Commercial Satellite Programmes Drive Rad-Hard Technology Evolution

Satellite electronics is a specialized market with a unique set of environmental and performance requirements. A modern communications satellite is a costly undertaking, ranging around $250 million, with launch and insurance costs adding to the expense. The system then must operate for some 15 years in an extremely hostile environment of vacuum, temperature extremes, and intense radiation.

Communications satellites are flown in geosynchronous orbits for optimal Earth coverage. These systems, then, can’t be maintained or repaired directly (Fig. 1). That 15-year lifetime is for entirely unattended operation. Accordingly, the component parts must be reliable and radiation-hard while remaining at least somewhat affordable.


1. Communications satellites operate in geosynchronous orbit for maximum coverage, making maintenance and repair very difficult. Parts must be able to withstand 15 years in the harsh space environment.

Component procurement can be carried out using several models. The most traditional model uses a combination of stringent process controls, screening, and acceptance testing to produce a part with ensured quality, reliability, and radiation hardness. These parts can be upscreened commercial parts or specifically designed hardened parts.

MIL-PRF-38535, which defines the Qualified Manufacturer List (QML) system, controls this category. QML Class V components are the most frequently used in space applications. Other approaches include upscreening of commercial parts, which is used extensively but requires an accurate cost model to determine if an actual cost reduction over QML parts is being realized, and shielding or spot shielding at the part or box level.

The extensive screening requirements for a Class V part result in very high average selling prices (ASP), and the drive toward commercial rather than government space programmes has resulted in cost reduction pressure. This is forcing increased consideration of commercial, non-Class V parts. A standard approach has not yet evolved, but the emergence of a downscoped QML equivalent is likely to include plastic packages, revised radiation hardness criteria, reduced screening, and acceptance testing requirements and reduced process controls.

Reprogramming In-Orbit

Components used in space applications have predominantly used hermetic packages. Until recently, plastic packages have seen minimal penetration, depending on the criticality and dollar value of the system. This is changing, in this case driven by rapidly increasing input/output (I/O) counts for large-scale digital parts such as FPGAs.

These parts are huge enablers since they can change logic configurations in software rather than hardware—and do so in-orbit. FPGAs routinely have thousands of I/Os, so conventional chip and wire technology is simply inadequate. The widespread market acceptance of FPGA devices for space systems (as indeed has happened in all electronic systems, not just space systems) has in turn driven the acceptance of nonhermetic packages in the form of ceramic substrate-based flip-chip column grid arrays (CGA).

A user/supplier task group developed the Class Y appendix to MIL-PRF-38535, which is nearly complete. It includes nonhermetic CGA devices but does not cover conventional injection molded plastic packaging or chip-scale packaging.

Mounting cost reduction pressure from the commercial markets may well force yet another class to the QML system to accept plastic parts. Nonhermetics differ from conventional hermetic packages in outgassing, contamination control, mechanical properties, and radiation hardness. Class Y has introduced a number of revised screening and qualification requirements to deal with these issues.

New Understanding Of Radiation

The understanding of the effects of various radiation environments on space systems has evolved over the years and has changed as minimum dimensions and circuit complexity have progressed. It is now widely known that some components show an enhanced sensitivity to low-dose-rate radiation.

Historically, ionising radiation tests were performed at perhaps 100 rad(Si)/s, using gamma rays emitted by the radioactive isotope 60Co. This represents an approximation at best of the space environment, which consists mostly of electrons and protons.

In the early 1990s, U.S. researchers noted enhanced low-dose-rate radiation response in components containing bipolar junction transistors, with greatly increased parametric degradation at dose rates as low as 0.01 rad(Si)/s.

Enhanced low-dose-rate sensitivity (ELDRS) has turned into a key issue, as the actual dose rate in space is in many cases even lower than this. Many acceptance tests at these low dose rates also take an inordinate amount of time. For example, a 50-krad(Si) test takes more than eight weeks. Most suppliers of space parts now offer low-dose-rate testing in some form, either on a characterization basis or in some cases on a wafer-by-wafer acceptance testing basis.

Single-Event Effects

The understanding of single-event effects (SEE) due to protons or cosmic rays has also progressed greatly. SEE hardness in commercial parts has improved as minimum dimensions have fallen and as silicon on insulator (SOI) technology has become more prevalent.

Passive isolation addresses effects such as single-event latchup. The use of large logic devices has led to reductions in the supply voltages required for these devices, and the understanding of single-event transient phenomena in power management devices has evolved in response.

As an example, buck dc-dc converters known as point-of-load regulators (POLs) commonly supply dc power to FPGA devices. The voltages involved can be as low as 1.0 V, and a cosmic-ray induced transient on the output of a POL can result in immediate destructive damage to the FPGA.

This is an interesting issue in the context of commercial space applications. While commercial part upscreening procedures can ensure ionizing radiation hardness, most types of SEE hardening require architectural or process modifications, and upscreening is not always effective.   

Power Distribution Systems

General satellite electrical system architecture including signal processing, RF functions, and power conditioning and distribution has not changed materially. Power distribution systems have trended toward higher bus voltages to reduce copper mass, and this has combined with lower subsystem and component supply voltage requirements to lead to multiple levels of distribution bus.

Box-to-box communications have progressed from earlier protocols such as RS-422 to high-speed protocols such as low-voltage differential signalling (LVDS), which is now a standard way of communicating in space systems. A longer-range trend is the adoption of fiber optic (FO) interconnects, which has been ongoing but hampered by hardness concern over the electro-optical (EO) components used.

A final trend is the gradual improvement of part radiation hardness as process technology advances. As an example, deep-submicron CMOS processes have very thin gate dielectric layers, and the oxide volume available for charge trapping in the total dose environment is greatly reduced.

This leads to commercial parts that are inherently hard to high levels of total ionizing dose, which is a part of the changing dynamic in the space market. The parts are hard but are commercial and are not specified, screened, or guaranteed for the various environments. Intersil is introducing a line of Class V-compliant operational amplifiers built in a commercial bonded wafer SOI process (Fig. 2). These parts display excellent total dose hardness without any design or process changes.


2. Intersil’s ISL70218 features a low 850-µA supply current that’s ideal for power-sensitive satellite applications.

Conclusion

The hardened electronics business is seeing many changes, some driven by the move toward commercial rather than government space programs in the United States, the trend toward nonhermetic packaging as driven by greatly increased I/O counts, and the improvements of commercial part performance in various radiation environments.

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