Commercial off-the-shelf (COTS) components offer a number of benefits for high-reliability applications. Although consumer silicon tends to define the limits of performance in today’s climate, designers can still achieve leading-edge functionality in new designs. Furthermore, component purchase price is usually lower than for corresponding devices backed up by a Certificate of Conformity (C of C) to high-reliability standards. On the downside, some vendors don’t offer extended support for COTS components, or guarantee performance under harsh operating conditions.
Where qualified components are available, vendors have historically borne the costs associated with qualification, which is reflected in higher purchase prices. However, shortening component lifecycles are pushing some vendors to not bother qualifying components to high-reliability standards. This is reflected in the steady reduction of military and aerospace grades, from 9% of the overall electronics market in 1984, to just 0.9% in 2005 to 2006 (Fig. 1).
Hence, COTS components are becoming pervasive in high-reliability projects, both on merit and through necessity. Entire sub-assemblies may be built using these devices. As COTS content in any given design increases, OEMs must fulfill whole-life component-management actions normally associated with the supply of qualified components.
Three elements are essential in this process: ensure that the selected parts satisfy all performance requirements of the end application; ensure continuity of supply throughout the lifetime of the end product; and rigorous test and inspection to compensate for the absence of a vendor-issued C of C. Designers must pay proper attention to these activities to ensure adequate supplies of fit-for-purpose components and sub-assemblies throughout the CADMID (Concept, Assessment, Development, Manufacture, In-Service, Disposal) cycle.
Whole-Life Component Management
The whole-life plan should commence at the concept phase, beginning by establishing base criteria for the performance of available devices. Factors including operating temperature range, resistance to shock and vibration, and radiation hardness should be considered. More than merely surviving, the component must remain fit-for-purpose under the expected conditions. This front-end process can be structured into a formal quality criteria plan for the components.
There’s also an impact on design, as well as on procurement and testing. This takes into account improvements in device technologies that will deliver advantages such as higher memory capacities and lower supply voltages. With these enhancements in mind, designers should begin planning for technology insertion during subsequent design and production phases.
The next stage is a top-down selection process to determine a Design Parts List (DPL) as a roster of components from which design teams may choose. It’s important to begin monitoring for a healthy supply of devices as soon as they’re added to the DPL, since COTS components have much shorter lifecycles than their intended system.
Military or aerospace equipment is usually in service well beyond a decade, and the end of service (EoS) date may further extend beyond the designed-for obsolescence to reduce costs. In contrast, COTS component lifecycles are frequently less than one year, and device types are now being made obsolete at a rate of several thousand types per month. With military and aerospace design cycles routinely lasting from five to eight years, it’s likely that many of the COTS components on the DPL will become obsolete before production even begins (Fig. 2).
Component engineering staff has a role in protecting high-reliability product programmes from the threat of component non-availability. An obsolescence strategy is necessary to ensure continuity of supply for whichever components are used—not only for development and production, but also for lifetime maintenance. In practice, this means implementing many of the obsolescence-management best practices that have become established in recent years. This may include strategies such as advance procurement and long-term storage of components, which are critical to the design and cannot be replaced.
Test And Inspection
An appropriate component qualification plan or specification is also necessary to ensure compliance against performance and durability criteria of the specific application. In effect, this compensates for the lack of provenance of COTS parts.
There’s no guarantee that successive batches of components will be manufactured or assembled in the same facility, let alone with the same process. Thus, OEMs must implement a rigorous testing and inspection regime, bearing in mind that reliability requirements are inherently more exacting than in a commercial environment.
A component failure preventing part of the payload of a communications satellite from fulfilling its mission, for example, can have serious financial consequences for its owner. Failure of an aircraft flight-control system, on the other hand, will jeopardise a mission and may cause loss of life.
Dealing with extremes of temperature and increased levels of radiation are two of the most serious challenges, particularly for military and aerospace applications. The levels of radiation present at high altitudes, for example, result in electronic components being susceptible to random changes in logic state.
The risk of a so-called single-event upset (SEU) is increasing as CMOS process technology shrinks to 90nm and beyond. A suitable radiation-testing regime must validate specified components against a linear energy transfer threshold appropriate for the application. In practice, this is directly related to the altitude at which the device needs to function. In addition, the components’ performance must be assessed in terms of the total amount of incident ionising radiation.
As far as extremes of operating temperature are concerned, an initial qualification will be required if a component is used outside its specified temperature range, to determine whether it will continue to perform at the limits expected. This should include the effects of repeated temperature cycling. Further data may be required to establish whether the device’s performance needs to be derated in light of the more exacting temperature conditions. The overall design may need to be adapted to accommodate this level of performance.
The test regime should also consider performance under more demanding conditions of shock and vibration than what’s guaranteed by the manufacturer. Typically, however, components tend to be more physically robust than purchasers initially expect. In practice, the temperature or radiation requirements usually need much more careful consideration.
When using COTS components for harsh environments and demanding applications, the qualification process is, in effect, an up-screening—testing lower-grade devices to elevated specifications. Traditionally, engineers have turned to a reliability handbook such as MIL-HDBK-217 to create a suitable schedule.
Common practices, for instance, include an initial visual and mechanical screening and dc electrical check, followed by detailed constructional analyses such as X-ray inspection, electron microscopy, C-scanning acoustic microscopy (C-SAM), and micro-sectioning. A hermeticity test is also usually applied. MIL-HDBK-217 is now obsolete, though, as assembly techniques have moved on. Instead, engineers tend to rely on several resources like Def Stan 0041, MIL QPL, and NASA specifications.
Accelerated life testing is then used to predict component requirements throughout the production and maintenance phases of the project, and helps to ensure sufficient qualified components throughout its duration. Devices are monitored for critical parameters to identify out-of-spec parts, as well as total functional failures. Other tests specific to the end product’s environment may also be applied. From this process a qualified batch may be drawn, and then a lot should be pulled from the batch and placed in long-term storage, with safeguards and watchdogs. As the project progresses, acceptance testing of further batches of components is also a key part of the process.
The best response to the transfer of component-management responsibilities toward the procurer demands a complex mix of skills on the purchasing side. Sourcing hard-to-find components via the gray market, for example, requires both contacts and experience. Moreover, considerable investment is required in specialist facilities for testing, inspection, and storage.
Since whole-life management of COTS components is now critical to successful project completion, external component procurement, testing, and storage providers can help OEMs maximise the benefits of COTS design whilst at the same time avoid the pitfalls.