Thermal Management Considerations for PCBs

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Thermal Management Considerations for PCBs- Measurement techniques and heat conduction

Thermal Resistance

  • TSP Method (temperature sensitive parameter)
  • Meets military specifications
  • Use forward voltage drop of the calibrated diode to measure a change in Tj due to known power dissipation

Thermal resistance calculation

  • Recall the formula for junction temperature:
    TJ = (PD x qJA) + TA
  • Rearranging equation, thermal resistance calculated by:
     qJA=DTJ/PD=TJ-TA/PD

where TJ is junction temp, TA is ambient temp and PD is power dissipation

TSP Calibration

  • TSP diode calibrated in constant temperature oil bath, measured to ±0.1°C
  • Calibration current low to minimise self-heating
  • Normally performed at 25°C and 75°C

Temperature coefficient

  • Temperature coefficient is known as the K-factor
  • Calculated using K=T2-T1/VF2-VF1 at constant IF

where:

K=Temperature coefficient (°C/mV)
T1,2 = lower and higher test temperatures (°C)
VF1, F2=Forward voltage at IF and T1,2
IF=Constant forward voltage measurement current

Calibration graph

  • K-factor measured from the inverse of slope

Thermal resistance measurement

  • Constant voltage and constant current pulses were applied to test the device
  • Constant current pulse is the same value as used to calibrate the TSP diode
  • This is used to measure forward voltage
  • Constant voltage pulse used to heat test device
  • Constant voltage (heating) pulse much longer than constant current (measurement) pulse to minimize cooling during the measurement
  • Typically >99:1ratio
  • Measurement cycle starts at the ambient temperature
  • Continues until the steady state reached, i.e. thermal equilibrium
  • Thermal resistance is calculated by:
    qJA=DTJ/PD=K(VFA-VFS)/VH ´ IH where:
    VFA=forward voltage of TSP at ambient temp (mV)
    VFS=Forward voltage of TSP at equilibrium (mV)
    VH=Heating voltage (V)
    IH=Heating current (A)

Test ambient

  • Measurement of qJA
  • Devices soldered to special thermal resistance test boards
  • 8-9 mil (200-225µm) standoff from board
  • Placed in a box of known volume (1cu ft if you’re American!)
  • Temperature rise measured

Air flow tests

  • Ambient test can also use moving air
  • Air flow passed over device at known constant rate
  • Required for calculations involving active cooling
  • Similar setup to static ambient test

Test setups

qJC Tests

  • Test device held against an infinite heatsink
  • This comprises a massive, water-cooled copper block, kept at 20°C
  • In this way, qCA (case-ambient) is very close to zero, so any measurement is purely qJC (junction-case)
  • SO devices mounted with bottom of package against heatsink, using thermal grease for good conductivity
  • PLCC devices mounted upside down, with top of package against heatsink
  • Spacer used on bottom side to prevent heat loss from here

PLCC qJC test setup

qJC data

  • Power dissipation has an effect on thermal resistance
  • Must be considered when calculating cooling requirements

Other factors affecting qJC

  • Lead-frame design, pad size
  • Larger pads reduce thermal resistance for given die size
  • Lead-frame material – Alloy 42 or copper

qJA data

  • Air flow also affects qJA
  • Important consideration for forced-air cooling

Heatsinks

  • Purpose of a heatsink is to conduct heat away from a device
  • Made of high thermal conductivity material (usually Al, Cu)
  • Increased surface area (fins etc) helps to remove heat to ambient
  • Interface between heatsink and device important for good thermal transfer

Interface roughness

  • Surface roughness at interface between two materials makes a huge difference to thermal conductivity
  • Various different contact configurations on microscopic scale

Surface roughness

Surface roughness

  • Air gaps act as effective insulators
  • Need some interstitial filler
  • Many types available, including greases, elastomers, adhesive tapes
  • Seen by consumers e.g. in PC processor heatsink/fan kits

Interstitial filler materials

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Solid interfaces

  • Conforming rough surfaces can have high conductivity:

Heat Conduction in a PCB

  • PCB is layered composite of copper foil and glass-reinforced polymer (FR4)
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Heat conduction in PCB

  • Can treat this layered structure as homogeneous material with two different thermal conductivities
  • Heat flow within plane is kIn-plane
  • Heat flow through thickness of plane is kThrough

Conductivity Equations

thermal management

where t is thickness of given layer and k is thermal conductivity of that layer

Sample results

  • Total PCB thickness is 1.59mm
  • PCB comprises only copper and FR4 layers
  • k of copper is 390 W/mK
  • k of FR4 is 0.25 W/mK
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Conclusions from results

  • Even for thin copper layers, kIn-plane is much greater than kThrough
  • As FR4 has very low thermal conductivity, a continuous copper layer will dominate heat flow
  • Because of this, thermal conduction is not efficient where no continuous copper path exists

Refining calculations

  • Trace (signal-carrying) copper layers have much less effect on heat transfer than planes
  • Trace layers can normally be excluded from calculations
  • If required, conductivity of trace layer can be calculated from where fi is fractional copper coverage

Summary

  • TSP Method for measuring junction temperatures
  • Thermal resistance test methods – junction-air and junction-case
  • Effects of power dissipation and airflow on thermal resistance
  • Interface resistance
  • Use of interstitial materials to decrease this
  • Heat conduction in copper-clad PCB dominated by in-plane transfer
  • Trace layers have only a small contribution to total conduction
  • FR4 is a good insulator!

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