Tin Whiskers Revisited
Q. I am new to the world of electroplating and had a discussion recently regarding tin whiskers. Needless to say, I did not have much to contribute to this discussion! Can you tell me more about the tin whisker phenomenon? N. B.
A. I receive many questions about the phenomenon of tin whiskers. Tin whiskers continue to be a very important area of plating research. I last wrote about this phenomenon approximately 4 years ago. Here is what I wrote at that time:
“Welcome to the world of tin whiskers! The phenomenon 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, has used 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 problem, type ‘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 µm).
- 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 titled The Electrodeposition of Tin and its Alloys, authored by Manfred Jordan, has a good discussion of this problem. Unfortunately this book is no longer in print, but you should be able to find a copy by doing a search on the Internet.
An excellent paper presented at SUR/FIN 2007 covered some of the latest developments in this area. Written by J. Barthemes et al., the paper, “Quantification Of The Influence Of All Important Physical And Chemical Tin Plating Bath Parameters On The Propensity For Whisker Formation,” discusses an extensive experimental program for trying to determine the critical variables in the plating process. The variables investigated were:
- Tin concentration
- Current density
- Plating bath temperature
- MSA concentration
- Additive concentrations
- Different nickel interlayers
- Post baking.
Space constraints do not allow for a discussion of the results, but you can try to get a copy of the paper. It gives much detail about the experimental results. Contact the National Association of Surface Finishers (NASF) at www.NASF.org and see if they can provide you with a copy.