thermal bottlenecks and shortcut opportunities in flotherm

Identifying Thermal Bottlenecks and Shortcut Opportunities

Robin Bornoff, PhD

Mechanical Analysis Division

November 2010

2© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com

Thermal Bottlenecks and Shortcut Opportunities

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


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

RBB, Thermal Bottlenecks and Shortcut Opportunities, November 2010


A Perfect Design?

A near perfect 3D thermal design for cooling a point source of heat via conduction would be:

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

360degree fixed

T_ambient and HTC

Point heat source



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 am

bientTa (degC

)

Heat source (W)

Cold am

bientTa (degC

)

Heat source (W)



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 am

bientTa (degC

)

Heat source (W)



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


http://upload.wikimedia.org/wikipedia/commons/a/a1/Bottleneck.svg


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



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/mK100 °C 35 °C



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…

180 W/mK

0.3 W/mK

100 °C 35 °C



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 GradientVector (degC/m)

Heat FluxVector (W/m2)



[** US Patent Pending]


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






[** US Patent Pending]


Thermal Bottleneck Field Example

Elevated Bn values are found where heat flow has to squeeze into the narrow section of the path



Composite Wall Revisited

Bottlenecks captured correctly in both cases, as both heat flux and gradT effects are included



Thermal Shortcut Opportunity Field Example

Best location to consider a new heat transfer path is above the copper block

Location of Maximum Sc



Application to System Level Model

‘Wall Unit’ (Installed FloTHERM Application Example)




1st Bn Modification – Push connectors through canned PCBs




2nd Bn Modification – Add thermal vias under Comp1




1st Sc Modification – Push connectors further down onto heatsink



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

TemperatureSc



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




2nd Sc Modification – Reduce heatsink fin length



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




Examination of the Sc field in the fin channels will indicate ineffective (low Nu) regions of the heatsink




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%




Examination of the Bn number will enable subsequent design modifications to be identified



A Note on Bn in Isotropic Solids…

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




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%




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%



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



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).



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


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