The Reliability of 302A Numerics

Abstract

To assess the long-term reliability of Western Electric 7-segment 302A numerics, accelerated forward bias-aging (10 mA, 60° and 125°C) and thermal cycling ('40° to 125°C) experiments have been performed. In treating the bias-aging data, we strictly differentiate between LED chip and digit failure and show that the times to chip failure — defined here as the times required to reach a normalized efficiency of r = η/η0 = 0.5 — are lognormally distributed. The analysis of the data was facilitated by a novel computer-graphics routine which provides for each digit on test a bar-by-bar description of the time-evolution of r. The median life and standard deviation at 125°C and 10 mA for chips are 2400 hours and ∼0.4, respectively. Furthermore, we find that the failure distribution for digits can be obtained from the chip distribution by a simple probabilistic consideration. The good accord demonstrated between the experimental data and the theoretical curve derived from the diffusion theory of red GaP LED degradation indicates that the predominant mode of degradation in bias-aging of 302A devices is that of the LED. The thermal cycling response of 302A numerics encapsulated in Hysol 1700 epoxy is excellent. Similar to bias-aging, the chip failure distribution is lognormal, and chip and digit failures are interrelated by the probabilistic law. For a temperature excursion between −40° and 125°C, the median number of cycles to failure is 23,350 for chips and 300 for digits. The cause of failure is identified by electrical testing as open wire bonds. Finally, the acquired data permit the estimation of the mean times to failure (MTTF) and failure rates beyond infant mortality of 302A numerics in a specific application such as Transaction telephone sets under realistic operating conditions. The overall reliability of these devices is excellent, characterized by an MTTF of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> hours and a maximum failure rate of less than 1 FIT for bias-aging over a 20-year service life. Failures due to broken wires are estimated to yield an MTTF of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> hours and a failure rate of ≤4 FITs at 20 years of service.