A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Thermal Interface Materials for 

Power Electronics Applications 

Preprint 

S. Narumanchi, M. Mihalic, and K. Kelly 

National Renewable Energy Laboratory 

G. Eesley 

Delphi Electronics 

Presented at Itherm 2008 

Orlando, Florida 

May 28–31, 2008 

Conference Paper 

NREL/CP-540-42972 

July 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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THERMAL INTERFACE MATERIALS FOR POWER ELECTRONICS APPLICATIONS 

Sreekant Narumanchi, Mark Mihalic, and Kenneth Kelly 

National Renewable Energy Laboratory 

1617 Cole Blvd. 

 Golden, CO 80401-3393, USA 

Phone: (303)275-4062 

Fax: (303)275-4415 

Email: sreekant_narumanchi@nrel.gov

Gary Eesley 

MC: D-16, Delphi Electronics 

One Corporate Center, 2705 Goyer Rd. 

Kokomo, IN 46904-9005, USA 

ABSTRACT 

In a typical power electronics package, a grease layer forms 

the interface between the direct bond copper (DBC) layer or a 

baseplate and the heat sink. This grease layer has the highest 

thermal resistance of any layer in the package. Reducing the 

thermal resistance of this thermal interface material (TIM) can 

help achieve the FreedomCAR partnership goals of using a 

glycol water mixture at 105°C or even air cooling. It is 

desirable to keep the maximum temperature of the 

conventional silicon die below 125°C, trench insulated gate 

bipolar transistors (IGBTs) below 150°C, and silicon carbidebased devices below 200°C. Using improved thermal interface 

materials enables the realization of these goals and the 

dissipation of high heat fluxes. The ability to dissipate high 

heat fluxes in turn enables a reduction in die size, cost, weight, 

and volume. This paper describes our progress in 

characterizing the thermal performance of some conventional 

and novel thermal interface materials. We acquired, modified, 

and improved an apparatus based on the ASTM D5470 test 

method and measured the thermal resistance of various 

conventional greases. We also measured the performance of 

select phase-change materials and thermoplastics through the 

ASTM steady-state and the transient laser flash approaches, 

and compared the two methodologies. These experimental 

results for thermal resistance are cast in the context of 

automotive power electronics cooling. Results from numerical 

finite element modeling indicate that the thermal resistance of 

the TIM layer has a dramatic effect on the maximum 

temperature in the IGBT package. 

KEY WORDS: Thermal resistance, steady state, transient, 

modeling, greases, PCMs, IGBTs, ASTM D5470, laser flash 

NOMENCLATURE 

A area, mm2

APEEM Advanced Power Electronics and Electrical 

Machines 

ASTM American Society of Testing and Materials 

BLT bond line thickness, mm, μm 

C specific heat, J/kgK 

CNTs carbon nanotubes 

DBC direct bond copper 

IGBT insulated gate bipolar transistor 

k, K thermal conductivity, W/mK 

NIST National Institute of Standards and Technology 

PCM phase-change materials 

Q heat flow, W 

R thermal resistance/impedance, mm2

K/W 

RTD resistance temperature detector 

T temperature, K, °C 

TIM thermal interface materials 

Greek 

Δ difference 

ρ density, kg/m3

INTRODUCTION 

This study falls under the thermal control work being done in 

the U.S. Department of Energy’s (DOE) Advanced Power 

Electronics and Electrical Machines (APEEM) area, which is 

part of the broader FreedomCAR and Fuel Partnership in the 

DOE Vehicle Technologies Program. The National Renewable 

Energy Laboratory (NREL) leads research and development 

activities in thermal control related to APEEM activities. The 

overall objective of these thermal control activities is to 

develop advanced technologies and effective integrated 

thermal control systems aimed at meeting the FreedomCAR 

goals. These goals address key requirements for power 

electronics, such as target values for the volume, cost, and 

weight of various subcomponents. 

Importance of Thermal Interface Materials 

This section demonstrates the importance of thermal interface 

materials (TIMs) in power electronics applications. Figure 1 

shows the different layers constituting a typical insulated gate 

bipolar transistor (IGBT) package in an inverter. The silicon 

die is soldered to the direct bond copper (DBC) layer, which is 

composed of an aluminum nitride layer sandwiched between 

two copper layers. This DBC layer is soldered to a copper 

baseplate, and the grease layer serves as the interface between 

1
the baseplate and the heat sink. This thermal grease can be up 

to 100 microns thick (bond-line thickness, or BLT) and, 

depending on the formulation, it has a thermal conductivity in 

the range of 0.4 to 5 W/mK. In an actual IGBT package, 

overall size, layout and processing can cause the DBC 

substrate to have a camber of up to 100 microns. This implies 

that, even though the intent is to keep the BLT to a minimum, 

the TIMs sometimes fill gaps of 100 microns or greater. In 

most inverter configurations, a clamping mechanism is used in 

the package. This results in the TIM being subjected to a 

pressure of the order of 0.17 to 0.34 MPa. Typically, the 

inverter has a heat sink in which the coolant flows through pin 

fins in a channel flow configuration. 

