Rad-hard systems may not need rad-hard components

Designing for the radiation environment in space doesn't always mean using rad-hard parts, says Juergen Fedrich of SBS Technologies.

Designing for the radiation environment in space doesn't always mean using rad-hard parts, says Juergen Fedrich of SBS Technologies.

Space is no longer the sole domain of well-funded governments.

Increasingly commercial applications, such as telecom satellites, are finding homes above the earth.

But with the increasing commercialism of space, there is a migration away from the use of traditional rad-hard components.

Design teams are now asking, "How rad-hard do components need to be for the system to work?".

In general, this has to be defined on a project to project basis depending on its mission, environment, budget and criticality.

In mission critical systems, such as satellite guidance computers, true rad-hard components need to be used.

Other systems may be able to use radiation enhanced, radiation tolerant, or even commercial parts if the system and board-level design consider radiation.

In all cases, designing with radiation in mind is essential.

Designing for radiation need not be limited to space-bound systems, either, atomic power plants and aircraft components have to function under the highest radiation levels.

Further, the semiconductor industry's move to decrease device structure sizes, reduce power requirements and increase speed, has lead to increased radiation sensitivity for all applications.

Thus, a radiation-induced device failure could become a major problem, even at sea level.

The need to account for radiation effects, combined with budget restrictions, can tax the ingenuity of design teams.

System-level tolerance can be built in using redundancy and fault-tolerant system design along with shielding.

At the board level, however, designers have only the following options: mitigation at chip level by using components, which do meet the radiation requirement; mitigation at board level by using design techniques to achieve a radiation-tolerant board; or, most likely, a combination of both approaches.

Designers can add radiation tolerance to their boards by choosing to use rad-hard components.

Rad-hard component qualification does not guarantee that the device is insensible to radiation, however.

It means that the device has endpoint electrical parameters, which are tested and certified by the manufacturer, that consider radiation.

Thus, the device specifications that the board designer works with already account for the effects of radiation, building radiation tolerance into the board design automatically.

Rad-hard components are built to withstand up to a specified level of radiation.

For the die itself, radiation hardness is achieved by using special materials and processes.

Silicon on insulator (SOI), for instance, is a process with increased radiation tolerance.

At the component level, it is possible to increase hardness by using special shielding techniques for the case.

For example, using several layers of different material significantly improves shielding of the component case.

The disadvantage of using rad-hard devices is their high price.

Furthermore, not all devices are available in rad-hard versions, especially newer commercial devices, which makes a fully rad-hard design difficult to achieve.

When no better devices are available or when the subsystem is considered as noncritical, designers can use commercial components in their design.

The major effects to be considered when using commercial components are total ionising dose (TID), single event upset (SEU) and single event latchup (SEL).

Total ionising dose causes degradation of a device's electrical parameters, such as propagation delays and threshold voltages.

Commercial parts tolerate TID ranges between 3 and 30krad (Si).

Using commercial devices under high TID for longer time is impractical, as the devices will degrade by absorbing more and more radiation.

The single event upset is a radiation-induced charge pulse that can occur anywhere within the component's circuitry.

SEU on digital logic may result in a signal glitch, but can have a greater impact on memory cells or registers.

In such devices, the SEU can cause the content of one cell to be flipped.

The single event latchup (SEL) is a more serious effect.

In this case, the ionisation caused by the radiation triggers parasitic transistors, causing an internal short circuit that is self-perpetuating.

On commercial devices, a latchup causes high current draws resulting in high internal temperatures.

In the worst-case scenario, the device may melt down.

For devices that are not critical, with respect to overall system functionality, power-cycling the device's supply can cure a latchup.

Board designers can take advantage of this effect by monitoring the ICC of a device to detect a latchup, then shutting power down for a set period.

Generating an interrupt during this event will inform the (supervising) system, that a latch-up condition was seen by the affected hardware and corresponding steps (eg system boot) can take place.

An example of rad-hard components operational can be seen in the International Space Station (ISS), where an open architecture based on the VMEbus is being used.

The ISS should be finished in 2006.

With such an extended duration in space while the station is being completed, reliability in board designs is critical.

All hardware has to meet a 10-year in-orbit life expectancy and require minimal repair and maintenance.

SBS' Government Group, which specialises in designing and producing hardware for harsh environments, has supplied a number of boards and systems for use in the ISS.

While this example is for a space application, the same approach can be used for any design that might be subject to radiation effects.

That may be more designs than people think.

With the industry's move to reduce the size of device structures, radiation is becoming an important element to consider on sea level applications.

By using the right technology, qualified parts and proper design, it is possible to create the right solution for radiation-sensitive applications.

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