Installation of a Transistor in a Heat Sink

How a Heat Sink Works

• A heat sink is a thermally conductive appendage attached to the case of a transistor or other electronic component. This appendage carries waste heat away from the electronic component and dissipates this heat across the heat sink. The heat sink is made of a thermally conductive material, such as aluminum or copper.

Why Heat Sinks May Be Required

• Transistors will operate only within a specified temperature range. If a transistor overheats, it will fail quickly. A heat sink transfers excess heat from a transistor into the air surrounding the heat sink, thus keeping the transistor within its operational parameters.

How to Attach a Heat Sink to a Transistor

• Many transistors contain bolt holes that may be used to attach a heat sink. Clip-on heat sinks are available for smaller transistors, such as small-signal and general-purpose transistors. Heat-conducting paste must be applied to the joint where the heat sink comes in contact with the transistor case. This prevents air pockets from isolating the heat sink from the transistor and allows the heat sink to transfer the maximum amount of heat away from the transistor.

Other Considerations

• The metal case on some transistors (such as a transistor in a TO-3 package) can be used as an electrical terminal; in this situation it is necessary to use an electrically isolative separator between the heat sink and the transistor case. The most common separators of this type are made of mica, or silicone rubber and fiberglass. Heat-conducting paste must be applied to the joint between the heat sink and the isolative separator, and to the joint between the separator and the transistor. The separator must be sandwiched between the heat sink and the transistor so that the heat sink does not act as an electrical terminal.