed the fibers into the adhesive or tacky substance with a portion of the fibers extending out of the adhesive.
The adhesive is cured and the space between fibers if filled with a curable encapsulant. A second piece of release liner is placed over the fiber ends. Then the fibers in the adhesive with the release liner over the fibers in the adhesive with the encapsulant in the spaces between the fibers is compressed to a height less than the normal fibers’ length and clamped at the compressed height.
Thereafter the encapsulant is cured while under compression to yield a free fiber tip structure with the fiber tips extending out of the encapsulant.
One possible solution to meet the heat dissipation needs
of microprocessors and other processing devices is to employ an active cooling
system—e.g., a liquid based cooling system that relies, at least in part, on
convective heat transfer initiated by the movement of a working fluid—rather
than (or in combination with) heat sinks and other passive heat removal
components. Disclosed herein are embodiments of a cooling system for an
integrated circuit (IC) device—as well as embodiments of a method of cooling an
IC device—wherein the cooling system includes a liquid metal thermal interface
that is disposed between a die and a heat transfer element, such as a heat
spreader or a heat sink. Embodiments of a method of making a liquid metal
thermal interface are also disclosed.
This patent is for a liquid metal thermal interface for
an integrated circuit die. The liquid metal thermal interface may be disposed
between the die and another heat transfer element, such as a heat spreader or
heat sink. The liquid metal thermal interface includes a liquid metal in fluid
communication with a surface of the die, and liquid metal moving over the die
surface transfers heat from the die to the heat transfer element. A surface of
the heat transfer element may also be in fluid communication with the liquid
metal.
Per Figure 2, the cooling system 200 is coupled with an IC die 10. During operation of the IC die 10, the die may generate heat, and the cooling system 200 is capable of dissipating at least some of this heat, such as may be accomplished by transferring heat away from the IC die 10 and to the ambient environment. The IC die 10 may comprise any type of integrated circuit device, such as a microprocessor, network processor, application specific integrated circuit (ASIC), or other processing device.
The present invention is a thermal interface for coupling
a heat source to a heat sink. One embodiment of the invention comprises a mesh
and a liquid, e.g., a thermally conductive liquid, disposed in the mesh. The
mesh and the thermally conductive liquid are adapted to contact both the heat
source and the heat sink when disposed there between. In one embodiment, the
mesh may comprise a metal or organic material compatible with the liquid. In
one embodiment, the liquid may comprise liquid metal. For example, the liquid
may comprise a gallium indium tin alloy. A gasket may optionally be used to
seal the mesh and the liquid between the heat source and the heat sink. In one
embodiment, the heat source is an integrated circuit chip.
In another aspect of the invention, a method for cooling
a heat source with a heat sink is provided. In one embodiment, the method
includes providing a thermal interface having a mesh and a liquid disposed in
the mesh. The thermal interface is interposed between the heat source and the
heat sink, such that the mesh and the liquid are in contact with the heat
source on a first side of the thermal interface and in contact with the heat
sink on a second side of the thermal interface.
In another aspect of the invention, a method of fabricating a thermal interface for assisting the thermal transfer of heat from a heat source to a heat sink is provided. In one embodiment, the method includes providing a mesh. A liquid is disposed in the mesh in sufficient quantity to substantially fill the mesh. The liquid comprises liquid metal. Optionally, the liquid metal may subsequently be frozen in place.
Advanced Thermal Solutions, Inc. (ATS) engineers have received several questions from customers about the phase-change material that comes standard on the base of all ATS heat sinks. Engineers have asked whether imperfections on the surface of the grey foil that protects the phase-change material, such as dents or wrinkles, have a significant impact on the thermal interface material’s thermal performance. Do these imperfections have any impact at all? Should the liner be removed?
ATS heat sinks come with Chomerics T766 phase-change material standard. (Advanced Thermal Solutions, Inc.)
