A Test of Parylene as a Protective System for Microcircuitry

Parylene has been suggested for use and, in some cases, is used as a protective system for electronic components and assemblies, such as microcircuits. unfortunately, the amount of environmental test data for such parylene-protected components is quite limited and, to the authors' knowledge, almost no long-term test data exists. The principal objective of the test program reported in this paper was to determine whether it is at all reasonable to use parylene as a protective system for microcircuits in place of a hermetic seal in high-reliability equipment. A radiation-hardened circuit containing nichrome resistors was selected as a test specimen because of its considerable sensitivity to humidity-induced failure. The ceramic encased circuits were mounted on carrier cards, delidded, and coated with parylene C at three different facilities (Vendors A, B, and C). The cleaning cycle, the adhesion promotor, and the thickness of the parylene coating appifed differed from vendor to vendor, depending on the practices considered appropriate by the particular coating facili-ty at the time. The circuits were operated in a ring-counter configuration during humidity tests. Nonparylened units both with and without lids were also tested as controls, and other parylene-coated control units were kept in a dessicator. Units which had been coated by Vendor C had a hifgher failure rate than even the unprotected units. This is attributed to the adhesion promoter used. The failure modes exhibited by Vendor-C parts were: (1) Nichrome voiding at the nichrome/aluminum interface. (2) Nichrome voiding in the bulk of the nichrome resistors.