Metal whiskers are not a new problem. But finding them in new places can be.

Metal whisker formation on plated and metal surfaces in electronics has been found on a variety of surfaces: tin; SnPb (yes, in a few cases); SnAgCu; silver; copper; zinc and gold. Each whisker formation has a variety of shapes and structures. A whisker appears to be a single crystal of metal extruded from the metal surface for a long whisker, while a nodule appears to grow from a cluster of metal crystals. Figure 1 shows that a variety of shapes can grow in a small space. We have seen whiskers growing from plated surfaces, and at the NASA Web site (http://nepp.nasa.gov/whisker/) all the aforementioned conditions are described. But a new whisker has been found growing out of the SAC alloy.


We are working on an investigation where whisker formation is not on a plated surface, but from a SAC solder paste alloy. More information will be presented in future case studies, but for now the issue of creating a tin whisker from the surface of a soldered condition is possible, and we are working to find the root cause material and processing variables. Multiple assemblies have grown whiskers, as seen in Figures 2 to 4. This is not a report on the detailed study, but a first blush with the issues that whisker formation and returned field hardware inspection must start with a magnified inspection of the returned hardware, especially if no trouble found (NTF) conditions exist. We have not seen long whiskers growing from SnPb solder on product in the field for five years in this study, but product built in the past year shows conditions of long whisker growth on MOSFET SMT product.


At this time, little is known about this whisker formation trend, other than we are seeing a difference in surface residues in the areas where whiskers are found versus areas where they aren’t.

Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. His column appears monthly.