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:
- Connections to the mounted chips for power and signal lines.
- A base to attach the active die and any associated components.
- A means of removing unwanted heat.
- 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).
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While early implementations of plastic packages solutions offered some cost reduction, the use of internal assembly lines hindered potential cost savings. A more attractive approach is the use of high-volume packaging facilities, where significant capital costs, knowledge, and innovations are shared among multiple product lines. This is directly analogous to the significant cost reduction experienced by IC design companies who chose to use world-class external foundries.
The latest family of RF power products from Agere is housed in over-molded plastic Power Small Outline Packages (PSOPs) from Amkor Technologies (West Chester, PA). Nearly one billion of these packages have been produced to date, with the high yields, low cost, and high reliability required by the RF electronics market.
The small, thin housings were designed to operate reliably in the harshest environments. Particular attention was focused on material sets and assembly processes to address user issues such as flatness, coplanarity, wire sweep, delamination, solderability, and cost. Over-molded plastic technology as applied to RF power transistors offers another important technology and business advantage: this packaging places RF power transistors into the accepted mainstream manufacturing process required by the electronics and communication industries.
The company's PSOP-packaged transistors approach the performance of the same devices mounted in more-expensive ceramic housings, delivering excellent results through 2.1 GHz at significant cost savings. These reliable over-molded plastic packages also meet both current and 2006 environmental standards while being compatible with lead-free board assembly technologies. In addition, the company has shown that a new generation of highly conductive epoxies can be used to perform the die-attach operation, replacing the lead-based solders of earlier solutions. Agere subjects its over-molded ICs to an extensive set of tests to ensure that the low-cost over molded part is a suitable replacement for traditional ceramic parts. Early in the second quarter of 2004, the over-molded LDMOS parts will meet the thermal, electromagnetic, mechanical, and physical qualifications outlined in Fig. 6.
With time, over-molded plastic RF power transistors will lead to significant cost reductions for wireless base-station equipment designers and network operators. It is also feasible that the reduced power and space requirements will enable operators to deploy base stations in new and novel ways. The packaging technology innovation is certain to spur other packaging innovations. For example, Agere is already working on novel air-cavity solutions for higher-output-power devices. In addition, these packaging concepts can be extended to integrated modules for even greater cost reductions without compromising performance.
ACKNOWLEDGMENTS
The authors would like to thank their colleagues at the Analog Products Division of Agere Systems for their work supporting this new package development.
- Ceramic power-transistor packages such as these can amount to about one-half the cost of the device.
- Over-molded plastic packages (left) can effectively and reliable encapsulate devices at power levels of 60 W and more (right).
- These cross-sectional views compare over-molded plastic- (left) and ceramic-packaged (right) power LDMOS devices.
- Through careful, design, layout, and bond-wiring techniques, the electrical performance of a plastic package (left) can approach that of a more expensive ceramic package (right).
- Using high-conductivity epoxy for die attachment (right), improved thermal performance can be achieved in a plastic package compared to a similar device attached with low-conductivity epoxy.
- Over-molded plastic-packaged LDMOS devices are being developed this year to meet these requirements.