Webinar on Fan Characterization

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, Jan. 24 from 2-3 p.m. ET and will cover fan characterization and deployment in an electronics system. Learn more and register at https://qats.com/Training/Webinars.

Low-profile thermal solutions required for cooling high-density boards

Advancements in the telecommunications, Internet of Things (IoT), broadcast, biomedical, and other industries demand more power, more data processing, and more capabilities. Engineers have been required to fill boards to the brim in order to meet the ever-increasing call for more and this high-density board design requires creative thermal management solutions.

Low-profile solutions
With today’s high-density boards, low-profile solutions are required to ensure proper thermal management even in tight spaces. (Advanced Thermal Solutions, Inc.)

Standard heat sink sizes are too large for many telecom systems, module and blade servers, or IoT gateways, where card-to-card and internal spacing is limited, and may not be designed to handle the lower airflow that is the result of numerous components packed into tight spaces. With space and airflow at a premium, engineers need low-profile solutions that are lightweight, compact, and will not sacrifice thermal performance.

Advanced Thermal Solutions, Inc. (ATS) has several low-profile heat sink options that will give engineers greater flexibility in designing boards and systems while still managing heat.

Ultra-Low-Profile blueICE™ Heat Sinks

ATS blueICE™ heat sinks are specially designed for low airflow systems where space is limited. The heat sinks range in height from 2-7 mm and the spread-fin array maximizes surface area to enhance thermal performance even in low airflow systems. Their thermal resistance is as low as 1.23 °C/W within an air velocity of 600 ft/min.

Ultra-low-profile blueICE heat sinks are specially designed for high thermal performance in tight spaces and low airflow. (Advanced Thermal Solutions, Inc.)

The heat sinks are lightweight, ranging from 4-30 grams, and no mechanical attachment is required. Thermal tape is all that is needed to attach blueICE™ heat sinks to a component, which further reduces weight and assembly time and saves valuable space on the board.

In systems where boards are packed tightly together, low-profile heat sinks can provide the necessary thermal performance without significantly adding to the height of the components on the board. Also, the design of blueICE™ heat sinks removes heat from devices even with lower airflow.

Low-Profile maxiFLOW™ Heat Sinks

ATS has also made low-profile versions of its ultra-high-performance maxiFLOW™ heat sinks, available with either maxiGRIP™ or superGRIP™ mechanical attachments. The low-profile, spread-fin array maximizes surface area and enhances convection cooling, while attachment technology offers secure hold without a significant increase in footprint or the need to drill holes in the board.

Low-profile maxiFLOW heat sinks with superGRIP attachment technology maximizes surface area for higher thermal performance. (Advanced Thermal Solutions, Inc.)

Low-profile maxiFLOW™ heat sinks are designed for component heights ranging from 1.5-2.99 mm and the specially-designed fin array increases the surface area to provide the highest thermal performance per volume occupied when compared to other heat sinks on the market.

Using maxiGRIP™ or superGRIP™ heat sink attachment technology also gives design engineers more flexibility because of their easy assembly and removal. There is no damage to the board, which is important because of dense PCB routing and the potential need for rework.

Heat Pipes and Vapor Chambers  

In situations where low-profile heat sinks will not fit, ATS has heat pipes and vapor chambers that will transport heat away from a component and can be attached to a heat sink or the system chassis/enclosure to dissipate the heat to the ambient. These innovative cooling solutions will meet even the toughest thermal challenges.

Heat pipes can be used to move heat from devices to heat sinks, the chassis, or system enclosure to remove the heat to the ambient. (Advanced Thermal Solutions, Inc.)

ATS has expanded its line of high-performance, off-the-shelf round and flat heat pipes to provide the broadest offering on the market. Engineers can avoid the extra cost of custom lengths by selecting from the more than 350 product numbers that ATS has created. Flat heat pipes are available in lengths of 70-500 mm, with widths of 4.83-11.41 mm, and heights of 2-6.5 mm.

Vapor chambers are essentially flat heat pipes and provide another low-profile option for spreading heat. (Advanced Thermal Solutions, Inc.)

