Silicone Encapsulant Needs -

an LED Manufacturers Perspective

Ralph C. Tuttle

Cree, Inc.

3 February 2016

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RALPH C. TUTTLE

Cathode Anode

LED Chip Wire Bond

Silicone Lens

Substrate

ENCAPSULANTS USED ON LED’s are SILICONES

Phosphor

(LED Cross-Section)

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Si O Silicon-Oxygen Chain

(SiO)n

• These silicone encapsulants are made using an Si-O-Si-O-Si-O chain (SiO)n as their basic structure

O Si O Si O Si

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Polyorganosiloxanes (“siloxanes”) are Si-O chains where a hydrocarbon group is covalently bonded to the Si

Siloxane (R2SiO)n

(R = functional hydrocarbon group)

O Si O Si O Si O Si

R R R R

R R R R

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One typical classification of encapsulation materials are methyl (CH3) siloxanes

Methyl Siloxane [SiO(CH3)2]n

O Si O Si Si O

H – C – H

H

H – C – H

H

H – C – H

H

H – C – H

H

H – C – H

H

H – C – H

H

Methyl Group CH3

– C – H

H

H

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Another classification of encapsulation materials are phenyl (C6H3) siloxanes

O Si O Si Si O

Phenyl Group C6H5

Phenyl Siloxane [SiO(C6H5)2]n

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WHY DO WE USE SILICONE ENCAPSULANTS

• Protect LED chip form being damaged

• Light extraction (couple light from chip)

• Can withstand low wavelength (high energy) light

• Stable over a wide temperature range

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LIGHT EXTRACTION

Ө1

Ө2

n1

n2

ӨC

Ө2

Critical Angle - when exceeded, all emitted

light will be internally reflected in the chip.

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LIGHT EXTRACTION

InGaN: n = 2.5

Air: n = 1.0

For an LED chip in air the critical angle is 23.6°

2.5 =n1

1.0 =n2

ӨC

Ө2

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Methyl Silicone: n = 1.40 ӨC = 34.0°

Phenyl Silicone: n = 1.55 ӨC = 38.3°

Silicones have higher refractive indices.

The higher the refractive index, the more light

can be coupled from the chip.

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However….

Long term lumen maintenance (L70) during LM-80

testing shows high index phenyl siloxane is

WORSE than with low index methyl siloxane.

phenyl siloxane

methyl siloxane

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Substrate

Solder Mask “Dam”

Phosphor + Silicone

LED Chips **Scale = None

(COB Cross-Section)

Temperature Resistance

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Silicone

Substrate

Phosphor

Blue photons emitted from the LED chip strike phosphor particles and

are down converted to longer wavelengths.

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Silicone

Substrate

Phosphor

Because the silicone is an insulator, the heat generated from

the phosphor particles can cause the silicone to crack.

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Temperature (lower) (higher)

Sto

rag

e M

od

ulu

s (G

’)

DMA Analysis of Silicones – Visco-Elastic Behavior

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Rubbery/Elastic State

Glass Transition Region

Glassy Plateau

Silicones may have glass transition temperatures within

the typical operating temperatures of the LED itself.

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Rubbery/Elastic State

Glass Transition Region

Glassy Plateau

An LED built using this type of silicone could have reliability issues in cold weather applications. The stress in the silicone as it transitions from the “rubbery” state to the “glassy” state could cause damage to the die attach or the chip integrity itself.

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Silicone Encapsulant Needs:

• High Refractive Index (couple light from chip)

• High ni with good lumen maintenance!

• Resistant to high temperatures

• Low glass transition temperature

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Thank You!