Mica is a naturally occurring group of minerals characterized by an ability to split readily into flat, thin films, with the specific type known as “muscovite” mica being preferred for capacitor applications. As a dielectric, mica offers excellent stability over time and applied voltage, a low temperature coefficient, high temperature tolerance, very good dielectric strength, and low loss characteristics over a wide frequency range. Aside from being excellent dielectric materials, mica (a naturally occurring mineral) has almost nothing in common with PTFE (a synthetic fluoropolymer) but because there’s at least one capacitor product series on the market that uses PTFE instead of mica for some capacitance values, both materials get mention in the heading…
Construction of mica capacitors varies depending on application, though similarities can be found with ceramic and film types. Whether the mica is used as monolithic sheets cleaved from a chunk of raw material, or as a “paper” made from many small flakes, an electrode/terminal attachment layer (usually silver) is deposited on two sides, and then either used singly (like a single-layer ceramic device) layered together like an MLCC, or wound like a film capacitor. Early devices produced prior to the development of metallization processes mechanically clamped sheets of mica together with electrode foils. Like other clamped capacitors, the stability and reliability of these devices was inferior to more modern types, and clamped mica capacitors have thus been obsolete at least since world war two.
Range of available values:
The chart above illustrates the range of capacitance and voltage values for Mica/PTFE capacitors available from Digi-Key at the time of writing.
Common usages & Applications:
Mica capacitors are a technological contemporary of the vacuum tube, and have historically been a device of choice where a stable, high-quality capacitance was needed. Like the vacuum tube, newer technologies offering better price/performance ratios have gained dominance and relegated mica technology to niche markets where uncommon stress factors such as nuclear radiation, extreme temperatures, or high voltage stresses justify the cost of a mica device.
Common Failure mechanisms/Critical design considerations:
Modern mica capacitors tend to be quite reliable by virtue of the stability of materials used in their manufacture, and for most application purposes can be treated similarly to a C0G ceramic device. Mechanically-induced failures stemming from vibration, impact, thermal cycling, etc. are all possible as with other capacitor types, and electrode corrosion due to moisture ingress is also a potential problem.