We have developed and applied new methods to
estimate the functional life of miniature, implantable, wireless
electronic devices that rely on non-hermetic, adhesive
encapsulants such as epoxy. A comb pattern board with a high
density of interdigitated electrodes (IDE) could be used to
detect incipient failure from water vapor condensation.
Inductive coupling of an RF magnetic field was used to pro-
vide DC bias and to detect deterioration of an encapsulated
comb pattern. Diodes in the implant converted part of the
received energy into DC bias on the comb pattern. The capac-
itance of the comb pattern forms a resonant circuit with the
inductor by which the implant receives power. Any moisture
affects both the resonant frequency and the Q-factor of the
resonance of the circuitry, which was detected wirelessly by
its effects on the coupling between two orthogonal RF coils
placed around the device. Various defects were introduced
into the comb pattern devices to demonstrate sensitivity to
failures and to correlate these signals with visual inspection
of failures. Optimized encapsulation procedures were validat-
ed in accelerated life tests of both comb patterns and a func-
tional neuromuscular stimulator under development. Strong
adhesive bonding between epoxy and electronic circuitry
proved to be necessary and sufficient to predict 1 year pack-
aging reliability of 99.97% for the neuromuscular stimulator.

• Accelerated life-test methods and results for implantable electronic devices with adhesive encapsulation