Tower 8 – X6 anomaly

1. Tower 8 – X6 anomaly MRB Meeting – 24 August 2005

2. Description and reference documentation • Description of nonconformance • detected and monitored on layer X6 during tower8 TVAC test • noise increases and strip occupancy rises above required level of 10-4 for all strips in a ladder • anomaly appears when temperature exceeds 25C (see fig 2) • the anomaly appears on different ladders with different frequencies • same anomaly detected in tower7, layerX17 • Suspected Root cause • still unknown • Reference NCR List • NCR 551 • NCR 544: similar failure on tower7-x17, eventually fixed by replacing anomalous tray • NCR 562: investigations performed on tray removed from tower7 with similar anomaly

3. Nonconformance detection method TE301 TE301 TE302 Test TE301 and TE302 from standard CPT suite: entire ladders appear noisy or dead This behaviour is tipical of missing bias on the Si

4. Tower 8 additional testing • Failed ladders • Ladder 0: 4 • Ladder 1: 0 • Ladder 2: 5 • Ladder 3: 31 A noise monitor was set-up and run for layerX6: continuous runs of TE302 Green stripes are TE302 successfully passed - Red stripes are failed TE302, red dots are nb of failed ladders on X17 (right axis) The failure appears only at high temperature T~25C seems to be the failure start temperature

5. Layers bad channels during TVAC • X6: out from the failure regions where entire ladders fail, signal strips are stable, i.e. no signal wire-bonds are permanently lost • It is therefore very unlikely that the HV is lost via an intermittent wire-bond

6. Tray history and further investigations • Investigations were performed on tray M010, the tray that showed the same anomaly in tower7 and was eventually removed • Both trays were among the very first MID trays produced at the time of tower0: • they were reconsidered for use in flight towers as documented in NCRs 503, 509 • they had excellent performance and had no cut on the bias circuit • an additional successful test was run in vacuum before using these trays in flight towers • Other 2 tower0 trays (B002, L023) had been re-qualified (NCRs 468, 472), installed in flight towers (5-position00, 4-position01) and gave no problem during environmental tests • The last tower0 tray still considered flight (L015) is now ON-HOLD

7. Further investigations (M010) • The tray was installed in the INFN thermal chamber, connected to the EGSE readout system and monitored at various temperatures (see NCR 562 for full descriptions) • Cable dependance: • the anomaly was observed with either 1 or 2 readout cables connected to the tray • Temperature/vacuum: • the failure was reproduced in the same range of temperatures (T>~25C) and vacuum is not responsible for the failure • Bias voltages: • High voltage on the Si: no instabilities were found when monitoring the voltage with a scope probe connected after each ladder protection resistor • Electronics low voltages: no instabilities were found when monitoring each line (AVDDA, AVDDB, DVDD) with a scope probe connected to the tracker breakoutbox • Grounding: • a good grounding was assured by adding a connection between the tray inner honeycomb and the two tray MCMs; a res < 1W was measured between the two MCMs and no instabilities were found when monitoring the ground line with a DVM and the scope probe

8. Further investigations (M010) • A trigger rateand occupancy monitor was set up and run in the attempt to evaluate the impact of the failure on tower performance: the pictures show a failure detected in a dedicated run taken at 45C: • the failure was active for ~10% of the run-time • the single ladder noise occupancy is O(10), so the single strip noise occupancy is O(10-2), i.e. 100 times larger than requirement • The impact of such a failure to the tower performance would be negligibleat the level of: • level-0 trigger rate that require 6 layers to be in coincidence) • Track reconstruction: few noise hits, most of the time clearly off-track, would not disturb reconstruction software

9. Conclusions and possible Corrective actions • Investigations conclusions • no smoking gun found, root cause is still unknown • Corrective action 1 (not preferred) • Leave tower 8 as is: • Pros: • No risk for tower disassembly and reassembly • no delay in delivery schedule • no need for a new environmental test run • Cons: • even if observations on M010 seem to indicate an acceptable or even negligible impact on the tower performance, there is no way to test the stability of such behaviour with time and extrapolate to the LAT lifetime • Corrective action 2 (not preferred) • Perform further studies on M010 and try to identify a root cause. If identified try to fix tower8 w/o removing trayM007 • Pros: • No risk for tower disassembly and reassembly • No need for a new environmental test run • Cons: • There is currently no idea on the possible root cause • Even if a fix was performed, it would have to be tested by heating the tower to a higher temperature or even running a full TVAC

10. Conclusions and possible Corrective actions • Corrective action 3 (preferred) • Disassemble tower8, remove and replace tray M007 • Pros: • Definitive fix • Known procedure already executed on other towers (A, 1, 7) with minor differences (the tower assembly jig is required), available as annex to NCR 551 • Cons: • Controlled risk for tower disassembly and reassembly • Some delay in the tower delivery for extra assembly • need for a new environmental test run