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Thermalyze Thermal Image Analysis Software

Model Board Comparison (MBC)

Note: Model Board Comparison is included only with EL and is an optional item otherwise

Model Board Comparison (MBC) is a software tool used to identify defects on printed circuit board assemblies. Many circuit board defects such as shorts, defective ball grid arrays, and stressed components cannot easily be identified easily using conventional inspection and failure analysis methods such as in-circuit test, functional test, optical inspection, and X-ray inspection. Much time is spent debugging boards with these types of defects and often these boards end up in the scrap pile. MBC provides an alternative method of fault detection that can isolate these types of defects, thus filling the gaps between conventional inspection and debugging techniques.

MBC can detect very small temperature differences between functional and defective boards that are nearly impossible to detect using any other temperature measurement method. Entire boards can be inspected at once, regardless of component density, and without contacting the board. Tens of thousands of individual detector pixels in the infrared camera act as virtual test probes.

How MBC Works

Creating the Golden Board Model

MBC involves creating a thermal model of one or more circuit boards that operate correctly, called golden boards. Golden boards are powered and their thermal behavior is used to create a thermal model that represents how defect-free boards should behave thermally. Statistical models can be created from many golden boards or as few as one golden board.

Powering the Boards

Boards can be powered for a short period of time for analog or simpler board designs, or for longer periods that comprise the complete bootup cycle of more complex boards. Even more sophisticated boards may need to be powered using diagnostic or functional tests in order to exercize all components on the boards.

Testing Defective Boards Against the Model

After the golden boards have been tested and the thermal model has been created, defective boards can then be tested in the same manner. The thermal behavior of the faulty boards is compared to the golden board model and any thermal deviations from the model are used to isolate faulty or marginal components, as well as open and shorts on solder, traces, vias, and connectors. User-defined acceptance criteria determines how closely a test board must match the model to be considered defect-free. When troubleshooting scrapped boards, for example, MBC is often used to detect and locate faults such as power-to-ground shorts and bad components.

Image Subtraction

MBC tests are performed using a software process called image subtraction wherein a reference thermal images is captured immediately prior to powering the board. This reference image is then subtracted from each captured thermal image during the test, creating a sequence of "subtracted" images that display temperature changes as the device is powered. Subtracted images represent temperature changes from the moment power was applied to the board. Image subtraction improves test sensitivity and accuracy by reducing the affects of ambient (room) temperature changes between tests.

Isolating Defects

MBC Test Results

The results of an MBC test consist of a sequence of images that represent differences in thermal behavior between the test board images and the golden board model at different point in time throughout the test. At the start of the test, there are no thermal differences as both the test board and model are unpowered and the subtracted images show no temperature changes. As the test progresses, areas of temperature differences may appear, identifying components or traces on the test board that are higher or lower in temperature than on the golden boards.

Some types of defects, such as short circuits, can often be immediately identified at the end of the test and located to within a very small area of the board. Other failure modes, such as open or faulty components, may produce secondary thermal effects resulting in a number of suspect components. In these cases, the defect site is often identified as the location where anomalous thermal behavior is first observed after powering the board.

If the first thermal anomaly is not the true defect site, an engineer or technician should examine the sequence of test result images to identify areas of thermal differences from the model. These areas are potential defect sites. The engineer or technician should have a good knowledge of board functionality and may need access to the board's electrical schematics. By examining the magnitude of thermal anomalies and the time in which they occur during the test, the engineer should be able to narrow down the search to a small region of the board containing only a few components.

Complex Fault Isolation Example

Following is an example of tracing a more complex fault to its source. Upon initial examination of the test result images, an area of the board containing a number of components is discovered to be lower in temperature than the model. This may indicate that a component is not functioning properly. After inspecting the suspect components under X-ray, no internal defects are found. Upon review of the board's schematics, it is discovered that the functionality of the components are controlled by the I/O output from a neighboring component. Although the test result images gave no indication of thermal problems with this component, it was found to be the true failure site after swapping out the component and retesting the board.

MBC Defect Search

As test boards are successfully troubleshooted, a description of each defect can be associated with the test board images to create a defect database. After each defective test board has been added to the defect database, current test boards can be compared to the database to identify a close match. As more and more defective boards are added to the database, troubleshooting time can be significantly reduced.

MBC Test Procedure

Step 1 - Create the Golden Board Model

Create a golden board model by testing known good boards. These images show the temperature changes that occurred on the board during the test.

Step 2 - Test Defective Boards Against the Model

Defective test boards and then tested and their thermal behavior is compared with the model. Areas on the board that are different from the model are highlighted and may indicate defects.

Step 3 - Locate the Fault

Locate the defect by overlaying a transparent visual picture of the board over the test results image.

MBC Software Interface

A number of conveniently organized tabs provide access to functionality such as running new tests, reviewing past tests, reviewing test results, and managing golden and defect boards.

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