Fundamentals of Thermal Conductivity Measurement via ASTM 5470

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Fundamentals of Thermal Conductivity Measurement
via ASTM 5470

• by
• Dr. John W. Sofia

Analysis Tech Inc. 2007
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Trends in Electronic Cooling

• Increasing module and device heat fluxes
• Declining thermal design-margins

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Trends in System Costs

• Decreasing cost-performance ratio
• Increasing cost of system failure to end user

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Major Causes of Electronic Failures
Source U.S. Air Force Avionics Integrity Program

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Definition of Apparent Thermal Conductivity
ASTM 5470 is Ohms law applied to one-dimensional
heat flow.
Conductivity defined only for heat flow between
parallel, isothermal surfaces (ASTM 5470)
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Apparent versus Actual Thermal Conductivity
Hot Test Surface
Contact Resistance Rt
Q
Sample
L
Q
Contact Resistance Rb
Q
Cold Test Surface
kactual kapp If Rt Rb 0
Actual conductivity of sample equals apparent
conductivity only if contact resistances are zero
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Best Method for Conductivity Measurement
- best-fit data line -
RA
Intercept (Rt Rb)A
Sample Thickness
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Best Method for Conductivity MeasurementRequireme
nts

• Sum of Contact resistances must be the same for
  all samples
• (stacked samples have higher total
  contact resistance)
• Fractional composition must be the same all
  samples
• (supporting matrices or meshes cause problems
  here)
• Variable thicknesses must have uniform bulk
  properties
• (fabrication can yield thickness-dependent
  properties)

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Test Error Failure to Use Constant Pressure
(Type 2 Material)
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Contact Impedance Versus Pressure (Type 3
material, hard rubber, dry contact)
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Characteristics of Surface Contact Resistances
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Treating Surface Contact Resistances
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Type 1 Samples (ASTM 5470)
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Best Method for Conductivity Measurement(Type 1
Materials)
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Type 2 Samples (ASTM 5470)
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Best Method for Conductivity Measurement(Type 2
Materials)
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Type 3 Samples (ASTM 5470)
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Best Method for Conductivity Measurement(Type 3
Materials)
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Repeatability of TIM Measurements
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Accuracy of TIM Measurements
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TIM Tester 1300 Features

• Automatic operation under WinTIM Software
• Electronic sample thickness measurement -
  "as-tested"
• Sample-temperature control batch testing
• Automatic pressure control batch testing
• Controlled-thickness controlled-pressure test
  modes
• In-house calibration procedures
• English or metric unit selection
• Requires chiller, compressed air, PC computer

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TIM Tester 1300
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TIM Tester Pressure Range Options

• Kit 1 5 to 95 psi (60 to 650 kPa) /- 3
  psi
• Kit 2 10 to 170 psi (100-1100 kPa) /- 5 psi
• Kit 3 10 to 380 psi (100-2600 kPa) /- 10 psi
• Kit 3 recommended for mostly type 3 materials
• Kits 1 2 recommended for mostly type 1 2
  materials
• Pressure accuracy is /-2.7 of maximum pressure
• Kits can be changed by end-user and are available
  in complete set of 3 kits.

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