To assess the impact of thermal resistance, we performed a 3-

D finite element analysis using ANSYS. The domain used for 

the 3-D simulations of the Toyota Prius inverter is shown in 

Figure 2. Figure 2(a) shows the actual Prius inverter [1]; 

Figure 2(b) shows the representation of the inverter in 

ANSYS; Figure 2(c) shows the simulation domain, which 

consists of 1 IGBT-diode pair; and Figure 2(d) gives some 

representative temperature contours in the simulation domain. 

Making use of symmetry boundary conditions helps to reduce 

the size of the simulation domain. For the results presented 

here, 40 W heat dissipation is applied in the diode and 160 W 

heat dissipation in the IGBT. This translates to a heat flux of 

120 W/cm2

 in the IGBT and 95 W/cm2 in the diode. The area 

of the IGBT is 134.2 mm2

 (13.7 x 9.8 mm), the area of the 

diode is 0.42 mm2

 (6.6 x 6.4 mm), the DBC top area is 78.7 

mm2

 (32.3 x 24.4 mm), and the heat sink convective area is 

1464 mm2

 [2]. A convective heat transfer coefficient of 15,000 

W/m2

K was imposed on the base of the baseplate, as shown in 

Figure 2(c). The coolant temperature is 105°C. This represents 

glycol-water coolant at 105°C, since using glycol-water at 

105°C is an important FreedomCAR goal. Table 1 shows the 

thickness and thermal conductivity of the different layers in 

the package. The stack-up of the different layers is in 

accordance with Figure 1. 

DBC 

Baseplate 

Fig. 1 Layers Constituting the IGBT Package 

in an Inverter (not to scale) 

Thermal 

grease 

Heat sink 

Silicon chip Solder 

Pin-fins Coolant 

flow Q heat 

(a) (b) 

(c) 

(d) 

Fig. 2 (a) Prius Inverter [1], (b) Representation of the Prius Inverter in ANSYS, (c) Simulation 

Domain, (d) Sample Temperature Contours in the Simulation Domain 

A 

A 

Diode IGBT 

2
Figure 3 shows the impact of the TIM thermal resistance on 

the maximum die temperature. The baseline case shown 

corresponds to an average-performance grease, as measured at 

NREL. For the analysis, the grease thermal resistance was set 

at 100 mm2

K/W. The 5x TIM case corresponds to a TIM 

thermal resistance of 20 mm2

K/W (5 times lower than the 

baseline), the 10x TIM case corresponds to 10 mm2

K/W, and 

the 20x TIM corresponds to 5 mm2

K/W. Note that the thermal 

resistance values include the material bulk thermal resistance 

as well as the contact resistance. The numerical results for 

temperature are accurate (mesh-independent) to within 1%. 

Figure 3 shows that, for the baseline case, there is a significant 

temperature jump across the TIM (approximately 23°C). For 

the 5x TIM case, the temperature rise across the TIM is 7°C, 

for the 10x TIM case, the temperature rise is 4°C, while for 

the 20x TIM, the temperature rise is only 2°C. Interestingly, 

reducing the thermal resistance beyond 10x does not 

significantly change the maximum die temperature. This 

implies that, once the thermal resistance of the interface 

material is no longer the dominating resistance (<5 mm2

K/W), 

it does not matter if its resistance is reduced further. It must be 

mentioned that these conclusions are dependent upon the 

package configuration. However, our analysis results for a 

different package also lead to the conclusion that the TIM 

stops being a thermal bottleneck below a resistance of 5 

mm2

K/W. It is also worth noting that the baseline case 

corresponds to a TIM thermal resistance of 100 mm2

K/W. It is 

possible that this resistance could be even higher in an actual 

inverter, depending on the type and thickness of grease used. 

Under such conditions, the impact of TIM resistance on the 

temperature results would be even more dramatic. We will 

demonstrate this in a subsequent section. 

Table 1. Thickness and Thermal Conductivity of 

Different Layers in the Package 

State-of-the-Art of Thermal Interface Materials 

In this section, we briefly review the state of the art of thermal 

interface materials. The total thermal resistance of an interface 

material is composed of the bulk resistance and the contact 

resistances. Accordingly, the thermal resistance of an interface 

material can be written as follows: 

R = Rc + BLT/kTIM , (1)

where Rc is the contact resistance, BLT is the bond-line 

thickness of the interface material, and kTIM is the bulk thermal 

conductivity of that material. A high thermal conductivity can 

reduce the bulk thermal resistance of the interface material, 

but the contact resistance must also be minimized. Contact 

resistance is an area that has received significant attention in 

the literature (e.g., [3-5]). 

