Electrical Reliability of Silver Filled Epoxies for Die Attach

Silver-filled epoxies used in manufacturing for die attachment have sometimes been found to exhibit high resistance approximately 24 hours after warming to room temperature. In this study devices from good and bad lots and commercially available silver-filled epoxies have been examined. Comparison of secondary electron images with absorbed current micrographs indicates that conduction occurs through a percolative network. The sensitivity of resistivity as a function of silver concentration to formulation shows that conductivity through the percolative network is strongly affected by a complex interplay between the silver flake and the epoxy matrix. The amount of insulating amorphous carbon layer immediately surrounding the silver flake, as demonstrated by Raman scattering, is related to the storage time and conductivity. The bulk conduction mechanism probably involves a combination of direct contact of silver flakes and tunneling through the insulating layer. This insulating layer can be electrically broken down to form a mechanically fragile pathway. Secondary electron micrographs demonstrate that the silver flakes are randomly oriented throughout the bulk but, near interfaces between the metallization or air, they tend to align parallel to the interface. The minimum free energy configuration near the interface appears to be driven by the specific surface areas and the interfacial free energies. The orientational ordering near the interface undoubtedly alters the resistivity in that region.