The use of commercial parts in electronics military systems is certainly not a new concept and has been in use for many years. However, there is an increasing trend of the reduced availability of military specified component parts in many military applications due to the fact that the manufacturers of the military versions ("Mil Spec") either no longer exist or are no longer interested in such cost expenditures for so little return. The military customer represents less than 1% of the semiconductor market today.

The need to use COTS (commercial-off-the-shelf) components in military systems has not reduced the requirement for reliable operation in harsh environments. It is the EMPF's challenge to make use of COTS components either through the determination of methods for upgrading them or by determining if the characteristics of the "commercial" parts reliably meet or exceed those listed in the specifications.

The question of how to validate the use of these components must be addressed with every new design. At the component level, up-screening is an effective way to ensure that commercial parts will meet the electrical and environmental parameters required by the intended system. Extended, or life-cycle, screening will provide some assurance that the parts will have long-term reliability. The practice of such testing is known as Ruggedization.

Testing
Some of the tests that apply are often referred to as environmental stress screening (ESS) and are performed at the EMPF. These include thermal cycling, high and low temperature exposure, highly accelerated stress testing or screening (HAST or HASS), highly accelerated life testing (HALT), vibration, shock, humidity, salt fog, rain, explosive atmosphere, low pressure (altitude) and fungus testing. Excluding HAST/HASS and HALT, those listed would be typical of environmental testing for a military contract design of a printed circuit board or electronic unit, and are therefore a part of the COTS qualification process at the EMPF.

Accelerated test methods such as HAST allow exposure for the purpose of simulating the advanced aging of the device under test (DUT). HAST includes relative humidity (RH) up to 98% and temperatures up to 120ºC with simultaneous pressure (versus simply 80ºC temperature and 85% RH in a normal thermal chamber).

Testing is necessary since MTBF calculated from MIL-HDBK-217 falls short. This is mostly due to the fact that new component models have not been updated in the MIL-HDBK and therefore it is not always possible to calculate a valid MTBF.

Finite Element Analysis (FEA) is utilized to determine the reliability (the stresses, strains, structural integrity, and fatigue life) of components, and is advisable as an accompaniment to testing.
Analysis of Failures
Any failures found during testing (exposure to environmental conditions) may be subject to failure mode analysis. This typically includes both optical inspection and micro-sectioning of the part for SEM (scanning electron microscope) analysis in order to find the cause of failure.

One example of a failure is a printed circuit board fracture with the failure mode occurring 1) when the heavy heat sink is residing on the outer two inch perimeter of the board, 2) at temperatures below -20ºC, and 3) when subject to 20g shock. Another example of a failure is when an integrated circuit produces internal open or short circuiting over extended periods of time. Here, methods such as HALT will assist in mimicking those conditions. SEM analysis may then reveal that an overlay of dissimilar metals has caused intermetallic layer growth and fractures to form an open circuit, or tin whisker growth to form a short circuit.

EMPF Testing and Failure Analysis
The EMPF's environmental and materials labs allow for exposure testing and failure analysis required of COTS parts used in military applications. A list of the testing services available can be found here.

Example of COTS Components Ruggedization For a DoD Application
Assisting the Standard Missile program, the EMPF up-screened components associated with the EMMA (Electronic Miniaturization for Missiles Application) program for the Autopilot system. The usage of COTS components reduced manufacturing costs by 66% and reduced system volume by 80% over the 20 year old previous design using Mil Spec Parts. The remaining volume was then able to be used for the Flight Termination System (FTS).

Vibration and thermal cycling (-55 to +125ºC) was performed for components with various packaging types that included SOT, TQFP, PBGA, TBGA, flip chip, and LFCSP while mounted on three different substrates. Other devices including amplifiers, A/D converters, power MOSFETs and memory components were subjected to environmental exposure followed by functional testing and analysis.

Limiting the Testing Required
The comparison of a MIL-STD-883 specified part with its commercial equivalent may reveal the limited nature of the commercial part's intended use. However, the full military specified version may not be required either. Depending on the circuit, military platform and intended environment, the specifications for testing must be decided specific to the application.

By determining if “Mil Spec” parts are absolutely necessary for the system's performance requirements, the ruggedization testing can be limited in scope. The particular application in the circuit, physical position with respect to thermal dissipation and mission duration further limit or add to the testing required for any given part. Thermal cycling is a typical starting point for testing, as real aging includes exposure to daily changes in temperature. A second choice is often vibration, as it will reveal the mechanical fragility of an item, particularly in regard to its packaging.

The electrical requirements of a COTS part may fall short of those of its military predecessor. However, if the part is intended as a portion of a redesign, it may be specified less critically while associated new circuitry makes up the balance of tolerances. These tolerances may have actually been required of the circuit instead of the part. The compromises involved in choosing similar rather than identical replacements can be discovered in SMT magazine, November 2000: "The Use Of Commercial Components:in a High Reliability Environment".

Conclusion
Up-screening components before assembly and age-simulation for failure rate prediction provides the reassurance of risk reduction. The key to reducing the scope of reliability testing for COTS parts in a military system is to determine what specifications truly must be "Mil-Spec" to meet the performance requirements of the application. Secondly, the re-design of associated circuitry to alleviate the critical nature of a single part may be an option should a drop-in COTS item be unavailable. This maintains the integrity of the next higher assembly. When the COTS part itself must adhere to certain specifications, the performance parameters under harsh operating conditions must be verified.