Thermal resistance calculation Recall 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 Power dissipation has an effect on thermal resistance

Must be consideredwhen calculatingcooling requirements

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

Surface roughness at interface between two materials makes a huge difference to thermal conductivity

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

Can treat this layered structure as homogeneous material with two different thermal conductivities

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

Heat Transfer Mechanism in Electronic

packagesConduction

heat transfer occur within the solid

Convection - heat transfer by external fluid or gas that

surround the surface

Natural Convection

heat transfer based on the principle that hot air rises

Forced Convection

heat transfer by forced air blown across the surface

Radiation

heat transfer calculation based on energy released by

radiation.

Challenges

Resolving heat flow in densely-populated boards requires large, high-fidelity models

Vendor-supplied jc and jb values are idealized

Thermal interaction between closely-spaced, powered components invalidates single-component characterizations

Sizing problem

Footprint of electronic devices is shrinking

Higher component density on boards

Less thermal mass available

Complex cooling solutions required

Traces/vias control the model size

Restrictive length-scales compared to overall device length-scale

ANSYS Icepak

Background - ANSYS Icepak

Fluid dynamics software for thermal management of electronic systems

Vertical application

Built on CFD solver

Coupling to FEA solvers available

Time-saving capability

Package-level analysis to system-level analysis

Differing levels of detail available

Build entire models using Icepak objects

Import ECAD and MCAD data

Object-based approach

Blocks, plates, sources

Packages, PCBs, heatsinks, fans, blowers, etc.