glast lat project april 27, 2006: lat pre-ship review presentation 7 of 12 ncrs and waivers 1 glast...

NCRs and Waivers 1

GLAST LAT Project April 27, 2006: LAT Pre-Ship Review

Presentation 7 of 12

GLAST Large Area TelescopeGLAST Large Area Telescope

LAT Pre-Shipment Review

NCRs and Waivers

Rich BrightSystems Engineering

Stanford Linear Accelerator Center

Gamma-ray Large Gamma-ray Large Area Space Area Space TelescopeTelescope

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NCR Introduction 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

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NCR Category Definitions NCR Category Definitions

Open NCR’s 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

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Significant Open NCRsSignificant Open NCRs

NCRDate

OpenedAffected

Hardware Problem Description Status NCR Category535 6/20/05 TKR Monitor trend data for TKR 4 thru LAT testing (spin-off NCR) Monitor Known Feature624 8/27/05 ACD Fluctuations in temperature reading (originally an ACD PR) Monitor Monitor for

Verification625 8/27/05 ACD AcdVetoHitmapPha failed (originally an ACD PR) Monitor Known Feature626 8/27/05 ACD ACDMonitor script high counts during T/V (originally ACD PR) Monitor Monitor for

Verification684 10/3/05 TKR Noise Occupancy failures due to intermittent hot strips. Disposition Required Known Feature685* 10/3/06 ACD KldCal runs had excessive counts for GARC11, GAFE13 Disposition Required Known Feature

718 10/27/05 ACD VETO threshold for channel 1123 can't be set below minimum value of 0.45pC (originally an ACD PR)

Monitor Known Feature

809 1/17/06 Integration EPU crate s/n GLAT 2522 spontaneous reset Analysis Completed Minor Documentation

829 2/10/06 ACD ACD coherent noise at 1000 system clock ticks after each event collected in flight config.

Disposition Required Known Feature

855 3/23/06 FSW LATC verify occcasionally reports errors in CRC, SPT, ARC Disposition required Under Investigation

880 4/12/06 Integration SIU Spontaneous Reboot New Under Investigation

881 4/12/06 Integration EPU Spontaneous Reboot Analysis In Progress Under Investigation

890 4/19/06 Integration Radiator heater isolation tests fails on +Y Radiator New Hardware Discrepancy

*Closed but listed for Pre-PSR NCR review continuity

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NCR 535NCR 535Tower FM-4, Layer Y4 MarginsTower 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 Vdd=2.51 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 doesn’t 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.

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NCR 624NCR 624ACD Temperature SensorACD 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

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NCR 626NCR 626ACD Rates During TransitionsACD 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

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NCR 685NCR 685ACD GARC 11 GAFE 13 Have Excess CountsACD GARC 11 GAFE 13 Have Excess Counts

• Issue– AcdHldCal runs 134001115, 134001116 and 134001117 all

had large excess counts (>>20) 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

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NCRs 809/881/882 NCRs 809/881/882 EPU/SIU RebootsEPU/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

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NCR 855NCR 855LATC Verify ErrorsLATC 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

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NCR 890NCR 890Radiator ShortRadiator 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.

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NCR 625NCR 625ACD Veto Hitmap PHA FailureACD 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.

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NCR 684NCR 684Tracker Noise FlaresTracker 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!)

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

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NCR 829NCR 829ACD Coherent Noise at 1000 System Clock TicksACD 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.

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TKR Bad StripsTKR Bad Strips

• Three major categories:1. Hot strips:

– Historically anything >104 occupancy, but strips well above this level can still be useful and should not be masked unnecessarily!

– Small numbers, with no trending issues.2. 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.3. 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.

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SLAC Trending, All TowersSLAC Trending, All Towers

Bad Channel Trend

0

200

400

600

800

1000

1200

1400

1600

hand-off 2TWR 4TWR 6TWR 8TWR 16TWR LICOS

Test phase

TkrFMATkrFMBTkrFM1TkrFM2TkrFM3TkrFM5TkrFM4TkrFM6TkrFM7TkrFM9TkrFM10TkrFM11TkrFM12TkrFM13TkrFM14TkrFM15

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SLAC TrendingSLAC Trending

Disconnected Channel Trend

-5

0

5

10

15

20

25

30

35

2TWR 4TWR 6TWR 8TWR 16TWR LICOS

Test phase

TkrFMATkrFMBTkrFM1TkrFM2TkrFM3TkrFM5TkrFM4TkrFM6TkrFM7TkrFM9TkrFM10TkrFM11TkrFM12TkrFM13TkrFM14TkrFM15

(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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SummarySummary

• 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

1. the high cost of redoing 1/3 of the MCM production

2. 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 NCRsOpen NCRs

NCRDate

OpenedAffected

Hardware Problem Description Status NCR Category458 5/10/05 TEM/TPS Observed GTCC and GTRC phasing errors within the TEM FinalMRB required Known Feature529 6/16/05 TKR TEM register test on PDU1 and GASU redundant failed,Twr 3 Monitor Known Feature

