The increasingly harsh demands of space
28 February 2011
The importance of high-reliability technology used in space was highlighted back in August 2010 by the problems affecting GOCE. Bob Hunt, Head of Strategic Technology for C-MAC MicroTechnology explores the subject.
This flagship European Earth observation satellite was on a mission to make precise maps of how gravity varies across the globe when it was struck by a computer glitch that left it unable to transmit its data down to Earth.
Although this event was in fact a software glitch, a hardware failure earlier in the year highlighted the difficulties of dealing with reliability issues in space. Similarly, last July the International Space Station ran into trouble when half the cooling system suddenly shut down, necessitating spacewalk repairs.
The message is clear; reliability is crucial in components designed for harsh environments, and one of the most extreme environments can be found in the high radiation levels of space.
The majority of electronic components operate in relatively benign environments, running well within their design specifications for many years. In contrast, the components and assemblies destined for use in very harsh environments (wide temperature ranges, high vibration conditions and confined spaces for example) are often employed in mission-critical applications where failure is catastrophic.
While everyday electronic components are often discarded without a second thought when they fail, the electronic systems used in space must be absolutely reliable in order to meet the objectives of the mission. Failure is not an option
The cost of failure multiplies exponentially in space applications. Satellite programmes cost millions and, as the GOCE issues demonstrate, even small errors can undermine the very reason they were sent into space, especially when under the scrutiny of taxpayers. The long life of systems sent into space requires components to be completely reliable. They need to function for the life of the flight as failure would not only be expensive and detrimental to reputations, but could also mean the early end of a mission, or even a risk to life.
The rigours of space and the high altitude atmosphere create a particular set of conditions. Protection from radiation is widely recognised as critical to the survivability of electronics in satellite systems. High levels of cosmic radiation on satellites threaten to influence, shorten the lifespan or even destroy components, so package design with radiation hardened devices and with filtering or shielding is crucial to ensure the high reliability of a system. That’s why radiation hardened (or rad-hard) products are essential in most applications.
In addition to ionising radiation, thermal energy is a big concern. The choice of packaging materials and design partitioning will significantly influence the operating temperature and hence performance of the product and these design steps must be addressed diligently in order to avoid junction failures of the active semiconductors, which for silicon devices are constrained by the technology to a continuous maximum operating temperature of 150°C – 200°C. Packaging impacts heavily on device junction temperature and must be designed to ensure that manufacturers' recommended maximum temperatures are not exceeded.
Weight is also a huge concern when launching satellites or other spacecraft, so naturally electronics need to be as compact as possible, while maintaining total reliability.
Electronic packaging technologies have been advancing rapidly over the last few years as manufacturers continue to design smaller, lighter, more effective solutions to meet the increasingly complex demands of fast-paced industries including defence and aerospace.
Defence is an industry that necessitates absolute reliability from its electronics; it can genuinely be a matter of life or death on a day-to-day basis. For hermetic products, this is reflected in the rigorous tests dictated by military standards such as MIL-PRF-38534 and MIL-STD-883. The space industry has learned from these markets, and so MILPRF-38534 Class K has become the defacto space grade test and qualification standard for hybrid microcircuits and the engineering teams responsible for these standards ensure that electronic component test methods maintain pace with the fast developing packaging technologies.
To complement the package, the interconnect substrate of choice for such demanding applications remains conventional 96% aluminium oxide, and this provides a rigid, stable and robust platform for the interconnect and circuit elements.
Technology pushing the limits
However, for the appropriate application, an alternative ceramic technology, low temperature co-fired ceramic (LTCC), has become increasingly relied-upon. LTCC technology has proved itself in a range of applications, including high-volume, harsh environment automotive systems such as engine and gearbox management, and it is now becoming the technology of choice; particularly for high frequency electronic applications as it provides a number of benefits for multi-layer interconnect.
LTCC enables the embedding of passive components such as resistors, inductors and capacitors within the multi layer structure along with the conductor’s traces.
These techniques result in minimising the length of interconnects, improving integration and robustness, and further reducing circuit geometry. It also has the advantage of low development costs and the possibility of cost effective low and medium volume manufacture.
Multi-layer substrate interconnects manufactured using this co-fired technique can be processed using similar methods as the more conventional thick film technology.
When they have been fired, laser trimmed and singulated, they can then be populated using standard surface mount and advanced chip-and-wire techniques to establish the full electronic functionality.
Despite the increase in the usage of COTS component grades for aerospace and defence electronics, the packaging of choice for the extreme environments of space continues to be hermetic cavity enclosures of metal or ceramic.
The commercial market struggles to meet the parameters for space, primarily because of the naturally occurring radiation, the temperature extremes and rapid temperature excursions, not to mention the harsh mechanical stresses occurring during launch. In particular, one aspect affecting hermetic component reliability is the internal atmosphere within the package cavity.
This is heavily affected by the sealing process controls and the choice of materials used for the substrate and component attachment. Inappropriate organic materials can out-gas corrosive contaminants over time and destroy the integrity of the internal cavity. Ceramic is non-porous and hence provides the basis for hermetically sealed packages, avoiding the out-gassing issues associated with PCB epoxy laminates.
Out-gassing of moisture is a key concern as it can ultimately lead to failure due to resultant leakage currents, which in themselves can cause failure, but can also lead to corrosion, open circuits and other associated failure mechanisms.
The correct choice of material together with tightly controlled vacuum bake and sealing processes ensures that the low moisture environment is maintained over the full lifespan of the product.
Reliability is always relevant
The need for high-reliability electronics that can perform in harsh environments is similar across the space, defence and aerospace markets; it's just the drivers that differ. The space industry has learned a great deal from these other sectors, and as such, has adopted materials proven in applications used in different industries.
As the cost of electronics becomes more significant in the overall budget, so will the drive to find more cost effective solutions for key applications. Cost-reduced quasihermetic modules are finding an increasing number of applications where metal enclosures have been previously used.
Specialist high-reliability packaging manufacturers with extensive experience across markets are ideally placed to address ongoing electronic design challenges in all environments, particularly the challenges of space.
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