Tin Whiskers Again

Question:

Our plating shop does bright tin plating for electrical connectors. We are getting dark spots on the surface of the parts after plating. One of our senior platers is telling me that this is the ”tin whisker” phenomenon. What can you tell me about tin whiskers? M I.

Answer:

The dark spots on the bright tin plate are probably not tin whiskers. They are more likely due to improper rinsing after plating or excess copper in the tin plating bath.

Tin whiskers are a very different phenomenon. First of all you can determine if you have a whisker problem by examining the plated part under magnification. There has been a lot written about this phenomenon since it can cause catastrophic failure of electronic components.

In an earlier column the following was written about tin whiskers:

“Welcome to the world of tin whiskers! This phenomenon of tin whiskers was first reported in the 1940s. Typically, the whiskers will grow from the surface and, once they are long enough, can cause shorting out of electronic components. The classic way of preventing this is to incorporate some lead in the tin plate. Hence, much of the electroplating of tin, particularly in the electronics area, uses tin/lead alloys. The deposit typically contains 1–3% lead.

In the last 10–15 years great effort has been made to remove lead from all things including tin electroplates. Much effort has been expended in trying to find solutions to this problem. Obviously the whisker problem is particularly critical in today’s world of tightly packed electronic components.

If you want to learn more about the tin whisker problem, go to your computer and type in “tin whiskers” in your search engine. I did just that and came back with many hits. There are a few suggestions that can help you reduce whisker formation:

• Use tin alloy deposits instead of pure tin. Tin/nickel alloys are one such example. • Reflow the tin coating after deposition. Reflowed deposits tend not to form whiskers. • Avoid very thin tin deposits (less than five microns). • Avoid bright tin deposits if possible. • Avoid mechanical stress of your parts during assembly operations, stress seems to increase the chance of whisker growth.

A book, The Electrodeposition of Tin and its Alloys by Manfred Jordan, has a good discussion of this problem. Unfortunately this book is no longer in print but you may be able to find a copy by searching the Internet."