Integration, cooling and packaging issues for aerospace equipments

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1 Integration, cooling and packaging issues for aerospace equipments C. Sarno, C. Tantolin Packaging Department, TBU Navigation Thales, Aerospace Division Valence, France Abstract Packaging becomes an important issue in aerospace equipments because of high integration and severe environmental constraints. In order to develop products which respond to the specifications at a minimum cost, Thales performs both mechanical and thermal simulations. The simulation level depends on the phases of design (preliminary or detailed). The major challenges are encountered on thermal management problems with power higher than 100 W at the module level and with local hot spot greater to 100 W/cm². Under these conditions, standard cooling approaches using forced air are no longer applicable. To challenge these points Thales has launched European collaboration research programs: COSEE for the development of two phases cooling systems and NANOPACK for the development of thermal interface materials. Keywords-electronics, cooling, heat pipe, avionics, packaging, integration I. INTRODUCTION The packaging and thermal management of electronics equipment has become an important issue because of increased power levels and the simultaneous miniaturization of the devices. With the advent of denser device packaging and faster intrinsic speeds, cost, reliability and size have been improved, but unfortunately packaging and thermal management have not followed at the same speed. As a result, it may be difficult to use the latest technology available (microprocessors for example) in avionics conditions. In the coming years, the electronics industry will face significant thermal management problems in the use of both existing and emerging highly integrated electronic components and modules. The heat removal capabilities of existing cooling techniques are being overtaken in three different areas: increased card, blade or module total heat dissipation (heat power levels higher than 100W), microprocessors presently dissipate 10W and will reach 30 W to 50 W in the coming years increased local heat densities for highly integrated components (heat flux greater than 10 W/cm² and up to 100 W/cm²) maximum use of low-cost plastic components or COTS components (Component On The Shelf) in severe avionics applications. These objectives, up to twice as high as the current practices, are associated with size limitations and higher reliability requirements. As a results packaging plays a major role in the design of the futures equipments II. PACKAGING DESIGN The main causes of failure in airborne equipments are due to: thermal problems, vibrations, thermo-mechanical induced stress and other environmental constraints as fluid resistance, sand and dust The resulting objective of the packaging design is then to develop a product which responds to the specification at a minimum cost and in one shot, it means: to be able to anticipate the behavior of the equipment regarding the above mentioned specifications to make the good choice for the architecture and for the technologies used in the equipment to identify the weaknesses of the design and margins regarding fatigue effects. THERMAL DESIGN SIMU THERMAL INTERFACE MATERIAL THERMAL EXCHANGES EXP SPECIFICATION ANALYSIS INTERCONNECT TECHNOLOGIES MULTI-LEVEL INTEGRATION PACKAGING DESIGN DOCUMENT Figure 1. Packaging design procedure MECHANICAL DESIGN SIMU EXP STRUCTURAL MATERIAL MECHANICAL BEHAVIOR /DATE EDAA

2 In order to achieve these goals, our main solutions are to have an excellent knowledge of the involved technologies and to perform multi-physics simulations. Fig. 1 shows the design procedure used in most of our applications. Generally the mechanical and the thermal designs are performed in parallel. A. Mechanical design The complexity of the mechanical analysis will depend on the equipment function; basically the presence of sensors or sensitive equipments will require more complex simulations and justifications. The tool used in Thales is the ANSYS finite element code [1]. The effort may vary from some ten of hours to some several thousand of hours. In term of mechanical behavior the approach is to check that the resulting levels seen by the sensitive equipment remain below the admissible level (known or verified by tests) all along the spectrum of analysis and under the solicitation generated by the carrier in the zone where the equipment is located. Example of such approach