APPLICATION NOTE
www.onsemi.com

Power Packages Heat Sink Mounting Guide
AND9859/D
SUMMARY
This document provides guidelines for mounting heatsinks for the proper thermal management of power
semiconductor devices in field applications. This document

describes heat-sink mounting methods, considerations,contact thermal resistance, and mounting torque for various
packages.

HEAT SINK MOUNTING CONSIDERATIONSPower Dissipated
Package

Silicon DieR junction-caseR case-heat sink

Heat Sink
R heat sinkR heat sink-ambient

Figure 1. Thermal Resistance Model of an Package Assembly with a Heat Sink
Thermal Resistance and Heat Sink Mounting

remaining space is filled with air. Since air is a good
insulator, the air gap significantly increases the thermal
resistance. This is called a contact thermal resistance,Rθ case-heat sink and is shown in Figure 1. The contact thermal
resistance is a function of the roughness of the contacting
surfaces and the contact pressure. The technique ofmounting a heat sink is a key factor in minimizing thecontact thermal resistance.

The thermal performance of a package with a heat sink ischaracterized by a junction-to-ambient thermal resistance,Rθja, which is the sum of junction-case (Rθjc), case-heat sink(Rθcs), heat sink (Rθsink), and heat sink-ambient (Rθsa).
Thermal resistance components are shown in Figure 1.
Air convection is usually the dominant heat transfer
mechanism in electronics. The convection heat transfer
strongly depends on the air velocity and the area of theheat-transferring surface. Since air is a good thermal
insulator, it is important that a heat sink is used to increase
the overall heat transfer area to the ambient, i.e., the overall
thermal performance, Rθ heat sink-ambient, as shown in
Figure 1. This is especially true for power device packages.
Applying the heat sink provides an air gap between the
package and the heat sink due to the inherent surface
roughness, as shown in Figure 2. Since the two roughsurfaces can only make contact at certain points or areas, the© Semiconductor Components Industries, LLC, 2014

May, 2022 − Rev. 3

Package
Air Gap
Heat Sink

Figure 2. Surface Model Showing the Air Gap
between a Package Case and a Heat Sink

1

Publication Order Number:
AND9859/D

AND9859/D
MOUNTING TECHNIQUE

The first way to reduce the contact thermal resistance is toincrease the contact pressure, which is the joining force. Thismaximizes the contact area between the two surfaces.
Increasing the mounting torque in the fastening screw orusing a clip with a high spring constant provides for larger
contact areas, providing solid conduction heat flow paths
compared with air conduction through the air gap.
The second method is to fill the air gap with an interface
material, such as silicone grease or an insulation pad, having
better thermal conductivity than air. Table 1 shows the
typical thermal resistance values of several interface
materials [1]. Thermal information for various interface
materials is provided by leading manufactures [2, 3, 4].

Screw Mounting

Screw mounting is a traditional assembly method
accomplished with the fastening of a screw, nut, and washer.
Although it takes long to assemble in a mass production
environment, it is easy and quick to disassemble. Applying
the proper mounting torque is the key factor in obtaining
adequate contact pressure along the contact surfaces of the
package and the heat sink to minimize the contact thermal
resistance. With a low mounting torque, the contact thermal
resistance increases due to bad thermal contact underinsufficient contact pressure. With a high mounting torque,the package head and mounting tab show large deformations
such that the package may be lifted away from the heat sink,
as shown in Figure 3. This also increases the contact thermal
resistance. Hence, an appropriate range of mounting torquevalues must be applied to produce minimal thermal
resistance. This prevents package destruction or changes indevice characteristics.