The focus here is on automotive power electronics cooling, 

but thermal interface materials play a key role in other 

microelectronics and high-power applications, as well [5]. The 

continual increase in cooling demand for microprocessors has 

led to an increased focus on improving thermal interface 

materials. Figure 4 shows the state of the art of some thermal 

interface materials. Significant advances have been made in 

the development of thermal greases/gels, phase-change 

materials (PCMs), solders, and carbon nanotubes (CNTs) as 

interface materials. Greases, gels, and PCMs are the most 

Material/ 

Layer

Thickness 

(mm) 

Thermal 

Conductivity

(W/mK) 

Silicon IGBT 0.51 116.0 

Silicon diode 0.32 116.0 

DBC top copper 0.41 393.0 

AlN substrate 0.64 170.0 

DBC bottom 

copper 

0.41 393.0 

Copper baseplate 3.0 393.0 

Aluminum heat 

sink 

6.0 235.0 

100

110

120

130

140

150

160

0 2 4 6 8 10 12

Distance across the package (mm)

Temperature (0C)

Baseline TIM

5x TIM

10x TIM

20x TIM

Baseplate IGBT 

location

Heat sink 

base 

backside 

Fig. 3 Impact of TIM Thermal Conductivity on 

the Maximum Temperature in the Die 

0

2

4

6

8

10

12

14

16

Greases Gels PCMs Singlesided CNTDoublesided CNT

Thermal resistance (mm2K/W)

Fig. 4 State-of-the-Art of Some Thermal Interface 

Materials (CNT: carbon nanotubes, PCMs: phase-change 

materials; greases, gels, PCMs: [5]; CNTs: [6]) 

3
widely used, and their thermal performance has reached 10 

mm2

K/W [5]. However, it should be noted that the BLT 

determines the performance to a significant extent. The high 

performance of greases/gels and PCMs is for a BLT of less 

than 25 microns. 

In the recent past, significant progress has been made in 

characterizing the performance of CNTs as TIMs [6]. Figure 4 

illustrates that CNTs yield a thermal resistance as low as 4 

mm2

K/W [6] under certain conditions. 

Challenges for Automotive Power Electronics 

Despite the promising performance indicated in Figure 4, a 

number of barriers have to be overcome before a TIM can be 

used in automotive power electronics. In the automotive 

industry, greases are still used predominantly as interface 

materials. Some problems with greases include pump-out 

during repeated thermal cycling, dry-out over time [5] (aging 

effects), and nonuniform application. One reason for their 

widespread use is that greases fill up the microscopic voids 

and cavities between mating surfaces very well. This results in 

a fairly low contact resistance (the first term on the right-hand 

side in Eq. (1)) at even moderate pressures (lower than 0.2 

MPa). We elaborate on this aspect in a subsequent section. 

In addition to thermal performance, a TIM should have a long 

life, be reliable, account for manufacturing variations (e.g., 

camber in the package of the order of 100 microns), be easy 

to apply, and be inexpensive. In addition, it is often found that 

the in situ performance of TIMs is different from the 

performance during material characterization through one of 

the several techniques prevailing in the literature. This also 

brings up the critical issue of the lack of a universally accepted 

standard for thermal resistance measurements. The ASTM 

D5470 approach [7] is the closest to being an accepted 

standard for thermal resistance measurements. However, it has 

its shortcomings and is not universally accepted. 

The several aspects mentioned in the previous paragraphs 

form the motivation for our research. Typically, in the 

literature, results for different materials are reported with 

different techniques. Our intent is to report results for different 

materials with one single methodology (based on ASTM 

D5470) and create a consistent, objective database of the 

thermal performance of materials over different conditions. 

Our goal is also to understand all the parameters and 

mechanisms that affect the thermal performance of any TIM. 

Understanding in situ behavior and the reliability (e.g., impact 

of thermal cycling, “aging” effects) of TIMs are also 

important goals. Mechanistic understanding of the thermal 

degradation of TIMs is currently an open area of research [5]. 

The ultimate goal of our research is to fabricate/identify a lowthermal-resistance material that would be suitable for 

automotive power electronics applications. 

In this paper, we focus on characterization of the thermal 

performance of several greases, PCMs and thermoplastics 

using the ASTM D5470-based steady-state approach. For 

some select cases (PCMs and thermoplastics), comparisons 

are also presented between the steady-state approach and the 

transient laser flash approach. The implications of the results 

for power electronics package cooling are discussed. 

EXPERIMENTS, MODELING, AND DISCUSSION 

Test Apparatus for Thermal Resistance Measurements 

 Steady State. NREL acquired, tested, redesigned, and 

improved a test stand, based on the ASTM D5470 test method 

[7, 8], for measuring thermal resistance. The intent of 

acquiring the test stand was to characterize the thermal 

performance of novel as well as state-of-the-art thermal 

interface materials that are suitable for automotive 

applications and to develop an objective, consistent database. 

Figure 5 shows the test apparatus in the Electrical Systems 

Laboratory at NREL as well as details of the components of 

the test stand. Figure 5(a) shows the overall test setup; Figure 

5(b) shows various components of the apparatus. These 

include the aluminum hot plate with cartridge heaters 

embedded in it (Figure 5(c)), the aluminum cold plate with 

silicone oil circulating through it, the copper spreader blocks, 

the copper metering blocks, the resistance temperature 

detectors (RTDs) for accurate temperature measurement, and 

the pneumatic press for applying load on the sample under 

test. 

