Heat Sink
Selection
Thermal Management of Electronics
San José State University
Mechanical Engineering Department
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
http://www.dansdata.com/images/c3ezra/viasink220.jpg
http://www.amatteroffax.com/images/inventoryimages/691916.JPG
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: 15-25 W
– Cost range at 10,000 pieces: $5.00-$10.00
Heat Sink Categories

Active Heat Sinks
– Fans are designated for its
own use
– Reliability is dependent on
moving parts
– Height at heat input: 35http://cache-www.intel.com/cd/00/00/14/97/149748_149748.jpg
80mm
– Normal load limit: 10-160W
– Cost range at 10,000 pieces:
$10.00-$20.00
– One big negative: if your fan
dies, you need to replace the
entire unit
http://www.newegg.com/Product/ProductList.asp?Brand=1647&N=2010110062
+50001647&Submit=ENE&Manufactory=1647&SubCategory=62
Heat Sink Types

Stampings
– 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
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
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.
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
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.

Heat Sink Materials and Finishes
Aluminum (6063 or 6061) is most
common, followed by copper (which is 46x 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.

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
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
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

Flow Bypass

The amount of flow
bypass will be
greatly dependent
on the cross
sectional geometry
and the pressure
drop across the heat
sink
Lee, S. “Optimum Design and Selection of Heat
Sinks”. Pg. 815
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
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
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
Attachment Means

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



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
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.