Jean-Louis Malinge - EEWeb

TECHNICAL Roger Stout ARTICLE
Back in the days when most electronics components
were packaged in metal cans (or manufactured as axial
leaded devices with equipment housings generally
spacious and un-crowded), matters of device level
thermal management were relatively straightforward
for design engineers. Sadly, thermal management has
become much more of a challenge nowadays – both to
understand and to tackle; with multi-pin plastic packaged
devices in ever-shrinking formats being incorporated
into high component density portable electronics with
considerable pressure being placed on available space
and overall power efficiency.
The following article details some of the key areas of
concern in regard to device level thermal management
in modern electronic products, with explanations of the
fundamental theory behind it as well as some practical
considerations.
First principles
Senior Research Scientist
Thermal
Basics of
Complex
Devices
As we all know from basic thermodynamics, heat energy
will flow from a higher temperature region to a lower
temperature region (this being done through either
conduction, convection, radiation or often a combination
of these). Furthermore, the bigger the temperature
difference witnessed, the greater the flow of heat will be.
Historically, for discrete devices, the ‘junction’ referred to
in the term junction temperature (TJ) was the PN junction
of the device. Though this is true for basic rectifiers,
bipolar transistors, etc, the junction now generally refers
to the hottest point within the device. As we move towards
more complex device constructions where different
parts of the silicon have different functions at different
times, locating the precise position of this point can be
very difficult.
A common misconception often held by engineers is that
a device’s thermal resistance is an intrinsic property of
the package in which it is enclosed. Among the reasons
why this does not hold true is the fact that there is no
isothermal surface, making it impossible to define a
‘case’ temperature. Though the metal can devices of the
past had a relatively good approximation of an isothermal
surface, modern plastic packages tend to exhibit quite
large gradients. Furthermore, in modern package types
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