When pressure is applied, the phase-change material (and the metal foil) conform to both surfaces, completely removing air gaps or voids to maximize heat sink performance. The phase-change material will “attain minimum bond-line thickness” and “maximum surface wetting,” according to information from Chomerics, to limit the thermal resistance path and ensure almost no thermal contact resistance between the device being cooled and the heat sink. For the T766, the phase-change temperature is listed as 55°C. The liner should remain in place when placing heat sink on the device it is intended to cool (see the video below).[1]
Should engineers be concerned about the appearance of the metal foil lining? Do the dents or wrinkles in the lining impact the performance of the phase-change material and potentially impact the efficiency of the heat sink?
To reassure engineers that the appearance of the metal foil would have a negligible impact on the thermal performance of the TIM, the Chomerics Research and Development Department released the results of tests that the company performed on the T766 conformable metal foil. [2] Chomerics studied the impact on thermal impedance when the foil was wrinkled, dented, and even folded.
Researchers tested materials that were not wrinkled, lightly wrinkled, moderately wrinkled, and severely wrinkled under different pressures (20 psi, 50 psi, and 100 psi). The results (shown below) demonstrated that even when wrinkled “to a far greater extent than would be expected in actual handling” thermal impedance never increased more than 0.02°C-in22/W. The report explained, “For 50 W of power, through one square inch of material, that’s only 1.0°C change!”
The dent test was created using a wooden tongue depressor and included a sample with five dents per square inch and a second with 15 per square inch. As was demonstrated in the wrinkle study, the dents smoothed out during the testing process and showed a minimal impact on thermal impedance. “Once again, the thermal impedance did not increase by more than 0.01°C-in2/W. For 50 W of power, through one square inch of material, that’s only 0.5°C change! The metal foil carrier is so conformable that the dents were almost unidentifiable after testing with 100 psi of pressure.”
The final test was performed on T766 that was folded. One sample was folded under on one edge and the second was folded to overlap down the center. The results indicated that small folds of up to 5% of the pad’s area does not significantly impact thermal impedance. A large fold, which tripled the thickness of the foil in the center of the sample, had a significant impact on the thermal impedance of the material.
The report concluded, “T766 will perform extremely well even when the pad is wrinkled or folded, or the foil is scratched or dented. The high conformability of the metal foil carrier will allow it to smooth out and erase almost any imperfection.”
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
For decades, thermal interface materials (TIMs) have been used as pathways allowing heat to flow from one location to another. TIMs are often part of cooling systems that remove heat from component dies by dissipating it into heat spreaders, such as heat sinks, and ultimately out of the dies’ surrounding enclosures.
As a product line, TIMs have continuously evolved, driven by market needs for higher thermal conductivity, lower thermal impedance, new applications and lower costs.
Figure 1. Thermal Interface Materials are Used Both Inside and Outside Chip Packages. (Indium) [1]
From a broad view, most TIMs fall into three material categories. Some are made from elastomers or other polymers with a thermally-conductive filler added. Other TIMs are solder-based. Like elastomers, these solder-based TIMs are soft and conformable to mating surfaces, filling air gaps that compromise thermal transfer. Finally, newer carbon-based TIMs have emerged that demonstrate superior performance, though many of these are not yet commercialized.
Here is a brief look at recent developments within the major TIM categories:
A New TIM Filler
A new generation of polymer-based TIMs uses boron nitride nanosheet (BNNS) fillers to enhance thermal conductivity. BNNS is a two-dimensional crystalline form of hexagonal boron nitride (h-BN), also known as white graphene. BNNS ranges in thickness from just one to a few atomic layers. It has a similar geometry to its all-carbon analog graphene, but some very different properties. For example, graphene is highly electrically conductive while BN nanosheets are electrical insulators.
Figure 2. Edges of boron nitride nanosheets are atoms of all boron, all nitrogen, or alternating elements. (Wikimedia Commons)
Hexagonal boron nitride (h-BN) has other desirable properties, including a large surface area, high-thermal transport, and chemical inertness. The thermal conductivity of bulk h-BN can reach 400 W/mK at room temperature. [3]
A recent study from Rice University, which continues to expand on its original simulations of graphene’s effect on nanoscale heat transfer, demonstrated that an h-BN thin film composed layer-by-layer of laminated h-BN nanosheets can enhance lateral heat dissipation on a substrate, in this case glass. Thermal performance improved with the BN coating due to its anisotropic thermal conductivity. It had a high in-plane thermal conductivity of 140 W/mK for spreading and a low cross-plane thermal conductivity of 4 W/mK to avoid a hot spot beneath the tested device. [4]
Researchers have also created simulations showing that 3-D structures of h-BN planes connected by boron nitride nanotubes could transfer heat (move phonons) in all directions, whether in-plane or across planes. The number and length of the nanotubes connecting the h-BN layers have an effect on heat flow: more and/or shorter pillars slow conduction, while longer pillars speed heat transfer along.
