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 Plastic (ACP) Packages
In order to overcome the limitations of the ACC package while keeping its performance advantages, Ampleon has introduced a new family of packages known as Air-Cavity Plastic (ACP2). Their structure is similar to ACC but the lid and the ringframe are made of polymers instead of ceramic. This enables the ringframe to be glued to the flange rather than brazed, and reduces the stress and distortions of the flange. This in turn allows the use of thinner matching capacitors, reducing RF losses both at the gate and the drain. The result is higher gain and efficiency compared to the ACC format. In addition, the assembly process uses strips rather than individual headers, reducing handling time and allowing more variation in lead shape and length while simplifying logistics. The improved ACP3 package has a further key enhancement by replacing the traditional CPC flange with a Cu flange which gives a 30 % improvement in thermal performance as well as simplifying the board level assembly to provide a highly effective cost-efficient RF package solution.