Packaging is critical to achieving maximum performance from RF power transistors. Since RF power transistors are among the most expensive components in a power amplifier (PA), and the PA is the most expensive component in a cellular base station, there is obvious motivation to reduce the cost of the transistor without sacrificing performance. The answer lies with over-molded plastic packaging technology, which is well accepted for other power integrated-circuit (IC) applications but only recently has improved sufficiently to serve the needs of RF power transistor developers. The technology provides the needed technical performance at costs roughly one order of magnitude less than the existing approach.

Innovations that drive the price/performance of RF power semiconductors have enormous potential to impact the future of 2.5G and 3G wireless networks (Fig. 2). High-power RF transistors have traditionally been housed in leaded ceramic packages. Within a base station, power transistors sit on printed-circuit boards (PCBs) that slide into a cellular base station in much the same manner as line cards in telecommunications central-office equipment. A typical cellular/wireless base station has about eight to ten PAs. Power transistors are the largest cost item in the PA and therefore are a very significant contributor to the total cost of all base stations. In addition, roughly 30 percent of the problems with base stations are related to PAs; hence, their reliability is critical to the successful operation of wireless networks.

Because of the performance and reliability demands on RF power transistors, they have traditionally been housed in packages that combine a thermally and electrically conductive metal base with a ceramic ring to isolate the input and output leads. The base is made of a copper-tungsten alloy and is covered with metals to allow attachment to the ceramic ring by means of a high-temperature brazing process. Due to the ceramic ring, this first-generation solution is known as a ceramic package. Additionally, the package base is gold plated to allow the die to be attached by means of a second high-temperature process. For its only form of environmental protection, the package is capped with a ceramic lid that is glued to the ceramic ring and the input and output leads.

With the ceramic approach, the packaging represents about one-half the total cost of a finished power transistor product (Fig. 1). Of course, for an RF transistor, the packaging not only protects the die, it also provides electrical connections and a thermal path for excess heat. In fact, packaging can be the gating factor in achieving high performance while meeting the cost objectives in many microelectronics and computer systems. Essentially, an RF package must provide:

1. Connections to the mounted chips for power and signal lines.
2. A base to attach the active die and any associated components.
3. A means of removing unwanted heat.
4. A structure to protect and maintain the integrity of connections and chips while providing a platform for handling and external markings for identification.

One obvious way to reduce cost is by eliminating the costly ceramic ring, and its cumbersome and expensive brazing process. Fortunately, innovations in polymer materials (plastics) have made this practical (Fig. 2). There are two classes of synthetic polymers: thermoplastic and thermoset materials. Thermoplastics are processed by means of heat and pressure alone, without a chemical reaction. Upon cooling, thermoplastics either crystallize or transform to a glassy state. Thermosetting polymers (epoxies, bakelite, formica) chemically react upon the application of heat, causing an increase in the molecular weight. This chemical reaction leads to full conversion of all reactive groups to produce a polymer with substantial hardness, high heat distortion temperature, and both good chemical and physical resistance.

When encapsulating a semiconductor chip with a polymer, the chip is typically connected to the package lead frame via wire bonding; subsequently they are encapsulated in a polymeric insulator, which serves as a dielectric insulator and shields against environmental degradation. Figure 3 offers a comparison of ceramic and over-molded plastic packages.

Over-molded packages do alter RF performance. In a ceramic package, chips and bond wires sit in an air cavity. In a plastic package, the polymer material surrounds and is in contact with the devices and bond wires. Since polymers have a higher dielectric constant than air, parasitics are marginally higher in a plastic package, resulting in slightly decreased output power and gain compared to the same device in a ceramic package. However, by using appropriate device design, layout, and wire-bonding techniques, the impact of plastic-package parasitics could be minimized to less than 0.5 dB (Fig. 4).

In March 2003, Agere Systems (Allentown, PA) unveiled a new line of 21 breakthrough transistors targeting the wireless base-station PA market. Based on traditional ceramic packaging, these products enabled much cooler, smaller, and less expensive wireless base stations. By achieving new thermal performance levels, these semiconductor products offered the potential to cut in half the number of cooling fans in base stations, offering service providers lower capital costs and operating expenses while also reducing noise pollution. The company is now migrating to next-generation RF devices assembled in high-volume over-molded plastic packages (Fig. 5).