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|Identification of Cracked MLCCs on Printed Circuit Board Assemblies using Time Domain Reflectometry|
|Keywords: Multilayer Ceramic Capacitors, Time Domain Reflectometry, Non-destructive Inspection|
|Cracking of multilayer ceramic capacitors (MLCCs) typically occurs during or after the solder assembly process, after which the capacitors are interconnected to other components on typically highly populated circuit boards. This limits the success of conventional methods, such as measurement of insulation resistance or capacitance, or X-ray imaging, to characterize MLCCs and to identify cracks within them. While electrical measurement techniques work on electrically isolated MLCCs, interference from other interconnected components on the board renders them useless in these cases. In our earlier investigations, harmonic analysis and methanol testing were evaluated as characterization tools. The results show the capabilities of these methods to identify isolated cracked MLCCs. For interconnected MLCCs, though, there is still a need in the industry for a non-destructive characterization method. Time domain reflectometry (TDR) is a non-destructive technique that allows the localization of impedance discontinuities in the time domain. Based on the ratio of the reflected to the incident radio frequency signal, the nature of the impedance discontinuity can be determined. Probing close to the examined MLCC reduces or eliminates interference from other elements of the circuit, which enables interconnected capacitors to be analyzed. In the first stage of this study the capability of TDR to identify isolated cracked MLCCs of two different sizes were examined. The results showed that clear discrimination between healthy and cracked MLCCs is possible. In further experiments, interconnected cracked MLCCs on PCB assemblies were evaluated and compared to healthy specimens on identical PCB assemblies. The results show that TDR holds promise for the applicability to identification of cracked interconnected MLCCs.|
|Thomas Fritzler, Graduate Student
University of Maryland
College Park, MD