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Contract No. DE-AC36-08GO28308

Thermal Performance and 

Reliability Characterization of 

Bonded Interface Materials 

(BIMs)

Preprint

D. DeVoto, P. Paret, M. Mihalic, 

and S. Narumanchi

National Renewable Energy Laboratory

A. Bar-Cohen and K. Matin

Defense Advanced Research Projects Agency

Presented at ITherm 2014

Orlando, Florida

May 27-30, 2014

Conference Paper

NREL/CP-5400-61107

August 2014
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Thermal Performance and Reliability Characterization of Bonded Interface Materials (BIMs)

Douglas DeVoto, Paul Paret, Mark Mihalic, and Sreekant Narumanchi 

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

Email: sreekant.narumanchi@nrel.gov

Avram Bar-Cohen and Kaiser Matin

Defense Advanced Research Projects Agency

675 N. Randolph Street, Arlington, VA 22203

ABSTRACT

Thermal interface materials (TIMs) are an important 

enabler for low thermal resistance and reliable electronics 

packaging for a wide array of applications. There is a trend 

towards bonded interface materials (BIMs) because of their 

potential for low thermal resistance (<1 mm2

-K/W). However, 

due to coefficient of thermal expansion mismatches between 

various layers of a package, thermomechanical stresses are 

induced in BIMs and the package can be prone to failures and 

integrity risks. Deteriorated interfaces can result in high 

thermal resistance in the package and degradation and/or 

failure of the electronics. The Defense Advanced Research 

Projects Agency’s (DARPA) Thermal Management 

Technologies (TMT) Program has addressed this challenge, 

supporting the development of mechanically compliant, low 

resistivity nano-thermal interface (NTI) materials. Prior 

development of these materials resulted in samples that met

DARPA’s initial thermal performance and synthesis metrics. 

In this present work, we describe the testing procedure and 

report the results of thermal performance and reliability 

characterization of an initial sample set of three different NTIBIMs tested at the National Renewable Energy Laboratory.

KEY WORDS: thermal interface, bonded interface material, 

accelerated testing, temperature cycling, aging, thermal 

resistance, transient technique, steady-state technique

NOMENCLATURE

A metering block cross-sectional area, m2 

BIM bonded interface material

cp specific heat, J/g-K 

DARPA Defense Advanced Research Projects Agency

k thermal conductivity of metering block, W/m-K 

l thickness of the test sample, cm

NREL National Renewable Energy Laboratory

NTI nano-thermal interface

R thermal resistance, mm2

-K/W

t time, s 

T temperature, °C

TIM thermal interface material

TMT Thermal Management Technologies

x bondline thickness, mm

Δx1 distance between T1 and T2 or T3 and T4, m

Δx2 distance between T2 or T3 and sample interface, 

m 

Greek

a thermal diffusivity, cm2

/s

λ thermal conductivity, W/m-K 

ρ bulk density, g/cm3

Subscripts

avg average

s top top of the interface material

s bot bottom of the interface material

top top metering block

bot bottom metering block

1,2,3,4 locations of temperature measurement in the 

metering blocks

INTRODUCTION

Modern electronic packages continue to increase in 

processing power by shrinking transistors in each subsequent 

generation of the silicon devices. With these advances, 

thermal management of the package becomes more 

challenging. A successful cooling solution must address the 

needs of a chip that operates at higher power levels and higher 

heat fluxes. Hot spots in the chip can result in localized heat 

fluxes that are several times greater than the chip’s average 

heat flux. Package and die stacking designs will limit access to 

the back side of chips for cooling, thus creating higher heat 

density packages. Innovative thermal management solutions 

are needed to ensure that future chip package architectures can 

operate at their maximum performance potential.

DARPA’s TMT Program aims to address these thermal 

design concerns through several focus areas: novel air-cooled 

heat sinks, two-phase heat spreaders, TIMs/BIMs, and 

thermoelectric coolers. In an electronic package, the TIM/BIM 

is a critical component that fills up the air gaps between 

various layers thereby providing a path for heat dissipation. 

