LAT FSW System Checkout TRR

1
GLAST Large Area Telescope LAT Pre-Shipment
Review NCRs and Waivers Rich Bright Systems
Engineering Stanford Linear Accelerator Center
2
NCR Introduction

• Presentation focus is on the significant hardware
  NCRs that remain open
• Several NCRs are planned to be left open for
  Environmental Testing at NRL
• Open NCRs continue to be worked towards closure
• Several NCRs are expected to be unresolved in
  time for LAT shipment to NRL
• NCRs have classified into categories for
  discussion purposes
• NCR Summary List of all open NCRs is presented
  for reference

3
NCR Category Definitions

• Open NCRs classified into categories for
  discussion purposes

Category Definition Count
Hardware Discrepancy H/W Issue that does not meet design specification or intent 3
FSW Discrepancy Identified FSW bug, FSW JIRA in work 3
Monitor for Verification Likely test issue, trending for repeat 2
Known Feature Specification not violated, but trending or changes required to accommodate behavior 9
Minor Documentation Issue will close with final documentation 12
Spares NCR is against flight spares only 8
EGSE NCR has been isolated to EGSE 1
Data Processing Identified Data Processing bug 3
Under Investigation Cause of the anomaly is under investigation 3
4
Significant Open NCRs
5
NCR 535Tower FM-4, Layer Y4 Margins

• Issue
• 1 of 2 GTRCs in layer Y4 of FM-4 failed margin
  tests
• IS worked up to 53 clock duty SB up to 55
• IS worked down to Vdd2.51 V SB down to 2.50 V
• Analysis
• The GTRC is known to have weak timing margins in
  its memory access. The clock termination on the
  cables was changed from 100 ohms to 75 ohms to
  alleviate this, and MCMs were screened for clock
  duty cycle. Nevertheless, this 1 out of 1152
  GTRCs slipped through and doesnt quite meet our
  spec when installed in the final system.
• No failure has been seen to date at the nominal
  operating points (2.65V and 50), including
  during Rome T/V testing.
• Resolution Plan
• Pair with a TEM/TPS with a relatively high
  measured Vdd
• Keep the NCR open to monitor at high T in T/V
  tests
• Impacts on On-orbit performance
• None expected. Even in the worst case, if this
  GTRC gives repeated errors, the MCM could be read
  from the other cable, with no loss of channels.

6
NCR 624ACD Temperature Sensor

• Issue
• NCR opened to track GSFC PR ACD-02334-004 for LAT
  TV testing
• ACD Thermal Monitoring system readout for
  Yp_Inshell_S initially read 23 deg C at startup
  and started fluctuating between 5 deg C and -50
  deg C
• Analysis
• Thermistor operated properly after pump-down and
  anomaly was not observed throughout T/V test nor
  during ambient pressure checkout post-T/V
• Not indicative of a thermistor failure
• Likely cause is connection between the ACD and
  the readout outside the T/V chamber
• Resolution Plan
• Monitor during LAT TV
• Impacts on On-orbit performance
• There are two thermistors right next to each
  other, and the redundant thermistor always
  functioned without anomalies
• Potential loss of redundancy for this thermistor

7
NCR 626ACD Rates During Transitions

• Issue
• NCR opened to track GSFC PR ACD-02334-016 for LAT
  TV testing
• Observed high count rates exceeding 1000 Hz in
  the ACDMonitor script during two of the four
  transitions from hot to cold during the ACD TV
• Analysis
• Temperature range was -10 C to -15 C
• Because hardware counters were used, we only know
  that it was one of the data channels from
  phototubes attached to tile 320 - i.e. GARC 6,
  GAFE 16 or GARC 7, GAFE 17
• By the time the temperature had stabilized at -25
  C, the rates had returned to their normal values
  of less than 100 Hz
• No problems have been seen with either phototube
  signal in any functional test at any temperature.
• Resolution Plan
• Monitor during LAT TV
• Impacts on On-orbit performance
• Potential need to mask inputs from a phototube
  for tile 320
• Tile 320 is located near the base of the ACD on
  the X side
• Loss of one signal is acceptable for ACD
  performance