Figure 5(c) shows how the different layers stack up. The 

cylindrical metering blocks are 31.75 mm in diameter and 25.4 

mm long. Each metering block has two RTDs, embedded 25.4 

mm into the block, for temperature measurement. The 

metering block is made of oxygen-free copper with a thin 

nickel coating to prevent oxidation of the copper and also to 

prevent erosion or corrosion of the block. The average 

roughness of the surface of the block (facing the TIM) is about 

0.5 microns. Surface roughness plays a critical role in 

determining the contact resistance [5] between the block and 

the TIM. The dimensions of the square copper spreader blocks 

are 50.8 x 50.8 mm, the square aluminum hot plate measures 

101.6 x 101.6 mm, and the aluminum cold plate measures 

152.4 x 88.9 mm. 

Load control for the pneumatic press is done via Labview. The 

required load is set by the user, and a proportional control with 

a deadband and adaptive gain has been implemented in 

Labview. The RTD temperatures are also acquired through 

Labview. The thermal resistance of the sample under test is 

determined as follows: 

 R = ΔT Asample/Q , (2) 

 Q = (Qtop + Qbottom) /2, (3) 

 ΔT = (T2 – 0.333*(T1-T2)) – (T3 + 0.333* (T3-T4)), (4) 

 Qtop = -k*Ablock*(T2- T1)/ ΔL, (5) 

 Qbottom = -k* Ablock*(T4- T3)/ ΔL, (6) 

4
where ΔT = the temperature difference across the sample 

(Figure 5(c)), Q = average power going through the sample, 

Asample = area of the sample, Qtop is the power going through 

the top metering block (in the –y direction (Figure 5(c)), 

Qbottom is the power going through the bottom metering block 

(in the –y direction (Figure 5(c)), k is the thermal conductivity 

of the oxygen-free copper (393 W/mK), Ablock is the area of 

the metering block (791.7 mm2

), T1, T2, T3 and T4 are the 

temperatures measured by the RTDs (Figure 5(c)), and ΔL is 

the distance between RTD probes 1 and 2 (15.24 mm) as well 

as probes 3 and 4. 

Fig. 5 Test Apparatus for Thermal Resistance 

Measurements; (a) Facility Setup at NREL, (b) the 

Apparatus, (c) Stack-up of the Different Layers

(a) 

(b) 

1 

2 

3 

4 

1,2,3,4: RTDs 

Copper 

metering blocks 

Thermal 

interface 

material 

Hot plate 

Cold plate 

Copper 

spreader block 

Copper 

spreader block 

Cartridge 

heater 

ΔT 

Channels for 

silicone oil 

Force (c) 

+y 

 Transient. Another method for measuring thermal 

conductivity is referred to as the transient laser flash method 

[9]. In this approach, a short laser pulse is used to illuminate 

the surface of a sample, and the resulting transient temperature 

rise at the opposite surface is measured as a function of time. 

Analytic models for this temperature are then fit to the 

measurement, yielding the thermal diffusivity of the sample, 

α = ρC/K p , (7) 

where K, ρ, and Cp are the thermal conductivity, density, and 

specific heat capacity of the material. Independent 

measurements of ρCp are used to calculate K from the 

diffusivity value. This is a somewhat indirect method relative 

to the steady-state method, but it has the advantage of rapid 

measurement over a large temperature range. 

In most cases of interest for thermal interface materials, we are 

really more concerned with measuring the total thermal 

resistance of the TIM sandwiched between substrates of 

interest. If the TIM-BLT is thermally thin relative to the 

adjoining substrates, it is possible to determine thermal 

resistance directly via the transient method [10]. This is based 

on analytical models derived for laser flash testing of 

multilayer samples and avoids the need to determine ρCp . 

As more commercially available instrumentation has been 

developed for this transient method, it is becoming more 

widely used to characterize TIMs. In addition to the steadystate testing method, we also employed the laser flash method 

on some of our samples. Our purpose is to validate this 

technique relative to the steady-state measurement and provide 

a thorough and consistent evaluation of thermal interface 

materials. In the test results presented in this paper, the TIM is 

sandwiched between the DBC aluminum nitride substrate and 

aluminum. The cross-sectional area of the DBC substrate and 

aluminum layers are 10 x 10 mm2

. The thickness of the DBC 

substrate is 1 mm, while the thickness of the aluminum layer 

is 0.5 mm. 

 Experimental Uncertainty. The uncertainty in the thermal 

resistance measured from the steady-state test method (Eq. 

(2)) was estimated by the method of Kline and McClintock 

[11]. The accuracy of this methodology is dependent upon the 

temperature and heat flow in the metering blocks being onedimensional. For most of the measurements, the discrepancy 

between the heat flux in the hot and cold metering blocks 

(Equations 5 and 6) was less than 4%. The accuracy of the 

RTD probes, during the initial calibration, with respect to a 

NIST-traceable probe was within 0.04 K. Some drift in the 

accuracy is possible but is difficult to estimate. The overall 

5
uncertainty in the resistance measurements is estimated to be 

±10% (at 95% confidence levels). The repeatability of the 

experimental results was within ±4%. 

