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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Ladder Wirebonds break
MRB Meeting – 3 december 2004
L. Latronico
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Description and reference documentation
• Description
– 3 different ladders belonging to 2 tower A trays showed
signal strips interrupted at half their length
– this could be inferred from noise measurements
(compatible with half strip capacitance) and CR hitmap
occupancy
– trays could not be inspected as they were already
assembled into tower A
• Root cause
– from comparison with measurements taken during
acceptance test at G&A and after thermal cycle we could
identify thermal cycle as the origin of the break
• Reference NCR List
– INFN/PI_318 (Tray Heavy 020)
– INFN/PI_319 (Tray Bottom 003)
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Acceptance tests at G&A
•
•
Findings
– top plots: noise profiles
show no disconnected or
interrupted channels
– bottom plots: CR hitmaps are
uniformly efficient
Reference documentation
– Bottom 03 tested on
11/26/2004 (EGSE session
GA-alpha-105)
– Heavy 020 front tested on
11/24/2004 (EGSE session
GA-alpha-89)
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Bottom 03
Heavy 20 Front
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Post thermal cycle tests
•
•
Performed on stack of trays for bottom03 (11/28
- EGSE session INFN/PI-beta-15):
– Noise profile shows a group of channels
with lower noise. these channels were not
detected as threshold for disconnected
channels is ENCstrip < 500 e- and they are
just about above that threshold
– CR hitmaps shows a less than 100%
efficient region correspondingly
Heavy 20 Front was not tested before tower
assembly for a mistake:
– After a standard flow-chart thermal cycle (4
cycles) a HV short was found on ladder 0 of
back side (signal bonds touching ladder
edge - NCR INFN/PI-303)
– The tray was immediately brought to G&A
(11/29) for repair without testing front side
– The repaired back side was tested OK at
G&A
– The tray came back to INFN (11/29) and went
to a second thermal cycle (2 cycles) but only
the repaired side (back) was tested
afterwards (11/30), in parallel to the start of
tower assembly, and assuming that the front
side had been tested before
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Bottom 03
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Tower A findings
Tower 1st test (11/26/2004 - INFN/PI-gamma-9) clearly revealed the problem –
Mid 82 shown for reference as a good layer
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Mid 82 Back
Bottom 03
Heavy 20 Front
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Offline analysis confirm the problem
1D CR hitmap Y0 (Bottom
03)
~30 strips half dead
~ some strips broken at ¼
length?
1D CR hitmap X0
Few dead channels
2D scatter plot
X0 vs Y0:
• dead channels
are inefficient all
along strip length
• broken wire
bonds along
ladder are
efficient for part
of strip length
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Offline analysis confirm the problem
1D CR hitmap Y4
(Heavy 20 Front)
~1.5 ladders half dead
1D CR hitmap X4
Few dead channels
2D scatter plot
X4 vs Y4:
Break again at
half strip
length
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Offline analysis from tower 0
•
Went back to tower 0 data
– The only tray showing
interrupted strips had very
few live channels (NCR
INFN/PI-92)
– The problem was there
already
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Conclusions and Proposed dispositions
• Conclusions
– Wirebonds connecting different SSDs inside a ladder broke
in 4 ladders attached to 3 different trays of stiff type (heavy
and bottom)
– Break tend to appear at the centre of tray
– Root cause is very likely the same effect that broke
wirebonds between strips and FE and led us to remove
encapsulation
– We cannot remove encapsulation on ladders
• Proposed dispositions (for both NCRs)
– use trays as they are for tower A: there was no availability of
spare trays of heavy or bottom type, therefore trays were
assembled into tower A to keep up with the schedule
– Review temperature ranges for thermal cycles and thermalvacuum tests:
• Proposed range is -20C - +50C
• Qualification program on tower0 trays
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GLAST LAT Project
Ladder wirebonds break - MRB 03 Dec 2004
Tower A first 100 events
Despite this issue, Tower A was assembled and test in a week and it’s a good tower!
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