Extreme Environment Advancements
05/01/2007
Extreme-environment electronics operate in conditions outside the parameters of mainstream electronics. Space exploration, oil and gas drilling, electric vehicles, and automotive electronics are all growing areas for technology advancements.
For example, lunar temperatures range from -180oC (night) up to +120oC (day), to -230oC in permanently shadowed craters. While radiation exposure is low, 100 krad, solar activity creates potential for radiation-induced single event effects. The Mars environment is -120oC to +85oC with radiations levels of 300 krad. At the other environmental extreme is Venus with a surface temperature of +485oC and a pressure of 90 bars.
A team lead by the Georgia Institute of Technology is developing silicon-germanium (SiGe) integrated electronics for extreme environments under the Radiation Hardened Electronics for Space Environments (RHESE) program. The objective is to develop electronics to operate directly in lunar and Martian environments without temperature-controlled warm boxes. The wide temperature ranges, and the change in mechanical properties (particularly polymers) at low temperatures, are packaging challenges for these devices. At the low temperature extreme, creep, diffusion, intermetallic formation, and grain growth are not an issue. However, at 120oC, these can become problems depending on the materials selected. With a 300oC ΔT, even small coefficient of thermal expansion (CTE) mismatches can result in high stress levels at interfaces between dissimilar packaging materials.
Silicon carbide (SiC) and gallium nitride (GaNi) device technologies are being developed for Venus’ environment. The surface temperature of Venus does not vary, so thermal cycling is not a major consideration in the packaging design. However, the choice of packaging materials for 485oC operation is significantly limited. For example, nickel (Ni) is commonly used as a barrier between Cu and Au in conventional packages. At 300oC, these materials interdiffuse - Ni is not an effective barrier for long-term exposure. Diffusion, intermetallic formation, and grain growth are critical factors and must be considered when selecting metallizations. SiC devices have been demonstrated to operate at temperatures up to 600oC and high-temperature MEMS sensors have also been fabricated in SiC.
Down-hole electronics are used for well logging during drilling and to optimize production post drilling. The well environment is corrosive and hot, and the electronics are subjected to high shock and vibration levels during drilling. The operating life is relatively short (~1000 hours) for drilling tool electronics, but can extend to 20-30 years for down-hole tools used to monitor the well in the production phase. Bulk Si is used at the lower temperature ranges, while silicon-on-insulator (SOI) devices are used at the higher temperatures. Aluminum wire-bond pads typically used on SOI devices are incompatible with gold thermosonic wire bonds at temperatures above 200oC for long periods of time. However, aluminum wire-bond pads plated with electroless nickel/electroless gold have shown good mechanical and electrical performance after 10,000 hours at 300oC with thermosonic gold wire bonds.
More electric vehicles are being developed for military and commercial use. In some cases, the high-temperature operating environment of these vehicles is due to the engine’s ambient environment, or dissipation from the power electronics. Even at 95% efficiency, a 50 kW power system must dissipate 2500 Watts. SiC power devices are being developed for these high-temperature, high-power applications. Thermal cycling is an issue; for example, copper is required for its high current-carrying capacity and ceramic is needed as a high-temperature insulator, but the CTE mismatch between Cu and ceramic (Al2O3 or AlN) results in ceramic fracture when thermal cycled between -55oC and +250o-300oC.
Time itself is an extreme environment. Automotive electronics have a life expectancy of 10+ years, while many military systems will remain in the field for 25+ years. While these applications are currently exempt from EU RoHS legislation, the switch to lead-free assembly is inevitable. Studies conducted by Auburn University Ph.D. student Hongtao Ma demonstrate that creep resistance of SAC 305 and 405 lead-free alloys continues to decrease with room-temperature aging for up to 63 days. Thus the alloy properties continue to change with time, even at room temperature. We do not have 25+ years worth of storage data for lead-free solder alloys. This presents an unknown risk for electronics that must last for long periods of performance.
While extreme environment electronics are a small share of the electronics market, research opportunities are significant and challenging.
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R. Wayne Johnson, Ph.D., may be contacted at Auburn University, 200 Broun Hall, Auburn, AL, 36849; 334/8440-1880; E-mail: [email protected].