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.
Extruded metal heat sinks are among the lowest cost, widest used heat spreaders in electronics thermal management. Besides their affordability, extruded heat sinks are lightweight, readily cut to size and shape, and capable of high levels of cooling.
Metal
Choices
Most extruded heat sinks are made from
aluminum alloys, mainly from the 6000 alloy series, where aluminum dominates. Trace
amounts of other elements are added, including magnesium and silicon. These
alloys are easy to extrude and machine, are weldable, and can be hardened.
Common alloys for extruded heat sinks are the 6063 metals. These can be extruded as complex shapes, with very smooth surfaces. 6061 aluminum is also used for extrusions. Its tensile strength (up to 240 MPa) is superior to 6063 alloys (up to 186 MPa). In addition to heat sinks, these aluminum alloys are popular for architectural applications such as window and door frames. [1]
Figure 1. An extruded aluminum heat sink with a black anodized finish. [2]
Aluminum alloys are popular for extruding as heat sinks because they provide both malleability and formability. They can be easily machined and are as little as one-third the density of steel. This results in extrusions that are both strong and stable, at a reduced cost relative to other materials.
Figure 2. Heated aluminum alloy billets are pushed through a die to produce extruded length heat sinks and other parts. [3]
The aluminum extrusion process starts
with designing and creating the die that will shape the heat sink profile. Once
this has been done, a cylindrical billet of aluminum is heated up in a forge to
high temperatures, generally between 800-925°F
(427-496°C). Next, a lubricant is added to the aluminum to prevent it from
sticking to any of the machinery. It is then placed on a loader and pressure is
applied with a ram to push heated aluminum through the die.
During this process, nitrogen is added in order to prevent oxidation. The extruded part will pass completely through the die and out the other side. It has now been elongated in the shape of the die opening. The finished extrusion is then cooled, and if necessary, a process of straightening and hardening creates the finished product.
They can be cut to the desired lengths, drilled and machined, and undergo a final aging process before being ready for market. [4]
Finished heat sinks typically come
with anodized surfaces, which can enhance their thermal performance.
Alternatively, a chromate finish provides some corrosion protection, or can be
used as a primer before a final paint or powder coating is applied. [5]
Shapes of
Extruded Heat Sinks
Extruded heat sink profiles range
from simple flat back fin structures to complex geometries for optimized
cooling. They can be used for natural (passive) or forced convection (active)
with an added fan or blower. Extruded profiles can also include special
geometries and groove patterns for use with clip or push pin attachment
systems. [6]
Figure 3. Extruded heat sinks are available in many shapes and lengths. [6]
Extrusions are also available in
bulk lengths, e.g. 8 feet, which can be cut to different lengths per customer
needs.
Optimizing
Thermal Performance
6063 aluminum alloy has a thermal conductivity of 201-218 W/(mK). Higher tensile strength 6061 aluminum’s thermal conductivity ranges from 151-202 W/(mK).
Besides choosing the aluminum alloy, selecting an optimal extruded
heat sink should factor in its overall dimensions and weight, the specified thermal
resistance, and the extrusion shape (flat-back, flat-back with gap, hollow,
double-sided, etc.). [7]
Extruded heat sinks can be designed with very thin, and thus more, fins than other sink types. They can be extruded with aspect ratios of around 8:1, which can greatly optimize heat sink performance. A heat sink’s aspect ratio is basically the comparison of its fin height to the distance between its fins.
In typical heat sinks the aspect ratio is between 3:1 and 5:1. A high aspect ratio heat sink has taller fins with a smaller distance between them for a ratio that can be 8:1 to 16:1 or greater.
Figure 4. Different thin fin extruded heat sinks mounted on a PCB. [8]
Thus,
a high aspect ratio heat sink provides greater density of fins in a given
footprint than with a more common size sink. The great benefit is the increased
amount of heat dissipating surface area it provides due to its additional fins.
Further, these heat sinks do not occupy any more length or width. The result is
a more efficient heat sink with higher performance per gram in the same
footprint. [9]
Applications
An extruded heat sink is used mainly to increase the surface area
available for heat transfer from high-power semiconductor devices, thus
reducing a given semiconductor’s external case temperature, as well as its
internal junction temperature.
Figure 5. Extruded heat sinks mounted on processors by clips (left) and push pins (right). [10]
This allows the semiconductor devices to perform at their highest
level, with maximum reliability. Such semiconductor devices include (but are
not limited to) RF power transistors, RF power amplifiers, Power MOSFETs,
IGBTs, inverter power modules, and thyristor modules.