Historically, polymeric interface materials such as greases, 

gels and phase change materials [1, 2] have been used in 

various packaging applications. However, the trend towards 

miniaturization and increasing power densities has resulted in 

the need for development of low-resistance packaging 

configurations with TIM/BIM layer resistances on the order of 

1 mm2

-K/W or less. An additional and important requirement 

is also that the TIM/BIM should be reliable. For 

environmental reasons, non-lead-based solutions are being 

developed and include lead-free solders [3-7], high-pressure 

sintered silver [8, 9], low-pressure/temperature sintered silver 

[10, 11], as well as thermoplastic adhesives [12]. None of 

these materials though have yielded thermal resistances below 

1 mm2

-K/W for large-area attachments (> 50 mm X 50 mm 

cross-sectional area footprint) while demonstrating reliability 

under harsh accelerated testing conditions. The goal of the 

DARPA NTI Program is to develop materials with thermal 

resistance on the order of 1 mm2

-K/W while demonstrating 

robustness and reliability. In this work, we report the 
2 

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characterization of thermal performance and reliability of 

BIMs synthesized by several performers in DARPA’s NTI 

Program. Standardized test samples were bonded by the 

performers for accelerated thermal testing. Their initial 

thermal performance was characterized by the xenon flash 

transient measurement technique. Xenon flash measurements 

were taken at periodic intervals during and after the 

completion of accelerated testing. Rigorous finite element 

analysis was used to revise a steady-state experimental setup 

for measuring the thermal resistance of the samples between 

standardized copper test blocks. 

DESCRIPTION OF TEST SAMPLES

A variety of interface materials were evaluated to meet

DARPA’s thermal and reliability requirements. They include 

aligned carbon nanotubes, laminated graphite and solder, and 

copper nanosprings. GE has established an assembly 

technique that forms metal nanosprings by the glancing angle 

deposition process [13]. The number of springs, diameter of 

spring wire, radius of winding, number of windings, and 

overall spring length can be controlled by the glancing angle 

deposition process. This allows the process to engineer the 

desired shear and compressive compliance within the interface 

while also optimizing for minimal thermal resistance.

Teledyne developed a bonding process that vertically aligns 

graphite platelets within the contact area between two surfaces

[14]. The platelets are first aligned and compressed into thin 

layers before a solder binds the graphite layers to each other 

and to the surfaces. The Georgia Institute of Technology has 

led an effort to develop a low temperature process that grows 

and aligns carbon nanotubes as a thin interface material [15]. 

The thermal performance and reliability of the performers’ 

interface materials were characterized by utilizing 10-mm X 

10-mm cross-sectional footprint samples of silicon bonded to 

copper via the BIMs, shown in Fig. 1. The silicon diodes used 

for creating the bonded samples were 350 μm thick and were 

provided with a backside metallization of aluminum/titanium/

nickel/silver. The copper coupons were 1 mm thick and were 

not provided with any metallization. Surface preparation and 

additional metallization processing were allowed for the teams 

to optimize the bond strength with their interface materials. 

After bonding, bondline thicknesses varied amongst the 

performers’ samples from 70 to 325 μm.

Fig. 1 Silicon and copper coupons

INITIAL THERMAL RESISTANCE MEASUREMENTS 

Bonded samples were evaluated for thermal performance 

using a Netzsch LFA 447 Nanoflash instrument. The 

Nanoflash operates following the ASTM E-1461-13 test 

standard [16]. A xenon flash pulse directs energy towards the 

underside of a test sample. An infrared detector with a 7.8 mm 

aperture records the sample’s top-side rise in temperature as a 

function of time. This technique is demonstrated in Fig. 2.

