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Thermal imaging aids electronics failure analysis

An Optotherm product story
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Edited by the Electronicstalk editorial team Feb 22, 2007

Long wavelength thermal imaging system enables engineers and technicians to quickly locate short circuits and other defects on printed circuit board assemblies.

OptoTherm's long wavelength thermal imaging system for electronics failure analysis enables engineers and technicians to quickly locate short circuits and other defects on printed circuit board assemblies.

Two of the system's tools, find shorts and model board comparison, simplify the process of detecting and locating troublesome defects.

Short circuits can be very difficult to troubleshoot.

Although an in-circuit test may indicate that a short exists, many times it cannot isolate the defect.

Technicians and engineers can spend many hours locating a single short, particularly interlayer shorts.

Using the find shorts tool, power-to-ground shorts, high resistance, low resistance, and even interlayer shorts can be easily identified.

The process of finding shorts involves powering the board for a short period of time (say 10 seconds) and then locating temperature increases resulting from the heat dissipated within the short due to I2R losses.

High resistance shorts (greater than 10ohm) usually exhibit temperature increases of at least 1C.

Low resistance shorts dissipate smaller amounts of power (heat) than do high resistance shorts and are somewhat more difficult to detect.

A 0.5ohm short, for example, may only exhibit a temperature increase of 0.2C.

When locating shorts, the OptoTherm EL system operates in differential temperature mode, displaying thermal images that represent temperature changes from the moment power was applied to the board.

Using this approach, even a temperature change as small as 0.1C is sufficient to identify a short.

Additional features include automated board power control and the ability to automatically disconnect power from the board to prevent damaging sensitive components located near the short.

Many printed circuit board defects such as defective ball grid arrays and stressed components cannot be identified easily using conventional methods such as in-circuit test, functional test, automated optical inspection, and automated X-ray inspection.

Many hours are spent debugging boards with such defects and often these boards end up in the scrap pile.

Model board comparison (MBC) provides an alternative method of fault detection that can isolate these defects, thus filling the gaps between conventional debugging techniques.

An MBC test involves creating a thermal model of one or more known good boards by powering the board(s) and analysing the temperature changes that occur on each board and on its components.

Defective boards are then tested against the model in order to identify thermal differences that indicate defects.

Using this approach, entire boards can be inspected at once, regardless of component density, and without contact with the board.

When troubleshooting scrapped boards, the most common defects are associated with power-to-ground shorts and bad components which can be readily identified using MBC.

While conducting an MBC test, circuit boards must be rigidly and repeatably positioned using a sturdy camera stand and circuit board fixture.

Board power is controlled using the system's digital inputs and relay outputs.

Boards are powered for the length of their bootup sequence or until component temperatures begin to level off.

Like the find shorts tool, an MBC test is also performed in differential temperature mode.

Performing MBC tests in differential mode improves the sensitivity of the test and minimises the effects of ambient temperature changes from test to test.

For these reasons, MBC can detect and locate subtle temperature anomalies that are nearly impossible to detect using any other method.

There are three stages in conducting a model board comparison.

First, create a model by analysing one or more known good boards.

Then test a defective circuit board and compare the results against the model: points on the board with temperatures that are different from the model are highlighted and indicate defects.

Finally, locate the source of the defect by overlaying the visual image of the board over the thermal image: cross-hairs mark the location of the defect.

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