are illustrated on the Ariane Navigation Unit (Fig 2), where the power supply has been designed so that its main resonant mode be located around 500 Hz as specified in the initial frequency allocation plan. Another example (Fig 3) is the design of the mechanical filtering function and dampers of an inertial measurement unit. PCB response expected Rack response measured Power supply Figure 3. Inertial Reference System During the thermal design analysis, we generally perform thermal simulations with the use of computational fluid dynamics software (FloTHERM from Mentors Graphics [2]). Basically, we consider three levels for the simulation which correspond to the three phases of the design (Fig 4): level 1, Equipement level, Preliminary design phase: the simulation just takes care of the rack external constraints. Dissipative PCBs are simulated with volumetric sources. This first algebraic or numerical approach helps us to select the most appropriate cooling technologies (free convection, conduction, forced convection) given a level of power in the package and the available cooling options. The global feasibility with associated design complexity is stated. level 2, PCB level, Preliminary and detailed design phases: The PCB are represented but the functional areas are just modeled as dissipative surfaces. This simulation level gives the PCB temperature and allows the optimization of the mechanical design (copper layers, specific drains, thermal wedge lock ). Level 1: Equipment Figure 2. Ariane Navigation Unit Level 3: Component B. Thermal design From a thermal point of view, the objectives are to take care of the equipment environment (cooling technologies, ambient temperature ) in order to keep the electronics component temperature under an acceptable level (typically 125 C for the junction temperature or 85 C for the ambient temperature). Level 2: PCB Figure 4. From the equipment to the component level Resistive network model

3 level 3, Components level, detailed design and validation phases: All the dissipative components are modeled including their packaging technologies. Thales internal models database are used. This level allows us to reach the junction temperature for each component. The temperature will be used as an input data for the safety and reliability calculations. Typical Mean Time Between Failure (MTBF) for aerospace applications is about 40,000 h.. III. EXISTING COOLING TECHNIQUES The main principles, schematically depicted in Fig. 5, are implemented to cool down the components on a PC board in the aerospace domain: the heat load generated by the components dissipation is directly transferred from the components and the board to a cooling fluid according to various thermal transfer modes, namely radiation and free convection in the air and forced convection with the air (direct air flow). the heat load is conducted from the components to the board, then from the board to heat exchanger where they are rejected to a fluid. In this case, the exchanger can be secured at the board ends or directly on the entire rear face of the board. To minimize the thermal resistance on the board, the board can be fitted with a thermal drain heat pipes. Another alternative is from the components via high thermal conductivity materials to the module shell and then via the heat exchanger surface to a gas or liquid coolant. On the board itself, the thermal transfer mode is conduction (conduction cooled, liquid flow through, air flow around). Among all these techniques the most widespread is direct air cooling. Air is available on most of the platforms, it is simple to implement, and it does not require complex and expensive sealing devices. CONDUCTION COOLED DIRECT AIR FLOW AIR OR LIQUID FLOW THROUGH AIR FLOW AROUND IV. FUTURE COOLING TECHNOLOGIES As mentioned already, with the increase of heat densities following Moore s law, we will face tremendous new challenges in which components heat densities are surpassing 10 W/cm² and will reach 100 W/cm². The standard approach using typical ARINC600 standard cooling conditions in the electronic bay (220 kg/h/kw mass air flow rate in forced convection) are no longer applicable. This global airflow rate cannot cope with the hot spot problems (up to ten time the standard air flow rate would be required). It is necessary to use novel technologies which will be able to offer alternate solutions and which will be compatible with high-integrated electronics and at affordable costs. This challenge is shared between the main European and US aerospace equipment