Board Level Thermal Testing Methods

A thermal measurement on a finished assembly is the only way to know whether the design removes heat as intended, and it is usually done with a thermocouple, an infrared camera or both. The measurement is simple and the interpretation is where the care is needed.

The result depends on the load, the ambient and the time allowed, so all three have to be defined before the measurement means anything.

Test Conditions

The load should be the worst case the product will see, which is often a continuous maximum rather than a typical value. A measurement at a partial load flatters the design.

The ambient should be the maximum of the specified range rather than the laboratory temperature, and the enclosure should be the real one if the product is used in a housing.

Thermocouple Measurement

A thermocouple measures the temperature where it is attached, and the attachment determines the result. A bead that is not in contact with the surface measures the air.

The wire also conducts heat away from the joint, which lowers the reading. A fine wire and a short run reduce the error, and the attachment should be recorded so that the measurement can be repeated.

Infrared Measurement

An infrared camera gives a picture rather than a point, which is useful for finding the hot spot that nobody expected. The reading depends on the emissivity of the surface, so a bare metal surface reads low.

The emissivity should be corrected or the surface prepared with a known coating, and the result should be compared with a thermocouple for at least one point.

Steady State

The measurement should be taken after the assembly has reached a stable temperature, which can take a long time on a product with thermal mass. A reading taken early is on the way up and is always lower than the final value.

The criterion for steady state should be defined as a rate of change rather than as a fixed time, since the two differ between products.

Hot Spot Identification

The point of interest is usually the junction of the hottest component, and on a board it may not be where the schematic suggests. An infrared image finds it, and a thermocouple confirms it.

The result should be compared with the component limit and with the derating policy, rather than with the ambient alone. The margin is what matters.

Thermal Cycling

A single steady state measurement says nothing about behaviour through a cycle. Where the product switches load repeatedly, the measurement should include the transient to catch the overshoot.

The transient is often the condition that sets the limit, particularly when the thermal path has mass. This is the same interaction described for thermal via placement.

Recording and Comparison

The measurement should be recorded with the load, the ambient, the attachment method and the time to steady state, so that a later measurement can be compared. A figure without the conditions is not a result.

The comparison against a previous build is what shows whether a design change helped, which is the purpose of the exercise.

Records

The records belong with the design documentation, since the thermal performance is a design characteristic rather than a process one. They should be retained with the revision they apply to.

The process evidence described for manufacturing processes covers the assembly side of the same product.

Process Control and Verification

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.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

FAQ

Why does a thermocouple read low? Usually because the wire conducts heat away or because the bead is not in contact with the surface.

Is an infrared camera accurate enough? It is accurate when the emissivity is corrected, and it is best used to find the hot spot rather than as the final number.

How long should the soak be? Until the rate of change is small, which is a defined criterion rather than a fixed time.

Should the test be run at the maximum ambient? It should, since the margin at the maximum ambient is what the design has to achieve.