8
NCR 685ACD GARC 11 GAFE 13 Have Excess Counts

• Issue
• AcdHldCal runs 134001115, 134001116 and 134001117
  all had large excess counts (gtgt20) for GARC 11,
  GAFE 13.
• Analysis
• Issue has been determined to be a result of the
  test methodology
• The test method under LATTE results in a test
  that goes too close to the noise threshold of
  this channel and results in retriggering.
• The revised test under LICOS uses an alternate
  strategy to test the functionality and will not
  have this problem.
• Resolution Plan
• No Defect
• Impacts on On-orbit performance
• None

9
NCRs 809/881/882 EPU/SIU Reboots

• Issue
• Three instances of CPU reboots
• First was EPU reboot during file upload after the
  primary boot code for that EPU was successfully
  completed
• Second two were SIU reboot followed by EPU reboot
• Analysis
• First instance cause is likely software related
  rather than hardware
• Second instance appears to be an SIU reboot
  requested by the operating system, followed by an
  EPU timeout likely due to disruption in traffic
  with the SIU
• Resolution Plan
• Plan in place to gather additional data if
  another reboot occurs
• Impacts on On-orbit performance
• Loss of data until LAT re-initialization is
  complete

10
NCR 855LATC Verify Errors

• Issue
• Infrequent register setting verification errors
  as registers are set up for physics runs
• Analysis
• Frequency is 16 errors out of 800 runs, with over
  200 additional readouts.
• Shows in Readout Controller Chips even though the
  Front End chips dominate the chip count
• Have seen read and write verify errors
• Register tests through LATTE did not show this
  problem
• Resolution Plan
• Gathering data using combination of LATC dumps
  and augmented register reporting in LATC dumps to
  isolate the cause
• Impacts on On-orbit performance
• TBD

11
NCR 890Radiator Short

• Issue
•  Radiator heater isolation tests fails on Y
  Radiator connector JL-128
• Analysis
• Measured resistance for the return leg of the
  radiator redundant survival heaters on JL-128 to
  the face sheet of the radiator to be 121 ohms
• The day after it measured 55 ohms, and it again
  changed to 58 ohms 
• Time Domain Reflectometer (TDR) test performed
  reveals
• The secondary return is different from the
  primary return
• There are two "possible" discontinuities, one 6.1
  feet and one that is 13.4 feet from the connector
• Resolution Plan
• Determine the cause of the failure and repair
• Impacts on On-orbit performance
• If not repaired, loss of one half of the Y
  radiator redundant survival heaters.

12
NCR 625ACD Veto Hitmap PHA Failure

• Issue
• AcdVetoHitmapPha failed in GARC 11, GAFE 17 under
  high level charge injection.
• Analysis
• Root cause is unknown. This is a test script
  that we no longer use as the functionally that it
  tests is covered in other tests, though not as
  explicitly. The main purpose of this test is to
  confirm that the PHA and veto data are consistent
  with each other and with the software scalars.
• For particle data we have scripts to check the
  consistency between the PHA, Veto and GEM data
  explicitly.
• This type of error could have been caused by
  using settings or charge injection reset
  procedures that are known to cause retriggering.
• Resolution Plan
• Monitor that Veto and PHA data are consistent in
  particle data runs.
• No plans to re-run this particular script.
• Impacts on On-Orbit performance
• None.

13
NCR 684Tracker Noise Flares

• Issue
• 8 (of 612) layers in 17 Trackers have shown
  infrequent, sporadic flares of increased noise
  occupancy. The 8 layers are uncorrelated.
• The flares are correlated across channels in a
  given ladder, with many or all channels in the
  ladder firing at once.
• There is no evidence that the problem was
  statistically worse in T/V than in atmosphere,
  but we cannot rule out a small effect.
• Analysis
• Monitor in cosmic-ray data in FM-8 and in 16
  towers.
• The affected regions are fully ON and sensitive
  immediately before and after a flare. This ruled
  out intermittent bias connections as a cause.
• Even during flares, all recent runs still satisfy
  all noise specifications.
• Study in FM-8 versus HV level and humidity
• Unfortunately, we could not get the problem to
  recur at all in FM-8, so we did not reach any
  conclusion.
• Resolution Plan
• Continue to monitor the effects in 16-tower
  cosmic-ray data, especially in T/V testing.
• Impacts on On-orbit performance
• The observed noise is very far from a level that
  would have any impact at all on performance. An
  increase by much more than an order of magnitude,
  including spreading to other trays, would have to
  occur to begin to see impacts. (Overall, the TKR
  noise performance is phenomenally good!)