Fig. 2 Xenon flash measurement technique

Under adiabatic conditions, this allows for the thermal 

diffusivity of the sample to be calculated by the following 

equation:

a = 0.1388 ∙ l2

t50

(1)

where:

a = thermal diffusivity

l = thickness of the test sample

t50 = the time at which 50% of the temperature rise has 

occurred

Previous knowledge of a sample’s bulk density and specific 

heat also allows calculation of its thermal conductivity, as 

shown in the following equation:

λ(T) = a(T) ∙ ρ(T) ∙ cp(T) (2)

where:

T = temperature

λ = thermal conductivity

ρ = bulk density 

cp = specific heat 

With knowledge of the sample’s bondline thickness, the 

thermal resistance of the interface layer can be calculated:

R = x

λ (3) 

where:

R = thermal resistance

x = bondline thickness

λ = thermal conductivity

Prior to testing, all samples were sprayed with DGF-123 Dry 

Graphite Film Spray. This uniform graphite coating allows for 

consistent absorptivity of the xenon flash pulse and emissivity 

to the infrared detector between test samples.
3 

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Initial thermal resistance measurements are summarized in 

Table 1. The performers will be designated as Performer A, B, 

or C for all discussions of results. 

Table 1. Initial sample thermal resistance (mm2

-K/W) 

Sample 

Number

Performer

A 

Performer

B 

Performer

C 

1 28.8 3.3 8.4

2 16.7 3.4 3.4

3 13.3 2.7 2.2

4 4.6 2.6 2.1

5 13.4 2.9 1.2

6 9.8 84.1 0.9

7 3.6 19.1 2.4

8 4.3 78.4 1.5

9 4.9 18.6 1.4

10 46.8 21.5 1.2

11 15.6 13.3 1.3

12 4.9 11.9 0.8

13 3.9 3.6 2 

14 4.7 4.8 --

15 4.4 3.7 --

Fig. 3 Initial sample thermal resistance (mm2

-K/W)

Performer A synthesized a sample with a bondline thermal 

resistance of 3.6 mm2

-K/W, and approximately half of the 

samples were measured to have thermal resistances lower than 

5 mm2

-K/W. However, a significant number of samples were 

measured with high resistances. This indicates the potential for 

a low thermal resistance material but also that synthesis 

variations are present in the production process. Performer B 

samples followed a similar pattern with one sample measuring 

2.6 mm2

-K/W, and half of the samples measuring below 5 

mm2

-K/W. Again, the remaining samples yielded high thermal 

resistances. Performer C produced samples that measured less 

than 1 mm2

-K/W and was the only team that produced 

samples with little deviation in thermal resistance 

measurements.

In addition to transient thermal measurements with the 

Nanoflash apparatus, acoustic microscopy images were taken 

of the bondlines for qualitative evaluations of the interfaces.

Fig. 4 Acoustic images of samples from Performer A (top 

left), Performer B (top right), Performer C (bottom 

left), and lead solder as a reference (bottom right)

In general, darker areas indicate a strong bond between the 

silicon and copper coupons while lighter areas denote the 

likely presence of voiding or delamination. Performer A and B 

samples typically had large areas of discontinuity while 

Performer C samples consistently showed minimal variation in 

bond quality (Fig. 4). The presence of lighter, poorer bond 

areas in samples correlated with higher thermal resistance 

measurements. For reference, a sample bonded with leadsolder is shown with a high percentage of voiding.

RELIABILITY TESTING AND CHARACTERIZATION

The bonded samples were subjected to accelerated tests in 

the form of temperature cycling as well as thermal aging at an 

elevated temperature. In thermal aging tests, the samples were 

exposed to a temperature of 130°C for 300 hours. In thermal 

cycling tests, the samples were subjected to temperatures from 

-40°C to 80°C at low (3°C/minute) and high (25°C/minute) 

ramp rates. Transient thermal measurements with the 

Nanoflash apparatus were performed to characterize the 

thermal performance of all samples prior to, during, and after 

accelerated testing. Acoustic microscopy was used to monitor 

the condition of the interfaces during the same analysis 

intervals. 

Samples thermally aged at 130°C were inspected every 100 

hours. After 300 hours, Performer A samples all showed an 

increase in thermal resistance, as shown in Table 2 and Fig. 5. 

In several cases, the thermal resistance of the bondline within 

samples approached 100 mm2

-K/W, indicating that a failure of 

the interface would occur shortly if thermal aging continued.

0.1

1

10

100

1 3 5 7 9 11 13 15

Thermal Resistance (mm2-K/W)

Sample Number

Performer A

Performer B

Performer C
4 

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One sample that initially was measured at 4.6 mm2-K/W did 

measure below 10 mm2

-K/W after the aging test concluded.

