Influence of medical sterilization on ACA flip chip joints using conformal coating

Health care is a field in which electronics is increasingly used. If electronics is used in a device that is implanted, it needs to withstand a harsh environment. Moreover, before implantation the device needs to be sterilized, and this should not impair the function or reliability of the device. Since implantable applications are small in size, electronics packaging solutions need to be space saving. Conformal coating is needed to shield the electronics from conditions inside the human body, and on the other hand to protect the body from the materials used in electronics. This paper studies the effect of medical sterilization on the reliability of adhesive joints. Flip chip technology with anisotropically conductive adhesives (ACA) was used and epoxy and parylene C were used as the conformal coating materials. Differential scanning calorimetry (DSC) was used to ascertain the curing degree of the adhesive after bonding. Sterilization was performed using gamma radiation sterilization and the reliability tests were conducted in a constant humidity test, at 85 °C and 85%RH. The results clearly indicated that parylene C coating is the most reliable choice and gamma radiation does not affect the reliability of parylene C coated devices. However, the gamma radiation increased the reliability of the non-coated test lot.