manufacturers that are investigating new cooling options. Within Thales, European collaboration research programs have been launched on both civil and military sides about the thermal management. One of the most promising and investigated route involves phase change systems like heat pipe [3] (HP), loop heat pipe [4,5,6,7] (LHP) or thermosyphon loop. These technologies have been investigated through the European COSEE project for avionics applications. A. COSEE Project The COSEE Project (Cooling Of Seat Electronic Boxes and cabin Equipment) aims at the cooling of electronics equipment for the IFE (In-Flight Entertainment Systems). New generations of In-flight Entertainment Systems are required to provide more and more services (Audio, video, Internet, flight services, multimedia, games, shopping, phone, etc ) at an affordable cost. But unlike other avionics systems installed in temperature controlled bays most of the IFE equipment and boxes are installed inside the cabin, they may be buried in small enclosed zones and they are not connected to the aircraft cooling system (ECS). This situation creates thermal management issues that may affect the reliability, the safety and the cost of the equipment. The most critical equipment is the seat electronic box (SEB) installed under passenger seat. To face the increasing power dissipation, the use of fans will be required with the following drawbacks: extra cost, energy consumption when multiplied by the seat number, reliability and maintenance concern (filters, failures ) The objectives of the project was therefore to develop and evaluate an alternate advanced cooling technique to the fans, based on two phase change passive systems, adequately integrated inside the seat structure and taking benefit of the seat frame as a heat sink or of the aircraft structure when installed in the ceiling. Some samples of seat electronics boxes and IFE architecture are given in Fig 7. Figure 5. Cooling modes Fig 6 gives some exemples of Thales equipment using forced cooling convection. The thermal dissipation still increases: from 10 W/module, its will reach 20/30 W/module in the near future and 60 W/module in the next developments. In the same time, the module sizes are reduced or at the best remain unchanged. Airbus A340/A380 SSJ 100 Figure 6. Computer racks

4 Fig 9 gives an overview of the PCB equipped with HP. In order to test the thermal performance of these two phases technologies, we used dummy PCB with resistive components. This approach allows a correct management of the heat dissipated in each part of the demonstrator. Several thermocouples regularly distributed over the thermal path gives the thermal performance of the cooling systems. SEB Figure 7. IFE Architecture and SEB samples A community collaboration has been established due to the multidisciplinary nature of the problems to be solved and to the fact that the necessary expertise and knowledge do not lie in a sole nation (phase change simulation and design, heat pipe manufacturing, equipment cooling, seat integration and development, aircraft interfaces). The project involved Airbus, airlines, seat manufacturers, research centers and industrial companies (Fig 8). HP LHP AIRLINES (F, D,) SEAT MANUFACTURER 1 AVIO INTERIORS (I) THERMAL RESEARCH CENTER INSA LYON (F) END USER CLUB MAINTENANCE ORGANISATION SEAT MANUFACTURER 2 BRITAX (UK) Aeronautical Research and Test Institute VLZU (CZ) LHP MANUFACTURER EURO HEAT PIPES(B) INSTITUTE OF THERMAL PHYSICS ITP (RU) EQUIPMENT MANUFACTURER THALES AVIONICS (F) Figure 8. COSEE partnerships structure AIRBUS (D) SEAT MANUFACTURER 3 RECARO (D) UNIVERSITY OF STUTTGART USTUTT (D) The cooling of the SEB has been realized with the help of two kind of phase change systems : heat pipe and loop heat pipe. first heat pipe were used to transfer the heat from the dissipating components and the edge of the SEB. In order to increase the heat transfer and to reduce the thermal contact resistance, thermal interface materials (TIM) are used between each mechanical parts. then loop heat pipe were used to transfer the heat from the SEB side to the seat mechanical structure which is used as a heat sink and is cooled by natural convection with the ambient air. LHP are particularly interesting when the heat is transferred over large distance under small temperature differences. Figure 9. SEB equipped with HP and LHP Outside the SEB, two LHPs transfer the heat from the seat to the mechanical structure composed of two mains aluminum rods. The results [8] obtained in this configuration are