14
NCR 718 ACD Channel 1123 Veto Threshold Min. is
0.45 pC

• Issue
• Can not set the VETO threshold for GARC 1, GAFE
  13 (aka tile 123, pmt 1)below 0.45pC (about 2/3
  of a MIP) The nominal setting for would be
  0.25pC ( 0.2 - 0.3 of a MIP).
• Analysis
• The root cause is not known. Trying to set any
  VETO threshold below 0.45 pC results in the same
  actual threshold trigger point. Likely this is
  an issue with the front-end electronic in this
  channel.
• This channel is in the 3rd row of side tiles and
  will _NOT_ be part of the normal operating mode
  ACD veto. However, it may be used in any ACD
  triggered operations. In any case, it is still
  possible to set the threshold of fire well below
  a MIP, so this will have a minimal effect even in
  the ACD triggered operations.
• Resolution Plan
• Use as is. Monitor for degradation. Make plans
  to treat this channel specially in offline
  calibration and analysis.
• Impacts on On-Orbit performance
• None in regular operation. Minimal
  in-efficiency in ACD-triggered operation.
  Doesn't not affect any science requirements.

15
NCR 829ACD Coherent Noise at 1000 System Clock
Ticks

• Issue
• ACD shows coherent noise at 1000 system clock
  ticks after each event.
• Analysis
• The root cause is not known. The pedestal value
  in each channel varies with time since previous
  event.
• Pedestal is shifted down 30 bins at 500 ticks
  after previous event, and up 15 bins at 1000
  ticks. These shifts correspond to about -0.08
  MIPs to 0.04 MIPs.
• Zero suppression threshold is nominally 15 bins
  above nominal pedestal, so upward drifts in
  pedestal can cause excess numbers of hits near
  1000 ticks after previous event.
• Preliminary analysis has not seen similar effects
  in the VETO and CNO lines, suggesting that any
  effect there are too small to cause noticeable
  changes in performance.
• LAT design requires ACD to be efficient above
  0.2-0.3 MIPs, i.e. margin still exists.
• Resolution Plan
• Use as is. Quantify effect for each channel and
  correct offline.
• Determine temperature dependence of effect.
• Impacts on On-Orbit performance
• Still under investigation
• Possible reduced efficiency for very small pulses
  (0.1 MIPs or less) that occur 500 - 750 ticks
  after previous event
• Possible excess noise occupancy at on-orbit
  background rates. Mitigate by raising zero
  suppression thresholds to 25 bins above
  pedestal. This would dramatically reduce the
  number of the excess hits with minimal effect on
  science performance.

16
TKR Bad Strips

• Three major categories
• Hot strips
• Historically anything gt10?4 occupancy, but strips
  well above this level can still be useful and
  should not be masked unnecessarily!
• Small numbers, with no trending issues.
• Dead strips do not respond to internal charge
  injection.
• Either a dead amplifier or a broken SSD strip
  connected to the amplifier (usually the latter).
• Very small numbers, with no trending issues.
• Disconnected strips broken wire bond or trace
• between ladder and amplifier, mostly due to MCM
  encapsulation debonding from silicone
  contamination.
• or between SSDs within a ladder, due to Nusil
  encapsulation debonding in thermal cycles.
• The majority of the bad strips is in early
  towers, and the delamination definitely
  propagates somewhat with time.

17
SLAC Trending, All Towers
18
SLAC Trending
(Zero represents the state of the tower during
the hand-off test.)
The increases are almost entirely in the fully
disconnected category, suggesting some creep of
the encapsulation delamination on some MCMs.
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Summary

• The problem of encapsulation delamination has
  been well known and discussed for a long time,
  including the increase during Tracker T/V
  testing, but the project elected to use the
  affected MCMs as-is because of
• the high cost of redoing 1/3 of the MCM
  production
• and the belief that future degradation would
  never reach a level at which the science would be
  compromised.
• Nothing is different today
• There is some evidence that the problem areas
  have expanded very slightly during LAT
  integration, but
• It is impossible to be sure at any time what
  channels are really disconnected, because the
  wires in delamination regions often make
  electrical contact even when the mechanical bond
  is gone.
• No disconnected channels have appeared in
  previously unaffected regions of MCMs.
• We can expect that the problem regions will
  expand during LAT environmental testing, but if
  comparable to the Tracker environmental testing,
  the degradation will not be significant with
  respect to science performance.

