Thermal Bottlenecks and Shortcut Opportunities in FloTHERM

Identifying Thermal
Bottlenecks and
Shortcut Opportunities
Robin Bornoff, PhD
Mechanical Analysis Division
November 2010

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Thermal Bottlenecks and Shortcut
Opportunities
?
© 2010 Mentor Graphics Corp. Company Confidential
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 The following slides present a new
approach to electronic thermal
simulation results post-processing
 Electronics thermal simulation is used
primarily to observe temperature
behaviour to judge thermal compliance
— “What is my design doing?”
 Little insight is available as to why the
temperatures are what they are
— ”Why is my design doing that?”
 If more insight were provided,
remedial thermal design changes could
be identified more readily
— “How can I fix my design?”
RBB, Thermal Bottlenecks and Shortcut Opportunities, November 2010

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Heat Always Flows…
 Heat is dissipated on the active surface of an IC
 The heat then travels out:
— Towards a colder ambient
— Through various objects and obstructions
— Via Conduction, Convection and Radiation
— How ‘easily’ the heat finds its way to the ambient
determines the temperature rise (above ambient) at the
heat source
– …and at all points in between

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A Perfect Design?
 A near perfect 3D thermal design for cooling a point source of
heat via conduction would be:
360degree
fixed
T_ambient
and HTC
 The (fixed and cool) ambient temperature is everywhere
equally near to the heat source
 The heat would find it equally easy/difficult to leave no matter
what path it took
Point heat
source

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Thermal Bottlenecks
 A real electronic thermal system is never so simple
 There are many different paths, with different resistances, the
heat has the option of following on its way to the ambient
 Some paths carry a lot of heat, others offer high resistance to
the heat flow
 A thermal bottleneck is a path where a lot of heat flows AND
where the resistance is high
— Relieving the bottlenecks should decrease all ‘upstream’
temperatures
Cold ambient
Ta (degC)
Heat source (W)

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Shortcut Opportunities
 A real electronic thermal system is never so simple
 There are many different paths, with different resistances, the
heat has the option of following on its way to the ambient
 Some paths carry a lot of heat, others offer high resistance to
the heat flow
 A shortcut opportunity is where an additional heat flow path
might be added so as to bypass the heat to cooler areas
 Again, reducing the temperature rises
Cold ambient
Ta (degC)
Heat source (W)

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A Traffic Analogy…
 A traffic bottleneck is a
section of road in which
congestion is observed
— Lots of vehicles
— Forward motion is
impeded
 Relieving the source of
the congestion will
reduce driver ‘traffic jam
frustration’, where:
= dT

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A Traffic Analogy…
 A traffic shortcut
opportunity is a road
that would take you
to your destination
quicker, should it be
built.
 Building the shortcut
will reduce driver
‘long journey
frustration’, where
again:
= dT

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100 °C 35 °C
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Why not Heat Flux?
 Heat Flux shows where the heat is going,
but not where it finds it difficult. Consider
a composite wall …
0.3 W/mK
400 W/mK
137 W/mK
11 W/mK

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Why not Temperature Gradient?
 GradT shows where there is a change in temperature, but
doesn’t show high heat flux areas. Consider a parallel
composite wall…
100 °C 35 °C
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180 W/mK
0.3 W/mK

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Bn Definition
 The bottleneck, Bn, number is defined
by considering the ‘dot product’ of
heat flux and temperature gradient
vectors at any one point
— Magnitude of heat flux x Magnitude of
temperature gradient x |cos()| **
 The temperature gradient is taken to
be symptomatic of thermal resistance
 Bn is large when:
— Heat flux is large
— And the temperature gradient is large
— And the two vectors are aligned
– i.e. the heat experiences a resistance in
the direction of heat flow

Temperature
Gradient
Vector (degC/m)
Heat Flux
Vector (W/m2)
Temperature
Gradient
Vector (degC/m)
Heat Flux
Vector (W/m2)
[** US Patent Pending]

[** US Patent
Pending]
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Sc Definition
 The shortcut, Sc, number is defined
by considering the ‘cross product’ of
heat flux and temperature gradient
vectors at any one point
— Magnitude of heat flux x Magnitude of
temperature gradient x |sin()| **
 The temperature gradient is taken to
be the indication of an opportunity to
divert the heat to cooler areas
 Sc is large when:
— Heat flux is large
— And the temperature gradient is large
— And the two vectors are perpendicular
– i.e. the heat is passing parallel to locally
colder areas

Temperature
Gradient
Vector (degC/m)
Heat Flux
Vector (W/m2)
Temperature
Gradient
Vector (degC/m)
Heat Flux
Vector (W/m2)