Table 1. LIST OF CONVENTIONAL INTERFACE
MATERIALS AND THEIR PROPERTIES
Thickness(in)

ThermalResistance(5C−in2/W)

Mica / Grease

0.003

0.10

BN-Filled Silicone Sheet

0.010

0.17

Kapton MT / Phase Change

0.002

0.20

Kapton MT/BN-Filled Silicone

0.006

0.25

Alumina-Filled Silicone Sheet

0.010

0.45

Material Type

Screw
WasherPackageHeat Sink

Mounting Surface Condition

Bad Thermal
Contact

Nut

The contact surfaces must be flat and clean. The following
surface conditions are recommended for the heat sink:• Flatness ≤ 16 m (reference length 0.8 mm)• Surface finish ≤ 0.02 mm

Figure 3. Bad Assembly in Screw Mounting

A fastening torque can be transferred to the axial force ofthe screw, as described in Equation 1: [5]A milled or machined surface is satisfactory if preparedwith tools in good working condition. The heat sink
mounting surface also must be clean and free from particles,
damage, or corrosion. Drilling is preferable to punching in
the machining of the heat sink through-hole. Inappropriate
punching can cause a depressed and irregular surface near
the mounting hole. This can cause local distortion of the
package and poor thermal performance due to uneven
thermal contact.

F+ǒd m * l sec 2T@dm1 ) d m sec Ǔ(eq. 1)

where l, dm , m, and 2a are the pitch, mean diameter, friction
coefficient, and thread angle (2a = 60° for standard metricthreads), respectively.
The axial force in the screw results in the contact pressure
between the package case and the heat sink surface. Table 2
shows the typical coefficients of sliding friction for common
material combinations [5].

Table 2. FRICTION COEFFICIENTS FOR THREADED PARTS
Nut Material
Screw Material

SteelBronzeBrassCast Iron

Steel, Dry

0.15−0.25

0.15−0.23

0.15−0.19

0.15−0.25

Steel, Machine Oil

0.11−0.17

0.10−0.16

0.10−0.15

0.11−0.17

Bronze0.08−0.12

0.04−0.06

www.onsemi.com
20.06−0.09

AND9859/D
Clip Mounting

the copper patch on the aluminum heat sink without screws,
nuts, rivets, or clips. Figure 6 shows the vertical structure ofa package assembled with PowerSite technology, whichremoves the problem of pressure dependency associatedwith mounting hardware and fasteners becoming loose. The
copper patch improves thermal performance compared withmanually applied thermal pads. However, it needs special
equipment for assembly and rework requires solder reflow.

The clip mounting method has become popular because it
is a simple and reliable mounting process. The process is fast
and appropriate for mass production assembly. The screw
mounting is applied at the die center-off on one end of the
package during fastening. This causes uneven contactpressure and a bad thermal contact. But the clip is mounted
on the package center over the die, and this results in more
uniform contact pressure and good thermal contact.
Although clip mounting reduces the chances of over orunder tightening the fastening screw, it is hard to place thepackage correctly.

Solder (0.076 mm)
Copper
(0.036 mm)

Clip

Component
Metal Tab

Adhesive(0.004 mm)

Kapton MT(0.025 mm)Heat Sink

Figure 6. Package Assembly with
PowerSite Technology

HEAT SINK MOUNTING GUIDE

Application of clip force(at the package center above the die)Screw Mounting

Screw mounting is a common assembly technique for
Fairchild power packages. The screw mounting techniques
and torque values are a function of several assembly
variables: contact surface condition, washer, thermal grease,contact pressure, and mounting torque. The thermal
resistances for packages, such as discrete (TO220, TO220F,
TO3PF, TO3P, TO264, TO247, and TO126) and module(7PMGA, HA, IA, and SPM27−AA) are measured. The
mounting torques recommended for optimal thermal
performance are analyzed below.

Figure 4. Example of Clip Mounting
Rivet Mounting

The rivet method enables quick assembly in mass
production, but it is difficult to disassemble and control theforce applied to the rivet. The riveting force must be
controlled to avoid shape distortion, which increases thecontact thermal resistance between the package case and theheat sink surface.