535 6/20/05 TKR Monitor trend data for TKR 4 thru LAT testing (spin-off NCR) Monitor Known Feature594 8/2/05 TKR TKR Layer efficiency for Y9 and Y17 are lower at SLAC than

Pisa.Analysis In Progress Known Feature

624 8/27/05 ACD Fluctuations in temperature reading (originally an ACD PR) Monitor Monitor for Verification

625 8/27/05 ACD AcdVetoHitmapPha failed (originally an ACD PR) Monitor Known Feature626 8/27/05 ACD ACDMonitor script high counts during T/V (originally ACD PR) Monitor Monitor for

Verification684 10/3/05 TKR Noise Occupancy failures due to intermittent hot strips. Disposition Required Known Feature718 10/27/05 ACD VETO threshold for channel 1123 can't be set below minimum

value of 0.45pC (originally an ACD PR)Monitor Known Feature

741 11/11/05 Integration HIB Cables to short or wrong orientation. QA to Close Minor Documentation

743 11/13/05 Integration EMI shield mechanical tolerances not met. QA to Close Minor Documentation

774 12/8/05 Integration Cannot mate harness LAT-DS-02092-53. MRB Required Spares777 12/13/05 Integration DAQ-ACD Internal RF shield to GASU cable rework. MRB Required Spares788 12/21/05 GASU GASU AEM temperature alarm MRB Required Minor

Documentation793 12/28/05 ACD GARC digital 3.3V current is slightly below allowed range Disposition Required Minor

Documentation

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Open NCRs (cont’d)Open NCRs (cont’d)

NCRDate

OpenedAffected

Hardware Problem Description Status NCR Category806 1/17/06 Trigger CAL is retriggering during SVAC run Disposition required Known Feature809 1/17/06 Integration EPU crate s/n GLAT 2522 spontaneous reset Analysis Completed Minor

Documentation811 1/23/06 Integration 1553 cables rejected for workmanship MRB Required Spares814 1/27/06 DAQ Glass body of the Thermistor is cracked Disposition Assigned Spares829 2/10/06 ACD ACD coherent noise at 1000 system clock ticks after each event

collected in flight config.Disposition Required Known Feature

836 2/27/06 Mechanical MLI bar bracket does not mount flush, has insufficient thread engagement

Disposition Required Minor Documentation

840 3/8/06 DAQ RunControl software problems observed on spare PDU during T/V test (NCR #794); opened NCR to track FSW updates

Disposition required Spares

843 3/9/06 DAQ Design Change to DAQ-ACD Internal Cable Disposition Required Spares845 3/16/06 FSW TEM register set to incorrect values;TCC event timeout was 0 Disposition required FSW Discrepancy

851 3/16/06 DAQ One or more EPU resets unexpectedly due to large number of events and interaction with end of run activities

Disposition required FSW Discrepancy

852 3/16/06 DAQ LCB errors observed during muon run Disposition required Data Processing854 3/23/06 FSW Digi report occasionally shows ACD header parity non-zero Disposition required Data Processing855 3/23/06 FSW LATC verify occcasionally reports errors in CRC, SPT, ARC Disposition required Under

Investigation859 3/23/06 FSW CAL LCI data compression less than expected, results in very

large datagramsDisposition required FSW Discrepancy

863 3/29/06 Integration TkrNoiseAndGain Continuous Busy Analysis in progress Minor Documentation

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Open NCRs (cont’d)Open NCRs (cont’d)

NCRDate

OpenedAffected

Hardware Problem Description Status NCR Category865 3/29/06 Integration Strange packets in run 077001868. New Data Processing875 4/10/06 Mechanical Manufacture Part# not listed on packing slip Disposition Assigned Minor

Documentation878 4/11/06 Integration ACD cables have an excessive amount of pressure on a step up

transition on the ACDAnalysis In Progress Hardware

Discrepancy879 4/11/06 Integration LAT Power On in configuration #2 Limit Failures Disposition Assigned Minor

Documentation880 4/12/06 Integration SIU Spontaneous Reboot New Under

Investigation881 4/12/06 Integration EPU Spontaneous Reboot Analysis In Progress Under

Investigation882 4/12/06 DAQ GLAT2525 power up software crash due to Result FIFO not

empty.Analysis In Progress Spares

883 4/13/06 Integration RunControl software occasionally crashed during PDU TV Testing

Analysis In Progress Spares

884 4/15/06 Integration VSC errors observed during LAT power on New EGSE885 4/17/06 Mechanical Incorrect part (screw) received from vendor New Minor

Documentation886 4/17/06 Mechanical Two different length of screws intermingled in vendors

packaging New Minor

Documentation887 4/17/06 Integration Primary and redundant thermostats on the +Y Radiator were

wired backwards during fabriaction at Lockheed MartinAnalysis In Progress Hardware

Discrepancy890 4/19/06 Integration Radiator heater isolation tests fails on +Y Radiator New Hardware

Discrepancy892 4/23/06 Integration Insufficient screw length New Minor

Documentation

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LAT Waivers (1/2)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

433-0356Test Point Short Circuit Isolation

LAT is required to operate within spec if any testpoint 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

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LAT Waivers (2/2)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 GSFC’s approval with no compromise to product reliability. Submitted

433-XXXTracker Flex Cable and MCM Coupon Failures

Several flex cables and MCM’s 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-XXXRadiator 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 StatusSC-LAT ICD Status

ICN # Title Description Status

-096Unregulated 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 IntegrationThis is an appendix to the ICD that is meant to capture agreements for Observatory I&T activities.

