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Accurate prediction of the cooling requirements of new consumer products
within as short a time scale as possible is of paramount importance to
the success of Motorola. Tom Lee, Ben Chambers and Mali Mahalingam, from
the companys Advanced Packaging Development Center, compared the accuracy
of rapidly obtained results using FLOTHERM to those obtained from lenghtly
experimental evaluations. The numerical results were in excellent agreement
with the experimental data.
Handheld portable products comprise a large portion of Motorola's manufacturing
business and represent one area where a Computational Fluid Dynamic (Computational
Fluid Dynamics) software such as FLOTHERM can play a potentially important
role in the design process. In this particular case study, Motorola chose
a generic representative of its handheld products, for which experimental
data already existed and thus could be compared to the results from the
Computational Fluid Dynamics simulation.
Problem Description
Typically, handheld portable products are housed in sealed cases and
rely predominantly on free convection for cooling. In general, the heat
flow path in these types of products is a series of conduction and convection/
radiation paths. Much of the heat generated in a IC is first conducted
into a PCB and from here into the outer case walls. Heat may be transferred
through gaseous conduction, convection and/or radiative process, depending
on the specification of the enclosure. The heat must then be conducted
through the case walls and finally removed to the environment by free
convection.
 Flotherm
Model
This case was modelled as a sealed enclosure comprising of a series of
plates 0.8mm thick with a thermal conductivity of 0.6W/moC.
The interior of the case consisted of a single circuit board containing
all the IC packages and other components. Several baffles were present
as an integral part of both the front and rear case halves and these effectively
blocked the air flow between compartments.
The circuit board was modelled using a block element with a thermal conductivity
of 20W/moC, with larger circuit elements (IC packages, amplifier
module, filter elements, etc..) also being modelled using block elements.
Simulation Results
The simulation predicted the external flow to approach velocities of
up to approximately 0.2m/s, while the internal flows were nearly negligible
(.02m/s), suggesting that the predominant mode of heat transfer inside
this case design occurs through gaseous conduction. The maximum temperature
occurring in the amplifier module was predicted to be 98.5oC,
which compares to an experimentally determined temperature of 92.6oC
for the same component, giving a difference of just 6.4% between predicted
and measured temperatures.
 Conclusions
Results of this case and others performed by Motorola demonstrate the
capability of FLOTHERM readily solved both internal and external fluid
regions as well as the associated conduction problem. All available comparisons
of simulated results were within 10% of experimental data, indicating
that the tool will be a valuable asset in a design setting for predicting
thermal performance.
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