BTeV Pixel Substrate

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BTeV Pixel Substrate

• C. M. Lei
• Fermi Accelerator National Laboratory
• November 20, 2001

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Design Spec.

• Exposed to gt10 Mrad Radiation
• Exposed to Operational Temp about 15C
• Under Ultra-high Vacuum, 10E-4 torr or better
• Serve as a Dimensionally Stable Support
• Serve as a Heat Sink to remove heat 60W
  (0.5W/cm2, Heat Generating Area by ROCs 119
  cm2, Heat Conduction Area 66 cm2)

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Material Spec.

• Rad-Hardness and Low Rad Length
• Low Out-gassing Rate
• Light and Stiff
• High Thermal k and Low cte

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Design Approach

• Address Cooling Needs First
• Heat Removed Basically by Conduction to Coolant
  Q kA T/ L
• Maximize kA while Keeping Thickness L Small

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Design Types

• Cooling Tubes Array with Added Substrate (Fuzzy
  Carbon Design)
• Cooling Chamber as Substrate (Beryllium Design)

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Cooling-Tube Array w/ Added Substrate

• Need Manifolds and many Joints
• Need to build up Substrate on Array
• Allow Porous Substrate Low Rad L
• Seamless Tubing Array
• Generate a Temp Drop across the Array/Substrate
  Interface
• Effective Heat Transfer Area limited

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Cooling Chamber as Substrate

• Need to Machine Integral Cooling Channels
• Need to make a Large-Surface Quality Joint at the
  Interface
• Huge Area for Heat Transfer
• Wall Substrate, Min. Thickness
• Allow Smaller Temp Drop due to 1 less Joint
  Impedance

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Material Choices
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Fuzzy Carbon Technology

• High k fibers bonded to non-permeable tubing with
  carbon joint

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Why Baseline Design?

• Low mass
• Low rad L (0.17 per plane)
• Low cte and match with Si
• Reasonably good thermal k
• Low out-gassing

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Fuzzy Carbon Design

• All carbon heat exchanger design
• Pixel sensors to be supported directly by fuzzy
  carbon fibers
• Tubing flattened in cooling area and bonded
  together to form a stiffened array
• Leak-tight carbon joints between tubing
  manifold to be toughened by resin mixed with
  carbon nanotubes (preferred, but regular epoxy OK)

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Problems We Had

• Tubing and joint broken easily
• Many fibers not in contact with tubing
• Non-straight tubing
• Small diameter thin wall tubing used
• Tubing array used as the sole support
• Brittle carbonized joint between tubing and
  manifold (just good for sealing purpose)

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Prospective Solutions

• Use larger diameter, thicker wall tubing, and
  fused together to form a solid cooling array
• Use C superstructures next to cooling array and
  connect them to manifolds to form a rigid
  rectangular back-frame support
• Toughen the manifold/tubing joints with
  carbonized nanotube-resin or regular epoxy
• Use tooling fixture to make parts straight
• Warrant no missing of radial C fibers in contact
  with tubing because of the solid tubing array

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Dog-Leg Tubing
Seems the dog-leg corner can be cooled down too
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The Back-Up
Be Substrate without Top Plate
Be Substrate Bonded Assembly
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Be Substrate Design

• Back-up design because of high cte
• 4X 0.5-mm-deep channels
• Huge cooling area directly under hot sensors
• Cooling strips along the channels
• Overall thickness 3.066 mm
• Ave. rad L per plane 0.32

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Flow Test on Be Substrate
_at_ 960 cc/min
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Loading on Bond Area
Coolant Contact Area 8.9 in2 For P 80 psi, F
710 lbf Bonding Area 5.8 in2 Tensile Stress
in Bond 122 psi If Peeling occurs and only the
inner perimeter takes up the load PIW 47.7
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Choices of Structural Adhesives
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FEA on Be Substrate

• Heat Load from ROC .5 W/cm2
• Heat Load from Sensor .025 W/cm2
• Constant Coolant Temp -15C
• Coolant Pressure 40 psi
• Convective film Coef. 2000 W/m2C
• Radiation Effect Ignored (lt 1)
• Surrounding Temp 20C

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Temp Profile
3.8C
Coolant Temp -15C
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Temp Profile
3.8C
Coolant Temp -15C
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Temp Profile
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Displacement UY
.071
UY 0 at 4 corners
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Displacement UX
UX 0 this side
.018
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Displacement UZ
UZ 0 this side
.025 mm
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Resultant Stresses
(16,710 psi)
Be Sy 240 MPa
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Resultant Stresses in Epoxy Layers
(5,550 psi)
Stresses can be lowered significantly If epoxy
with lower E is used.
E 1 Msi
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Resultant Stresses in ROC
(5,440 psi)
Sy 120 MPa
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Resultant Stresses in Sensor
(5,470 psi)
Sy 120 MPa
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Temp Profile of 8-chip Module
In this model, bump bonds between ROC sensor
are added. Kapton HDI cable with epoxy are also
added. Results of temperatures, displacements
and stresses Are somewhat similar and less
because of smaller size of model.
Coolant Temp -15C
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Temp Profile on ROC
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Resultant Stresses in HDI Cable
(290 psi)
Tensile stress of Kapton 24,000 psi
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Resultant Stresses in Bump Bonds

• Bump bonds (0.01mm DIA, 0.01mm high) were modeled
  with 64,220 Beam Elements
• Min Principle Stress -104 Mpa (15,000 psi)
• Max Principle Stress 189 Mpa (27,400 psi)
• Tensile Strength of Indium 1.6 Mpa 13.7 Mpa
  (?)
• Stresses can be reduced significantly if 0.5mm
  wide epoxy can be glued around the ROC
• Reinforced Min Principle Stress -71 Mpa (10,300
  psi)
• Reinforced Max Principle Stress 44 Mpa (6,380
  psi)

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Resultant Stresses in Reinforced Epoxy
(4.600 psi)
(0.5mm wide epoxy around ROC perimeter)
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FEA Conclusions on Be Substrate

• For h 2000, Temperature Distribution OK
• For T 35C, Thermal Displacements OK
• Stresses on Epoxy is High
• Stresses on Bump Bond is not Acceptable
• Displacements and Stresses can be reduced if
  Smaller T allowed

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Choices of Thermal Conductive Epoxy
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Thermal Cycle Test
After 5 cycles between 15C and 20C, all 3
epoxies stay OK.
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2 Other Designs with Pocofoam
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The Pocofoam Coupon
Glassy C tubing array
Pocofoam substrate
Al rectangular manifold
Al Rods, together with Al manifold To form a
back-bone frame
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Rad L per Plane
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Future Plans

• Run thermal test and verify effective h with
  cooling-strip effect included
• Try to lower coolant temp to 10C or so
• Evaluate and Select Epoxies
• Do Thermal Cyclic Test on Si Dummies with
  Bump-Bonds
• Do Bump-bond Testing
• Develop prototypes for all tubing-array designs