Recent Advances in Anisotropic Conductive Adhesives (ACAs) Technology: Materials and Processing
Anisotropic Conductive Adhesives (ACAs) have been widely used in semiconductor and display interconnection applications for chip-on-glass (COG), chip-on-flex (COF), chip-on-board (COB), flex-on-glass (FOG), flex-on-board (FOB), and flex-on-flex (FOF) interconnections due to their fine-pitch capability, simple and low temperature processes, and cost effectiveness.
In this presentation, more than 15 years of ACA materials and processing innovations invented at KAIST will be presented. New solder ACFs containing 10 ~ 40 micron diameter size Sn-Bi and SAC solder particles combined with the later described “Ultrasonic Bonding” method overcome the technical limit of conventional ACFs in terms of current handling capability and reliability, because solder ACFs form many tiny solder alloy metallurgical joints rather than physical contact based joints of conventional ACFs with metal conductor particles such as metal coated polymer balls and Ni particles. 120C less than 5 seconds curing epoxy based ACFs have been invented by using photo activation addition to ACF polymer resins.
After 355 nm UV exposure, the PA-ACFs become activated, can be thermally cured at 120C within 5 seconds. They can be used at the applications requiring extremely low temperatures processing such as touch screen panels (TSPs) and some temperature sensitive devices assembly. In addition, nanofiber ACFs, about 500 nm diameter polymer nanofibers with coupled conductive particles are fabricated using an electro-spinning method, and then ACFs with the nanofibers are successfully made by laminating NCFs(nonconductive films) on top and bottom side of the nanofibers.
This novel nanofiber ACF shows excellent electrical bump joint contact resistance, and fine pitch handling capability. Nanofiber ACF can completely solve the electrical shortage problems at 7 µm bump-to-bump gap and 20 micron ultra fine bump pitch of COG(Chip On Glass) and COF(Chip On Flex) electronic packaging applications In the ACA processing innovations, the room temperature operating Ultrasonic (U/S) bonding method was successfully invented as an alternative of conventional thermo-compression (T/C) bonding method.
By spontaneous heat generation at the ACF layer using vertical U/S vibration, U/S bonding achieved less than 3 seconds bonding times, reduced assembly thermal damage, good adhesion strength, good contact resistances, and reliability. US bonding can be successfully used for FOB(Flex On Board) and FOF(Flex On Flex) applications to eliminate socket connectors in mobile applications. The specially formulated non-conductive adhesives (NCAs) were invented for the 3D TSV(Through Silicon Via) chip stacking.
NCAs were applied on the wafer level before bonding so that it acts like both adhesive and underfill. Excellent SnAg bump joints were successfully formed with stable electrical interconnection at 40 um pitch 3D-TSV bump pitch. New ACA materials and processing technologies innovation invented at KAIST will be presented in terms of various packaging applications such as FOB, FOF, COF, COG, and 3D-TSV assemblies.