This invention relates to a novel mounting structure for mounting power semiconductor devices such as triacs or silicon controlled rectifiers, and more specifically relates to a novel insulation mounting structure which enables the mounting of the semiconductor device so that it is thermally coupled to an underlying heat sink but is electrically insulated therefrom.
Power semiconductor devices have numerous applications. By way of example, they can be used in wall box dimmers of the type shown in U.S. Pat. Nos. 3,746,923 and 3,801,874.
The mounting of such semiconductor power devices requires a relatively large heat sink to which the device is coupled so that the device can be properly cooled. It is also usually necessary that the semiconductor device be electrically insulated from the electrically conductive heat sink to avoid the danger of shock to a user who might come in contact with the heat sink. The mounting structure should also be of a nature to be easily manufactured and should have sufficient strength to allow handling of the semiconductor device as when attaching leads to the device terminal without causing mechanical damage.
Some semiconductor devices are manufactured with a semiconductor die contained within the device which is electrically insulated from its conductive outer surface. Such isolated devices can then be mechanically connected to the heat sink by any appropriate fastener, such as a screw or rivet. Thus, the device is inherently electrically insulated from the heat sink. These devices, however, are more costly than devices which do not have internal isolation and generally have poorer heat transfer characteristics than unisolated devices.
Another known method for mounting semiconductor devices employs a metalized ceramic disk which is first soldered to the device and then soldered to a subplate of conductive material which is fastened to the heat sink. The metalizing process and soldering process are costly and such devices tend to be sensitive to the heat of soldering. Additional thermal interfaces are also introduced.
It is also known to secure semiconductor devices to their heat sink by means of an epoxy cement, using a thin, electrically insulative cement layer. This process requires great care during the manufacturing process and a high degree of cleanliness for proper mechanical adhesion. Moreover, the adhesives are relevantly poor thermal conductors.
It is also known to provide mechanical clamps which will press the semiconductor device against a thin insulation layer inserted between the device and the heat sink. These clamps require a large area and are usually metallic so that they too must be electrically isolated from the device or from the heat sink.
Other connecting systems are known using mica disks with nylon shoulder washers and screws but these are ineffective because the mica is a poor heat conductor and the nylon shoulder washers will flow under pressure, thus reducing clamping force and heat transfer. An arrangement similar to this is disclosed in U.S. Pat. No. 3,801,874 in which a thin insulation sheet is disposed between a TO-220 outline semiconductor device and the heat sink. The opening in the mounting tab of the TO-220 device is aligned with an opening in the insulation sheet and an opening in the heat sink and a single conductive screw in employed to compress the three components together. An electrically insulative shoulder washer is required to ensure electrical insulation between the conductive screw and the conductive tab of the semiconductor package. A deformable insulation collar fills in the remaining air gaps in the mounting structure. Thus, a large number of separate parts is needed.