Three recently developed silver filled isotropic electricallyconductive adhesives from different manufacturers were selected forstudy. This work was focused on the use of isotropically conductiveadhesives for joining surface mount devices (SMD) on printed circuitboards for potential solder replacement. The purpose of this study is toevaluate the reliability and failure mechanism of the adhesive joints inhumid environments. 0805 chip components with pre-tinned terminals wereattached using the conductive adhesives to printed circuit board (PCB)test boards with passivated copper and tin/lead metallizations. Shearstrength and electrical resistance of the adhesive joints were measuredinitially after assembly and after 85° C/85%RH constant humidityexposure at different time intervals. Electrical resistance of theadhesive joints increases and shear strength decreases dramatically forone adhesive after the humidity exposure while the others showedrelatively smaller changes. Microstructure investigations by optical andscanning electron microscopy (SEM) reveal adhesive cracks initiallyafter the cure, and crack development after the humidity exposure.Adhesive joint thickness and adhesive wetting properties to thecomponent leads and PCB pad metallizations were also observed. Differentsilver flake sizes, distributions and orientations were alsoinvestigated. Transmission electron microscopy (TEM) shows differentdegrees of oxidation of the PCB pad metallizations due to moisturepenetration
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