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Tower 14 Layer Y0 Malfunction - MRB Meeting - 14 September 2005

Almost the entire ladder 3 in layer Y0, tower 14 appear dead to charge injection and noisy when random triggered, with some channels completely dead and others disconnected. Suspected root cause is a HV short induced by signal wire-bonds in contact with the silicon ladder edge. Possible corrective actions include reworking tower 14 bottom tray or replacing it with a new tray.

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Tower 14 Layer Y0 Malfunction - MRB Meeting - 14 September 2005

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  1. Tower 14 – Layer Y0 malfunction MRB Meeting – 14 September 2005

  2. Description and reference documentation • Description of nonconformance • Almost the entire ladder3 in layer Y0, tower 14 appear dead to charge injection and noisy when random triggered, with the exception of a small nb of channels (~15) that appear completely dead and a small nb of channels (~20) that appear disconnected • The failure is temperature dependent and disappears at temperature < 25C • A sharp current drop is observed when the failure disappears at low T; the drop corresponds to the current expected from a shorted ladder • The number of disconnected channels increases with time and temperature cycles • Suspected Root cause • HV short induced by signal wire-bonds in contact with the silicon ladder edge; such contacts are likely to be due to ladder movements induced by an air-bubble underneath the ladder • Reference NCR List • NCR 572 • NCR 430: similar failure on twr1-x16, eventually fixed by replacing anomalous tray • NCR 449: investigations performed on MID063 removed from twr1 show similar conclusions

  3. Noise anomalies observations Random triggers at hot soak Charge injection at hot soak Noise level at T<25C Consistent with: the ladder is lifted in vacuum and channels at the ladder right edge (~1500 and higher) are partly in contact with the silicon (white lines on bottom left plot), partly disconnected from the MCM (rainbow lines); the channels in contact cause a short on the HV line that is responsible for the behavior of most channels (purple lines)

  4. Noise anomalies trends The failure disappears at T < 25C The number of disconnected channels increases with thermal cycles

  5. ~ 0.71 mA DI = 0.35 mA ~ 100V/270Kohm ~ 0.36 mA Leakage Current observations • The current drop when the failure disappear corresponds to the current expected from a shorted ladder

  6. Possible Corrective actions • Constraints: • The last TVAC window is scheduled for september 26/29 for tower 8 retest and another tower • We only have one bottom tray available by that date • Corrective action 1 (preferred) • Assemble tower 15 and test it with tower 8 • Rework tower 14 bottom tray (see below), do not repeat environmental tests but still consider it as a flight tower after successful functional test • Pros: • We would have 2 flight towers for completion of the LAT and a spare for tower A replacement • Cons: • Tower 14 would not be env-retested. We could provide a vacuum test and a thermal test for the new bottom tray before installation • Tower 15 C5 is missing and there are no projections on delivery – we might consider getting the missing cable from tower 14 • Tower 14 rework: • Tower 14 has solithane • after inspection of bottom tray we could either remove the broken bonds or replace the tray • A new bottom tray would be available not sooner than 10-15 days • Corrective action 2 (not preferred) • Replace tower 14 bottom tray and test it with tower 8 • Pros: • We would fix the anomaly • Cons: • We could not perform environmental tests on tower 15 and would loose a spare flight tower (tower A replacement)

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