A Method of Grading Mica for use in High-Reliability Capacitors

The presence of iron or other impurities in mica is known to degrade the quality of mica capacitors. Insulation resistance, capacitance, and loss factor stability are frequently used as a measure of product quality. Mica, as a raw material, may be graded in quality by prescribing limits on structure defects and staining or color. These tests are slow, costly, and subjective in nature. Where high reliability is required, a more precise indicator of basic mica quality is desirable. A relatively simple and rapid technique based on the absorption of ultraviolet light has been developed. A dual beam spectrophotometer is used to make a comparison of the transmission properties of individual mica sheets with a reference standard. This allows the selection of high-quality individual pieces. Capacitors fabricated from mica showing low absorption have survived accelerated life tests. !n contrast, products intentionally fabricated from sheets showing anomalous absorption exhibit relatively short life. A continuous gradation in product quality has been demonstrated and a correlation between absorption properties and measured electrical parameters (insulation resistance, loss stability, and capacitance stability) has been established.