1. LED Lumen Maintenance
& Lifetime
Mark_McClear@cree.com
May 13, 2010

2. Copyright © 2010 Cree, Inc. pg. 2
Trivia Point: 50,000 hours is:
137 Years at 1 hour/day
68.5 Years at 2 hours/day
34.2 Years at 4 hours/day
22.8 Years at 6 hours/day
17.1 Years at 8 hours/day
11.4 Years at 12 hours/day
5.7 Years at 24 hours/day

3. Copyright © 2010 Cree, Inc. pg. 3
Semiconductor Reliability Testing
• Reliability test methods and acceptance criteria for
semiconductor components have been standardized
(JEDEC, EIAJ, others…) and practiced for decades
– Think: processors, regulators, microcontrollers, etc..
If you’ve ever flown in an airplane, driven in a car, or
spoken on a cell phone, you’ve depended on this
body of scientific work and testing…

4. Copyright © 2010 Cree, Inc. pg. 4
LED Reliability Testing
• LEDs are semiconductor
components that happen to emit light
• Most LED manufacturers conduct the traditional
standardized semiconductor component reliability testing on
their LED lamps (http://www.cree.com/products/pdf/XLamp_Reliability.pdf)
• The Illumination Engineering Society of North America
published IES LM-80-2008 18 months ago to characterize
the Lumen Maintenance aspect of LED semiconductor
components
• Note: Lumen Maintenance ≠ LED Lifetime. Another
standards committee – TM-21 – is working on that aspect of
the problem

5. pg. 5Copyright © 2010 Cree, Inc.
Lumen Maintenance Definition
40%
50%
60%
70%
80%
90%
100%
110%
0 5 10 15 20 25
Operating Time (k hrs)
LumenOutput(%)
L70 = 10,000 hours
Definition: change in light output of a light source over operational
life, relative to initially measured light output
Lxx = time to xx% of original light output
• L70 = time to 70% of original light output
• L50 = time to 50% of original light output
Lumen Maintenance: Hypothetical HID Lamp
Traditional light
sources gradually
dim then fail
catastrophically
(“burn out”)

6. pg. 6Copyright © 2010 Cree, Inc.
LED Lumen Maintenance Critical Parameters
1. TAIR
Ambient Air Temperature
2. TJ
Junction Temperature
3. TSP / TC / TS
Solder-Point Temperature /
Case Temperature
4. IF
Forward Current /
Drive Current

7. Copyright © 2010 Cree, Inc. pg. 7
Low temp (25ºC) testing is a good surrogate for
the LED chip depreciation – 1-2% @ TJ = 65ºC
LEDs Last Forever! [under ideal conditions]
At lower ambient air temperatures, LEDs hardly depreciate at all

8. pg. 8Copyright © 2010 Cree, Inc.
High Air Temperature Degrades Encapsulant
• Cree now understands that the silicone-based encapsulants
used in the industry degrade when exposed to high
temperatures
• Degradation comes from organic pendant groups (e.g. CH3,
C6H5, -OH) that can off-gas or be trapped in the matrix
• The higher the air temperature, the more the encapsulant
will degrade, the more light lost
OSiOSiOSi
H–C–H
H
H–C–H
H
H–C–H
H

9. pg. 9Copyright © 2010 Cree, Inc.
• The out-diffusion of volatiles from the silicone
encapsulant causes the refractive index of the
encapsulant to decrease
• As the refractive index decreases the critical angle
increases allowing less light to be emitted from the lamp
(Snell’s Law)
Un-powered LEDs Depreciate at High Temp

10. pg. 10Copyright © 2010 Cree, Inc.
1,000 2,000 4,0003,000 5,000 6,000 7,0000
100
85
90
105
95
%LF
Hours on Test
Typical Behavior of LEDs on LM-80 Testing
Low Air Temp
High Air Temp

11. pg. 11Copyright © 2010 Cree, Inc.
Consistently Repeatable Behavior

12. pg. 12Copyright © 2010 Cree, Inc.
Cree XR-E XLamp
55°C LM-80 Testing

13. pg. 13Copyright © 2010 Cree, Inc.
Cree XR-E XLamp
55°C LM-80 Testing

14. pg. 14Copyright © 2010 Cree, Inc.
Cree XR-E XLamp
85°C LM-80 Testing

15. pg. 15Copyright © 2010 Cree, Inc.
Cree XP-E XLamp
55°C and 85°C
LM-80 Testing

16. pg. 16Copyright © 2010 Cree, Inc.
Using the actual low temp test data at 6,000 hours, the projected L70 value is 63,917 hours.
Projecting LED Lifetime
Measured Data

