NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

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Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Mechanical Characterization 

Study of Sintered Silver Pastes

Preprint

Paul Paret, Joshua Major, Douglas DeVoto 

and Sreekant Narumanchi

National Renewable Energy Laboratory

Yansong Tan and Guo-Quan Lu

Virginia Polytechnic Institute and State University

Presented at American Society of Mechanical Engineers (ASME)

2018 International Technical Conference and Exhibition on 

Packaging and Integration of Electronic and Photonic Microsystems 

(InterPACK 2018)

San Francisco, California

August 27–30, 2018

Suggested Citation

Paul Paret, Joshua Major, Douglas DeVoto, Sreekant Narumanchi, Yansong 

Tan, and Guo-Quan Lu. 2018. “Mechanical Characterization Study of Sintered 

Silver Pastes: Preprint.” Golden, CO: National Renewable Energy Laboratory. 

NREL/CP-5400-71381. https://www.nrel.gov/docs/fy19osti/71381.pdf.

Conference Paper

NREL/CP-5400-71381

October 2018
NOTICE

This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Vehicle

Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the 

U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, 

acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish 

or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

This report is available at no cost from the National Renewable 

Energy Laboratory (NREL) at www.nrel.gov/publications.

U.S. Department of Energy (DOE) reports produced after 1991 

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov.

Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795.

NREL prints on paper that contains recycled content.
1

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MECHANICAL CHARACTERIZATION STUDY OF SINTERED SILVER PASTES 

BONDED IN A DOUBLE-LAP CONFIGURATION

Paul Paret

National Renewable Energy 

Laboratory

Golden, CO, USA

Paul.Paret@nrel.gov

Joshua Major

National Renewable Energy 

Laboratory

Golden, CO, USA

Douglas DeVoto

National Renewable Energy

Laboratory

Golden, CO, USA

Sreekant Narumanchi

National Renewable Energy 

Laboratory

Golden, Colorado, USA

Yansong Tan

Virginia Polytechnic Institute and 

State University

Blacksburg, VA, USA

Guo-Quan Lu

Virginia Polytechnic Institute 

and State University

Blacksburg, VA, USA

ABSTRACT

Sintered silver-based bonded interfaces are a critical 

enabling technology for high-temperature, compact, highperformance, and reliable wide-bandgap packages and 

components. High-pressure (~40 MPa) sintered silver 

interfaces have been implemented commercially, most notably

the commercial products offered by Semikron. To reduce 

manufacturing complexity, there is significant industry interest 

in pressure-less sintered silver interfaces. To this end, current 

formulations of sintered silver paste are comprised of purely 

nano-sized silver particles or a combination of nano- and 

micro-sized silver particles/flakes. It is essential to quantify the 

mechanical properties and determine the reliability of these 

interfaces prior to use in automotive power electronics 

applications. In this paper, research efforts at the National 

Renewable Energy Laboratory, in collaboration with Virginia 

Polytechnic Institute and State University and an industry 

partner, in optimizing the synthesis procedure and mechanical 

characterization of sintered silver double-lap samples are 

described. These double-lap samples were synthesized using 

pressure-less sintering techniques. Shear testing was conducted

at multiple temperatures and displacement rates on these 

samples sintered using two types of sintered silver pastes, one 

of them consisting of nano-silver particles and the other a 

hybrid paste or a combination of nano- and micron-sized silver

flakes, employed in a double-lap configuration. Maximum 

values of shear stress obtained from the characterization study 

are reported.

INTRODUCTION

Power electronics packaging in the rapidly growing power 

industry is becoming more critical than ever before. To improve 

performance, reduce volume and weight, and get higher 

efficiency out of electronics packages, wide-bandgap devices 

such as silicon carbide (SiC) and gallium nitride were proposed 

as a replacement for widely used silicon (Si) devices. A 

redesign of the traditional packaging for power modules, both 

in materials and design, is necessary to exploit the faster 

switching speed and to enable high-temperature operation of 

the wide-bandgap devices. For high-temperature operation 

(>200°C), materials that can withstand severe thermomechanical stresses and function without any failure for long 

periods of time need to be considered. In particular, for bonded 

interfaces, i.e., die-attach and substrate-attach, reliable 

operation at elevated temperatures is a major challenge as a 

failure would eventually result in a thermal runaway of the 

entire power electronics package.