Vapor chambers are essentially flat heat pipes that can be used in the base of heat sinks to spread heat. ATS has expertise designing vapor chambers and heat pipes into electronics systems to improve thermal management, especially with limited space and airflow. Their high thermal conductivity can move a lot of heat from devices and they can be easily attached to heat sinks to form cooling assemblies.

See how low-profile solutions are needed in IoT sensor-level infrastructure in the following video:

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.

Technology Review: Data Center Cooling

(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.

Data Center Cooling

This article explores recent patents and technical advancements from the data center cooling industry. (Wikimedia 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 data center cooling solutions.

There are many U.S. patents in this area of technology, and those presented here are among the more recent. These patents show some of the salient features that are the focus of different inventors.

Hybrid Immersion Cooled Server with Integral Spot and Bath Cooling

US 7724524 B1, Campbell, L., Chu, R., Ellsworth, M., Iyengar, M. and Simons, R.

An immersion cooling apparatus and method is provided for cooling of electronic components housed in a computing environment. The components are divided into primary and secondary heat generating components and are housed in a liquid sealed enclosure. The primary heat generating components are cooled by indirect liquid cooling provided by at least one cold plate having fins. The cold plate is coupled to a first coolant conduit that circulates a first coolant in the enclosure and supplies the cold plate. Immersion cooling is provided for secondary heat generating components through a second coolant that will be disposed inside the enclosure such as to partially submerge the cold plate and the first coolant conduit as swell as the heat generating components.

The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a method and associated hybrid immersion cooling apparatus for cooling of electronic components housed in a computing environment. The components are categorized as primary and secondary heat generating components and are housed in a liquid tight enclosure. The primary heat generating components are cooled by indirect liquid cooling provided by at least one cold plate having fins.

The cold plate is coupled to a first coolant conduit that circulates a first coolant in the enclosure and supplies the cold plate. Immersion or direct liquid cooling is provided for secondary heat generating components through a second coolant that will be disposed inside the enclosure such as to partially submerge the cold plate and the first coolant conduit as Well as the heat generating components.

In one embodiment, the cold plate and the coolant conduit each comprise extended external surfaces used to transfer heat from the second coolant to the first coolant. In one embodiment the second coolant cools the electronics via free convection or a combination of free convection and boiling while an alternate embodiment provides a sub merged pump to aid circulation. In yet another embodiment, the electronics are housed on an electronics board that is tilted at an angle to also aid circulation of the second coolant.

Thermal Caching For Liquid Cooled Computer Systems

US 7436666 B1, Konshak, M.

The present invention involves supplementing the liquid cooling path within a given rack of electronic components in order to reduce the temperature rise of critical components before damage or data loss can occur in the event of a primary cooling system failure. Liquid cooled racks generally have piping and heat exchangers that contain a certain volume of liquid or vapor that is being constantly replaced by a data center pump system. Upon a data center power or pump failure, the How stops or is diminished. The coolant, however, is still present within the rack components. Upon detection of a lack of How, an unexpected rise in the coolant temperature, or a significant reduction in How pressure, and before any liquid can be drained away from the racks, an electrically or hydraulically operated valve bypasses the supply and return of the data center cooling path, creating an independent closed loop supplemental cooling system within the rack itself.

One embodiment of a system for mitigating a failure of a data center’s liquid cooling system is shown. A high level block diagram of one embodiment of a system for thermal caching in a liquid cooled computer system is presented. Under normal circumstances, coolant is delivered to a rack housing a plurality of liquid cooled components by a data center liquid cooling system supply and return lines. The supply and return lines are coupled to the rack by quick disconnecting and the coolant is pumped through the conduits by one or more data center pumps. Within the rack a secondary cooling loop is established that is in fluid communication with the data center cooling loop.

Upon entering the rack the data center supply line enters a monitoring device. In one embodiment of the present invention the device monitors fluid flow, pressure, and/or temperature. Other characteristics of the fluid that can also be used to identify a failure of the data center cooling system as imposed on the supply line. Thereafter, and before allowing the coolant to access any of the electronic components housed within the rack, a one way valve is placed on the supply lid entering the rock. The valve allows fluid to pass from the data center cooling supply line into the rack but prevents flow from regressing toward the supply line should the flow stop and/or an adverse pressure gradient is experienced.

