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GLAST Large Area Telescope:
Gamma-ray Large Area Space Telescope GLAST Large Area Telescope: Tracker Subsystem WBS 4.1.4 2C: Failure Analysis-Risk Assessment Hartmut F.-W. Sadrozinski Santa Cruz Institute for Particle Physics University of California at Santa Cruz Tracker Subsystem Scientist
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LAT TKR Failure Overview
Details of the Failure Mode Analysis are in the LAT document: LAT-SS TKR Reliability Analysis Applicable LAT documents: LAT-SS LAT Science Requirements Document LAT-SS LAT TKR Subsystem Level III Specification LAT-SS Level-4 Electronics requirements LAT-TD LAT EE Parts List Design against failure: “Over-design” Modularity Segmentation (many channels) Horizontal Isolation, minimize Cross-strapping Redundancy Production to reduce failure Approved parts, manufacturing with approved vendors and QC Tight parts specifications (SSD, Polyswitches), testing, screening
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Over-Design against Failure
Expected LAT Performance exceeds the requirements of the LAT Science Requirements Document LAT-SS-00010: 20% Margin in effective area 30% Margin in PSF (10GeV) “Over-design” is an effective risk mitigation! LAT-SS LAT TKR Subsystem Level III Specification Reliability: The reliability of the tracker shall be at least 96% in five years. Reliability is the probability that the tracker will not experience a reduction in operability below 90% due to failure of its components. Operability is the percentage of tracker channels that are operational. This allows for loss one of 16 towers. We build with a < 1% operability goal …
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LAT Overview Modularity 16 identical towers 16 TEM
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LAT TKR Overview Segmentation Many trays provide signal, Redundancy
8 cables read out tower 4 are redundant
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LAT TKR Isolation Mechanical Electrical No Inter-Tower Interfaces
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LAT TKR Tray Overview Segmentation
4 ladders per SSD layer; bias feeds isolated by resistors
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LAT TKR Tray MCM Overview
Redundancy Every MCM has 2 readout controllers and 2 readout cables Hence every FE chip has two control and data paths Isolation Power fused with Polyswitch resettable devices
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Component Failure Rates
for TKR EEE Parts Component Quantity Failure Rate Years per Fail Comment Capacitor, Ceramic, LV 56,448 173 Capacitor, Tantalum 3,456 1530 Capacitor, Ceramic HV 4,608 2,200 Due to two in series Connector, Micro-D 1,280 33/330 The second number is two in parallel at tray Poly- Switch, 407 Does not take into account 2 in parallel Printer Wire Board 576 2.5 Dominated by via failure. Nearly all are mitigated with redundancy. GTRC 1,152 4,000 Data from MOSIS GTFE 13,824 107 Resistor 55,296 29.5 Flex Cable 128 96.3 Total Failures 2.1 Years/ failure Dominated by PWB via failure, which is mitigated by redundancy in most cases.
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Component Failure Rates for LAT TKR
TKR Sub Assemblies Number Allowable Failure Rate with insignificant Loss of Mission Objectives (Category 4 or larger) Observed Failures in BTEM Tower Remarks Tower 16 / LAT none Tower Cable 2 / side (8 / Tower) 1 / side (4 / Tower) 1 during assembly, none during testing Redundant pair connects 8 MCMs Si Plane /MCM 36 / Tower 1 / Tower Assumes isolation between different MCM’s. (Polyswitches) GTRC ASIC 2 / MCM (72 / Tower) 1 / MCM Redundant pair on each MCM Si Ladder 144 / Tower 2 / tower Ladder Bias isolated by resistors GTFE ASIC 864 per Tower 5 / Tower Readout redundant Coupling Cap 55926 / Tower 20 / Tower Assumes affects ¼ GTFE Single Channel 500 / Tower 27 mostly during assembly
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Failure Severity Definition
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Risk Assessment
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TKR Failure Modes and Mitigation
Component Function Failure Type or Cause Failure Effect Mitigation Perform. after Mitigation Category Probab. Code Risk Tower Gamma Trigger, Gamma Reconstruction, Background Rejection Failure of non-redundant TEM part 1) Effective Area reduced by ~1/16 2) Background rejection effected None 94% of nominal 3, 2R 3 low Failure of redundant TEM part Redundant TEM parts Nominal 5 Tower Cables Transmit Trigger Transmit Data Supply Power 1) Connector Failure 2) Broken Trace Trigger, Data, Power is interrupted Redundant Cables, doubled power pins 4 Si Plane Detect Charged particles Adjacent Planes touch SSD or Wire Bonds break Mechanical design, Encapsulate Wire Bonds
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TKR Failure Modes and Mitigation (cont.)
Component Function Failure Type or Cause Failure Effect Mitigation Perform. after Mitigation Category Probab. Code Risk MCM Bias and Signal distribution and processing (1 per Si plane) Cable Failure Small effect on Effective area and background Rejection Redundant Cables Nominal 4 5 Redundant low Isolated Surface Mounted Parts Failure 5 Redundant Non-isolated Surface Mounted Parts Failure Potentially one ¼ Tower effected Small (?) effect on Effective Area and Background Rejection Polyswitches 3 PWB Failure Small effect on Effective Area and Background Rejection none Ladder Detect Charged particles Bias Voltage Failure Ladder bias isolated
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TKR Failure Modes and Mitigation (cont.)
Component Function Failure Type or Cause Failure Effect Mitigation Perform. after Mitigation Category Probab. Code Risk Readout ASIC (GTRC) Digital controls and data processing Power Failure Negligible effect on Effective area and background Rejection Redundant, Redundant power bonds Nominal 4 5 Redundant low SEE “ Design Frontend ASIC (GTFE) Analog data processing Power Failure Switch readout path, 3 Coupling Caps Decouple ASIC input from SSD current Develop Short Part of ASIC noisy/ineffective Increased Trigger rate Mask Channels Single Channel noisy Detect Charged particles Shorted Cap Noisy ASIC Increased noise in data and Trigger rate Channel dead Dead ASIC Channel Decrease in sensitivity in Trigger and Data Remove from Data Stream 4
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