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Open NCRs
21
Open NCRs (contd)
22
Open NCRs (contd)
23
LAT Waivers (1/2)
CCR Title Description Status
433-0311 DC Voltage Tolerance LAT is required to tolerate 0-40V DC. Due to MOSFET switches at power feed inputs, LAT can tolerate minimum 15V, excluding transient events. Approved
433-0357 DC Voltage Tolerance 2 LAT required to tolerate 0-40V DC. After a voltage drop analysis, it was found that the TEM MOSFET switches would receive too low a voltage with the DAQ feed voltage at 15V. To operate the TEM's safely, the input voltage needs to be 18.5V minimum. Submitted
433-XXX J-STD vs NASA STD LAT circuit card assemblies uses J-STD-001 as the workmanship standard instead of NASA-STD-8739.3. Submitted
433-0356 Test Point Short Circuit Isolation LAT is required to operate within spec if any test point is shorted to ground. A shorted external clock select pin would render the redundant GASU inoperable. Submitted
433-0358 GTFE TID LAT is required to perform TID testing on all GTFE ASIC lots. The final two lots were not tested since previous lots exhibited such large margins. Submitted
24
LAT Waivers (2/2)
CCR Title Description Status
433-XXX 24AWG STD Strength Cu High strength Cu alloy is required for 24AWG wire. LAT uses standard strength Cu wire. As reported by the LAT PCB, standard strength 24AWG wire has been used on previous NASA projects with GSFCs approval with no compromise to product reliability. Submitted
433-XXX Tracker Flex Cable and MCM Coupon Failures Several flex cables and MCMs are installed on the LAT although they have failed coupons. Submitted
433-XXX Tracker Environmental Test With Non-Flt or Missing Cables Several tracker towers went through environmental test with a subset of missing or non-flight flex cables. The replacement flight cables were not subjected to component-level vibe and will not see twelve tvac cycles. Submitted
433-XXX Radiator Low-Level Sine Vibe Instead of a low-level sine test, the radiators were subjected to alternative tests. To be submitted
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SC-LAT ICD Status

