1. MEP configuration pros/cons
Wedge-locks vs
Board-in-Frame
updated by D. Pankow

2. MEP configuration discussion
Why the recent reviewer uproar over e-box design & analyses…
Several unexpected / unexplained vibration failures (including SSL)
Most failures involve large Actels (e.g. CQ352) …0.008”w x 0.006”t leads are very fragile)
BACKGROUND (the reviewers commonly accepted reference)
“Vibration Analysis for Electronic Equipment, 3rd Ed.” Steinberg (2000) J. Wiley
His reference database has never been published or reviewed (and is likely quite dated)
Requires box mode >2X highest PWB mode to avoid near resonance coupling
Most of this can be predicted (FEM or analytic) from dynamics & published material data
The PWB first mode predicts are too conservative (assumes no stiffening by components)
COMMONLY USED PACKAGING
PWB motherboard with board side Wedge-Loks
Wedge-Loks provide 2 PWB fixed edges, backplane & board fronts are 2 PWB pinned edges
Individual stacked board frames
Frame ledge with screw support only is quite marginal (4 pinned PWB edges)
Adding Epoxy edge fillets yields highest board first mode (4 fixed PWB edges ← Best Fn)
Stacked board frames provide marginal box shear stiffness
MAVEN PC-104 board to board connectors were unreliable, so changed to wired backplane

3. MEP configuration discussion
HISTORICAL REVIEW
THEMIS (wedge-loks) – post launch analyses indicate design was adequate
Random ASD: Hz → Steinberg: ~100 Hz PWBs → ~200 Hz box
RBSP (wedge-loks) – post CDR analyses revealed adequate design (APL reviewed)
Random ASD: Hz → Steinberg: ~100 Hz PWBs → ~200 Hz box
MAVEN (board frames) – significant re-design & evaluation was needed (+ D CDR)
Random ASD Hz → Steinberg: ~300 Hz; PWBs → ~600 Hz box
Mass Ds: (6.62 kg original kg increase) ● 306 g MDM harness cover
318 g – shear panels ●70 g for 100 box screws ●178 g - DFB stiffener ●100 g - MAG stiffener
Recent MAVEN / SSL based advances or “review gains”
Finite fatigue life has now been accepted by both APL & GSFC reviewers
Assume 4X accumulated vibration exposure with < 25% fatigue damage
PWB’s epoxy fillets to board frames to increase board resonant frequency.
50/50 Boron Nitride filled EA-9309 Epoxy for improved thermal conductivity
SSL tested epoxy bond strength margin > 12
EM board “tap test” or sine signature now needed for margin verification

4. DON’T Actel CQ352 package soldered to primary PWB
MEP configuration discussion
DON’T Actel CQ352 package soldered to primary PWB
DO same Actel ! in CCGA package on plug-in daughterboard
(non flight parts shown)
Flown by GSFC groups – draft CCGA standard released
batch soldered by JPL or a few approved vendors
high 1st mode is not a worry, main PWB rolls off inputs
Non-flight flash parts can be plugged in for development
Success oriented approach
‘One Shot’ fab – rework unsettling
Rigid ceramic body limits max PCB deflection to ≈0.010”

5. DON’T PWB’s parallel to panel
MEP configuration discussion
DON’T PWB’s parallel to panel
SPP parallel PWB: 1.25 G2/Hz
Steinberg calls: no easy solution !
Likely > 300 Hz all PWB; > 600 Hz box
All PWB’s may need stiffeners
≈ Mass Cost: 5.94kg now kg added
7 PWB 0.18 kg each
Shear panels & 0.37 kg
SPP perpendicular: 0.04 G2/Hz
Like THEMIS, Easier than RBSP !
Steinberg calls: > 100 Hz PWB; > 200 Hz box
Wedge-loks or epoxied frames adequate
PWB frames with 8 (corner & side) skewers may be adequate.
DO PWB’s perpendicular to panel
CAUTION: box Random Test Specs can change

