COTS Components in Space Systems: Benefits and Risks

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 for large volumes or low reliability/low Radiation application where risks might be taken.
• Performance advantage if the performance is not obtainable by classical High-Reliability components
• Shorter lead times but consider quick obsolescence cycle of COTS components limited shelf life.

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.

COTS Main Issues:

Reliability

• 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

Mounting process

• Typically plastic packages, often BGA, Flip Chip, etc.
• Finish problems (tin whiskers)
• Low experience in the qualification of processes

Radiation effects

• COTS need radiation behavior assessment, which is a long, costly and uncertain process
• 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

Cost Comparison

• 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
• Need to consider the full cost of ownership if cost is the driver

COTS Parts Requirements

For Commercial encapsulated active Monolithic parts (Integrated circuits and Discrete) the ECSS-Q-ST-60-13 can be adopted 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).

ECSS-Q-ST-60-13C provides 3 levels of qualification dependent on application risk:

• Class 1: High Reliability/Low Risk – full up screening: up screened part could be used as a Grade1.
• Class 2: Low to Moderate Risk – based on Justification Document (JD), some tests could be removed, screening at PCB level.
• Class 3: High Risk – based on Justification Document (JD), evaluation could be reduced to CA and Radiation tests;

Justification Documents (JD) :

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)

PURE TIN issues:

Parts with pure tin finish are allowed, provided they pass the JESD-201 class 2 requirements. If not, the supplier shall issue a risk analysis with mitigation plan to prevent pure tin whiskers induced failures.

The Risk analysis with mitigation plan shall include as a minimum :

• The termination type (as identified in the Justification report)
• Incoming termination control
• Assembly process compatibility (backward)
• Design analysis, PCB layout and spacing between exposed electrical conductors
• Assembly melting conditions (temperature, soldering flux, Annealing etc..)
• Whiskers proof barrier : e.g. conformal coating type / thickness
• Rework : tin lead electrolytic plating

 

Related articles:

• COTS PARTS DATABASE
• COTS PARTS Post Tags

 

CONCLUSION:

The risks that must be ascertained when using COTS in Space must include:

1. Supplier selection to insure good product quality and reliability
2. Total Cost of Ownership including any upgrade screens/qualification
3. Radiation Sensitivity

Use of COTS Parts in Space applications a complete characterization over the full environment intended is required.

For Expertees and specific Consultancy Contact us.

 

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