LITESURF Plating Technology - Interview with Dr. Erika Crandall, Senior R&D Product Development Engineer. In this video section, you can learn about metallic whiskers and what is their significance.
What are metallic whiskers and what is their significance?
Metallic whiskers are crystalline filamentary eruptions that can go grow from various metal films. They are often thought to be spontaneous growth, but the matter of fact is they grow due to stress. Now the stress could be internal stress source in the film itself, or an external source applied such as bending the film, scratches on the surface, etc.
Basically, as stress builds up in the film… whiskers grow outward from the surface to relieve the stress within. Whiskers always grow from the root of the whisker, not from the tip, so, very similar to hair. In fact, their form is also very similar to hair, in the fact that they are extremely thin and very long, up to millimeters long. However, they are orders of magnitude smaller than the human hair, making them almost impossible to see with the naked eye.
Now they are metallic growths, and that means they are electrically conductive. And herein lies the threat they pose to our electronic systems. These whiskers can grow long enough to branch from one electrical contact to its neighboring electrical contact, resulting in an electrical failure, usually in the form of a short circuit. Now not all metals have this proclivity to whisker. So for example some materials that are more known for their whiskering characteristics include Cadmium, Zinc, and for our discussions today in case Tin.
A key example of this is press-fit pin applications. Here we have a tin-plated pin that is pressed into a through-hole on a circuit board with a diameter smaller than that of itself. Here it is very clear that the stress motivating whisker growth in this case in the external stress from the through hole on the plated pin. Now, not all press-fit pin applications have this same whisker risk associated to it. For example, what I mean is let’s say a whisker has found itself between two neighboring contacts.
If the voltage across this whisker is large enough, than the resulting current that flows through the whisker can also easily be large enough to essentially disintegrate the whisker away, resolving the problem immediately.
So for power or digital applications, whisker growth is not as big of a concern. However, for signal or analog applications this is where whisker growth becomes a much larger threat.
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