DOE Advanced Materials Panel: High Operating … · High Operating Temperature Interconnects for...

an Alent plc Company

v. 3

DOE Advanced Materials

Panel

High Operating

Temperature Interconnects

for LED Applications

Ravi M. Bhatkal, Ph.D.

Vice President, Energy Technologies

Alpha, an Alent plc Company


Outline

• Introduction

• Key issues in LED lighting

• Role of high performance, high reliability

electronic and thermal interconnects

• Key platforms to address high thermal, high

reliability requirements

– High temperature creep fatigue resistant solders

– Sintered die attach materials

• Conclusions


Hierarchy of LED Lighting System

Level 0

Level 1

Level 2

Level 3

Level 4

Level 5

Substrate / Wafer / Device

Package

Light Engine (POB)

Luminaire/Module

Power Driver

Sensing and Control Systems

LED Chip

Die Attach Material

Package Submount

LED Package

Substrate / Board

Solder Print

LED Package

Substrate / Board

Solder Print


Key Issues in LED Lighting

• Efficiency and Brightness Improvement

– Lumens per Watt

• Higher efficiency of electrical power to light and lumen output per package reduce energy consumption.

• High heat dissipation

• Good light extraction and directionality

• Reliability and Lifetime Improvement

– Lumen maintenance over life

• Reliability of the package and the system determines lifetime of the package and system, and lowers maintenance costs.

• Vibration and high temperature thermal cycling performance

• Cost/Lumen Over the Lifetime

– $/Lumen and Lumens/$

• Increased efficiency and brightness, and increased reliability and lifetime reduce the cost per lumen over life.


• Convey power and information efficiently and

reliably over the rated life.

• Get the heat out faster and reliably over the

rated life.

• Enable more light output, consistently, for

longer time for the same package and system

footprint.

Role of Electronic and Thermal Interconnects


Key Issues in High

Temperature Operation


Ceramic submount

CTE ~ 5-7 ppm/K

PCB CTE (ppm/K)

Stainless Steel ~ 12

Copper ~ 16

Aluminum ~ 25

Different expansion/contraction with temperature swings

Stress Relief via Strain in:

Solder Joint

Dielectric

Failure of Solder Joints Via

Thermo-Mechanical Cycling

7


Thermo-Mechanical Fatigue of Solder Joints

•Cycling of thermal stresses with each temperature cycle

•Progressive mechanical fatigue/degradation of solder

joint

•Loss of function via loss of mechanical/electrical/thermal

contact

8

Stress Strain Hysteresis Loop of Solder


Impact of Materials Stack on Reliability

• Reliability/lifetime is dependent on:

– The material stack selected (submount /package,

solder, board material and dielectric, and respective

geometries),

– The manufacturing process conditions and

– The use environment (temperature swings and

harshness of conditions)


Materials Stack Impact e.g. Al MCPCB

• Key materials stack variables

– ΔCTE between the package/submount and the board material

• E.g. ΔCTE between AlN submount and Al MCPCB is 20

– Geometry of the materials stack

• Length scale of the package/submount

– Larger package, higher stress

• Thickness of solder joint

• Thickness of dielectric

– Moduli of materials

• Moduli of submount, solder and dielectric


Creep Resistant Alloy Features

• Solder joints with improved mechanical and thermal fatigue/creep and vibration resistance and bond line uniformity, can be accomplished by a micro-structural control approach.

• Advanced alloys have been developed with special additives for improved thermal stability.

• Preform, solder paste and wire


LED Package-on-Board Reliability

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200

pe

rce

nt

of

po

pu

lati

on

wit

h t

he

ligh

t st

ill o

n

Thermal Cycles (-40C to 125C, 30 minute dwell time)

Failure Rates of Thermally Cycled LEDs

Dielectric A, Solder A

Dielectric B, Solder A

Dielectric A, Solder B

Dielectric B, Solder B

Creep Resistant Alloy

•High power ceramic submount LEDs assembled on Al MCPCBs with solder pastes with creep

resistant alloy and SAC305 alloy

•Assemblies thermally cycled -40 to 125C with 30 min dwell time

Creep resistant

alloy improves

reliability


What Does This All Mean?