Figure 3. 3-D structure of highly thermally conductive h-BN sheets connected by BN nanotubes. (Shahsavari Group/Rice University)
Solder-Based TIMs
With ever-increasing power and heat dissipation needs across the electronics industry, solder-based TIMs may be better suited to take the heat away from dies than thermal grease where electrical insulation isn’t required.
Issues with thermal grease include:
Grease has a low bulk thermal conductivity of 3-12 W/mK. Some solder-TIMs provide a high bulk thermal conductivity of 87 W/mK.
Over time, thermal grease tends to pump-out and migrate away from the center of the power die. It gets hotter and can fail prematurely. There is no pump-out with a solder-TIM.
Over time, grease tends to bake-out and dry (becomes powdery), thereby increasing thermal resistance and reducing heat-dissipation effectiveness. With solder-TIMs, there is no bake-out. [5]
Figure 4. SMA-TIMs conform to surface disparities over time to increasingly reduce thermal resistance. (Indium Corp.) [6]
Recent solder-based TIMs developed by Indium Corporation include a new SMA-TIM (soft metal alloy). This is made from an indium solder base and offers uniform thermal resistance at lower applied stresses in compressed interfaces. It is provided as a compressible metal foil that can be used as a TIM between a heat source and a heat sink, heat spreader, or heat pipe.
The malleability of the indium minimizes surface resistance and increases heat flow (conductance). Over time, the malleability of the solder helps fill the interface gaps even better. Thus, thermal interface resistance decreases over time as opposed to thermal grease where the thermal interface resistance increases over time. [7]
Another newer indium-containing material has been developed for use in the TIM 1 position, between the die top and its case. The material is part of a system, developed by Indium Corporation, called mdTIM. It provides a thermal conductivity of 87 W/mK.
While pure indium metal has a superb thermal transfer rate, air or gas pockets (voids) can degrade the performance of the material. These voids are created by entrapped air or gasses produced by flux component evaporation that fail to escape during reflow.
Indium’s mdTIM uses a patented system of materials and reflow technology does not use flux so there are no outgassing issues.
Carbon-Based TIMs
The very high thermal conductivity of pure carbon has long made it attractive for use in TIMs. Today’s carbon-based TIM fillers include diamond, carbon nanotubes (CNT), graphite and graphene. Often these fillers are dispersed in a spreadable (grease-like) polymer matrix.
In some cases, different forms of carbon fillers are being combined. For example, highly thermally conductive CNT have been mixed with less expensive carbon substrates like graphite and graphene to reduce costs but still deliver very high thermal conductivity.
Recent research has been made with graphite nanoplatelets (GNP) in thin thermal interface layers. These studies concerned the through-plane and in-plane alignment of GNP in a spreadable matrix. When dispersed, the GNP fillers take a naturally in-plane alignment, meaning the great majority of heat flow is in parallel to an interface. However, at the same time, the desired through-plane heat transfer from one surface to the other is much less. [8]
Figure 5. The top SEM images are graphite nanoplatelets with in-plane alignment. Bottom images show hybrid mix of GNP with a 45% volume of Al2O3 spheres. [8]
A solution was found by adding spherical microparticles. Spherical Al2O3 and Al filler particles were tested. The hybrid filler formulations resulted in enhanced through-plane thermal conductivity by disrupting the natural in-plane alignment of the GNP. This led to the disruption of the GNP in-plane alignment and the improvement of the through-plane thermal conductivity of the tested thermal greases.
