Fabrication of TPG* Thermal Management Parts

LED Headlamp Cooling Enabled by Thermal Pyrolytic Graphite (TPG*) Materials via S-Bond Joining

By          Dr. Ronald Smith, S-Bond Technologies
Dr. Wei Fan, Momentive Performance Materials Inc.

Automotive LED headlights present a thermal management challenge where conventional aluminum- and copper-based heat sinks limit the maximal power loading to LEDs. Thermal Pyrolytic Graphite (TPG*) materials are now being designed into high power LED headlight assemblies to help thermally manage LED (light emitting diode) heat. TPG* materials contain millions of highly-oriented stacked graphene planes forming excellent in-plane thermal conductivity (>1500 W/m-K) with very low density (2.25 g/cm3). TPG-metal composites can simultaneously achieve high thermal conductivity from the TPG core and high mechanical strength from the metal shell.

In testing conducted by Momentive Performance Materials Inc. (“Momentive”) on prototype headlights, data suggests that replacing aluminum fins with metallized TPG plates can reduce total system thermal resistance by 27%, and by inserting a TPG core underneath LED dies, an additional 24% reduction in thermal resistance can be achieved. Furthermore, testing of this integrated headlight assembly demonstrated that TPG material-assisted heat dissipation at these two strategic locations can allow for a twofold increase in power load to the LED.

Various new bonding processes, including diffusion bonding, epoxy bonding, S-Bond soldering and brazing, have been developed to integrate TPG material into metal-encased TPG-metal composites, such as aluminum, copper, tungsten copper, molybdenum copper and ceramics made through encapsulation processes. TPG-metal composites typically behave like solid metal and can be further machined, plated or bonded to other components to meet various requirements.

TPG-metal composites have been incorporated into Momentive’s TC1050* heat spreaders, TMP-EX heat sinks and TMP-FX thermal straps where they can quickly conduct heat away from the heat-generating sources; therefore, greatly increasing LED cooling efficiency and life. A baseline LED conventional headlight assembly (see Figure 1) shows the LED die bonded to a heat spreader and thermally connected to a cooling fin and active fan system. Thermal analysis of this baseline LED headlight indicated that integrating high-thermal conductivity TPG material into the aluminum heat sink base and aluminum heat sink fins could make the greatest impact in increasing heat dissipation of the LED headlight.

HeadLamp1Figure 1.  The baseline LED headlight (VLEDS 9006) illustrating key components for  thermal management.

Next, TPG materials were integrated into various parts of a LED headlight assembly. In this study, three proposed designs were validated in simulation, built and then tested, as shown in Figure 2.

In Design 1, the fin heat exchanger from the baseline assembly was replaced with straight aluminum fins with similar heat dissipation performance.

HeadLamp2

 

Figure 2.  New designs with TPG material integrated at heat sink fins and heat sink base to improve heat dissipation. Image: Momentive

 

In Design 2, the aluminum fins were replaced with Momentive’s TMP-FX thermal straps metallized with S-Bond solder. The high-thermal conductivity TPG material spread the heat more uniformly across the entire fin structure; thus, utilizing the fin surface more efficiently. The thin S-Bond coating not only protected the TPG material from moisture and abrasion, but also enabled soldering of TPG material directly to the aluminum base to minimize any interface heat resistance.  Temperature mapping through measurement, as shown in Figure 3, revealed that a thermal resistance of 4.7 oC/W, which was 27% less than the measured resistance of Design 1, was achieved.

HeadLamp3Graph

Figure 3. Measured LED junction temperature (subtracted by ambient temperature)
as a function of input power per LED for the three headlight designs. Note: Test data. Actual results may vary.

In order to facilitate the heat flow from the LED die to the heat sink fins through the narrow neck area, a T-shape TPG tile with 2 mm thickness, as illustrated in Figure 2 as Design 3, was embedded into the aluminum base. This TPG tile, which presented a thermal conductivity 8 times higher than that of aluminum, was first metallized and then brazed into an aluminum enclosure with a T-shape cavity at nearly 600 oC. High temperature braze joints between the TPG material and aluminum and between the aluminum enclosure components provided excellent thermal interfaces and high bonding strength. More importantly, the braze bond endured the downstream S-Bond soldering temperature when the LED dies and TPG fins were attached. This TPG-embedded aluminum heat sink base is a TC1050 heat spreader that can be made via brazing or diffusion bonding processes. With the heat dissipation assist from the TPG tile in the heat sink base, in combination with the S-Bond-joined heat sink fins, the measured thermal resistance of Design 3 was 3.0 oC/W, another 24% reduction as compared to Design 2 (see Figure 3).

