Just as with their semiconductor counterpart the integrated circuit, one of the most common failures for a PCB is excessive current leakage. There is a myriad of possible reasons for this type of failure, all of which must be examined in light of the customer’s reported application and the history of the device. A board which handles extremely high voltages in an industrial application, for example, is far more likely to have been damaged by an electrical arc between adjacent traces than a battery-powered toy robot would be; similarly, one would expect the mass-produced children’s toy to suffer from manufacturing defects and poor quality control more often than an essential PCB from the fire suppression system of a military vehicle. Even the environment that a PCB has been exposed to must be taken into account, since high amounts of moisture or ionic contaminants can easily accelerate corrosion or create conductive paths between neighboring traces, creating a short circuit. Thermal imaging can easily detect the majority of current leakage defects, though the insulative properties of the layers of a PCB can sometimes obscure smaller leakage sites. Once a site has been identified, tools like electron microscopy or elemental analysis may be used to analyze the defect and determine the root cause of failure.
Open circuits are also fairly common PCB defects for an analyst to encounter. Just like current leakage, an open circuit can be caused by any number of different factors. Excessive physical stress – like the vibration and torsion an electronic assembly might be subjected to in a vehicle – can, over time, wear down electrical connections until they finally break. Poor quality control during manufacturing and assembly may result in improperly constructed circuit boards with malformed vias and traces, or in unacceptable soldering work, creating brittle electrical connections between the PCB and its components. Oftentimes, a thorough visual inspection performed in construction with x-ray imaging is enough to locate these types of failure; should the location of the open still remain elusive, there are specialized analysis techniques like Time Domain Reflectometry (TDR) that can precisely pinpoint an open circuit on a board.
A solid understanding of common PCB defects is essential to performing successful analysis. It is, therefore, always beneficial to find a team with an extensive background in PCB technology and manufacturing to tackle the failure analysis of a defective printed circuit board.
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelors of Science degree in Electrical Engineering.