1. GLAST LAT ProjectMRB 09/14/2005 L. Latronico1 Tower 14 – Layer Y0 malfunction MRB Meeting – 14 September 2005

2. GLAST LAT ProjectMRB 09/14/2005 L. Latronico2 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. GLAST LAT ProjectMRB 09/14/2005 L. Latronico3 Noise anomalies observations Charge injection at hot soak Random triggers at hot soak 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) Noise level at T<25C

4. GLAST LAT ProjectMRB 09/14/2005 L. Latronico4 Noise anomalies trends The failure disappears at T < 25C The number of disconnected channels increases with thermal cycles

5. GLAST LAT ProjectMRB 09/14/2005 L. Latronico5 Leakage Current observations ~ 0.71 mA ~ 0.36 mA  I = 0.35 mA ~ 100V/270Kohm The current drop when the failure disappear corresponds to the current expected from a shorted ladder

6. GLAST LAT ProjectMRB 09/14/2005 L. Latronico6 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)