Dry Mounting

Generally speaking, increasing the mounting torques at an
initial stage reduces the junction-to-heat sink thermal
resistance or the contact thermal resistance, because thecontact area also increases with the mounting torques.
Further increasing the mounting torque increases the contact
thermal resistance due to excessive deformation of the
package. This results in a bad thermal contact, as shown in
Figure 3. Figure 7 shows variations of the thermal
resistances of each package under various conditions. The
Rθjc of TO220 follows this tendency very well: the lowest
Rθjc appears in the mounting torque of 4 kgf ⋅ cm to
6 kgf ⋅ cm. Beyond that range, the Rθjc rises again, which
may be due to deformation of the package. The Rθjc of TO3P,
TO264, TO247, and TO126 decreases as the mounting
torque increases, not showing an increase in Rθjc in the rangeof measurement. TO3P, TO247, and TO264 can beassembled to heat sink properly with mounting torque ofaround 9 kgf ⋅ cm. The TO220 Rθjc is low enough witha mounting torque of 6 kgf ⋅ cm to 8 kgf ⋅ cm.

Package
Rivet

Heat Sink

Figure 5. Example of Rivet Assembly
Soldering

In general, devices mounted to aluminum heat sinks mustbe either screw or clip mounted. A new technology,
PowerSitet [6] allows packages to be soldered directly towww.onsemi.com
3

AND9859/D(a) TO126(b) TO220(c) TO220F(d) TO3P(e) TO3PF(f) TO264(g) TO247(h) 7PM−GA(i) 7PM−HA(j) 7PM−IA(k) SPM27−AA

Figure 7. Thermal Resistance vs. Mounting Torque for onsemi Power Packages
www.onsemi.com
4

AND9859/D
so it is not appropriate to define an optimum mounting
torque range. But a low mounting torque may allow thescrew to loosen due to vibration or other external loads. This
results in the weak contact pressure between the package
surface and the heat sink. Therefore, it is reasonable to set
the mounting torque range similar to the range for packages
such as TO220 and TO3P. The appropriate mounting torqueof TO220F and TO3PF in dry mounting can be 4 kgf ⋅ cm to
5 kgf ⋅ cm and 6 kgf ⋅ cm to 8 kgf ⋅ cm, respectively.
The IGBT module package does not show the effect of
reducing the thermal resistance above 20 kgf ⋅ cm
(Figure 7(h)~(j)). From the results, the torque of
18~20 kgf ⋅ cm is recommended for 7PM−GA and
22~26 kgf ⋅ cm is recommended for 7PM−HA, IA.
However, SPM27−AA does show a tendency similar to
TO3P or TO264. SPM27−AA thermal resistance doesn’t
decrease beyond 5 kgf ⋅ cm.
Besides optimization of thermal resistance, anothercriterion for mounting torque is that physical or functional
characteristics of a product shall not deteriorate while thepackage is tightened to the heat sink. A device under test canbe considered a failure if it exhibits any of the following after
package mounting: external crack visible with naked eye,electrical function failure, or de-lamination on the activeside of die. To determine maximum recommended torque of
TO220 and TO220F packages, a torque test was performed
for FDP047AN08A0 and FDP032N08 in TO220 and
FQPF13N50CF and FDPF20N50 in TO220F. These
products were selected as test vehicle because a product with
big die is prone to fail under the loading condition of screw
mounting. In the test, no package cracking and electric
failure happened until 7 kgf ⋅ cm. Table 3 shows thatde-lamination on the die didn’t change after screw
mounting. 7 kgf ⋅ cm can be regarded as the maximum
torque for TO220 and TO220F.

