Accelerated Aging Tests of Bubble Devices

Bubble devices are being manufactured by Western Electric Company, as well as a number of other companies, and have been used in the Bell System since 1978. Accelerated aging tests which include temperature-humidity-voltage bias exposure, temperature cycling, vibration and shock, and electromigration degradation of the Al-4.5% Cu conductors have been performed since the earliest bubble device was constructed. Many of the failure modes that occurred in the early designs have been eliminated or reduced in severity by changes of materials or design. Copper corrosion on the fiberglass-epoxy circuit board has been greatly reduced by the use of an improved epoxy casting resin, magnet cracking has been totally eliminated by spring mounting the magnets rather than by cementing them in place, bias field variation after temperature cycling has been greatly reduced by proper magnet design, and vibration and shock problems have been eliminated by the use of mounting studs on the external shield. Electromigration studies show that the Al-4.5% Cu alloys, electron-beam-evaporated on garnet substrates, will withstand current densities of 1.5(10)7 amp/cm2 for a good lifetirme when the duty cycle is low (~ 1.5%). Results of the accelerated tests on the most recent design conclude that the bubble device is comparable to Western Electric plastic-encapsulated semiconductor integrated circuits.