(C l P i t St bilit ) (Color Point Stability) o or o n a y · o or o n a y LED Package Reliability...

o or o n a y

LED Package Reliability

(C l P i t St bilit ) Ralph C. Tuttle

Cree, Inc ralph_tuttle@cree.com

(Color Point Stability)

Color Bins Shown

CIE 1931 Color Space

ANSI C78.377 Color Bins Shown

Observed Color Shift with LEDs

Green Shift Yellow Shift

Blue Shift

Color Shift with Plastic Packages

The white material in the package cavity reflects much of the light emitted from the LED chips.

The overall chromaticity (color) of the LED is a combination of all of the light emitted directly from the chips through the phosphor and that which is reflected internally in the package.

Short distance through phosphor Long distance through phosphor = cooler color temperature = warmer color temperature

These photons will continue to These photons will not be be emitted from the package reflected off the package walls.

Over time the materials in these packages will discolor.

An overall blue shift and lumen depreciation results

These photons will continue to These photons will not be be emitted from the package reflected off the package walls.

Over time the materials in these packages will discolor.

LM-80 Test – PPA Plastic Packages

Tsp = 85ºC

LM-80 Test – PPA Plastic Packages

Tsp = 85ºC

Storage Test – PPA Packages

LEDs placed un-powered in reliability test chamber.

Storage Test – EMC Packages

LM-80 Test – EMC Packages

Storage Test – Ceramic Packages

Blue Color Shift with Plastic Packages

Phosphor Chip Coated LED

Phosphor Silicone Lens

LED Chip

Anode Cathode

Wire Bond

Substrate

Phosphor Chip Coated LED (High Temp Operation)

Phosphor Chip Coated LED

At high temperatures cracking and delamination of the phosphor can occur at the chips surface. The light scattering increase in the angle at which the emitted light travels through the phosphor. The path of the blue photons through the phosphor is increased causing a shift towards the warm end of the spectrum.

High temperature LM-80 testing has been used to accelerate the color shift in high power LEDs.

125°C

105°C

Yellow Color Shift

Can models be developed to project color shift over time?

Color Shift during WHTOL testing

60/90 WHTOL Testing

Tsp = 60ºC Tair = 60ºC 90% RH 90% RH 1 hour on 1 hour off

Green Color Shift

Red Phosphor degrades causing a shift of the color point.

im ortant.

Summary

• Color shift can happen with LEDs.

• There are many applications where it is not critical.

• There are many applications color point stability is important. p

• Materials used in LED package construction will result in different color point stability characteristics.

• There is no method currently available to project color shift.

(C l P i t St bilit ) (Color Point Stability) o or o n a y · o or o n a y LED Package Reliability...

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