Adhesives%20for%20Electronics

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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

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Z-Axis/ Anisotropic Conductivity
Conductivity in the Z direction only (through the
thickness of the adhesive)
Z
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Flex to ITO Bonding
Flex Circuit
LCD
Copper trace
ITO Trace
ZAF
Glass
Polymer particle preferred with ITO traces
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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

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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 low electrical resistance)
• Proper pitch
• minimum space between traces to avoid shorting

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Thermocompression Bonding
Applied pressure
Compliant layer
Bonding Thermode
Flex Circuit
ITO Glass
Z-Axis Conductive Film
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Pressure Uniformity
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Temperature Profile
Ram pressure
Compliant layer
Bonding Thermode
Flex Circuit
ITO Glass
Thermocouple
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Overlap Area
Flex Circuit
LCD
Copper trace
ITO Trace
ZAF
Glass
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 W
  /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

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Thermocompression Bonder
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
  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

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4 Wire Test Circuit
Test Pads
Circuit Traces
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4 Wire Test Pads
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 of
  environmental 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 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

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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