With the advancement of overmolded plastic packaging technology, developers of high-power RF transistors now have a reliable and cost-effective alternative to traditional ceramic packaging. This innovation is changing the landscape for RF power devices, offering performance that can match or even exceed ceramic solutions.
Packaging plays a crucial role in achieving optimal performance from RF power transistors. These components are among the most expensive parts of power amplifiers (PAs), which themselves are the most costly elements in cellular base stations. Therefore, reducing transistor costs without compromising performance has become a top priority. Supermode compression molding technology offers a promising solution that has already been widely adopted in other power IC applications. It provides the necessary technical performance at a fraction of the cost compared to conventional methods.
Innovations in RF semiconductor packaging are expected to influence the development of 2.5G and 3G wireless networks. Historically, high-power RF transistors were packaged in leaded ceramic enclosures. In base stations, these transistors are mounted on printed circuit boards (PCBs) that act as line cards within telecom central office equipment. A typical base station includes around 8 to 10 PAs, with power transistors being the most expensive part of each PA. Their cost represents a significant portion of the total base station cost. Moreover, about 30% of base station failures are related to PAs, making their reliability essential for the smooth operation of wireless networks.
To meet the performance and reliability demands of RF power transistors, they are typically housed in packages that combine a thermally conductive metal base with a ceramic ring to isolate input and output leads. The base, made from a copper-tungsten alloy, is bonded to the ceramic ring through a high-temperature brazing process. The package is gold-plated to attach the die using another high-temperature process. A ceramic cover is also used to protect the internal components.
Ceramic packaging accounts for nearly half the cost of a finished power transistor (see Figure 1). Beyond just protecting the die, the package must also provide power delivery and heat dissipation. Packaging is not only key to performance but also a major factor in cost reduction across many microelectronics systems. An effective RF package should:
1. Connect the power and signal lines of the installed chip
2. Attach the base of the active die and all related components
3. Provide a method for heat dissipation
4. Offer structural protection and maintain connection integrity while enabling easy identification
The way to reduce costs lies in eliminating expensive ceramic rings and complex copper welding processes. Fortunately, advances in polymer materials—such as thermoplastics and thermosets—make this possible. Thermoplastics are processed by heating and pressure without chemical reactions, while thermosets undergo chemical changes when heated, resulting in a rigid, durable structure.
When a semiconductor is packaged with a polymer, the chip is connected via wire bonding and then encapsulated in a polymeric insulator that acts as both an electrical insulator and a protective layer. Figure 2 shows a comparison between ceramic and overmolded plastic packages.
Overmolded packaging does affect RF performance. In ceramic packages, the chip and bond wires are in air, whereas in plastic packages, the polymer surrounds them. Since the dielectric constant of the polymer is higher than air, parasitic effects increase, slightly reducing output power and gain. However, with proper design, layout, and wire bonding techniques, these effects can be minimized to within 0.5 dB (see Figure 3).
In March 2003, Allertown introduced 21 new breakthrough transistors for the wireless base station market. These products, based on traditional ceramic packaging, allowed for smaller, cooler, and more affordable base stations. The thermal performance of these devices was so advanced that it reduced the number of cooling fans in base stations by half, lowering capital and operating costs while also reducing noise pollution.
Agere Systems is now transitioning to next-generation RF transistors packaged in high-capacity overmolded packages (see Figure 4). Early implementation of plastic packaging solutions helped reduce costs, but internal assembly lines limited further savings. High-capacity packaging allows shared investment, knowledge, and innovation across multiple product lines, similar to how integrated circuit companies benefit from external manufacturing.
Agere’s latest RF power product line is packaged in Amkor Technologies’ Power Small Outline Package (PSOP) in West Chester, PA. This package has achieved nearly 1 billion units in production, meeting the high throughput, low cost, and high reliability required by the RF electronics market.
This compact, thin package operates reliably in harsh environments. Manufacturers focus on material selection and assembly processes to address issues like flatness, coplanarity, line sweeping, delamination, solderability, and cost. Supermode compression molding also offers additional advantages: it enables mainstream manufacturing processes accepted by the electronics industry.
Amkor’s PSOP-packaged transistors perform comparably to those in more expensive ceramic packages, delivering superior test results in the 2.1GHz band while significantly cutting costs. These reliable overmolded plastic packages meet current and 2006 environmental standards and are compatible with wireless board technology. Agere has also developed highly conductive epoxy materials for core tube bonding, replacing traditional wire bonds.
Agere’s overmolded LDMOS components, launched in Q2 2004, meet the thermal, electromagnetic, and physical requirements outlined in Table 1. Over time, overmolded RF power transistors will help reduce costs for base station designers and network operators, while also saving power and space. This opens up new deployment possibilities for base stations.
Packaging innovations will drive progress in other areas as well. For example, Agere is developing a new venting solution for high-output power devices. Additionally, these packaging concepts can be extended to integrated modules, further reducing costs without sacrificing performance.
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