Fortronic Oxford 2014

www.osram-os.com
LED Lifetime and reliability
Colin Beale | 9th September | Fortronic Manchester
Today’s topics
Page
1. LED Lifetime – Definition and influences
03
2. Investigating LED lifetime and reliability:
LEDs from the inside out
06
3. Investigating LED lifetime and reliability:
Solder joints, encapsulates and coatings
14
4. Investigating LED lifetime and reliability:
Enclosures and the application environment
18
5. Conclusion – Is your application doomed to fail?
21
2
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
LED Lifetime:
Definition and influences
LED lifetime:
Definition and influences
MTTF figures for single LEDs are usually in excess of a million hours but external
causes that could impact lifetime and reliability the LED are not
taken into account
LED end of life (EOL) for lighting is defined as the time after which the luminous flux of 50% of
a batch of LEDs (B50) has reached 70% (L70) of its’ original value. At EOL…...
– The LEDs are still usable (i.e. they have not failed catastrophically)
– The degradation in light output is only just noticeable
Limiting factors for LED lifetime (manufacturer controlled)….
– Degradation of the optical properties of the LED package (extraction efficiency)
– Phosphor degradation (for white and converted colour LEDs)
– Package and LED die robustness and reliability
Limiting factors for LED lifetime (application controlled)….
– The average temperature of the LED die (TJ, the junction temperature)
– The (forward) current being driven through the LED (IF)
Plus external influences that may or may not be application controlled….
4
Fortronic Oxford – March 2014 | OS S NE | CB
LED junction temperature (TJ) and drive current:
The effect on lifetime
DURIS P5 200mA, 6k hour TM21 lifetime predictions
DURIS E5 6k hour TM21 lifetime predictions at TS = 85°C
– 80mA: 104k hours (TJ = 94°C)
– 120mA: 54k hours (TJ = 100°C)
– 150mA: 37k hours (TJ = 104°C) ←
5
Fortronic Oxford – March 2014 | OS S NE | CB
Note how drive current impacts on the important TJ
value whilst TS remains constant
LED Lifetime:
External influences
Factors external to the LED can have a
dramatic effect on lifetime and even
lead to catastrophic failures
The obvious causes…...
– Mechanical damage
– Electrical overstress
– Thermal overstress
– Static damage during or post assembly
The less obvious causes….
– Atmospheric corrosion on a
macro or micro scale
– Chemical attack accelerated
by encapsulation
– Humidity
6
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
Investigating LED lifetime and reliability:
LEDs from the inside out
Investigating LED lifetime and reliability:
LEDs from the inside out
Starting from the LED die itself we will work outwards to the surrounding
atmosphere investigating various factors that can influence LED lifetime and
reliability and how the risks can be minimised
Lens / encapsulation
(clear or phosphor filled)
Leadframe
Outer moulding
Substrate
LED die + phosphor layer for white /
colour converted LEDs
8
Fortronic Oxford – March 2014 | OS S NE | CB
LEDs – From the inside out:
1 - The LED die and contacts
Type and main cause of degradation or failure…
– Growth of ‘dark line’ defects (non radiative recombination areas)
• Temperature
– Contact failure or degradation of contact to LED die back or top
surface and / or leadframe
• Excessive temperature cycling, mechanical damage to or force via the
lens, environmental influences
– Damage to die structure and / or surface metalisation. Die
cracking, fused bond wire
• Electrical overstress (excessive current)
– Change of LED characteristics, rapid reduction in light output
• Static damage
Minimise the risks…
– Good thermal management
– Avoid mechanical damage or excessive
force on the lens
– Good ESD protection in production / handling
– Protect against over current
9
Fortronic Oxford – March 2014 | OS S NE | CB
LEDs – From the inside out:
2 - The phosphor layer or phosphor filled encapsulate
Type and main cause of degradation...
– Reduction in phosphor conversion
efficiency (particularly red phosphors)
causes colour to shift towards blue with
consequent reduction in light output
• Temperature
• Very high DC drive current
• Humidity
Minimise the risks…
–
–
–
–
Thermal management
Avoid very high drive currents if possible
Protect against high humidity
Check real lifetime test reports e.g. LM80
for evidence of good colour stability of
warm white colours e.g. 3000K
– Choose a quality LED supplier with a
good track record
10
Fortronic Oxford – March 2014 | OS S NE | CB
LEDs – From the inside out:
3 - The outer moulding of over moulded type LEDs
Type and main cause of degradation...
– Reduction in the reflectivity of the inner
surfaces of the white moulding causes a
non reversible reduction in the package
extraction efficiency of a few percent
during the first few 100 to 1000’s of hours
• Short wavelength (blue to green) radiation
• High radiant flux
• PPA outer package material
Minimise the risks….
– Check that the LED predicted lifetime
(based on standard methods such as
TM21) meets your expectations.
– Choose an overmoulded LED that uses a
superior lifetime outer package material
such as the OSRAM DURIS S and
DURIS P series (shown opposite)
11
Fortronic Oxford – March 2014 | OS S NE | CB
LED Lifetime and reliability:
The OSRAM mid power white LED options
DURIS
E Series
1k hours
42 days
10k hours
1 year 2 months
25k hours
2 years 10 months
35k hours
4 years
50k hours
5 years 8 months
75k hours
8 years 7 months
100k hours
11 years 5 months
12
Fortronic Oxford – March 2014 | OS S NE | CB
S Series
P Series
LEDs – From the inside out:
4 - The clear or phosphor filled encapsulate
Type and main cause of degradation...
