Hermetic passivation of chip-on-board circuits

Results of a screening study and evaluation of silicon nitride, silicon dioxide, and a combination of the two as passivation for integrated circuit assemblies are presented. Screening test samples consisted of silicon wafers coated with various combinations of aluminum and spun-on polyimide. A series of chemical, mechanical, and environmental tests was carried out. The passivation coatings were applied by four different vendors using CVD (chemical vapor deposition), plasma-enhanced CVD, and room-temperature reactive plasma. The most promising material was then applied to a variety of more complex samples, including triple track resistor assemblies, and active integrated circuits which were electrically tested to evaluate results. Silicon nitride at a nominal thickness of 5000 Å applied by the room-temperature reactive plasma process was judged to be the best passivation material