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Reliability of Level 1 and Level 2 Packaging in Solid-State Lighting Devices

Lynn Davis, PhDFellow, RTI International

December 8, 2016

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AGENDA

§ Background

§ Nomenclature

§ SSL Device Failure modes

§ LEDs and LED Modules– Types and Construction– Failure Modes

§ SSL Device Drivers– Common typologies– Failure modes

§ Conclusions

§ Acknowledgements

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The Impact of Solid-State Lighting (SSL)

From DOE SSL Research R&D Project Plan 2016.

• Solid-state lighting technology is completely changing the lighting industry business model.• Currently built on lamp and ballast

replacement every 2-5 years.• SSL will enable low maintenance operation

for 5 – 20 years.• Limited current infrastructure and knowledge

to support this change.

• Total market penetration is < 6%.

• RTI is working closely with DOE and the lighting industry to address critical market impediments to the adoption of SSL technologies.• Models for reliability & lifetime• Accelerated Life Testing procedures• Technical consulting

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Types of LEDs: Direct Emitters

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Types of LEDs: Phosphor Converted LED (pcLED)

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Blue DirectEmitter Phosphor

Emissions

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Types of LEDs: Hybrid LED

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Red Direct Emiiter

pcLED

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Types of LEDs (Level 1 Packages)

Phosphor + Binder Layer

Silicone lens

High-Power LED HP-LED)

Mid-Power LED (MP-LED)

From Tuttle & McClear, LED Magazine Feb. 2014.

Chip-On-Board LED (COB-LED)

Chip Scale Package LED (CSP)

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Costing Breakout for HPLEDs

U.S. Department of Energy, Solid-State Lighting R&D Plan, June 2016.

• Cost of LEDs has been dropping significantly over the last 5 year.

• For HP-LEDs, significant cost reductions are expected at the wafer level.

• Some reductions expected in HP-LED packaging cost, but of packages as % if total LED costs may rise.

• MP-LEDs follows similar trend but phosphor costs higher and wafer processing costs lower as a % of total costs.

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LED Modules and Arrays (Level 2 Packaging)

FR-4

FR-4 Metal CoreMetal Core

• Provides connection to driver circuit and facilitates integration in product• Part of optical system so white solder mask is usually used• Provides thermal management and protection from mechanical and

environmental influences

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Module Reliability: Impact of Soldering Voiding on LEDs

§ Acoustical microscopy examination of solder joints revealed the presence of voids (< 20%) in the interior of the joint.

§ Potential impact– Reduced heat transfer rate from LED?– Decreased mechanical strength of solder

joint

§ John Pan (Cal Poly) provided data indicates that the effect of voiding on solder thermal performance is negligible for void volumes < 25% and weakly correlated for void volumes > 25%.

§ Cleaning of LED modules post reflow should follow manufacturers guidelines.

Voids in solder joint

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Aging in White Solder Mask

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lute

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lect

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Wavelength (nm)

Initial 4500 hr 75C Operation Bake

Initial

After Aging

Aging of some solder masks

produce:

Luminous flux lossChromaticity shift

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SSL Luminaire Lifetime & Reliability

§Debate over what is more likely to fail in SSL devices

– LEDs vs. electronics

§Usage environments and product expectations can differ greatly (e.g., disposable vs. “appliance” luminaires and lamps)

§ Accelerating failure modes of SSL products in a meaningful way is difficult

What is Life?SSL luminaires do not always fail in a “lights out” fashion as with other lighting sources

Possible SSL failures:• Lights Out Failure – nothing happens

when switch is thrown• Lumen maintenance – lighting levels

reduced below a lower limit• Color shift – Change in color of light• Energy consumption – change in

electrical properties

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Typical Phosphor-Converted LED (pcLED)

CID 1976 Color Space

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Blue Emitter

Yellow Emitter

Yellow Shift

For color shifts along the blue-yellow line, the peak shapes and peak maxima are unchanged, but the relative intensities change.

Blue Shift• Possibly caused by a drop in

yellow emissions, especially if not at phosphor saturation.

• Characterized by large drop in v¢and modest negative shifts in u¢.

Yellow Shift• Possibly caused by an increase in

yellow emissions (e.g., down to greater down conversion) or a drop in blue emissions.

