1. The encapsulation problem of the LAT heavy trays
Gamma-ray Large Area Space Telescope
The encapsulation problem of the LAT heavy trays
Alessandro Brez
INFN - Pisa

2. The tray encapsulation
DAM Nusil 1142
Fill Nusil 2502
ladder
TMCM

3. The problem Lateral shift On axis shift
On all the heavy trays, on both sides, we had bonding failures after thermo cycling.
Cause: adhesion failure of the FILL material from the ladder surface (CTE) with subsequent induced over stress on wires

4. Possible solutions
We had no failures (not a single missing connection) in the MID trays, even after 18 thermo cycles (+50,-30oC)
Something very special with heavy trays. There is no time to model the problem or to find and test a new fill material.
The only practical solution:
NO ENCAPSULATION
The free wires are much more adaptive to CTE induced movements (loop).

5. Qualification program
From EM trays:
Some EM trays have no encapsulation but
Most of the EM ladders are from dummy wafers
The pitch adapters of the EM TMCMs are different from flight units
Several bonding wires have been damaged during the tower thermo-vacuum assembly (thermo couples placed over the trays)
These trays have been mounted and dismounted many times
There are layers with a complete series of good bondings:
They survived to the tower vibe tests
They could be tested again in Centrotecnica (is it significant? , they survived to 2 previous tests)

6. Thermo or thermo vacuum test
In principle we can perform a leakage current test
In climatic chamber
In thermo-vacuum chamber
test