One of the most challenging cases of PCB failure analysis is the search for an open circuit. Navigating the maze of metal interconnects with probes and an ohmmeter is time-consuming and, frustratingly, often ends without bearing fruit when an analyst encounters a component like a ball-grid array (BGA), with concealed connections that prevent further probing. At this point, the analyst is stuck; removing the component by desoldering would remove any evidence of an open circuit, and a blind cross section has low odds of success unless the component has large numbers of open solder joints. In such occasions, dye penetrant testing can be used to detect any solder defects, revealing broken or non-wetted joints at the expense of further testability.
Dye penetrant (also referred to colloquially as Dye and Pry) is a PCB failure analysis technique that involves submerging a failing circuit board in a brightly colored dye, then subjecting the sample to alternating vacuum and pressure to force the dye into any cracked or improperly formed solder interfaces. The sample is then pulled from the dye and allowed to dry, at which point any suspect component is unceremoniously (but very delicately) ripped from the board, exposing the interconnect surfaces. Both the component-side and board-side of the interconnecting surface are inspected for any traces of dye; finding dye on any surface where a connection is supposed to be made is indicative of an open circuit. The inspection can be time-consuming, due to the sheer number of surfaces that must be inspected, so the type of dye used must be carefully chosen with proper attention paid not only to material properties like viscosity but also to its visibility; IAL uses a deep crimson dye, which is not only easy to see under a microscope but is very fashion-forward in the inevitable event of an analyst spilling dye on themselves.
Though dye penetrant testing is useful for both PCB failure analysis and adding unintentional sartorial flair, it can also be applied to other types of device with equal measures of success. Hermetically sealed packages can be subjected to the dye to determine whether a good seal has been achieved; traditional plastic encapsulated devices can be tested as a method of determining the presence and extent of delamination propagating from the edges of a package, perhaps in lieu of (or in conjunction with) acoustic microscopy. When used in this fashion, dye penetrant can even reveal improper wirebonding, especially over the leadframe!
Though useful, dye penetrant is but one of many tools in an analyst’s PCB failure analysis repertoire and is not always the ideal choice. Since it is inherently destructive, dye and pry testing cannot be used in cases where further electrical testing is desired; furthermore, in cases where there is contamination is suspected, the dye would serve to effectively mask any potential residues that could be the root cause of the problem. It is therefore necessary to have an experienced analysis team examine the project before determining whether dye penetrant testing is indicated.