Adhesives for Electronics 2000

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3M Bonding Systems Division

Adhesives for Electronics

Reliability Study of Sub 100 Micron Pitch, Flex-to-ITO/glassInterconnection, 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

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Z-Axis/ Anisotropic Conductivity

Conductivity in

the Z direction only

(through the thickness

of the adhesive)

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Flex to ITO Bonding

Copper trace

ZAF

ITO Trace

Glass

Flex Circuit

LCD

Polymer particle preferred with ITO traces

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Cellular Phone Applications for ZAF

•Flex to ITO glass

–5000 series - specificproduct dependingupon pitch

•Flex to PCB

–7303 or 5000 seriesdepending upon pitch

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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 lowelectrical resistance)

•Proper pitch

–minimum space between traces to avoid shorting

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Thermocompression Bonding

ITO Glass

Bonding

Thermode

Applied pressure

Flex Circuit

Z-Axis Conductive

Film

Compliant

layer

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Pressure Uniformity

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Temperature Profile

ITO Glass

Bonding

Thermode

Ram pressure

Flex Circuit

Thermocouple

Compliant

layer

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Overlap Area

Copper trace

ZAF

ITO Trace

Glass

Flex Circuit

LCD

Overlap Area = ZAF width x

Trace width

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Overlap Area

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Particle Count

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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 1029cycles

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

•85 C and 85 % RH for 1028 hours

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Thermocompression Bonder

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Thermode

Compliant layer

Circuitry

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Temperature Profile

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LCD Visual Inspection

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Proper Bond Formation

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Particle Compression - Top

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Particle Compression - Side

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4 Wire Resistance Test

•Test designed to subtract out the resistance of thecopper 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 resistanceis influenced by ITO resistance

–Samples must be removed from the environment fortesting

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4 Wire Test Circuit

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Test Pads

Circuit Traces

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4 Wire Test Pads

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Power

Supply

Voltmeter

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100 Micron Reliability

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80 Micron Reliability

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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 ofenvironmental aging

•5 peeled parts per environmental condition

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Typical Peel Test

2.5 mm/min

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Peel Reliability

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Shorting Test

•Flex with 30 micron gapbonded to non-conductiveglass

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

•128 flex samples remadewith proper bonding

•12,800 gaps measuredwith no shorts

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Conclusions

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

•Proper bonding conditions and equipment are importantwhen 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 peeltesting are important tools in monitoring process control