Trends and Challenges in the Thermal Management Field

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1 Trends and Challenges in the Thermal Management Field

2 Challenges in thermal management are not new and they are not going to be going away anytime soon. Patent search shows patents addressing thermal issues going back to the 1920 s and before. Patent from 1922 shown for optimizing the cooling of a transformer

3 Technically we are still in the middle ages! Kurzweil shows what we can expect from the computing market over the next few decades As far as we think we have evolved, he shows today s computing capabilities as that of between and insect and a mouse I expect this growth in capability will continue to challenge thermal designs.

4 Other aspects of technology show a similar logarithmic trend CPUs, GPUs, Storage, Bandwidth and DNA sequencing all are on the same path

5 Trends in the thermal world are not unique A market develops due to needs of the latest technologies As these technologies move into main stream markets companies grow up to serve the market Once mainstream the pressure on cost drives the supply chain to a Low Cost model Once this point is reached very little resources are spent on R&D This leaves a hole in the market. R&D of new technologies starts, often at small start ups, and the cycle starts all over again. Up to now the thermal market has progressed through; Stampings and machined parts Extruded parts Fabricated parts, fin pack, skived fins, bonded fin, etc. Hybrid parts Copper and aluminum assemblies Heat pipes So, what is next?

6 Where are the challenges? Typical resistance stack ups include up to 8 steps Resistance in the silicon TIM 1 1st interface With such a long series of resistances there are many area for improvement Getting the heat out of the devices TIM materials Moving the heat Enhancing the heat transfer co-efficient Resistance in the lid TIM 2 2 nd interface Spreading resistance TIM 3 interface to the fins Fin to Air resistance Air Temperature rise

7 DARPA is looking into a few of the key areas Thermal Interface Spreading resistances Enhanced heat exchangers

8 TIM Materials Over the past 40 years, CMOS, tele-communications, active sensing and imaging, and other technologies have undergone tremendous technological innovation. Over this same period, the performance of TIMs in consumer electronics and in DoD systems has undergone relatively little improvement. DARPA NanoThermal Interfaces (NTI) The primary goal of this program is the development and demonstration of ideas based on novel materials and structures that can provide significant reductions in the thermal resistance of the interface layer (often called the TIM) between the backside of an electronic device and the next layer of the package.

9 TIM Materials Typical resistance of common TIM materials are shown In higher power density applications the thermal interface can significant These numbers, getting as low as 0.2 oc/w/cm 2 sound reasonably low, the issue is one of power density Even the best greases, at 50w/cm 2 still have a 10 degree delta-t which is often 25% or the total thermal budget

10 TIM Materials The obvious solution is to move towards soldered interfaces The key drawback for this approach are economics Adds cost at the semiconductor packaging stage. The packaging houses opt for lower cost and kick the problem downstream. Alternatives such as Carbon NanoTube based materials are in development. This only looks at increasing the thermal conductivity of the TIM material and does not address the underlying issue of the interface of the substrate and the TIM material where most of the challenge lies. Near term solution Be smart, interfaces can make or break a design Recognize this early in the design phase Design out as many interfaces as possible CONTROL the remaining interfaces. Chose the right material

11 Moving the heat Today s high spreader materials Copper, graphite, diamond, heat pipes, vapor chambers Range in conductivities from 400 to 8000 w/mk DARPA - Thermal Ground Plane (TGP) Thermal Ground Plane (TGP) Proposed research should investigate innovative uses of 2-phase cooling, as in common heat pipes, where the benefits include very high thermal conduction and extreme reliability in a light-weight, thin, 2-D package that is also engineered to match the thermal expansion of semiconductor substrates.

12 Moving the heat

13 Moving the heat There are a wide variety of composite materials around labeled as spreader materials but most of these are really developed for their thermal expansion properties. The few that do have high thermal conductivities, such as diamond or pyrolitic graphite are too expensive for most of today s consumer/commercial products.

14 Moving the heat

15 Moving the heat Near term reality Selective use of copper in aluminum heat sinks Heat pipes Vapor chambers Loop thermosyphon Pumped single and two phase

16 Enhancing the heat transfer co-efficient This has been an area of study for as long as most of us can remember. Cross cutting Wavy and dimpled fins Off set fins The issue for most electronics applications is the increased pressure drop resulting in decreased airflow and increased fan power requirements. No free lunch! DARPA MACE program Micro technologies for Air-Cooled Exchangers (MACE) The primary goal of this program is the development and demonstration of air-cooled exchangers that offer significant reductions in thermal resistance (from case to air) and significant reductions in the total electrical power used to force the air through the system.

17 Enhancing the heat transfer co-efficient DARPA MACE program initial designs are incorporating piezoelectric stirrers and synjets to enhance the heat transfer coefficients. An interesting study but likely years away from a commercial use. Diagram from Thermacore Thermacore

18 Enhancing the heat transfer coefficient Thermacore

19 Lets take a look at more near term challenges and solutions One area ripe for an improvement is in the area of air movers. Over the last 20 years there has been steady improvement in fan design giving us long lasting, high performance air movers The issue of noise, pressure capability and efficiency continues to be a challenge Cooling electronics using air quickly reaches a limit due to noise constraints. Improvements in noise, pressure capability and efficiency is one area that is very active. Recent studies have shown; Reducing fan power requirements offers significant energy savings» It has been estimated that 20% of the total power supplied for a high end server is consumed by cooling fans The Green Grid Opportunity- Decreasing Datacenter and other IT Energy Usage Patterns, WP#2 February 16, Higher performance thermal solutions offer short payback in reduced energy usage due to reduced demand on the air movers» Marlin Vogel

20 There are several new products being presented recently claiming to make improvements, the most interesting to me are the miniature blowers being developed by Bergquist, Nidac and others The other interesting technology are Synjets based on a quite old technologies of bellows and venturi Ancient water bellows

21 Liquid cooling Liquid cooling is often treated as the black sheep of cooling even though it offers the best performance next to direct immersion or spray cooling. A lot of this is due to the G5 issues a few year ago. (That was a poor design) Liquid cooling is simply a way to move heat efficiently The drawback for liquid cooling are primarily in two areas. First and foremost is the risk of a leak. From observations, the leak risk is more a human factor than a technical factor. Liquid cooling systems can be designed to be leak free. Tubing, fittings, etc., are available that meet the requirements for reliability. The problems occur when costs cutting is done or when user induced damage occurs. The second concern is over cost. A liquid cooling system may cost several times that of a passive system but the cost/benefit analysis needs to be done at the system performance level and not the component level. We will continue to see the adoption of liquid cooling. One key example of this is IBMs move back to using liquid cooling. For a good look at a liquid cooling system that meets todays requirements take a look at the Coolit systems.

22 Summary Trends & Challenges in Thermal Management

23 Summary Trends & Challenges in Thermal Management TIM materials There is room for significant improvements in these interfaces Equally important is the use of the right material Compression, pressure, and bond line thickness are all critical factors The wrong material can result in performance less than what would be expected. Air Movers The noise, pressure capability and energy usage are all areas being addressed by fan manufactures Better performing thermal solutions reduce the amount of air required Heat Spreaders/Movers Current technology, heat pipes and vapor chambers meet most of the demands for today s designs except the CTE issues Pressure for thinner products is resulting in improvements in the basic designs/construction Liquid cooling Advances in products designed for electronics applications has resulted in performance, reliability and costs that meet the needs of the market