Table 2. Performer A thermal aging results (mm2

-K/W) 

Number 

of 

Hours

Performer A Samples 

1 2 3 4 

0 28.8 16.7 13.3 4.6

100 68.3 30.7 27.9 5.1

200 72.2 22 36.6 6.5

300 86.5 24.3 43.3 7 

Fig. 5 Performer A thermal aging results (mm2

-K/W)

Performer B samples also showed an increase in thermal 

resistance, as shown in Table 3 and Fig. 5. This increase was 

less significant compared to Performer A, and all samples 

maintained a thermal resistance at or under 6 mm2

-K/W. 

Table 3. Performer B thermal aging results (mm2

-K/W) 

Number 

of 

Hours

Performer B Samples 

1 2 3 13

0 3.3 3.4 2.7 3.6

100 3.7 4 2.9 3.9

200 5 4.6 3.5 4.4

300 6 3.7 3.9 4.9

Fig. 6 Performer B thermal aging results (mm2

-K/W)

While all samples from Performer C were initially measured 

below 10 mm2

-K/W, a significant increase in thermal 

resistance was measured under aging conditions (Table 4 and 

Fig. 7). Initial measurements did not prove to be indicators for 

which samples would show a large increase in thermal 

resistance. The degradation mechanism was not identified as 

the acoustic imaging did not reveal a structural change within 

the bonded interface.

Table 4. Performer C thermal aging results (mm2

-K/W) 

Number 

of 

Hours

Performer C Samples

1 2 3 4 

0 8.4 3.4 2.2 2.1

100 8.9 3 14.2 47.8

200 12 4.2 56.9 175.6

300 23 5.5 75 254.8

Fig. 7 Performer C thermal aging results (mm2

-K/W)

Under thermal cycling conditions, samples were subjected to 

temperature extremes from -40°C to 80°C and transitioned 

between the extremes at a low 3°C/minute ramp rate. Samples 

were inspected every 10 cycles with transient thermal 

measurements and acoustic imaging. Performer A samples 

showed an approximately 50% increase in thermal resistance 

from samples that were initially measured at 4 mm2

-K/W. 

1

10

100

1,000

0 100 200 300

Thermal Resistance (mm2-K/W)

Number of Hours Completed

1 2

3 4

1

10

100

1,000

0 100 200 300

Thermal Resistance (mm2-K/W)

Number of Hours Completed

1 2

3 13

1

10

100

1,000

0 100 200 300

Thermal Resistance (mm2-K/W)

Number of Hours Completed

1 2

3 4
5 

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Samples that were initially measured at 10 mm2-K/W or 

higher showed a significantly larger increase in thermal 

resistance. Performer A sample results are summarized in 

Table 5 and Fig. 8. 

Table 5. Performer A thermal cycling (low ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer A Samples

5 6 8 13

0 13.4 9.8 4.3 3.9

10 20.9 13.1 3.8 4.3

20 23.2 16.3 4.4 4.9

30 31 22.5 5.4 5.5

40 28.4 23.6 5 5.8

50 37.9 29.5 6.1 6.3

Fig. 8 Performer A thermal cycling (low ramp rate) results 

(mm2

-K/W)

Performer B samples showed significant degradation from 

thermal cycling conditions, with one sample’s silicon diode 

completely separating from the copper coupon. Inspection of 

the delaminated interface that remained on the copper coupon 

revealed multiple fracture lines within the interface, as shown 

in Fig. 9. 

Fig. 9 Performer B sample failure

The thermal resistance measurements for the remaining three 

samples, presented in Table 6 and Fig. 9, indicated that 

additional failures via the interface would have occurred if the 

number of cycles in the test had been extended.

