Case Studies in PCB Failure Analysis
The modern printed circuit board is a veritable labyrinth of components, vias, and conductive traces, routing electrical signals from point to point through convoluted pathways that span large areas of onboard real estate. While electrical signals have no problem navigating this maze of circuitry, the human eye cannot always follow the same route as traces dive into buried layers deep within the board. Naturally, this greatly increases the difficulty for an analyst tasked with performing PCB failure analysis. Though the unassisted eye may not always be able to detect an issue on a PCB, an analyst has access to many different tools and techniques that can allow the analyst to “see” the defect. As shown in the following two case studies in which both samples were reported as failing for shorts, these tools can be invaluable to a successful analysis.
The first case study involves a high-voltage board found in a large RF power device. The customer reported board damage resulting from a short-circuit condition. The board was visually inspected as received to document any damage and look for possible defects. Areas of the board were severely blackened and burnt, preventing any sort of comprehensive visual inspection in these areas; any defect that may have been in these locations would most likely have been consumed in the event that caused the damage. The only visual cue that could not be explained by smoke or fire damage was an accumulation of a powdery residue between the contact points for the positive and negative voltage supplies on the board. Energy dispersive spectroscopy was used to analyze the residue; large concentrations of sulfur, an ionic contaminant that could potentially create a conductive path, were detected. These contaminants most likely existed in the environment, but were concentrated near the positive and negative supplies over time due to the higher electric field in that area, until eventually they led to a malfunction and the destruction of the board.
The second PCB failure analysis case study, a circuit board from a consumer-grade home electronic device, was received with a short-circuit reported between two pins of an IC. Following an unfruitful visual inspection, the suspect IC was removed , along with a handful of other components connected to the same node, and the board was retested. The short-circuit was still present even with the components disconnected, implying that the short was hidden somewhere inside the circuit board. The board was powered up, then inspected with thermal imaging, revealing a hot spot at an area near the lands for the IC reported as failing. Upon review of the board’s layout, it was discovered that the traces for the two failing pins of the IC actually crossed over one another at the hot spot location; cross-section of the hot spot revealed board delamination causing a metal-to-metal short between the layers of the PCB.
Given the multi-layered, hyper-dense nature of modern circuit boards, PCB failure analysis must rely not only on the visual acuity of the analyst, but also on a diverse set of tools and techniques that can reveal defects that may not be identifiable by the naked eye. These tools and techniques allow the analyst to work with even the most complex of circuit boards and deliver defect free services, on time, every time.
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.