Electrochemical impedance spectroscopic study of encapsulated triple tracks test (TTT) circuits

Electrochemical impedance spectroscopy (EIS) is effective for studying the reliability of protective coatings on microelectronic packaging devices. Triple track test (TTT) circuits were evaluated as a function of exposure time at 121°C and 2 atm pressure. Specimens were deliberately contaminated by various chemicals and particles before coating. The impedance of some specimens decreased after the first 10 hours in the Pressure Cooker Test (PCT) while some lasted more than 150 hours in the PCT without an appreciable change in the impedance values. Impedance of the specimens changed appreciably due to water uptake in the epoxy coatings. The ionic contamination trapped under the coating caused faster failure in the TTT with respect to clean specimens. Water diffused to the polymer/gold interface causing a change in the dielectric properties of the specimen, shown as a shift in the EIS spectrum. This shift was observed in EIS spectra for most of the specimens which were exposed for more than 150 hours. The change suggests water that penetrated the coating made a conductive pathway to the interfacial region, a common cause of debonding between the metal (or ceramic) polymer interface. In one case (specimens contaminated by SiO₂ particles) the shift was not observed-suggesting that SiO₂ particles do not accelerate the adhesive failure at the metal (or ceramic)/polymer interface. The calculated saturated volume fraction of water at the metal (or ceramic)/polymer interface was 1.1±0.1