In some power conversion circuit applications, large diodes, rectifiers, diode modules and even high-power resistors (thick film, etc.) can also require thermal contact with an extruded heat sink. For cooling DC-DC power converters and power modules, extruded heat sinks are available for full, half, quarter and one-eighth brick sizes
In this article, Qpedia will explore some innovative thermal management products that have recently hit the market. These new thermal products encompass a variety of thermal management applications from CPU coolers to thermal interface materials (TIM) to sensors and test instruments to advanced materials and concepts.
Metal Oxide Microparticle Thermal Compound
NT-H2 is a second generation, hybrid thermal compound from Noctua. The compound uses a mixture of metal oxide microparticles for lower thermal resistance and reduced bond-line thickness at typical mounting pressures. Up to 2°C lower temperatures were attained In Noctua’s standardized internal testing at various platforms and heat loads. NT-H2 thermal compound does not require a break-in period. Because of its long-term stability, it can be used on the CPU for up to five years.
Both pastes are electrically non-conductive and non-corroding, so there is no risk of short circuits and they’re safe to use with all types of heat sinks. NT-H2 thermal compound will come in standard 3.5g and extra-large 10g package that come with a supply of pre-moistened wipes for effective surface cleaning.
Form in Place Liquid Thermal Gap Filler is Peel-able
The new BergquistGap Filler TGF1500RWoffers thermal interface material (TIM) re-workability without sacrificing thermal conductivity or automation performance in a single material. TGF 1500RW is a one-part, cure-in-place liquid gap filler that can be used with automated dispensing equipment for high-volume manufacturing operations.
Because the material is applied as a liquid, it is ideal for miniaturized, high-density assemblies and complex architectures, penetrating small gaps for complete coverage. Once cured, the material provides optimized surface contact and thermal transfer with a 1.5 W/m-K thermal conductivity and delivers excellent low and high temperature mechanical and chemical stability.
While traditional cure-in-place TIMs generally require very high force for disassembly, TGF 1500RW peels away cleanly from contact surfaces to safeguard delicate componentry and preserve product value.
All-In-One Liquid Cooler for Demanding Workloads
Asetek has introduced its highest performance all-in-one (AIO) liquid cooler, designed in collaboration with Intel. The Asetek 690LX-PN liquid cooler is approved for the Intel Xeon W-3175X processor. The AIO liquid cooler features extreme performance to enable overclocking up to 500 watts (W) and provides stable operation for demanding workstation and content creation workloads.
The 690LX-PN cooler uses the new Gen6-s pump from Asetek – the same core technology that is used to cool some of the world’s fastest supercomputers and includes a highly efficient copper radiator. A large cold plate covers the entire CPU package and ensures optimal plate-to-package thermal contact.
The 690LX-PN liquid cooler is approved to cool the new 28-core/56 thread Intel Xeon W-3175X processor, which offers up to 4.3 GHz single-core turbo frequency.
Data Center Racks
Enhance Thermal Management
The NRSe series of data center racks from NetRack helps achieve effective air flow management at the rack level. These racks are manufactured out of steel sheet punched, formed, welded and powder coated with highest quality standards under stringent ISO 9001-2008 Manufacturing and Quality management system.
Thermal management with air seal kit and blanking panels and bottom brushed access with enhanced grounding and bonding assures 100 per cent compatibility with all equipment, conforming to general industry standards. Features like Intelligent Locking and Rack Monitoring add to easy management of data centers.
Silicon-Based Microchannel Heat Sink
Imec, a research and innovation hub in nanoelectronics and digital technologies has introduced a silicon-based compact microchannel heat sink that enables high heat flux dissipation. When assembled to a high performance chip for cooling, the Imec heat sink achieves a low total thermal resistance of 0.34-0.28 K/W at less than 2 W pump power. It is directly integrable in the semiconductor infrastructure.
The Si-based microchannel heat sinks are fabricated separately and then interfaced to the back side of a heat-dissipating chip. Using an optimized Cu/Sn-Au interface, very low thermal contact resistance is achieved between both parts.
Because the fluidic performance and thermal behavior can be predicted with a high degree of accuracy, the new microcooler can be tailored according to external system constraints such as space and liquid supply.
Air to Air Heat Exchangers for Closed Loop Systems
The new Pfannenberg PKS Series Air to Air Heat Exchangers use the Pfannenberg Kinetic System next generation cooling technology that out-performs conventional heat exchangers. The units are lightweight and easy to install, while air to air technology takes advantage of a cooler ambient environment when closed-loop cooling is required, sealing against gas, humidity, and dust.
