Dielectric Breakdown by Metallic Particles in COF Packaging for Display Driver IC

An electrical test is the last step to detect electrical failures in manufacturing of chip-on-flex (COF) packaging for display driver IC (DDI). However, we observed several cases that good COF packages proven by the electrical dc test were electrically short-circuited later during the display test or aging test at the panel level. Cross-sectional analysis by a focused ion beam (FIB) showed that metallic particles existed between two leads in the defective COF packaging. From the FIB images obtained by sequential milling, we found that there were sub-micrometer gaps filled with underfill material between the lead and the metal particle. The gap size ranged from 50 to 500 nm. We also observed that there were voids, delamination, and cracks in the underfill layer of the gap. Our observations suggest that the underfill material in the sub-micrometer gap between the lead and the metal particle should be degraded under high electric field during the display test and finally short-circuited as a result of dielectric breakdown.