The Reliability of 302A Numerics

The 302A and 302C red numeric displays designed* by Bell Laboratories and manufactured by Western Electric--Reading consist of 8 red GaP chips arranged as 7 segments into a single digit and a right-hand decimal point. Each chip is mounted in a separate reflector and connected with either a common cathode (302A) or common anode (302C). In both cases, the device is encapsulated in Hysol 1700 epoxy.t In general, the effects of long-term thermomechanical, environmental, and electrical operating conditions on device performance are determined concurrently with device development. The major objective of this work was to obtain reliability information on 302A numeric display devices by means of forward bias-aging and thermal cycling at accelerated rates, which enables us to predict their long-term behavior, beyond infant mortality, when used in typical Bell System applications. First, we provide an outline of the experimental procedures employed in the acquisition of failure data by high temperature forward bias-aging and wide temperature-range thermal cycling. Then, the handling, treatment, and graphic display of the copious amount of information generated by the bias-aging experiments are discussed. Second, the time evolution of the relative luminescent efficiency (r = 77^0) of 302A numerics during bias-aging is compared to the predictions of the diffusion theory of red GaP degradation. 1 Moreover, the failure distribution is established for the entire sample of chips. Next, we determine the failure distribution for thermal cycling and attempt to locate the thermomechanical weak points of a bonded chip.