Designed for indoor, outdoor, remote and washdown applications that require a closed loop system to protect electronics, the PKS Series of exchangers are ideal for keeping rain and dust from sensors and drives on outdoor systems and protecting against corrosion and contamination.
As their speeds increase, the heat dissipation from high performance processors requires more innovative cooling techniques for heat removal. One such technique, jet impingement, provides one of the highest heat transfer coefficients among cooling methods. This property of jet impingement has been used by Advanced Thermal Solutions, Inc. (ATS) in a 1-U server application. Jet impingement has also been applied to ATCA chassis.
Cooling high-powered 1U servers requires new thermal management techniques, including advanced air jet impingement. (Wikimedia Commons)
A Ujet-1000™ 1-U chassis made by ATS was assessed in the company’s thermal test lab. Four identical heat sinks were tested under conventional parallel flow and ATS’ proprietary Therm-Jett™ impingement flow. The results showed a 20-40 percent improvement in the thermal performance of the heat sinks.
The
Ujet-1000™ is a 1 to 2 KW 1-U chassis system (depending on component case and
ambient temperature) designed for the most demanding, telecom and server
applications. Lab tests demonstrated that four heat sinks on four simulated
chips located on a PCB achieved 0.16 to 0.18oC/W thermal resistance.
The power dissipation of each simulated component was maintained at 200 W. On
the other hand, the thermal resistance of the same heat sinks with parallel
flow, using the same fans, was almost 20 to 40% worse.
The new ATS Therm-Jett™ technology uses a specially made duct with an impingement plate beneath it to create jet impingement on top of the components and heat sinks. The tremendous increase in heat transfer coefficient leads to significant reduction of thermal resistance compared to the other conventional 1-U systems. A Therm-Jett™ system can be built for any specific configuration. The impingement duct is less than 5 mm thick and is located in the chassis on top of the motherboard. In addition to high heat transfer coefficient, fresh air is distributed between all heat sinks at inlet temperature.
In
contrast, in conventional cooling systems, the upstream heat sinks and
components receive air at inlet temperatures which are cold and gradually warm
up as the air moves downstream. The increase of air temperature effectively
reduces the cooling effect of the air downstream.
The other advantage of Therm-Jett™ is that there is no need to make a special duct for each heat sink, thus freeing the motherboard for other components. Even by adding ducts, other components such as memory cards, resistors and capacitors located upstream of the heat sinks on the PWB would deprive the heat sink of the flow at its most critical point, which is close to the base.
Figure
1 shows a CAD drawing of the real system under test. The cooling is provided by
eight 40 mm high capacity double fans located in the midsection of the 1-U
chassis. The power to the heat sinks was provided by four heaters attached below
and dissipating 200 W each. The heat dissipation of the power supply was simulated
by attaching a rectangular heater strip under the power supply which dissipates
about 100 W. A “U” shape frame made of aluminum was located under the hard
drives. The power of four hard drives was simulated by placing a rectangular
heater under the “U” shape frame which dissipates about 80 W.
Four
thermocouples were placed in holes at the center of the base of the heat sink
downstream. The holes were filled with thermal grease to minimize the interfacial
resistance. Three thermocouples were attached to the aluminum “U” frame, and
their average temperature was recorded as an approximate temperature of a real
hard drive. One thermocouple was also attached to the base of the power supply
to measure its approximate temperature. All temperature measurements were taken
using J type thermocouples.
Figure 1. Schematic of the Ujet-1000TM and the Therm-JETT™ cooling duct. (Advanced Thermal Solutions, Inc.)
Figure 2 shows the exploded view of the Ujet-1000™ chassis.
Figure 2. Exploded view of the Ujet-1000TM and the Therm-JETT™ cooling duct. (ATS)
Figure 3 shows a conventional cooling system for a 1-U system. In this system, the air flow from the fans is parallel to the heat sinks.
Figure 3. Conventional Cooling System in a 1-U Application. (ATS)
Results
Figure 4 shows the schematic set up of the conventional cooling system. In this configuration, eight (8) blowers move the air in parallel to the heat sink fins.
Figure 4. Configuration of conventional layout in a 1U system for temperature measurement. (ATS)
Figure 5 shows the implementation of an ATS Therm-Jet, which provides jet impingement on the same four heat sinks, and the location of impingement holes with respect to the heat sinks.