55

60

65

70

75

80

85

0 10 20 30 40 50 60

Distance (mm)

Temperature (0C)

M odeling - 25 um grease

Experiments -25 um grease

M odeling - 150 um grease

Experiments- 150um grease

Fig. 6 Comparison of Experimental and Modeling Results

Test Results with Conventional Thermal Greases 

In this section, we discuss experimental results for various 

commercially available greases. Greases are predominantly 

used in the automotive industry despite well-known problems 

related to pump-out and dry-out. The intent here is to 

understand the mechanisms underlying the thermal 

performance of greases and also to create a consistent, 

objective database against which to compare the performance 

of any alternative TIM. 

Figure 6 shows a comparison of experimental and thermal 

modeling results with a conventional grease (Wacker Silicone 

P12). The average temperature in the metering blocks is 

plotted as function of the vertical distance, i.e., moving from 

the cold plate (0) to the hot plate (50.8 mm). 

Results are shown for two different grease thicknesses: 25 and 

150 microns. The step change in temperature at 25.4 mm is 

across the TIM. The modeling results indicate that the model 

is capturing the trend of the experimental results. Of course, in 

the modeling, there are a number of uncertainties, such as the 

heat loss coefficients on the boundaries and the contact 

resistance between the various layers in the apparatus. 

However, between the modeling results for the 25- and 150-

micron grease layer cases, the only difference is the resistance 

of the interface material. This shows that the model is 

capturing the changes in the interface material quite well. In 

addition, the model shows that the experimental results are 

following the expected trends. 

Since grease is still the most predominantly used interface 

material in the automotive power electronics industry, we 

performed experiments with some brands of commercially 

available thermal greases. Some of these greases (Aavid 

Thermalloy Thermalcote 251G, Wacker Silicone P12) are 

currently in use in automotive inverters. These experiments 

Fig. 7 Thermal Resistance vs. Thickness for Different Greases 

(dashed line: target value of 3 mm2

K/W) 

0

50

100

150

200

250

300

350

400

0 0.05 0.1 0.15 0.2

Thickness (mm)

Thermal resistance (mm2K/W)

Thermaxtech-xtflux-GA

Arctic Silver 5

Thermalcote-251G

Shinetsu-X23-7762-S

Wacker Silicone P12

Keratherm KP77

Dow Corning TC5022

0

50

100

150

200

250

Wacker

Silicone P12

Thermalcote

251G

Arctic Silver 5

ThermaxtechX

t-Flux-GA

Dow Corning

TC-5022

Shinetsu X-23-

7762-S

Resistance (mm2K/W)

Bulk resistance

Contact resistance

Fig. 8 Contribution of Bulk and Contact Resistances for 

Various Greases (at 75 microns BLT and an average sample 

temperature of ~75°C) 

were performed over a sample temperature range of 55° to 

80°C, a thickness range of 25 to 150 microns, and an applied 

pressure of 0.17 to 0.34 MPa. Figure 7 shows the thermal 

resistance as a function of grease thickness at an average 

sample temperature of ~75°C for the various greases. In the 

near future, trench IGBTs will soon be the norm. These trench 

IGBTs have a maximum temperature rating in the range of 

150° to 175°C. Hence, it is important to characterize the 

performance of these greases at elevated temperatures of the 

order of 150°C. We plan to do this in the near future. 

To maintain the thickness of the grease layer, we used glass 

spheres acquired from Mo Sci Corporation. The volume of the 

glass spheres is a negligible fraction of the total volume of the 

grease between the metering blocks. Figure 7 shows that, as 

the grease thickness increases, the thermal resistance also 

6
increases linearly, as expected. For any given grease thickness, 

the thermal resistance may also change as a function of 

temperature. We found this change to be as much as 8%, in 

some cases, over a temperature range from 55° to 80°C. 

However, not all greases exhibited this behavior. 

For any given grease listed in Figure 7, the inverse of the slope 

of the linear curve gives the thermal conductivity of the 

material, and the y-intercept gives an estimate of the contact 

resistance. From a thermal perspective, the Shinetsu-X23-

7762-S and Dow Corning TC-5022 greases are highperformance materials. For a 25-micron thick layer, the 

thermal resistance is 13 and 15 mm2

K/W for Shinetsu and 

Dow Corning, respectively. Even for a 150-micron thick layer, 

the thermal resistance is only about 45 and 47 mm2

K/W for 

Shinetsu and Dow Corning, respectively. In our study, we are 

exploring a TIM thickness range of 25 to 150 microns, 

because in actual packages, manufacturing constraints may 

cause unavoidable gaps of 100 to 150 microns. 

Figures 7 and 8, as well as Table 2, show contact resistances 

for the various greases. Individual differences in the greases 

may be a result of how well they wet the surfaces and fill up 

the microscopic surface irregularities [5]. The Shinetsu-X23-

7762-S and Thermaxtech-Xtflux-GA greases have the lowest 

contact resistances. However, for all the greases, the contact 

resistances are in the range of 6 to 20 mm2

K/W. Though not 

presented in this paper, the contact resistance of TIMs can be 

estimated theoretically [3-5]. It would be interesting to 

compare modeling results for contact resistance with the 

experimental results obtained here. Arguably, the contact 

resistance of greases is quite low, which may be one reason 

for their widespread use despite significant drawbacks. Figure 

8 shows that, as the BLT of the grease increases, the bulk 

resistance of the grease dominates over the contact resistance. 

