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Mounting Considerations for International Rectifier's Power Semiconductor Packages.

Posted: 20 Jun 2003     Print Version  Bookmark and Share

Keywords:power 

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1

Application Note AN-1012

Mounting Considerations For International Rectifier's Power

Semiconductor Packages

by Pamela Dugdale and Arthur Woodworth, International Rectifier

Introduction

It is important that power semiconductors are

correctly mounted if full functionality is to be

achieved. Incorrect mounting may lead to both

thermal and mechanical problems. The aim of

this Application Note is to describe good practise

in the mounting of power semiconductors.

Making Good Thermal Contact

One of the major considerations when mounting

all power semiconductor packages is the

dissipation of heat. This is because the junction

temperature of the die and the glass transition

temperature of the plastic limit the performance

of the device. Indeed there are maximum

allowable temperatures above which the device

functionality cannot be guaranteed. The way in

which a device is mounted can have a large

effect on the thermal contact between the header

and the heat sink and hence on the ability of the

package to dissipate heat. This is often referred

to as the contact thermal resistance and is quoted

in datasheets. A full discussion of all of the

components that make up the thermal resistance

of a power semiconductor package is given in

International Rectifier Application Note AN-997

"Mounting Guidelines for the Super-247". In the

present note we shall concentrate on the thermal

resistance between the case and the heat sink as

this is the most dependent on the mounting

technique.

Package Case

Heatsink

Air Gap

Figure 1. Cross section showing the source of

the thermal contact resistance between a

package case and the heat sink.

The physical source of the contact resistance is a

result of the fact that surfaces are never perfectly

flat. The recommended flatness for the mounting

surface is 0.02mm in 10mm. Particular attention

should be paid to ensuring that no damage occurs

during the manufacture of mo unting holes. Even

for two well-prepared surfaces contact only

actually occurs at several points separated by

large air gaps. This is shown in Figure 1. As air

is a very good thermal insulator this is

undesirable and increases the thermal resistance.

There are two ways of reducing the volume of air

trapped between the surfaces. One is to increase

the force holding the two surfaces together and

the other is to improve the quality of the contact

area by filling in the gaps. In the case of the

former this can be done by either applying a

force above the die with a clip or by increasing

the torque on the screw, which mounts the tab to

the heatsink. The way in which the thermal

resistance varies as a function of the torque on

the mounting screw or the clip force is shown in

Figs. 2 and 3. Over tightening the mounting

screw may lead to deformation of the

semiconductor package and hence an increase in

the thermal resistance. This is shown in Fig. 2

which gives the thermal resistance as a function

of torque for the PowIRtabTM, showing a

minimum at 1.1Nm for dry mounting and 0.8Nm

for wet mounting. Other package types behave in

a similar way. Details of how to correctly mount

down a semiconductor to avoid this and other

problems are given in this application note.

2

Application Note AN-1012

0

0.5

1

1.5

0 0.5 1 1.5

Torque / Nm

ThermalResistanceC/W

Dry Mounting

With Heatsink Compound

Figure 2. Contact thermal resistance as a

function of mounting screw torque for a

PowIRtabTM package mounted to a heat sink

showing both dry mounting and mounting using

heat sink compound.

0

0.5

1

1.5

0 100 200 300 400

Force / N

ThermalResistanceC/W

Dry Mounting

With Heatsink Compound

Figure 3. Contact thermal resistance as a

function of clip force for a PowIRtabTM package

mounted to a heat sink showing both dry

mounting and mounting using heat sink

compound.

The second technique requires the use of a heat

sinking compound. This is usually a silicone

grease loaded with electrically insulating,

thermally conductive material such as alumina.

The purpose of the grease is to fill the gaps

without increasing the distance between the two

surfaces. If the layer of grease is too thick then

the thermal resistance will be increased. To

determine the correct amount of grease for a

particular application a series of experiments

should be performed. Several power packages

and heat sinks should be assembled using

different amounts of grease applied evenly to one

side of each mounting surface. This can be

achieved using a small rubber roller. When the

amount is correct a very small amount of grease

should appear around the perimeter of the device

as it is slowly torqued or clip mounted to the heat

sink. Excess compound should be carefully

removed.

