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Particles & Fields Data Processing Unit (PFDPU) Electronics Box Mechanical Evaluation Bill Donakowski , SSL David

Particles & Fields Data Processing Unit (PFDPU) Electronics Box Mechanical Evaluation Bill Donakowski , SSL David Pankow , SSL Paul Turin, SSL Scott Tucker, et.al., LASP

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Particles & Fields Data Processing Unit (PFDPU) Electronics Box Mechanical Evaluation Bill Donakowski , SSL David

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  1. Particles & Fields Data Processing Unit (PFDPU) Electronics Box Mechanical Evaluation Bill Donakowski, SSL David Pankow, SSL Paul Turin, SSL Scott Tucker, et.al., LASP NOTE: Maven has requested in depth review support from GSFC on this topic. Our point of contact is Seke Gordo (code 542). This review is ongoing and still has some open items.

  2. PFDPU Mechanical Evaluation Evaluation Approach • Experience has shown that the largest components offer the greatest risk • Ceramic substrates are quite stiff as compared to flexible circuit boards • Most likely failure mode is (KOVAR) fatigue of corner leads on large components • Fatigue strains are dictated by the accumulated flexures of the PCB. • Steinberg’s methods predict max. board deflection for infinite fatigue life • He’s fitted his accumulated PCB test data to provide useable empirical relations • These results may also be applied to yield & finite fatigue life predictions • Infinite life is appropriate for many terrestrial applications, while the typical NASA launch vibration (& testing) exposure is very brief. • PCB (four sided) edge support naming conventions • Pinned – edges constrained in translation, but not in rotation • Fixed – edges constrained in both translation and rotation Working Reference • Steinberg “Vibration Analysis for Electronic Equipment” 3rd Ed. J.Wiley, 2000

  3. PFDPU Mechanical Evaluation Summary of Evaluation Relations (after Steinberg) • First Mode of a given PCB design (fixed edges): • Fn = (2p/3) (D/r)½ [3/A4 + 2/A2B2 + 3/B4]½• D = Eh3 / [12 (1-m2)] • r = W/g / [AB] • A & B – PCB dimensions • h – PCB thickness • W - weight • Max PCB deflection for Infinite Fatigue Life: • Z∞ = 0.00022 B / [C h r √L] • C – component factor (table) • r - position factor (table) • L – component length • Maximum PCB test deflection of a given PCB design: • Z3s = 36.8 √P / Fn5/4• P - ASD test level (G2/Hz) near Fn • Accumulated fatigue damage using “Miner's Rule”: • Rn = (FnST) [0.683/N1s + 0.271/N2s + 0.043/N3s ] • Nns = [Z∞/Zns]b x 1E7 cycles • ST – accumulated exposure time • Yield Strength Fraction: • Fy = (Se / Sy)(Z3s / Z∞) • Se - endurance strength • Sy – yield strength

  4. PFDPU Mechanical Evaluation OBSERVATIONS • GSFC-STD-7000 (GEVS) and NASA-STD-5001 pose two design constraints • 1.6 min. safety factor on metallic yield in vibration from GEVS table 2.2-3 • 4X exposure time calculations to account for scatter in typical life data • Designing for “Infinite Life” is an admirable goal, but overly restrictive • Accepting modest fatigue damage (perhaps < 25%) poses minor risks • Plot below illustrates design reserves for retesting & programmatic unknowns • 1.6 min. GEVS yield safety factor → 62% of yield (red line) • Finite life only in the box level PFM test • Subsequent tests are at lower levels FM Level Req’d PFM Level

  5. PFDPU Mechanical Evaluation PFDPU Box Construction • Each PCB is bonded into a ledged aluminum frame • Eleven slices stack together & bolt to base plate • Internal stacking connectors provide “backplane” • Two threaded “skewers” hold top corners together

  6. PFDPU Mechanical Evaluation PFDPU PCB Artwork MAG PCB DFB PCB REG PCB – no concerns DAP PCB NOTE: No large components on BEB board

  7. PFDPU Mechanical Evaluation IIB Board: includes two perpendicular daughter boards w/ frame (and no large components) DCB Board: includes a partial daughter board on standoffs

  8. PFDPU Mechanical Evaluation Original PCB Evaluation Results • COMMENTS • * The DCB board has a daughter card and many other features that make a proper (FEM) evaluation questionable. Having the EM board previously shown, the plan of action was to measure this board frequency, and make adjustments to obtain a first mode of > 200 Hz • • These evaluations assume good workmanship practices in building these PCBs.

