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

3M Bonding Systems Division Z-Axis/ Anisotropic Conductivity Conductivity in the Z direction only (through the thickness of the adhesive) Z

3M Bonding Systems Division Flex to ITO Bonding Copper trace ZAF ITO Trace Glass Flex Circuit LCD Polymer particle preferred with ITO traces

3M Bonding Systems Division 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

3M Bonding Systems Division 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

3M Bonding Systems Division Thermocompression Bonding ITO Glass Bonding Thermode Applied pressure Flex Circuit Z-Axis Conductive Film Compliant layer

3M Bonding Systems Division Pressure Uniformity

3M Bonding Systems Division Temperature Profile ITO Glass Bonding Thermode Ram pressure Flex Circuit Thermocouple Compliant layer

3M Bonding Systems Division Overlap Area Copper trace ZAF ITO Trace Glass Flex Circuit LCD Overlap Area = ZAF width xTrace width

3M Bonding Systems Division Overlap Area

3M Bonding Systems Division Particle Count

3M Bonding Systems Division 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

3M Bonding Systems Division Thermocompression Bonder Thermode Compliant layer Circuitry

3M Bonding Systems Division Temperature Profile

3M Bonding Systems Division LCD Visual Inspection

3M Bonding Systems Division Proper Bond Formation

3M Bonding Systems Division Particle Compression - Top

3M Bonding Systems Division Particle Compression - Side

3M Bonding Systems Division 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

3M Bonding Systems Division 4 Wire Test Circuit Test Pads Circuit Traces

3M Bonding Systems Division 4 Wire Test Pads Power Supply Voltmeter

3M Bonding Systems Division 100 Micron Reliability

3M Bonding Systems Division 80 Micron Reliability

3M Bonding Systems Division Peel Test IPC TM –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

3M Bonding Systems Division Typical Peel Test 2.5 mm/min

3M Bonding Systems Division Peel Reliability

3M Bonding Systems Division 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

3M Bonding Systems Division 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