In the last decade, the obsolescence of electronic components including passive components such as resistors, capacitors, inductors as well as integrated circuits (ICs) has become a challenging issue for many OEMs and design houses in the avionics, military and space (AMS) market. Many of the precision resistors and ICs that were designed into systems from the early 90s and into the beginning of the 21st century are, unfortunately, becoming obsolete or difficult to get for reasons beyond the control of these companies. The reasons can vary. They range from the rejection by specific industries of tin-lead terminations or pressure from purchasing and management to reduce the cost of the bill of materials (BOM), often by searching for substitutes from Asia.
Whatever the specific causes, the results have been quite negative. Many AMS manufacturers have seen unintended consequences in their shift from traditional established-reliability precision resistors qualified by the Defense Electronics Supply Center (also called DSCC) to commercial-off-the-shelf (COTS) resistors. Pressure to reduce prices has actually hastened obsolescence and counterfeiting while encouraging some suppliers to publish less definitive datasheets that mask the very real differences between their low-priced products and military-qualified devices with full supportive data. The OEMs can mitigate the confusion with the only solution that really works—taking the time to verify the history of the precision resistor manufacturers, confirm the truth and availability of the technical data and support they provide, and order samples for testing as needed and verifying conformance at incoming inspection.
Origins of the problem
In the 1960s, the start of the exponential growth of electronics in military systems, communication satellites, and space exploration brought about the need for specific base-line performance standards for high-reliability components. Design engineers needed to be certain that components would perform the exact same way with known reliability regardless of manufacturer. That was the only way to be sure that systems operated the same way over different life cycles. It also ensured that replacement parts could be used without changing fundamental performance criteria and with no reduction in reliability.
Beyond standardizing performance, military specifications specified the testing protocols underlying the data used in statistical analysis to define various levels of reliability. Any manufacturer who qualified to these specifications and maintained the continuing testing could supply parts against these specifications. All manufacturers who qualified were considered to be equal. Different specifications were issued for different technologies to avoid distinctly different characteristics within the same specifications. If one manufacturer had a unique product with far superior performance he could only qualify to the lesser performance levels of the established specs - which were highly influenced by the common performance levels of multiple manufacturers. Applications that needed the superior performance levels still had to write separate specifications to define the critical performance parameters.
Military-qualified components came at an added cost. Qualification and maintenance were an expensive burden on the manufacturers and they had to be included in the component price. The cost burden included qualification and maintenance testing, administration and reporting costs, equipment and QC costs, and the cost of the tens of thousands of components whose lives were used up through the test protocols.
As the use of qualified parts expanded into nearly every military and space project, the quest for new systems and extraordinary performance began to be moderated by the need to reduce the cost of the required components. So interest developed in using standard commercial components rather than fully qualified parts. Eventually these became known as commercial-off-the-shelf, or COTS components.
COTS components are less expensive because there are no uniform test and documentation protocols as there are in the military specifications. Suppliers no longer have to be on the Qualified Suppliers List (QPL). It is highly desired that a supplier have the experience and background to support a QPL program but it is no longer a requirement. As a consequence, price considerations have begun to dominate design decisions. Manufacturers’ claims become less controlled by objective standards and even datasheets become more of a reference than a guarantee, providing a looser interpretation of specifications. Before long, deception by omission and incomplete descriptions begin to creep in.
Designers can now mix component technologies in the same applications even though different technologies have different defining characteristics. Precision resistors, for example, may have the same resistance value and tolerance but vary widely in temperature coefficient of resistance (TCR), power TCR (PCR), thermal EMF, current noise, harmonic distortion, ESD, thermal stabilization, and end-of-life cumulative deviations (or total error envelope), none of which is automatically included in the resistor’s identification by value and tolerance. Replacements or substitutions are made by form, fit, and function (FFF). Resistors are then purchased to fit the equipment’s initial function, ignoring the performance and reliability effects of the other, unmentioned, characteristics. Data from which to make statistical predictions regarding failure rate or mean-time-to-failure (MTBF) is lacking. COTS also presumes that replacement parts are available when needed. It presumes continued supply by the manufacturer.
With established-reliability (ER) and EEE-INST-002 components, all critical performance characteristic were specified and demonstrated along with documented reliability. With COTS, the design and purchasing decisions are focused on price, manufacturer claims, and BOM costs. Equipment reliability becomes separated from parts and production costs. Purchasers are encouraged to buy from the cheapest source, equipment reliability is no longer linked to parts purchased, and repair costs increase along with field failures.
EEE-INST-002 Bulk metal foil resistors