• Characterized by large increase in v¢ and modest positive shifts in u¢.

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Typical Phosphor-Converted LED (pcLED)

CIE 1976 Color Space

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Blue Emitter

Yellow Emitter

For color shifts that deviate from the blue-yellow line, the peak shapes and/or peak maxima do change.

Green Shift• Possibly caused by oxidation of a

nitride phosphor that produces a shift to lower l of phosphor emissions.

• Characterized by a negative shift in u¢ and modest changes in v¢.

Red Shift• Rare for pcLED systems.• Characterized by a positive shift

in u¢ and modest changes in v¢.

Red Shift

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PAR38 Lamp Models§ 32 different LED models

§ Luminous Flux Range: 440 –1530 lm

§ Power: 8.6 – 24.5 W

§ Luminous Efficacy Range: 47 -99 L/W

§ Test started in March 2013.

§ Simple optical design with reliance on clear secondary optics. Minimal use of reflectors.

§ Lumen and chromaticity maintenance dominated by LED behavior.15

12-64 12-66

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60 W Equivalent A-Lamp Models

§ 15 different LED models

§ Rated Luminous Flux Range: 800 –850 lm

§ Rated Power Range: 9.5 – 13.5 W

§ Rated Luminous Efficacy Range: 59 – 86 LPW

§ Test started in January 2014.

§ Complex optical designs to achieve isotropic radiation pattern. Extensive use of diffusers and opaque lenses.

§ Optical plastic degradation may impact lumen and chromaticity maintenance.

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LED Packages Breakout in CALiPER Studies

60W Eq A Lamps PAR38 LampsCALiPER 20.5

HB-LED 7 18

COB LED 0 7

Plastic Leaded Chip Carrier (PLCC) 6 6

Hybrid 1 1

Remote Phosphor 1 0

Total 15 32

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CSM-1 Behavior in PAR38 Lamps

§Characterized by a persistent shift in the blue direction. Follows Blue-Yellow line.§Rate of shift is rapid at first but slows down at time progresses.§LEDs often become more efficiency when first turned on producing more blue photons.§Likely causes of CSM-1 behavior:

– Drop in quantum efficiency of phosphor

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Solais PAR38 Lamp in Extended 45 C Test (PNNL)

Planckian locus

Sample 1

Sample 2

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Sample 5

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CSM-2 Behavior in PAR38 Lamps

§Direction of shift deviates from Blue-Yellow line in the green direction (Du¢ change).§Rate of shift is rapid at first but slows down as time progresses.§Examination of the spectral changes demonstrates that the emission peak of the phosphor is shifting to lower wavelength by < 5 nm.

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CSM-3 Behavior in PAR38 Lamps§ Initial shift is in the blue direction, followed by a reversal to a yellow shift.§Time of the reversal varies depending on operation conditions and LED.

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v'u'

12-100 PAR38 Lamp in Extended 45 C Test (PNNL)

Planckian locusSample 1Sample 2Sample 3Sample 4Sample 5

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B

C

BB/BA BC/BB YB/YA YC/YB

Lamp model

Average Average Average Average

12-75 1.026 0.974 0.978 0.98312-81 1.010 0.963 0.923 0.977

12-100 1.019 0.779 1.001 0.928B-Blue peak max Y-Yellow phosphor max

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Possible Cause of CSM-3 Behavior§High temperature & CTE mismatches produces stress at die-phosphor interface.§High temperature can also degrade the mechanical compliance of binder in phosphor layer.§Result is cracking and delamination in the phosphor layer which changes the optical path of blue photons.

Reference, DOE Webinar, LED Color Stability – 10 Important Questions, 2014.

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CSM-4 Behavior in PAR38 Lamps§Short, initial shift in the blue direction, followed by a reversal to a yellow shift, followed by a second blue shift.§Time of the reversal varies depending on operation conditions and LED.§CSM-4 behavior was only observed in lamps with PLCC LED packages suggesting that it is associated with some plastic molding resins.