  9. PFDPU Mechanical Evaluation PFDPU Box Evaluation • Steinberg calls for an octave (2X) separation between boards and box • Charts below show potential board – box resonant interactions (using Miles Eq’n) • Lead fatigue is governed by board displacements shown in the right chart. • The 2X criterion reduces box-board mode interactions to quite modest levels Acceleration Displacement

  10. PFDPU Mechanical Evaluation MAVEN PFDPU Design Details 2X Skewer Rods 11 X Individual Frames (Al 6061 T6, .100” thick) SolidWorks model of PFDPU Attach Frame Bolted to S/C Each frame interlocks with adjacent

  11. PFDPU Mechanical Evaluation MAVEN PFDPU FEM Analysis Details Fixed Constraint at Box Base (no other constraints in model) 11 x Individual PCBs/11 Individual Frames Exploded View

  12. PFDPU Mechanical Evaluation Top of Box Frames Bonded (Skewer Rods hold frames together) • FEM software • SolidWorks ‘Simulation’ 2011 • SolidWorks Premium 2011 x64, SP 1.0 • Includes FEM Simulation 2011 x64 SP 1.0) • Analysis Details • 2-D Shells and Mesh • All 11 Frames and PCBs modeled • Frames fixed at bottom (mounting to S/C Deck) • Frames bonded to each other at Top (skewer rods) Bottom of Frames Fixed (PFDPU Base Plate not included - mounts to spacecraft) Material Properties

  13. PFDPU Mechanical Evaluation First Mode (side view of PFDPU) Second Mode (top view of PFDPU) Numerical Results (modal mass was ~68% of total mass) (many similar modes)

  14. PFDPU Mechanical Evaluation PFDPU FEM Modeling Discussion • Box Modes (only) are the desired results in this effort • Software can provide up to 25 modes starting from the lowest value • Using actual PCB physical properties will only yield a plethora of board modes • PCB modulus (E) was increased 20X to avoid the box modes (i.e. very stiff boards) • Higher order box modes were frame flexing or (Al) end panel diaphragm flexing • A subsequent test run with the actual E(pcb) did not provide any box modes • This did provide a useful result where PCBs clustered around 176 Hz vs. 185 Hz • The 5% decrease in Fn is from the “less than rigid” nature of the stacked frames • This is a conservative result because stiff boards contribute to the modal mass • Actual ‘floppy’ boards will contribute less to the modal mass at these frequencies • PCB modes can be increased to ≈300 Hz before interactions are a concern

  15. PFDPU Mechanical Evaluation Part 2: Review of, and Modifications to STEINBERG’S APPROACH Issues to be considered: • EM – DCB board test results • Validation of PCB frequency predicts • PCB mode shape vs. fatigue amplitude discussion • PCB material damping

  16. PFDPU Mechanical Evaluation EM-DCB BOARD TEST (¼ G sine sweep) • The partial daughterboard & many connectors defy proper analysis TEST RESULTS • Fn = 211 Hz (185 Hz Steinberg predict for a nominal – single board) • Qmax = 46 (measured near the board center) • Qcg = 4/p 2Qmax • A measured Q at some arbitrary point is not meaningful, Qcg is needed • Qcg = 18.6 (14.5 predict using Steinberg’s QSt = √Fn)

  17. PFDPU Mechanical Evaluation • VIABILITY OF PCB FREQUENCY PREDICTS • NASA-SP-160 (1969) is one of the most complete analytic works available • <available on line by searching ntrs.nasa.gov> • A 5% frequency reduction is needed to reflect the less than rigid stacked frame FEM. • Minor corrections are needed for the EM – DCB test results • The test board had eight screws, but was not glued into it’s board frame • DCB test board was FR-4 while FMs will use higher modulus polyimide PCBs • Isola P95 is one example of a higher modulus board material

