hbLEDSW7.10 © 2010 TechSearch International, Inc.

High Brightness LEDs: Assembly and Materials Challenges/Opportunities

E. Jan VardamanDr. Frank Bachner

TechSearch International, Inc.

www.techsearchinc.com

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Overview• HB LEDs have high

potential growth rates, BUT to reach the full market potential

– Must be reliable– Must have good

performance (lumens/$, lumens/watt)

– Must have lower prices

• Key patents and technology around the diode itself, phosphor, LED epitaxial layer, electrical contact material on die

• Material selection, packaging, and assembly are becoming the key differentiators in LED performance, reliability, and cost

Source: OSRAM GmbH

hbLEDSW7.10 © 2010 TechSearch International, Inc.

High Brightness LED Module Structures• Materials to fabricate LEDs

– InGaN– AlInGaP

• LED mounted on various substrates– Leadframe– Silicon– Ceramic (AlN, Al2O3)– Metal core board

• Die attach• Interconnect• Application of phosphor• Encapsulation of LED

– Epoxy mold– Silicone

• Focus light– Glass lens– Plastic lens

• Mount LED package or mount LED directly to metal core board

• Cooling

Source: LUMILEDS

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Issues for LED Packaging and Assembly

• There are no standard packages– No standard footprints

• Optimize (try not to degrade) optical performance of LED– Reflectivity– Transmissivity– Index of Refraction

• Thermal dissipation (how do we get the heat out more efficiently)– Die attach material– Metals used in package (leadframe, metal core board, AlN,

alumina)• Reliability of package over lifetime

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Thermal Issues• LED emits light, but heat is not radiated• Controlling the LED Tj to 50 to 100°C is critical to performance

– Change in junction temperature changes the wavelength of emitted light (making color control difficult)

– With high Tj, the active regions of LED and electrodes of the device can degrade—leading to changing light output of the device

• Good thermal dissipation is required because heat can degrade the phosphors in the LED package as well as packaging materials

– Yellowing of encapsulation materials– TCE and Tg of materials can change (reduces lumens and decreases

efficiency)

Source: CoreSEM

hbLEDSW7.10 © 2010 TechSearch International, Inc.

LED Lifetime with Junction Temperature

If the junction temperature changes from 74oC to 63oC, the lifetime increase from 15,000 to 40,000 hours

More light output in same used hours

More longer time if junction temp decrease

Source: CoreSEM

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Material Selection Key in Thermal Dissipation

LED package(Die attach + Heat slug)

Metal plate

TIM

Heat sink

SolderPCB bonding pad (Cu)

LED package

Metal Core PCB(CoreSEM)

Heat sinkH

eat f

low

Thermal bottleneck(dielectric layer)

Source: CoreSEM

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Copper Circuit Layer

CoreSEM’s Metal PCB (CoolRATE®)

High thermal conductive

dielectric layer

Advantage : High thermal transfer characteristics of dielectric layer (4~10 W/mK thermal conductivity grade)

Very low thermal resistance with thin dielectric layer (50~120um)High operating temperature and good adhesionEasy control of dielectric breakdown voltageEasy thickness control from 0.5 mm ~ 2 mm

Aluminum plateAluminum plate Aluminum plate

Source: CoreSEM

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Substrate Solutions for HB LEDs

Copper Signal LayerThermal DielectricCopper or Aluminum Plate

Single Layer

Copper Signal LayerThermal Dielectric

Copper or Aluminum Plate

Two-Layer

Copper Signal LayerThermal Dielectric

Copper Signal LayerThermal DielectricCopper or Aluminum Plate

Copper Signal LayerThermal Dielectric

Copper or Aluminum Plate

Multilayer(one of many variants)

Copper Signal LayerThermal DielectricCopper Signal Layer

Thermal Dielectric

Most Common

Less Common Thermal DielectricCopper Signal Layer

LTCCMLamina

DBC

IMSBergquist

MCS SteelHeatron

Graphtech Z Spreader

MCS AluminumFerro/Heatron

hbLEDSW7.10 © 2010 TechSearch International, Inc.

MCS on Aluminum for Thermal Cooling

Aluminum 3003 alloy( 4 to 5 mm thickness)

DL13-168

ConductorCN33-479 (lead free)

@ 500°C (12 min) peak firing

CN50-101 @580oC

Dielectrics3 coats & 3 fire

@ 600°C ( 12 min) peak firing

DL13-169

DL13-169

OverglazeOG15-313@ 500°C (12 min)peak firing

Aluminum thickness of 4-5 mm prevents warping

hbLEDSW7.10 © 2010 TechSearch International, Inc.

T-clad Metal Core Substrate

Source: Bergquist

hbLEDSW7.10 © 2010 TechSearch International, Inc.

hbLEDSW7.10 © 2010 TechSearch International, Inc.

One Big Die

• Some companies use one large die– Lighting applications– Die range from 0.7mm to 1.5 mm on a side– Power dissipations 1-4 Watts

• Packaging structure vary– LED package– Package mounted on a metal core board

Source: Lumileds

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Trend to COB• Many companies mounting LEDs directly to the metal core board

– Simpler structure– Less packaging materials– Fewer thermal interfaces

• Small die size (0.2-0.4mm, 0.1-0.3 W)• Example with multiple LEDs

– LEDs are wire bonded– Security bond with stud bump

Source: T. Onishi

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Problems with LED Packaging Materials

DelaminationDisconnectionCrack

Reduced LumensShort circuitVisual defects

Moisture Reflow

DelaminationDisconnection

Reduced LumensShort circuit

Heat Cycle-40˚C~100˚C/200cyc

Resin color shiftMetal oxidation

Reduced LumensIncreased current leakage

Biased Temp. & Humidity85˚C/85%/20mA/300hr

Chip degradationReduced LumensLow Temp. Bias

Resin color shiftReduced LumensRoom Temp. Bias25˚C/20mA/1000hr

CauseProblemTest

Source: SANYU REC Co., Ltd.

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Packaging Material Needs

• Encapsulants (Silicone or Epoxy) material properties affect the number of lumens out of the package– Materials with high reflectivity– Materials with a high index of refraction– Materials must stable with exposure to elevated temperatures, UV, and

other wavelength radiation• Lenses (Glass or Plastic)

– High transmission rate– High index of refraction– Low moisture absorption– Low thermal expansion

• Die attach materials– Solder (Pb-free)

• Thermal Interface Materials– Bond well – Conduct well– No degradation with age or use

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Current Encapsulation Materials

Poor adhesionSurface tackinessHigh moisture permeabilityExpensivePoor physical strength

Poor UV light durabilityColor change due to heatPoor heat shock stability

Cons

Good radiation durabilityHeat stableGood electrical propertiesGood shock stability

Good adhesionGood electrical propertiesPhysically toughFairly good heat stabilityLess expensive

Pros

SiliconeEpoxy

Source: SANYU REC Co., Ltd.

hbLEDSW7.10 © 2010 TechSearch International, Inc.

Conclusions• Industry is experiencing rapid growth, but need reliable, long lasting, low cost

products• No standard packages—footprints of LED packages not even the same• Continued room for improvement in packaging and assembly

– New designs– New materials– New assembly methods

• Understanding optical, thermal, and material science trade-offs– Key to making reliable, low-cost products

Source: T. Onishi