Thermal simulation conducted on Designs 1, 2 and 3 with input power of 15 W per LED (30 W total) is illustrated in Figure 4. The simulation results showed that the TPG S-Bond soldered fins in Design 2 reduced the temperature gradient along the fins; hence, increasing the effective area dissipating heat to the air circulation. In the case of Design 3, the T-shape TPG tile in the heat sink base further decreased the temperature gradient between the LED die to the heat sink fins. Compared with Design 1, the combination of the TPG heat sink fins and TPG heat sink base in Design 3 allowed a significantly lower LED temperature.

HeadLamp4thermalimage

 

Figure 4. Simulated temperature profiles of LED headlights with Design 1, 2 and 3, respectively. Image: Momentive

These studies, conducted by Momentive on its TPG materials and components, have shown that S-Bond metallization and soldering presents a key element to the manufacture of LED headlamps with improved cooling.

If you want to learn more about joining TPG materials, or to otherwise fabricate thermal management components, please Contact Us at S-Bond Technologies or Contact Momentive for data and information on their TPG materials and/or devices.

*TPG and TC1050 are trademarks of Momentive Performance Materials Inc.

 

Fabrication of Hybrid Cu-Al Finned Heat Sinks

DSCN7334Copper has superior cooling capacity than aluminum and is the preferred heat sink material for telecommunications and high power electronics. However, the weight and cost or copper limits the size of the heat sink packages. Therefore for larger electronic enclosures a hybrid design, using copper for a localized heat sink joined to an aluminum frame with good thermal contact can significantly improve the cooling performance of a heat sink package.

Joining copper to aluminum poses it challenges. Cu and Al cannot be welded easily due to the intermetallics that form when Cu alloys with Al in the weld pool. Alternatively, brazing cannot be done since melt point of aluminum is below the typical Cu-Ag braze filler metals (silver solders) used to braze copper. These issues leave “soldering” as the metal filler joining process of choice. But alone, soldering of Cu to Al has challenges. Solders, typically Sn-Ag based, cannot easily wet and adhere to aluminum without first plating the aluminum with nickel or using very aggressive chemical fluxes which themselves are incompatible with soldering to copper.

S-Bond Technologies, working with its customers has demonstrated its active solder, S-Bond 220-50, join Cu to Al in all configurations. The figures below show an example of where a copper- finned heat sink assembly was S-Bond joined into a finned aluminum package. In this assembly, the Cu-fins were individually S-Bond soldered into copper heat sink base, after which the Cu fin-base assembly was then S-Bond joined into the aluminum base at 250C. This soldering temperature well below the softening temperatures for the aluminum frame and low enough that the thermal expansion mismatch between Cu and Al did not distort the bonded assembly when cooling.

Hybrid heat sinks, combing the thermal benefits of copper with lightweight aluminum are taking advantage of the capabilities of active solder joining. For tough dissimilar materials and copper and aluminum bonding challenges, Contact us.

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S-Bond Helping Convert Braking into Energy in Formula One Race Cars

Kimi_Raikkonen_2009_Belgium_3How do Formula One high performance cars covert braking energy, later into acceleration to conserve fuel and gain speed… using the The Kinetic Energy Recovery System (KERS). Formula One race car makers have adopted this technology to improve racing performance. When braking, a fly wheel is activated to run a generator that charges on-board batteries. Read more about S-Bond Helping Convert Braking into Energy in Formula One Race Cars

Solder Bond vs. Epoxy Bond as Thermal Interface Materials

Thermal interface materials are materials used in creating heat conductive paths at interfaces between components and thus reduce thermal interface resistance. These materials permit more effective heat flow between separate components where heat is being generated to a heat dissipation components such as solid state transistors to heat sink or a high frequency device connected to a heat spreader. Thermal interface materials’ purpose is to fill the air gap that occurs at contact interfaces with more thermally conductive compounds to permit more effective heat flow than poorly conductive air.

