A Bayesian Demonstration of Reliability for Encapsulated Implanted Electronics

The long term performance of implanted medical devices is critically affected by their packaging materials. Traditional hermetic enclosures are a hindrance to miniaturisation which might be avoided by polymer encapsulation. However, much uncertainty remains regarding such an approach for chronic implants. It is difficult to extract meaning from lifetime experiments performed on arbitrary test structures and before many highly reliable devices have actually failed. In this study, we describe an accelerated aging experiment on devices that employ test structures fabricated on a silicone encapsulated 0.35 μm CMOS technology. Samples are aged at 47, 67 and 87 °C under a constant 5V DC bias. After over 300 days of aging, of the 36 samples under test, there have only been four failures. Three failures are attributed to the interconnect to the measurement system, with the remaining failure due to delamination of the silicone encapsulant over wirebonds. By employing a Bayesian reliability analysis of the near-zero failure data we demonstrate a 1st failure percentile for the CMOS components of greater than 92 days when aged at 87 °C, calculated at a 95% confidence level, which approximately corresponds to over 8 years at 37 °C.