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

Transcription

1 Reliability of Level 1 and Level 2 Packaging in Solid-State Lighting Devices Lynn Davis, PhD Fellow, RTI International December 8, RTI International is a registered trademark and a trade name of Research Triangle Institute.

2 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 2

3 The Impact of Solid-State Lighting (SSL) 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%. 3 From DOE SSL Research R&D Project Plan 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

4 Types of LEDs: Direct Emitters Spectral Radiant Flux (W/nm) Wavelength [nm] 4

5 Types of LEDs: Phosphor Converted LED (pcled) Spectral Radiant Flux (W/nm) Blue Direct Emitter Phosphor Emissions Wavelength (nm) 5

6 Types of LEDs: Hybrid LED Spectral Radiant Flux pcled Red Direct Emiiter Wavelength [nm] 6

7 Types of LEDs (Level 1 Packages) High-Power LED HP-LED) Silicone lens Chip-On-Board LED (COB-LED) Phosphor + Binder Layer From Tuttle & McClear, LED Magazine Feb Mid-Power LED (MP-LED) Chip Scale Package LED (CSP) 7

8 Costing Breakout for HPLEDs 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. 8 U.S. Department of Energy, Solid-State Lighting R&D Plan, June 2016.

9 LED Modules and Arrays (Level 2 Packaging) FR-4 Metal Core Metal Core FR-4 9 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

10 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%. Voids in solder joint 10 Cleaning of LED modules post reflow should follow manufacturers guidelines.

11 Aging in White Solder Mask 100 Absolute Reflectance (%) After Aging Initial Aging of some solder masks produce: Luminous flux loss Chromaticity shift Wavelength (nm) Initial 4500 hr 75C Operation Bake 11

12 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 12

13 Typical Phosphor-Converted LED (pcled) v' 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 Blue Emitter u' CID 1976 Color Space 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.

14 Typical Phosphor-Converted LED (pcled) v' Yellow Emitter Red Shift 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 Blue Emitter u' Red Shift Rare for pcled systems. Characterized by a positive shift in u and modest changes in v. CIE 1976 Color Space 14

15 15 PAR38 Lamp Models 32 different LED models Luminous Flux Range: lm Power: W Luminous Efficacy Range: L/W Test started in March Simple optical design with reliance on clear secondary optics. Minimal use of reflectors. Lumen and chromaticity maintenance dominated by LED behavior.

16 60 W Equivalent A-Lamp Models 15 different LED models Rated Luminous Flux Range: lm Rated Power Range: W Rated Luminous Efficacy Range: LPW Test started in January Complex optical designs to achieve isotropic radiation pattern. Extensive use of diffusers and opaque lenses. Optical plastic degradation may impact lumen and chromaticity maintenance. 16

17 LED Packages Breakout in CALiPER Studies 60W Eq A Lamps PAR38 Lamps CALiPER 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

18 CSM-1 Behavior in PAR38 Lamps 18 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 Solais PAR38 Lamp in Extended 45 C Test (PNNL) y Planckian locus Sample 1 Sample 2 Sample 3 Sample 4 Sample x

19 CSM-2 Behavior in PAR38 Lamps 19 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.

20 20 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. B B /B A B C /B B Y B /Y A Y C /Y B Lamp model Average Average Average Average B-Blue peak max Y-Yellow phosphor max v' PAR38 Lamp in Extended 45 C Test (PNNL) C A B Planckian locus Sample 1 Sample Sample 3 Sample 4 Sample u'

21 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,

22 22 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.

23 Impact of Aging on some MP-LEDs Spectral Radiant Flux (W/nm) Wavelength (nm) Initial 14,000 hr 23

24 24 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.

25 25 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 hr than in PAR38s. Major CSM for both HPLED and PLCCs is CSM-1. One instance of CSM-4 in a PLCC package. Number of LED Lamp Models Color Shift Modes for CALiPER Retail A Lamps CSM-1 CSM-2 CSM-3 CSM-4 Complex Color Shift Mode (CSM) COB HPLED PLCC Hybrid Remote

26 System Design and Performance Ambient: Dim to 75% = 0.46 w/ft 2 Whiteboard: 100% On CCT = 3825 K 51 fc 51 fc 26 DMX Central Control Example: q Scheduling q Events q Reporting q Administrative Tasks

27 Cost Estimates of LED Lighting Devices 27 U.S. Department of Energy, Solid-State Lighting R&D Plan, June 2016.

28 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. 28 At higher operating conditions or longer times, a yellow shift occurs and will continue for some LED packages designs.

29 SSL Driver Structure Input Power Filter and Condition AC to DC Conversion (Rectify) Shaping and Power Factor Correction Switched Mode Control (Regulation) Final Output Power Filtering Components Fuse Capacitors Inductors MOV Components Diode Bridge Capacitors Resistors Diodes Components Control IC Capacitors Inductors Resistors Components Control IC MOSFET Inductor Capacitor Diodes Transformer Components Electrolytics Film Caps Inductors 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. 29 Overall product reliability is only as good as the weakest link.

30 LED Drivers Many Typologies: Buck, Flyback, Boost, Combination of through-hole and SMT technologies 30

31 Failure Modes Analysis 75/75 31

32 4 MOSFET Electrical Measurements Failure (75/75) hr. # I D (ma) Leakage Current Increase V GS (V) New - A C/75% C/75% C/75% 32 Electrical analysis shows large increase in leakage current. C-SAM shows catastrophic damage likely caused by excess currents. Possible TDDB mechanism.

33 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. 33

34 Electrolytic Capacitors Often cited as a leading cause of failure in SSL drivers. Common failure models for electrolytic reliability include: Voltage stress (V op /V rated ) 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. 34

35 35 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.

36 Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-EE 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. 36