Traditionally, solders have predominantly been used as 

interface materials in power electronics packages in the 

automotive industry. With the push towards replacing Si with 

wide-bandgap devices, high-temperature-compatible materials 

have gained significant interest. Among these materials, 

sintered silver, along with transient liquid-phase bonding, is a 

leading candidate for replacing solders in novel power 

electronics packaging designs. In the past, sintered silver made 

from micron-sized silver particles demonstrated excellent 

reliability under accelerated thermal cycling conditions [1]. 

However, the drawback of the high bonding pressure (30 – 40 

MPa) requirement resulted in its limited applications and 

necessitated the research and development of novel and simple 

bonding techniques of sintered silver. To this end, sintered 

silver paste comprising nano-silver particles were developed 

[2] and more recently, hybrid sintered silver paste, involving a 

mixture of nano- and micron-sized silver particles/flakes, are 

gaining traction among various researchers and the industry. 

With these formulations, sintering can be achieved through 

purely a temperature-driven diffusion of the silver particles 

without applying any pressure. While micron-sized sintered 

silver paste has made inroads into commercial power 

electronics packages [3], more work is required to establish the 

mechanical properties and thermomechanical reliability of 

large-area pressure-less sintered silver pastes before adopting 

them for commercial applications. A comprehensive review of 

the different types of sintered silver pastes available in the 

industry is provided by Siow [4]. In this study, the merits and 

demerits of different types of sintered silver pastes available in 

the industry are compared and the author predicts that these 

pastes will become mainstream in the power electronics and 

microelectronics packaging industry over the next few years.
2

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A common test method used to measure the integrity and 

strength of a bonded interface material is the shear stress test. 

In the past few years, several studies were conducted to 

determine the shear strength and other mechanical properties of 

sintered silver joints. Khazaka et al. [5] conducted a detailed 

study reviewing the different sintering parameters that affect 

the shear strength of nano-silver joints. A comprehensive 

literature survey of the reliability studies performed by various 

researchers is also provided. The authors attribute the variation 

in reported electrical, thermal, and mechanical properties of 

sintered silver in the literature to the microstructural variation 

of the material being tested, mainly material density and grain 

size. A similar study was conducted by Li et al. [6] on samples 

fabricated by bonding multiple SiC dies on direct-bond-copper 

substrates. Bond line thickness, substrate metallization, and 

synthesis pressure were varied among the different tested 

samples. In another study to determine the mechanical 

properties of sintered silver comprising micron-sized silver

particles, Caccuri et al. [7] synthesized bulk specimens with 

similar microstructure and investigated the effect of joint 

density on its mechanical properties. Kahler et al. [8] worked 

on a sintered silver paste comprising a mixture of both micronsized and nano-sized particles and achieved shear strength 

values close to 40 MPa. Chua et al. [9] characterized the 

evolution of porosity and microstructure of pressure-less 

sintered nano-silver joints bonded on direct-bond-copper, 

copper-plated, and silver-plated substrates. Test samples were 

aged at 300°C for 1,000 hours, and shear strength was 

measured. In a similar study [10], Egelkraut et al. investigated 

the impact of thermal aging on the shear strength and 

microstructural evolution of nano-silver pastes and compared it 

to other high-temperature solder alloys.

In addition to shear test, another key aspect of a bonded 

interface material developed for electronics packaging 

applications, especially at high temperatures, is 

thermomechanical reliability. Similar to the study on 

mechanical characterization, research on assessing the 

thermomechanical reliability of sintered silver joints provides

critical information to power module manufacturers and 

electronics packaging design engineers. Work on this end has 

increased in the past few years as indicated by the rising 

number of publications highlighting the thermomechanical 

reliability of sintered silver joints. Bai and Lu [11] conducted a 

thermal cycling experiment between 50°C and 250°C on 1.706 

mm x 1.380 mm dies bonded to silver- and gold-plated directbond-copper substrates, where the samples survived for 4,000 

