Military Grade Chip Resistor Screening And Reliability

A chip resistor bought on price and a chip resistor bought against a military specification look the same and measure the same on the bench. The difference appears in the qualification data behind them, in the screening that each unit has passed, and in the termination design that decides how the part behaves in a sulfurous or a humid environment. Where a board has to work in an aircraft or a defence system, those differences are the reason for the premium.

This article explains what the specification actually requires, how the failure rate levels are established, and where the money is well spent and where it is not.

What The Military Specification Adds

A commercial resistor datasheet describes the part and the conditions under which the ratings apply. A military specification adds a quality system: a defined set of tests, a defined sampling plan, a defined screening sequence applied to every unit, and a documented failure rate derived from life testing. The specification number identifies the whole package rather than the part alone.

The practical consequence is traceability. Each lot can be traced to the tests it passed, and a change in materials or process has to be notified rather than made quietly. For a programme that has to demonstrate reliability to a customer or a regulator, that traceability is the deliverable, and it is what a commercial part cannot provide at any price.

Verified Failure Rate Levels

The failure rate levels are established by running a large sample at rated conditions for an extended period and counting the failures. The result is expressed as a percentage per thousand hours, and the levels are assigned letters that correspond to progressively tighter limits. A part qualified at the tighter level has demonstrated a lower failure rate, and the level is a property of the qualified product line rather than of an individual unit.

Two cautions apply. The first is that the demonstrated rate is only as good as the sample size and the test duration behind it. The second is that the rate applies under the conditions of the test, and a part used outside those conditions is outside the qualification. Reading the level as a warranty rather than as a statistical statement about a defined test leads to the wrong conclusion.

Screening And Group A Testing

Screening is applied to every unit rather than to a sample. It typically includes a visual inspection, a measurement of the resistance at the reference temperature, a thermal shock or a temperature cycle, a burn in at elevated temperature, and a second measurement to detect the units that drifted outside the tolerance during the exposure. Units that drift are removed, which is why the delivered distribution is tighter than the incoming one.

The screening also removes the early failures that would otherwise appear in the first hours of service. A part that survives the burn in has demonstrated that it does not contain the marginal joints or the contaminated film that cause infant mortality, and the remaining population has a lower and more predictable failure rate. This is the same logic that applies to the burn in of an assembled board, applied at the component level.

Sulfur Resistance And Termination Design

A chip resistor terminates in a metal system that is soldered to the board, and the termination is a layered structure. In a sulfurous environment, sulfur can diffuse into a termination that contains silver and form a compound that is insulating, which raises the resistance of the joint and eventually opens it. The test method for this behaviour exposes the part to a sulfur vapour at elevated temperature and measures the change in resistance.

The countermeasure is a termination that includes a barrier layer, usually nickel, between the silver and the solder, so that the sulfur cannot reach the silver. A part described as resistant to sulfur has a termination designed and tested for it, and the claim is verifiable by the test method rather than being a marketing statement. Where the product is used in an industrial or an automotive environment with sulfur bearing lubricants or rubber, the termination design is a functional requirement rather than a refinement.

Derating And Thermal Design

The rated power is quoted at a defined ambient, commonly seventy degrees Celsius, and the permitted dissipation falls above that temperature along a derating curve. The curve is provided by the manufacturer and it applies to the part in a defined mounting condition, so a part on a small pad without thermal vias will run hotter than the curve assumes.

Applying the derating rule is not a substitute for a thermal design. The temperature of the part is the sum of the ambient, the rise from the board and the rise from its own dissipation, and the third term depends on the copper attached to the terminations. Where the part is expected to run warm, the pad area should be increased and the trace width that carries the current should be checked with a current capacity calculation, because the resistance of the trace and the temperature of the part are coupled.

When The Specification Is Justified

The premium is justified when the consequence of a failure is severe or when the environment is aggressive: aerospace, defence, medical, and industrial equipment that operates in a corrosive atmosphere or in a location that cannot be serviced. It is also justified when a customer requires the traceability for their own qualification, even if the environment is benign.

It is not justified simply because a board is described as industrial. For a general purpose product, a commercial part from a reputable manufacturer with a sound termination design will outlast the equipment, and the money is better spent on the thermal design and on the inspection that catches assembly defects. The verification of the assembly is the part of the quality chain that is under the designer control, and the practice described for design and quality characteristics and for pad design is where most of the field reliability is actually won. Where the environment is corrosive, protection at the assembly level, as described for conformal coating, is often the more effective measure.

Additional Considerations for This Build

Practical attention to MIL-PRF-55342 pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating MIL-PRF-55342 explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, sulfur resistance is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

FAQ

Does a military specification guarantee that the part will not fail? No. It states that a defined population demonstrated a failure rate under defined test conditions, and that each unit passed a screening sequence. It is a statistical and documentary assurance, not a guarantee.

What does sulfur resistance actually protect? It protects the termination from sulfur diffusing into the silver layer and forming an insulating compound that raises the joint resistance. A nickel barrier in the termination is the usual countermeasure.

Is screening the same as qualification? No. Screening is applied to every unit to remove early failures. Qualification is a test programme on a sample that establishes the capability of the product line.