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Steve Munns at Linear Technology Corporation explores factors influencing the continued use of hybrid modules and why the most recent technology developments have been so widely adopted for military equipment design
Hybrid circuits were developed back in the late 1950s with funding from the U.S. Army to increase the integration of transistor circuits for the production of miniature radios and computers. Today, more than 50 years later, the military still use hybrid circuits, but the problems they sought to address have evolved.
In the late 1950s, advances in computing using discrete transistors made huge strides forward, but circuit boards were becoming increasingly complex, sometimes with thousands of interconnected transistors, diodes, resistors and capacitors. A solution was needed to improve the density and reliability – Government agencies funded various hybrid circuit ideas.
The military was especially interested and active in this field. In 1958, the US Army funded RCA to develop their ‘micro-module’ concept. This took the approach of uniform-sized cubes externally configured so that they could be clipped together. Internally, small wafers of the various discrete components were vertically stacked and interconnected at their edges.
Component density by volume was more than doubled and reliability increased by a factor of six, which delighted the military who invested further over the coming few years. In 1962, according to an article in Electronic Engineering Times, a ten-component module cost $52, around 2.5 times the cost of a conventional discrete PCB solution.
Despite its cost, the RCA micro-module was very successful but short-lived and the birth of the integrated circuit (IC) no doubt contributed to its demise. At nine times the cost of a hybrid solution, early ICs were often the beneficiary of military or Government funded programs and one notable 1962 project was the Raytheon-built Apollo Guidance Computer for NASA.
With the rapid evolution of ICs, their advantages over hybrids and modules were soon recognised. In that regard, it seems surprising that hybrid circuit technology survived at all. However, the military often had broader considerations, including product stability and long-term availability, reliability, utility versus innovation and complex operational requirements. These factors together with the specific technical advantages of hybrids and modules have undoubtedly contributed to their continued usage over the last 50 years.
The path to integration
In the timescale spanned by this article, ASIC technology has revolutionised the industry. Initially, gate arrays of a few hundred gates offered the military a route to improved digital integration and as gate densities rapidly advanced and development tools were refined, the days of the hybrid circuit looked numbered.
In the late 1980s, military equipment designers recognising the success of digital ASICs, attempted to apply the same techniques to mixed-signal circuits. Their motives were dominated by the need for miniaturisation, as more and more complex systems were demanded by the military, which had big budgets at that time. But difficulties adapting full custom design tools for customer use and the complexity of analogue design, meant that mixed-signal ASIC remained very resource intensive and highly reliant on semiconductor manufacturer design teams for, in effect, full-custom designs.
While analogue ASIC design tools and technology have made significant progress, the breadth of real-world analogue problems are still difficult to implement with off-the-shelf semi-custom circuits. Thus, hybrid circuits have continued to offer a means of integrating various high-performance analogue and signal-chain functions into a single package when off-the-shelf products will not do the job.
Military and aerospace systems are generally designed on the basis of modular sub-systems. For example, line replaceable units (LRUs) simplify service and operational support. Interconnecting LRUs rely on standards such as the MIL-STD-1553 bus interface. Implementing these functions in hybrids, modules, ASIC macros or on standard format circuit boards has become the preferred approach and they are, in effect, application specific standard products (ASSPs).
This highlights two important factors. First, there is nothing to be gained by re-inventing the wheel and designers are more usefully employed focusing on the core intellectual property of the system. Secondly, the military and aerospace industry is a small user of semiconductors by today’s standards and developing solutions at the module or board level is a more realistic proposition than a monolithic IC-level ASSP.
Traditionally, the performance requirements of power supply modules has also aligned well with hybrid module technology where the use of hermetic metal can packaging supports the power density and thermal management needs of high-temperature, high-reliability military applications.
With increasing power requirements from large FPGAs and microprocessors, the drive for more efficient power architectures and point-of-load (POL) regulation has given rise to new module solutions. Applications such as radar have also long-relied on hybrids and modules for RF and microwave solutions. Only in recent years have there been monolithic IC products that begin to address some of these needs.
