Infrared inspection of printed circuit boards (PCBs) uses heat that is emitted. This requires the presence of some current. Infrared inspection can provide information not available from optical inspections due to variations of heat emitted in different parts of the PCB. The circuit boards can be compared to known standards. Image subtraction using computer software is another technique for infrared inspection.
Temperature Profiles
Flaws in printed circuit boards can cause variations in temperature. Insufficient solder at a joint increases electrical resistance. This causes an increase in temperature that is detectable by an infrared camera. Behaviors of circuit boards being inspected can be compared to boards known to be good. The good circuit board can be an actual part or a computer simulation designed for inspection purposes.
Assumptions About Heat Emissions
The theory of inspecting PCBs with a known standards is based upon the assumption that the emission of heat from the part being inspected is both known and predictable. The validity of this assumption can be affected by materials and techniques used in the manufacture of the PCB. The board can be made with components using various semiconductor materials with varying emissions of heat. The ideal for inspection would be the knowledge of expected heat emission at every point on the board.
Image Subtraction
Image subtraction is based upon a software that creates an ideal thermal profile. The thermal profile can be compared to or subtracted from the actual inspection results. The profile can be used from a computer or directly loaded into the infrared inspection camera. This technique reduces the significance of variations from one board to the next.
Active and Passive Infrared Inspection
In all infrared PCB inspection, a current must be present to provide the necessary heat. When using active infrared inspection, the PCB is energized as it would be in actual use, and heat emitted from the components is measured. In passive infrared inspection, a current is sent along a path in the board to a heat sink. Under proper conditions, the passive technique can provide acceptable sources of thermal emissions to allow infrared inspection.
Limitations of Infrared Thermal Imaging
Calibration of the infrared detector can be quite complex. Calibration based upon the different components on the PCB is possible. However, such calibration is not considered practical unless the components have been covered with a uniform coat of constant emissivity paint. This means the technique is better suited for finding defects in a pass or fail format, while lacking the precision to analyze the defect.