Final logistical details in work

-100 LAT ImpedanceIncorporate into ICD the as-measured LAT differential impedance.

Measured – Data being evaluated

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

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DC Voltage ToleranceDC Voltage ToleranceRequirement

433-IRD-001 SC-LAT IRD3.2.4.1.3.2 DC Voltage ToleranceThe LAT shall tolerate without damage or degradation DC voltages greater than 0 volts and less than 40.0 volts.

Departure From RequirementThe 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.

Justification1. 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 MOSFET’s are not a single point failure. The primary and redundant feeds do not use the same set of MOSFET’s. If there is an anomaly on the primary feeds that damage the primary MOSFET’s, the redundant feed and MOSFET’s can be used to continue the mission.

5. The EGSE contains low voltage protection to protect the LAT during ground test.

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DC Voltage Tolerance #2DC Voltage Tolerance #2Requirement

433-IRD-001 SC-LAT IRD3.2.4.1.3.2 DC Voltage ToleranceThe LAT shall tolerate without damage or degradation DC voltages greater than 0 volts and less than 40.0 volts.

Departure From RequirementThe 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.

Justification1. 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.

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J-STD vs. NASA STDJ-STD vs. NASA STD

Requirement 433-MAR-001 Mission Assurance Requirements 5.2 Workmanship and Printed Circuit Board CouponsThe GLAST Project will use the following NASA and commercial workmanship standards for LAT:

a. NASA-STD-8739.3 – Soldered Electrical Connections

Departure From RequirementJ-STD-001 is used as the workmanship standard instead of the preferred NASA-

STD-8739.3

JustificationJ-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 CCA’s. Minor differences in the J-STD and NASA STD will not affect the products reliability.

29



Test Point Short Circuit IsolationTest Point Short Circuit IsolationRequirement

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 RequirementBackground: 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.

Justification1. 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 I&T 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 I&T 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.

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GTFE TIDGTFE TIDRequirement

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 RequirementThe deviation from 433-SPEC-001 is to not perform TID testing on the two additional fabrication runs for GTFE (T2CY#3 and T2CY#4) which are required to complete the TKR MCM production.

JustificationThe 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.

T2CY#1 fab'd Nov '02, TID tested Jun '03T2CY#2 fab'd Jan '04, TID tested Mar '04 T2CY#3 fab'd Mar '05T2CY#4 fab'd May '05

Given 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 (T2CY#3, fab'd Mar '05 and T2CY#4, 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.

NCRs and Waivers 31



24AWG STD Strength Cu24AWG STD Strength CuRequirement

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 RequirementStandard 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.

JustificationThe 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 GSFC’s 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.

NCRs and Waivers 32



Tracker Flex Cable and MCM Coupon Failures (1/3)Tracker Flex Cable and MCM Coupon Failures (1/3)

Requirement433-MAR-001 Mission Assurance Requirements5.2 Workmanship and Printed Circuit Board CouponsIPC-6012 Qualification and Performance Specifications for Rigid printed boardsIPC-6013 Qualification and Performance Specifications for Flexible boards

Departure From Requirement

Installed on the LAT are seven Tracker flex cables and three MCM’s (multi-ship modules) that had coupon failures. Justification

Flex Cables

The NCR’s and EC’s 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).

NCRs and Waivers 33




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 CABLESTable 1 - FLEX CABLES

Tower Flex Cable # NCR # EC # Coupon Failure

A C2 S/N 4 241 50143 Barrel separation; annular ring <0.002"

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 <0.002"

C4 S/N 1 358 40516 Annular ring <0.002"

C6 S/N 6 358 50130 Vias missing on coupon

NCRs and Waivers 34




MCMs

The MCM’s 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.

Tower/Tray MCM # NCR # EC # Coupon Failure

Tower 6, Tray H004 2354 158 50110 Delamination

Tower 15, Tray M175 13078 553 50431 Annular ring <0.002"

Tower 15, Tray M192 13085 553 50431 Annular ring <0.002"

Table 2 - MCM’s

NCRs and Waivers 35



Tracker Environmental Test With Non-Flight or Tracker Environmental Test With Non-Flight or Missing Cables (1/3)Missing Cables (1/3)

Requirement433-IRD-001 SC-LAT IRD 3.2.2.8.4 Component Evaluation Random VibrationThe 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 RequirementSeveral 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.

NCRs and Waivers 36




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.

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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

glast lat project april 27, 2006: lat pre-ship review presentation 7 of 12 ncrs and waivers 1 glast...

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