Non Inductive Micro Thick Film Resistor Selection

A resistor that has to fit into a space of a few square millimetres, survive a wide temperature range and behave predictably at high frequency is a different component from the chip resistor that sits beside it on the same board. The thick film element is printed on a ceramic substrate and enclosed in a plastic body, and the construction is what allows a metal film performance in a package small enough to fit between other parts.

This article looks at what such a part contains, how its ratings are defined, and how to read the ordering code so that the tolerance and the temperature coefficient that arrive are the ones the design needs.

What The Small Body Contains

The resistive layer is a metal oxide film, often ruthenium based, printed and fired onto a substrate of high purity alumina. The film is trimmed to value by cutting a path through it, and the trim geometry determines both the final resistance and the behaviour at high frequency. The body is then encapsulated in a plastic shell and fitted with axial leads.

The alumina substrate serves two purposes. It is an electrical insulator with a low loss, and it is a thermal conductor that carries heat away from the film into the leads. Because the film is thin and the substrate is small, the thermal mass is low, which is why the part responds quickly to a change in dissipation and why the derating curve is a practical document rather than a formality.

Why A Non Inductive Element Matters

A wound resistor behaves as an inductor because the winding is a coil, and at high frequency the impedance rises above the resistance. A helical trim in a thick film element does something similar, which is why a non inductive part uses a trim pattern that cancels the magnetic field of adjacent turns rather than a simple spiral.

The result is a part whose impedance stays close to its resistance over a wide band. That property is useful in a snubber, in a current sense path that carries fast edges, and in any position where the resistor is expected to damp a resonance rather than to add one. It also makes the part predictable in a measuring circuit, where the difference between the resistance and the impedance would otherwise appear as a frequency dependent error.

Power Derating In A Tiny Package

The rated power is quoted at a defined ambient, commonly seventy degrees Celsius, and above that temperature the permitted dissipation falls along a derating curve until it reaches zero at the maximum operating temperature. A part rated at sixty three milliwatts at seventy degrees cannot dissipate that much at a hundred and fifty degrees, and the curve is what states the usable figure.

The maximum working voltage is a separate limit and it is not simply the product of the power and the resistance. It is capped at a value defined by the package, and the applicable limit is the smaller of the two. In a high resistance value the voltage limit is reached long before the power limit, which is why a high ohmic part in a high voltage position has to be selected on the voltage rather than on the power.

Resistance Tolerance And Temperature Coefficient

The tolerance states how far the delivered value may be from the nominal at the reference temperature, and it is selected by the ordering code. A one percent part costs more than a five percent part, and the choice should follow the error budget rather than a general preference. Where the resistor sets a reference or a gain, the tolerance contributes directly to the initial accuracy of the circuit.

The temperature coefficient states how much the value changes with temperature, and it is quoted in parts per million per degree. The low ohmic values have a larger coefficient because the lead and termination resistance, which is largely insensitive to temperature, forms a smaller proportion of the total. For a divider that has to stay accurate over a wide range, the coefficient matters more than the initial tolerance, because the initial error can be calibrated out while the drift cannot.

Reading The Ordering Code

The part number encodes the body size, the resistance value, the tolerance, the temperature coefficient and the packaging. A typical string contains a body designator, a four character value in which a letter stands for the decimal point and the multiplier, a letter for the tolerance, a letter for the coefficient, and a suffix for the packaging. Reading it correctly is what prevents a shipment of parts with the right value and the wrong drift.

The packaging suffix deserves attention in production. A taped and reeled version is required for machine placement, while a loose part is intended for hand assembly. A tape that is specified for one lead spacing will not feed a machine that expects another, and a reel that is opened and left in a humid environment can oxidise before it is used.

Where The Part Fits

The usual application is a high voltage divider, a current sense element, or a damping resistor in a snubber, in each case in a position where the board area is limited and the voltage is significant. The combination of a small body, a high working voltage and a non inductive element is what makes the part suitable for those positions rather than a general purpose chip.

Where the requirement is a precise stable value in a benign environment, a metal film part with a lower coefficient is often the better choice. Where the requirement is mechanical robustness in a small space and the resistance value is not critical, a thick film chip is cheaper. The thick film element earns its place in the middle, and the choice should be made from the position rather than from the technology. The surrounding design follows the usual practice for sizing conductors for current and for the fabrication of the board, and the soldering conditions are covered in the comparison of leaded and lead free processes.

Process Control and Verification

On a design of this kind, resistance tolerance 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

Is a non inductive resistor always needed for a snubber? It is needed where the object is to damp a resonance rather than to add inductance. A wound part in that position can make the ringing worse rather than better.

Why does the maximum working voltage matter more than the power rating for high values? Because the package sets a voltage ceiling that is reached at a lower current than the power limit when the resistance is large. The applicable limit is the smaller of the two.

Which matters more, tolerance or temperature coefficient? It depends on whether the error can be calibrated. An initial error can be trimmed out, while a drift over temperature cannot, so the coefficient deserves the attention in a wide range application.