TO3

TO-3 or "metal can" is commonly used for very large transistors, silicon-controlled rectifiers, and, occasionally, integrated circuits. The component is mounted on a metal plate with a metal can crimped on top of it, providing good heat conduction and durability. TO-3 packages usually have three leads, though devices with four leads exist and more leads are possible. The leads pass through the metal plate and a glass seal, except for one lead which is usually attached to the case itself. They have two tabs with holes for mounting to a heatsink. Other packages tend to be used instead for newer designs, such as the TO-220 or the D2PAK which have similar thermal resistances [http://www.linear.com/designtools/packaging/therresist.pdf] but have a smaller footprint.

Typical applications

TO-3 packages are heatsinkable, and thus can be used in projects where a large amount of power is being drawn. The sides of the package are metal tabs with holes used in mounting the component to a heatsink. Thermal compound is also used to provide greater heat transfer.

The case is usually connected electrically to the internal circuitry. A mica insulator may be required to electrically isolate the component from the heatsink if the heatsink is grounded or is mounted to multiple devices. Mica insulators are favorable because they have good thermal conductivity while acting as an electrical insulator.

Advantages

*Can be used in high-power and high-current applications where equivalent components of other cases may be susceptible to damage.
*Almost all surfaces are metal for better heat conduction and durability.
*Mounting with the tabs ensures the component to be held firmly in place.
*Metal-glass seal is hermetic to protect the semiconductor from liquids and gases

Disadvantages

*More costly than equivalent components of other cases.
*Component size and lead placement makes soldering to a circuit board difficult -- off-board space must be dedicated to the components and/or their heatsinks, thereby increasing production costs.

Common components that use the TO-3 package

Common Voltage Regulators:
*LM317, voltage regulator
*LM340, voltage regulator

Common Transistors:
*2n3055, NPN power transistor

External links

* [http://www.national.com/ms/HE/HERMETIC_PACKAGES.pdf Description of several hermetic case tiles from National Semiconductor(pdf)]