This is the presentation made by LED's Engin's Head of Europe, Gerrit-Willem Prins, at LpS 2013, a leading lighting industry conference held in Austria. The show is organised by the same team responsible for LED Professional magazine. Many thanks to the organisers.

1. Threats to LED lifetime performance and
proven techniques to secure reliability
Gerrit-Willem Prins
VP, Europe
Presentation made at the LpS Symposium,
September 26, 2013

2. LED lighting adoption
• LEDs will replace traditional light sources when
performance + reliability is met
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

3. LED lighting adoption
• Design needs for
accent + downlighting
–
–
–
–
–
Lumen density
Beam control
Efficiencies – lm/W
Light quality
Long lifetime
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

4. Packaging design: compact,
multi-die LED light source
Glass lens
Blue die
Dark spots
from defects
Silicon
Eutectic bond
Localised hot spots
Yellow phosphor

5. 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

6. Stability and consistency of light
Results based on
independent
LM80 test results
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

7. Stability and consistency of light
Results based on
independent
LM80 test results
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

8. Threats to reliability of LED sources
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
Browning
• Degradation of optical material Δ emission spectrum
Functional failure
Cracking
• LED thermal or electrical overstress
• Mechanical failure
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

9. 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

10. 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

11. 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
Localized hot
spots
with rise in junction temperature
Dark spots
from defects
LED device under test
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

12. 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

13. 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 Silicone
layer
LED Engin LZC White Emitter
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

14. Package design considerations
• 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
LED Engin LZC White Emitter
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

15. Package design considerations
• Die attach material
and process
– Eutectic Die attach
• Die attach material
Eutectic die
attach
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
LED Engin LZC White Emitter
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

16. Package design considerations
• 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
LED Engin LZC White Emitter
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

17. Package design considerations
• Primary lens design
– Select glass primary lens
over silicone lens
• Interlocking design adds
Solid
interlocking
glass lens
structural package integrity
• No degradation from self-heating
• Added moisture protection barrier
reduces browning of silicone
LED Engin LZC White Emitter
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

18. System design to maximise heat flow
Considerations to capitalise on high flux emitter for long-term reliability
High flux emitter
Metal Core PCB
 High power density
 Low thermal resistance




Metal acts as a heat spreader
Lowest thermal resistance
ESD protection
Avoid over winding mounting screws to prevent
LED substrate cracking from MCPCB warping
Heat Sink
 Interface material
and surface flatness key
to minimise air pockets
 Sized properly to dissipate
heat from system
Pedestal Design
 Emitter in direct contact with base material
eliminating dielectric barrier
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

19. 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

20. Other system design considerations
• 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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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

21. Other system design considerations
• Mechanical
– Mounting techniques should
prevent stress on package
that can lead to cracks
and delamination
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

22. 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
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Threats to LED lifetime performance and proven techniques to secure reliability
Gerrit-Willem Prins, LED Engin

23. Thank you
For more information, visit www.LEDEngin.com
Email: prins@ledengin.com
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