• SC-LAT ICD EIY46311-000C is released
• The table below lists pending changes

ICN Title Description Status
-096 Unregulated Power Voltage For shorts periods of time, the SC will be unable to provide the minimum 25V for the unregulated feeds. The voltage may get as low as 23V. SASS voltage drop analysis in process
-099 LAT Integration This is an appendix to the ICD that is meant to capture agreements for Observatory IT activities. Final logistical details in work
-100 LAT Impedance Incorporate into ICD the as-measured LAT differential impedance. Measured Data being evaluated
26
DC Voltage Tolerance
Requirement 433-IRD-001 SC-LAT IRD 3.2.4.1.3.2
DC Voltage Tolerance The LAT shall tolerate
without damage or degradation DC voltages greater
than 0 volts and less than 40.0 volts. Departure
From Requirement The MOSFET's used to implement
the Variable Conductance Heat Pipe (VCHP)
switches in the Heater Control Boxes and the
input switches in the Power Distribution Unit
(PDU) can tolerate a continuous minimum supply
voltage of 16V. A lower continuous voltage
causes the MOSFET's to be in a partially
conducting state. When in this partially
conducting state, the increased MOSFET
source-drain impedance causes the device to
dissipate more power which increases its
temperature to the point of damage.
Justification 1. The MOSFET's do tolerate
transient events such as the SC
enabling/disabling the VCHP or DAQ power
feeds. 2. The VCHP and DAQ feeds are regulated
at 28/-1V so a continuous lower voltage would be
an anomaly. 3. The SC uses DC/DC converters
that turn off when their input voltage is below
the minimum 16-V input voltage to generate 28V at
their output. In other words, the DC/DC
converter output is either 0V or 28V and never in
between. 4. The MOSFETs are not a single point
failure. The primary and redundant feeds do not
use the same set of MOSFETs. If there is an
anomaly on the primary feeds that damage the
primary MOSFETs, the redundant feed and MOSFETs
can be used to continue the mission. 5. The EGSE
contains low voltage protection to protect the
LAT during ground test.
27
DC Voltage Tolerance 2
Requirement 433-IRD-001 SC-LAT IRD 3.2.4.1.3.2
DC Voltage Tolerance The LAT shall tolerate
without damage or degradation DC voltages greater
than 0 volts and less than 40.0 volts. Departure
From Requirement The MOSFET's used to implement
the TEM Power Supply (TEM) switches can tolerate
a continuous minimum supply voltage of 16V. A
lower continuous voltage causes the MOSFET's to
be in a partially conducting state. When in this
partially conducting state, the increased MOSFET
source-drain impedance causes the device to
dissipate more power that increases its
temperature to the point of damage. Accounting
for the voltage drops within the LAT Power
system, the minimum input voltage to the LAT is
18.5V. Justification 1. The LAT MOSFET
switches require a minimum voltage of 16.15V.
This translates to 18.5V at the SC-LAT power
input after accounting for power system voltage
drops. 2. The MOSFET switches do tolerate
transient events such as enabling/disabling the
input power feeds. 3. The SIU/VCHP and DAQ
feeds are regulated at 28/-1V so a continuous
lower voltage would be an anomaly. 4. The
Electrical Ground Support Equipment (EGSE)
contains low voltage protection to protect the
LAT during ground test.
28
J-STD vs. NASA STD
Requirement 433-MAR-001 Mission Assurance
Requirements 5.2 Workmanship and Printed
Circuit Board Coupons The GLAST Project will use
the following NASA and commercial workmanship
standards for LAT a. NASA-STD-8739.3
Soldered Electrical Connections Departure From
Requirement J-STD-001 is used as the workmanship
standard instead of the preferred NASA-STD-8739.3
Justification J-STD-001 is used by DoD as a
primary standard for reliable electrical
connections. SLAC suppliers are qualified to
meet the requirements of J-STD-001 and
J-STD-001CS (Space Applications Electronic
Hardware Addendum). Workmanship has been
verified by inspection of the CCAs. Minor
differences in the J-STD and NASA STD will not
affect the products reliability.
29
Test Point Short Circuit Isolation
Requirement 433-IRD-001SC-LAT IRD 3.2.4.6.8
Short Circuit Isolation Test point short-circuit
isolation shall also be provided. The
Observatory shall operate within specification in
the event any test point is shorted to the power
bus, ground, or another test point, and upon
removal of the short. Departure From
Requirement Background Test port JL-39 once
provided additional capability to exercise the
instrument and testbed that was not possible
through flight interfaces. Namely, clock and
