Ensuring minimal humidity levels in hermetic implant housings

The electronic circuitry of active implantable devices is commonly protected against the risk of water-induced corrosion by using gas-tight (hermetic) packages, preventing moisture from the host body to reach the electronics. However, when closing the package, one has to ensure that the packaged components do not contain moisture that could rise humidity inside the package to critical levels by outgassing. For our miniature metal/ceramic packages, we found a drying procedure of 120°C at 180 mbar absolute pressure for one hour, followed by a dry helium purge sufficient to keep the relative humidity below 2.5% over a time span of 300 days at 80°C, corresponding to over 15 years at 37°C. The additional integration of a desiccant inside the package permits to keep the relative humidity below 0.1%, the detection limit of the integrated sensor. This sensor was selected based on an evaluation of 17 commercially available humidity sensors.