mica insulator
A sheet of electric insulator material made from mica placed between a power semiconductor device and the heat sink.
mica insulator: thermal path with zero electrical conductivity
A mica insulator is a thin sheet of layered silicate mineral placed between a power semiconductor (such as a transistor or diode) and a metal heat sink. Its job is to block electrical current flowing to the heat sink while allowing heat to transfer away from the junction. The semiconductor generates both heat and electrical potential; the mica keeps those isolated from each other.
Mica's insulating strength comes from its crystalline structure, which forms parallel atomic layers that resist electron flow perpendicular to the plane. Electrical breakdown voltage for typical mica sheets ranges from 2 to 4 kV per millimeter of thickness. At the same time, mica conducts heat reasonably well for an insulator, with thermal conductivity around 0.3 to 0.4 watts per meter-kelvin. This unusual combination makes it nearly irreplaceable for this application in power electronics.
Specification and installation
Mica insulators are sold in standard thicknesses from 0.05 mm to 0.5 mm, cut to fit common semiconductor packages. The sheet is clamped between the device case and the heat sink using thermal compound or grease on both faces to fill microscopic air gaps that would trap heat. Aluminum oxide and boron nitride sheets serve similar functions but are more brittle; mica remains flexible and less prone to cracking under thermal cycling.
Moisture absorption is a real failure mode. Mica that has absorbed humidity loses insulating strength and can cause leakage current to the heat sink at high voltages. Storage in sealed packaging with desiccant is standard practice. In humid climates, mica insulators must be kept dry from receipt through installation, or thermal conductive adhesives designed for electrical insulation should be substituted.
The name derives from the mineral itself, which has been mined and split into sheets for centuries. Industrial mica insulation emerged in the mid-twentieth century as power transistors and thyristors demanded a lightweight, mechanically simple thermal interface that did not require expensive potting compounds or epoxy encapsulation.