Lesson Info:

• Lesson Number: 0676
• Lesson Date: 1999-02-01
• Submitting Organization: GSFC
• Submitted by: Wil Harkins

Subject:

Description of Driving Event:

This Lessons Learned is based on Reliability Practice No. PD-ED-1201; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.

EEE parts derating can be established as either design policies or from reliability requirements. In general, NASA has taken the approach of establishing derating policies that cover all applications of the various part types in space flight equipment. These policies are available in MIL-STD-975, "NASA Standard Parts List." Table 1 provides typical derating factors from that document. If derating is to be determined from a reliability requirement, the reference document is MIL-HDBK- 217, "Reliability Prediction of Electronic Equipment." MIL-HDBK-217 contains the information necessary to quantitatively estimate the effects of stress levels on reliability.

Technical Rationale:

The reliability of a EEE part is directly related to the stresses caused by the application, including both the environment and the circuit operation. MIL-HDBK-217 contains specific part failure rate models for a wide variety of part types. The models include factors for calculating the effects of various stresses on the failure rate and thus on part reliability. The types of factors include (for example): environment, quality levels, voltage, frequency, and temperature. Given the extensive tables of factors in MIL-HDBK-217, one can formulate reliability predictions for piece parts.

As shown in Figure 1, the plot of piece part failures versus an application stress level such as temperature, voltage, or current indicates decreasing failure rates for lower levels of stress. Therefore, a part's reliability in an application can be increased by decreasing the maximum allowed stress levels from the absolute maximum for which a part is rated.

[D]

Lesson(s) Learned:

Derating lowers the probability of failures occurring during assembly, test, and flight. Decreasing mechanical, thermal, and electrical stresses lowers the possibility of degradation or catastrophic failure. Derating policy documents such as those prepared by NASA and DoD, and generally required in their contracts, allow the designer to avoid lengthy and involved calculations by mandating the derating of specific characteristics and parameters.

Recommendation(s):

Practice:

Derate applied stress levels for electrical, electronic, and electromechanical (EEE) part characteristics and parameters with respect to the maximum stress level ratings of the part. The allowed stress levels are established as the maximum levels in circuit applications.

Evidence of Recurrence Control Effectiveness:

This practice has been used on all Goddard Space Flight Center (GSFC) flight programs.

Documents Related to Lesson:

Mission Directorate(s):

• Exploration Systems
• Science
• Space Operations
• Aeronautics Research

Additional Key Phrase(s):

• Configuration Management
• Flight Equipment
• Ground Equipment
• Hardware
• Launch Vehicle
• Parts Materials & Processes
• Payloads
• Safety & Mission Assurance
• Spacecraft

Additional Info:

Approval Info:

• Approval Date: 2000-03-09
• Approval Name: Eric Raynor
• Approval Organization: QS
• Approval Phone Number: 202-358-4738