Table 2. Comparison of Various Greases as to Thermal Resistance, Conductivity, Composition, and Cost 

Overall, the results in Figure 7 suggest that high-performance 

greases can yield resistances as low as 13 and 33 mm2

K/W, 

for a BLT of 25 and 100 microns, respectively. Our analysis 

indicates that the interface material completely stops being a 

bottleneck below a resistance of 3 mm2

K/W for a conventional 

Grease Thermal Resistance @ ~75°C (mm2

K/W) Thermal 

Conductivity 

(W/mK) 

Composition Cost 

($/g) 

Contact 

Resistance 

(mm2

K/W) 

23.5 μm 

(Total 

resistance) 

75 μm 

(Total 

resistance) 

150 μm 

(Total 

resistance) 

Expt. 

@~75°

C 

Mfg. 

Data 

Wacker 

Silicone P12 

13.6 59.1 150.8 294 0.54 0.8 Dimethylsiloxane(s) with 

filler (trade secret) 

0.07 

Aavid 

Thermalloy 

Thermalcote 

251 G 

19.6 77.5 207.9 393.3 0.4 0.39 Polydimethylsiloxane (30%-

60%); zinc oxide 

(30%-60%) 

0.04 

Arctic Silver 5 7.9 30.9 91.0 166.1 0.94 8.7 Silver, boron 

nitride, zinc oxide, 

aluminum oxide, 

ester oil blend 

2 

Thermaxtech 

Xt-flux-GA 

6.0 36 101.6 197.2 0.78 7 Silicone oil, highpurity particles 

(info. not 

available) 

1.2 

Dow Corning 

TC-5022 

8.7 16.7 23.8 47.8 4.0 4.0 Dimethyl, 

methyldecylsiloxane (1%-

~5%), metal 

oxides, treated 

fillers (trade 

secret) 

0.43 

Shinetsu X-23-

7762-S 

6.3 12.6 26.5 46.6 3.7 4.4 Aluminum 

(>60%), zinc 

oxide (<30%), 

unknown solvent 

(trade secret) 

0.9 

7
IGBT package configuration. The results here suggest that, 

while the commercially available greases tested in this study 

can be used to obtain very good performance, none 

approaches 3 mm2

K/W for practical BLTs. In addition, 

reliability is a concern with a majority of the greases. Aspects 

related to reliability include performance after thermal cycling 

(pump-out), aging effects (dry-out effects on viscosity, 

thermal performance), and uniform application. 

0

10

20

30

40

50

60

Honeywell

PCM45-G

Loctite

Powerstrate

Xtreme PS-X

Cookson Staystik

1172

Btech TP-1

Resistance (mm2K/W)

NREL ASTM

method

Delphi laser flash

method

Table 2 is a summary and comparison (including costs and 

chemical composition) of most of the greases that were tested. 

The cost numbers are given as $/gram of grease, but this 

number can be highly influenced by the quantity purchased. A 

typical IGBT package footprint in an inverter is about 13,560 

mm2

. Assuming a grease thickness of 75 microns gives a total 

grease volume of about 1000 mm3

. Given that the density of 

the grease could vary from 1600 kg/m3

 (Thermalcote 251G) to 

4100 kg/m3

 (Arctic Silver 5), the weight of grease required for 

a single inverter varies from 1.6 to 4.1 grams. 

Fig. 9 Comparison of Steady-State and Transient Laser Flash 

Approaches for Thermal Resistance Measurement of PCMs 

and Thermoplastics 

phase. Upon cooling, the PCM has adhesive properties and 

forms a bond between the two bounding layers. From the cost numbers in Table 2, this means that the cost of 

grease for a single inverter can vary from $0.06 to $8.40, 

depending on the type of grease used. Again, estimating costs 

can be very tricky, and some of the cost numbers provided in 

Table 2 could change drastically based on the quantity 

purchased (e.g., Arctic Silver 5 and Thermaxtech-Xtflux-GA). 

Thermoplastics are also of interest in the automotive industry 

because, upon curing, they function as adhesives and hold the 

bounding layers of the package together without any external 

clamping mechanism. The curing process involves keeping the 

bounding layers and the thermoplastic at a temperature of 

150°C under a pressure of 0.2 MPa for about half an hour, and 

then allowing the assembly to cool down to room temperature. 

For the results shown in Figure 9, the average sample 

temperature is about 62°C and the pressure is 0.17 MPa. 

The Wacker Silicone P12 ($0.16/inverter application) and 

Thermalcote 251G ($0.06/inverter) greases are certainly less 

expensive than the Shinetsu X23-7762-S ($2.25/inverter) or 

the Dow Corning TC-5022 ($1.40/inverter). However, the 

thermal performance of Thermalcote 251G and Wacker 

Silicone P12 is also considerably inferior to that of the 

Shinetsu or Dow Corning greases. For the steady-state method, the TIM is placed between the 

two metering blocks (Figure 5). For the transient laser flash 

method, the TIM is placed between the aluminum and DBCaluminum nitride layers. This aspect itself could cause some 

of the discrepancy in the results from both the steady-state and 

transient laser flash methods. 