Similarly, a two-way valve is placed on the coolant return line that returns heated coolant from the electronic components to the data center coolant return line. The two way valve is in communication with the monitoring device and capable of receiving a signal that indicates a failure in the primary or data center liquid cooling system.

Upon receiving such a signal from the monitoring device, the two-way valve closes either electrically or hydraulically and diverts the return coolant from the electronic devices to the supplemental or secondary liquid cooling loop. The supplemental liquid cooling loop, which is housed entirely within the rack, thereafter operates independent of the data center liquid cooling system.

Modular High-Density Computer System

US 7688578 B2, Mann, R., Landrum, G. and Hintz, R.

A modular high-density computer system has an infrastructure that includes a framework component forming a plurality of bays and has one or more cooling components. The computer system also has one or more computational components that include a rack assembly and a plurality of servers installed in the rack assembly. Each of the one or more computational components is assembled and shipped to an installation site separately from the infrastructure and then installed within one of the plurality of bays after the infrastructure is installed at the site.

Historically, a room oriented cooling infrastructure was sufficient to handle this cooling problem. Such an infrastructure was typically made up of one or more bulk air conditioning units designed to cool the room to some average temperature. This type of cooling infrastructure evolved based on the assumption that the computer equipment is relatively homogeneous and that the power density is on the order 1 -2 Kilowatts per rack. The high density racks mentioned above, however, can be on the order of 20 Kilowatts per rack and above. This increase in power density, and the fact that the equipment can be quite heterogeneous leads to the creation of hot spots that can no longer be sufficiently cooled with simple room-oriented cooling systems.

By separating the computational components of a high density computer system from the requisite cooling, power distribution, data l/O distribution, and housing infrastructure components, significant advantages can be achieved by embodiments of the invention over current configurations of such computer systems. The computational components can be assembled and tested for proper operation while the infrastructure components are shipped separately in advance of the computational components. Thus, the infrastructure cooling components, data and power distribution components, and framework components can be delivered and their time-intensive installation completed more easily and prior to receiving the tested computational components.

Moreover, the shipping of the computational components separately from the infrastructure components achieves less laborious and less costly delivery and handling of the components. Finally, it makes it easier for the data center to upgrade or add additional infrastructure and infrastructure components to a current installation.

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.

Effective cooling of high-powered CPUs on dense server boards

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

The main goal of electronics thermal management is to efficiently remove enough heat from a device’s active region so that it stays within its rated temperature. Providing effective cooling presents different design challenges, not all of which involve the chip itself. Some thermal challenges are related to the system in which the chip resides. A common example is cooling a device positioned on a crowded printed circuit board (PCB). The congestion of components restricts airflow and space, which makes the use of many conventional cooling devices difficult.

Dense Server boards

Figure 1. A dense motherboard from Gigabyte Technologies featuring an Intel P55 chipset. [1]

Optimizing PCB for thermal management has been shown to ensure reliability, speed time to market and reduce overall costs. With proper design, all semiconductor devices on a PCB will be maintained at or below their maximum rated temperature. Applying thermal management can sometimes be problematic for dense boards employing fine pitch devices. (Pitch is the space between the center of one BGA ball to the center of the next one.)

But if certain layout guidelines are not followed and considerations are not given to a PCB’s thermal performance, the device and the overall system can suffer from sub-par performance and reliability in the field. [2]

Today’s circuit boards are often assembled with increasing density with the goal of making smaller, lighter systems, or to provide more processing power in demanding applications such as data centers and IoT (Internet of Things) applications. PCB designers must use proven layout techniques to ensure effective thermal performance for the board and its components.

Figure 2. Crowded boards have limited space from where chip cooling air can be drawn. (Wikimedia Commons)

Part of the trend toward higher density boards is related to the industry’s adoption of increased server density. This means increasing the power of the chips, putting more chips per rack unit, and filling up the racks as much as possible. Rack power has transitioned from a normal of about 4 kilowatts to 70 kilowatts per rack.

