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Satellite Design Considerations
Given all of the hazards in space operations, there are many challenges for satellite designers. While some aspects, such as solar radiation cycles, are well understood and can be predicted, the mechanism behind tin whisker growth still eludes scientists, so there is no standard test for identifying potential problems.
SunWire engineers intend to address reliability issues as discussed herein.
Radiation, Deep Charging, and Particle Collisions There are a variety of ways to address the radiation effects on electronics in satellites, both physical and logical [1].
PHYSICAL SOLUTIONS
Radiation Hardened (RADHARD) chips are often manufactured on insulating substrates instead of the usual semiconductor wafers.
Bipolar integrated circuits generally have higher radiation tolerance than CMOS circuits.
Magnetoresistive RAM, or MRAM, is considered a likely candidate to provide radiation hardened, rewritable, non-volatile conductor memory.
Shielding the package against radioactivity to reduce exposure of the bare device.
Capacitor-based DRAM is often replaced by more rugged (but larger and more expensive) SRAM.
Choice of substrate with wide band gap (e.g. silicon carbide or gallium nitride), which gives it higher tolerance to deep-level defects.
Shielding the chips themselves by use of depleted boronborophosphosilicate glass passivation layer protecting the chips.
LOGICAL SOLUTIONS
Error correcting memory uses additional parity bits to check for and possibly correct corrupted data. Since radiation effects damage the memory content even when the system is not accessing the RAM, a "scrubber" circuit must continuously sweep the RAM, reading out the data, checking the parity for data errors, then writing back any corrections to the RAM.
Redundant elements can be used at the system level, such as three separate microprocessor boards to compute and compare answers to a calculation. Logic may be added such that if repeated errors occur from the same system, that board is shut down.
Redundant elements may be used at the circuit level. A single bit may be replaced with three bits and separate "voting logic" for each bit to continuously determine its result. This increases chip size by a factor of 5, but it has the advantage of being "fail-safe" in real time. In the event of a single-bit failure (which may be unrelated to radiation), the voting logic will continue to produce the correct result without resorting to a watchdog timer.
A watchdog timer will perform a hard reset of a system unless some sequence is performed to indicate the system is alive. One example is a write operation from an on-board processor. During normal operation, software schedules a write to the watchdog timer at regular intervals to prevent the timer from running out. If radiation causes the processor to operate incorrectly, it is unlikely the software clear the watchdog timer. The watchdog eventually times out and forces a hard reset of the system.
The SunWire satellites will utilize a combination of logical and physical solutions to address radiation concerns. Given the extended lifetime of the satellite, durability is a key factor.
All circuit designs will contain space-certified RADHARD components. Various logical redundant systems will be implemented depending on the relative importance of each subsystem. A watchdog timer system will be installed as a "last resort" technique to force a hard system reset if data is corrupted due to radiation.
As a primary line of defense, all sensitive electronic components will be physically insulated in protective enclosures. This tiered protection approach will address radiation, deep charging,and particle collision issues.
Satellite Charging and Discharging
The SunWire satellites will contain ground paths as needed to maintain a uniform potential across the satellite.
Outgassing
Outgassing poses a threat to the Sunwire system because the expelled substances can condense on solar cells and obscure them. Typically, moisture, sealants, lubricants, and adhesives are the most common sources, but even metal and glass can release gases from cracks or impurities.
For most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly. Cleaning surfaces or baking individual components or the entire assembly before use can drive off volatiles.
Sunwire will address the potential for outgassing by using space certified components. NASA maintains a database of outgassing data for selecting spacecraft materials online [2].
The electronics aboard the Sunwire satellites will be manufactured using standard aerospace procedures on approved, space-ready circuit materials by Rogers Corporation (Duroid, Ultralam, TMM). Additionally, Rogers provides space certified solutions for gasketing/gap filling, packaging designs (thermal), vibration & acoustic solutions, and vibration isolation.
Tin Whiskers
The uncertainties associated with tin whisker growth make it extremely difficult to predict if/when tin whiskers may appear. However, there extensive research by NASA scientists have revealed some suggestions for reducing the risk of tin whisker induced failures [3].
While RoHS standards make it difficult to acquire and implement leaded components and solder in today's circuits, the only proven solution to the tin whisker is to avoid the use of pure tin plated components. Studies have shown that alloying tin with a second metal, such as lead (>3%), reduces the propensity for whisker growth.
When simple avoidance of pure tin plating is not a viable option (such as in cases where its use is discovered late in system integration/test), then the following approaches may also be considered to reduce risk.
Solder dip the plated surfaces using a tin-lead solder to completely reflow and alloy the tin plating
Re-plate the whisker-prone areas
Add a conformal coat or foam encapsulation over the whisker prone surface. The choice of coating material, thickness and possible degradation with time/environmental exposure can impact the effectiveness of the coating.
The SunWire design team will collaborate with tin whisker expert David Pinsky at the Raytheon Reliability Analysis Lab to determine the severity of tin whisker threats to the system and to confirm that proper precautions have been taken [4].
Macroscopic Threats: Space Debris and Meteorites
Certain mechanical issues must be considered to protect the satellite from space debris and meteorites. Research suggests that the prominent meteorite threat size in GEO is 2 mm. Thus, all components will undergo a rugged mechanical reliability test to validate protection against 2 mm meteorites. The mirrors, solar cells, and external structures are of particular concern.
The physical structure of the radiation protection will provide sufficient protection for the electronics components.
REFERENCES
[1] http://en.wikipedia.org/wiki/Radiation_hardening
[2] http://outgassing.nasa.gov/
[3] http://nepp.nasa.gov/WHISKER/background/index.htm
[4] https://www.reliabilityanalysislab.com/ral_TechLibrary_TinWhisker.asp
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