The key to the good performance of the Shinetsu grease seems 

to be a high content of very conductive particles and an 

unknown solvent that enhances performance. This solvent is 

considered to be a trade secret. The solvent evaporates over 

time (a few weeks or months), so the viscosity of the grease 

increases over time. As a consequence, the Shinetsu grease is 

not as easy to work with as the Thermalcote 251G and Wacker 

Silicone P12 greases. It is not clear how reliable the Shinetsu 

grease will be over time. We plan to characterize aspects 

related to the reliability of the TIMs in the near future. 

It is important to note that, unlike the experiments with 

greases, the BLT of the PCMs and thermoplastics was not 

controlled during the experiments. The lack of BLT control 

also is closer to a real-world application. For the PCMs, we 

found a reasonable match between the results (Figure 9) from 

the two approaches (within 20%); for the thermoplastics, 

however, there is noticeable discrepancy in the results (within 

30%). For the thermoplastics, there is a difference in the 

processing conditions (curing process) for the tests with the 

two different approaches. Figure 9 shows that the PCMs have 

very low thermal resistances (~8-10 mm2

K/W); however, their 

reliability and suitability for automotive applications remains 

to be proven. 

Test Results for Phase-Change Materials and 

Thermoplastics 

We also performed measurements on some PCMs and 

thermoplastics through both the steady-state and the transient 

laser flash approaches. Figure 9 shows the results for two 

phase-change materials (Honeywell PCM45-G and Loctite 

Powerstrate XtremePS-X) and two thermoplastics (Cookson 

Staystik 1172 and Btech TP-1). Some studies in the literature [5] indicate that the performance 

of a PCM degrades when it is exposed to elevated 

temperatures over a period of time. The environment in 

automotive power electronics is harsher than it is in computer 

microelectronics [5]. 

The PCM changes phase (solid to liquid) at a temperature of 

the order of 50°C. The PCMs are placed between the two 

bounding layers, and the temperature of the PCM has to be 

about 50°C at least once in order for the PCM to change 

8
Figures 7, 8, and 9 gave an indication of the thermal resistance 

numbers associated with the various TIMs. It is worthwhile to 

explore the impact of these numbers on the maximum die 

temperature in a realistic power electronics package. We do 

this through the 3-D ANSYS finite element simulations 

presented earlier. The domain for the simulations remains the 

Fig. 10 Predicted Maximum IGBT Junction Temperature as a 

Function of the Total TIM Thermal Resistance for the Various 

Greases 

Fig. 11 Temperature Across the Cross Section of the IGBT 

Package (AA in Figure 2(C)) for Two Extremes of the TIM 

Resistance 

same as that in Figure 2. The results for temperature are again 

accurate (mesh-independent) to within 1%. Figure 10 shows 

the maximum IGBT junction temperature as a function of total 

TIM thermal resistance for each of the three grease 

thicknesses considered in this study (25, 75, and 150 microns). 

The lowest grease thermal resistance from Table 1 is 12.6 

mm2

K/W; the highest is about 393.3 mm2K/W. Figure 10 

shows that the difference in the maximum IGBT temperature 

for the lowest and highest thermal resistance cases is almost 

58°C, which is a significant difference. 

Figure 11 shows the temperatures across a cross section (AA 

in Figure 2(c)) through the IGBT package for the Prius 

inverter for the two extremes in TIM thermal resistance (12.6 

and 393 mm2

K/W). For a TIM resistance of 12.6 mm2K/W, 

the temperature jump across the TIM is only 4.7°C. However, 

when the TIM resistance is 393 mm2

K/W, the temperature 

jump across the TIM is about 65°C. This suggests that, even 

within greases, one has to be careful of the type and thickness 

of grease that is applied, since this can have a significant 

impact on the die temperatures, as demonstrated in the results 

presented here. 

100

120

140

160

180

200

220

0 100 200 300 400 500

TIM Total Thermal Resistance (mm2

K/W)

Predicted maximum IGBT temperature (0C)

25 microns

75 microns

150 microns CONCLUSIONS 

This paper demonstrates that TIMs pose a significant 

bottleneck to heat removal from the IGBT package. From a 

DOE Vehicle Technologies Program and automotive industry 

perspective, a reliable, cost-effective, and high-thermalperformance TIM can help to meet some program goals. 

These goals could include cooling with 105°C glycol-water, or 

even air cooling, and extracting maximum heat flux from the 

device, whether it be conventional silicon-based, trench IGBT 

or silicon-carbide-based, and keeping the maximum 

temperature within acceptable limits. 

We acquired, tested, redesigned, and improved a test stand 

based on the ASTM D5470 steady-state test method to make 

thermal resistance measurements of several TIMs. 

Experiments were performed with several conventional 

thermal greases, and their performance was characterized over 

a range of temperatures and BLTs. The Shinetsu and Dow 

Corning greases are superior in thermal performance over 

other greases that were tested. These results suggest that care 

has to be taken in choosing the type of grease and the 

thickness of the grease that is applied, since its impact on die 

temperatures is significant. Results also suggested that the 

contact resistance of greases is fairly low. 