voltage margining, external trigger and front-end
simulator controls. All these signal wires have
been removed for the flight instrument except for
pri/red external trigger, redundant external
clock select and redundant external clock. Only
the redundant external clock select can adversely
affect the instrument if this pin is shorted to
ground. The effect is the redundant GASU looks
for an external clock instead of the internal LAT
clock. Once the short is removed, the internal
LAT clock is again selected. Connector JL-39
pin11 is the external clock select for the
redundant GASU. If this pin is shorted to
ground, it will disable the redundant GASU clock
rendering the redundant GASU inoperable. Note
that all other test point pins are double
buffered by the LVDS translator and FPGA pin
buffer. Justification 1. Only the redundant
GASU clock is affected. The primary GASU
external clock select pin is not present on
JL-39. All other test point signals are double
buffered and poses no risk to the instrument. 2.
The ability to diagnose if an anomaly is clock
related is significantly reduced without the
capability to adjust the clock frequency.
Without this capability, depending on the nature
of the anomaly, a credible outcome is the
disassembly of the LAT so the electronic boxes
can be accessed. This is true for all test
phases including Observatory IT and launch
base. 3. Processes are in place to safeguard
against shorts. The JL-39 cable assembly is
manufactured and tested per NASA STD 8739.3 and
8739.4. During the LAT IT phase, JL-39 is
subject to safe-to-mate/EICIT tests prior to each
mate. Also, a visual inspection is performed for
every mate/demate including the final
installation of the flight cover. 4. Once on
orbit, the risk of a short is very low. The
majority of the risk, although still considered
low, is due to ground operations. To mitigate
this risk, the redundant clock will be tested
after the final installation of the JL-39
protective cover ensuring that no short exists
prior to launch.
30
GTFE TID
Requirement 433-SPEC-001 Mission System
Specification 3.3.6.2.1.2.3 Ground Testing If
test data do not exist, ground testing shall be
required. For commercial components, testing
shall be performed on every flight procurement
lot. Departure From Requirement The deviation
from 433-SPEC-001 is to not perform TID testing
on the two additional fabrication runs for GTFE
(T2CY3 and T2CY4) which are required to
complete the TKR MCM production. Justification Th
e successful radiation hardness assurance program
testing for TID, SEE, and SEU, for the 9 GLAST
LAT ASICs demonstrated considerable engineering
margin. The ASICs were fabricated in Agilent
0.5um CMOS.The Tracker GTFE ASIC has now been
tested twice, for different date codes.T2CY1  
fab'd Nov '02, TID tested Jun '03T2CY2  fab'd
Jan '04, TID tested Mar '04 T2CY3   fab'd Mar
'05T2CY4   fab'd May '05Given that the
radiation requirements for the mission are below
10krad even with large engineering margins, and
that all ASICs in the Agilent process passed with
flying colors, and that we have tested ASICs up
to 40 krad without degradation, I propose that we
forgo further testing on the two additional
fabrication runs for GTFE (T2CY3, fab'd Mar '05
and T2CY4, fab'd May '05), which are required to
complete the TKR MCM production. These ASICs are
produced with identical masks in a very well
qualified and controlled process not testing
them for TID can be considered extremely low
risk.
31
24AWG STD Strength Cu
Requirement 433-MAR-001Mission Assurance
Requirements 5.2 Workmanship and Printed Circuit
Board Coupons NASA-STD-8739.4 PARA 7.3 (12)
High strength copper alloy shall be used for
AWG 24 and smaller conductors. Departure From
Requirement Standard strength copper alloy
wires, MIL-W-22759/11-24 were used in
cable/harness assemblies instead of high strength
copper alloy fro 24 gauge wires. NASA-STD-8739.4
Para 7.3 (12) specifies the use of high strength
copper alloy wires for 24 AWG or smaller size.
Reference drawings LAT-DS-05054 and
LAT-DS-03453. Justification The LAT used
standard strength wire for 24AWG and larger wire
and high strength wire for 26AWG wire. Both of
these parts were approved by the LAT Parts
Control Board in 2004. As reported by the LAT
PCB, standard strength 24AWG wire has been used
on previous NASA projects with GSFCs approval
with no compromise to product reliability. Use
of standard strength 24AWG wire does not create
an unnecessary risk to the reliability of the LAT
cables using this wire.
32
Tracker Flex Cable and MCM Coupon Failures (1/3)

