What is TO-3 package? TO-3 package means metal can. 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  but have a smaller footprint. TO-3 package image.