Lumen Maintenence Overview

XLamp XR-E Lighting-Class LED Lumen Maintenance

July 2009

Copyright © 2009, Cree, Inc. pg. 2

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..


LED Reliability Testing

• LEDs are semiconductor components that happen to emit light

• Cree conducts all the traditional standardized semiconductor component reliability testing on all of our LED lamps (http://www.cree.com/products/pdf/XLamp_Reliability.pdf)

• Cree has been active in leading the industry standardization of an additional test series – IES LM-80-2008 – to characterize the Lumen Maintenance aspect of LED semiconductor components


Lumen Maintenance Definition

40%

50%

60%

70%

80%

90%

100%

110%

0 5 10 15 20 25

Operating Time (k hrs)

Lum

en O

utp

ut (%

)

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: 400W metal halide lamp

Traditional light sources

gradually dim then fail

catastrophically (“burn out”)


LED Lumen Maintenance History

<1990 LEDs were used as indicators.Lifetime did not matter too much.

2001 Lumileds (Philips) introduces first High Power LED. Lifetime testing gets more serious.

2004 LRC/ASSIST publishes first work on the topic.Basic science understood.

2006 LRC/ASSIST publishes ASSIST recommends test procedure for lifetime testing of High Power LEDs, which becomes the basis for IESNA LM-80-2008 standard.

2007 Philips Lumileds publishes white paper, extending the knowledge of the industry on the topic (IF).

2008 IES LM-80-2008 is published.

2009 Cree publishes reliability white paper (TAIR).

1990’s LEDs used for traffic signals and exit signs.First basic studies on LED lifetime performed.


LED Lumen Maintenance Critical Parameters

1. TAIRAmbient Air Temperature

2. TJ Junction Temperature

3. TSP / TC / TSSolder-Point Temperature / Case Temperature

4. IFForward Current /Drive Current


TJ & TSP Linked by Thermal Resistance

1. TAIRAmbient Air Temperature

2. TJ Junction Temperature

3. TSP / TC / TSSolder-Point Temperature / Case Temperature

4. IFForward Current /Drive Current

TJ = TSP + ( [Rth j-sp] x [VF] x [IF] )Thermal

ResistanceForward Voltage


Cree’s Test Configuration Per IES LM-80-2008

Temperature of solder pad of lamps is independently actively controlled by fluid flowing through heat sink.

Temperature of ambient around lamps is actively controlled by air flowing through chamber

Lamps are mounted to MCPCB’s.

• During test, the temperature of the solder pad of the lamps and the air around the lamps is the same

• Per LM-80, − For 55ºC testing, the TSP of the lamps and air are both at 55ºC

− For 85ºC testing, the TSP of the lamps and air are both at 85ºC


Low temp (25ºC) testing is a good surrogate for the LED chip depreciation – 1-2% @ TJ =

65ºC

40,000 Hour / 4.5 Year XLamp Long-Term Data

• At lower ambient air temperature, LEDs hardly depreciate at all.

• Low ambient air temperature (25ºC) is a good surrogate for LED chip depreciation – 2% @ TJ = 65ºC


High Air Temperature Degrades Encapsulant

• Cree now understands that the silicone-based encapsulants used in the industry degrade when exposed to high temperatures.

• As they degrade over time, less light is transmitted through the encapsulant.

• The higher the air temperature, the more the encapsulant will degrade.


Cree Lumen Maintenance 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.

• Good quality LEDs in well-designed fixtures will last for decades under normal use and operating conditions.


Cree Lumen Maintenance Model

• 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 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

• Mean L70 lifetime predictions are shown grouped by TAIR or by IF


L70 Lifetime Prediction – TAIR = 35ºC


L70 Lifetime Prediction – IF = 350 mA


Summary

• Testing and characterizing LED lumen maintenance is an evolving science based on decades of work from the component semiconductor industry

• We have fully characterized the XLamp XR-E LED lamp and have developed an accurate predictive model for its lumen maintenance

• Key inputs to long-term lumen maintenance are:– LED junction temperature (TJ) – calculated from TSP

– Forward drive current (IF)

– Air temperature around the lamp (TAIR)

• From these, L70 for the LED lamp canbe predicted with confidence

Lumen Maintenence Overview

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