We all know of large, precision-guided weapons. Weighing about 3000 lb and with a unit price over $1 million, the latest-generation cruise missiles can be launched from the safety of a ship and travel over 1000 miles with the necessary precision to minimize collateral damage. They include advanced electronics such as datalinks, radar altimeters, inertial guidance and digital processing. These large, precision-guided weapons give the U.S. and allied forces a significant advantage—the long range keeps the operators away from harm, while the high precision reduces the risk of collateral damage. They represent the culmination of decades-long technology development and, while ideal for delivering large payloads, the existing technologies are heavy and expensive.
The options for surgical precision at the infantry level are far fewer, leading to the emerging need for extremely compact munitions that contain the technology for precision guidance. Commonly referred to as “smart bullets,” these precision-guided munitions require a new breed of RF and digital electronics that is not only compact, but modular enough to support a wide range of applications. Developing a precision-guided capability small enough to fit into a munition that weighs less than 0.1 percent of a cruise missile, for a fraction of the cost, requires a new approach built from the ground up. This requirement for smaller precision-guided munitions is forcing the defense electronics industry to find novel ways of building extremely compact, low-cost systems. To amortize development costs and reduce production time, a standard electronics architecture that supports a variety of applications is required. This framework must include extremely dense integration, high-reliability and a modular design. Instead of racks of sensor and processing hardware, all of the electronics must be small enough to fit into the palm of a hand and be as easy to upgrade as removing and replacing a circuit card.
STACK OF QUARTERS
To address these requirements, Mercury Systems is developing a novel architecture that incorporates its expertise with compact hardware, dense integration, modular design and high-reliability. The SpectrumSeries™ Compact Multi-Band Platform combines multiple board layers using a solderless, high-reliability approach. With a diameter as small as 25 mm, six layers can be combined with a total height also about 25 mm. Using pin-and-socket interconnects, the technology-agnostic solution combines surface-mount technology (SMT) boards, alumina substrates, chip-and-wire assembly, hermetic ceramic cavities and printed antennas. This flexible architecture provides the framework for a variety of applications, such as a simple single input/output radio or a complex monopulse radar with integrated patch antenna.
As new applications require smaller electronics, it becomes more challenging to maintain a modular design. However, a modular approach is key to reducing development time through technology re-use and ensuring a future-proof system through easy upgrades. By making changes to a limited number of design elements, a modular product can easily be redesigned to accommodate a different frequency band or use an improved component. This approach enables the most cutting-edge products, since it is possible to incorporate the rapid technology growth from Moore’s Law.
To achieve this modular design framework, the SpectrumSeries Compact Multi-Band Platform consists of multiple, compact layers that are individually manufactured and tested, then easily combined with solderless contacts. By formalizing the interconnection between layers, the designer can use a variety of technologies. Sensitive chip-and-wire components are placed in hermetic packages—a technology that has been proven on multiple programs and is small enough to fit on a single layer. This high performance, low loss approach uses bare die and wire bonds. Vias and hermetic feedthroughs on the bottom of the package provide high frequency connections to the rest of the module and outside world. Control and digital components are placed on SMT boards that use standard, automated assembly to reduce cost. For complex applications, multilayer boards allow complex signal routing for advanced digital devices. In addition to simple SMT components, these types of boards can include printed patch antennas (see Figure 1) or complex ball-grid array devices.
Critical to successfully implementing this technology are the board-to-board interconnects. DC and digital contacts are through a flex harness and DC pins around the circumference of the module. This nail-and-socket approach enables high pin count while supporting easy assembly. SMP-style RF connectors are capable of high frequency operation for both board-to-board and external connections. Mercury’s patented coaxial-to-microstrip transition technology enables high frequency orthogonal connections in an extremely compact space (see Figure 2).
This modular, technology-agnostic framework provides a starting point for any new design. As the library grows, the design process is simplified by combining pre-existing and custom layers, reducing development time, lowering cost and enabling easy product modification.
DENSELY INTEGRATED AND COMPACT
Minimizing the size and weight of the module requires more than just a modular framework; it requires densely integrated components and interconnects. While this is critical to designing a complex sub-assembly no larger than a stack of quarters, it also presents challenges, such as maintaining sufficient electrical isolation. The RF circuity is often the most sensitive to noise and radiated signals. To isolate the RF and provide environmental protection, the RF section is packaged in a hermetic ceramic or metal housing (see Figure 3), allowing the use of bare die, which requires significantly less space than individually packaged devices. Inside the package, metal walls provide channel-to-channel isolation and reduce electromagnetic cavity effects. Using advanced modeling during design and automated assembly during manufacturing, process variation is reduced, minimizing tuning time and cost.
To maintain performance during extremely harsh operating conditions, a few special considerations are required. Plastic encapsulated microelectronics on the SMT boards receive a conformal coating to provide environmental protection, which is smaller and lower cost than a hermetic cavity. Additionally, encapsulation of the sensitive RF components with a low dielectric material increases reliability and enables operation in extremely high G environments. Whether bare die or SMT, advanced manufacturing capabilities enable dense integration by tightly controlling device placement, which reduces the required spacing between devices and helps keep cost low.
MODULAR, MULTI-USE FRAMEWORK
This compact modular framework provides the flexibility to support a range of applications. For example, by stacking multiple RF cards, the module can support multiple frequency bands. By integrating the RF and digital blocks, the platform can integrate a complete sensor-chain solution: RF layers acquire and disseminate the signal, mixed-signal layers digitize the signal and the digital layers perform signal processing.
This scalable and modular approach supports a new breed of precision-guided munitions, as well as being used for other applications requiring compact and reliable hardware. For example, Group I unmanned aerial vehicles, with a maximum weight of 20 lb, require extremely compact payloads. This approach of integrating modular layers of RF, digital and control circuitry in a compact and ruggedized form factor applies broadly across countless applications. The modular nature of the SpectrumSeries Compact Multi-Band Platform enables solutions to be developed rapidly. Through compact modularity, the architecture is optimized to rapidly bring the latest technology to where it is most needed.