1. Heat Sink Selection Thermal Management of Electronics
San José State University
Mechanical Engineering Department

2. Heat Sink Categories Passive Heat Sinks
Used in natural convection applications or in applications where heat dissipation is not dependent on designated supply of air flows
Height at heat input: ~10mm to large
Normal load limit: 5-50 W
Cost range at 10,000 pieces: $0.50-$10.00

3. Heat Sink Categories Semi-Active Heat Sinks
Leverage off of existing fans in the system
Usually a passive heat sink set in front of a fan to produce impingement or vertical flow
Height at heat input: ~10mm
Normal load limit: W
Cost range at 10,000 pieces: $5.00-$10.00

4. Heat Sink Categories Active Heat Sinks
Fans are designated for its own use
Reliability is dependent on moving parts
Height at heat input: 35-80mm
Normal load limit: W
Cost range at 10,000 pieces: $10.00-$20.00
One big negative: if your fan dies, you need to replace the entire unit

5. Heat Sink Types Stampings Casting
Copper or aluminum sheets stamped into desired shapes
Low cost solution to low density thermal problems
Casting
High density aluminum or copper/bronze pin fins are produced with sand, lost core, or die casting (May or may not be vacuum assisted)
Allows for maximum performance with impingement cooling
More expensive but can make odd shapes
Al alloys used have lower conductivity than Al alloys used for extrusions

6. Heat Sink Types Extrusions
Allows for the formation of elaborate 2-D shapes capable of dissipating large heat loads
Cross-cutting to produce rectangular pins may increase performance by 10-20% but the extrusion rate will be slower
Design limits are usually set by
Fin height-to-gap aspect ratio
Minimum fin thickness-to-height
Maximum base to fin thickness
*Fin height-to-gap aspect ratio up to 6 and minimum fin thickness of 1.3mm are attainable with standard extrusion
*10 to 1 aspect ratios and a fin thickness of .8mm are attainable with special die design features

7. Heat Sink Types Bonded/Fabricated Fins
Thermally conductive aluminum filled epoxy is used to bond planar fins on a grooved extrusion plate
Greater fin height-to-gap aspect ratio is achieved with this method (~20-40)
Cooling capacity is increased without increasing volume requirements
But epoxy has a much lower thermal conductivity, acting as an increased resistance.

8. Heat Sink Types Folded Fins
Aluminum or copper sheet metal is folded into fins and then attached to a base plate or directly to the heat surface via brazing or epoxying
Due to the availability and fin efficiency folded fins are not suitable for high profile heat sinks
Allows for the fabrication of high performance heat sinks when extrusion or bonded fins are unacceptable
When are bonded or extrusion fins not acceptable?

9. Heat Sink Geometries
Rectangular fins have better performance than square fins, whose back edges have poor air flow past them.
Rectangular fins also have better performance than round fins; however, pressure drop is also higher for rectangular fins.
Round fins are good if you don’t know which direction your airflow will be from or if airflow may not be straight through the heat sink (it’s omni-directional).
For natural convection/radiation, solid black anodized fins mounted in a vertical direction tend to work best.

10. Heat Sink Materials and Finishes
Aluminum (6063 or 6061) is most common, followed by copper (which is 4-6x more expensive, 3x as heavy, by has 2x the conductivity)
It is difficult to alter the surface of copper to improve radiation.
External finish of aluminum is usually anodize or chromate of various colors.

11. Design Considerations
The following cases always increase thermal performance - True or False?
Longer fin heights
Longer heat sinks in the direction of air flow
Increased number of fins

12. Design considerations
All of the cases are false
Longer fin heights mean increased surface area but with a fixed volumetric flow rate performance may actually decrease with fin height
Longer heat sinks in the direction of flow and more fins both mean increased surface area but both have adverse affects on pressure drops and flow bypass, and the average heat transfer coefficient goes down

13. Flow Bypass
Flow bypass occurs when the flow duct is larger than the cross sectional dimensions of the heat sink
Velocity of the approaching fluid may be much greater than the velocity through the channels
Most commonly occurs with semi-active heat sinks

14. Flow Bypass
The amount of flow bypass will be greatly dependent on the cross sectional geometry and the pressure drop across the heat sink
The figure above – The derivation of this would take too long to fit in the allotted 25 minutes but this is a good example to show the adverse affects of adding fins. The u[f] term on the y-axis is the normalized fin flow velocity and the Reynold’s Number on the x-axis is the is the approach velocity times the length of the heat sink over the kinematic viscosity.
Lee, S. “Optimum Design and Selection of Heat Sinks”. Pg. 815

15. Radiation in Heat Sinks
Radiation in heat sinks is usually accounted for with an open-sided U-shaped channel
3 surfaces are considered opaque and the other 3 are transparent
The 3 opaque surfaces should be analyzed for the amount of energy that is emitted, absorbed, and reflected
Kraus, A. Pg. 292

16. Thermal Performance Graphs
Provided to show the thermal performance of a heat sink under ideal natural and force convection conditions
Ideal conditions mean:
Mounted to a heat source the same size of the heat sink
At sea level
Mounted with the fins in the direction of air flow
The heat sink is painted or anodized black
Zero Flow Bypass

17. Thermal Performance Graphs
Natural Convection Curve
Extends from the lower left to the upper right
Temperature rise vs. Q
Forced Convection Curve
Extends fro them upper left to the lower right
Thermal resistance versus air velocity
Lee, S. How to Select a Heat Sink. Pg. 5

18. Attachment Means Clip to device Clip to PCB Snap-on stampings
Double-sided tapes
Solder or adhesives
Thermal interface resistance minimized using grease or pad and by making surfaces as flat as possible

19. References
Lee, Serri. “Optimum Design and Selection of Heat Sinks”. IEEE Transactions on Components, Packaging, and Manufacturing. Part A, Vol. 18, No. 4. December 1995.
Lee, Serri. How to Select a Heat Sink. Aavid Thermal Technologies, Inc. Laconia, New Hampshire.
Kraus, A. The Heat Sink Design Procedure.
Markstein, H., “Optimizing Heat Sink Performance”, Electronic Packaging and Production, v 35, n 10, Sept 1995, p. 38.