As strong as the weakest link:

Transcription

1 As strong as the weakest link: Reliability from the LED system perspective Rudi Hechfellner, Dir. Of Applications, Philips Lumileds Mark Hodapp, Senior Application Engineer, Philips Lumileds Design for reliability Agenda Long term LED reliability (from a LED manufacturers point of view) lumen maintenance, wear-out failure probability and long term color stability Methods of scaling single LED behavior to a system with multiple LEDs The impact of LED behavior on a luminaires light output performance Best known practices for designing reliable lighting sub-systems Notice LEDs from different manufacturers will have different reliability profiles Different material systems Different manufacturing methods etc. 1

2 System Reliability LED Reliability R system = R electrical * R connections * R LEDs * R optical * R thermal * R mechanical Reliability Influencing Factors LED design - Electrical parameters - Heat transport - ESD layout - Package Assembly - Storage condition - Soldering process - Handling (pick & place; ESD) System Reliability Customer application - Heat transport - PCB layout and material - ESD protection - Electrical circuit Environment - Ambient temperature - Temperature cycles - Humidity - Pollution - Light radiation Source: OSRAM OS Application Note Reliability and Lifetime July

3 Chronological progression of system failure rate Source: OSRAM OS Application Note Reliability and Lifetime July 08 Design for Reliability Workshop, October 25, 2010 LED Reliability Power LED Reliability Lumen Maintenance Wear-out behavior Color stability Design for Reliability Workshop, October 25,

4 Lumen Maintenance Not all LEDs behave the same way! Data collection following IESNA LM-80 Data projection following IESNA TM21 For LUXEON Rebel Drive current and Junction Temperature affect lumen maintenance. Normalized Light Output Ambient Temperature T A has no confirmed impact on lumen maintenance ln b L70 a 35,000 hour 25,000 hour Cool-White LUXEON Rebel stressed at 85 C, 0.35A (Tjunction 98 C) Normalized to 1 at 24 hours a = e-6 b = ENERGY STAR 35,000 and 25,000 hour limits after 6,000 hours of stress 6x test time* *Philips Lumileds recommends extrapolations of 6x the test time Extrapolated Data = -1.7% at 10,000 hours L70 = 159,000 hours Ta within -5C of Ts, in accordance with LM80. Sample size = 78 units, y-axis error bar = +/- 3 standard d i ti 0.6 1,000 10, ,000 1,000,000 Hours End of Life Failure Analysis LED End of Life failure rates can be modeled using the same general principles as silicon-based semiconductors. Failure rate versus operating time can be determined for several different stress conditions [i.e. different combinations of T J and I F ]. By design, white LUXEON Rebel LEDs fail short. Probability of Failure End of life behavior Catastrophic failure rates LUXEON Rebel using TFFC Temperature 1 > Temperature 2 > Temperature 3 Temperature 1 Temperature 2 Temperature ,000 10, ,000 1,000,000 Hours Curves are 90% LCL worst case. Curve shifts to the left at higher temperatures. 4

5 LED Reliability Drive Current Junction Temp. Lumen Maintenance Failures System Reliability Wear-Out Failures End of Life Rate & Confidence Interval 1.2 LED Lumen Maintenance versus Time (not to scale) LED Com mponent Lumen Mainten nance LED1 LED2 LED3 LED4 LED5 0.0 Hours of Operation 5

6 LED Sub-System Lumen Maintenance versus Time 1.2 LED1 Sys stem Lumen Maintenance e System LM [90%] limit System LM [70%] limit system LED2 LED3 LED4 LED5 0.0 Hours of Operation System Reliability LED Reliability R system = R electrical * R connections * R LEDs * R optical * R thermal * R mechanical Design for Reliability Workshop, October 25,

7 How to build a reliable system Best practices base on our experience Real world examples to make your system more reliable and more robust Solder joint voiding criteria Voids in solder Small open via in thermal pad acts as air vent during reflow X ray image Rebel on FR4 IPC A-610d: Components with Bottom Thermal Plane Terminations Philips Lumileds recommendation : 25% voids max 7

8 LUXEON Rebel solder voids Influence of voids on thermal resistance LUXEON Rebel examples Voids % has no impact on thermal resistance 11 Voids : 0% Voids: ~20% Rth (junct ion to heatsink) (K/W) micron solder joint 100 micron solder joint 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% percentage of voids in solder joint Measured Rth values shows no significant difference versus voiding % Process steps with risk of mechanical stress Reflow Warping during heat cycles (material) Separating the PCB from the panel V-cut Breaking Final assembly Mounting warped PCB Uneven force distribution for fixing PCB Positioning of screws 8

9 Reflow soldering A PCB is a layered stack-up of different materials having different Coefficient of Thermal Expansion (CTE). Temperature cycling can lead to elastic deformation of the PCB (as with a bi-metal ) In the reflow oven, at max temperature the board is bent the most. With cooling down, the PCB will flatten and further stresses components. Note Maximum curvature will depend on board material and geometry. The amount of relaxation depends on the material combination. Reflow oven Bent boards in the reflow oven Relaxation of the board will differ per material choices. Board design The PCB layout influences warping Different copper surface area (top vs. bottom layer) will bend during reflow Top side of PCB with traces Bottom side - one large continuous copper area 9

10 Board design Equal copper areas on top and bottom side balance forces Board won t bend Back side copper islands Separation of boards Separation of the board from the panel has to be done with care Strip 1 Strip 2 Bending in long direction of the PCB Twisted in short direction of the PCB 10

11 De-panelling Routing of Bridges V-cut Mounting the PCB Mounting a bent PCB increases risk of component cracks. This is not limited to LEDs (also applies to resistors, capacitors etc.) 11

12 Screw positions for board assembly Thermal Interface Material (TIM) is often used between board and heat sink (for defined thermal contact) If the holes are positioned on the edges of the board the TIM will be pressed together and the board will bend, especially for thick, soft TIM (A). Board TIM Heat sink If the holes are positioned more to the center the forces will be distributed more equally and the bending of the board is less (B). See also next slide. PCB screw positions experiment PCB test boards with different screw positions. After assembly and removal of the PCB the contact areas of the TIM are visible. For larger distances between the holes the center area did not make contact, the PCB is bent and doesn t have solid thermal contact. TIM material 1 TIM material

13 Secondary optics materials Transparent materials evaluated PMMA (Polymethyl methacrylate) PMMI (Polymethylmethacrylimide) PC (Polycarbonate) PMMA / Lucite PC / DSM PMMI / Evonik PA (Polyamide) COP (Cyclic Olefin Polymers) COC (Cyclic Olefin Copolymer) PA / EMS COP / ZEON COC / Topas Important material parameters For transparent materials the most important parameters are: Maximum operating temperature Can this material be used in this application? Brittleness Important for screw holes or click fingers Optical transmission (over time) Can have influence on the Lumen Maintenance of the application Refractive Index key optical design parameter Material cost Moldability Quality, process time, accuracy 13

14 Sample overview transparent materials PMMA PMMI PA COC COP PC Best choices for secondary optics materials PMMA gives the best overall performance for use in outdoor application where the maximum operating temperature is below 85 C. Based on lens quality (molding process), UV resistance, cost, and molder selection. PMMI gives the best overall performance where the maximum operating temperature is above 85 C. 14

15 Board Assembly - Pick and Place Nozzle Choice Special Feeders Conclusion Simulations around the LED Sub-system are useful to determine system reliability One step further than relying simply on an LM-80 test report LED Sub-system integration is important Mounting, assembly and component placement etc. Understanding this can have a significant impact on the sustainability of the business 15