and 6,000 cycles, respectively. This study considered a 50% 

drop in shear strength as the failure criterion. Zhao et al. [12] 

studied the impact of thermal aging on the shear strength of 

sintered nano-silver joints using 13.5 mm x 13.5 mm Si chips 

bonded on a bare copper plate. Knoerr et al. [13] performed 

thermal cycling and power cycling accelerated tests on sintered 

nano-silver joints made with different sintering parameters and 

developed a lifetime prediction model using the Coffin-Manson 

approach. The footprint of the samples was 14 mm x 14 mm,

and 20% growth in the crack area was considered to be the 

failure criterion. Rajaguru et al. [14] developed a computational 

design optimization model of a power electronics module to 

improve the reliability of its sintered silver joint at the interface 

between a SiC chip and the substrate. In this work, a reducedorder modeling approach was adopted to analyze the impact of 

design variations on the reliability of the sintered silver joint. 

Henaff et al. [15] formulated a lifetime prediction model of 

nanoscale sintered silver with inputs from accelerated tests and 

thermomechanical modeling.

In the past, research at the National Renewable Energy 

Laboratory (NREL) was focused on characterizing the 

reliability of sintered silver joints synthesized from micronsized silver particles [1]. Accelerated thermal cycling from -

40°C to 150°C was conducted on 50.8-mm x 50.8-mmfootprint test samples, in which Si3N4 substrates were bonded 

to copper baseplates with sintered silver material. The sample 

synthesis was done at Semikron. A bonding pressure of 30 – 40 

MPa was applied that resulted in good quality bonding with 

minimal voiding. Under thermal cycling, samples survived 

without failure for 2,300 cycles. A 20% increase in crack area 

was considered as the failure criterion. More recently, research 

focus at NREL has shifted to characterizing novel formulations 

of sintered silver pastes. In collaboration with an industrial 

partner and Virginia Tech (VT), efforts to evaluate the shear 

strength and reliability of nano and hybrid sintered silver 

pastes, in which micron- and nano-sized silver particles are 

embedded, are ongoing. In this paper, we focus on the 

mechanical characterization of sintered silver. Sample 

preparation, synthesis techniques and test procedures are 

outlined. In the Results section, shear stress values of sintered 

silver as a function of ambient temperature and applied 

displacement rates are plotted. The reliability test plan is also 

described.

SAMPLE DESCRIPTION

1. Sample Geometry

A modified double-lap sample configuration [16, 17], as 

shown in Fig. 1, was selected to measure the mechanical 

strength of sintered silver pastes. The sample consists of a 5-

mm-thick copper coupon in the middle that is bonded to thinner 

copper coupons (2-mm-thick) on either end using sintered 

silver. The simple layout of this sample configuration enables 

an effective characterization of the joint material without the 

occurrence of any unintended peel stress at the interfaces. By 

applying a compressive force on the top of the middle coupon, 

as shown in Fig. 1, shear forces are developed in the sintered 

silver joints. The copper coupons were fabricated in the 

laboratory machine shop with a prescribed surface finish of 

RMS 16. After fabrication, the coupons were plated with a 4-

μm-thick silver layer. The footprint of the samples was 12.7 

mm x 12.7 mm.
3 

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Fig. 1. Double-lap sample configuration for mechanical 

characterization

2. Sample Synthesis

After plating, the copper coupons were bonded in a

double-lap configuration with sintered silver material as 

described in the previous section. Two types of sintered silver 

pastes were included in this study. In one set of double-lap 

samples, the copper coupons were bonded using a sintered 

silver paste that was purchased from an industrial partner, while 

the other set of samples was synthesized at VT using a sintered 

silver paste purchased from NBE Tech.

a. Samples bonded with sintered silver paste purchased

from the industry partner

The synthesis of a double-lap sample at NREL using this 

sintered silver paste was achieved in two stages. First, the paste 

was applied using a 150-μm-thick stencil printer in a circular 

pattern with an 8-mm diameter onto a thin copper coupon that

was then placed in a slot within a sample holder. To ensure 

proper alignment within the sample, the thicker middle coupon 

was carefully dropped on top of the thin coupon. After 

repeating this procedure for multiple samples, the sample 

holder was placed inside a solder reflow station. Figure 2 

shows the sintering profile that was adopted for this sintered

silver paste. Sintering was performed in an inert gas 

atmosphere, and no pressure was applied on the coupons at any 

stage of the sintering profile. Once the sintering was completed 

and a bond formed between the two copper coupons, the entire 

process was repeated on the other side of the thicker copper

coupon to complete the synthesis of one double-lap sample. 

