Fundamentals of Transient and Steady State Component Thermal Characterization

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Fundamentals of Transient and Steady State
Component Thermal Characterization

• 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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Lifespan Vs. TJ

• Control of TJ Goal of electronic cooling
• Higher TJ shorter device life

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Electrical Measurement of Junction Temperature
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Phase 11 Thermal Analyzerwith WinTherm Graphical
Control Software
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Bath Method of Junction Calibration
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Linear Junction Calibration of TSP
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Nonlinear Junction Calibration of TSP
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Linearity as Function of Sense Current Level
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Identical Device Calibrations
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Diode Component Test Method
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Thermal Die Test Method
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Electrical Transient in Junction Temperature Plot
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Transient versus Steady State

• Transient heating ends when the initial, starting
  conditions become irrelevant to the data
  collected.
• At equilibrium, the passage of time or duration
  of the test is independent of the data collected.
  
• Transient ?T/Q impedance _at_ specific duration
• Equilibrium ?T/Q resistance

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Equilibrium and Transient Effects
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Junction Temperature Step-ResponsePlotted Using
Linear-Time Axis
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Junction Temperature Step-ResponsePlotted Using
Log-Time Axis
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Heating Curve as Thermal Cross Section of
Component
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Die-Attach Test Histogram
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Discrete Dynamic Model of Component(multistage
RC model)
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Heating Curve Data versus Optimized 4-Stage Model
Simulation (in red)
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Model Simulation for Square Wave Heating at
Various Duty Cycles and Frequencies
Ton power-on duration pulse duration Toff
power-off duration T Ton Toff cycle
period Duty Cycle Ton / T
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LED Heating Curve and Structure Function
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Heating Curve Comparison of Component Mounted on
3 Different PWBs
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Structure Function Comparison of Component
Mounted on 3 Different PWBs
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Thermal Resistance and Impedance

• Definitions, concepts, and applications
• Common misconceptions
• Correct test design implementation
• Data interpretation

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Definition of Thermal Resistivity
Resistivity defined only for parallel heat flux
between parallel isothermal surfaces
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Definition of Rja
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Definition of yjt
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Definition of Rjc
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Liquid-Cooled Fixture Base
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Spring-Loaded Thermocouple Probe
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Definition of Rjb
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RJB Fixture
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Heat Flow In Still Air
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Still Air Fixture
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Heat Flow In Forced Air
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Wind Tunnel
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Thermal Resistance Imperatives
Thermal resistance parameter defined by

• Shape of the heat flux network
• Reference temperature site

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Plot of RJA versus Airspeed
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Plot of ?JC versus Airspeed
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Plot of RJA versus Power
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Plot of RJC versus Power
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BGA ?JC versus Board Heating
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Fixturing Effects

• Fixturing should be nearly identical to intended
  application
• Results are fixture dependent

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Environmental Effects

• Forced or natural convection
• Fluid temperature (gas / liquid)

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Reference Temperature Effects

• Precisely defined reference site
• Reference temperature appropriate to thermal
  resistance desired
• Precisely defined measurement method
• Properly identified measurement parameter
• Low gradient reference site

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Power Level Effects

• Thermal resistance is specific to power level
• Highest sensitivity in natural convection

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Two-Wire Power Measurement
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Four-Wire Power Measurement
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