presented on Fig 10. This graph gives the temperature difference between a representative point of the PCB and the ambient air according to the total dissipated power inside the SEB. Three cases are considered: without LHP: the SEB is only cooled by natural convection and is not linked to the seat mechanical structure, with LHP horizontal: the seat is in horizontal position and the two LHP are connected to the mechanical structure with LHP (22 tilt): the seat is tilted up the 22 in order to evaluate the sensitivity to the tilt of the plane We can note that the use of loop heat pipe induces a tremendous improvement in the thermal management of the PCB: increase of 150% of the heat dissipation capability: from 40W up to 100W with a constant PCB temperature (about 60 C difference between the PCB and the ambient). for a same dissipated power, for example 40W, the use of HP and LHP allow 32 C decrease on the PCB temperature without the use of fans

5 Tpcb1 - Tair ( C) AVIO Seat with LHP from ITP Power dissipated by Loop heat pipes : 58 W SEB Power (W) SEB Power (W) Figure 10. Thermal results Without LHP With LHP (horizontal) With LHP (22 tilt) Additional tests were performed in order to check the conformity of the cooling systems with the mains avionics specifications. These tests include: linear acceleration (up to 9 g 3 minutes in each axis) vibrations (according to DO160 Curve C1) climatic tests (performance evaluated between 25 and 55 c ambient temperature) thermal shock (-45 C/+55 C, 5 C/min) The seats have been submitted to all the different tests without damage. The loop heat pipes have had a good thermal behavior. In addition to these operational tests, long-term investigations were conducted by the phase change manufacturers without damages. In these first tests, the seat mechanical structure was made from aluminum. We have also tested seat made of carbon composite structure. Compared to the aluminum, this material has a rather poor thermal conductivity, thus the results are slightly under those obtained with aluminum: increase of 80% of the heat dissipation capability (from 38W up to 70W with a constant PCB temperature) for a same dissipated power (40W) the use of HP and LHP allow 20 C decrease on the PCB temperature. Nevertheless these results are of great interest in term of maximal power dissipation and reliability. Two-phase systems have a great potential for use in future avionics products. B. NANOPACK Project The thermal efficiency of cooling systems needs to integrate all the thermal resistances, which are encountered on the thermal path from the components to the ambient. According to the high integration of avionics application and regarding to the high heat flux density, one of the bottlenecks of the thermal path is thermal interface resistance. In order to reduce these resistances, thermal interface material (TIM) are widely used in the electronics industries. Thales is coordinating an European project named NANOPACK (Nano Packaging Technology for Interconnect and Heat Dissipation) which is a European large-scale research project aiming at the development of new technologies and materials for low thermal resistance interfaces and electrical interconnects. It explores the capabilities offered by nanotechnologies such as carbon nanotubes, nanoparticles and nanostructured surfaces, and by using different enhancing contact formation mechanisms, compatible with high volume manufacturing technologies. The objectives are to develop materials having an intrinsic thermal conductivity up to 20 W/m.K and leading to thermal resistance lower than 5 K.mm²/W with bond line thickness lower than 20 mm. This FP7 project is involving the main European players in the nanotechnology field. The NANOPACK Consortium is made up from 14 partners from 8 European countries and is well balanced by including 5 large companies, 5 universities or public bodies and 4 small or medium enterprises (list of partners on The results to date are [9,10,11,12]: High thermal performance on adhesives products. Two remarkable products have been developed: silver flakes and micro silver spheres have been dispersed in monoand multi-epoxy matrix. The respective thermal conductivities are 6 and 9.5 W/m.K. These adhesives are electrically conductive ( Ω.cm). The shear strength of the mono-epoxy matrix products has been measured to 14MPa which is also remarkable and suggests excellent mechanical and reliability properties. A surface modification technique based on micromachined hierarchical nested channels (HNC) has proven its