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.

 

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

S-Bond Repair of Brazed Aluminum Heat Exchangers

S-Bond Technologies has developed and demonstrated a “solder welding” process that is finding application in the repair of brazed aluminum heat exchangers.

Aluminum heat exchangers and cold plates many times are brazed either by dip brazing or vacuum brazing. In these operations, aluminum braze filler metals are added to joint areas as pastes, brazing foils or braze alloy clad aluminum sheets. Depending on the complexity of the braze joint and the assembly, braze joints can on various occasions after the brazing cycle, be found to leak. Leaks at this point cannot be re-brazed since the aluminum braze filler metals cannot be melted without melting the entire component due to the interdiffusion of the silicon from the filler metal into the base metals. Thus brazed aluminum heat exchangers are normally weld repaired… but with limited success. Many times the weld will “chase” the crack and not seal it and the locally high silicon in the braze joint can also create inconsistent welds and if thin walls are part of the aluminum heat exchanger the high local temperatures from the welding process can “blow” holes in the thing gage. With these limitations on weld repair, solder repair is more viable.

Solder repair is viable since it has low heat input and is conducted below 250˚C, provided the solder filler metal can wet and adhere to the rework areas. Conventional soldering aluminum normally requires Ni-plating and or aggressive fluxes which complicate the rework procedures. S-Bond active solders bond to directly to aluminum and fill in crevices in aluminum surfaces without the need for flux and or preplating, thus it can be used to directly fill machined out leaks on braze joint in aluminum. The process consists of 1) locating leak areas (bubble testing is typically used) 2) grinding out the areas through and adjacent to the leaks, 3) deburring and degreasing the machined areas, 4) heating the plate locally or in its entirety to hold the repair area at 250˚C 5) melt the S-Bond filler active solder with the heat in the heat exchanger 6) spread the S-Bond solder into the joint to mechanically activate the solder to enable it to wet and adhere to the aluminum repair areas. NOTE: is has been found that ultrasonic activation using an ultrasonic solder tip can improve the reliability of the repair soldering process. The figures below illustrate the S-Bond solder aluminum repair on aluminum surfaces.


Please Contact Us to see how S-Bond Solder repair of aluminum can be used to salvage leaking aluminum heat exchangers and save on rework costs.

S-Bond Joined Phase Change Materials (PCM’s) Heat Sinks

S-Bond® active solders are being used extensively as a high conductivity bonding solution for foam cored phase change materials foam cored heat sinks. Increasingly, thermal management in electronics is the limiting factor in performance and/or life of electronics as higher power and higher speed in electronic devices generate more intense heat. High brightness LED’s, high speed/high bandwidth telecommunications, avionics, satellites and solid state conversion devices all have transient and steady power states where intense heat is generated and needs to be channeled away from the electronic device to prevent performance loss or permanent damage. The electronic industry is relying on a host of devices from conventional heat sinks with fins and fans to heat pipes and vapor chambers to more exotic materials and composites that include pyrolytic graphite or diamond. S-Bond materials and processes have been proven to be a good solution when bonding these various components and materials with a metallic “thermal interface material (TIM)” rather than filled polymeric bonding agents.

When electronics have a high transient heat output thermal engineers are using “phase change” heat sinks. Such heat sinks utilize “phase change materials (PCM’s)” that when exposed to heat absorb it very quickly and effectively as the material “changes phase”… either going from solid to liquid or liquid to vapor. Materials with high latent heats of fusion or latent heat of vaporization at or near the maximum temperatures that electronics are being used in the core of such heat sinks. In PCM heat sinks during the phase change there is the potential to rapidly absorb a high heat load… that can later be more slowly released to the atmosphere with cooling fins as the phase change is reversed and the heat is released away from the electronic device.

Two of the most used PCM’s are paraffin and water… each has a high latent heat of fusion or heat of vaporization, respectively. The challenge in the use of PCM’s is to overcome their relatively low thermal conductivities. For example, in heat sinks with paraffin as the PCM, when the heat transfers from the electronic device into the heat sink package, the outer layer of paraffin melts and then slows the transfer of heat into the solid paraffin core. To offset this heat flow limitation, designers are incorporating metallic or graphitic foams into the core of the heat sinks. The foams’ cells separate the PCM’s into small reservoirs that are surrounded by high thermal conductivity cell walls that then transfer the heat to a small PCM filled pore in the foam and therefore quickly melts the paraffin or vaporizes the water. Later in the “reverse” cycle, the conductive foam “cell walls” transfer the heat out of the PCM filled pores to solidify or condense the full volume PCM in the heat sink.

S-Bond joining has found excellent application in paraffin based PCM heat sinks in combination with graphitic foams (PocoFoam® or K-Foam®). S-Bond can effectively bond to graphite and graphite foams to heat sink package materials such as aluminum, copper and many heat sinks composites such Al-SiC, Al-Gr, Cu-W or Cu-Gr. In such graphite core/paraffin heat sinks. S-Bond Technologies has S-Bond metallized the Gr-Foam preparing for it to be soldered directly to the heat sink package. After S-Bond metallization, various S-Bond solders and processing can be used to bond the graphite foam to the components of the heat sink.

Heat Sink for Laser Diode Packages

Figure 1. Paraffin “PCM” / Graphite Foam cored heat sink for laser diode packages.

Figure 1 shows a PCM core finned Aluminum heat sink box used to cool high power laser diode packages mounted on the flat side opposite the side with the fins. The aluminum box contains a core of graphite foam bonded to the walls and base of the aluminum enclosure. The aluminum enclosure is then heated to 100˚C and filled and infiltrated with paraffin PCM’s. After filling the enclosure is sealed and the assembly is a PCM heat sink. When the laser diodes are on for intermittent periods of time the graphite foams thermally bonded to the wall of the enclosure heat the PCM… later when the heat load from the diodes are off, the bonded fins with air convection assist, cool and solidify the paraffin PCM to get the heat sink ready for the next thermal cycle. S-Bond active solder joining enabled the graphite foam to have an excellent thermal interface to the enclosure without filling the graphite foam and compromising the graphite foam’s ability to hold and transfer heat quickly to the paraffin PCM.

Heat Sink for Hot Fluid Channeled into Core

Figure 2. Graphite foam cored PCM heat sink for hot fluid channeled into the core.

Figure 2 shows another type of PCM cooling. The alternating bonded fluid circulating aluminum tubes bonded with S-Bond sandwiched between S-Bond metallized and bonded graphite foam plates. The stack is later encased in an enclosure and paraffin PCM is infiltrated into the foam to make a large ~ 24” x 24” x 12” PCM heat sink. When heated fluids circulate in the aluminum tubes the PCM filled graphite foam core rapidly absorbs the heat from the fluid.

Graphite foam wrapped heat pipe PCM heat sink

Figure 3. Graphite foam “wrapped” heat pipe PCM heat sink.

Figure 3 illustrates another style of PCM heat sinks that are mounted around a central heat pipe. In this design, S-Bond metallization of the faces and ID’s of the graphite annular rings permitted a graphite foam outer core to be become an effective PCM heat sink for a heat pipe cored thermal management device.

Contact us to evaluate how S-Bond can be used to enable your thermal management components to be made and how phase change material (PCM) heat sinks can be effectively incorporated into your designs.

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