COTS Components in Space Systems: Benefits and Risks
Space is becoming a more and more competitive, asking continuously for higher performance figures while reducing the overall Cost from missions. Such a trend has consequences at all levels down to the selection and procurement of building blocks and Electronic components.
The usage of COTS (Commercial Off The Shelf) or PEM / PED (Plastic Encapsulated Microcircuits) of Electronic parts in High Reliability applications offers many benefits:
- Cost advantage, only for large volumes or low reliability/low Radiation application where important risks might be taken.
- Performance advantage if the performance is not obtainable by classical High-Reliability components
- Shorter lead times and lower risk of part unavailability (consider quick obsolescence cycle of COTS components limited shelf life).
However, there is still a big gap between Space and Terrestrial application of components, and proper methodologies have still to be developed and approved in order to allow a more systematic usage of Commercial Off The Shelf (COTS) components and modules for Space applications.
There is a wide spectrum of approaches to parts screening. At one end of the spectrum, EEE parts used in critical space systems in general are subjected to 100% parts-level inspections and testing to provide high assurance of quality and reliability.
Electronic components designed for Terrestrial application (such as Automotive and other Industrial sectors) show high reliability levels when produced in massive quantities and being subject to ad-hoc qualification schemes (i.e. AEC-Q).
As a first step, the criticality of the equipment, subsystem or system needs to be determined, which will then determine which set of guidelines should be used.
It is important to calculate the Total Cost of Ownership of the parts, which consists of the procurement cost and the costs associated testing and upgrading to meet Program Reliability requirements.
The requirements for selection, control, procurement and usage of EEE (Electrical, Electronic and Electromechanical) parts for Space projects are defined by the ECSS standards according to a defined class for the Project.
COTS Main Issues
- Generally no traceability, quality inspections, temperature range etc.
- But manufacturer reliability data are often very good COTS built for Automotive/Aero/Defense markets can have good reliability features and are a good starting point.
- Typically need incoming lot screening and tests
- Typically plastic packages, often BGA, Flip Chip, etc.
- Finish problems (tin whiskers)
- Low experience in the qualification of processes
- COTS need radiation behavior assessment, which is a long, costly and uncertain process Total dose is not a great concern: often the requirements are limited (10-20 Krad), shielding is effective, latest silicon technologies (e.g. 65 nm on SOI) are inherently TID resistant
- SEE effects are the major problem, in particular SEU/SEFI for digital devices.
- SEE tests are expensive and difficult to perform and interpret the results
- SEE are not destructive, but generate transient errors that affect the availability of the computer
- COTS are not designed to be SEE hard, ECC (if any) is limited to large memory blocks (caches, RAM), but registers, IO and control logic is unprotected. Different mitigation methods can be implemented, but at last a quick reset and restart can be needed.
- When using COTS computers, it must be born in mind that full SEE immunity, i.e. 100% availability, which is typical of classical P/F computers, cannot be realized: occasional short interruptions must be taken into account at system level, and availability becomes a key performance indicator that must be specified and considered in the design.
- Automotive parts are inexpensive but large minimum order quantity purchases can be required -into the thousands.
- No radiation data available for automotive EEE Parts
- Additional screening costs (including radiation assurance) may be required to meet mission requirements before automotive parts can be used in low risk space applications
- Need to consider the full cost of ownership if cost is the driver
Applicable Documents :
ECSS-Q-ST-60-13C ( Commercial EEE components) is applicable to commercial encapsulated active Monolithic parts (integrated circuits and discrete).
While PEM / PED device’s the Mechanical, Environmental and Electrical testing, as well as Construction Analysis shall be based on NASA Documents :
- PEM-INST-001 (Instructions for PEM Selection, Screening and Qualification)
- EEE-INST-002 (Instructions for EEE Parts Selection, Screening, Qualification and Derating).
Automotive Grade EEE Parts are qualified in accordance with Automotive Electronics Council (AEC) specifications “AEC Q” :
- AEC-Q100 Stress test qualification for integrated circuits
- AEC-Q101 Stress test qualification for discrete semiconductors
- AEC-Q200 Stress test qualification for passive components
Justification Documents (JD) with Part detailed Quality / Reliability analysis to obtain information regarding design, workmanship, counterfeit:
- PPAP (Production Part Approval Process)
- Constructional Analysis (C.A.) systematically per lot
- Radiation test report for sensitive active devices
- Dispositions to prevent Tin whiskers induced failures.
- Special design rules (Derating)
- ESCC (European Space Components Coordination) : https://spacecomponents.org/
- AEC (Automotive Electronics Council) : http://www.aecouncil.com/
- NTRS (NASA Technical Reports Server) : https://ntrs.nasa.gov/search.jsp