Low Temperature Bonding Based on Solder for Flexible IoT Platforms

Kwang-Seong Choi, Haksun Lee, Hyun-Cheol Bae, and Yong-Sung Eom

2014. 10. 16.

IT Materials and Components Lab.

Kwang-Seong Choi, Slide 1

Outline

Kwang-Seong Choi, Slide 2

I. IoT (Internet of Things)

II. Motivations

III. Bumping Process using Solder Bump Maker (SBM)

IV. Bonding Process using Fluxing Underfill

V. Resistance Comparison

VI. Summary

IoT (Internet of Things)

Kwang-Seong Choi, Slide 3

Source: Microsoft

Supply Chain of IoT

Kwang-Seong Choi, Slide 4

Source: Microsoft

Device + Service + Software

Kwang-Seong Choi, Slide 5

Source: Microsoft

Device + Service + Software

Kwang-Seong Choi, Slide 6

Source: Microsoft

Devices for IoT

Kwang-Seong Choi, Slide 7

Source: Internet

Packaging Technologies of Devices for IoT

Kwang-Seong Choi, Slide 8

Source: Internet

Human

Packaging Technologies of Devices for IoT

Kwang-Seong Choi, Slide 9

Source: Internet

Human

Low cost Miniaturization High performance

Human-compatible Low cost High performance

Sensors on Human Body

Kwang-Seong Choi, Slide 10

Source: Rogue Valley Microdevices

Blood monitoring Blood levels of sodium,

glucose, kidney func-tion, etc

Smart contact lens Controlling intra-ocu-

lar pressure levels Mounted on a flexible

substrate

Treatment and predic-tion of cardiac disor-ders

3D elastic membrane made of a soft, flexi-ble, silicon material

Soft Assemblies of Sensors for the Skin

Kwang-Seong Choi, Slide 11

Source: Sheng Xu, et al. Science 344, 70 (2014)

Motivations

Kwang-Seong Choi, Slide 12

Example of Flexible DisplaySource: Internet

One of the Technical Key words: Flexible

• Candidate Substrates

– PET (Tg = 67 to 81 )℃

– PEN (Tg = 120 )℃

– PES (Tg = 185 )℃

• Requirements for Interconnections

– Lower Bonding Temperature

e.g. Lower Than 150℃

Motivations

Kwang-Seong Choi, Slide 13

ACF used as Interconnection Material for Display

Process Flow of ACF Bonding

Source: Internet

• Characteristics of ACF Bonding

– Randomly distributed particles

– Mechanical Contacts

– High Electrical Contact Resistance

– Reliability Issues

• Characteristics of ACF Bonding

– Randomly distributed particles

– Mechanical Contacts

– High Electrical Contact Resistance

– Reliability Issues

Motivations

Kwang-Seong Choi, Slide 14

How to Reduce the Bonding Temperature?

How to Reduce the Electrical Contact Resistance?

Glass or FPCB

FPCB or Glass or IC

Heat, Pressure

Electrode

ACF

Conductive particles

Glass or FPCB

FPCB or Glass or IC

Glass or FPCB

FPCB or Glass or ICFluxing underfill

Electrode

Heat, Pressure

InSn solder

Glass or FPCB

FPCB or Glass or IC

ACF Bonding

Bonding using Solder and Fluxing Underfill

• Bumping Process?

• Bonding Process?

Bumping Process using Solder Bump Maker (SBM)

Kwang-Seong Choi, Slide 15

Bumping Process

SnAgCu305

Ref.: Haksun Lee, et al, “Novel interconnection technology for flex-on-glass (FOG) applications,” EMPC 2013.

Bonding Process using Fluxing Underfill

Kwang-Seong Choi, Slide 16

Bonding Process

Ref.: Haksun Lee, et al, “Novel interconnection technology for flex-on-glass (FOG) applications,” EMPC 2013.

Cross-sectional SEM of Bonded Joints

Solder Bump Maker (SBM) Technology

Kwang-Seong Choi, Slide 17

Component Functions and Requirements

Polymer matrix- Carrying solder powder- Proper viscosity with temperature- Proper surface tensions

Deoxidizing agent and Additives

- Elimination of the oxide layer on the sol-der powder

- Minimal chemical reactions- No void formation- No out-gassing

Solder powder- Sn3.0Ag0.5Cu (SAC305), SnBi, 52InSn- Solder bump formation- Size distribution

How to Reduce the Bonding Temperature?

Kwang-Seong Choi, Slide 18

Applying Nano-sized Solder Powder

Behavior of Melting Temp. of Nano powder

Source: Prof. Lee, SNUST

How to Reduce the Bonding Temperature?

Kwang-Seong Choi, Slide 19

Applying Solder Alloy with Lower Melting Point

Solder with Lower Melting Point

Kwang-Seong Choi, Slide 20

Tm : 84.35C

Solder Powder

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(> 38um) (20 ~ 38um)

(< 20um) (< 20um) 2nd Hopper

100um

100um

100um

100um

Design of Test Vehicle

Kwang-Seong Choi, Slide 22

FPCB

Substrate FPCB + Cu OSP

Pad Length 2650 µm

Pad Pitch 243 µm

Pad Width 86 µm

Maskless Printing Process using SBM

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Solder Bump Array

Kwang-Seong Choi, Slide 24

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

Bump number

Mea

sure

d H

eig

ht

[um

]

Number of bumps 46

Minimum height 23.1 µm

Maximum height 33.4 µm

Average height 27.44 µm

Standard Deviation 2.39

SEM Images of Solder Bump Array

Kwang-Seong Choi, Slide 25

Bonding Conditions

Kwang-Seong Choi, Slide 26

(a) Dispensing (b) Bonding (c) Post Cure

Bonding

Temp Ramp Hold Time Force

Upper 130ºC0.2ºC/sec. 180 sec. 200gf

(4gf / bump)Lower 130ºC

Post Cure(Hot Plate)

Temp Ramp Hold Time Force

Upper -

- 600 sec. 0gfLower 130ºC

FU130-LI20

Cross-Sectional SEM Image

Kwang-Seong Choi, Slide 27

Resistance Comparison

Kwang-Seong Choi, Slide 28

Solder ACF

Contact Re-sistance 0.21 mΩ 18.82 mΩ

ACFSolder

Summary

Kwang-Seong Choi, Slide 29

For the flexible IoT flatform, the low temperature bonding is re-

quired to obtain the reliable interconnections.

Solder Bump Maker and Fluxing Underfill technologies are in-

troduced, and their features are low temperature process and low

electrical contact resistance.

Kwang-Seong Choi, Slide 30

Thank you!Thank you!