The most widely specified and used standard governing such ‘high-performance’ parts is the US defence standard known as MIL STD. A huge variety of electronic components – active, passive, emech, interconnect, hybrid and even sub-systems – are covered by various different MIL-STDs which govern the manufacture and testing of the parts in question.
That may well be all well and good, but there is a problem. MIL-STD numbers are associated with a specific component.Developing MIL-STDs for new parts is a long process, and once the standard is developed, for a component manufacturer to get that part listed is a long and costly process.
What this means is that engineers working on designs that need high performance, high reliability components will be limited to older technology components, as new smaller, lighter, faster – maybe even cheaper – parts will not yet have had a MIL-STD created for them, or if it has, the manufacturer may not yet have received approval.
Luckily a couple of alternatives are emerging.
COTS+
Many engineers will have heard the term COTS – Commercial Off-The-Shelf. In the USA, COTS is a Federal Acquisition Regulation (FAR) term for commercially-available items and services that can be bought and used under government contract. It has come to be a widely used term, and the concept appears especially appealing in the light of defence budget cuts and restrictions. However, to put it plainly, components designed for use in the commercial world are often unsuitable for the rigours of military, space and aerospace applications.
COTS+ is a development from COTS which can address the disparity between the requirement for a lower cost, more easily available device and one which is fully-approved to meet harsh environment use. COTS+ programs start with an enhanced part – often an automotive grade device that may have, say, an elevated operational temperature range – then add specific tests as necessary for an application.
It is vital that defence and security engineers ensure the reliability and the lifetime support of their designs especially when it comes to projects that can run for many years. Increasingly we are seeing devices with in-service lifetimes of many decades being built with components with lifetimes of just months.
One issue with COTS+ is that it requires rigorous best practice processes if designers are to leverage both its benefits and mitigate its risks.
All COTS+ programs vary and some have different names. An example is KEMET’s ‘COTS program’ – which we would describe as COTS+. KEMET regularly supplies ‘up-screened’ products by working with customer drawings and imposing specified design and test requirements. Their COTS program offers the same high quality and high reliability components as up-screened products, but at a lower cost to the customer. This is accomplished by eliminating the need for customer-specific drawings to achieve the reliability level required for customer applications. A series of tests and inspections have been selected to provide the accelerated conditioning and 100% screening necessary to eliminate infant mortal failures from the population.
KEMET’s C0G dielectric features a 125°C maximum operating temperature and is considered ‘stable.’ The Electronics Components, Assemblies & Materials Association (EIA) characterises C0G dielectric as a Class I material. Components of this classification are temperature compensating and are suited for resonant circuit applications or those where Q and stability of capacitance characteristics are required. C0G exhibits no change in capacitance with respect to time and voltage and boasts a negligible change in capacitance with reference to ambient temperature. Capacitance change is limited to ±30 ppm/°C from -55°C to +125°C.
KEMET’s COTS testing includes voltage conditioning and post-electrical testing as per MIL–PRF–55681. For enhanced reliability, KEMET also provides the following test level options and conformance certifications:
A: Testing per MIL–PRF–55681 PDA 8%
B: Testing per MIL–PRF–55681 PDA 8%, DPA per EIA–469
C: Testing per MIL–PRF–55681 PDA 8%, DPA per EIA–469, Humidity per MIL–STD–202, Method 103, Condition A
Various other manufacturers have COTS+ programs including Amphenol, AVX, Harwin, Honeywell, Souriau, TE, TT and Vishay.
AVX, for example, recently released a new COTS+ version of its TCR Professional Series polymer tantalum chip capacitors specifically for use in critical military systems. The new TCB Series COTS+ polymer tantalum capacitors feature conductive polymer electrodes, which enable extremely low ESR and reduce the potential for ignition failure.
Defence Supply Center
The Defense Supply Center, Columbus (DSCC) qualification is another way to address the issue of needing to use approved parts that are latest generation technology offerings. In many ways it is very similar to MIL-STD, including being administrated from the same United States Department of Defence, Defence Logistics Agency(DLA) site (Columbus, Ohio).
Even more than that, DSCC approvals very often reference MIL-STDs for testing and procedure. Where they differ is that they are much more flexible and quicker to implement. It is, for example, much quicker to bring on a DSCC standard for an increased capacitor voltage range – ie from 50V to 100V – than it would be under the MIL-STD scheme. So designers get to work with the latest technologies rather than having to wait for the standards to catch up.
Currently there are DSCC approval covering passive and semiconductor parts, and it looks likely that it will migrate to cover electromech and connectors shortly.
TTI, as is the case with most large distributors, works with a number of suppliers that offer parts for high reliability applications. We would urge designers to look beyond the limitations of the MIL-STD portfolio to see whether a COTS+ device or a component carrying DSCC approval might not offer the performance they need.
Four branches for different technologies The DLA Land and Maritime, Sourcing and Qualifications Division is organised into four branches, each with responsibility for different technologies. DLA Land and Maritime-VQC handles primarily microelectronics; VHQ manages hybrid microcircuits; VQP has responsibility for passive electronic and construction items; while VQE is responsible for semiconductors, printed wiring boards and electron tubes. |
Author profile:
Lee Thompson is Director Industry Marketing Europe, Military, Aero & Space, TTI
