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.

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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.

 

Rotating Graphite:Metal Seals

2014-06-20 036Carbon/graphite compressor seal rings are employed in many compressors and more and more metal backed graphite seals are being used in higher efficiency compressors. Frictional heating of seals can degrade metal backed graphite seals, therefore good thermal contact between the graphite seal ring and the metal backing is needed to improve cooling of the seal. S-Bond Technologies has developed active soldering methods for graphite bonding which is now being used to manufacture rotating metal backed graphite seals.

In the process, graphite rings are initially S-Bond metallized to create a chemically bonded solder to graphite interface. The sequence of bonding is shown in the figures below. After metallization the metal rings’ bonding surface are pre-tinned with S-Bond filler metals, via heating, melting of the solder surfaces on the metal followed by mechanical activation (ultrasonic solder tip agitation). The metallized graphite ring surface, at 250C is pressed against the pre-tinned metal backing ring surface and then the assembled ring is cooled to solidify the solder joint.

These S-Bond solder joined graphite to metal backing ring seals have endured 1,000’s of hours of running in natural gas compressors and are providing the customer with improved graphite – metal backed ring seal performance.

Please Contact Us for your graphite to metal bonding solutions.

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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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Carbon:Carbon Joining for Fermi Lab’s Particle Physics Detectors

2013-03-25 054S-Bond Technologies’ active solder joining solutions have been used by by Fermi National Accelerator Laboratory for joining carbon:carbon and pyrolytic graphite in its particle accelerator program.  The improved Forward Particle Detector (FPIX) is to be used in Compact Muon Solenoid (CMS) and used for high-resolution, 3D tracking points, essential for pattern recognition and precise vertexing, all embedded in a hostile radiation environment.

The challenge posed by Fermi Lab to S-Bond Technologies, was to create high thermal conductivity metallic bonds between the ends of thermally pyrolytic graphite (TPG) blades and carbon:carbon composite end walls of a turbine nozzle like assembly. Figure 1(a-b) shows ¼ of a full assembly that was built using S-Bond joining and S-Bond 220 solder and processing. S-Bond’s active solder processing was successfully demonstrated and is in the technical roadmap for assembling Fermi Lab’s particle accelerator FPIX.  In this work, regular adhesives were not conductive and would off-gas in the extremely high vacuum environments, hence the selection of the active metal solder filler, S-Bond 220.

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Figures 2 – 4 show various assembly steps that all utilize the S-Bond carbon:carbon joining process described elsewhere in our technical blog. The process started with the S-Bond metallization on the TPG blades, Figure 2,  and the C:C nozzle endwall slots

2013-02-26 407where the blades were inserted, Figure 3. After metallization, the blade and nozzle segments were inserted into a heated alignment press, Figure 4. After heating and insertion, the nozzle segment/ TPG blade assembly was cooled and removed. Figures 1a – b, above, showed the final fully bonded assembly.

 

 

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Please contact us if you have challenging graphite / carbon joining applications where S-Bond Technologies active solders can solve your joining problems.

Active Solders For Solar Panel Manufacture

In recent years, active solders have made their way into use in solar panel manufacture. To understand where S-Bond solder alloys are being used one has to understand the solar panel construction. Solar panels consist of arrays of solar cells, soldered together. A solar cell consists of three basic elements, top contact, base and rear contact, as shown in Figure 1. From the back of each solar cell, electrical contact needs to be made between these surfaces to close the circuit and provide an electron path as photons emit electrons in the semiconductor polysilicon photovolatic (PV) cell which migrate to the back of the exposed cell surface, as illustrated.

Solar Cell IllustrationFigure 1. Illustration of solar cell.

Read more about Active Solders For Solar Panel Manufacture

S-Bond Helping Recycle Water on International Space Station (ISS)

International Space StationCarrying water to the space station is a real challenge and cost, hence recycling water is critical. Waste water, sweat and other ISS water is constantly recycled in a complex system that evaporates and condenses clean water for reuse. For more information on the space station recycling system see the following link: Water Recovery System.

Read more about S-Bond Helping Recycle Water on International Space Station (ISS)

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

Bonding Graphite Felts To Metals

Graphite and carbon felts are increasingly being used for applications in solar, LED, medical, semiconductor, automotive and energy storage systems.

S-Bond Technologies (SBT) has customized it S-Bond (S-B) processes to bond graphite felts to metals for such applications. In the process, graphite felts are first S-B metallized using a paste and vacuum process to create a metallurgical bond between graphite and the SBT active solder filler metals. After S-B metalizing of the graphite, only the tips of the felt surfaces are metallized and thus solderable to the opposing metal plates that are coated with SBT active solders.  The S-B metallization process does not fill the open cells of the graphite felts with S-B alloys, hence its advantage for bonding open cell structures.
Read more about Bonding Graphite Felts To Metals

Applications For Aluminum Soldering

Aluminum soldering is used in making small area electrical and/or thermal connections or seals to other metal or ceramics, while aluminum bonding is used to join large areas either for thermal and/or structural purposes. Aluminum soldering finds applications in sensors, electronics, and electrical power where aluminum contact and/or wire leads are being utilized. Aluminum soldering has also been used as a means to seal and/or repair aluminum heat exchangers.

We have been contacted many times for assistance in solving the problem of small contact to aluminum without the use of aggressive chemical flux or cases where the chemical flux for aluminum was not compatible with the metals of the opposing side of the joint. Additionally, in many electronic packages the use of corrosive aluminum soldering fluxed are limiting When faced with these choices, active fluxless solders such as S-Bond become a good solution. Read more about Applications For Aluminum Soldering

Epoxy Bond vs. Solder Bond Applications

Bond assembly can be done via 1) mechanical attachment, 2) adhesive bonding of which epoxy bonding is one form of adhesive, 3) soldering bonding using lower melting filler metals (< 450˚C), 4) brazing using filler metals melting above 450˚C, and 5) welding such as resistance welding bonding, ultrasonic welding and friction weld bonding that uses locally melted parent metal.

Bonding is done for a variety of technical reasons a) mechanical attachment, b) thermal contact, c) electrical contact d) gas or liquid seal, or e) any or all combinations thereof. The choice of bonding method will then depend on the intrinsic properties of the bonding filler materials ( i.e. hermetic, electrical conductance, thermal conductance, thermal coefficient of expansion, adhesive bond strength related to the intrinsic fillers’ mechanical properties, and their adhesive and cohesive strengths). Read more about Epoxy Bond vs. Solder Bond Applications