17. pg. 17Copyright © 2010 Cree, Inc.
Using the actual low temp test data at 10,600 hours, L70 increases to 109,903 hours.
Projecting LED Lifetime
Measured Data

18. pg. 18Copyright © 2010 Cree, Inc.
When using the low temp 34,800 hour data, the L70 increases to > 200,000 hours!
Projecting LED Lifetime
Measured Data
Exponential Curve-fit is inaccurate and exceedingly conservative

19. pg. 19Copyright © 2010 Cree, Inc.
So, How to Translate Lumen Maintenance to Lifetime…?
• Cree has accumulated millions of XLamp XR-E LED lamp
device hours of long-term data under both LM-80-compliant
conditions and other test configurations
• The effects of TAIR, TJ, TSP and IF on long-term lumen
maintenance in this system have been closely studied and
are well understood
• Cree has observed that the lumen maintenance
characteristics of the XLamp XR-E white LED lamps are
different in the first 5,000 hours (called Period A) than in
the time period following 5,000 hours (called Period B)
• A “best fit” algorithm was developed to accurately model
this lumen maintenance behavior, based on the critical
parameters TAIR, TJ, TSP and IF
• This algorithm is likely to be different for every LED lamp
system (e.g. XLamp XP, MC, Rebel, Dragon, NS6, etc…)

20. pg. 20Copyright © 2010 Cree, Inc.
Cree LED Lifetime Model
• Degradation in first 5,000 hours is mostly due to degradation in
the silicone encapsulant
• After 5,000 hours, this mechanism drops out and the slower chip
degradation dominates
• We see no “B10” early life failures or catastrophic failures in our
XLamp systems

21. pg. 21Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 35ºC
If = 350 mA
If = 700 mA
If = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Given the Air (ambient) temp inside the fixture, drive current,
and junction temperature, the lifetime is projected
Note: 35ºC Air Temp

22. pg. 22Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 45ºC
If = 350 mA
If = 700 mA
If = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 45ºC Air Temp

23. pg. 23Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 55ºC
If = 350 mA
If = 700 mA
If = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 55ºC Air Temp

24. pg. 24Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 65ºC
If = 350 mA
If = 700 mA
If = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 65ºC Air Temp

25. pg. 25Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 75ºC
If = 350 mA
If = 700 mA
If = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 75ºC Air Temp

26. pg. 26Copyright © 2010 Cree, Inc.
XLamp XR-E Lumen Maintenance Projection
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAIR = 85ºC
If = 350 mA
If = 700 mA
If = 1000 mA
Note: 85ºC Air Temp

27. pg. 27Copyright © 2010 Cree, Inc.
XLamp XR-E Lifetime Projection
There’s Nothing Magic about 50,000
• Models err on the conservative side
• As much as we’d like it, there is no
“one-number” answer (like CRI)

28. pg. 28Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - IF = 700mA
Tair = 45C
Tair = 55C
Tair = 65C
Tair = 75C
Tair = 85C
XLamp XR-E Lumen Maintenance Projection
Note: 700mA IF

29. pg. 29Copyright © 2010 Cree, Inc.
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
110,000
120,000
130,000
140,000
150,000
60 70 80 90 100 110 120 130 140 150 160
MeanL70Lifetime(hours)
Junction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - IF = 1000mA
Tair = 45C
Tair = 55C
Tair = 65C
Tair = 75C
Tair = 85C
XLamp XR-E Lumen Maintenance Projection
Note: 1000mA IF

30. pg. 30Copyright © 2010 Cree, Inc.
LED Lifetime Is Irrelevant
System Lifetime is What Creates Value
LED Lamps: Practically never
fail; depreciate very slowly in a
well-designed system
Optical Components: Can
(rarely) yellow over time and
lose light; system design choice
Driver: Can be the weak
link in the system if cheap
components are used
Heat Sink: Linchpin of the entire system.
If this is poorly designed, all the other
components can be compromised

31. Copyright © 2010 Cree, Inc. pg. 31
LED Lumen Maintenance & Lifetime Summary
• Testing and characterizing LED lumen maintenance is an
evolving science based on decades of work from the
component semiconductor industry
• LED Lumen Maintenance ≠ LED Lifetime
• We have fully characterized the XLamp XR-E LED lamp and
have developed an accurate predictive model for both the
Lumen Maintenance and Lifetime of this lamp system
• Key input variables to Lumen Maintenance and Lifetime are:
– LED junction temperature (TJ) – calculated from TSP
– Forward drive current (IF)
– Air temperature around the lamp (TAIR)
• From these, L70 and Lifetime of the LED
lamp can be predicted with confidence
• Different LEDs will likely have different lifetime models (TM-21)
• LED Lifetime ≠ Luminaire Lifetime: need systems approach

32. pg. 32Copyright © 2010 Cree, Inc.