TO-3 at Motorola around 1955. The lead spacing was originally intended to allow plugging the device into a then-common tube socket. The metal package can be attached to a heat sink, making it suitable for devices dissipating several watts of heat. Thermal compound is used to improve heat transfer between the device case and the heat sink. Since the device case is one of the electrical connections, an insulator may be required to electrically isolate the component from the heatsink. Insulating washers may be made of mica or other materials with good thermal conductivity. The case is used with high-power and
Glass-to-metal seal Glass-to-metal seal Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic components. Properly done, such a seal is hermetic (vacuum tight, good electrical insulation, special optical properties e.g. UV lamps). To achieve such a seal, two properties must hold: Thinking for example about a metal wire in a glass bulb sealing, the metal glass contact can break if the CTEs (coefficient of thermal expansion) are not well aligned. For the