Reliability and failure mechanism of isotropically conductive adhesives joints

Three recently developed silver filled isotropic electrically conductive adhesives from different manufacturers were selected for study. This work was focused on the use of isotropically conductive adhesives for joining surface mount devices (SMD) on printed circuit boards for potential solder replacement. The purpose of this study is to evaluate the reliability and failure mechanism of the adhesive joints in humid environments. 0805 chip components with pre-tinned terminals were attached using the conductive adhesives to printed circuit board (PCB) test boards with passivated copper and tin/lead metallizations. Shear strength and electrical resistance of the adhesive joints were measured initially after assembly and after 85/spl deg/C/85%RH constant humidity exposure at different time intervals. Electrical resistance of the adhesive joints increases and shear strength decreases dramatically for one adhesive after the humidity exposure while the others showed relatively smaller changes. Microstructure investigations by optical and scanning electron microscopy (SEM) reveal adhesive cracks initially after the cure, and crack development after the humidity exposure. Adhesive joint thickness and adhesive wetting properties to the component leads and PCB pad metallizations were also observed. Different silver flake sizes, distributions and orientations were also investigated. Transmission electron microscopy (TEM) shows different degrees of oxidation of the PCB pad metallizations due to moisture penetration.