Ultra-fine pitch chip-on-glass (COG) bonding with metal bumps having insulating layer in the side walls using anisotropic conductive film (ACF)

We developed a new ultra-fine pitch chip-on-glass (COG) bonding technique using insulated metal bumps and anisotropic conductive film (ACF). An insulating layer was formed by spin coating a photosensitive insulating polymer and subsequently exposing it without any mask. The shape of the insulating layer coverage on the side walls of the metal bumps can be controlled by changing the exposure time and the viscosity of the photosensitive insulating polymer. In our experiment, we successfully fabricated COG joints with a 25 μm pitch using Au bumps with an insulating layer on their side walls and a conventional single-layer ACF. When the bumps were covered with the photosensitive insulating polymer, a few conductive particles were trapped between neighboring bumps and many conductive particles were embedded between bumps and pads. The electrical resistance between neighboring bumps was measured by the two-point probe method. The resistances were measured only in uncoated specimens. The measured resistance indicates that electrical shorting between neighboring bumps occurred in uncoated specimens. Therefore, electrical shorting was successfully prevented by the insulating layer on the side walls of the bumps.