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The Crucial Influence of Thermal Interface Material in Power Electronic

Design

Dr.-Ing. Martin Schulz, Infineon TechnologiesScott T. Allen, Henkel Electronic Materials LLCDung Phan, Henkel Electronic Materials LLCDr. Wilhelm Pohl, Hala Contec

Speaker:Giuseppe CaramellaHenkel Electronic MaterialsBelgium

March 21, 2013 The Crucial Influence of Thermal Interface Material in Power Electronic Design2

Available Thermal Interface Materials (TIMs)• Thermal interface solutions for the electronics market have been available

for decades – aka generic thermal grease

• Advancements in thermal interface materials• Explosive growth in consumer electronics, early 1990’s - 2000’s• Personal computing and handheld electronics• Home entertainment, gaming and the internet

• TIM developments driven by the specific needs of the consumer market• Simplicity of application and increased performance

• Low cost labor for OEMs or home installation – overclockers• High performance greases, more elaborate filler types

• Rapidly changing designs and short term needs• Increased acceptance of disposable consumer electronics

• Numerous grease and phase change materials (vendors) emerged• “Flavor of the day” greases


TIM Selection for the Power Electronics Market• Power electronics designers and assemblers have had to rely on

commonly available thermal interface materials

• Material selection based on data sheet values• End performance doesn’t always meet expectations• Unexpected TIM performance and poor application methods• Lifetime predictions fall short due to uncertainties in the stability data

• Improvements in performance and longevity could be achieved by utilizing a TIM solution specific to the needs of the power electronics market

• Focus will be on a robust TIM pre-applied by the module manufacturer• Problems from end user application methods eliminated• Lifetime predictions ensured through rigorous testing• Enhanced thermal performance not available from off the shelf TIMs


The Challenge for the Power Module Manufacturer• Find the best thermal interface material, suitable for power electronics,

while avoiding detrimental features

Thermal Interface Materials

available to the market

SiliconebasedSuffers from

Dry-Out

Insufficient thermal range

Solid or Separates Electrically

Conductive

Ask a TIM manufacturer to fill the gap


What is so special in Power Electronics?

Chip Area [mm²] 263 190

Power Density [W/cm²] 50 100 - 200

Expected Lifetime [Years] < 5 up to 30

Cost of replacement [US$] < 200 up to 1,000,000

Ambient Temperature [°C] 20 - 40 -50 65

Case Temperature [°C] < 75 85 110

IGBT 4


Thermodynamics


Thermal Cycling - Consequences• Forced air-cooled heat sink, 2 Minute cycle, 50% duty cycle• Current tuned to achieve Tjmax~150°C

Thermal transfer remains intact only if the material stays in place

Datasheet values for thermal conductivity are no more than an indicator

Wetting ability matters

Creeping ability matters

Long term stability matters

Chip-Temperature increase of >20K due to pump-out of thermal grease within

630 cycles/32 hours test time


Basic Physics

A: Path through TIM

B: Metal-to-Metal Contact

Heat fromDevice

Dissipate to Ambient

mKW

th 10<λ

mKW

th 100>λ

• Maximize λth for TIM

• Minimize areas with path A

• Maximize areas with path B

• Achieve smallest possible bond lines


0

0,001

0,002

0,003

0,004

0,005

0,006

0,007

GPG Filler A.1 Filler A.1+B Filler A.2 Filler A.2+B

Measured results from ASTM High Pressure Testing

Ther

mal

Res

ista

nce

[K/W

]Optimizing the Filler Components

-15%

-47%

-74%


Long-Term Stability• Throughout the development, High-Temperature-Storing (HTS) was

found suitable to achieve reliable results. 125°C, 1000h

Gradual increase due to aging effects

Triggered effect after a certain time

Stable behavior as demanded

IFX-TIM with improved performance


Long-Term Stability


Convincing 3rd Party Results

100

110

120

130

140

150

160

0 1k 2k 3k 4k

Chip Te

mp. [°C]

Test time [h]

