Chip Scale Review - July August 2018
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Chip Scale Review July • August • 2018

[ChipScaleReview.com]

Die-attach materials and LED functional performance

By Gyan Dutt, Nicholas Herrick, Sathish Kumar, Pavan Vishwanath, Ranjit Pandher

[Alpha Assembly Solutions]

ie attach materials play a key

role in functional performance

and reliability of light-emitting

diodes (LEDs). Selection of a suitable

die-attach material for a particular chip

structure and application depends on several

considerations. These include performance

(light output and thermal dissipation at

operating temperature), reliability (lumen

maintenance and mitigation of thermo-

mechanical stresses over lifetime), packaging

process (equipment, throughput and yield)

and cost factors. Eutectic gold-tin, silver-

filled epoxies, solder, silicones and sintered

materials have all been used for LED die

attach. Often, use of a particular material

technology results in trade-offs between

different performance attributes.

In this study we used different die-attach

materials (solder, silver epoxy and silicone-

based) to package lateral, vertical and flip-chip

LEDs. For vertical and flip-chip structures

(which require electrically conductive die

attach), solder, silver epoxy and silver-filled

silicone materials were used. Lateral dies

(on nonmetallized sapphire substrates) were

bonded with silicones (both conductive and

insulated) as well as silver epoxy. The optical

and electrical performance of the assembled

packages was then characterized by junction

temperature, luminous flux and efficacy

measurements in an integrating sphere

(according to IES LM-79-08).

The paper describes these results and

discusses the effect of die-attach material

properties and chip structure on LED

functional outputs. The results clearly

indicate that while optical characteristics of

the die-attach material dominate the light

output of lateral LEDs, junction temperature

management (via die-at tach thermal

conductivity) is important for achieving

optimal functional performance for higher

power vertical and flip-chip LEDs.

Introduction

LED die packaging still accounts for

~1/3 the cost of the packaged LED. The last

few years have seen several trends in LED

packaging, namely: 1) flip-chip and chip-scale

packaging (CSP) adoption; 2) growth of a

packaged mid-power segment (0.2-0.5W) with

lateral dies and chip-on-board (COB) modules;

and 3) increased focus on infrared (IR) and

ultraviolet (UV) LEDs for new applications

(with higher power vertical structures). All

these trends, with different chip structures, are

driven by underlying need for higher efficacy

(lm/W), lower cost (lm/$) and longer lifetime

in a smaller form factor.

Die attach is the first layer that comes into

contact with the LED die and its thermal

performance and stability has a direct

impact on LED light output, light extraction

and lumen maintenance over time. The die-

attach material and (more importantly) the

process together have a significant effect on

the cost of ownership of the package (and the

light engine).

With several material technologies available

for die attach (like eutectic gold-tin/Au80Sn20,

silver-based conductive adhesives, solder- and

silicone-based adhesive), selecting a suitable

material for a particular chip structure (lateral,

vertical and flip chip) is essentially about

making trade-off decisions between different

process or performance attributes.

In this study we attached different structure

LEDs (lateral, flip chip, and vertical) with

different die-attach materials (silver epoxy,

solder and silicone). These packages were then

subjected to industry standard optical and

electrical performance tests.

Die-attach materials overview

The descriptions below provide an overview

of die-attach materials.

Eutectic gold-tin or AuSn.

Euctectic gold-

tin (80/20 Au/Sn by wt) has been the “gold

standard” die-attach material for high-reliability

applications. For LED die attach it is used either

as a pre-coated layer on the LED backside, a

preform, or in the form of solder paste. The

typical process is flux-less with an automated

bonder with scrubbing capability. Lately, AuSn

solder paste as well as flux-assisted processes

(with pre-coated dies or preforms) have been

used for higher throughput. Although the cost

of ownership of AuSn die attach is much higher

than other materials, it is still the material of

choice for high-power applications due to its

high thermal conductivity (57W/mK) and

proven reliability (high creep and fatigue

resistance with secondary reflow compatibility).

Conductive adhesives.

Conductive

adhesives (mostly silver-filled epoxies)

constitute the largest class of thermal die-attach

materials (by unit number) for semiconductor

power packages. They are compatible with

the existing back-end packaging equipment

and provide an attractive cost/performance

balance (typically up to 50W/mK thermal with

secondary reflow compatibility). Because they

stick to bare silicon, they are the preferred

material of choice for dies without back-side

metallization (like GaN on silicon).

Solder.

Solder (mostly SAC-based) provides

exceptional value with low cost and fast

assembly process with reasonable thermal

performance (50-60W/mK). Lately, there has

been a trend to make the flip-chip structure

compatible with solder on surface mount

technology (SMT) lines. However, because

SAC solder melts in the 217-221ºC range, its

use is limited to applications where either high-

temperature stability is not required in operating

conditions, or during further processing (like

secondary reflow). SnSb-based solder with a

melting point range between 245-251ºC can

survive second reflow below 240ºC.

Silicone-based adhesives.

Silicone-based

adhesives (filled with ceramic-based fillers

for heat dissipation) have been the die-attach

technology of choice for low-mid power

sapphire-based lateral LEDs. These materials

are transparent, have excellent adhesion and

high-temperature stability (resistant to color

degradation). The stamping (pin transfer)

process has been adopted to achieve very thin

bond lines at relatively high throughput for LED

die attach.

LED die structures overview

This section discusses the experimental

elements of our study.

LEDs.

For lateral die structure, green

LEDs on a sapphire substrate from III-V

semiconductor (TCE13-525, 330x330µm)

were bonded on a star-shaped extruded

copper pedestal. This design did not include

D