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Impact of Aging on some MP-LEDs

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Initial 14,000 hr

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Summary of Color Shift Behavior of PAR38 Lamps

§The major CSM for HB-LEDs is CSM-3.§The major CSM for PLCC packages is CSM-4.§CSM-1 and CSM-2 is found in some HP-LED & COBs. Possible that CSM-3 behavior will occur with longer test time or more aggressive conditions.

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Summary of Color Shift Behavior of Retail A Lamps§Much less reversal in color shift direction.

– Test duration– Power per LED is lower

§Small green shift is more evident in the first 24 hrthan in PAR38s.§Major CSM for both HPLED and PLCCs is CSM-1.§One instance of CSM-4 in a PLCC package.

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CSM-1 CSM-2 CSM-3 CSM-4 Complex

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Color Shift Mode (CSM)

Color Shift Modes for CALiPER Retail A Lamps

COB

HPLED

PLCC

Hybrid

Remote

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System Design and Performance

DMX Central Control Example:q Schedulingq Eventsq Reportingq Administrative Tasks

Ambient: Dim to 75% = 0.46 w/ft2Whiteboard: 100% On

51 fc

51 fc

CCT = 3825 K

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Cost Estimates of LED Lighting Devices

U.S. Department of Energy, Solid-State Lighting R&D Plan, June 2016.

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Summary of Color Shift in LEDs§Color shift at the LED package

level depends on many factors including:

– Package type– Materials of construction– Operating conditions

§ In many cases, initial color shift is a small blue shift. This may be the only shift observed under very mild conditions or short times.§ At higher operating conditions or

longer times, a yellow shift occurs and will continue for some LED packages designs.

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SSL Driver StructureInput Power

Filter and Condition

ComponentsFuse

CapacitorsInductors

MOV

AC to DC Conversion

(Rectify)

ComponentsDiode BridgeCapacitorsResistorsDiodes

Shaping and Power

Factor Correction

ComponentsControl ICCapacitorsInductorsResistors

Switched Mode

Control (Regulation)

ComponentsControl ICMOSFETInductor

CapacitorDiodes

Transformer

Final Output Power

Filtering

ComponentsElectrolyticsFilm CapsInductors

• For highest efficiency, most SSL drivers are switched mode power supplies containing several electrical circuits.

• The susceptibility of each circuit to voltage transients can differ widely. Impacted by design of Input Power Filter and Conditioning circuits.

• Failure in other circuits can be manifested as “lights out” failure, flickering, or reduced luminous flux.

• Overall product reliability is only as good as the weakest link.

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LED Drivers – Many Typologies: Buck, Flyback, Boost, …

Combination of through-hole and SMT technologies

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Failure Modes Analysis – 75/75

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MOSFET Electrical Measurements– Failure (75/75)

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I D(m

A)

VGS (V)

New - A 147 75C/75% 146 75C/75% 144 75C/75%

4500 hr.

Leakage CurrentIncrease

#144

• Electrical analysis shows large increase in leakage current.• C-SAM shows catastrophic damage likely caused by excess currents.• Possible TDDB mechanism.

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Change in MOSFET Switching Waveforms

Control 4000 hours of 7575

Degradation of PFC caps and inductors produced higher level of “ringing” and transients in aged device.

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Electrolytic Capacitors

§ Often cited as a leading cause of failure in SSL drivers.

§ Common failure models for electrolytic reliability include:

– Voltage stress (Vop/Vrated)– Voltage transients– Temperature

§ Driver manufacturers are aware of the limitations of electrolytic capacitors and take appropriate actions.

– Derating of T & V (often 2X or more)– High quality caps (105 C rating min.)– Use in low voltage and/or low ripple circuits.

Use film caps where possible.– Avoid placement near heat sources on either

side of the board.

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Overall Conclusions

§ Even under overstress conditions, LED luminaires exhibit a high level of robustness.§ The reliability of LED luminaires should be considered

from a systems perspective. The “weak link” determines reliability.

LED luminaire reliability involves more than the LEDs

§ There have been several collaborative efforts among industry participants to share information on this critical issue.

Collaborative efforts continue to gain momentum

§ Still an opportunity for additional voices from the industry to help “understand the issues surrounding true lifetime and reliability.”

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Acknowledgements

§ This material is based upon work supported by the Department of Energy under Award Number DE-EE0005124.

§ Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.