TO-5 Package

The TO-5 is a type of 'metal can' package (also known as 'metal header' package) for semiconductor devices. It is hermetically sealed to protect the device from environmental factors such as contaminants and moisture.  The TO-5 is commonly used in housing transistors and integrated circuits with low lead counts such as operational amplifiers.

The TO-5 is made primarily of metal, with the microchip mounted on the die pad of its metal header and then sealed with a metal cap by high-current welding. The TO-5 looks like a TO-52 but it is bigger than the latter. 

Most TO-5 packages come with three terminals, since the TO-5 is commonly used in three-terminal devices such as transistors.  However, TO-5 packages with more terminals than this are also quite common.

Table 1. Properties of a Typical 3-lead TO-5 Package