High current electronic components like microcontrollers can generate a significant amount of heat. To keep the board temperature lower, it is usually best to mount these components near the center of the board. Heat can diffuse throughout the board and the temperature of the board will be lower.

Many components in this situation, such as GPU, will require a dedicated cooling system, such as a fan sink. But simply installing a fan sink on top may not provide the needed level of cooling. It is good practice to quantify system flow bypass on the fan sink, and to also consider the proximity of components neighboring the fan sink. The mass airflow rate is the true measure of available coolant, along with the air velocity.

Obstructions in the intake or exhaust of the fan (e.g. neighboring components) must be carefully considered as their presence will impact the performance of the fan sink. The size and position of adjacent components can impact the fan’s performance. [3]

Figure 3. The QuadFlow CPU cooler draws air from four sides, passing it through cooling fins and expelling warm air. (Advanced Thermal Solutions, Inc.)

One new and effective solution for cooling hot components on congested PCBs is the QuadFlow CPU cooler from Advanced Thermal Solutions, Inc. (ATS). The liquid-free cooler features a high-power blower that draws in air from four different directions. So, while proximate components may block local air in a couple of directions, the QuadFlow fin fields will pull in air from the other directions to make sure that the component is being cooled.

QuadFlow coolers are just 29 mm tall, so they will fit into standard 1-U racks and there are several options for base material (aluminum, copper, or vapor chamber) depending on performance, weight, or cost requirements. [4]

Before applying any thermal management hardware, the smartest engineering activity may be investing is various PCB design services. These include CFD studies on boards at the CAD stage to wind-tunnel testing of actual or dummy boards in conditions that simulate air distribution in real-world applications. Services are available for characterizing boards using research-quality instruments, heat and air velocity sensors, and PCs.

Figure 4. FloTHERM image reveals hotter and cooler regions on a PCB. (Advanced Thermal Solutions, Inc.)

Dummy or working PCBs can be tested in isolation or installed in their own packaging domain. Computational simulations can be made of engineered designs using computational fluid design packages such as 6SigmaET, FloTHERM and CFDesign.

These services are available from ATS, whose engineers can design board layouts to improve cooling airflow in dense systems. Natural airflow can be enhanced to individual hot components and to active cooling systems that rely on airflow for effective performance. Often, these studies head off more expensive cooling solutions by showing that minor changes to component layouts or to the volume of airflow will resolve thermal problems. [5]

References
1. https://www.techpowerup.com/103375/gigabyte-unwraps-latest-p55-series-motherboards
2. https://www.embedded.com/design/configurable-systems/4395845/Ultra-fine-pitch-devices-pose-new-PCB-design-issues
3. https://www.hpe.com/us/en/insights/articles/why-youll-be-using-liquid-cooling-in-five-years-1710.html
4. https://www.qats.com/cms/2013/06/21/how-system-flow-affects-fan-sink-performance/
5. https://www.qats.com/Consulting/PCB-Board-Layout

Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. On Thursday, Jan. 29 from 2-3 p.m. ET the webinar will cover “Methodologies for Fan Characterization and Deployment within a System.” Learn more and register at https://qats.com/Training/Webinars.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/Consulting/Custom-Cooling-Solutions or contact ATS at 781.769.2800 or ats-hq@qats.com.

Are More Efficient Heat Sinks Really Costlier?

The question arises, why pay more for a higher-efficient heat sink that is also smaller and lighter in weight, especially in cases whereby a simple, larger but heavier cast or extruded heat sink can also do the job? In most cases, the single piece part price is the main driver as to why engineers and purchasers stay with lesser effective solutions. This is generally because they believe that they are saving money for the company in the long-run.

But, is this really true?

It is very easy to compare the price of two single heat sinks, a highly efficient one versus a standard extruded type with the same thermal performance. But such a simplistic comparison does not take into account the flow performance, effect on neighboring and downstream components, weight, volume usage, etc.

More Efficient Heat Sinks

(Advanced Thermal Solutions, Inc.)

