LED ENGIN LpS 2013 presentation

Threats to LED lifetime performance and proven techniques to secure reliabilityGerrit-Willem PrinsVP, Europe

Presentation made at the LpS Symposium, September 26, 2013

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LED lighting adoption

• LEDs will replace traditional light sources when performance + reliability is met

Threats to LED lifetime performance and proven techniques to secure reliability

Gerrit-Willem Prins, LED Engin

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LED lighting adoption

• Design needs for accent + downlighting– Lumen density – Beam control– Efficiencies – lm/W– Light quality – Long lifetime



Dark spots from defects

Localised hot spots

Blue die Yellow phosphor

Glass lens

Eutectic bondSilicon

Packaging design: compact, multi-die LED light source

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LED reliability expectations



Colour quality + consistency

Stability of light levels over time

Long service life

Good beam quality for accent + downlighting

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Stability and consistency of light



Results based on independent LM80 test results

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Stability and consistency of light



Results based on independent LM80 test results

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Threats to reliability of LED sources



Browning

Cracking

Lumen degradation of the source• Degradation of optical material absorbs more light• Cracking and delamination of optical material• Degradation of the LED chip reduces light emission

Colour temperature shift over time• Degradation of optical material Δ emission

spectrum

Functional failure• LED thermal or electrical overstress• Mechanical failure

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Contributing factors – heating of optical material



• High-index silicones can boost initial efficiency, but they are typically more susceptible to degradation due to heat

• Common effects are a breakdown of the silicone causing diminished light extraction and CCT shift over time

Photonics.com

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Contributing factors – heating of optical material



Solutions: • Silicone materials which balance index of refraction

and stable high transmission rates at temperature

• Direct phosphor on die-top application

• Efficient thermal designs extracting heat away from the silicone

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Contributing factors – thermal stress on die



• Material CTE mismatch at higher temperatures puts stress on the material lattice

• Micro stress fractures develop over time that don’t generate light

• Result: Lifetimes decrease with rise in junction temperature

LED device under test

Dark spots from defects

Localized hot spots

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Contributing factors – thermal stress on die



Solutions: • Thermally match materials CTE

• Maximise overall system thermal efficiency

to extract heat away from die junction

• Minimise localised hotspots from cracks, voids and delamination in interface materials throughout the system’s thermal path

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Package design considerations



• Silicone selection– Lower index silicone

over higher index• LED initially less bright

but benefits from stability over time

• Doesn’t brown or crack in high temperatures and humidity

• CCT stable over time

Thin Siliconelayer

LED Engin LZC White Emitter

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• Phosphor adhesion process– Deposit phosphor directly

onto die-top• Direct contact with die;

no adhesive layer• Thermally efficient path

to back of die• Reduces localised heating of

silicone from phosphor emissions• No voids under phosphor layer

that create localised hot spots

Phosphor die top


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• Die attach material and process– Eutectic Die attach

• Die attach material and process are key in thermal path to substrate

• Eutectic die attach have high thermal conductivity compared to Ag epoxies

• Minimises voids between die and substrate to avoid localised heating

Eutectic die attach


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• Substrate design– Multi-layer ceramic

• Multiple layers of metal into ceramic substrate offer lowest thermal resistance

• CTE-matched to die creates stress-free environment – no localised hot spots

• Maximises power density and reliability


Multi-layer ceramic substrate

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• Primary lens design– Select glass primary lens

over silicone lens• Interlocking design adds

structural package integrity• No degradation from self-heating• Added moisture protection barrier

reduces browning of siliconeLED Engin LZC White Emitter

Solid interlocking glass lens

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System design to maximise heat flow



Considerations to capitalise on high flux emitter for long-term reliability

Metal Core PCB Metal acts as a heat spreader Lowest thermal resistance ESD protection Avoid over winding mounting screws to prevent

LED substrate cracking from MCPCB warping

Pedestal Design Emitter in direct contact with base material

eliminating dielectric barrier

Heat Sink Interface material and surface flatness key to minimise air pockets Sized properly to dissipate

heat from system

High flux emitter High power density Low thermal resistance

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Other system design considerations



• Driver design considerations• Protect against surge currents• Ensure accuracy of output current • Driver should not add to the

overall system heat• Drivers with thermal feedback

and protection can protect the LED from thermal overstress

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• Optics– Optimise for specific sources

for best lumens in beam results

– Positioned properly for most efficient light output

• Off-center lenses can put stress on primary lens

• Method of clamping lens to leadframe or substrate can create torque and cracking of the substrate

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• Mechanical– Mounting techniques should

prevent stress on package that can lead to cracks and delamination

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Conclusions



Reliability is key to the value proposition of LED lighting

Reliability designs include:– Design for thermal efficiency

–Materials selection–Process Design

Seek ways to maximise Lux on Target and light quality over time

Thank you

23For more information, visit www.LEDEngin.com

Email: [email protected]

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LED ENGIN LpS 2013 presentation