A new COF bonding technique using Sn bumps and a non-conductive adhesive (NCA) for image sensor packaging

A chip-on-flex (COF) bonding technology using Sn bumps and a non-conductive adhesive (NCA) was investigated. Two types of Sn bumps, square bumps and hemispherical bumps, were fabricated. The COF bonding was performed at 180degC for 5 s at 90 MPa after the NCA was dispensed. To evaluate the reliability of the COF joints, a high temperature storage test (150degC, 1000 h), thermal cycling test (-25degC/+125degC, 1000 cycles), and temperature and humidity test (85degC /85% RH, 1000 h) were performed. The bondability and reliability were evaluated by measuring the contact resistance of each bump. The contact resistance results showed that all COF joints were successfully fabricated. Good bondability was ascribed to the easy deformation of the soft Sn bumps and to metallurgical bonding in the bump/pad interfaces. However, several failed joints were detected in the COF joints fabricated using square Sn bumps after reliability test. The reliability of the COF joints fabricated using hemispherical bumps was excellent. NCA trapping in the bump/pad interface in the square bump specimens had an influence on the reliability of those joints. This technique has been successfully applied to image sensor packaging.