Analysis of high-voltage RF overmolded plastic packaging technology

With the advancement of overmolded plastic packaging technology, developers of high-power RF transistors now have a reliable alternative to traditional ceramic packaging. This innovation not only enhances performance but also significantly reduces costs, making it a game-changer in the industry. Packaging plays a crucial role in achieving optimal performance for RF power transistors. As one of the most expensive components in power amplifiers (PAs), which are themselves the most costly part of cellular base stations, reducing transistor costs without compromising performance is a top priority. Supermode compression molding has emerged as a promising solution, widely used in other power IC applications and recently refined to meet the specific needs of RF power transistors. This method delivers the required technical performance at a fraction of the cost of conventional packaging techniques. Innovations in RF power semiconductor technology can greatly influence the future of 2.5G and 3G wireless networks. Historically, high-power RF transistors were housed in leaded ceramic packages. These are mounted on printed circuit boards (PCBs) within base stations, much like line cards in telecom central offices. A typical base station contains around 8 to 10 PAs, with power transistors representing a significant portion of the total cost. Moreover, approximately 30% of base station issues are linked to PA failures, highlighting the importance of reliability in these systems. RF power transistors are typically packaged using a combination of a thermally conductive metal base and a ceramic ring, which separates input and output leads. The base is made from copper-tungsten alloy and is attached to the ceramic ring via high-temperature brazing. The package is gold-plated to facilitate die attachment through another high-temperature process. A ceramic cover is then sealed onto the ring and leads, offering environmental protection. Ceramic packaging accounts for nearly half the cost of a finished power transistor, as shown in Figure 1. Beyond just protecting the die, the package must provide electrical connections, heat dissipation, and structural integrity. Essential functions include: 1. Connecting power and signal lines of the installed chip 2. Attaching the active die and related components 3. Providing a path for heat dissipation 4. Protecting the chip and maintaining its integrity during operation To cut costs, manufacturers are moving away from expensive ceramics and complex copper welding processes. Advances in polymer materials—specifically thermoplastics and thermosets—have made this shift possible. Thermoplastics can be reshaped by heating, while thermosets undergo irreversible chemical changes when heated, resulting in strong, durable structures. When a semiconductor is packaged with a polymer, the chip is connected to the lead frame via wire bonding and then encapsulated in a polymeric insulator. This provides both electrical insulation and environmental protection. Figure 2 shows a comparison between ceramic and overmolded plastic packages. Overmolded packaging does affect RF performance slightly. In ceramic packages, the chip and bond wires are in air, whereas in plastic packages, they are surrounded by a material with a higher dielectric constant. This increases parasitic effects, leading to a small reduction in output power and gain. However, with proper design and layout techniques, this impact can be minimized to less than 0.5 dB, as seen in Figure 3. In March 2003, Allertown introduced new RF transistors for wireless base stations, based on traditional ceramic packaging. These devices produced less heat, were smaller, and more cost-effective. They improved thermal performance, reducing the number of cooling fans needed and lowering operational costs and noise pollution. Agere Systems later adopted next-generation overmolded packages, as shown in Figure 4. Early use of plastic packaging reduced costs, but internal assembly lines limited further savings. High-capacity packaging solutions, however, offer better scalability, sharing investment and expertise across multiple product lines. This approach mirrors the success of integrated circuit companies that outsource manufacturing to external partners. Agere’s latest RF power products are packaged in Amkor Technologies’ Power Small Outline Package (PSOP), manufactured in West Chester, PA. This package has achieved nearly 1 billion units in production, meeting the high throughput, low cost, and reliability demands of the RF market. Its compact, thin design ensures reliable operation in harsh environments. Amkor’s PSOP offers performance comparable to ceramic-packaged devices, with superior test results in the 2.1GHz band and significant cost savings. These overmolded packages meet current and 2006 environmental standards and are compatible with wireless board technology. Agere also introduced advanced epoxy materials for core tube bonding, replacing traditional wire bonds. Overmolded LDMOS components launched in Q2 2004 meet all thermal, electromagnetic, and physical requirements outlined in Table 1. Over time, overmolded RF transistors will reduce equipment costs for base station designers and operators. They also help save power and space, enabling new deployment strategies. Packaging innovations will drive progress in other areas too, such as Agere’s development of new venting solutions for high-output devices. These concepts can also be applied to integrated modules, further cutting costs without sacrificing performance.

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