There is a wide variety of thermal interface materials (TIM’s); thermal greases, phase change polymers, thermal tapes, gap filling pads, filled epoxies and solders. All having various costs, performance and manufacturing challenges. Read more about Solder Bond vs. Epoxy Bond as Thermal Interface Materials

Aluminum Bonding and Heat Management in Manufacturing

Abstract CPUThink of the smartphone you hold in your hand, or of your tablet or laptop. The amount of processing power they contain dwarfs offerings from just a few years ago. Theoretically, the parts should be much hotter from doing so many more operations. They are not and one of the biggest reasons why is aluminum soldering and aluminum bonding applications.

Basics of Thermal Management in Manufacturing

Many of the advancements for electronics manufacturing, and other industries that benefit from aluminum soldering, revolve around miniaturization. Parts are smaller, which in some cases mean they are more fragile. In the case of solid-state lighting and LEDs, the advancement in efficiency comes at a price as well: the area of the product where the light is created can hit high temperatures of up to 150 degrees Celsius at the absolute maximum, according to Cree LED data sheets. Read more about Aluminum Bonding and Heat Management in Manufacturing

Joining of Heat Pipes and Vapor Chambers

Figure 1. Illustration of vapor chamber heat spreader with CPU heat source

Figure 1. Illustration of vapor chamber heat spreader with CPU heat source

Heat pipes and vapor chambers are used to transfer and/or spread heat from concentrated heat sources such as high brightness light emitting diodes (LEDs) and high computing speed CPUs. These active thermal management devices are enclosures/tubes that have porous wick materials lining the walls that provide condensation surfaces and small connected pores that via capillary force, transfer condensed fluids that were originally vaporized at heat source surfaces. When the vapor is transported via convection to the cooler surface to condense, the fluid is then channeled back to the heat source surfaces in a continuous cycle, in effect pumping the heat out of the package without using external power surfaces. Figure 1 illustrates a vapor chamber used to cool a mounted CPU.

Figure 2. Light emitting diode package bonded to vapor chamber

Figure 2. Light emitting diode package bonded to vapor chamber

Thermal management is critical in the life and performance of such electronic components that all employ a variety of thermal interface materials (TIMs). With increased power and speed, the polymer-based TIMs being used today are limiting and metal bonding with solders is growing in application. Conventional Sn-Ag soldering temperatures can overheat the thermal fluids in heat pipes and chambers while Indium (In) solders are expensive and do not bond as well as active solders. Responding to this need, engineers at S-Bond® Technologies have announced its latest alloy, S-Bond® 140 as an effective TIM for bonding CPUs or LEDS to heat pipes and vapor chambers.  The Bi-Sn-Ag-Ti alloy can wet and join to all metals including aluminum and to most ceramics and glasses.  S-Bond® 140 is lead free, does not require plating and flux thus keeping electronic and LED packages clean.

Figure 3. S-Bond 140 bonded heat pipe assembly

Figure 3. S-Bond 140 bonded heat pipe assembly

Figure 2 illustrates a high brightness LED array that has been bonded to a Ni-plated copper vapor chamber with S-Bond 140 solder. This technique provides a high strength and high thermal conductivity metallic solder bond. Figure 3 is another example showing S-Bond 140 solder bonding copper heat pipe tubes (water as the phase change fluid) into aluminum slots to enhance the cooling from the heat pipe to the aluminum package base without plating and flux.. Normally when soldering heat pipes over 200°C, the water in the heat pipe goes to vapor and the resultant pressure distends/distorts the thin copper tube walls. Lower temperature metallic solders, such as S-Bond 140.

Contact us for more information and to order our S-Bond products.

S-Bond Joining of High Brightness LEDs

S-Bond active solder joining is emerging as an effective method to bond heat sinks to the back of High Brightness Light Emitting Diodes (HBLEDs). Active solders can wet and adhere to many of the thermally conductive ceramics (AlN, BeO, etc.) that are being used in HBLED’s and enable effective and thermally stable and conductive joints. Read more about S-Bond Joining of High Brightness LEDs