• From the test results, one can conclude that:

– The creep resistance of the solder is a significant factor in

minimizing failures in solder joints due to strains incurred in

thermal cycling.

– The relationship between the modulus of the dielectric to the

modulus of the solder over the temperature range in the thermal

cycle can be an effective way to manipulate the strain away from

the solder joint in thermal cycling, hence reducing failures due to

solder joint fatigue.


Emerging Technology

Sintering Materials for LED

Die Attach


Ag Properties Relative to Other Materials

• Sinterable Ag materials can be sintered at low temperatures (180-300C) with or without pressure to form an Ag interface

• After processing, sintered Ag acts as bulk Ag with a melt point of 962°C

• Thermal conductivity can be as high as 200 – 250 W/m-K

• No intermetallic phases formed, pure diffusion bond

Sintered Ag Technology for Die Attach-Basics

Melting

Point

Thermal

Conductivity

Electrical

Resistivity

Material Composition °C W/m°K µΩcm

Silver 100Ag 962 419 1.6

High Lead 92.5Pb/2.5Ag/5Sn 287-296 26 17

SAC 305 96.5Sn/3Ag/0.5Cu 217-228 55 14.5

Au/Sn 80Au/20Sn 280 57 16

Melting Range Density CTE

Tensile

Strength Modulus

Thermal

Conductivity

Electrical

Resistivity

Material ºC g/cm3 ppm/ºC Mpa Gpa W/mºK µΩcm

Silver 962 10.5 20 140 76 419 1.6

88Au/12Ge 356 14.7 13 185 69 44 -

92.5Pb/2.5Ag/5In 300 11.0 25 31 - 25 -

92.5Pb/2.5Ag/5Sn 287-296 11.0 29 29 - 25-27 -

88Pb/10Sn/2Ag 268-299 10.8 29 23 - 27 8.5

80Au/20Sn 280 14.5 16 276 59 57 16.0

96.5Sn/3.5Ag 221 7.4 30 38 50 58 12.5

96.5Sn/3Ag/0.5Cu 217-228 7.4 22 40 30 55 14.5

During sintering, particles to coalesce and form solid conductive structures


Silver sintering film advantages:

• No die tilt, highly controlled and uniform

bond line.

• No bleed-out of the die attach material,

ensuring die-footprint-conformal die attach.

• Placement of an array of die closer

together without movement or "die float".

• Ultra-high thermal conductivity, enabling

thermal resistance reduction, and therefore

lower junction temperatures, or higher

drive currents.

• Very high die shear strength and thermal

cycling/thermal shock reliability.

Ag Sinter Film Technology for Die Attach

Sintered Silver Film

• Uniform, dense microstructure

• Low modulus joint

• No intermetallic formation, pure diffusion interface


Sintered Ag - Reliability

Sn3.5Ag

Ag Sinter

% B

on

d A

rea I

nta

ct

% B

on

d A

rea I

nta

ct

Ag Sintering Better in

Thermal Cycling

Sintered Ag

SAC305


• Pressureless Sintering

• Die-Shear Strength: >25MPa

• Thermal Cycling: Passes 1000 cycles with -40 to 125C

• Passes 1000 hrs aging at 175C with no change in die-shear strength

• Thermal Shock : Passes 1000 cycles from -55 to +125C

• Passes Moisture Sensitivity Level (MSL) Level 1

• Applicable to Both Vertical and Flip Chip Die

CSAM Images - Thermal Shock Test

Ag Sinter Paste - Vertical & Flip Chip LED Attach


Conclusions

• Electronic and thermal interconnects are present

in all levels of LED manufacturing

• Critical in efficient, reliable conveyance of power,

information and heat

• Traditional solder alloys were not designed for

high temperature operating conditions where

thermo-mechanical fatigue is dominant failure

mechanism

• Emerging technologies in solder alloys and

sintered materials are beginning to address this

issue


Acknowledgments

• Bawa Singh

• Oscar Khaselev

• Ranjit Pandher

• Shamik Ghoshal

• Sathish Kumar

• Siuli Sarkar

• Mike Marczi

• Julien Joguet

• Gustavo Greca

• Monnir Boureghda


Thank You

Questions?

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