Costs and other factors pose development challenges to TIMs with carbon-based heat transfer schemes. But given the high thermal conductivity and various configurations available from carbon-based materials, these will likely be at the heart of many upcoming performance advancements in TIMs.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
The design of a printed circuit board (PCB) is a complicated process that requires engineers to consider a number of different issues before the board is ready to move beyond prototype and into production. Engineers must think about the physical constraints of a board on component size and placement, the electrical interaction between components, the signal loss through wires and traces, and the thermal management of each component and the system as a whole. [1]
ATS maxiFLOW heat sink with superGRIP attachment on a PCB. (Advanced Thermal Solutions, Inc.)
With all of that to consider, it is no wonder that many designs go through several iterations before moving into the production stage. Since the process is already complex and there is a certain amount of trial-and-error in designing a PCB, engineers will look for ways to avoid unnecessary rework that will add significant cost to the project in terms of both time and money.
As noted in a previous article, the type of heat sink attachment technology that an engineer chooses will impact the ease with which a design can be reworked and the amount of damage to the board that will be caused if a change needs to be made.
Push pins, threaded standoffs and z-clips require holes or anchors be drilled into a board, which leaves permanent damage if a component needs to be moved to a new location and could also impact signal routing. There is even the possibility of a short in installation, which also would damage the board. [2]
Non-mechanical attachments such as thermally conductive tape and epoxy are not guaranteed to provide the optimal thermal management because there is “risk of die damage and poor thermal performance due to uneven heat sink placement,” according to a case study from the Altera Corporation. [3]
The case study also said that thermal tape and epoxy have “high risk of damaging the device or PCB” when compared to mechanical attachment technology coupled with thermal interface material (TIM) or phase change material (PCM). In fact, to remove a heat sink attached with epoxy requires an even temperature of 115-120°C.
As the video below shows, removing thermal tape from a heat sink (even one that is not attached to a board) requires a lot of work and tools. If the heat sink is attached to a component, the process to remove it could damage the board or other devices in the vicinity:
A recent chart from NEMI (National Electronics Manufacturing Initiative) indicated that the cost of assembly can be very high per I/O (input/output) on the PCB – considering some of the new BGAs have hundreds of I/O and there are dozens of BGAs on the board, the cost can be prohibitively expensive to put together a board irrespective of the product sector. [4] Obviously, full reworks necessitated by the use of damaging heat sink attachments raise those costs exponentially.
Board assembly roadmap from NEMI showing the conversion costs by product sector. [4]
Advanced Thermal Solutions, Inc. (ATS) has created a mechanical attachment technology that makes rework easy and allows engineers to make changes to the design without damaging the PCB or the components. superGRIP™ is a two-part attachment system with a plastic frame clip that fastens around the edge of the component and a metal spring clip that fits between the fins of the heat sink and quickly and easily attaches to the frame.
As the video below demonstrates, superGRIP™ can be installed and removed with common household tools and will provide a steady, firm pressure to ensure optimal thermal performance of the heat sink and the reliability of the device:
The advantage of superGRIP™ is not limited to its ease of use and the time and money that will be saved in reworking a PCB design. The pressure strength and security of the superGRIP™ attachment system allows the use of high-performance phase change materials that can improve heat transfer by as much as 20 times over standard thermal tapes. [4]
superGRIP™ comes with Chomerics Thermflow T-766, a foil PCM with a thickness of 0.0035 millimeters that has an operating range of -55°C to 125°C. According to Chomerics, the T-766 and other traditional non-silicone thermal interface pads “completely fill interfacial air gaps and voids. They also displace entrapped air between power dissipating electronic components. Phase-change materials are designed to maximize heat sink performance and improve component reliability.” [5]
Chomerics added, “Upon reaching the required melt temperature, the pad will fully change phase and attain minimum bond-line thickness (MBLT) – less than 0.001 inch or 0.0254 mm, and maximum surface wetting. This results in practically no thermal contact resistance due to a very small thermal resistance path.”
The combination of frame and spring clip provides uniform force over the heat sink and ensures no movement to optimize the impact of the PCM, while not damaging the solder holding the BGA component in place on the board. ATS engineers designed the attachment technology so that the in-plane and normal forces of both the frame and the spring clip hold the heat sink without stressing the solder even through NEBS (Network Equipment Building Systems) shock and vibration testing. [6]
Save time, save money, and avoid unnecessary headaches during the design phase by using ATS superGRIP™ technology.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.
