As previously discussed, a cross section of a printed circuit board can be an excellent way to qualify a new process and determine whether a product is being produced to specification. The data about layer spacing, plating thicknesses, and interconnect quality that can be obtained through a well-targeted cross section is invaluable in determining whether appropriate manufacturing procedures are being followed. The cross-section is not only useful for determining the acceptability of a given product, however; indeed, PCB cross section analysis is often one of the only ways to identify certain types of PCB defects.
Generally speaking, since most circuit boards are densely woven constructions of metal and dielectric, it is crucial to identify a site that is the most likely point of failure before beginning any PCB cross section analysis. A cross section will only give information about a single “plane” of a device; the absence of a defect in a given plane by no means rules out the existence of defects altogether, since any anomaly could just as easily be in one of the infinite number of other possible cross section planes. The first step in any PCB cross section analysis is, therefore, an attempt to isolate the failure to as narrow a region as possible, using the circuit layout and schematic in conjunction with point-to-point electrical testing and specialized tools like thermal imaging to pinpoint a defect before the cross section even begins.
Once a site has been chosen, the PCB cross section analysis will continue in the same fashion as for a board qualification. The sample will be ground and polished until the desired site has been exposed, then the sample will be inspected for any anomalies. Burnt dielectrics, cracked copper traces, improperly wetted solder joints, and stray metal filaments are some of the many defects that can be revealed by cross section. Once the defect has been found, an analyst will determine the most likely cause of failure - for example, whether a cracked trace is most likely due to a manufacturing defect or improper handling by an end user.
Though powerful, PCB cross section analysisis only one of many tools for approaching failures on a printed circuit board. Other techniques, like printed circuit board delayering or “dye-and-pry” (the use of a brightly colored dye to reveal any cracking or delamination on solder interfaces) might be more appropriate for a given failure mode. It is important, therefore, to entrust difficult-to-find PCB failures to labs with a wide breadth of experience with the various approaches to PCB 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.