• Requirement
• 433-MAR-001 Mission Assurance Requirements
• 5.2 Workmanship and Printed Circuit Board
  Coupons
• IPC-6012 Qualification and Performance
  Specifications for Rigid printed boards
• IPC-6013 Qualification and Performance
  Specifications for Flexible boards
• Departure From Requirement
• Installed on the LAT are seven Tracker flex
  cables and three MCMs (multi-ship modules) that
  had coupon failures.
• Justification
• Flex Cables
• The NCRs and ECs listed in Table-1 contains
  the detailed test results and disposition to use
  as-is. The risk was judged to be very low for
  the following reasons
• 1. In all cases the nonconforming cable was
  paired on a tower side with a conforming cable.
  The two cables on a tower side are completely
  redundant. If one cable were to fail, 100 of
  the channels could be controlled and read via the
  other cable.
• 2. None of the cables (conforming or
  nonconforming) suffered any failures whatsoever
  before, during, or after the tower environmental
  testing. That holds true also for 7
  nonconforming cables that were temporarily used
  during tower environmental testing in later
  towers (and subsequently replaced by conforming
  cables).

33
Tracker Flex Cable and MCM Coupon Failures (2/3)
3.The nonconformances themselves were judged
to be relatively low risk, in that the vias are
unlikely to fail even with the imperfections. 4.
The vast majority of the vias in the cable are
doubled up for redundancy, so failure of a single
via is unlikely to affect the performance.
Table 1 - FLEX CABLES
Tower Flex Cable NCR EC Coupon Failure
A C2 S/N 4 241 50143 Barrel separation annular ring lt0.002"
2 C3 S/N 1 350 50116 Barrel separation
  C4 S/N 6 350 50129 Barrel separation
  C6 S/N 5 350 50130 Vias missing on coupon
3 C2 S/N 6 358 50226 Annular ring lt0.002"
  C4 S/N 1 358 40516 Annular ring lt0.002"
  C6 S/N 6 358 50130 Vias missing on coupon
34
Tracker Flex Cable and MCM Coupon Failures (3/3)
MCMs The MCMs are deemed OK to use as-is are
as follows 1. All 3 MCMs passed through the
entire flight-hardware test regimen with no
failures, including 21 thermal cycles plus
168-hour burn-in, 4 tray thermal cycles, and the
tower-level vibration and thermal-vacuum cycles.
2. The nonconforming annular rings are judged
to be low risk, since they were greater than
0.001 in any case. 3. The coupon delamination
is more serious, but the MCM tests indicate that
the actual PWB is okay. In the worst case,
complete failure of that MCM would knock out 7 of
the 36 layers in that tower, reducing performance
in one tower, but not killing it. However, the
MCMs have redundancy built in, such that a single
failure, such as opening one via, would in the
worst case result in the loss of just a single
front-end chip (64 channels out of 1536 on that
layer). These 3 MCMs were designated
use-as-is because of the severe impact on
schedule and cost that would result in trying to
replace them versus the low risk of a failure
that, in the worst case, would not prevent the
instrument from satisfying its science
requirement.
Table 2 - MCMs
Tower/Tray MCM NCR EC Coupon Failure
Tower 6, Tray H004 2354 158 50110 Delamination
Tower 15, Tray M175 13078 553 50431 Annular ring lt0.002"
Tower 15, Tray M192 13085 553 50431 Annular ring lt0.002"
35
Tracker Environmental Test With Non-Flight or
Missing Cables (1/3)
Requirement 433-IRD-001 SC-LAT IRD 3.2.2.8.4
Component Evaluation Random Vibration The
evaluation of components (with components as
defined in the GEVS-SE) shall use the generalized
random vibration power spectral density in
GEVS-SE. 433-MAR-001 Mission Assurance
Requirements Prior to its delivery to the
Government/spacecraft integrator, all temperature
sensitive instrument components will be subjected
to a minimum of eight (8) thermal-vacuum
cycles. Departure From Requirement Several
tracker towers did not have the full complement
of flight flex cables for tower environmental
test. This means several flex cables did not
receive component level vibe and will have eight
thermal vacuum cycles instead of the required
twelve.
36
Tracker Environmental Test With Non-Flight or
Missing Cables (2/3)

• Justification
• Due to the unavailability of a full complement of
  flight flex cables, seven tracker towers had a
  subset of missing or non-flight flex cables
  installed for tower environmental testing. See
  Table 1 for the list of missing or non-flight
  cables. This is considered low risk because
• The tracker design contains flex cable-redundancy
  in that the flex cables are paired on a tower
  side to read the same data. Only one cable from
  each pair is required to meet tower performance.
  In most cases a non-flight cable was substituted
  for each missing flight cable during the test
  phase (and later removed), in order to facilitate
  a comprehensive test of the tracker towers. In
  several instances no non-flight cable was
  available and the testing was done with just a
  single flight cable on a tower side, a
  configuration in which nevertheless 100 of the
  detector channels could be read out. In all
  cases, tower testing was not carried out until
  there was at least one flight cable on each tower
  side. Therefore, if the flight cables added post
  testing were to fail, all sides of the towers
  could still be read out via the flight cables
  that were included in the testing.
• No failures were encountered for all the flight
  flex cables that were subjected to environmental
  testing.
• The tracker tower comprehensive performance tests
  (CPT) were performed successfully after the full
  complement of flex cables was installed.
• The flex cables will be subjected to LAT and
  Observatory level vibration and to eight thermal
  vacuum cycles before launch.

37
Tracker Environmental Test With Non-Flight or
Missing Cables (3/3)
Non-Flight or Missing Flex Cables
Tower Flex Cable Status for Env Test
7 C6 Missing
9 C0 Non-Flight
  C3 Missing
  C7 Non-Flight
10 C0 Non-Flight
  C3 Missing
  C7 Non-Flight
11 C1 Non-Flight
  C2 Non-Flight
12 C5 Missing
13 C2 Missing
  C5 Missing
15 C2 Non-Flight
  C5 Missing