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.
Why are metallic whiskers a risk to automotive manufacturers today?
Why are they such an issue today, they’ve actually been around since as early as the 1940s. But with the use of lead, in tin-lead finishes, tin whisker growth itself was essentially mitigated, and the whole topic was left in the dark and forgotten about for many years. It wasn’t until just a little over a decade ago, when they started bans and restrictions on the use of lead, this resulted in the use of pure tin films, and that is when tin whisker growth really became a big issue. Now when we couple this with the fact that press-fit pin applications are only going to be increasing in our electronic systems due to their high reliability in connection, along with the growing miniaturization trends, of course to save on room and weight, this really becomes a concern for all electronic systems out there. And especially when we look to the automotive sector. Here, our cars have hundreds of pins in every vehicle being produced today.
And this is only going to increase itself, the more we implement high functionality accessories in our cars, and of course, as electric cars, they already have started, but as electric cars slowly and continually replace our motorized cars this is going to become an even bigger issue.
And not to mention that automotive transportation is the number one most widely used form of transportation for passengers today, making this reliability of these systems extremely important to the point of where manufacturers really cannot afford the risk of whisker induced failures.
What actions must automotive manufacturers take to avoid whiskers?
Automotive manufacturers need to find a new plating solution that eradicates the whole whisker risk. Tin itself being such a highly motivated whiskering producer, it’s probably best to avoid the use of pure tin wherever possible, but this is not so easily done.
Any new replacements needs to be also a sustainable, non-toxic and environmentally friendly material.
What is LITESURF Plating and how does it solve the challenge of tin whiskers?
TE Connectivity’s LITESURF deposit, it is a Bismuth-based deposit, and therefore it is a harmless non-toxic plating. It was specifically produced to easily replace out tin in any production plating line. This comes with many advantages against whisker growth.
First, being that here we have now a Bismuth film, replacing what used to be a Tin film. So there is no more Tin. No more Tin, no more Tin whiskers. Second, Bismuth is not a highly motivating whisker producer like Tin is. And third being that the conductivity of Bismuth is one-tenth that of Tin. Now, remember the whole reason these whiskers pose such a threat in our electronic systems lies in the fact that they are electrically conductive. So the fact that Bismuth is one-tenth the conductivity of Tin, simply means that even if a Bismuth whisker did grow, it is just that much less of a concern or threat as it stands today using Tin.
Now this LITESURF plating it has been deposited and tested on signal press-fit pins where it has shown itself to reduce the whole whisker risk by over 1600 times that of tin today. Making it in any case a very promising solution for our future not just for press-fit pin applications but for the reliability of Automotive all together.
