Electronic Materials Letters, Vol. 7, No. 3 (2011), pp. 271-275

DOI: 10.1007/s13391-011-0917-x

Prediction of Reliability on Thermoelectric Module through Accelerated Life

Test and Physics-of-Failure

Hyoung-Seuk Choi,1,2,* Won-Seon Seo,

1 and Duck-Kyun Choi

2

1Center of Energy Material, Green Ceramic Div., Korea Institute of Ceramic Engineering & Technology(KICET),

Seoul 153-801, Korea2Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Korea

(received date: 29 October, 2010 / accepted date: 12 February, 2011)

Thermoelectric cooling module (TEM) which is electric device has a mechanical stress because of temperaturegradient in itself. It means that structure of TEM is vulnerable in an aspect of reliability but research onreliability of TEM was not performed a lot. Recently, the more the utilization of thermoelectric cooling devicesgrows, the more the needs for life prediction and improvement are increasing. In this paper, we investigatedlife distribution, shape parameter of the TEM through accelerated life test (ALT). And we discussed abouthow to enhance life of TEM through the Physics-of-failure. Experimental results of ALT showed that thethermoelectric cooling module follows the Weibull distribution, shape parameter of which is 3.6. The accel-eration model is coffin Coffin-Manson and material constant is 1.8.

Keywords: thermoelectric module(TEM), accelerated life test(ALT), reliability, physics-of-failure analysis

1. INTRODUCTION

When the voltage is applied at the both ends of thermo-

electric material, the one side of it emits heat and the other

side absorbs heat. It is called Peltier effect. By this effect,

TEM can convert electrical energy and thermal energy to

each other. TEM consist of n-type and p-type thermoelectric

semiconductor. It is mostly made by Bi2Te3. (Fig. 1)

In this paper, Our study has been done on TEM made by

Bi2Te3 which is generally applied to TEM for room tempera-

ture. TEM is designed that the p-n thermoelectrical semicon-

ductor pairs are connected in series and attached to ceramic

substrates in both sides and lead wires are connected at the

both ends of the series. (Figs. 2 and 3)

2. THE FAILURE MECHANISM OF TEM

TEM emits heat and absorb at each side simultaneously. It

causes temperature gradient (Fig. 4) which causes expansion

and contraction of the substrate. And finally it stresses the

internal parts of the thermoelectric device and Soldering.

The structure of TEM in which 127 p-n semiconductor pair

connected in series, is a weak structure from the viewpoint

of reliability because one failure of device bring about fail-

ure of entire module

3. THE FAILURE ANALYSIS AND LIFE MODEL

To examine the failure mechanisms of the TEM, the fail-

ure analysis was implemented such as Visual Inspection, X-

ray and SEM, EDS about failed modules. (Fig. 5) First, the

failed sample was tested by naked eye and OM (Optical*Corresponding author: hschoi@kicet.re.kr©KIM and Springer

Fig. 1. Schematic diagram of thermoelectric devices.

272 H.-S. Choi et al.: Prediction of Reliability on Thermoelectric Module through Accelerated Life Test and Physics-of-Failure

Electron. Mater. Lett. Vol. 7, No. 3 (2011)

Microscopy) and electrical characteristics were measured.

Second, the non-destructive and destructive tests were done.

Third, after completing the tests, the failure mechanism of

the modules could be found by failure analysis data.

The results of X-ray test shows the failure sites such as

void, crack and positioning error that can be discovered in

process (Fig. 6). Much attention must be paid in the manu-

facturing process because the defect of solder can make a

role of starting point of crack propagation. Fig. 7 shows that

the thermoelectric devices were dislocated by the periodic

bending stress. The fact that dislocations mostly occurred on

the edge of TEM confirms that the failure is because of shear

stress by thermal expansion.

If TEM is used for a long time, the diffusion phenomenon

can be found that the carrier spreads to solder, flux, Cu. To

confirm this, we analyzed the elements by using SEM, EDS.

(Fig. 8) As a result of this analysis, Diffusion phenomenon

didn't occur.

We could define the main failure mechanism of the TEM

as follows. Bending stress generated by temperature gradient

bring about shear stress concentration on the brittle solder

parts. Because TEM operates long time, fatigue fracture

occurs and crack propagates and finally, device is broken.

Other failures such as short and carrier diffusion are

known to happen in the field occasionally but it couldn't be

found in our failure analysis. So we decided to focus to fail-

ure by mechanical stress. This failure mechanism can be

Fig. 2. Inside of TEM.

Fig. 4. Shear stress mechanism of TEM.

Fig. 3. Actual appearance of thermoelectric modules.

Fig. 5. Failure analysis flow chart.

H.-S. Choi et al.: Prediction of Reliability on Thermoelectric Module through Accelerated Life Test and Physics-of-Failure 273

Electron. Mater. Lett. Vol. 7, No. 3 (2011)

explained by Coffin- Manson Model. Coffin- Manson equa-

tion is a kind of Inverse Power Model. It is used the for

fatigue failure of the metal by thermal cycling. It is

expressed like Eq. 1.

(1)

N: Number of cycle

α : constant

β : material constant

We can set up the accelerated model along Eq. (1).

