Adhesive Testing for the BTeV Pixel Detector
The basic unit of the BTeV pixel detector is a multi-chip module which is comprised of a silicon sensor module bump-bonded to a number of readout chips. The pixel module will then be glued to a high intensity interconnect (HDI) cable using electrically conductive adhesive, and then onto a substrate using another kind of adhesive with reasonable thermal conductivity. This report is mostly addressed to the need of the latter--the substrate adhesive. The aim of this technical note is to summarize the testing efforts and results of this substrate adhesive covering a period since 2001 till the end of 2004. The substrate will serve two purposes: mechanical support and cooling of the modules. Stresses and strains will be generated when there is a thermal change on the substrate. In addition, since there are many kinds of materials, with different coefficient of thermal expansion (CTE), being glued together to form the complete detector assembly, the substrate may get distorted due to the CTE mismatches. As stress is directly proportional to the material modulus, a significant amount of effort was concentrated in understanding the adhesive modulus. There are other constraints which need to be considered as well. For instance, the detector will be placed in a vacuum close to the beam, and it will be exposed to significant radiation during operation. As there are so many requirements on the adhesive, it is certainly not that easy to find one that meets all the demands. With a reasonable screening that the adhesive candidates being radiation hard and have low outgassing, searching for suitable adhesives was focused on those with low modulus. That is because (1) a mechanically reliable and fail-proof adhesive structure with low stress is needed, and (2) the leaking current characteristics of the modules will increase if mechanical stresses are too high. However, much of the technical information needed is usually not available from the vendor and therefore testing on our own is needed to verify the compliancy. The demands for good thermal and electrical properties would be lessened in favor of the more important properties if conflicts arose in our decision. As the adhesive layer would be as thin as about 0.05 mm, the temperature drop across it would be small, so a lower number in thermal conductivity would be tolerable. Also, having a greater coefficient of thermal expansion (CTE) value for the adhesive would be acceptable, as long as the modulus is low enough so that high thermal stress would not be generated within the pixel module. For our testing purposes, some of the samples in these testing groups have been exposed to ionizing radiation. It is known that ionizing radiations incident on organic materials causes the formation of free radicals by rupturing covalent bonds. These radicals are chemically active and can form new bonds, altering the structure of the polymeric material and resulting in changes of its appearance and of the chemical, physical, and mechanical properties. It is thus important that some samples be subjected to ionizing radiation. Mechanical testing should be conducted on a group of samples to ensure that the mechanical properties are still acceptable after heavy radiation dosages. The results of these tests will portray a more accurate idea as to whether the material that will meet the expectations of the experiments in our unique environment.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76CH03000
- OSTI ID:
- 878915
- Report Number(s):
- FERMILAB-TM-2339-E; TRN: US0701196
- Country of Publication:
- United States
- Language:
- English
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