– Reduction in the transmissivity of the
encapsulate or lens causes a non
reversible reduction in light output in a
few 100 to 1000’s of hours
• Clear or phosphor filled epoxy
• Short wavelength (blue to green) radiation
• High radiant flux
Minimise the risks….
– For blue, green and white LEDs ensure
the LED uses Silicone and not epoxy for
the clear or phosphor filled encapsulate
– If epoxy is unavoidable, reduce the drive
current to reduce the radiant flux from
the LED die
13
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
Investigating LED lifetime and reliability:
Solder joints, encapsulates and coatings
Investigating LED lifetime and reliability:
The environment and protection
Now that we’ve covered the LED we will work outwards from the LED to the
surrounding atmosphere to investigate how various factors will influence the LED
lifetime and reliability and how the risks can be minimised
Enclosure
Protective coating
15
Fortronic Oxford – March 2014 | OS S NE | CB
Solder joint
Investigating LED lifetime and reliability:
Solder joint quality and reliability
Potential issues and their main causes...
– Solder joint cracks and fractures in the
field over time
• Mismatch between the coefficient of thermal
expansion (CTE) of the LED substrate and
metal core PCB
• Ceramic substrate LEDs
• Large physical LED size
• Extreme temperature variations
– Voids in solder joint to thermal pad
increases thermal resistance causing
LED overheating
• Incorrect solder paste stencilling
inhibits outgassing
Minimise the risks….
– Use smaller size ceramic based LED
– Change PCB material to non MCPCB
– Ensure solder paste stencil meets
manufacturers guidelines
16
Fortronic Oxford – March 2014 | OS S NE | CB
Investigating LED lifetime and reliability:
Encapsulation and coatings
Potential issues and their main causes...
– Solder joint fracture and / or light degradation
• Severe mismatch between the coefficient of thermal expansion (CTE) of the encapsulate and the PCB or
expansion / contraction during curing places severe force on LED
– Reduction in light output and / or large colour shift (white)
• Encapsulate / coating releases chemicals when curing that are detrimental to LED die
• VOCs used in PCB production trapped between LED and encapsulate / coating which degrade when
subject to short wavelength radiation (blue to green)
– Reduction in light output and / or large colour shift (white) within a
relatively short timescale
• High illuminance levels of short wavelength radiation (blue to green) degrades encapsulate transmissivity
or UV trace dyes often found in coatings
– Small colour shift (white) of random LEDs
• Delamination of encapsulate / coating from silicone lens / surface of LED
Minimise the risks….
– Ensure compatibility of encapsulate / coating, mechanically, chemically and optically
– Ensure VOCs are completely evaporated from PCB prior to encapsulation
– If application is sensitive to small colour shifts leave a void between the LED lens
and encapsulate / coating
17
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
Investigating LED lifetime and reliability:
Enclosures and the application environment
Investigating LED lifetime and reliability:
The application environment
Potential issues and their main causes
– Corrosion of silver plated leadframes and / or die
surface leading to rapid light output degradation and
potential catastrophic failure
•
Silicone encapsulation is porous
•
Corrosive gasses are present e.g. Road tunnels, car parks
•
Sulphurous gasses react with silver producing
black Silver Sulphide
– Degradation rate of light output is higher than
expected, decreasing expected lifetime
•
High humidity increases the light output degradation rate of
some types of LED die
Minimise the risks
– Protect the LEDs in a sealed enclosure or by
coating / encapsulation (observe relevant risks!)
– Use a corrosion and humidity resistant LED such as
the OSRAM DURIS P5 or OSLON series
– For LEDs that are not blue, green or white use epoxy
encapsulated types
19
Fortronic Oxford – March 2014 | OS S NE | CB
The enclosure design
Potential issues and their main causes
– Fully sealed enclosures trap any
chemicals released by materials used in
the enclosure construction creating a
micro environment. Certain chemicals
can attack LEDs and cause rapid light
output degradation e.g.
•
Adhesives and their activators
•
Certain rubber gasket materials
•
VOC’s used to clean PCBs
Minimise the risks
– Avoid fully sealed enclosures if possible
– Check the compatibility of the
adhesives, gaskets and materials used
in the construction of your enclosure
with LEDs
20
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
Conclusion:
Is your application doomed to fail?
Conclusion:
Is your application doomed to fail?
Most definitely not!!
– There are thousands of LED applications that, after careful consideration and design
have been operating successfully and without issue for a number of years. Many of
these are in severe and hostile environments.
– Take steps to minimise the risks that have been covered in this presentation
– Consider using only quality LEDs from reputable manufacturers that can support
reliability claims with documented test results
– If you are in doubt, consult your LED supplier’s application team.
– Speak to the suppliers of the materials you plan to use in your enclosure or for
encapsulation. Are they ‘LED friendly’?
– OSRAM OS pride themselves on their applications support. We have over 40 years of
experience both in manufacturing robust and reliable LEDs and the quality issues that
can arise in certain applications.....talk to us
22
Fortronic Oxford – March 2014 | OS S NE | CB
www.osram-os.com
Thank you for your time and attention
Enjoy your day!
We Shape the Future of Light
Visit us @ www.osram-os.com