An alternative to grease is the use of thermally

conductive pads. These are conformal under

pressure and tend to fill the air voids in the same

way as grease. The main advantage of this

approach is the ease of handling, however as

many of these pads are silicone based the

thermal resistance of this solution tends to be

higher than it would be if grease were used. In

addition to this, the silicone pads provide

electrical isolation, which can be either an

advantage or disadvantage depending on the

specific application. An alternative approach is

to use a phase change material. These materials

are solid until they are heated to temperatures in

the range 50o

C to 60o

C at which point they start

to flow, filling the air gaps. Such materials are

available on their own or as coatings on silicone

or other thermally conductive pads. For the case

when electrical isolation is not required, the

phase change material is available as a thin

coating on aluminium foil. The relative

performance of thermal grease, phase change

materials and silicone conductive pads and is

given in Table 1. These results were measured by

International Rectifier engineers at the Assembly

R&D lab.

Interface

Material

Electrical

Properties

Thermal Resistance

Junction to Heat

Sink

Heat sink

Compound

Non -

Conducting

0.7o

C/W

Phase Change

Thermal

Compound

Non -

Conducting

1.2o

C/W

Aluminium Foil

Coated in

Phase Change

Compound

Conducting 1.2o

C/W

Thermal

Interface Pad

Isolating 1.6o

C/W

Thermal

Interface Pad

Conducting 1.6o

C/W

Table 1. Performance of thermal interface

materials.

Correct Mounting Procedures

In this section the correct mounting procedures

for the TO220 package will be discussed. This

will include advice on clip mounting, screw

mounting and the requirements for isolation. By

3

Application Note AN-1012

using the TO-220 as an example most of the

common issues relating to the mounting of

power packages will be covered. Problems that

are specific to the other power packages,

including the International Rectifier PowIRtabTM

and Super packages, will be discussed in the next

section.

General Rules

7 The TO-220 package should always be

fastened to the heat sink before

soldering the leads to the PCB.

7 When bending the leads they must be

clamped tightly between the package

and the bending point to avoid strain on

the package. The leads must be bent at a

minimum distance of 2mm from the

plastic for standard packages, and 3mm

from the plastic for the TO-273. The

TO-274 is not normally lead formed

because on the design of the leg.

7 The leads should be bent no more than

90o

and should never be bent more than

once.

7 The radius of curvature should not be

less than the thickness of the leads and

ideally should be greater than 2 times

the thickness of the leads.

7 Lateral lead forming is not advisable.

The pin spacing of the leads should be

adhered to when mounting to a PCB.

7 It is recommended that in the case

where a device is rigidly secured to a

PCB and also to a heat sink mounted on

the PCB, a bend is put in the leads to

allow for differences in thermal

expansion.

7 Care should be taken not to cause any

mechanical damage to the package or

any surface finishes.

7 Heat sink compound or some other

thermal interface material should be

used.

Screw Mounting

7 For the TO-220 a M3 screw should be

used. Self-tapping screws should not be

used.

7 It is recommended that a rectangular

washer is inserted between the screw

head and the mounting tab. Care must

be taken to ensure that the washer does

not damage the plastic body of the

package during the mounting process.

7 The recommended mounting torque is

1.1Nm. This should not be exceeded.

7 When electrical isolation is required

insulating pads and insulating bushes

should be used.

Figures 4 and 5 show the suggested mounting

hardware for the TO-220.

M3SCREW

RECTANGULARWASHER

DEVICE

MICAINSULATOR

HEATSINK

INSULATINGBUSH

PLAINWASHER

SPRINGWASHER

M3NUT

Figure 4. Screw mounting through a heat sink

using a nut.

MICAINSULATOR

DEVICE

RECTANGULAR WASHER

INSULATING BUSH

M3SCREW

HEATSINK

Figure 5. Screw mounting into a tapped heat

sink.

4

Application Note AN-1012

Clip Mounting

This section is applicable to the TO-220 and

Super220TM (TO-273) packages.

7 Using clip mounting ensures that the

force is applied above the silicon and

that the thermal contact is good.

Figure 6. The effect of the position of the applied

force.

7 Increasing the force supplied by the clip

will ensure reduced thermal resistance.

This is shown in Figure 3. However the

increased cost of stronger clips is not

always worth the performance

improvements and a minimum force of

20N and typically up to 50N is

recommended.

