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 the current flow and thick enough dielectric to prevent arcing between neighboring traces.
Of course, the PCB cross-section is 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.