(2)

AF: Acceleration factor

∆T1 : Temperature difference of thermal shock in acceler-

ated life test

∆T2 : Temperature difference of thermal shock in the field

condition

4. ACCELERATED LIFE TESTING

We made QFD(Quality Function Deployment) design for

the life test of TEM.(Table 1 and 2) As the results of QFD

design, on/off test was decided as the primary factor of ther-

moelectric module failure.

Accelerated life test has three ways, first, accelerating the

number of cycles, second, accelerating the external stress,

third, strengthening the failure criteria.

We used first, and second acceleration methods. The flow

chart for ALT is in Fig. 9 changing the bias of voltage

changes the cooling and heating direction. We performed

temperature cycle test by changing direction of voltage. Test

matrix is Table 3.

N α

T∆------- ⎝ ⎠

⎛ ⎞β

=

AF T1∆T2∆

--------- ⎝ ⎠⎛ ⎞

β

=

Fig. 6. The X-ray result.

Fig. 7. Dislocation of devices by shear stress.

Fig. 8. FE-SEM/EDS data.

274 H.-S. Choi et al.: Prediction of Reliability on Thermoelectric Module through Accelerated Life Test and Physics-of-Failure

Electron. Mater. Lett. Vol. 7, No. 3 (2011)

As a result of accelerating test, It is found that the TEM

follows Weibull distribution the shape parameter of which is

3.6 (Fig. 10).

And we applied Coffin-Manson Model as a life model

(Eq. 1). So we get the material constants, á and â value

5.329, 1.78914 each. Considering the material constants are

2 and 3 by Coffin-Manson Model, the results of the life test

is appropriate. Life equation is Eq. 3.

(3)

(4)

5. ANALYSIS OF RELIABILITY

Figure 11 is the cycle (N) graph by Eq. 3 and Fig. 12 is the

relationship between AF and temperature. Temperature

range ∆T = 50°C(40°C to 90°C) is the field condition. So,

we can estimate field life as 4,245 cycles.

Generally, life distribution of the electric components is a

exponent distribution. But the value 3.6 is big as shape

parameter of weibull distribution. It means that wear-out

mechanism prevails and we could confirm this in the failure

analysis. TEM follows weibull distribution in contrary of

assumption that would follow the exponential distribution

because it is electronic component. It is because main failure

mechanism is wear out mechanism

N4 651 000, ,

T∆-------------------------⎝ ⎠⎛ ⎞

1.78914

=

AF T1∆T2∆

--------- ⎝ ⎠⎛ ⎞

1.78914

=

Table 1. Two level quality function deployment - requirements(stresses and performance) and failure mode matrix

Failure mode Requirements (stress and performance) short circuitChange the value of

R and ZCorrosion Fracture, crack

Temperature ○ ◎ ◎ ○

Humidity ◎ ◎ ◎ ◎

Overvoltage ○ ○ ○ ○

stress ○ ○ ◎ ○

Migration ○ ○ ○ ◎

Thermal shock ◎ ◎ ◎ ○

Points 22 24 16 12

Table 2. Failure modes and test methods matrix

Test methods Failure ModeEnvironmental Test

On/Off TestThermal shock Humidity vibration Salt spray Overload

short circuit ○ ○ ○ ◎ ◎ ○

Change the value of R and Z ○ ○ ○ ○ ◎ ◎

Corrosion ○ ○ ◎ ○ ◎ ◎

crack, fracture ○ ◎ ○ ○ ○ ○

Poins 12 9 9 14 8 16

Table 3. Test Matrix

Factor Run Temperature(°C) No. of Sample

1 20°C~90°C 8

2 10°C~90°C 8

Fig. 9. Accelerated life test methods.

H.-S. Choi et al.: Prediction of Reliability on Thermoelectric Module through Accelerated Life Test and Physics-of-Failure 275

Electron. Mater. Lett. Vol. 7, No. 3 (2011)

5. CONCLUSION

In this paper, we examined the failure mechanism of ther-

moelectric modules. We performed accelerating life test and

found the life distribution and acceleration model. Experi-

mental results of ALT showed that the thermoelectric cool-

ing module follows the Weibull distribution, shape

parameter of which is 3.6. Even though TEM is a semicon-

ductor component, its shape parameter is larger than the

other electric devices. The reason for it is identified as the

fatigue fracture caused by cyclic bending. The acceleration

model is coffin Coffin-Manson and material constant is 1.8.

ACKNOWLEDGMENTS

This work was supported financially by the Midium Term

Strategic Technology Development Program of the Korea

Ministry of Commerce, Industry and Energy.

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3. W.Q. Meeker and G. J. Hahn, How to Plan an Accelerated

Life Test - Some Practical Guidelines, Vol. 10, ASQC Basic

References in Quality Control: Statistical Techniques

(1995).

4. T. L. Tran, and J. S. Sherif, IEEE Second International Con-

ference on Engineering Software Standards, Montreal, Can-

ada (1995).

Fig. 10. Accelerated Life data of TEM.

Fig. 11. Relationship between number of cycle and temperature.

Fig. 12. Relationship between acceleration factor and temperature.