Achieving performance expectations for highfrequency components requires proper implementation. In the case of RF power transistors, for example, the device-to-heatsink interface establishes the heat transfer path and RF ground. In application note PKG001, "Proper Mounting of RF Power Transistors," HVVi Semiconductors details the heatsink surface preparation, maximum torque required to tighten the screws, and the proper procedure for connecting leads.
The heatsink surface must be flat and free of foreign debris and excessive oxidation. In addition, the heatsink's mounting surface should be no more than 0.4 mils/inch. A surface finish of 0.03 mils or better is required for optimal thermal dissipation. The note recommends two thermal compounds from Laird Technologies (www.lairdtechnologies.com). It also cautions engineers against using too much compound.
Mounting procedures are explained in detail. The application note dictates that a split washer should be used to provide the necessary thermal expansion and compression of the bolted joint. The document concludes with advice on connecting the leads. The solder tip should not exceed 20C above the melting point of the solder. Nor should it be applied to device leads for more than 20 s at the rated temperature. It is also essential that engineers remember that the direction of the lead deflection is key to controlling stress.
HVVi Semiconductors, inc., 10235 S. 51st St., Ste. 100, phoenix, aZ 85044; internet: www.hvvi.com.