3M Bonding Systems Division Adhesives for Electronics Reliability Study of Sub 100 Micron Pitch, Flex-to-ITO/glass Interconnection, Bonded with an Anisotropic

3M Bonding Systems Division

Adhesives for Electronics

Reliability Study of Sub 100 Micron Pitch, Flex-to-ITO/glass Interconnection, Bonded with an Anisotropic Conductive Film.

Cameron T. Murray, Peter B. Hogerton, Theary Chheang, Robert L. Rudman

3M Company Bonding Systems Division, 230-2E-11, St. Paul, MN 55144


Z-Axis/ Anisotropic Conductivity

Conductivity inthe Z direction only(through the thicknessof the adhesive)

Z


Flex to ITO Bonding

Copper traceZAF ITO Trace

Glass

Flex Circuit

LCD

Polymer particle preferred with ITO traces


Cellular Phone Applications for ZAF

• Flex to ITO glass

– 5000 series - specific product depending upon pitch

• Flex to PCB

– 7303 or 5000 series depending upon pitch


Making Reliable Bonds

• Proper thermocompression bonding technique

– Flex alignment to ITO/glass or PCB

– Proper thermal cure profile

– Proper pressure

• Proper overlap area

– guaranteed minimum number of particles (to provide low electrical resistance)

• Proper pitch

– minimum space between traces to avoid shorting


Thermocompression Bonding

ITO Glass

BondingThermode

Applied pressure

Flex Circuit

Z-Axis ConductiveFilm

Compliantlayer


Pressure Uniformity

Bonding Stage

Thermode

Pressure Senisitive Paper(Prescale Film)


Temperature Profile

ITO Glass

BondingThermode

Ram pressure

Flex Circuit

Thermocouple

Compliantlayer

380C/3.9s 285C/16.2s

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20 25 30seconds

de

gC


Overlap Area

Copper traceZAF ITO Trace

Glass

Flex Circuit

LCD

Overlap Area = ZAF width x Trace width


Overlap Area

0.01

0.1

1

10

0 20000 40000 60000 80000 100000

5552R Resistance vs. Overlap areaR

esis

tanc

e (o

hm

s)

High

Low

Average

Overlap Area (microns2)


Particle Count

0

50

100

150

200

250

0 20000 40000 60000 80000 100000

5552R Particle Count vs. Overlap AreaP

artic

le C

ount

Overlap Area (microns2)

Reliaiblity minimum = 20 pac men

High

Low

Average

3M recommendedmimimum overlap area

Safe Zone


Reliability Study

• Fine Pitch Flex bonded to ITO/Glass (10 /square)

– 80, 100 micron pitch

– 285 interconnections per condition

– Peel Testing and Electrical Resistance

• Environmental Testing Conditions

• 100 C for 1024 hours

• 125 C for 1025 hours

• 60 C / 95 % RH for 1029 hours

• -40 C to 100 C Thermal cycle (1 hour/cycle) for 1029 cycles

• -55 C to 125 C for 1820 cycles - Thermal shock

• 85 C and 85 % RH for 1028 hours


Thermocompression Bonder

Thermode

Compliant layer

Circuitry


Temperature Profile

380C/3.9s 285C/16.2s

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20 25 30seconds

de

gC


LCD Visual Inspection

3M ZAF flow front after bonding

Thermodefootprint - inspectin this region.

LCD Display Area

ITO trace


Proper Bond Formation

Flex Circuit

ITO/glass

Flex Circuit

ITO/glass

ZAF

ZAF

Case 1: Fillet formed and shorts observed

Case 2: No fillet formed and no shorts observed

Fillet


Particle Compression - Top

ShapeA


Particle Compression - Side


4 Wire Resistance Test

• Test designed to subtract out the resistance of the copper traces and the ITO glass:

– Requires specialized flexible circuit design

– Automated testing jig/ computer data collection

– 57 measurements per sample / 5 samples per condition

– Microvoltmeter and power supply

• Test limitations

– Perfect circuit design not possible - measured resistance is influenced by ITO resistance

– Samples must be removed from the environment for testing


4 Wire Test Circuit

Test Pads

Circuit Traces


4 Wire Test Pads

PowerSupply

Voltmeter


100 Micron Reliability

0

5

10

15

20

25

018

535

352

068

885

610

24

018

735

452

268

985

710

25

056

112

168

224

1029

032

082

013

2018

20

019

035

852

569

386

010

29

018

935

752

469

286

010

28

5552R Resistance Stability

B

Res

ista

nce

(o

hm

s)

Hours HoursHours Cycles Cycles Hours

125 C-40 to

+ 100 C-55 to

+ 125 C60 C /

95 % RH85 C/

85 %RH100 C

100 micron


80 Micron Reliability

0

5

10

15

20

018

535

352

068

885

61

024 0

187

354

522

689

857

102

5 0 56 112

168

224

102

9 032

082

01

320

182

0 019

035

852

569

386

01

029 0

189

357

524

692

860

102

8

5552R 80 micron pitch

B

Re

sis

tan

ce

(o

hm

s)

Hours HoursHours Cycles Cycles Hours

125 C-40 to

+ 100 C-55 to

+ 125 C 60 C / 95 % RH 85 C/ 85 %RH100 C


Peel Test

• IPC TM-650 2.4.9.1

– 2.5 mm / min peel rate

– 90 degree angle

• Reported values = maximum value on plot

• Peel testing performed at the conclusion of environmental aging

• 5 peeled parts per environmental condition


Typical Peel Test

0

2

4

6

8

10

12

14

0

200

400

600

800

1000

1200

1400

0 500 1000 1500 2000 2500 3000 3500 4000

90 Degree Peel

Pee

l Stre

ngth

(N)

Peel S

trength (grams)

Distance (microns)2.5 mm/min


Peel Reliability

0

250

500

750

1000

1250

1500

Init

ial

10

0C

dry

12

5C

dry

-4

0C

/10

0C

-5

5C

/12

5C

60

C/9

5%

RH

85

C/8

5%

RH

Init

ial

10

0C

dry

12

5C

dry

-4

0C

/10

0C

-5

5C

/12

5C

60

C/9

5%

RH

85

C/8

5%

RH

Pe

el A

dh

es

ion

(g

/cm

)

Pee

l Ad

he

sion

(N/cm

)

15

0

10

5

80 100 m m


Shorting Test

• Flex with 30 micron gap bonded to non-conductive glass

• Initially shorts found - related to fillet formation over edge of ITO glass

• 128 flex samples remade with proper bonding

• 12,800 gaps measured with no shorts

Flex Circuit

ITO/glass

Flex Circuit

ITO/glass

ZAF

ZAF

Case 1: Fillet formed and shorts observed

Case 2: No fillet formed and no shorts observed

Fillet


Conclusions

• Fine pitch circuitry can be bonded with Z-Axis Films into a highly reliable interconnection

• Proper bonding conditions and equipment are important when making a reliable bond

– proper placement of ZAF to prevent shorting

– proper particle compression

– proper thermal profile for good mechanical (peel) performance

• Visual inspection combined with electrical and peel testing are important tools in monitoring process control

Documents

3M Bonding Systems Division Adhesives for Electronics Reliability Study of Sub 100 Micron Pitch, Flex-to-ITO/glass Interconnection, Bonded with an Anisotropic