Choosing and Fabricating a Heat Sink Design for Thermal Management, Part 3

In part 1 and 2  of our 3 part series, “Choosing and Fabricating a Heat Sink Design for Thermal Management” we discussed different heat sink manufacturing methods and the pros and cons of each method.  Today in our final piece, we want to discuss the salient features of high performance heat sinks.

A desirable cooling solution for modern electronics is a lightweight heat sink with low thermal resistance at low air velocities. Because the noise from moving the air through electronics enclosures is an issue, the low thermal resistance at low air velocities is an attractive feature. Hence, two parameters are key when considering a heat sink: high fin count and management of air flow movement through the fin field.

As the number of fins increase, the air flow resistance of the heat sink also increases. This implies that by managing the flow through the fin field, significantly higher thermal performance (lower case-to-ambient resistance) can be attained. The following CFD CAD outputs show the impact of design on flow through a fin field for three heat sinks with the same geometrical volume but different fin structures: Folded Fin, Straight Fin and the ATS maxiFLOWâ„¢

Fold Fin CFD

Straight Fin CFD

ATS maxiFLOWâ„¢ CFD

Smoke Flow Visualization for a Straight Fin Heat Sink

The three Computational Air Flow Visualizations of an Unducted Heat Sink Showing the Premature Egress of Flow from the Fin Field, and Smoke Flow Visualization for the Straight Fin Heat Sink. The maxiFLOWâ„¢ Heat Sink Has the Least Egress and the Best Thermal Performance

With a wide variety of heat sinks available the question of which is the best always daunting for an application engineer trying to solve the thermal issue. The following chart shows that by just adding fins you are not going to get a better performing heat sink. The selection is application dependent.

The chart clearly shows that even in a system with a fan tray (fans in parallel or series), as the number of fins increases, the pressure drop and subsequently the air flow through the fin field diminishes accordingly. Therefore, it is always best to calculate the base temperature of a heat sink in a given application to see whether the device thermal requirements are satisfied.

Below, we show an analytical model for calculating the case temperature of a heat sink base. A control volume is placed on a single fin of a heat sink that resides on a component in a PCB channel with an adjacent PCB on top.  H and V refer to Horizontal and Vertical Surfaces, Respectively.


Let us apply conservation of energy to this control volume and place the appropriate heat transfer terms in this equation with P referring to the power coming to the heat sink from its base.

Assume a high efficiency fin, hence, Tfin = Tf,t = Tb = Tc , and Rr is the radiation resistance.  To calculate Tref , assume that the heat sink is facing the adjacent board with power dissipation of Padjacent

And

Where, Tm and Tc are defined by

And

Where

Solve for Tc,

The above equation provides an analytical expression for calculating a heat sink’s base temperature based on its in-situ boundary condition. As shown in our heat sink operating point chart above, the air velocity, V, approaching the heat sink is not only affected by the number of fins, but also by the system and PCB configuration. Once V is obtained, Tc can be calculated to ascertain whether the device thermal performance meets the stated objectives.

Summary

Thermal management of today’s and tomorrow’s electronics requires superior performing to meet the operational needs of a successful product. Combining innovation in both design and manufacturing has led to the development of next generation heat sinks.. Despite the availability and high volume production of unique and exceptionally performing heat sinks, the thermal requirements of the component and the respective heat sink at the in-situ level must first be addressed before a heat sink or its fabrication becomes is considered. The combination of analytical and computational tools has enabled engineers to asses this need before they consider a heat sink solution.

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