IR and PCBA Design Trends

IR technology has responded effectively to the challenges posed by recent PCBA design trends. Because the technology does not require physical probes to establish contact, it can be used to test very small and densely packed assemblies without the need for special test pads to be designed into the circuit. The system's field-of-view (FOV) also can be expanded for testing large boards. Area-array devices and high-density interconnects also are imaged easily.

IR technology also has proven effective in detecting power-to-ground shorts, which usually become evident immediately after power-up. Unlike conventional ICT testing, however, IR can isolate the location of the short. Measuring the change in thermal energy is accomplished while safeguarding the circuitry through current limiting, thereby protecting the assembly from damage. This assists the disposition process, particularly if the short is in an area containing a high degree of parallel circuitry. Similarly, IR imaging can detect high-resistance shorts, a type of defect that may pass an initial FT but later may cause a field failure. In fact, anomalous heating in components that have passed functional tests can indicate latent defects.

Other areas where IR test may identify problems that are difficult to troubleshoot on surface mount assemblies include those stemming from intermittent defects due to circuit opens. Traditionally, failure-analysis technicians relied on "freeze spray" or similar techniques to isolate such defects. However, applied in conjunction with a fixture that marginally stresses the board to simulate a bow/twist condition (Ref: ITC-TM-650 for mx. bow and twist condition), IR testing can be a valuable tool in diagnosing this problem.

As with other test methods, it must be emphasized that IR technology, though powerful, is a tool and not a panacea. While test results will show the effects of a defect, it may not always show the defect itself. For example, a cut trace will block current flow to downstream components and register them cold, but this does not necessarily mean that the components are defective. A failure analysis technician still needs to interpret the test results and his interpretive skill influences his ability to quickly and effectively troubleshoot the PCBA. The other requirement is that current must flow for IR testing to "see" problems. However, even with these limitations, IR imaging technology has proven to be a powerful diagnostic tool, particularly when used with other FT and ICT tests.