Interface-adhesion-enhanced bi-layer conformal coating for avionics application

A flexible, smooth, low profile conformal coating was developed for encapsulation of a MEMS applied to sense static pressure on aircraft during flight testing. The encapsulant should protect the MEMS and the MCM from environmental conditions, i.e. mechanical shock, temperature fluctuation, engine fuel and oil contamination, and moisture/mobile ion permeation. Conventional electronics packaging schemes cannot satisfy this specific outdoor application, and a new encapsulation combination was designed for the requirement of reliability without hermeticity (RWOH). A bi-layer structure was selected due to the limitations of single materials. Pliable elastomeric silicones are flexible, water repellent, and abrasion resistant. The silicone encapsulant is applied to planarize the MEMS surface and function as durable dielectric insulation, stress-relief, and shock/vibration absorbers over a wide humidity/temperature range. To compensate for the deficiency of silicone on engine fuel/oil contamination, parylene C is deposited afterward. This bi-layer coating has excellent bulk properties, e.g. moisture and mobile ion barrier resistance, chemical compatibility, and electrical insulation characteristics. However, the poor parylene C-silicone adhesion greatly restricts its application. To address this problem, silane coupling agents were used as adhesion promoters. Significant adhesion improvements were achieved by placing an interlayer silane coupling agent to provide interfacial bonding to the silicone elastomeric surface and parylene C film. Another possible adhesion enhancement mechanism is also presented.