1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-15a71 Center Point of Contact: JPLa71 Submitted by: Wil HarkinsSubject: Part Junction Temperature Practice: Maintain part junction temperatures during flight below 60C. (Short-term mission excursions associated with transi
2、ent mission events are permissible.)Abstract: Preferred Practice for Design from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Reliability is greatly increased because the failure rate is directly related to the long-term flight Provided by IHSNot
3、for ResaleNo reproduction or networking permitted without license from IHS-,-,-temperature.Implementation Method:Establish in-specification design (and test) temperatures 75C and limit part junction temperatures (JT) to 110C which constrains permissible part junction temperature rise (DJT) to 35C. (
4、This practice has been verified on programs in place before the release of MIL-STD-975H. If the MIL-STD-975H junction temperature of 100C is used, junction temperature rise should be changed to assure that long-term flight junctions stay below 60C.)Technical Rationale:Basic reliability is directly r
5、elated to temperature and time, i.e., l = f(T,t). The following relationship is obtained either theoretically from the Arrhenius relationship (l= Aexp-Ea/k (1/T - 1/T0) or empirically from the data in MIL-HDBK-217E.refer to D description DGiven: a71 Specific parta71 Specific derating factora71 Speci
6、fic chemical activation energyThe curve shape is representative of all electronic parts (and most mechanical processes) in the range of temperature typified by space exposure. Simply stated, the higher the long-term flight temperatures, the lower the reliability:refer to D descriptionD Assume that a
7、 design and test temperature of 75C is chosen. In the graph from MIL-STD-883B observe that a 25C D T corresponds to a failure rate increase of more than an order of magnitude- i.e., 1000% difference. MIL-HDBK-217E has different values, but the factor is up to approximately Provided by IHSNot for Res
8、aleNo reproduction or networking permitted without license from IHS-,-,-3X on some parts (depends on derating criteria and parts qualification). The following example illustrates the effect of this relationship on design and test temperatures.Assume the following conditions as an example:Case A: T =
9、 75C in-specification design temperature for baseplateCase B: T = 50C in-specification design temperature for baseplateCase A and Case B: T = 25C long-duration flight temperature for baseplateJT= 110C limit for any exposure or analysisThen:Design/Test Parameters Case A Case BDesign Baseplate 75C 50C
10、 JTlimit110C 110C Permitted DJTrise35C 60C Flight Conditions refer to D descriptionD from Arrhenius Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD NOTE: In the example given, short-term ground test exposure on the order of 1-
11、2 weeks will use an insignificant amount of life in hardware designed for long-life and high reliability. For example, a 1-week thermal vacuum test at 75C provides a short-term high temperature screen in the actual circuit usage configuration to provide confidence for a long-term exposure under flig
12、ht conditions (JT60C), and uses only 0.018% of the parts capability. This demonstration is an important element in establishing pre-launch confidence in design adequacy.refer to D descriptionD (Click image for a larger view) References:1. Gibbel, M. and Clawson, J.F., “Electronic Assembly Thermal Te
13、sting Dwell/Duration/Cycling,“ Proceeding of the 12th Aerospace Testing Seminar, March 13-15, 1990.2. Gibbel, M. And Cornford, S.L., “Surface Mount Technology Qualification Methodology (Testing and Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Veri
14、fication),“ Proceeding of the NASA Surface Mount Technology Workshop, NASA Lyndon B. Johnson Space Center, Houston, Texas, July 28-29, 1992.3. Thermal Design Practices for Electronic Assemblies, Reliability Preferred Practice No. PD-ED-1226Impact of Non-Practice: Reliability of electronic parts will
15、 be reduced significantly if junction temperatures exceed 60C.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-15a71 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-,-,-
