REG NASA-LLIS-0782-2000 Lessons Learned Part Electrical Stress Analysis.pdf

1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-14a71 Center Point of Contact: JPLa71 Submitted by: Wilson HarkinsSubject: Part Electrical Stress Analysis Practice: Every part in an electrical design is subjected to a worst-case part stress analysis performed at the ant

2、icipated part temperature experienced during the assembly qualification test (typically 75 degrees C). Every part must meet the project stress derating requirements or be accepted by a formal project waiver.Abstract: Preferred Practice for Design therefore, a conservative temperature assumption is m

3、ade. Highly stressed parts are identified for possible replacement with more robust parts or for possible circuit changes. The final design is confirmed by analysis, with part temperatures based on a part level thermal analysis, and with voltages and currents derived from either specification limits

4、 or the results of worst-case circuit analysis.Numerous life tests have been performed on electrical parts that establish the relationship of part life to applied stresses. There is a very strong dependence. The life expectancy typically can be doubled or tripled by operating at half the manufacture

5、rs full rated (100 percent) stresses, which typically are commensurate with a 10,000-hour life expectancy. Complex, multiple year missions must achieve very low part failure rates to achieve mission goals; therefore, operation at derated conditions is mandatory. Although typical reliability predicti

6、ons are based on nominal stresses, circuit nonlinearities and part and voltage variations can cause large operating point variations. Therefore, it is essential that the conservative approach of using worst-case stresses be implemented as standard practice.Although average temperatures during a miss

7、ion may be nominal, typical qualification test philosophy results in test temperatures that stress the design to assure margin against possible flight contingencies (typically 75 degrees C). It is essential that negligible aging of the parts be introduced during protoflight testing to assure mission

8、 reserve life. For this reason, it is prudent to show that the deratings are met while operating in the worst qualification or protoflight test environment. Historical evidence has shown that significant (40 degree C) temperature rises can exist between the thermal mounting surface of an assembly an

9、d the part body if good thermal design of the assembly is not rigorously pursued. For this reason, the results of a part level detailed thermal analysis must be an input to the part stress analysis.In summary, the stress derating requirements of every part at worst-case circuit conditions and contin

10、gency temperatures must be met. This will ensure a design that will function with a high degree of confidence at these extremes. It also will force a conservative thermal design (small Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-temperature rises

11、), which will produce even greater mission life margins under the expected nominal flight conditions.References1. Part Junction Temperature, Reliability Preferred Practice No. PD-ED-1204.Impact of Non-Practice: The failure to perform part stress analysis likely will result in several overstressed pa

12、rts in the design. These will become the life limiting items of the design and produce unacceptably short-lived hardware. If the analysis is performed at nominal temperature and operating points, without derating, or without a detailed thermal analysis, there will be no margin for contingencies and

13、the nominal life expectancy also will be degraded.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-14a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-