OptoTherm EL thermal imaging system automated defect analysis

Infrared Cameras and Thermal Imaging Systems

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Model Board Comparison

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EL Infrared Circuit Board Inspection System

Model Board Comparison

Model Board Comparison^TM (MBC) is a software tool used to identify defects on PCBAs. Many printed circuit board defects such as shorts, defective ball grid arrays, and stressed components cannot easily be identified easily using conventional methods such as ICT, FT, AOI, and AXI. Many hours are spent debugging boards with such defects and often these boards end up in the scrap pile. MBC provides an alternative method of fault detection that can isolate these defects, thus filling the gaps between conventional debugging techniques.

How MBC Tests Work

MBC involves creating a thermal model of one or more known good boards (golden boards) by energizing them and analyzing the temperature changes that occur on each board over time. Boards can be energized by powering them during bootup or until component temperatures begin to level off. More sophisticated boards may require diagnostic or functional tests in order to energize all of the components on the board. Defective boards are then tested against the model in order to identify thermal differences that may indicate defects.

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. Thousands of infrared detector elements in the InfraSight EL thermal imaging camera act as virtual test probes. When troubleshooting scrapped boards, common defects detected using MBC include power-to-ground shorts and bad components.

MBC tests are performed using a software process called image subtraction. When operating in image subtraction mode, a reference thermal images is captured immediately prior to powering the board. This reference image is subtracted from each thermal image that is captured of the board when it is under power. These "subtracted" images are then displayed on the screen and represent temperature changes from the moment power was applied to the board. Image subtraction mode improves test sensitivity and minimize the effects of ambient temperature fluctuations from test to test.

MBC Test Result Images

The results of an MBC test consist of a sequence of images that represent differences in thermal behavior between the test board and model throughout the test period. At the beginning of the test, there should be no thermal differences as both the test board and model were unpowered at this time. As the test progresses, areas of higher temperature may appear, identifying components that are higher in temperature than the model. Similarly, areas of lower temperature may appear that identify components that are lower in temperature than the model.

In many cases, defects such as short circuits are immediately identifiable after conducting an MBC test. Short under BGAs can often be narrowed down to an area less than 1 x 1 mm. 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. Secondary thermal affects on other components may then appear in later test result images.

If a defect is not immediately identified as the first thermal anomaly, an engineer or technician should then carefully examines the sequence of test results images in order to track down the true defect site. This person should have a good knowledge of board functionality and may need access to the board's electrical schematics. By examining the magnitude of thermal discrepancies and the order in which they occur during the test, the technicians should be able to narrow down their search to a small region of the board containing only a few suspect components..

As an example, upon initial examination of the test result images, an area of lower temperature is displayed, possibly indicating that a component is not functioning properly. After inspecting the suspect component under X-ray, no internal defect can be found. Upon further analysis of the board's schematics, it is discovered that the functionality of the suspect component is controlled by the I/O output from a neighboring component. Although the test result images gave no indications of thermal anomalies on this component, after swapping out the component and retesting the board, it was found to be the root cause.

MBC Defect Search

As boards are successfully troubleshooted using MBC, a description of each particular defect can be associated with the MBC test result images to create a defect database. Then, after each board has been tested, the resulting images are automatically compared to the images in the database in order to find the closest defect match. As more and more defective boards are added to the database, troubleshooting time can be reduced even further.

Example

Step 1 - Create Model

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

Step 2 - Analyze

Test a defective board so that its thermal profile can be compared to the model. Areas on the board that are different from the model are highlighted and may indicate defects.

Step 3 - Locate Defect

Locate the defect by overlaying a transparent visual picture of the board.

Model Board Comparison Settings

Models are created by adding tests of known good boards to a group. Acceptance Criteria determines how the board is compared to the model. Board tests can be automatically saved to hard disk after each test.

Set the test length and the rate at which images are evaluated during a Model Board Comparison test.

Set the time that each relay will be activated during the test. Only one relay is needed when powering a board. Multiple relays can be used to perform more sophisticated tests such as powering different board planes or applying different loads at different times.