Figure 5. Configuration of an ATS Therm-JETT™ application in a 1U system for temperature measurement. (ATS)
Tables 1 and 2 show the experimental values obtained within a 1-U chassis made by ATS. The two sets of tests were done for both 12 and 6 volts to the fans. The thermal resistance data of all four heat sinks, hard drives and the power supply were obtained for both conventional cooling and jet impingement cases. The acoustic noise for each case was also recorded for comparison.
The data shows an improvement of thermal resistance of 22% to 42% for the heat sinks from jet impingement as compared to conventional cooling. The power supply shows a 10% improvement in the thermal resistance.
The
hard drive, though, shows a 20% degradation. This is due to the fact that, with
jet impingement, the pressure drop on the fans increases, consequently
decreasing the flow through the system. However in an actual system the
percentage will be smaller. That’s because the heat generated will be more
volumetric compared to the current setup where heat is generated on the surface
of the “U” shape aluminum piece located at the bottom of hard drives.
In that case, the decrease of flow through the system will have less impact. Additionally, the increase in hard drive temperature is less than 2°C in this experiment, which is generally not large enough to be a concern.
Table 1. Experimental test results comparing conventional and Therm-JETT™ results with 12 volts to the fans.
Table 2. Experimental test results comparing conventional and Therm-JETT™ results with six volts to the fans.
The
question might be raised as to whether the performance of the heat sinks could
be improved if we removed the impingement duct, increased the heat sink height
by the height of impingement duct and ducted the flow.
We
analyzed this situation and found that the improvement would be at most 5%, if
we assume that the heat sink is ducted and the pressure drop is the same in
both short and tall versions. To study this problem in detail one must consider
the fan curves instead of using a fixed volumetric flow rate. Interested
readers will find an article in a previous issue of the ATS Qpedia Thermal eMagazine
[1] with more information about this topic.
Table 3 shows the temperatures of the four heat sinks, the hard drive and the power supply. As we mentioned earlier, the heat sinks were mounted on 200 W devices, the power supply was dissipating 100 W and the hard drives were dissipating 80 W. The results are shown for jet impingement and conventional parallel air flow over 23.5 mm tall heat sinks, and ducted flow over heat sinks with 28.5 mm tall heat sinks. It can be seen that heat sink temperatures are significantly lower for jet impingement even compared with a taller heat sink with ducted flow.
Table 3. Comparison of temperatures for jet impingement and parallel flow with 23.5 mm heat sinks and ducted flow with 28.5 mm tall heat sinks.
Table 4 shows the improvement in temperature of the four processors between the jet impingement and the two cases of 23.5 mm heat sink and the ducted 28.5 mm heat sinks. By comparing the results for 6 and 12 volts to the fans, it can be seen that at lower voltage the jet impingement temperature difference is even more than with higher voltage to the fans. This implies low pressure drop fans can significantly benefit from the application of jet impingement.
Table 4. Comparison of temperature improvement for jet impingement and parallel flow with 23.5 mm heat sinks and ducted flow for 28.5 mm tall heat sinks.
Figure
6 is a graphical representation of Table 4. The figure shows the significant
temperature increases in the case of parallel flow and ducted flow for the heat
sinks compared to jet impingement technology. Components (heat sinks) 2 and 3
are hotter than components 1 and 2 because they are downstream and the approach
air temperature is higher for a ducted flow. In the jet impingement mode, the
impingement flow is at upstream temperature and therefore much cooler than the
air received in ducted flow.
In impingement mode, there is another flow coming axially toward the components, called cross flow. It is the interaction of cross flow and impingement that causes the cooling of the component (heat sink).
Figure 6. Heat sink temperature increase of parallel and ducted flow compared to jet impingement cooling. (ATS)
It
should be noted that the above experiment was done for a heat source that is
the same size as the heat sink base; hence, the spreading resistance is zero
because it is almost independent of the heat transfer coefficient. The
spreading resistance can be added to the above numbers for other sizes of heat
sources.
The
same concept of jet impingement has been applied to simulated components in ATCA
chassis. The results will be published in subsequent Qpedia articles. The data
improvement is promising. Even though conventional air-cooling technology is
fast approaching its thermodynamic limit, there are still numerous potentials for
air cooling which will enable this technology to be used in the years to come.
Reference
1. “Heat Sink Thermal Resistance as a Function of Height-Ducted Flow with Fan Curve,” Qpedia Thermal eMagazine, Advanced Thermal Solutions, Inc., January 2009.
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.
Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. This month’s webinar will be held on Thursday, April 25 from 2-3 p.m. ET and will cover the use of thermal interface materials to enhance heat sink performance. Learn more and register at https://qats.com/Training/Webinars.