Table 6. Performer B thermal cycling (low ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer B Samples

5 6 7 8 

0 2.9 84.1 19.1 78.4

10 4.9 149.8 23.7 99.7

20 8.8 229.4 87.9 fail

30 28.5 259.8 115.6 fail

40 14.5 279.2 129.7 fail

50 106.7 303.2 146.4 fail

Fig. 10 Performer B thermal cycling (low ramp rate) results 

(mm2

-K/W)

Performer C samples showed an increase in thermal resistance 

after thermal cycling; however, all samples were measured at 

or below 7 mm2

-K/W. Additionally, between approximately 

20 and 30 cycles, the rate of increase in thermal resistance 

tapered off, and measurements held constant for the remainder 

of the test. If cycling tests had continued, all samples from 

Performer C would possibly maintain their thermal resistance 

levels.

Table 7. Performer C thermal cycling (low ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer C Samples

5 6 7 8 

0 1.2 0.9 2.4 1.5

10 1.7 0.9 2.6 1.2

20 2.3 1.6 4.4 2.4

30 4 2.4 8.3 3.5

40 3.1 1.1 6.4 2.7

50 3.7 1.3 7 2.9

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

5 6

8 13

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

5 6

7 8

Fracture Line
6 

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Fig. 11 Performer C thermal cycling (low ramp rate) results 

(mm2

-K/W).

Samples were subjected to a second accelerated test with 

temperature extremes again cycling between -40°C to 80°C. 

Ramp rates were greater than 25°C/minute to impart a more 

severe thermal shock condition onto the samples. All samples 

from Performer A showed a significant increase in thermal 

resistance after thermal cycling under high ramp conditions, as 

shown in Table 8 and Fig. 12. 

Table 8. Performer A thermal cycling (high ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer A Samples

9 10 11 12

0 4.9 46.8 15.6 4.9

10 6.8 72.3 17.3 5.8

20 13 91 17.9 7.4

30 19.4 80.6 22.1 8.1

40 26.1 73.2 24.9 11.9

50 51 85.9 20.7 17.7

Fig. 12 Performer A thermal cycling (high ramp rate) results 

(mm2

-K/W)

Performer B samples under high ramp rate conditions 

reflected similar results to the results for samples under low 

ramp rate testing (Table 6 and Fig. 9). One sample exhibited a 

complete failure of the interface while other samples measured 

significantly increased bondline resistance values, as shown in 

Table 9 and Fig. 13. 

Table 9. Performer B thermal cycling (high ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer B Samples

9 10 11 12

0 18.6 21.5 13.3 11.9

10 25.2 42.8 33.5 23

20 36.4 63.6 97.2 63.9

30 48.7 80.5 123.9 78.1

40 62.3 87.1 fail 81

50 80.1 110.8 fail 95.3

Fig. 13 Performer B thermal cycling (high ramp rate) results 

(mm2

-K/W)

Apart from one test sample that indicated a defect from the 

synthesis process, the thermal resistances of Performer C 

samples stabilized after initial increases under cycling 

conditions. The three samples that maintained thermal 

resistances below 4 mm2

-K/W would likely remain intact if 

the number of testing cycles was extended.

Table 10. Performer C thermal cycling (high ramp rate) results 

(mm2

-K/W) 

Number 

of 

Cycles

Performer C Samples

9 10 11 13

0 1.4 1.2 1.3 2 

10 1.6 1.7 2.5 2.8

20 2.4 2.5 3.4 6.8

30 1.9 1.7 2.8 14.3

40 2.3 1.7 3.7 22.5

50 2.1 1.8 3.7 31.7

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

5 6

7 8

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

9 10

11 12

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

9 10

11 12
7 

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Fig. 14 Performer C thermal cycling (high ramp rate) results 

(mm2

-K/W)

For some of the samples/results presented above for aging as 

well as thermal cycling, acoustic images of the interfaces after 

the accelerated tests revealed voids/defects which correlated 

with the high thermal resistance measured. This has provided 

insight into the failure modes as well as their impact on the 

thermal resistance.

STEADY-STATE THERMAL CHARACTERIZATION

We have previously developed a steady-state thermal 

resistance tester for characterization of TIMs and BIMs [17]

[1]. The operation of the steady-state apparatus follows the 

method outlined in ASTM D5470-12 [18]. This technique has 

been employed by other research entities, and results have 

been documented [19]. The basic configuration of the 

apparatus is shown in Fig. 15. 