Product obsolescence is an acute problem for the military. A 30- to 50-year program life is commonplace so military and aerospace equipment suppliers are continually looking for ways to mitigate risk. Hybrids and modules have been one approach to try and isolate the defence industry from the rapid pace of change in the semiconductor industry. Memory modules are a specific area of interest because of the particularly short lifecycle of DRAM and SRAM technology. The concept of a standard form factor and pin-out can be maintained, while the memory dice inside the module can be updated.
On another level, the use of standard format embedded processor cards provide an alternative higher-level approach where space permits. However, the notion of standard form factors is central to many obsolescence management strategies and has certainly been a major influencing factor in the long life of hybrid and module solutions.
Hybrids and modules also benefit because a full-custom module can be used to conceal valuable intellectual property related to the hardware design, making it more difficult to reverse engineer. Simply looking at part numbers on packages will not be enough to decode the design hardware. Furthermore, some semiconductor dice are also not readily available to everyone. For example, Linear Technology controls the supply of dice very tightly and will not sell product without disclosure of the end customer and application.
The previous arguments for continued use of hybrids and modules in military systems remain valid. However, it is important to recognise that the commercial pressures on military equipment manufacturers are greater now than ever before, and in particular, costs and time to market.
Full custom hybrid designs are expensive and take a relatively long time to develop. More alternative monolithic IC solutions are becoming available every year. While new hybrid designs are still developed at major defence companies, there is a detectable trend towards out-sourcing manufacture as volumes decline.
The picture for commercial-off-the-shelf (COTS) modules is quite different. There is definite momentum toward module-based solutions driven by both technical and commercial considerations. Switching power supplies are one area that is particularly well suited to module implementation as efficient designs need specialist knowledge that is a scarce commodity in today’s military design teams.
Switching regulators such as Linear Technology’s µModule are one example of modern day COTS modules. Launched in 2005, the first product provided a complete 10A DC/DC regulator in a 15mm square surface mount package (Figure 1). Subsequently, a complete family of µModule power products have been developed, including a wide range of power levels, device architectures and features. The same package technology has been used to implement interface and signal chain functions.
Resembling a surface mount IC, each of these regulators include a complete system-in-a-package solution that simplifies design and minimises external components. Internally, the layout and design are optimised for electrical and thermal efficiency. Table 1 shows a selection of the extensive reliability data that has been accumulated. This includes severe board mount tests required for military applications.
COTS Module package styles
Military-grade hybrids frequently use ceramic multichip module packaging and power modules continue to utilise metal can packages. High power regulator modules in particular, traditionally come with a bolt-down or brick type configurations and are intended for rack level or LRU supplies. These tend to be bulky, heavy and are configured to supply multiple voltage rails to circuits and boards, distributed from a single point.
More recent developments, such as the Linear Technology µModule regulators, are well suited to the newer POL type power architectures where regulation may be organised to drop the input voltage in several smaller and more efficient steps.
In military systems with large FPGAs requiring tens of amps at around one volt, it is desirable to position the regulator close to the load to optimise regulation and to distribute heat more evenly. The high efficiency and excellent thermal properties of the LGA package (shown in Figure 2) makes it ideal in military systems. BGA options are also available.
Military Plastic (MP grade) versions of µModule products provide guaranteed data sheet performance through 100 percent electrical testing at -55ºC and 125ºC. All MP products also receive enhanced screening, reliability monitoring and tighter in-process controls.
Fifty years ago, hybrids and modules were a technology of choice to miniaturise electronic circuits and improve their reliability. As the semiconductor industry became commoditised and product lifecycles diverged from those of the defence industry, hybrids and modules found a new role in obsolescence mitigation. While ASIC became the preferred approach for integration of digital electronics, the hybrid module found a niche solving difficult analogue problems.
Meanwhile COTS modules have emerged in the form of Application Specific Standard Products, notably for power supplies, processors and interfaces. These have been widely adopted as military equipment providers react to a new competitive age and recognise the importance of focusing scarce design resources on core competencies.
Today, defence budget pressures and shorter design cycles may make full custom hybrids increasingly a legacy solution, but without a doubt, COTS modules are increasingly becoming a technology of choice for the military and aerospace industry.
Linear Technology Corporation