Figure 3 shows the layout of the different synthesis steps that 

were followed to make a double-lap sample. After the sintering 

process, the final bond line thickness was measured to be 

roughly between 50 μm and 70 μm.

Fig. 2. Sintering profile for sintered silver paste purchased from 

an industry partner

Fig. 3. Synthesis procedure of a double-lap sample

b. Samples synthesized at VT

In addition to using a different sintered silver paste,

double-lap samples were synthesized in VT in two different 

ways – one through a purely pressure-less sintering technique, 

and the other using a pressure-assisted technique. It is noted 

here that while this silver paste does not require any sintering 

pressure as such, the pressure-assisted technique was included 

in the study to investigate the impact of external bonding 

pressure on the shear strength of these samples. Samples were 

made using 3 MPa as well as 10 MPa sintering pressure.

The pressure-less sintering procedure at VT followed a 

similar pattern as the samples made at NREL. To complete the 

synthesis of one double-lap sample, each interface was sintered 

separately, using the profile shown in Fig. 4. A few trials and 

variations were conducted with the pressure-less as well as the 

pressure-assisted sintering techniques before finally selecting 

the sintering profile and the stencil thickness. Unlike the 

circular pattern at the interface for the samples bonded at 

NREL, the double-lap samples made at VT employed a square 

footprint geometry for the material at the interface. The final 

bond line thickness of the pressure-less and pressure-assisted 

samples was roughly 80 μm – 100 μm. In this paper, only the 
4

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characterization results of samples synthesized using the 

pressure-less sintering technique are covered.

Fig. 4. Sintering profile of samples synthesized at VT

Once the double-lap samples were synthesized, the 

material bonding quality was assessed using images taken with

C-mode scanning acoustic microscopy (C-SAM) at NREL. 

Figure 5 shows select C-SAM images of double-lap samples 

made at NREL and VT.

 

 

Fig. 5. C-SAM images of double-lap samples synthesized 

at NREL (top) and VT (bottom)

The actual bonded area of each interface of all the samples 

were computed using SPIP image analysis software. In general, 

most of the samples exhibited a decent bond region with a low 

amount of voiding, however, those samples with high amount 

of voiding (>30% by area) were omitted from the study.

EXPERIMENTAL SETUP AND TEST PLAN

Shear testing on the double-lap samples was conducted 

using an Instron 5966 dual column testing system. The doublelap sample configuration and the fixture design allowed 

operating the Instron system in compression mode to induce 

shear stresses at the material interfaces. A 10-kN load cell was 

used to measure loading on the sample, and the resulting 

displacement at the interfaces was measured using a noncontact video extensometer. After the fixture was positioned on 

the base mount, the crosshead beam was slowly jogged 

downwards until the cylindrical bar connected to the load cell 

just touched the plunger in the fixture. Once the testing started, 

loading would be applied to the plunger, which then pushed 

down the middle coupon, inducing a shear stress at the 

interface regions. The displacement of the joint material was 

measured by tracking the relative movement of two white 

indicator dots in the field of view of the video extensometer. 

These white dots were marked on the plunger and the sample 

holder. The Instron system also consisted of a thermal oven that

allowed testing to be conducted at different ambient 

temperatures. The temperature range of the oven was from -

100°C to 300°C. A picture of the Instron test system with the 

thermal oven is shown in Fig. 6.

