NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Accelerated Testing and On-Sun 

Failure of CPV Die-Attach

Nick Bosco and Sarah Kurtz

National Renewable Energy Laboratory 

Golden, CO

Presented at the 2010 Workshop on Accelerated Stress 

Testing and Reliability, 8-9 October 2010, Denver, Colorado

NREL/PR-5200-49243
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Introduction: CPV concepts 
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Introduction: CPV concepts 

 500-1000x concentration

 DNI = 1000 W/m2

 η ~ 39%

 Q = 30-60 W

~2cm
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

die-attach and temperature 

changes

0

1000

2000

3000

4000

5000

6000

7000

8000

0 10 20 30 40

number of counted cycles

minimum temperature range (C)

σ

o

=

E

c

∆α∆T

1− υ ( )c

Stress in the die attach is directly related to 

the magnitude of temperature change

In Golden, CO over 4000 temperature 

changes of > 20 C are experienced each year
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

thermal fatigue

infrared imaging of CPV cell through thermal cycling

initial 500 cycles 1000 cycles 1500 cycles

Has this sample failed?

Thermal fatigue will cause die-attach cracking ultimately leading to cell 

failure via thermal runaway
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

motivation

Determine reliability

test to failure

induce and recognize a representative failure

Where we started

IEC 62108 design qualification and type approval

thermal 

Dark I-V

issue: will this sequence induce and recognize a representative failure?
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

motivation

Outstanding issues:

 Can thermal cycling produce enough damage to 

precipitate a representative failure?

 If produced, would that damage cause failure 

through testing?

 How much damage is required to cause this 

representative failure?

 Can the application of current simulate an on-sun 

failure

 If so, what level of current is appropriate?

 benign to a “healthy” die-attach

 will cause cell failure in a “failed” die-attach
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

IEC 62108 10.6

-50

0

50

100

150

0 50 100 150

-0.4

0

0.4

0.8

temperature (C)

time (min)

current

1.25xIsc

0

110 C

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80

8 A

6 A 

4 A

temperature (C)

time (s)

Thermal cycling parameters as 

set by the IEC standard:

Applied currents approaching Isc 

are commonly believed to induce 

un-representative cell failure

Simulated cell temperature 

response for 3 levels of forward 

bias
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experiment

IR image

Current ramp

On-sun exposure

500 thermal

cycles*

Initial IR images show varying degree and location of voids

Non of the initial 17 assemblies failed the 4 A current ramp

*Bosco, N.S., Sweet, C.,Kurtz, S., “Reliability Testing the Die-Attach of 

CPV Cell Assembles”, 34th IEEE Photovoltaic Specialists Conference, 7-

12 June 2009, Philadelphia, Pennsylvania 

Tmin = −40C

T

max = 110C

t

d = 5min

f = 48 day

 Samples consist of ~ 1cm2 multi-junction cell on ~2 x 2 

cm substrate

 Cell assembly heating is via the application of forward 

bias current, Imax = 3A
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IR imaging

Δt = 100 ms

18

20

22

24

26

28

0 50 100 150 200 250 300 350 400

temperature

frame number

Δt = 20 ms

18

20

22

24

26

28

0 50 100 150 200 250 300 350 400

temperature

frame number

Δt = 200 ms

18

20

22

24

26

28

0 50 100 150 200 250 300 350 400

temperature

frame number

Δt = 2000 ms

18

20

22

24

26

28

0 50 100 150 200 250 300 350 400

temperature

frame number

2 mm

Apply 2A (~4V) and record an IR “video”
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thermal cycling

-50

0

50

100

150

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70 80

temperature (C)

current (A)

time (min)

Forward bias heating current and temperature response

Max current of ~3A

Excellent temperature control
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

current ramp

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

voltage

current

voltage (V) current (A)

time (s)

95

100

105

110

115

0 10 20 30 40 50 60 70

cell

heat sink 

oven

temperature (C)

time (s)

Tcell T heat sink

T oven

Experimental set-up Temperature response I V response

4A current ramp designed to replicate cycling conditions
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on-sun exposure

Open circuit Voltage monitored during exposure

~1000x Fresnel lens

Fixtured cell assembly 

Sample heat-sunk and exposed to ~1000x concentration

2-axis tracker
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

thermal cycling failures

1000 cycles

LF15

shorted at N=1303

500 cycles

LF18

shorted at N=806
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IR imaging failures

1750 cycles

500 cycles

LF7

500 cycles

LF11

1000 cycles

failed in IR imaging

1000 cycles

failed in IR imaging
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current ramp failures

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

voltage

current

voltage (V) current (A)

time (s)

LF21

500 cycles Failed current ramp @ 

3.4A
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on-sun failure

initial 500 cycles 1000 cycles 1500 cycles

I ramp: passed I ramp: passed I ramp: passed I ramp: passed failed on-sun
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thermal cycling

1250

LF23 LF1

250 cycles

These samples have 

passed on-sun screening 

and continue to be cycled

LF26

250

LF9

1250
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thermal cycling

Summary of Results
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

thermal cycling

IR imaging and shorted samples represent lower bounds to their crack area 

%

Summary of Results: sorted for increasing crack damage
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

thermal cycling

0

1

2

3

4

5

6

7

crack area percent

pass

IR

I cycling

I ramp

on-sun

measured crack area

Summary of Results: sorted for increasing crack damage
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

conclusions

 3 A current application for thermal cycling is benign to a healthy 

die-attach

 A corner crack ~3 area% of the cell is required to precipitate a 

representative failure.

 Die-attach damage severe enough to cause an on-sun failure is 

similarly detected via power cycling and the 4 A current ramp.