AhklikAs strong as the weakest link

IEEE Santa Clara Valley CPMT Chapter February 8, 2012

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A h k li kAs strong as the weakest link:

Reliability from the LED system tiperspective

Rudi Hechfellner, Dir. Of Applications, Philips LumiledsMark Hodapp, Senior Application Engineer, Philips Lumileds

Design for reliability

AgendaLong term LED reliability (f LED f t i t f i )– 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

NoticeLEDs from different manufacturers will have different reliability profiles

– Different material systems

– Different manufacturing methods etc.


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System Reliability ≠ LED Reliability

Rsystem = Relectrical * Rconnections * RLEDs * Roptical * Rthermal * Rmechanical

Reliability – Influencing Factors

LED design- Electrical parameters

Heat transport

Customer application- Heat transport

PCB layout and material

SystemReliability

- Heat transport- ESD layout- Package

AssemblySt diti

- PCB layout and material- ESD protection- Electrical circuit

EnvironmentAmbient temperature- Storage condition

- Soldering process- Handling (pick & place;ESD)

- Ambient temperature- Temperature cycles- Humidity- Pollution- Light radiation

Source: OSRAM OS Application Note Reliability and Lifetime July ‘08


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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 Power LED Power LED ReliabilityReliabilityReliability

Lumen Maintenance Wear-out behaviorWear-out behavior Color stabilityColor stabilityWear-out behavior Color stabilityMaintenance WW C yC yW C y



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Lumen Maintenance

Not all LEDs behave the same way!

Cool-White LUXEON Rebel stressed at 85°C, 0.35A (Tjunction 98°C)

Normalized to 1 at 24 hours1.2

b0.7ln

L70

a = -2.2816e-6b = 1.0054

*Philips Lumileds recommends extrapolations

Data collection following IESNA LM-80

Data projection following IESNA TM21

For LUXEON Rebel

• Drive current and Junction Temperature affect lumen maintenance.

• Ambient Temperature TA has no confirmed impact on lumen maintenance 0 7

0.8

0.9

1.0

1.1

No

rma

lized

Lig

ht

Ou

tpu

t

Extrapolated Data = -1.7% at 10,000 hours

ENERGY STAR 35,000 and 25,000 hour limitsafter 6,000 hours of stress

aL70

35,000 hour25,000 hour

6x test time*

of 6x the test time

maintenance.

0.6

0.7

1,000 10,000 100,000 1,000,000

Hours

Ta within -5C of Ts, in accordance with LM80.Sample size = 78 units, y-axis error bar = +/- 3 standard d i ti

L70 = 159,000 hours

End of Life Failure Analysis

• LED End of Life failure rates can be modeled using the same general principles as silicon-based semiconductors

Catastrophic failure rates LUXEON Rebel using TFFC

1

End of life behavior

semiconductors.

– Failure rate versus operating time can be determined for several different stress conditions [i.e. different combinations of TJ and IF].

• By design, white LUXEON Rebel LEDs fail short. 0.001

0.01

0.1

Pro

bab

ilit

y o

f F

ailu

re

Temperature1

Temperature2

Temperature3

Temperature1 > Temperature2 > Temperature3

0.0001

100 1,000 10,000 100,000 1,000,000

Hours

Curves are 90% LCL worst case. Curve shifts to the left at higher temperatures.


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LED Reliability

SystemReliability

Lumen Maintenance Failures

Wear-Out Failures

Drive Current

Junction Temp

End of Life Rate&Confidence Interval

Temp.

1.0

1.2

nan

ce

LED Lumen Maintenance versus Time (not to scale)

0.4

0.6

0.8

mp

on

ent

Lu

men

Mai

nte

n

LED1

LED2

LED3

LED4

LED5

0.0

0.2

LE

D C

om

Hours of Operation


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1.0

1.2LED1

LED2

LED3

LED4

LED5e

LED Sub-System Lumen Maintenance versus Time

system

0.4

0.6

0.8

stem

Lu

men

Mai

nte

nan

ce

System LM [90%] limit

System LM [70%] limit

0.0

0.2

Hours of Operation

Sys

System Reliability ≠ LED Reliability

Rsystem = Relectrical * Rconnections * RLEDs * Roptical * Rthermal * Rmechanical



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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

X ray image Rebel on FR4

Small open via in thermal pad acts as air vent during reflow

• IPC A-610d:– 8.2.14 Components with Bottom Thermal Plane

Terminations

• Philips Lumileds recommendation : 25% voids max


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LUXEON Rebel solder voids

• Influence of voids on thermal resistance

• LUXEON Rebel examples

– Voids % has no impact on thermal resistance

8

9

10

11

ion

to

hea

tsin

k) (

K/W

)

50 micron solder joint

100 micron solder joint

Voids: ~20%

7

8

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%percentage of voids in solder joint

Rth

(ju

nct

Voids : 0%Measured Rth values shows no significant difference versus voiding %

Process steps with risk of mechanical stress

• ReflowW i d i h t l ( t i l)– Warping during heat cycles (material)

• Separating the PCB from the panel – V-cut – Breaking

• Final assemblyMounting warped PCB– Mounting warped PCB

– Uneven force distribution for fixing PCB• Positioning of screws


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Reflow soldering

• A PCB is a layered stack-up of different materials having different Coefficient of Thermal Expansion g p(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

Reflow oven

Bent boards in the reflow ovenstresses components.

• Note– Maximum curvature will depend on board material and

geometry.– The amount of relaxation depends on the material

combination.

Relaxation of the board will differ per material choices.

Board design

• The PCB layout influences warping

• Different copper surface area (top vs bottom layer) willDifferent copper surface area (top vs. bottom layer) will bend during reflow

Top side of PCB with traces

Bottom side - one large continuous copper area


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Board design

• Equal copper areas on top and bottom side balance forces B d ’t b d– 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


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De-panelling

V-cutRouting of Bridges

V cu

Mounting the PCB

• Mounting a bent PCB increases risk of component cracks.– This is not limited to LEDs (also applies to resistors, capacitors

etc.))


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Screw positions for board assembly

• Thermal Interface Material (TIM) is often used between board and heat sink

BoardTIMHeat 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).

– If the holes are positioned more to the center the forces will be distributed more equally and the bending of themore 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.

Af bl d l f h PCB h• 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 2


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Secondary optics materials

Transparent materials evaluated

• PMMA (Polymethyl methacrylate)• PMMA (Polymethyl methacrylate)

• PMMI (Polymethylmethacrylimide)

• PC (Polycarbonate)

• PA (Polyamide)

• COP (Cyclic Olefin Polymers)

PMMA / Lucite PMMI / EvonikPC / DSMPMMA / Lucite PMMI / EvonikPC / DSM

• COC (Cyclic Olefin Copolymer)

COC / TopasPA / EMS COP / ZEON COC / TopasPA / EMS COP / ZEON

Important material parameters• For transparent materials the most important parameters are:

M i i– 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


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Sample overview transparent materials

PAPMMA PMMI PCCOC COP

Best choices for secondary optics materials

• PMMA gives the best overall performance for i td li ti h thuse 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.


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Board Assembly - Pick and Place

• Nozzle Choice

• Special Feeders

Conclusion

• Simulations around the LED Sub-system are useful to determine system reliabilityuseful 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

etcetc.

• Understanding this can have a significant impact on the sustainability of the business

AhklikAs strong as the weakest link

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