Fig. 6. Instron 5966 dual column test system

In this study, the double-lap samples were characterized at 

three different displacement rates: 10-2 mm/s, 10-4 mm/s, and 

10-5 mm/s, and at three different ambient temperatures: -40°C, 

30°C, and 200°C. The intent behind choosing these rates was to 

simulate quasi-static loading conditions at the joint interfaces, 

thereby qualifying the results obtained from these tests to be 

applicable in scenarios such as thermal cycling. In a study on 

developing a constitutive model for SAC solders [18], the 

authors used a closed-loop mechanism that monitored and 

controlled the displacement rates at the solder joint interfaces 

of double-lap samples to compensate for the compliance in the 

load train and to accommodate the rate-dependent behavior of 

solder joints. However, in this study, the testing rates were 

controlled at the Instron crosshead and it was assumed that the 

actual effective displacement rates at the sample interfaces 

were the same. For each instance of a displacement rate and 

temperature, a total of four samples were tested and the average 

shear stress values are reported in this paper.
5

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RESULTS AND DISCUSSION

The direct output of shear testing on a double-lap sample 

using an Instron test equipment is the load-displacement curve, 

shown in Fig. 7. The two peaks in the figure denote the failure 

points of the two interfaces in the sample, and only the results 

obtained up to the first failure point are considered for further 

analysis. In the case of a few samples, the load-displacement 

graph exhibited only a single peak, which could lead to the 

misconception that only one interface failed. However, both 

interfaces were observed to shear off for these samples during 

the test. To observe the failure mechanism in the sintered silver 

material, a test run was conducted on a sample synthesized at 

NREL (with sintered silver paste purchased from the industrial 

partner). Before testing, the sample was cross-sectioned, and 

the interface region was closely captured using a digital 

microscope. The resulting image obtained is shown in Fig. 8. 

Clearly, a cohesive fracture mechanism within the sintered 

silver layer can be observed in this image.

Fig. 7. Load-displacement graph

Fig. 8. Cohesive fracture observed in the sintered silver 

layer of a double-lap samples under shear testing

For a given sample, the load information obtained from the 

tests were converted to stress results using Eq. (1).

(1)

where,

F is the load

A is the total area of both sample interfaces measured from CSAM images using SPIP

1. Pressure-less samples (NREL)

As described above, the double-lap samples synthesized at 

NREL were bonded using a sintered silver paste purchased 

from an industrial partner. These samples were synthesized 

using the profile as shown in Fig. 2, and the C-SAM images of 

these samples showed very minimal voiding at the interface 

regions. The average values of the peak shear stress of these 

samples are plotted in Fig. 9. The error bars represent the 

standard deviation of the shear stress results. At each 

displacement rate, there is more than a 50% drop in the shear 

stress value as ambient temperature is increased from room 

temperature (RT) to 200°C. Also, at a given temperature, a 

decrease in the applied displacement rate leads to a 40% – 50% 

drop in the shear stress of the sintered silver joint. The wide 

variation in the individual results averaged to obtain the plotted 

values, particularly at RT, could be likely due to differences in 

the bond shape between the samples. Although the paste was 

printed in a circular shape, it spread out and, in a few instances, 

did not conform to a circular shape.

Fig. 9. Shear stress values of samples synthesized at NREL

In addition to the mechanical characterization on samples 

sintered at 250°C, shear testing was conducted on a few 

samples synthesized with a sintering temperature of 270°C. A 

comparison of shear stress results between samples sintered at 

250°C and 270°C is given in Fig. 10. Although a significant 

increase in shear stress values cannot be observed, these results 

indicate that on an average, a higher sintering temperature led 

to a slightly increased inter-particle bonding force within the 

sintered silver paste.
6 

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Fig. 10. Comparison of samples sintered at 250°C and 

270°C

2. Pressure-less samples (VT)

Similar to the samples synthesized at NREL, mechanical

characterization was conducted on double-lap samples 

fabricated at VT using the sintered silver paste from NBE Tech. 

For a given displacement rate and temperature, multiple 

samples were characterized, and the shear stress results were 

computed from the resulting load-displacement graphs. Figure

11 shows a plot of the impact of displacement rate and ambient 

temperature on the shear stress values of these samples. As seen 

in the figure, there does not exist much difference in the shear 

stress values between the samples tested at -40°C and RT. 

