Panel Mount Semiconductor Package
Transistor Package TO-3 Diamond Case style, 4-Pin
TO-3 Metal Can, 4-Pin Flange Mount Outline
Note the location of the terminals to the mounting holes in the graphic above, some devices may have a pin attached to the metal body of the TO-3 package. The picture below shows a 2N3055 in a 3-pin TO-3 package. This is a through-hole device, similar in size to a TO-23 package. TO-3 packages with 4 terminals are used for voltage regulators, multiple transistors or other semiconductors that require more than three pins.
TO-3 Metal Can, 4-Pin Flange Mount Dimensions
Flange mount. The TO-3 style of package uses a diamond shape flange which is used to attach the component surface of the TO-3 package to a heat sink to achieve thermal management of the case temperature. The case of this device is not connected [in this example]. The TO-3 case style may also be called a case-mount device. Case temperature. Case temperature is that temperature measured at a specified point on the case of a semiconductor device. Because the TO-3 is completely metal, the actual measurement location may not matter that much. In most cases the junction temperature will be hotter than the case temperature. |
 TO-3 Diamond Case |
Normal TO-3 heat-sinks are a Black Anodized Aluminum Alloy with 2 mounting holes [for the mounting screws], so the 8-lead package should work with the same heat-sinks as the 3-lead version. As the location on the heat sink which corresponds to the TO-3 terminals is a cut-out. That is, there are no terminal holes, just an area with no material. However some heat sinks do have cut-outs for the two terminals, which would not work with the 4-pin package. A generic TO-3 heat sink is shown above, note that the TO3 mounts in a diagonal. In the general case, no cut-outs are shown at all.
Also refer to Heat Sink Manufacturers, for companies making transistor heat sinks.
Use this link for additional Transistor Derating Guide lines, listed by transistor part number.
The four pin TO-3 version is not as common as the 3 lead version, but resides in the same diamond shape metal package.
