Resistor Hot Spots: Distinguishing Geometry from Heat-Sink Problems

Confirm the heat input before interpreting the image

Record current and voltage at the resistor terminals throughout the test. Supply settings do not necessarily equal the specimen conditions because cables, contacts and current limiting can change the delivered power. A constant-voltage test can also change power as resistance changes with temperature. Use measured electrical input when comparing temperature rise.

For a simple ohmic resistor, power is voltage multiplied by current. A hypothetical 100-ohm pattern at ten volts dissipates one watt. If another pattern measures 125 ohms at the same voltage, it dissipates 0.8 watt. A lower observed temperature could therefore come from reduced heat generation. Compare at equal power to study heat removal, then verify the actual application drive separately.

Separate heat generation from thermal resistance

Geometry affects the distribution of electrical loss. A narrow neck, an inside turn or a constricted terminal can concentrate current and create greater local heating. The mounting affects how that heat travels through the substrate, interface and support to its surroundings. A local air gap can make an otherwise uniform electrical region appear hot.

A lumped estimate writes temperature rise as power multiplied by effective thermal resistance. This estimate is useful for comparing complete mounting arrangements, but one total number cannot predict every local peak. A spatial model needs the actual source distribution and thermal contacts. Treat the measured hot region as a clue about both terms rather than assuming the resistor material alone determines its temperature.

Change the support while preserving the electrical pattern

Measure the same specimen with a controlled reference mounting, then with the intended installation. Keep orientation, ambient airflow, power and temperature-observation method consistent. Define interface material, contact area, pressure or fastening method, and any electrical insulation between the substrate and support. Returning to the reference mounting helps identify irreversible specimen change.

If a broad hot region changes substantially with mounting while the narrow turn peaks remain in the same locations, both thermal and electrical features may be present. If a localized region follows a support defect when the substrate is repositioned, investigate the contact. Repositioning must preserve the permitted mechanical loading; bending a brittle substrate to improve contact would create another uncontrolled variable.

Change the current path without changing the heat sink

Where the mounting is repeatable, compare a geometry revision that addresses one suspected concentration. Examples include removing a local neck or increasing an inside-turn radius. Preserve overall resistance and power as closely as practical, or explicitly account for their changes. Inspect the printed result to confirm that the intended revision survived processing.

Use a low-excitation electrical measurement before and after powered testing. If a hot location also develops irreversible resistance change or visible damage, the evidence is stronger than temperature alone. However, a revised layout that moves the peak elsewhere may simply have redistributed loss. Compare the complete field, including terminals and neighboring elements, before declaring the original problem resolved.

Use the pattern of change to choose the next experiment

The most useful next experiment is the one that distinguishes two plausible causes. Keep a short hypothesis list and identify the observation that would support or weaken each. This avoids accumulating thermal images with different color scales and little diagnostic value.

Verify that the thermal image represents temperature

Printed film, exposed conductor and ceramic can have different emissivities and reflection behavior. The camera requires suitable emissivity and reflected-temperature settings to interpret the received radiation. A bright metallic terminal can reflect a nearby warm object and appear unlike the adjacent surface even without the corresponding temperature difference.

Keep focus, viewing angle, distance, temperature scale and analysis region consistent. Ensure that the region is large enough for the instrument's actual spatial measurement capability. Where a contact sensor or surface treatment is used to check the reading, evaluate how it changes heat flow or the surface itself. Preserve the original radiometric data and settings where available, rather than retaining only a colored screenshot.

Use time traces to distinguish fast and slow contributors

A short power step can reveal a local feature before the entire substrate warms, while a long dwell reveals the complete heat-removal path. Record several meaningful regions over time: suspected hot spot, neighboring film, substrate reference and support. Align those traces with actual electrical power so a supply transition is not mistaken for a thermal response.

The earliest observable temperature is limited by sensor response and spatial averaging. Do not infer an unseen peak from the first captured frame. Repeat the step only after the agreed initial condition is restored, and inspect for cumulative resistance change. A system that meets a steady-state temperature target can still require separate pulse evaluation because the spatial and temporal distribution of energy differs.

Connect the correction to a measurable acceptance condition

A geometry correction should have an inspectable feature and an electrical or thermal verification tied to the failure mechanism. A mounting correction should specify the interface and assembly condition that made the difference. Record both the original discrepancy and the controlled comparison, so a later supplier or assembly revision does not remove the condition that enabled the improvement.

For project review, provide the substrate drawing, power waveform and installation stack together. Include photographs of attachment surfaces and temperature data with measurement settings. This gives engineering a basis for deciding which experiment is needed next. The resulting power or temperature boundary remains specific to the reviewed construction and environment; it cannot be transferred from an unrelated packaged resistor or a different heat sink.