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  1. 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

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

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

  4. 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

  5. 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

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

  7. Pressure Uniformity 3M Bonding Systems Division

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

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

  10. Overlap Area 3M Bonding Systems Division

  11. Particle Count 3M Bonding Systems Division

  12. 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 3M Bonding Systems Division

  13. Thermocompression Bonder Thermode Compliant layer Circuitry 3M Bonding Systems Division

  14. Temperature Profile 3M Bonding Systems Division

  15. LCD Visual Inspection 3M Bonding Systems Division

  16. Proper Bond Formation 3M Bonding Systems Division

  17. Particle Compression - Top 3M Bonding Systems Division

  18. Particle Compression - Side 3M Bonding Systems Division

  19. 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

  20. 4 Wire Test Circuit Test Pads Circuit Traces 3M Bonding Systems Division

  21. 4 Wire Test Pads Power Supply Voltmeter 3M Bonding Systems Division

  22. 100 Micron Reliability 3M Bonding Systems Division

  23. 80 Micron Reliability 3M Bonding Systems Division

  24. 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 3M Bonding Systems Division

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

  26. Peel Reliability 3M Bonding Systems Division

  27. 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

  28. 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 3M Bonding Systems Division