Electronics Packaging for the Space Environment

1. Electronics Packaging for the Space Environment Zach Allen ECEN 5004: Fundamentals of Microsystems Packaging

2. Overview • Introduction • Overview • Space: extreme heat and cold • Packaging considerations for the vacuum of space • Packaging for the space radiation environment

3. Space: Extreme Heat and Cold • Definition of temperature: atomic and molecular motion • It is difficult to give space a “temperature” • The darkness of space: just a few degrees above absolute zero (2.7 Kelvin) • There are a few stray helium atoms floating around

4. Space: Extreme Heat and Cold • Temperature of an earth-orbiting satellite varies considerably • Sunward side: outside surfaces can exceed +140°C • Shadow side: outside surfaces can reach -150°C • Depending on period of orbit: satellite can see these temperature extremes several times per day • Huge challenge for packaging of electronics! • Electronics usually have maximum operating temperatures of -30°C and +70°C

5. Space: Extreme Heat and Cold • PWAs usually packaged within an aluminum or magnesium box • Shields electronics from extreme heat/cold transitions that outer surfaces of spacecraft experience • Good conductor of heat between PWAs/spacecraft • Aluminum: Lightweight (approx. $100,000/lb to put something into Low Earth Orbit)

6. Space: Extreme Heat and Cold • Key method of power dissipation on Earth-borne electronics assemblies is convection • Vacuum – heat dissipation methods: • Conduction: heat transfer from parts to satellite heat management system • Radiation: heat dissipation method for satellite thermal management system

7. Space: Extreme Heat and Cold • Spacecraft thermal control system • Series of pipes that conduct heat between different parts of the spacecraft • Heat dissipated through radiators on spacecraft • Heaters are used in some cases • PWA temperature is controlled to within operating limits

8. Space: Extreme Heat and Cold • NASA: developing the Space Technology 8 mission: • Launch in Feb 2009 • High-performance onboard CPU • Highly efficient solar panels • Ultra lightweight solar mast • “Thermal Loop” heat management system

9. Space: Extreme Heat and Cold • “Thermal Loop” heat management system • Based on loop heat pipe (LHP) design • Deployable radiators: radiate heat from both sides • Flow Regulator prevents heat from being transmitted back to the instruments • Variable Emmitance Coatings (VEC) change emmitance of radiators and eliminate need for supplemental heaters

10. Space: Extreme Heat and Cold • Summary of Spacecraft Thermal Management System: • Heat conducted: • From parts to boards • Boards to enclosures (usually aluminum) • Enclosures to spacecraft thermal management system • Radiated into space • Sometimes process happens in reverse (using onboard heaters)

11. Packaging Considerations for Vacuum of Space • “Outgassing” of materials occurs in vacuum • Ambient pressure of space is less than 10-6 Torr (10-9 atm) • Materials actually lose mass (evaporate) • Evaporated materials condense on nearby surfaces • Biggest risk is optical contamination • Best case: mission success is limited – fuzzy or blurry images • Worst case: mission failure

12. Packaging Considerations for Vacuum of Space • Two main parameters to characterize outgassing • Total Mass Loss (TML) ≤ 1.0% • Collected Volatile Condensable Material (CVCM) ≤ 0.1% • Sample of material is vacuum baked at 125°C, 10-6 Torr • End mass compared to initial mass (%TML) • Weight of a clean collector compared to weight of collector having condensed materials (%CVCM)

13. Packaging Considerations for Vacuum of Space Outgassing characteristics of selected materials from: http://outgassing.nasa.gov/cgi/sectiona/sectiona_html.sh

14. Space Vacuum – Tin Whiskers • Hair-like crystalline structures that grow from pure-tin surfaces • Typically 1mm in length, 1µm in diameter • Observed up to 10mm long • Exact cause is still unknown – physical stress may cause growth: • Compressive stress from screws or other fasteners • Bending or stretching • Scratches or nicks • Mismatch of CTE between Tin surface and substrate

15. Space Vacuum – Tin Whiskers • Hazard is undesired electrical connections • Whisker bridges gap between two isolated conductors – two scenarios: • Stable short circuit forms in low voltage, high impedance circuit (takes more than 50mA to fuse a whisker) • Metal vapor arc occurs when whisker vaporizes into plasma of highly conductive metal ions • Can be capable of conducting several hundred amps for several seconds

16. Space Vacuum – Tin Whiskers • Dr. M. Mason and Dr. G. Eng at The Aerospace Corporation performed research on Tin plasma arcs: • Sustained plasmas form at power supply voltages as low as 4V • Plasma duration increases with power supply voltage

17. Space Vacuum – Tin Whiskers M. Mason and G. Eng Tin Plasma Arc Experiment Setup Simulated tin whisker: 25 to 50µm diameter tin wire

18. Space Vacuum – Tin Whiskers M. Mason and G. Eng Tin Plasma Arc Experiment Results Tin Plasma Duration vs. Power Supply Voltage Tin Plasma Arc in Vacuum Chamber

19. Space Vacuum – Tin Whiskers DEC 2005, critical Shuttle Endeavour avionics box failed a test Tin plated card guides

20. Space Vacuum – Tin Whiskers • NASA experiment to contain Tin Whiskers with conformal (Uralane 5750) coating • Tin plated brass boards used as samples • Half of each sample conformal coated • Varying depths of conformal coating up to 2 mils thickness • After 2 more than 2 years of storage, Tin Whisker grew through 0.1 mil thickness conformal coating • After more than 3 years, no whiskers had made it through 2 mil conformal coating

21. Space Vacuum – Tin Whiskers NASA experiment: Tin Whisker penetrating 0.1 mil thick conformal coating

22. Space Vacuum – Tin Whiskers • Tin Whisker prevention techniques for the packaging engineer • Care in handling of assemblies containing Tin • Minimize physical stress/scratches • Conformal coat Tin plated surfaces with conformal coating of greater than 2 mils • Ensure all Tin plating has a minimum lead content of 3%

23. Space Radiation Environment • Single Event Effects • Caused by heavily ionized cosmic rays and high energy protons • Single Event Upset (SEU) • Soft errors such as bit flips • Not harmful to hardware • Single Event Latchup • Can cause device to draw more than specified current • Usually requires power reset to device • Displacement Damage • High energy particle displaces an atom from its crystal lattice • Permanently alters electrical properties of the device

24. Space Radiation Environment • Total Ionizing Dose • Cumulative long-term ionizing effect of protons and electrons • Effects • Threshold shifts • Increased device leakage (power draw) • Timing changes • Decreased functionality

25. Space Radiation Environment • Packaging Considerations • Aluminum plate • Blocks high energy electrons, low energy protons • Not effective against high energy protons • Tungsten plate • Up to 60% more dense than lead • Used for heavy shielding of parts • Radiation hardened parts often have redundant gates = more power consumption

26. Conclusion • Packaging of electronics for space environment poses many unique challenges • Keep weight down ($100,000/lb for Low Earth Orbit) • Electronic assemblies must meet stringent reliability requirements