Thermal%20grease%20application%20%20and%20QA

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Thermal grease application and QA
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Module FDR

• What is the thermal interface material ?
• Is it a grease ?
• Is it a solid ?
• How is it (the grease) applied ?
• What QA is done during production ?

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The problem

• Role of Thermal Compound
• During the mounting of SCT endcap modules onto
  the cooling structure, a thin film of a thermally
  conducting compound is applied to the cooling
  block with the aim of attaining the best possible
  thermal contact for the operational life of the
  SCT in ATLAS.
• Choice of Compound
• The final choice of compound has not yet been
  made. It may be a flexible pre-formed solid pad
  or a liquid grease dispensed in a controlled
  manner such that the quantity and disposition are
  well defined.

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Module Mounting

• Module Mounting
• Following the application of the interface
  compound, the module is mounted and the
  connections are electrically tested.
• The above is repeated until a certain number of
  modules have been attached (eg. All the lowers or
  all or a given ring-type).
• Sub-system Testing
• The (sub-)assembled disk is connected to a local
  evaporative cooling system and is installed in a
  suitable enclosure and tested.

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Operating Environment

• Thermal Effects
• The disks are assembled at normal room
  temperature (i.e. 22 C) and whilst the normal
  silicon running temperature is 7C there will
  be occasions when the module temperature falls to
  the evaporation temperature of the fluid (i.e.
  about 25C). The total temperature change is 50
  C.
• Humidity Effects
• Disks are assembled in an environment where the
  relative humidity is 50. When installed in
  ATLAS the disks will be operated in a dry N2 gas.

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Application of Thermal Compound

• Solid pre-form.
• The pads would probably exist in a pre-cut form
  on a large sheet. The in-plane dimensions of each
  pad could be checked optically. The thickness
  could be checked by measuring a few pads per
  sheet using a micrometer which would then be
  discarded.
• In addition to providing a good thermal interface
  the pad should be sufficiently tacky to resist
  movement on the block (main concern is rotation)
  whilst the module is mounted.

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• Thermal grease
• programme of work prior to the start of disk
  assembly would have to be undertaken to determine
  the optimal grease quantity and application
  technique.
• The main concern is that a balance has to be
  struck between giving the block just enough
  coverage to give maximal thermal contact and
  minimizing the risk of excess bleed of the grease
  onto the surface of the module.
• At the far-end the situation is somewhat
  different in that allowance has to be made for
  the contact to slide.

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1mm3 dot -gt 7mm _at_ 0.025 mm Coverage 83
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Module Mounting

• Hybrid End
• The module is fixed at the hybrid end using a
  suitable cone-shaped washer and nut.
• The design of the washer is such that the force
  exerted on the module is a known function of its
  deflection due to the nut.
• The nut is tightened such that the washer is
  somewhere near the mid-range of its deflection so
  that if there is relative movement during
  operation (eg. Changes in thermal compound
  thickness) there is still sufficient force on the
  module to hold it in place and to ensure a good
  thermal contact.

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• Far End.
• Differential movement between the module and the
  block due to the different CTEs of the disk and
  the module.
• Assuming that the modules CTE is about 5 ppm/C
  then for a 125 mm length the relative movement
  for a 50 C change in temperature is 31?m.
• However, over a longer period of operation in a
  dry atmosphere, CME effects will cause the
  carbon-fibre disks to shrink by about 0.2 mm/m or
  about 24?m over the length of a module.

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• Requirement
• The pressure exerted by the washer has to be low
  enough to allow the module to expand and contract
  safely whilst maintaining a good thermal contact.
  
• Work needs to be done to determine the thickness
  of grease, nut torque setting, etc..

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QA During Production

• Module Survey
• Measure Z of module
• Planarity
• Thermal QA
• Measure temperature on hybrid thermistor
• Thermal camera available as a development tool
  (Liverpool).

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QA and IR Imaging

• Aim
• Use thermal imaging camera to develop thermal
  compound application procedures that give
  consistent good thermal contact results.
• Apply these procedures during production
• Implement QA to ensure that correct procedures
  are followed
• Sample test using thermal camera ?