However, a considerable drop occurs when the ambient 

temperature is raised to 200°C, a trend that was observed in the 

samples synthesized at NREL. The fall in shear stress values 

with increasing temperature could be attributed to creep 

deformation becoming predominant at higher temperatures, 

which diminishes the material resistance to failure. Also, the 

applied displacement rate did not have a significant impact on 

the shear stress values at -40°C and RT. Only when the ambient 

temperature was increased to 200°C did the silver paste exhibit 

a strong rate dependence.

Fig. 11. Mechanical characterization results of samples 

synthesized at VT

While the decrease in shear stress of a sintered silver bond 

with rising temperature and decreasing displacement rate, as 

observed in this study, conforms to the trends that exist in the 

open literature [5, 19], more work is required to ascertain the 

exact behavior of the silver paste at the microstructural level 

under these conditions.

FUTURE WORK

Research study in characterizing the silver pastes under the

displacement rate of 10-5 mm/s is currently ongoing. 

Conducting shear tests at extremely slow rates and at elevated 

temperatures is a challenging task and a cautious approach in 

setting up the test equipment needs to be adopted. Shear stress 

versus shear strain graphs will be created from the existing set 

of data and the failure behavior of the sintered silver pastes in 

these tests, whether it is brittle or ductile, will be investigated. 

Preliminary characterization tests of double-lap samples 

synthesized using pressure-assisted sintering process produced 

promising results. A rigorous study of the shear behavior of 

these samples at multiple strain rates and temperatures will be 

conducted. 

In addition to mechanical characterization to determine the 

shear properties of sintered silver, a reliability evaluation study 

through accelerated thermal cycling will be initiated. Round 

samples of 25.4-mm-diameter with copper and Invar coupons 

bonded using sintered silver pastes were fabricated. Copper and 

Invar coupons were selected for this study to create a large coefficient of thermal expansion mismatch across the sintered 

silver joint. C-SAM images of these samples show negligible 

amounts of voiding in the joint region. These round samples 

will be subjected to a thermal cycle going from -40°C to 200°C 

and will be imaged using C-SAM periodically to monitor the 

occurrence of any failure mechanisms such as crack initiation 

and propagation. Also, lifetime prediction models will be 

developed by correlating the experimental results obtained 

from thermal cycling with strain energy density/J-integral 

results computed through finite element modeling analysis.

CONCLUSIONS

This paper focused on characterizing the shear behavior of 

two different sintered silver pastes bonded in a double-lap 

sample configuration. Samples were synthesized at NREL and 

VT using pressure-less sintering techniques and shear testing 

was conducted on these samples in an Instron testing system at 

NREL. Mechanical characterization was done at multiple 

temperatures and applied displacement rates, and the maximum 

values of shear stress obtained in each case are reported. In 

general, shear stress of the silver pastes in this study dropped 

with increasing temperature and decreasing displacement rates. 

Additional experiments will be conducted to infer the behavior 

of these pastes at the microstructural scale under these 

conditions. Future work in this project includes conducting 

thermal cycling experiments to evaluate reliability and 

developing thermomechanical models. Cycles-to-failure results 
7 

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obtained from accelerated tests will be correlated with strain 

energy density/J-integral values computed through modeling to 

formulate a lifetime prediction model for sintered silver.

ACKNOWLEDGEMENTS

This work was authored by Alliance for Sustainable 

Energy, LLC, the manager and operator of the National 

Renewable Energy Laboratory for the U.S. Department of 

Energy (DOE) under Contract No. DE-AC36-08GO28308. 

Funding provided by Susan Rogers, Technology Manager for 

the Electric Drive Technologies Program, Vehicle Technologies 

Office, U.S. Department of Energy Office of Energy Efficiency 

and Renewable Energy (EERE). The views expressed in the 

article do not necessarily represent the views of the DOE or the 

U.S. Government. The U.S. Government retains and the 

publisher, by accepting the article for publication, 

acknowledges that the U.S. Government retains a nonexclusive, 

paid-up, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, for 

U.S. Government purposes. We would also like to thank 

Andrew Wereszczak (Oak Ridge National Laboratory) for his 

guidance and insightful comments on sample preparation and 

design for reliability evaluation study.

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