Fig. 15 Steady-state thermal resistance testing apparatus

Heater cartridges are embedded into an aluminum hot plate 

while silicone oil is circulated through an aluminum cold 

plate. Four thermistors are embedded in metering blocks

which have the test sample between them. In testing of prior 

materials, thermal grease with glass beads of 25 μm diameter 

are applied between the metering blocks and test sample to 

ensure a consistent and known contact thermal resistance. The 

thicknesses of TIMs or multilayer test coupons are accurately 

measured before being placed within the apparatus. Due to the 

adhesive properties of the materials developed by the 

performers, metering blocks will be provided to them for 

direct bonding. This will eliminate the need for thermal grease 

and glass beads.

While the Nanoflash instrument using the transient thermal 

measurement technique provides measurements of the 

performers’ sample interfaces within an accuracy of ±3%, 

prior knowledge of the materials’ bulk densities and specific 

heats is a requirement. Additionally, the technique is 

measuring the resistance of the interface within the test 

samples where the diffusivity contribution of the silicon and 

copper coupons must be accounted for to obtain the 

performance of the bondline. Acoustic imagery has shown that 

variations of bond quality are present within the bonded 

interfaces. While the Nanoflash instrument only measures the 

thermal performance of interface within the central region of a 

test sample, the steady-state approach will average any quality 

variations by measuring the entire bonded interface. By 

allowing the performers to directly bond the metering blocks 

together with their interface material, a clean story on the 

bondline resistance can be obtained with only the bondline 

thickness needing to be measured.

The top and bottom heat fluxes through the metering 

blocks are first calculated through the following two 

equations:

Qtop = k ∙ A (T1 − T2)⁄∆x1 (4)

Qbot = k ∙ A (T3 − T4)⁄∆x1 (5)

where:

Qtop = top metering block heat flux calculation in W

Qbot = bottom metering block heat flux calculation in W

k = thermal conductivity of metering block 

A = metering block cross-sectional area

T1,2,3,4 = thermistor temperature measurements from hot 

plate side (1) to cold plate side (4)

Δx1 = distance between T1 and T2 or T3 and T4

The average heat flux is determined from the top-side and 

bottom-side metering blocks by:

Qavg = �Qtop + Qbot�⁄2 (6)

The temperature at the interface between the top metering 

block and the top-side of the interface material is determined 

by:

Ts top = T2 − ∆x2 ∙ ((T1 − T2)⁄∆x1) (7)

where:

Ts top = calculated temperature of the test sample’s top 

surface

Δx2 = distance between T2 or T3 and sample interface

The temperature of the interface between the bottom metering 

block and the bottom-side of the interface material is 

1

10

100

1,000

0 10 20 30 40 50

Thermal Resistance (mm2-K/W)

Number of Cycles Completed

9 10

11 13
8 

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calculated in a similar manner:

Ts bot = T3 + ∆x2 ∙ ((T3 − T4)⁄∆x1) (8)

The bulk and contact thermal resistance is calculated by:

R = ��Ts top − Ts bot� ∙ A� Q� avg (9)

A finite element analysis was performed to optimize the 

metering block geometry for an approximately 1 cm2 interface 

sample bond area and a 1 mm2

-K/W thermal resistance. The 

meshed model of the finalized geometry is shown in Fig. 16. 

Fig. 16 Finite element analysis model of the testing apparatus

90 W of heat were transferred into the hot plate by the 

cartridge heaters while an 800 W/m2

-K film coefficient at 

30°C was applied to the cold plate’s internal tube surfaces. 

Appropriate convection coefficients and radiation emissivity 

values were applied to all exposed surfaces. The crosssectional planes at which the four thermistors were embedded 

into the metering blocks were analyzed for temperature 

uniformity. It was found that temperature variations within the 

planes were approximately 0.13-0.16°C. This measurement 

uncertainty, along with thermistor calibration error, machine 

tolerances of the metering blocks, and deviation between the 

top and bottom meter block heat flux calculations due to 

radiation and convection losses, all contributed to 

measurement errors of the steady-state measurement method.