What makes a heat sink an efficient heat sink you may wonder? Such a heat sink is optimized for both flow and heat transfer and, hence, makes much better use of the volume available for cooling and the existing cooling air. The geometry of the heat sink is optimized by using thin fins and a special design to lower the air resistance, resulting in the highest possible velocity in the fin field, while minimizing the effect of bypass flow. Because of this, it also has a lesser intrusive effect to the flow, which has a positive effect on the neighboring and downstream flows. This produces lower pressure drops over the board and through the system. The result of a comparison test, presented by Lasance C.J.M. and Eggink H.J., demonstrates this. [1]

The shape and the number of fins create the available surface area. The more surface area in contact with cooling air, the more energy can be dumped into the air and, therefore, the lower the heat sink temperature. Most important, the component temperature will also be lower. Of course, this is only valid if the temperature of cooling air going through the fin field is still lower than the fin temperature; otherwise, no net heat transfer from the heat sink to the air will occur. As has been shown in the literature, there exists an optimum correspondence between the number of fins and the overall cooling effect of these fins.

To arrive at a better comparison, let us look to other related effects which will result in a system price increase due to the chosen cooling solution:

  1. Effect on the flow
  2. Heat sink weight
  3. Heat sink attachment
  4. Mechanical adjustments required to handle the weight on both a board and system level, and to fulfill mechanical requirements for shock and vibration
  5. Raw material usage to manufacture the cooling solution
  6. Required fan performance
  7. Product reliability
  8. Transportation cost
  9. End of life cost

Figure 1 shows a picture of a flow visualization test through a pin fin and the DUT. Compare the amount of flow entering the pin fin and the DUT, which in this case is a maxiFLOW™ heat sink. Most of the water “hitting” the pin fin is bypassing it because of the high air resistance of the fin field. Look at the flow that is left over downstream of the pin fin, which is lower than the upstream flow. Imagine what will happen if we put multiple pin fins in a row downstream of each other because we have to cool multiple devices in a row.

Fig. 1- Flow Test on Both a Pin-Fin (Left) and DUT (Right) in a Water Tunnel. The flow is from top to bottom. (Advanced Thermal Solutions, Inc.)

The first pin fin will have sufficient cooling but, the further we go downstream, the less effective the heat sink becomes. Limited air will be available for cooling, because most of the air is bypassing the heat sinks. What is the first reaction without knowing anything about the flow structure? We need a larger heat sink downstream to get the same cooling effect; or maybe we need to consider a more powerful fan system to drive more air through the system. However, if we would have started from a system level point of view instead of concentrating on a single heat sink, we would have studied the flow field and the interaction between the heat sink and the flow more closely, and we could have arrived at a better solution.

For example, in many cases the total number of heat sinks can be reduced, because other components are better cooled and probably do not require additional cooling.

Standard extruded heat sink profiles and cast heat sinks normally have a thick base and thick fins and are made of lesser thermally conductive aluminum alloys. The lower conductivity is a result of the additives that are included, to make the manufacturing of the product easier. The base and the fins tend to be thicker, because it is more difficult to manufacture thin and tall fins. Especially for natural convection, the optimum heat sink from a thermal point of view can easily be a factor of ten thinner than what is offered. The main design driver for these types of heat sink is the ease of manufacturing and not the overall thermal performance. The end result is a more voluminous and heavier heat sink that makes bad use of the available volume and has a negative effect on the flow.

To have a stable mechanical design, a stronger mechanical attachment to the component/board is required to handle the weight of standard heat sinks, as compared to high performance ones. An efficient light weight heat sink can still be attached by taping, glue, and other attachment methods, which use the component itself as an anchor.

Counting the weight of a standard heat sink and its attachment mechanism together, the overall product weight will be quite higher than for a design based on more efficient cooling solutions.

The same is valid for those LED designs that use the housing as a heat sink. The housing often is made by extrusion or casting processes, which limit the freedom of design. They are generally made of zinc aluminum (Zamac) with a thermal conductivity around 115 W/mK; whereas aluminum used for molding is between 100-150 W/mK; brass annealed is around 60 W/mK; aluminum alloy AL 6063 has a thermal conductivity of 201 W/mK and aluminum alloy AL 1050A reaches 229 W/mK.