  18. PFDPU Mechanical Evaluation • Steinberg on PCB Edge Conditions • PCB mode shape descriptions • Pinned edge mode shape is a half sine wave: Z(x.y) = [sin(px/a) sin(py/b)] • Fixed edge mode shape is a full sine wave: Z(x,y) = ½ [cos(2px/a) -1] [cos (2py/b) -1] • Local curvature differs by a factor of two at the board center • In his text, on pages 181-182 , this curvature difference is dismissed as small • Section 8.6 discusses edge wedge clamps (fixed edges) • His fatigue database presumably includes these types of PCBs

  19. PFDPU Mechanical Evaluation • Steinberg on PCB Edge Conditions (con’t) • Pinned vs. fixedPCB edge comparisons – an even more conservative approach • Heavy solid lines are the mode shapes – dashed lines are the local (2nd derivative) curvature • Rust line is the |fixed to pinned edge curvature ratio| • Fatigue amplitudes are defined by curvature between any two chip ends (arbitrary positions) • Red line @ 1.26 is the average of this curvature ratio between 20% & 80% of board dim’s • Large parts typically aren’t put in board corners (centered can be a smaller issue) • We introduce a modified (x 1.262 , for x&y) location factor: r’ = 1.60 r(Steinberg)

  20. PFDPU Mechanical Evaluation PCB Material Damping Properties • Inherent PCB material damping is proportional to 1/Q test results • Damping is the least understood material property, subject to many factors • SDOF models assume viscid damping, while data often shows hysteretic trends • Test data often reflects a weak amplitude dependence • Actual DCB test result was close to Steinberg predict ( Q = √Fn) • Design margin predicts are tolerant of variations in Q (below) test predict

  21. PFDPU Mechanical Evaluation • DCB Daughterboard has only a CCGA (ceramic column grid array)Actel • Considered a risk, but not as bad as the BGA (ball grid array) • Maven Thermal Expectations • Actel has provided an extensive thermal cycling test report • Typically 1000 cycles of +105 to -55C needed for failure • Coffin Manson predicts provided for other environments • PFDPU Thermal environments are expected to be mild • > 10C variation predicts for Martian orbits CCGA Contacts Detail • Maven Vibration Expectations • Small (2.5” x 2.7”) pinned edge board • ~ 1100 Hz first mode predict (DB only) • Z3s deflection ~ 0.004” predict • Z∞ deflection ~ 0.006” for infinite life • DB’s input will be rolled off ~ 12dB from the ~300 Hz first mode of its motherboard. • ~ 0.007” deflection at DBs center [R] from MBs 300 Hz 1st mode (~ 0.003” under CCGA)

  22. PFDPU Mechanical Evaluation Revised Design Margins • The revised results table below reflect: • Corrections based on the EM-DCB reduced vibration test data • Scaled DCB PCB frequency predicts • The 5% frequency reduction to account for the frame stacking • Enhanced location function (r’ = 1.60 r) accounting for larger board curvature

  23. PFDPU Mechanical Evaluation BOARD BY BOARD SUMMARY • DCB 1&2 (UCB) stiffened by partial daughter board: 0.062” PCB with glued edges • Test measurement of EM DCB resonant frequency proved adequate • IIB (UCB) w/ 2 T daughter boards and no large parts: 0.062” PCB with glued edges • REG 1&2 (UCB) NO large parts: 0.062” PCB with glued edges • DAP 1&2 (UCB): one large part: 0.062” PCB with glued edges • MAG 1&2 (GSFC) one large part: Considering (CAD-FEM) a local board stiffener • Fallback Option: 0.062” PCB with glued edges • BEB (LASP) NO large parts : 0.062” PCB with glued edges • DFB (LASP) Investigating (CAD-FEM) a local board stiffener • Fallback Option: 0.093” PCB with glued edges, or DFB-BEB-end plate partial skewer • PCB Edge Bonding (gluing) Plan • All board frames have a 0.080” wide shelf (all around) for good board support • Generous structural epoxy (Hysol EA-9309) fillets on both sides, before conformal coating

  24. APPENDIX LASP’s Board Stiffener

  25. Appendix: LASP’s DFB Stiffener Digital Filter Board Structural Analysis/Redesign EUV Mechanical David Normen / David Braun Structural Analyst / Mech. Design Eng.

