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 L 70 = 10,000 hours
Definition: change in light output of a light source over operational life, relative to initially measured light output
L xx = time to xx% of original light output
L 70 = time to 70% of original light output
L 50 = 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. 2008 IES LM-80-2008 is published. 2009 Cree publishes reliability white paper (T AIR ). 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 (I F ). 1990’s LEDs used for traffic signals and exit signs. First basic studies on LED lifetime performed.
LED Lumen Maintenance Critical Parameters 4. I F Forward Current / Drive Current 1. T AIR Ambient Air Temperature 2. T J Junction Temperature 3. T SP / T C / T S Solder-Point Temperature / Case Temperature
T J & T SP Linked by Thermal Resistance 1. T AIR Ambient Air Temperature 2. T J Junction Temperature 3. T SP / T C / T S Solder-Point Temperature / Case Temperature 4. I F Forward Current / Drive Current T J = T SP + ( [R th j-sp ] x [V F ] x [ I F ] ) Thermal Resistance Forward 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
For 55ºC testing, the T SP of the lamps and air are both at 55ºC
For 85ºC testing, the T SP of the lamps and air are both at 85ºC
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% @ T J = 65ºC
Low temp (25ºC) testing is a good surrogate for the LED chip depreciation – 1-2% @ T J = 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 T AIR , T J , T SP and I F 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 T AIR , T J , T SP and I F
Mean L 70 lifetime predictions are shown grouped by T AIR or by I F
L 70 Lifetime Prediction – T AIR = 35ºC
L 70 Lifetime Prediction – T AIR = 45ºC
L 70 Lifetime Prediction – T AIR = 55ºC
L 70 Lifetime Prediction – T AIR = 65ºC
L 70 Lifetime Prediction – T AIR = 75ºC
L 70 Lifetime Prediction – T AIR = 85ºC
L 70 Lifetime Prediction – I F = 350 mA
L 70 Lifetime Prediction – I F = 700 mA
L 70 Lifetime Prediction – I F = 1000 mA
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 (T J ) – calculated from T SP
Forward drive current (I F )
Air temperature around the lamp (T AIR )
From these, L 70 for the LED lamp can be predicted with confidence