Tag Archives: heat sink

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 maxiFLOW™

Fold Fin CFD

Straight Fin CFD

ATS maxiFLOW™ 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 maxiFLOW™ 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 R_r is the radiation resistance. To calculate Tref , assume that the heat sink is facing the adjacent board with power dissipation of P_adjacent

And

Where, T_m and T_c are defined by

And

Where

Solve for T_c,

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, T_c 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.

In part 1 of our 3 part series, “Choosing and Fabricating a Heat Sink Design for Thermal Management” we discussed different heat sink manufacturing methods. Today we want to discuss the pros and cons of each method so you can make an educated decision on which method to choose.

The pros and cons of each manufacturing technique:

The manufacturing process has a direct impact on a heat sink’s thermal performance. This stems from the number of fins that can be produced by a given manufacturing technique and from the interfacial resistances created when using that process. Bonded fin and swaging assembly techniques introduce a third material between the fins and the base. Single-fin assembly (SFA) places a third material in between the fins. Although SFA is a brazing process and the metallurgical joints very close to solid, nevertheless, all three of these techniques are impacted by the presence of the third material. Among other methods, stamped, extrusion and die-casting are perhaps the oldest technologies for high volume production. Most heat sinks on the market are made by such processes.

More recent heat sink fabricating methods were developed to meet the need for more fins on a flat surface. SFA, micro die-casting and forging produce higher performing heat sinks suitable for high power application. Of the three, SFA can produce the highest number of fins per linear length because of its manufacturing process. (Advanced Thermal Solutions, Inc. has made SFA heat sinks with 26 fins per cm, or 67 fins per inch.). Manufacturing technology has advanced enough to produce high aspect ratio as well as high fin count heat sinks.

In part 3 we’ll conclude our series on “Choosing and Fabricating a Heat Sink Design” with the salient features of high performance heat sinks.

Today we’ll be kicking off a three-part series on choosing and fabricating a heat sink design for thermal management.

For data and voice communications, speed is now the driving factor in the market. As a result, high frequency devices have become a major element for meeting consumer demands. Further, by combining higher packaging density and intelligent (software driven) PCBs there is much more power dissipation at both the component and board level.This situation creates great opportunities and significant challenges for thermal management and electronics packaging.

A first step in addressing thermal challenges is to consider heat sink choice and design. Looking at the market we find a strong presence of extruded heat sinks which can provide only limited thermal performance. Old or off-the-shelf extruded heat sinks do not meet the stringent temperature requirements of today’s ASICs. There’s a sore need for high performance heat sinks that can expand the envelope of heat dissipation in current and near term electronics. With many different types of heat sinks available, the question is: what type of heat sink is suitable for my application?

Heat Sink Types
The general function of a heat sink is the same, irrespective of its fabrication process. Thus we can distinguish between heat sinks by their manufacturing method.

Heat sinks fall into three broad categories:

1. Plate-fin: suitable for generally straight airflows
2. Pin-fin: suitable for omni-directional airflow
3. Foam-fin: suitable for ducted airflow (high pressure drop)

Excluding foam-fins, there are a number of high volume manufacturing processes for creating a heat sink fin field on a flat surface. Many articles are available that describe the details of such manufacturing techniques. Therefore, they are not covered here.  The following table below highlights the details of each manufacturing process:

In part two of “Choosing and Fabricating a Heat Sink Design for Thermal Management” we’ll discuss the pros and cons of each manufacturing technique.