Transistor Packages

TO-220 Plastic Case, Tab Mount

Case mount. [or Tab Mount] Is a style of package which provides a method of readily attaching one surface [the tab] of the semiconductor device to a [heat sink] to achieve thermal management of the case temperature. In this case the TO-220 uses a metal tab, and a major portion of the rear of the device is also metal; however other packages may be completely metal and use the term case mount. Other factors that may effect the mounting include using a Mica washer, copper washer, and thermal grease to aid in heat transfer.
When attached to a PWB, a copper pad should be located underneath the thermal tab of the TO-220. Note that the graphic to the left has a copper pad on the PWB but does not attach the tab to the board.

Case temperature. Case temperature is that temperature measured at a specified point on the case of a semiconductor device. The JEDEC specification will define the location of the thermocouple on the case.

Heat dissipation is handled by the metal tab; however during thermal measurements the case might be restrained with a metal clip holding the plastic body or a screw through the metal tab. Which ever mounting method is used will result in two completely different values in thermal impedance. As does the location of the thermocouple, under the die [hottest], next to the die [JEDEC], or on the tab [coolest].

TO-220AB 3-Terminal Package Drawing

Note the location of the Thermal Pad in the picture above. The bottom tab [term 4] may be electrically connected to the Drain [FET] or Collector [Transistor]. However not all styles of the TO-220 package use the tab as an electrical pin. In some cases the tab is just used as a heat sink. The surface mount version of a TO-220 is the TO-263 [D2PAK]. Note that part of the TO-220 body may lift off the mounting surface as pressure is applied to the tab by the mounting bolt, use conductive epoxy to secure the body of the TO-220 to the mounting surface, normally the printed wiring board [PWB].

The dimensions table for the TO220 package also provides a pin-out example of a FET, but the terminals could just as well identify the leads of a transistor.

The photograph to the far upper left shows two voltage regulators, a 7909 and a 7809 regulator, both mounted to a printed wiring board. The 7909 is a negative 9 volt regulator and the 7809 is a positive 9 volt regulator. However; the 78xx series covers a number of different voltage ranges; 7805 [5 volts], 7812 [12 volts] and so on, all in the same package. Note that in this particular case the tab is not attached to the printed wiring board, which implies that in this case those devices are not generating the heat which would require the heat-sink tab to be secured to a heat-sink [in this case a copper pad on the PWB].

Note some company data sheets use the case ITO-220 to refer to a TO-220 with an isolated tab, which would be a tab with no electrical connection to any of the pins. So an ITO-220 would be a package that uses the tab only as a heat sink.