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Thermal Bottleneck Field Example
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 Elevated Bn values
are found where
heat flow has to
squeeze into the
narrow section of
the path

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Composite Wall Revisited
 Bottlenecks captured correctly in both cases, as
both heat flux and gradT effects are included

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Thermal Shortcut Opportunity Field Example
 Best location to consider a new heat
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transfer path is above the copper
block
Location of
Maximum Sc

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Application to System Level Model
 ‘Wall Unit’ (Installed FloTHERM Application Example)
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 1st Bn Modification – Push connectors through canned
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PCBs

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 2nd Bn Modification – Add thermal vias under Comp1
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 1st Sc Modification – Push connectors further down onto heatsink
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A Note on Sc Numbers at Solid/Fluid
Interfaces…
 A qualitative correlation exists between the Sc number in
the air at solid/fluid interfaces and the local Nusselt
number at the surface
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TSecmperature

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A Note on Sc Numbers at Solid/Fluid
Interfaces…
 A qualitative correlation exists between the Sc number in
the air at solid/fluid interfaces and the local Nusselt
number at the surface
 Examining the Sc number in the air adjacent to a heated
surface will indicate areas of effective (high Nu) heat
transfer
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 2nd Sc Modification – Reduce heatsink fin length
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Application to Heatsink Optimisation
 Design modifications based on the Bn and Sc fields can be
seen as an alternative, or compliment, to ‘what if’ and
parametric optimisation methods, commonly employed for
heatsink design
 Consider an oversized aluminium heatsink placed in a
computational wind tunnel with a 100W heat source
under the base
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 Examination of the Sc
field in the fin channels
will indicate ineffective
(low Nu) regions of the
heatsink

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 The heatsink extrusion
profile can be reduced
to remove these
ineffective regions
 Overall thermal
resistance increases
slightly (5%)
 But volume decreases
by 88% and pressure
drop decreases by 23%

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 Examination of the Bn number will enable
subsequent design modifications to be
identified
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A Note on Bn in Isotropic Solids…
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 Take a point source of heat in an
isotropic solid material
 Heat flux and temperature gradient
decrease radially in the same way
 Resulting Bn field is spherical with
a more tightly defined boundary

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 The best Bn ‘profile’
that can be expected
in an isotropic solid is
hemispherical
 Central portion of the
base is thickened to
allow the Bn sphere
to be realised
 Overall thermal
resistance decreases
by 15%

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 The high Bn sphere is
the largest thermal
bottleneck in the model
 Addition of a copper slug
to cover the bottleneck
results in a decrease in
thermal resistance of
53%
 Making the entire
heatsink out of copper
reduced the thermal
resistance by only a
further 13%

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Remedial Actions?
 Bn and Sc distributions offer insights into the physics of a
thermal design
— From this you can infer what remedial design modifications
would be required
 Bottlenecks can be addressed by lessening the thermal
resistance of the high Bn region
— Increase the cross sectional area to heat flow
— Increase the thermal conductivity
— Decrease the length
 Shortcut opportunities can be leveraged by replacing the
insulating material with a conducting material
— Thermal vias
— Heat pipes
— Gap pads, glue etc. (instead of air)
 Shortcut opportunities in the fluid can also suggest design
changes
— Heat sinks when Sc indicates efficient convection off a surface.
— Removal of non efficient areas of heat sinks

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Simulation Workflow Advice
 The general strategy is to perform a simulation and inspect
both Bn and Sc fields to determine:
— which areas of the design have the largest thermal bottlenecks
— which areas would benefit the most from additional heat transfer paths
— which convective surfaces are operating efficiently.
 Armed with this information, targeted design changes can be
developed. The exact details of the changes will be
determined by other design constraints (electrical, mechanical,
cost) and how evolved the complete design is.
 Generally speaking, Sc changes should be considered first, as
the creation of a new heat transfer path can completely
change the heat flow topology for a design and cause pre-existing
bottlenecks to greatly diminish in importance (as the
new shortcut may bypass previously existing bottlenecks).

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Conclusions
 Evolving simulation from the ‘what’ to the ‘why’ and the
‘how’
 Identification of thermal bottlenecks and shortcut
opportunities provides an alternative approach to classical
parametric and design optimisation numerical studies
— Bn and Sc prompted design modifications can be identified readily
and effectively
 “If I had $10 to spend to make my design cooler, how
would I spend it?”
— Bn and Sc numbers provide an insight into why an electronics
system gets hot; where the cause of the problem exists
— A designer can then use their judgment as to how to most
effectively remedy the problem

Thermal Bottlenecks and Shortcut Opportunities in FloTHERM

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