7 Isolation is safely achieved by the use

of an insulating pad and without the use

of bushes.

7 Isolation between the bond wires and

the mounting clip is provided by the

mould compound. The minimum

thickness of plastic between a bond

wire and the clip is greater than the

thickness of the plastic on the header of

a fullpak device. This provides 4kV dc

isolation which is equivalent to 2.5kV

rms ac.

7 For heat sinks less than 5mm in

thickness saddle clips should be used.

These produce contact forces of

between 15N and 50N.

Figure 7. Saddle clip mounting of a

Super220TM package.

7 For heat sinks greater than 5mm in

thickness, U clips are used. These

produce contact forces of between

15N and 50N.

Figure 8. U-Clip mounting of a Super-220TM

package.

7 There are a number of proprietary clip

solutions where the clip is anchored in a

feature in an extruded heat sink. Forces

of between 25N and 50N can be

achieved.

Figure 9. Customized Clip Mounting of a Super-220

Application of force off-centre (i.e. bolting a device to heatsink) leads to

uneven themal contact. Using clip mounting ensures that the force is

applied above the silicon and that the thermal contact is good.

Interface material

5

Application Note AN-1012

Pop Riveting

7 It is recommended that press rivets

made of a soft material are used rather

than pop rivets.

7 The hole in the heat sink should be

smaller than the device mounting hole,

within acceptable tolerances. This

ensures that the rivet squeezes more

tightly on the heat sink than on the

device,

Soldering

A separate application note covers soldering

down power semiconductor packages "Surface

Mounting of Larger Devices." In general devices

that are being mounted to aluminium heat sinks

must be either screw or clip mounted. A new

technology, PowersitesTM

, allows solderable

power semiconductor packages to be mounted to

aluminium heat sinks.

Additional Information for Other Package

Types

7 Full-Pak devices do not require

isolation pads or bushes as the package

is isolated by design

7 The Super220TM (TO-273) does not

have a hole for screw mounting and

should be mounted using clips. The

`hole - free' design allows the package

to carry more current. Full instructions

for mounting the Super220TM (TO-273)

package are given in AN-1000

"Mounting Guidelines for the Super-

220"

7 The Super247TM (TO-274) does not

have a hole for screw mounting and

should be mounted using clips. The

`hole - free' design allows the package

to carry more current. Full instructions

for mounting the Super247TM (TO-274)

package are given in AN-997

"Mounting Guidelines for the Super-

247"

7 Mounting instructions for the PowIRtab

package are given in AN-1010

"PowIRtabTM Mounting Guidelines."

7 This is a high current package suitable

for mounting to bus bars.

Mounting SOT227

The SOT227 is a power module with some

special mounting requirements.

7 The package has two mounting holes

and four connection terminals.

7 Fasten the device to the heat sink before

connecting the leads.

7 The maximum allowable torque is

1.3Nm on the terminals and on the

mounting base.

7 M4 screws should be used with lock

washers. These are included with the

packages.

7 The separation of the centre of the

mounting holes is 30mm 1 0.2mm. Full

details of the dimensions are given on

the datasheet.

7 The first mounting screw should be

tightened to one third of the maximum

torque, the second screw should then be

tightened to the same torque. Full

tightening of both of the screws can

then be completed.

7 The case to sink thermal resistance at

the recommended mounting torque is

0.05C/W for a greased surface.

6

Application Note AN-1012

Links to Suppliers of Thermal Management and Mounting Accessories

Company Name Link Suppliers of......

Aavid Thermal

Technologies

www.aavid.com Interface materials,

clips, heatsinks.

Austerlitz Electronic

GmbH

www.austerlitz-

electronic.com

Interface materials,

clips, heatsinks.

Bergquist www.bergquistcompany.com Interface materials

Chomerics www.chomerics.com Interface materials,

PowerSitesTM

,

heatspreaders.

Fischerelektronik www.fischerelektronik.de Heatsinks and clips.

Fujipoly www.fujipoly.com Interface materials

Kunze-Folien www.kunze-folien.de Interface materials,

clips and customized

heat sinks.

R-Theta www.r-theta.com Heat sinks

Redpoint Thermalloy www.thermalloy.com Interface materials,

clips and heatsinks.

Thermagon Inc www.thermagon.com Interface materials

Warth International www.warth.co.uk Interface materials and

clips.





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