Anisotropic Conductive Films (ACFs) ACF = Thermosetting epoxy resin film + Conductive particles Chip or substrate 1 Heat Pressure ACF Substrate 2 Chip or substrate 1 ACF Substrate 2 Applications Chip-on-Board (COB) Chip-on-Glass (COG) Flex-on-Glass (FOG) Chip-on-Flex (COF) Flex-on-Board (FOB)
Background of Ultrasonic(U/S) ACF bonding ACF heating mechanism U/S vibration substrate 1 ACF resin ACF Substrate 2 Vibration Internal friction in polymer Heat generation
Thermo-compression vs. Ultrasonic T/C ACF bonding U/S ACF bonding Patent issued Heating tool U/S horn substrate Heat 1 Heat conduction ACF Substrate 2 Heating tool (2 ~25 ) ACF (18 ) T/C ACF bonding substrate 1 Local heat generation Substrate 2 U/S horn (R.T.) ACF (3 ) U/S ACF bonding Process temperature 2 ~25 Room temperature Bonding time 1 ~15 sec 1 ~ 5 sec Assembly productivity 1 > 3 times Thermal deformation
ACF U/S bonder 8 μm diameter Ni particles 4 μm thickness Test vehicles Flex-On-Board applications ACF FPCB PCB PCB.8mm thickness FR-4 Pitch : 2, 3, 4, and 8um Cu electrodes Flexible PCB Polyimide film Pitch : 2, 3, 4, and 8um ACF temperature ACF temperature ( o C ) 3 25 2 15 1 5 Bonding pressure: 3MPa 2 4 6 8 1 12 14 Bonding time (sec) Longitudinal vibration Power : 8W Frequency : 4KHz Amplitude 2 um 4 um 6 um 8 um 1 um 12 um
Flex-On-Board applications Electrical resistance Peel adhesion strength Daisy-chain resistance (Ohm) 3 28 26 24 22 2 18 16 14 12 1 Total daisy-chain resistance Total daisy-chain resistance Electrical Continuity X Thermo-compression 15 o C 2MPa 6sec 4% 5% 6% Stable at above 2MPa 1. 1.5 2. 2.5 3. Peel adhesion strength (gf/cm) 14 12 1 8 6 4 2 Cured 1 2 3 4 Time (sec) Bonding time 1 sec at 3 Mpa 26 o C 22 o C 18 o C Max. Peel adhesion strength : ~ 13gf/cm Bonding pressure (MPa)
Reliability evaluation Flex-On-Board applications -4 o C/1 o C thermal cycling 1 cycles 85 o C/85%RH test 1 hrs 125 o C storage test 1 hrs Cumulative distribution (%) 1 8 6 4 As-bonded 2 cycles 4 cycles 2 6 cycles 8 cycles 1 cycles 2 4 6 8 1 12 14 16 18 2 Resistance ( ) 14 failures Cumulative distribution (%) 1 8 6 Cumulative distribution (%) 8 14 failures 6 4 As-bonded 2 hrs 4 hrs 2 6 hrs 8 hrs 1 hrs 2 4 6 8 1 12 14 16 18 2 Resistance ( ) 1 4 As-bonded 2 hrs 4 hrs 2 6 hrs 8 hrs 1 hrs 2 4 6 8 1 12 14 16 18 2 Resistance ( ) 14 failures US bonded ACF joints showed stable electrical resistances in -4/1 o C TC, 85 o C/85%RH, and 125 o C storage tests.
ACA(Anisotropic conductive adhesive) Au coated polymer particles Test vehicles Touch Screen Panel applications 6 mm FPC (.16 mm thickness) Cu electrode Ag electrode 42 mm PC PET Bonding area 5.5 mm X 2 mm PC substrate ACA FPC(polyimide) ACA PET substrate 1.1 mm.4 mm ACF temperature ACA temperature ( o C) ACA temperature ( o C) 24 22 2 18 16 14 12 1 8 6 24 22 2 18 16 14 12 1 1 MPa pressure 4 2..5 1. 1.5 Time (sec) 2 MPa pressure 8 6 4 2..5 1. 1.5 Time (sec) 12 μm amplitude 1 μm amplitude 8 μm amplitude 12 μm amplitude 1 μm amplitude 8 μm amplitude
Touch Screen Panel applications Electrical continuity Cu electrode Stable electrical continuity ratio (%) Ω 1 9 8 7 6 5 4 3 2 1 Ag electrode PC substrate ACA FPC(polyimide) ACA PET substrate Bonding time 1 sec Stable resistance (< 25 Ω) Unstable resistance (> 25 Ω or open 8 1 12 14 16 ACA peak temperature ( o C) Pull adhesion strength Pull adhesion strength (gf) 3 25 2 15 1 5 1 MPa 2 MPa Bonding time 1 sec FPC tearing ACA joint failure 6 8 1 12 14 16 18 ACA peak temperature ( o C) Strong adhesion Poor adhesion failure) Bonding time : 1 sec 1 MPa Bonding pressure : 2 Mpa 2 MPa Max. Pull adhesion strength : ~ 3gf/cm
Touch Screen Panel applications Cross-section image bonding conditions : 2 MPa pressure, 5% amplitude, 1 second Reliability evaluation 25 bonding conditions : 2 MPa pressure, 5% amplitude, 1 second Conductive ball Ag electrode(pet) Cu electrode(fpcb) FPC(Polyimide) Resistance (Ohm) 2 15 1 5 Cu electrode(fpcb) Before reliability test After reliability test Ag electrode(pc) Test 1 Test 2 Test 3 Test 4 Test 5 Test 1 : Thermal shock test (- 4 /3 min ~ 8 /3 min, 5 cycles) Test 2 : Salt spray test (35, 5% NaCl, 48 hours) Test 3 : High temperature and high humidity test (6 /9% RH, 24 hrs) Test 4 : High temperature storage test (8, 24 hrs) Test 5 : Writing test (25 g, 2, times)
Potential application of U/S ACF bonding Multi-functional mobile devices Ultra-thin appliances Source : Apple Source : NOKIA Source : Samsung Side keys Sensors Camera Display High power LED backlit Battery
Conclusions U/S ACF bonding method was successfully demonstrated in Flex-On-Board and Touch Screen Panel applications U/S ACF bonding showed sufficient adhesion strengths, contact resistances, and reliability Flex-On-Board Bonding time 1 sec Adhesion strength ~ 13 gf/cm Daisy chain resistance ~ 16 Ω Touch Screen Panel Bonding time 1 sec Adhesion strength ~ 3 gf Electrical continuity ~ 1% Future work U/S bonding process optimization for various applications : FOF (Flex-On-Flex), Camera module interconnection Theoretical analysis on U/S heating mechanisms