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Cross section of 2N2222 transistor in metal TO-18 package, showing connection wires between external pins and die. Size comparison between (left to right) TO-39, TO-18, and TO-92 packages. In electronics, TO-18 is a designation for a style of transistor metal case. The case is more expensive than the similarly sized plastic TO-92 package.
The case is used with high-power and high-current devices, on the order of a few tens of amperes current and up to a hundred watts of heat dissipation. The case surfaces are metal for good heat conductivity and durability. The metal-to-metal and metal-to-glass joints provide hermetic seals that protect the semiconductor from liquids and gases.
Leaded chip carriers have metal leads wrapped around the edge of the package, in the shape of a letter J. Leadless chip carriers have metal pads on the edges. Chip carrier packages may be made of ceramic or plastic and are usually secured to a printed circuit board by soldering, though sockets can be used for testing.
A notable characteristic is a metal tab with a hole, used to mount the case to a heatsink, [3] allowing the component to dissipate more heat than one constructed in a TO-92 case. Common TO-220-packaged components include discrete semiconductors such as transistors and silicon-controlled rectifiers , as well as integrated circuits .
The typical TO-8 metal can package has a base diameter of 15.24 mm (0.600 in), a cap diameter of 12.29 mm (0.484 in), and a cap height of 7.62 mm (0.300 in). [1] The lead diameter is nominally 0.76 mm (0.030 in). The leads are arranged on a circle with a diameter of 7.16 mm (0.282 in). The minimum length of the leads is 10.16 mm (0.400 in).
Early semiconductor devices were often inserted in sockets, like vacuum tubes. As devices improved, eventually sockets proved unnecessary for reliability, and devices were directly soldered to printed circuit boards. The package must handle the high temperature gradients of soldering without putting stress on the semiconductor die or its leads.