THERMAL-VACUUM VERSUS THERMAL- ATMOSPHERIC TESTS OF ELECTRONIC ASSEMBLIES

Transcription

1 PREFERRED RELIABILITY PAGE 1 OF 5 PRACTICES PRACTICE NO. PT-TE-1409 THERMAL-VACUUM VERSUS THERMAL- ATMOSPHERIC Practice: Perfrm all thermal envirnmental tests n electrnic spaceflight hardware in a flight-like thermal vacuum envirnment (i.e., d nt substitute an atmspheric pressure thermal test fr the thermal/vacuum test). Mrever, if a cmprmise is thught t be necessary fr nntechnical reasns, then an analysis is required t quantify the reductin in test demnstrated reliability. Benefit: Assembly-level thermal vacuum testing is the mst perceptive test fr uncvering design deficiencies and wrkmanship flaws in spaceflight hardware. The margin beynd flight cnditins is demnstrated, as is reliability. Hwever, substituting an atmspheric pressure thermal test fr the thermal/vacuum test can effectively reduce electrnic piece part temperatures by 20 C r mre, even fr lw pwer density designs. The net result f this is that the effective test temperatures may be reduced t the pint where there is zer r negative margin ver the flight thermal envirnment. Prgrams That Certified Usage: Ranger, Mariners, Viking, Vyager, Magellan. Center t Cntact fr Infrmatin: Jet Prpulsin Labratry (JPL). Implementatin Methd: Establish a plicy fr spaceflight electrnic hardware that requires all assembly-level thermal testing t be perfrmed in a thermal/vacuum envirnment. Mrever, deviatin frm this plicy shuld require a waiver, supprted by quantitative analysis that cnsiders the effect n test demnstrated reliability. Technical Ratinale: Vacuum effects: A thermal/vacuum (T/V) test simulates the flight cnditin. Tw different physical phenmena ccur when a thermal/atmspheric pressure (T/A) test is perfrmed in lieu f a T/V test. They are "pure vacuum" effects and temperature level/gradient effects. The "pure vacuum" phenmena include crna and multipacting. Crna is f cncern in the pressure regin frm abut 0.1 t trr. Multipacting can JET PROPULSION LABORATORY

2 PAGE 2 OF 5 ccur starting frm the middle f the crna regin all the way t near hard vacuum cnditins. Pure vacuum prblems mst ften are assciated with radi frequency (RF) r high vltage circuits and devices. The additin f an ambient pressure gas alters key temperature levels and gradients. Fr a unit that is designed t be cnductively cupled t the spacecraft structure (shear plate), the prime thermal path frm the piece parts t the shear plate is via the bards and the husing. The intrductin f a gas int the "simulated" flight envirnment results in tw significant thermal alteratins. First, the dminant thermal paths frm key elements f the assembly (piece parts and slder jints, etc.) are altered because the gas creates a parallel path frm these elements t the chamber ambient via the ttal husing skin. Secndly, artificial parallel paths between the key elements t the flight heat sinking surface are added. These additinal parallel paths shrt ut any f the high thermal resistance paths that may be present in the design. The net result f this is a reductin in the temperature f the key elements at bth test temperature extremes. This test temperature reductin is referred t as the )T effect. A reductin in gradients between circuit elements als ccurs, which can lead t circuit perfrmance that is nt typical f flight. Fr example, a timing circuit may shw adequate perfrmance due t the reduced gradients, whereas the perfrmance in a flight-like vacuum cnditin culd be unacceptable. JPL Study Results: Analyses and testing have been specifically perfrmed at JPL since 1985 t quantify the effects f perfrming T/A testing in lieu f T/V testing. The results f these effrts are summarized in Table 1. Perfrming T/A testing in lieu f T/V testing reduces the temperature rise frm the thermal cntrl surface t key elements (bards, slder jints, parts, etc.) internal t the assembly. Nte that this effect reduces the perating temperatures f the key elements ver the whle temperature range (i.e., ht testing becmes less severe, while "cld" testing becmes clder). Reductins in the temperature rises can be n the rder f 15 C t 20 C r mre. Cmmnly, T/A testing reduces temperature rises by a factr f 2 t 4. Such reductins lead t margin demnstratins dramatically lwer than desired, and can easily cause negative test margin demnstratins. An electrnic assembly is manufactured by a series f chemical and mechanical prcesses. Design and wrkmanship failures related t the chemical prcesses are best described by the Arrhenius reactin rate equatin. Mechanical design and wrkmanship failures are mst ften a result f thermal fatigue and, t a lesser degree, vibratin. Bth the chemical and thermal fatigue failure mechanisms are a functin f temperature. Tables 2 and 3 quantify the temperature influence n these failure mechanisms.