efficiency to reduce the final bond line thickness by > 20% for the majority of TIMs on cm² interfaces. Gold nanosponge surface enhancement technique is also under evaluation to decrease interfacial contact resistance. Properties of randomly distributed and aligned Carbon Nanotubes (CNT) are currently studied. A specific process has been established to manufacture a metal-polymer composite with effective thermal conductivity as high as 20 W/mK. Development of thermal tester for thermal pastes and greases. The tester is build according to the ASTM standard D (achieved accuracy ±1 Kmm²/W) and also measures thermal interface material s thickness (with ±2μm accuracy). A specific apparatus has been set up to measure the electrical conductivity of electrically conductive adhesives (resistance >50μW with 5μW resolution). All these activities are also supported by high-end modeling and simulations using molecular dynamics and analytics methods. The benefits of the technologies will be evaluated in different applications to demonstrate improved performance of

6 microprocessors, for aerospace and automotive high power electronics and high power radio-frequency switches. V. CONCLUSION Packaging design in avionics applications requires a correct management of both mechanical and thermal issues. Simulation is an efficient way to improve and accelerate the design. Thales uses ANSYS finite element code for the mechanical approach and FLOTHERM finite volume code for the thermal approach. These codes help us to design at a minimum cost and in one shot. Nevertheless regarding to the integration constraints, standards solutions based on forced ventilation cannot cope with hot spot problems and new cooling technologies need to be explored. Thales has launched European collaboration research programs about thermal management. The first program mentioned concerns two phases systems for the cooling of a cabin equipments. The results exhibit a tremendous improvement in the management of the PCB with a strong increase of the heat dissipation capability of the equipment. Furthermore these systems exhibit a good behavior regarding to the mains avionics specifications. However this technology requires the use of many thermal interfaces; thus the optimization of the whole thermal path implies to improve the performance of the thermal interface material generally used in electronics applications. This approach is explored in the second European project NANOPACK. The first materials developed to date exhibited good thermal characteristics close to the objectives of 20 W/m.K. ACKNOWLEDGMENT This work was supported by the EC/FP6 COSEE Project (contract no ) and the EC/FP7 NANOPACK Project (contract no ). The authors gratefully acknowledge the support from the EC. REFERENCES [1] ANSYS, Structural Mechanics solutions, 6 p [2] FloTHERM, Optimizing Thermal Design of Electronics, 7 p., [3] G.P. Peterson, An introduction to heat pipe Wiley Interscience, 1994, 356 p [4] Y.F. Maidanik, Loop heat pipes Applied Thermal Engineering, 2005, Vol. 25, p [5] S. Launay, V. Sartre, J. Bonjour Parametric analysis of a loop heat pipe operation: a literature review. Int. J. Therm. Sci., Vol. 46, N 7, p (2007). [6] R. Revellin, J. M. Quiben, J. Bonjour, J. R. Thome, Effect of local hot spots on the maximum dissipation rates during flow boiling in a microchannel IEEE Transactions on Components and Packaging Technologies, Vol. 31 (2), p (2008) [7] S. Launay, V. Sartre, J. Bonjour, Simulation du fonctionnement d une boucle de fluide à pompage capillaire dans un système embarqué Congrès français de Thermique, SFT 2007, Ile des Embiez, 29 mai - 01 Juin 2007 [8] C. Sarno, C. Tantolin, S. Parbaud, Cooling of seat electronics boxes with loop heat pipe, IMAPS 2009, La Rochelle, France, 4-5 february, 2009 [9] S. Demoustier, A. Ziaei, C. Sarno, Nanopack-nanop packaging technology for Interconnect and Heat Dissipation, IMAPS 2009, La Rochelle, France, 4-5 february, 2009 [10] J. Liu, T. Wang and E.. Campbell, Thermal management technologies for electronics based on multiwalled carbon nanotube bundles, IEEE Nanotechnology Magazine, Vol 3 No 1, March 2009 [11] B. Smith, W. Glatz, and B. Michel, Mini- and Microchannels in Thermal Interfaces: Spatial, Temporal, Material, and Practical Significance, Electronics Cooling Magazine, Feb. 2009, Art. 2. [12] Y. Pennec, B. Djafari Rouhani, H. Larabi, J. Vasseur and A.C. Hladky- Hennion, Low frequency gaps in a phononic crystal constituted of cylindrical dots deposited on a thin homogeneous plate, Phys. Rev. B, 78, , 2008