Highly Accelerated EoL‐Test

General Purpose Grease 1 General Purpose Grease 2 New IFX-TIM

1000h Test 20 years of lifetime=̂

End of life if 150°C is reached


Lifetime Considerations• Predicted module lifetime relies on thermal performance stability• Changes in junction temperature can lead to unexpected failures

At end of test, MOD-3 yielded ΔTvjof 100K with predicted lifetime of 7.4·104 cycles

Improved TIM gave 30K reduction at end of test, for a predicted lifetime of 2.5·105 cycles


In Thermal Testing on LonGwin LW-9389

Casting a layer of material on a polymer film (release liner)Dry in 125°C oven for 1 hour

Cutting to the testing size 2.54 cm2 and peel of the release liner

Samples are ready for testing(0.195-0.120mm and

0.145-0.155mm)

LonGwin TIM tester

In House Thermal Testing


Starting thickness might affect thermal resistant at lower temperature and lower pressureIt looks like the material reaches to the saturated point at the pressures from 552 to 690 kPaLower starting thickness seems to have consistent thermal reaction ;and at 70, 85, and 100°C

material shows same thermal resistant range

Comparison of Thermal Resistant at Different Operating Temperatures and Starting Bonding Thickness

Thermal Resistant versus PressureStarting Thickness 0.150mm

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 100 200 300 400 500 600 700 800

Pressure (kPa)

Ther

mal

Res

ista

nt (°

C/W

)

55°C 70°C85°C 100°C

Thermal Resistant versus PressureStarting Thickness 0.2mm

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 100 200 300 400 500 600 700 800

Pressure (kPa)

Ther

mal

Res

ista

nt (°

C/W

)

55°C 70°C85°C 100°C

Comparison of Bondline vs Temperature


A Quick Look at Starting Assembly Thickness

Material shows same trend of melting rate and with corresponding thermal resistant. However, with thinner starting thickness seems transfer heat better.

Thermal Resistant versus BondlineAt 100°C

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Bond-line (mm)

Ther

mal

Res

ista

nt (°

C/W

)

starting thickness 0.200mm

starting thickness 0.150mm

Comparison of Bondline vs Start thickness


0

100

200

300

400

500

600

700

800

0.01 0.02 0.03 0.04 0.05 0.06 0.07

Bond-line (mm)

Pres

sure

(kPa

)

0.200

0.220

0.240

0.260

0.280

0.300

0.320

0.340

0.360

Ther

mal

Im

peda

nce(

°C*c

m2 /W

)

Pressure-BondlineThermal Impedance-Bondline

At 100°CStarting thickness 0.150mm

A Quick Look on Thermal Profile at 100°C Operating Temperature

Comparison of Bondline vs Start thickness

With a starting thickness of 0.15mm, material can reach to the bond-line thickness of ~ 0.02mm at 100°C and 700kPa to have a thermal impedance of ~ 0.24 °C*cm2/W


Thermal Conductivity at Different Operating Temperatures Sample thickness ~ 200µm, tested at different pressures 138, 276, & 552kPa ( or 20, 40, and 80psi)

At higher operating temperature (or high temperature makes material thinner with same operating press) Change in slope observed with higher temperature likely due to tighter packing of filler.

At 55°C material is around at melting point which is not completely melt and forming a thicker bond-line range contributing to a thermal conductivity of 2.94 W/(m°C).

Thermal Impedance versus BondlineCalculating Apparent Thermal Conductivity

y = 3.4054x + 0.04

y = 2.8072x + 0.1166

y = 1.9707x + 0.1203

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140

Bond-line (mm)

Ther

mal

Impe

danc

e (°

C*c

m2 /W

)

@55°C _ K=2.94 W/m*C@70°C _ K=3.56 W/m*C@85°C _ K=5.04 W/m*C

Affects of bondline on Conductivity


Conclusions regarding TIM• Acceptable TIM performance on a CPU does not mean the same will

be seen on an IGBT module

• Datasheet values may seem like a good indicator, but they do noteliminate proper verification in your actual application

• A dedicated, optimized thermal interface material outperforms general purpose solutions that are available to end users of power modules

Thank you!

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