These error sources and their tolerances are summarized in 

Table 11. 

Table 11. Error sources and their tolerances

Error Source Tolerance

ANSYS in plane temperature variation ±0.07-0.17°C

Thermistor, Inc. QT06005A thermistor 

calibration error ±0.03°C

Meter block diameter ±0.027*10-5mm

Distance between T1 and T2 or T3 and T4 ±0.027*10-5mm

Distance between T2 or T3 and interface ±0.027*10-5mm

Averaged heat flux error due to radiation 

and convection losses ±0.24-0.68W

These error sources were propagated through the steady-state 

measurement calculations and it was determined that the 

measurement uncertainty for one experimental test was 

approximately ±0.35 mm2

-K/W at 95% confidence intervals.

A testing apparatus with the optimized metering block 

geometry was constructed for experimental testing of the 

performers’ interface materials. This apparatus is shown in 

Fig. 17. 

Fig. 17 Constructed steady-state testing apparatus

The four holes in the necked-down region are the locations for 

the thermistors to be inserted. The complete test apparatus can 

be sealed under a vacuum bell jar for elimination of 

convective losses on the metering block surfaces. Experiments 

were conducted with a joined metering block where both the 

top-side and bottom-side metering block halves were 

machined together as one piece. The heater cartridges 

imparted 60 W into the hot plate, and silicone oil at 50°C 

flowed through the cold plate. A plot of the thermistor 

readings from these experiments is shown in Fig. 18. 

Fig. 18 Steady-state joined metering block measurements

The higher temperature measurements from the test conducted 

in a vacuum aided in the quantification of the convective heat 

losses. In all cases, the temperature drop across the four 

thermistor measurements followed a linear trend. Varying the 

application of vacuum as well as the orientation of the 

metering block (flipped block configuration) yielded 

measurement results to be within ±0.38 mm2

-K/W of the 

expected result (i.e. zero resistance). The results are 

summarized in Table 12. 

60

70

80

90

100

110

120

0 0.5 1 1.5 2 2.5

Temperature (°C) 

Distance from Bottom of Thermal Stack (cm)

60W, 50°C, Joined Block, No Vacuum

60W, 50°C, Joined Block, In Vacuum

60W, 50°C, Joined and Flipped Block, No Vacuum
9 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Table 12. Steady-state joined metering block calculations

Experimental 

Iteration

Average 

Heat Flux 

(W)

ΔT 

Across 

TIM

(°C)

Thermal 

Resistance 

(mm2

-K/W)

No Vacuum 36.18 0.11 0.38

In Vacuum 50.68 -0.12 -0.29

No Vacuum, Flipped 

Metering Block 38.26 -0.01 -0.03

After metering blocks are bonded together by the performers, 

steady-state tests will be completed.

CONCLUSIONS

The samples from Performer C showed very good thermal 

performance in the range of 1 to 3 mm2

-K/W, even after being 

subjected to 50 temperature cycles under low and high ramp 

rate conditions. The bonded samples from the other 

performers showed significant degradation after accelerated 

testing, even though a number of samples initially yielded 

thermal resistances less than 10 mm2

-K/W. Prior to 

accelerated testing, many of the samples from the performers 

met DARPA’s initial thermal performance metrics. Based on 

the lessons learned from the first round of testing, revised 10-

mm X 10-mm bonded samples were provided by the 

performers with altered processing/synthesis conditions. These 

samples are currently being characterized at the National 

Renewable Energy Laboratory. In addition, 12.5-mm diameter 

copper blocks will also be sent to one performer (and 

potentially other performers) for synthesizing the bonded 

interface between two copper blocks. Rigorous ASTM steadystate thermal resistance experiments will be performed at the 

National Renewable Energy Laboratory on these bonded 

copper blocks to confirm the thermal performance.

ACKNOWLEDGMENTS

The authors would like to thank the Principal Investigators 

Dave Altman, Jack Moon, Yuan Zhao and Dave Shaddock, as 

well as their research teams, for their submission of bonded 

test samples and their collaboration during this evaluation 

process of novel low-thermal-resistance interfaces.

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