Conversely, for the Rθjc of TO220F and TO3PF, thethermal resistances increase slightly as the mounting torqueincreases. This is related to the mounting surface flatness ofthe package, i.e., warping of the package during the molding
process in manufacturing. The mounting surface flatness of
TO220F and TO3PF is convex, while it is concave for
TO220 and TO3P as shown in Figure 8.
Epoxy Molding Compound

Lead Frame(a) TO220, TO3P(b) TO220F, TO3PF

Figure 8. Package Mounting Surface

The higher the mounting torque, the larger the
deformation of the package head and mounting tab (in thecase of the convex surface). The unbalanced deformation
around the through-hole causes the package to be lifted fromthe heat sink, which results in the bad thermal contact shownin Figure 3. For the concave surface, the space between the
package and the heat sink becomes smaller as the mounting
torque increases. This is helpful in heat transfer from thepackage to the heat sink. The mounting surface shape of the
package − convex or concave − depends on the package
structure and on the material properties (coefficient ofthermal expansion, elastic modulus, etc.). This indicates thatthe optimal mounting torque must be chosen for eachpackage by measuring the thermal resistance at various
mounting torques.
In the dry mounting of TO220F and TO3PF (Figure 7(c),(e)), there is no large dependency on the mounting torque,
Table 3. C−SAM IMAGES BEFORE AND AFTER MOUNTING
Before Mounting

After Mounting with 7 kgf V cm

TO220
(FDP047AN08A0)
TO220(FDP032N08)
TO220F(FQPF13N50CF)
TO220F(FDPF20N50)

www.onsemi.com
5AND9859/D
Thermal Grease

Thermal grease reduces the contact thermal resistance byfilling the minute air gap between the mating surfaces. The
experimental results shown in Figure 7 indicate thermal
grease is essential in heat sink mounting The thermal greaseused in the experiment is Wakefield Engineering’s thermal
compound 120 series, containing zinc oxide in a silicone oil
carrier. [4] Once the air, the main insulator along the contact
surface, is filled with thermal grease, the influence of
mounting torque on Rθjc decreases. Little declination or
dependency of the Rθjc on the mounting torque is observed.
WasherFigure 9. Effect of Insulating Material on Thermal
Resistance (TO264)

The use of a washer guarantees the proper assembly
between the screw and the part being fastened. It provides
a wider contact area to transfer the axial force of the screw.
However, it creates an additional contact surface, so part of
the fastening force is consumed as friction force at thesurface. A weakened assembly force results in a shrunkcontact area and can decrease thermal resistance. To
compensate, a stronger mounting torque is required tomaintain the same axial force as that of a screw without
a washer. Figure 7(b) shows the effect of the washer on thethermal resistance. Package mounting with a washer results
in a slight increase and more even distribution of thermal
resistance as compared to a mounting without a washer.
Widening the loading area with a washer may result ina more stable contact between the mating surfaces.

Pressure Loading

One of the advantages of a clip mounting is that it
distributes more uniform pressure over the entire mating
surface and results in a good thermal contact. Figure 10
shows the thermal resistance of TO220 under pressureloading. The pressure loading on the package center is thesame as the clip mounting in principle. It indirectly
represents the clip mounting technique. Upon analysis, theresults of Figure 7(a) show that the thermal resistance underpressure loading is lower and more even than that of a screw
mounting. It means that clip mounting is a more stable andsafe method than screw mounting.

Insulating Material

Mica and silicone pads are well known insulating
materials that are used between the package and the heatsink. Mica is usually assembled with the thermal grease on
both sides. Mica shows high thermal performance at low
cost, but it is apt to be messy, contaminate adjacent circuitry,and dry out over time. Clean installation is possible withsilicone pads. Reworking it is easy and the cost is low.
Silicone pads also have a few disadvantages. Their thermal
performance is not very good and the heat transfer may bepressure dependent. High mounting pressure can cause
cut-through failures in silicone pads. [6] Figure 9 shows thedifference in the Rθjc of TO264 between silicone pads andmica. The higher Rθjc is observed when a silicone pad isapplied. The dependency of the Rθjc on the mounting torqueis low in both cases.

