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Resistor datasheets: Always read between the lines

Posted: 16 Dec 2013     Print Version  Bookmark and Share

Keywords:resistors  datasheets  Tolerance  total error budget  ESD 

As an engineer, you should have a healthy scepticism that allows you to decipher incomplete and misleading specifications. The need is particularly urgent given the increasing number of counterfeit sources of "precision" resistors that unscrupulous suppliers may try to disguise on datasheets with superfluous text, figures, marketing fluff, and half-truths with respect to specifications. The problem is not necessarily that data is being withheld, but that it's being mixed up in a confusing way. As elsewhere in life, the devil is in the details.

Realising that design engineers are under great pressure to quickly assimilate the information in datasheets, we have prepared this article with the goal of offering a strategy that can help any person to reach the fundamentals in a precision resistor datasheet within a few minutes. To this end, we look at the key terms that turn up in these documents and review what they really mean.

Tolerance has many meanings, ranging from purchase tolerance (initial tolerance) to end-of-life tolerance (total error budget). The last thing you need to know in the component selection process is the part's initial tolerance. The first thing you need to know is the required end-of-life tolerance of the design. You need to evaluate all the components for their expected changes after all of the stresses and exposures they might experience over the planned life of the equipment.

Every component must have an error budget assigned to it, which if exceeded, will cause the equipment to fall out of its performance specification, or possibly fail. In addition to initial tolerance, the error budget includes allowable shifts through shelf life, assembly, TCR, shock, vibration, humidity exposure, thermal shock, thermal EMF, load-life drift, ESD, radiation, and harmonic distortion (harmonic distortion is a measure of a component's conformance to Ohm's law as a predictor of reliability.) You then need to select a particular resistor technology with the least amount of change through all the stresses, add up all the expected ΔRs, and subtract these from the end-of-life tolerance limit to arrive at the purchase tolerance.

The bill of materials (BOM) then specifies the resistor model by resistance, tolerance, and possibly by TCR (usually typical or restricted to a narrow temperature range). Although not delineated on the BOM, each ΔR limit for the specified resistor technology is critical to the application.

This is why precision resistors with the same resistance, tolerance, and TCR—but of different technologies—are not interchangeable: because their changes through service life are not the same. Substitution on the basis of these alone could jeopardise performance and mission success. Only the OEM engineer who did the error budget analysis can determine a suitable substitute. Unfortunately, today we see more and more attempts by certain electronic manufacturing services to replace precision resistors with cheaper solutions that put system performance at risk.

To be considered as substitutes, all performance characteristics through all stresses and exposures must be completely and exactly defined in specific detail to be sure they qualify without reducing reliability or shortening equipment life.

Particularly for high-precision circuits using high-reliability and tight-tolerance resistors, it is not enough for the manufacturer to measure the resistors before shipment. Its equipment must be calibrated and traceable to the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards). Its measurements must also have a guard band that confines measurement error to the specified tolerance and applies this to 100% of the resistors.

Also, the OEM must be assured that the resistors are within tolerance when received—not just "finessed" into a measured tolerance long enough to be listed as good for shipment (but not long after). Unfortunately, many OEMs' incoming inspection checks are not sufficiently calibrated to measure tight-tolerance resistors, so they must depend upon the suppliers' honesty. In addition, some companies have purchased components shipped directly to the production floor, bypassing incoming inspection completely. If the application is important, the supplier must be accessible for qualification inspection and periodic Q.A. audits.

The end customer must also evaluate whether a tolerance offered by a manufacturer is really practical. For example, some surface-mount thin-film chip resistors are offered in very tight tolerances for very low resistance values. That's impressive on the datasheet, but not compatible with assembly processes. As these resistors are mounted on the board there is a resistance change due to solder heat. The solder terminations melt, flow, and re-solidify with changed resistance values.

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