The traditional package for RF power transistors is the air-cavity package with a ceramic lid. The flange (or heatsink) material has evolved over the years and the most commonly used material today is CPC (Cu/Mo70Cu/Cu), a laminate of copper and coppermolybdenum. This material has been selected for its thermal properties, providing a low Rth (compared to the Cu-W used earlier) as well as a good CTE (Thermal Expansion Coefficient) match with the silicon used for the active dies and the internal matching capacitors. The package is made of three parts: flange, ringframe and lid. The flange is brazed with the ringframe at high temperature and the resulting component is known as a header. Active and passive dies are then soldered to the flange and wire bonds are used to create the matching circuits and the connections with the leads. The transistor is then closed by gluing the lid on top. The final step consists of testing the product for compliance to specification.
Air-Cavity Ceramic (ACC) Packages
Air-cavity ceramic packages have proven their reliability and performance over the years and exist in a variety of sizes and power levels. They present a number of advantages but also some disadvantages. They are assembled one by one, thereby demanding a high handling time, and the brazing of the ringframe with the flange creates stress and distortions. Also, each package variation (such as additional leads, or shorter leads) demands the creation of a unique header and there are limited economies of scale.