PCB Cross Section - Qualifying Today’s Cutting Edge Assemblies
The electronics industry is constantly striving to make products faster, smaller, and more power-efficient than previous generations. A task that may have required a dedicated desktop computer fifteen years ago can now be performed on a smartphone weighing several ounces, often while running several other processes at the same time. This increase in computing power is directly correlated to the level of complexity found in modern electronics; the intricacy of the circuits in a modern device, connected by the labyrinthine network of copper and dielectric material that make up a printed circuit board, is far beyond that found in the systems of yesteryear. Qualifying these incredibly dense assemblies can be an incredible challenge; fortunately, an approach for quickly examining the construction of these parts can be found in the PCB cross section.
PCB cross sections are often performed as part of a larger course of destructive physical analysis (DPA). A handful of boards (or several samples from a single board) are encapsulated in epoxy, which provides added structural integrity and prevents any damage to the sample from the cross-sectioning process. The sample is then polished with a set of increasingly fine abrasives, slowly removing material until an area of interest - a via between traces, a solder joint between the board and a surface mount component, or a connector between the board and the outside world, for example - has been exposed. Once the area of interest is in full view, the surface of the sample is buffed to a microscopically flawless shine, removing any artifacts from the rough grinding process and leaving behind only those features that are inherently a part of the specimen to be examined.
A well-executed PCB cross section can give both qualitative and quantitative data about a sample that is difficult (if not impossible) to obtain through other methods. Sectioning through a solder joint can give valuable information about the presence and thickness of any inter-metallic compounds formed during the soldering process, a metric which is often vital in understanding solder-related failures. Similarly, sectioning a sample that has been plated with metal (for example, a copper trace that has been plated with nickel) can identify issues with plating adhesion that may lead to failure in the field. Most importantly for DPA-style qualifications, however, is the ability to simultaneously view and measure the thicknesses of all the layers of the board, ensuring that there is sufficient conductive material to handle current flow and thick enough dielectric to prevent arcing between neighboring traces.
Of course, the PCB cross sectionis also a useful tool for failure analysis. A properly targeted cross section can identify cracked traces, poorly wetted solder, and any number of other physical defects. A lab providing PCB cross sections is therefore helpful in any phase of a design’s lifecycle - from its first inception and initial qualification, all the way to its inevitable failure and post-mortem analysis.
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.