6. MEP configuration discussion
PWB’s SHOULD BE TO BE FLIPPED 90°
MAVEN skewer locations
Alternate locations for higher stiffness
CURRENT MASS VALUES (June 2011)

7. MEP configuration discussion
COLLECTED COMMENTS
Reserve a (9th) common board center skewer position for PWB stiffening or shield support
Large parts should not be located close to the center of a PWB (max deflected curvature)
Screwed 1/32” hard anodized aluminum shield boards provide board to board EMI isolation
Maven frames were “dead mass” above ≈250 Hz where shear panels were needed
NOTE that any one “offending” PWB will drive the whole box design
Wired backplane can be used with Wedge-Lok concept (include F/R card edge supports)

8. HISTORICAL or BACK UP CHARTS

9. Wedge Lock Approach Wedge Lock design features Pros Cons
Boards have side entry into stack, secured by wedge locks
Uses a backplane for board interconnect
Pros
Fatigue lifetime analysis indicates that wedge locks provide stiffer boards (see chart next slide) to support large, flat pack ICs, at lower overall mass
The lower mass assertion needs some backup
Backplane is a really convenient method of board interconnect
Board integration (and de-integration) is simpler
Cons
Close tolerances are required to secure boards, and make backplane connector engage. Usually involves special assembly techniques, shims, etc.
Backplane connector interface is subject to large, lateral mechanical loads
Electrical shielding between boards, for radiated interference, is more difficult. This is especially true at high frequencies, and SPP requires HFR reception up to 20MHz.
Heat conduction is really only available via wedge locks, i.e. on two sides of board
Some team members simply don’t want this approach.

10. Board margin requirements
Pankow worked some general SPP cases
Board 6.3” x 9.3” x .062” thick
Same Actel (CQ352) and similar position seen on MAVEN DFB board for reference
Design life = 4X expected test durations + launch
PCB Frequency
Finite Life Design (GEVS < 62%)
Accumulated Fatigue Damage
Required Box Frequency
PCB Screwed to Frames 72
45%
24%
144
PCB Screwed and Glued to Frames
141
20% 0%
282
PCB Screwed with Center Support 99
41%
18%
198
PCB Wedge Locks 91
34% 5%
182
PCB Wedge Locks and Center Support
313
10%
626

11. Board-in-Frame Approach
Board-in-Frame design features
Boards are stacked, held together by skewers, can be either horizontal or vertical stack.
Uses external harnesses for board interconnect
Pros
Each board is in its own EMI-tight enclosure
That’s not to say we won’t have noise problems! It’s just one step in improving noise immunity.
Better thermal conduction, four sides available for heat transfer
On MAVEN, I think epoxy was required to make heat transfer really work well, and was needed to get board frequency up to desired level
Cons
Potential for differential motion between slices, fretting damage on frames
Harder to achieve required box stiffness, box frequency >2X board frequency
MAVEN required external stiffeners, which added mass

12. Board-in-Pan Approach
Board-in-Pan (5 sided box) design features
Boards are stacked, held together by corner skewers, can be either horizontal or vertical stack but horizontal is nicer.
Uses external harnesses for board interconnect
UMN approach to LNPS slice
Base/bottom of horizontal stack
Pros
Each board is in its own EMI-tight enclosure
That’s not to say we won’t have noise problems! It’s just one step in improving noise immunity.
Board can have one or more center post(s)
Better thermal conduction, four sides + posts available for heat transfer
On MAVEN, I think epoxy was required to make heat transfer really work well (more screws?), and was needed to get board frequency up to desired level
Cons
Potential mass hit if floor is thick enough to be structural
Potential for differential motion between slices, fretting damage on frames
Not with UMN’s nice V-grooves!
Harder to achieve required box stiffness, box frequency >2X board frequency
MAVEN required external stiffeners, which added mass