Figure 10. Thermal Resistance vs. Mounting
Pressure (TO220)

www.onsemi.com
6

AND9859/D• Clip mounting is a more stable and safer method thanCorrelation with Clip Mounting and Pressure Loading

Correlation between clip mounting and air pressure can beachieved by two ways. One is to calculate the pressure byclip mounting on the top surface of the package. The otheris to calculate applied force by air pressure loading on the top
surface of the package.
For pressure calculation in clip mounting, the contact area
between the package top surface and the clip must be known.
Actually, contact between the package top surface and clip
must not be in area but in contacted line. It’s very difficult
to calculate the pressure. Calculation requires structural
analysis. Air pressure and loading area are given in airpressure test methods and the force by pressure loading canbe obtained from the definition. Calculation of loading force
by air pressure is chosen to correlate clip mounting to airpressure test method. Calculation result of force value show
in Table 4.

screw mounting.
• The mounting torques in Table 5 are recommended forthe through-hole packages in screw mounting, regardlessof the application of thermal grease.
Table 5. MOUNTING TORQUES (in kgf·cm)

10

20

30

40

50

TypicalMaximum

TO220

4~5

7

TO220F

4~5

7

TO3P

8 ~ 10

TO3PF

6~8

TO264

8 ~ 10

TO126

6~8

TO247

8 ~ 10

7PMGA

18 ~ 2060

7PM−HA

22 ~ 26

7PM−IA

22 ~ 26

Table 4. CALCULATION RESULT OF FORCE VALUE
Pressure (psi)Package

Force (N)

14

27

40

54

68

81

Force (kgf)

1.4

2.8

4.1

5.5

6.9

8.3

REFERENCES[1]
Sorgo, M. D., “Comparison of Semiconductor
Mounting Techniques Reveals Thermal
Differences”, PCIM, May 2001, pp. 58 − 63.
[2]
Bergquist, 2002, Sil−Pad® Selection Guide.
[3]
Chomerics, Thermal Management Products.
[4]
Wakefield Engineering, Accessory Products
Catalog.
[5]
Shigley, J. E. and Mischke, C. R., “MechanicalEngineering Design”, 6th Ed., pp. 450 − 457.
[6]
Chomerics, PowerSitet Automated Power
Semiconductor Attachment.

The TO−220 package can use clip with clamping force of
2~8 kgf from view point of stable thermal resistance, but for
stable mounting, mounting force from 3 kgf to 6 kgf isrecommended.
SUMMARY OF MOUNTING GUIDE• In screw mounting, the thermal grease is essential inlowering the contact thermal resistance and having
consistent thermal performance.
• A washer slightly increases the thermal resistance.
• Choose an insulating material between the package andheat sink for optimal thermal performance.

PowerSite is a trademark of Parker Hannifin Corporation.
Sil Pad is a trademark of the Bergquist Company.
onsemi,, and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates
and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property.
A listing of onsemi’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. onsemi reserves the right to make changes at any time to any
products or information herein, without notice. The information herein is provided “as−is” and onsemi makes no warranty, representation or guarantee regarding the accuracy of theinformation, product features, availability, functionality, or suitability of its products for any particular purpose, nor does onsemi assume any liability arising out of the application or useof any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products
and applications using onsemi products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information
provided by onsemi. “Typical” parameters which may be provided in onsemi data sheets and/or specifications can and do vary in different applications and actual performance mayvary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. onsemi does not convey any licenseunder any of its intellectual property rights nor the rights of others. onsemi products are not designed, intended, or authorized for use as a critical component in life support systems
or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should
Buyer purchase or use onsemi products for any such unintended or unauthorized application, Buyer shall indemnify and hold onsemi and its officers, employees, subsidiaries, affiliates,and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or deathassociated with such unintended or unauthorized use, even if such claim alleges that onsemi was negligent regarding the design or manufacture of the part. onsemi is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Email Requests to: orderlit@onsemi.com
onsemi Website: www.onsemi.com◊

TECHNICAL SUPPORTNorth American Technical Support:
Voice Mail: 1 800−282−9855 Toll Free USA/Canada
Phone: 011 421 33 790 2910

www.onsemi.com
7

Europe, Middle East and Africa Technical Support:
Phone: 00421 33 790 2910
For additional information, please contact your local Sales Representative