An ac impedance in situ methodology for assessing high reliability performance of plastic encapsulated microelectronics in harsh environments

Conventional procedures for assessing environmental durability of encapsulated IC´s use periodic function testing after exposure to elevated temperature and high relative humidity (RH) for designated periods of time. This procedure is time-consuming, labor-intensive and costly. A test methodology using ac impedance allows early detection of degradation and has the potential to assess, without lengthy testing, if a part will perform reliably in a harsh environment. The ac impedance approach monitors the frequency dependence of the impedance of suitable circuit paths on a test chip or an IC. Development of diffusion paths in the encapsulant and any incipient corrosion processes that attack conductor paths and/or create leakage current paths alter the impedance spectrum significantly. This permits detection of the beginning stages of corrosion and moisture damage to the encapsulant long before the loss of functionality of the device. Results show that the ac impedance method offers the capability of real-time quantitative monitoring of the chemical degradation processes within the package.