3 PAGE 3 OF 5 Table 1. Summary f Analysis and Test Results fr the )T Effect Assciated with Perfrming T/A Testing in Lieu f T/V Testing POWER )T effect (Deg. C) ASSEMBLY TYPE DENSITY 2 W/cm Analysis Test Radar Transmitter RF (1) Radar Transmitter Pwer Supply Radi Receiver RF 0.10 < 9 Pwer Distributin Analg 0.01 < 5 Data Frmatter Digital/analg (2) Range Dispersin Digital/analg Cmmand Data Bay 3 Digital/analg Cmmand Data Bay 4 Digital Science Instrument Digital/analg Output Netwrk RF Ntes: (1) Unit nt blanketed during initial T/V test. Estimates fr the effect f this indicated that the lad n the heat exchanger was apprximately twice that dissipated by the unit. (2) Test perfrmed fr the )T effect part case-t-husing. Full )T effect shwn is a cmbinatin f test and analysis. Table 2. Arrhenius Reactin Rate Reductin Factrs fr Varius )T Effects and Activatin Energies. )T effect Activatin Energy (ev) (*) Deg. C *Assuming a 75 C shear plate plus a 35 C rise shear plate-t-part junctin. Lwer test levels lead t greater reductin ratis.

4 PAGE 4 OF 5 Table 3. Screening Strength Reductin Factr ("X" Factrs) fr Varius )T Effects and Shear Plate Temperatures. Shear plate )T Effect Degrees C (*) Temperature Deg. C *Fr cmpliant slder jints and cld test temperatures abve the glass transitin temperature fr all materials invlved. Als presented in Table 1 are varius ratinales generally in use in industry tday fr chsing a T/A test in lieu f a T/V test. The mst cmmn f these ratinales are either based n the pwer density f the unit r type f hardware (i.e. pwer supply, digital, RF, etc.) underging testing. The JPL study results clearly shw that these tw ratinales are nt valid. The current ratinale in use at JPL tday is that if analysis shws that the )T effect is less than 5 C n all piece parts, slder jints, etc., and there are n knwn pure vacuum effects, then perfrming a T/A test in lieu f a T/V test might be allwed depending f the criticality f the unit under test. The safest and simplest curse f actin is t T/V test everything. Impact f Nnpractice: Perfrming an atmspheric pressure thermal (T/A) test in lieu f thermal/vacuum (T/V) test reduces the ht temperature margin, screening strength, and test demnstrated reliability. Ht temperature margins can be cmprmised t the pint where there is a zer r negative margin between envirnmental test levels and the allwable flight level (e.g., a test with nly a planned 10 C margin and a T/A reductin effect f 15 t 20 C wuld result in a negative test margin). Screening strengths can be reduced by factrs f 2 t 4 r mre. Test demnstrated reliability can be reduced by factrs f 2 t 10 r mre. Related Practices: "Part Electrical Stress Analysis," PD-AP "Slder Jint Fatigue Cycles," t be published. "Envirnmental Factrs," PT-EC "Thermal Analysis f Electrnic Assemblies t the Piece Part Level," PD-AP-1306.

5 PAGE 5 OF 5 References: 1. Mark Gibbel, "Thermal/Vacuum Versus Thermal Atmspheric Testing f Space Flight Electrnic Assemblies," NASA Cnference Publicatin 3096, frm the 16th Space Simulatin cnference, Albuquerque, New Mexic, Nvember 5-8, 1990.