Anisotropic conductive film for fine-pitch interconnects

An anisotropic conductive film (ACF) can be utilized to simultaneously form mechanical bonds and electrical connections during flip-chip assembly. The electrical connection is created by trapping randomly dispersed metallized polymer spheres (MPS) in the ACF that are deformed during the bonding process. This work postulates that the reliability of interconnects formed with ACF depends on the degree to which the MPS are deformed. Silicon samples with fine-pitch electrical test structures were flip-chip assembled using an ACF and measured in-situ during environmental testing. Interconnects with MPS deformation below 60% proved more stable than interconnects with higher deformation during exposure to 85% relative humidity at 20 °C, 45 °C, 60 °C and 85 °C, as postulated. On the other hand, the stability of the interconnects did not show a dependence on MPS deformation during exposure to thermal shock cycling (TSC) (−55 °C / +125 °C, 7 s transit time, 700 cycles). The results suggest that deformation of MPS is a central factor with respect to reliability of ACF-bonded fine-pitch samples exposed to humid conditions, but the results also indicate that other failure mechanisms are more important for samples exposed to thermally unstable conditions.