  26. Appendix: LASP’s DFB Stiffener • LASP provides DFB and BEB board to Berkeley. DFB BEB

  27. Appendix: LASP’s DFB Stiffener • DFB’s FPGA did not meet LASP’s criteria for infinite life. • Steinberg Analysis for Electronic Equipment • Trade study to improve the FPGA’s structural performance. • Improved mounting does not predict infinite life, but should be acceptable.

  28. Appendix: LASP’s DFB Stiffener • Convert EMI Shield to stiffen board. • Add a center support. • Add simple support around the edges of the board. • Mat’l = Al 6061-T6 • FPGA add • Corner staking with Scotch Weld 2216. • Standard LASP lead wire forming. • Frame • Note change to 8 edge fasteners. • Board • No layout changes. EMI Shield and Stiffener Lead wire and corner stake elements

  29. Appendix: LASP’s DFB Stiffener CQ352 FPGA

  30. Appendix: LASP’s DFB Stiffener • Criteria: DFB/FPGA must survive all vibration tests + flight w/ margin. (≠  life) • ≥ 9 minutes of vibration. • < 100% Cumulative fatigue damage.

  31. Appendix: LASP’s DFB Stiffener • Stiffener and EMI shield are integrated as one part • DFB layout will use “EM” board-to-frame mounting hole pattern LASP supplied Stiffener LASP supplied DFB PWBA LASP supplied Frame Berkeley supplied Vent Assembly

  32. Appendix: LASP’s DFB Stiffener • Fasteners pass through frame and PWBA and threads into stiffener • Stiffener sits within the 0.070” keepout of PWBA EMI groove Stiffener, chromated aluminum, 178g PWB Frame, chromated aluminum

  33. Appendix: LASP’s DFB Stiffener • Copy of frame drawing so we can duplicate size and tolerances • Locate (2) clock signal connectors • Determine fix ~.050 height discrepancy of stackable connectors • Check on fastener head clearance between DFB & BEB

  34. Appendix: LASP’s DFB Stiffener Board damping is assumed to be fn (Steinberg) unless otherwise noted. Time to failure is 1/4th of the predicted per GEVS.

  35. BACKUP MATERIAL

  36. PFDPU Mechanical Evaluation MAVEN Zone 2 –Fwd Deck Vibration Levels (derived from Atlas 5 acoustic levels) Steinberg’s KOVAR Fatigue Plot Sy ≈ ≈ Se

  37. PFDPU Mechanical Evaluation EM-DCB BOARD TEST RESULT (¼ G sine sweep) • Partial daughterboard & the many other features defy proper analysis • Original result assumed a single uniform density board

  38. PFDPU Mechanical Evaluation DAVE STEINBERG’s ON-LINE PROFILE • Dave S. Steinberg is a well-known author and internationally recognized authority on the mechanical design, analysis, testing and packaging of cost effective sophisticated electronic equipment that must work with a high degree of reliability in harsh thermal, thermal cycling, vibration and shock environments. He has been involved in these areas related to commercial, industrial and military applications for many years. He recently retired from Litton Industries, where he was the manager of the Engineering Department for 15 years. • Mr. Steinberg is a visiting professor at the University of Wisconsin and for the past several years at UCLA where he has been presenting full semester post-graduate courses on similar electronic subjects related to the reliability of electronic equipment. • Mr. Steinberg has worked on many aircraft/missile/space programs, ships' electronic systems, automotive electronics and computers. These include the F-14, F-15, F-16, F-22, cruise missile, AMRAAM, ALCM, TALCM, SLCM, Titan 2 and 3, space shuttle, Mercury, Gemini, Apollo, communications satellites, DD963 destroyers, and the USS Forestall aircraft carrier. Typical companies include Lockheed, IBM, Litton, Intel, GM, Ford, Boeing, Wright Patterson AFB, Northrop, Cisco, Harris, United Aircraft, Hamilton Standard and Motorola, just to name a few. • MAJOR PUBLICATIONS (books) • “Vibration Analysis for Electronic Equipment” 3rd Ed. J.Wiley & sons, 2000 : 2nd Ed. 1988 • Cooling Techniques for Electronic Equipment, 2nd Ed.J.Wiley & sons, 1991 • Preventing Thermal Cycling and Vibration Failures in Electronic Equipment J.Wiley & sons, 2001

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