Heat spreading is an important factor in most LED applications, and drives the thermal design of LED cooling. If analysis shows heat spreading is important, the consequence is that for lower conductivity materials, the only option is to either increase the base thickness or embed heat pipes or vapor chambers, adding to cost and weight.

LED Thermal Solution

Ephesus LED lighting solutions, with ATS thermal management design, was used in the recent Super Bowl at U.S. Bank Field in Minneapolis.

Forged heat sinks are made of high conductivity aluminum, but the manufacturing method itself is very limited in design freedom. So, in general, to get a better performance, a larger heat sink is required. For natural convection, the use of thermally conductive plastic could be of interest because of its lower weight and greater design freedom. Plastics enhanced with carbon fibers could also be used but require special attention because of their non-orthogonal conduction behavior. Other options are designs that are a combination of highly efficient heat sinks and heat pipes, to either improve heat spreading when the heat sink is much larger than the source, or to transport the heat from the source to a remote heat sink.

Apart from the attachment of the heat sink to one or more component, the overall weight of the board, including the cooling solutions, affects the overall mechanical design. Additional mechanical features are needed to make the product mechanically stable and these features will add cost and weight and further limit the design freedom.

As discussed before, a more voluminous heat sink solution requires more raw material. The initial manufacturing process of aluminum, however, is energy intensive, something we would like to decrease in a world where reduction of energy consumption is key. Fortunately, it can be recycled without the loss of its properties and the recycling process uses only a fraction of the energy in the initial manufacturing process. Finally, there are manufacturing techniques such as bonding, folding and skiving, that do not suffer from these sustainability issues.

Furthermore, a lesser efficient heat sink such as a pin fin or standard extruded type of heat sink, will lead to higher air resistance and lesser optimized flow over the components/ board and through the system. To overcome the higher air resistance and allow for more flow to compensate for the reduced airflow, a more powerful fan or more fans are required. A more powerful fan can mean either a larger fan type or permitting the current fan to run at a higher rotational speed. However, doubling the fan speed means increasing the input power to the fan by a factor of 8. As a result, more heat is dissipated in the system, the power supply has a higher current usage and more power is dissipated in the fan itself. This will lead to a higher fan temperature, thus reducing its lifetime. On top of this, a higher fan speed and more flow will result in higher noise levels.

Optimizing your thermal design by optimizing around the heat sink could in some cases avoid the use of a fan at all, making up for the extra costs of a more sophisticated heat sink. The use of more efficient cooling solutions will lead to a more optimized overall thermal design of the system, influencing directly the thermally and thermo-mechanically related reliability issues of the overall system. The transportation cost of the cooling solution to the manufacturer of the system also has a price. This price is based on shipped volume and weight. Efficient cooling solutions are lower in volume and lower in weight, so will yield a reduction in transportation costs. The weight factor is also applicable for the final product, as a product equipped with lesser weight cooling solutions will be cheaper to transport.

Apart from transport issues, a human effect is applicable: take, for instance, an LED-based streetlamp. Lifting a 30 kg lamp and installing it on a pole, versus lifting a 20 kg lamp, speaks for itself. Additionally, the pole needs to be designed in such a way that it can handle the weight of the lamp, potentially reducing the costs of a lesser weight lamp. Every product has a certain economic and technical lifetime and will be recycled afterwards. The cooling solution need to be recycled too. The heat sink, lamp enclosure – in most cases made of aluminium – can be recycled in a cost and energy effective way; but the lesser mass we recycle, the better.

In summary, the conclusion must be that it pays off to focus on the costs of the total system, and not only on the costs of the individual parts. In times long gone by, it was standard practice that project leaders got bonuses for buying parts as cheap as possible. Needless to say, such an attitude cannot survive in a world where end-users buy total systems, not a collection of parts. However, in the case of heat sinks, we still notice a sub-optimal purchasing policy, often based on lack of knowledge and outdated protocols.

References
1- Lasance, C.J.M., Eggink, H.J., “A Method to Rank Heat Sinks in Practice: The Heat Sink Performance Tester,” Proc. 21st SEMITHERM, pp. 141-145, San Jose, CA.