100

120

140

160

180

200

220

0 2 4 6 8 10 12

Distance across the package (mm)

Temperature (0C)

Resistance = 12.6

mm2K/W

Resistance = 393.3

mm2K/W

The results also indicated that none of the tested greases were 

able to go below a resistance of about 33 mm2

K/W at BLTs of 

100 microns or greater. Practical inverter configurations are 

likely to require a TIM thickness of about 100 microns. 

Hence, a conservative goal would be to identify or fabricate a 

TIM that would have a thermal resistance of about 3 mm2

K/W 

at bond line thicknesses approaching 100 microns. The phasechange materials tested have a fairly low thermal resistance 

(~8-10 mm2

K/W), but it is likely that the BLT of PCMs during 

the experiments was fairly low (<25 microns). The match 

between the results from the ASTM steady-state method and 

those from the transient laser flash method is fair, given all the 

uncertainties involved. 

In addition to the TIMs considered in this study, other TIMs 

(such as metallic TIMs, graphite, and CNTs) will also be 

studied. For materials with promising thermal performance, 

we plan to characterize all aspects related to reliability in the 

automotive environment. These include the impacts of thermal 

cycling and aging on the TIM thermal resistance. The impact 

of elevated temperatures (~150°C) on the performance of the 

TIMs will be explored. We also plan to characterize the 

automotive power electronics package in situ performance of 

the most promising TIMs. 

9
10

ACKNOWLEDGMENTS 

The authors would like to acknowledge the support provided 

by Susan Rogers, Technology Development Manager for 

Power Electronics and Electrical Machines, Vehicle 

Technologies Program, DOE Office of Energy Efficiency and 

Renewable Energy. The authors would also like to thank Keith 

Gawlik and the power electronics team at NREL for helpful 

discussions and interactions. 

REFERENCES 

[1] R.H. Staunton, L.D. Marlino, J.N. Chiasson, and T.A. 

Burress, "Evaluation of 2004 Toyota Prius Hybrid Electric 

Drive System," Oak Ridge National Laboratory Technical 

Report, ORNL/TM-2006/423, 2006. 

[2] M. O'Keefe and K. Bennion, “A Comparison of Hybrid 

Electric Vehicle Power Electronics Cooling Options," IEEE 

Vehicle Power and Propulsion Conference, 2007. 

[3] C.V. Madhusudana, Thermal Contact Conductance, New 

York: Springer-Verlag, 1996. 

[4] M.M. Yovanovich and E.E. Marotta, “Thermal Spreading 

and Contact Resistances," in Heat Transfer Handbook, A. 

Bejan and A.D. Kraus, Editors, Hoboken, NJ: John Wiley & 

Sons, pp. 261−395, 2003. 

[5] R. Prasher, “Thermal Interface Materials: Historical 

Perspective, Status and Future Directions,” in Proceedings of 

the IEEE, Vol. 94, No. 8, pp. 1571–1586, 2006. 

[6] B.A. Cola, J. Xu, C. Cheng, X. Xu, T.S. Fisher, and H. Hu, 

"Photoacoustic Characterization of Carbon Nanotube Array 

Thermal Interfaces," Journal of Applied Physics, Vol. 101, 

No. 054313, 2007. 

[7] American Society for Testing and Materials, ASTM 

Standard D5470-01, 2005. 

[8] S.V.J. Narumanchi, Advanced Thermal Interface Materials 

to Reduce Thermal Resistance, NREL Technical/Milestone 

Report No. TP-540-40617, 2006. 

[9] R.C. Campbell, S.E. Smith, and R.L. Dietz, 

"Measurements of Adhesive Bondline Effective Thermal 

Conductivity and Thermal Resistance Using the Laser Flash 

Method," 15th IEEE Semitherm Symposium, pp. 83, 1999. 

[10] H.J. Lee, "Thermal Diffusivity in Layered and Dispersed 

Composites," Ph.D. Thesis, Purdue University, University 

Microfilms International, 1975. 

[11] S.J. Kline and F.A. McClintock, “Describing 

Uncertainties in Single-sample Experiments,” Mech. Eng.

(Am. Soc. Mech. Eng.), 75, pp. 3-8, 1953. 
F1147-E(09/2007) 

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Thermal Interface Materials for Power Electronics Applications: 

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14. ABSTRACT (Maximum 200 Words)

In a typical power electronics package for a vehicle, a grease layer forms the interface between the direct bond 

copper layer or a baseplate and the heat sink. This grease layer has the highest thermal resistance of any layer in 

the package. Reducing the thermal resistance of the grease or other thermal interface material (TIM) can help to 

achieve Department of Energy FreedomCAR goals to use a glycol water mixture at 105 deg. C or even air cooling. 

This paper describes progress in characterizing the thermal performance of some conventional and novel TIMs. 

Experimental results for thermal resistance, in the context of automotive power electronics cooling, indicate that the 

thermal resistance of the TIM layer has a dramatic effect on maximum temperature in the power electronics package.

15. SUBJECT TERMS

hybrid electric vehicles; inverters; IGBTs; heat transfer; oscillating jets; vehicle power electronics 

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