ELECTROCHEMICAL MIGRATION
Tin Whiskers Eliminated by Slight Alloying with Indium
The indium additive relieves stresses that drive whisker growth. by Indranath Dutta, Ph.D.
Copper components in electronic packages (e.g., lead-frames, interconnects, integrated circuit leads and press-fit connector pins, to name a few) are often electroplated with tin (Sn) to prevent tarnishing and to facilitate subsequent soldering. With passage of time during storage or service, long whiskers grow from these tin coatings, causing electrical shorts between neighboring circuitry, posing serious reliability risks.1-3 This problem is particularly problematic in long-life applications, and failures have been reported in many arenas, including aerospace, nuclear power plants, automotive electronics, and military electronics systems, causing damages worth millions of dollars.

A number of approaches to mitigate whisker growth have been pursued during the past 30 years, including additions of Pb, Bi, Au, Sb or Ge, or post-plating thermal treatments.4-8 These approaches mitigate whisker growth to varying degrees; however, none eliminates it. Over time, and under thermal-mechanical excursions, whiskers continue to grow.

Over the past five years, researchers at Washington State University have developed a robust method to eliminate, rather than reduce, whisker growth. The team showed the addition of 4 to 10 weight percent indium to Sn films produces no whiskers whatsoever under both isothermal aging and cyclic thermal shock loading.

The findings are detailed in a series of publications,9-16 culminating in the award of a US patent in December 2020 on the elimination of Sn whiskers by indium addition.17 The principal claim of the patent is pure Sn or Sn-based lead-free solder coatings containing 3 to 20 weight percent In are immune to whisker growth, either directly after deposition, or following a short post-deposition thermal treatment. Key results from the associated studies are summarized below.

the surface of a copper substrate with a 3μm-thick electroplated coating, after ambient temperature aging for 1.5 months
Figure 1. Surface of a copper substrate with a 3µm-thick electroplated coating, after ambient temperature aging for 1.5 months. The Sn version shows multiple whiskers.
an otherwise identical substrate with Sn-10%In, aged under the same conditions, showing a complete absence of whiskers
Figure 2. An otherwise identical substrate with Sn-10%In, aged under the same conditions, shows a complete absence of whiskers.
The addition of eight to 10% indium eliminates whisker growth outright (i.e., without any post-plating treatment), regardless of the method by which In is incorporated in Sn.9-17 In contrast, in control samples of pure Sn coated on Cu, numerous long (~25µm) whiskers grow under identical conditions. When only 4% In is added, a very small number of short (~2µm) whiskers grows from the untreated coating. But whisker growth is completely eliminated if the coating is subjected to a 30-minute heat treatment at 160oC following electroplating.15

The root cause of whisker elimination due to In addition was investigated.10-13 Indium gets incorporated in the surface-oxide film on Sn, introducing discontinuities in the oxide, thereby enabling relaxation of the compressive stresses in the film that drive whisker growth. In addition, In gets incorporated in the intermetallic compound (IMC) layer at the interface between the Sn coating and the Cu substrate, forming ternary Cu-Sn-In IMCs, and these may further reduce the stresses in the film.

A method to co-electroplate tin with indium from a methanesulfonic acid (MSA) electrolyte, which is the most widely used bath for electroplating of Sn, was developed.15 This method enables co-electroplating of Sn and In with only a small modification of the commonly used industrial practice for electroplating Sn.

The major advantage of In addition is it completely prevents the growth of whiskers, whereas all other currently available approaches slow whisker growth. As such, this technology proffers a way to completely solve the Sn-whisker growth problem in electronics using a lead-free approach.

Industry professionals interested in prevention of tin whisker growth from components may contact the author for more information.

graph illustrating numerous tin whiskers grow from a 1μm-thick electroplated tin coating on copper, but a Sn-10%In coating is immune to whisker growth
Figure 3. Numerous tin whiskers grow from a 1µm-thick electroplated tin coating on copper, but a Sn-10%In coating is immune to whisker growth.
Acknowledgements
This work was spearheaded by a former post-doctoral researcher (Lutz Meinshausen, now at Global Foundries in Germany) and a former graduate student (Susmriti Das Mahapatra, now at Intel Corp. in Arizona) working with the author. The researchers also collaborated with Professor B. S. Majumdar of New Mexico Institute of Mining and Technology. The work was supported by the National Science Foundation (CMMI-1335199/1335491).
References
  1. J.A. Brusse, CARTS 2002 – 22nd Capacitor and Resistor Technology Symposium, 2002.
  2. H. Leidecker and J.A. Brusse, “Tin Whiskers: A History of Documented Electrical System Failures – A Briefing,” Space Shuttle Program Office, http//nepp.nasa.gov/whisker, 2006.
  3. K.G. Compton, A. Mendizza and S.M. Arnold, “Filamentary Growths on Metal Surfaces – Whiskers,” Corrosion 7, 1951.
  4. M. Sobiech, J. Teufel, U. Welzel, E.J. Mittemeijer and W. Hugel, “Stress Relaxation Mechanisms of Sn and SnPb Coatings Electrodeposited on Cu: Avoidance of Whiskering,” Journal of Electronic Materials, 40, 2011.
  5. E. Chason, N. Jadhav, W.L. Chan, L. Reinbold and K.S. Kumar, “Whisker formation in Sn and Pb–Sn Coatings: Role of Intermetallic Growth, Stress Evolution, and Plastic Deformation Processes,” Appl. Phys. Lett., 92, 2008.
  6. P. Sarobol, A.E. Pedigo, P. Su, J.E. Blendell and C.A. Handwerker, “Defect Morphology and Texture in Sn, Sn-Cu, and Sn-Cu-Pb Electroplated Films,” IEEE Trans. Electron. Packag. Manuf., 33, 2010.
  7. J. A. Nielsen and T. A. Woodrow, “The Role of Trace Elements in Sn Whisker Growth,” Final report, SERDP project WP-1751, Contract no. W912HQ-10-C-0022, September 2013.
  8. M. Osterman, “Mitigation Strategies for Tin Whiskers,” www.calce.umd.edu/lead-free/tin-whiskers/tinwhiskermitigation.pdf, Aug. 28, 2002.
  9. L. Meinshausen, S. Bhashyantham, B. Majumdar, and I. Dutta, “Influence of Indium Addition on Whisker Mitigation in Electroplated Tin Coatings on Copper Substrates,” Journal of Electronic Materials, 45 (1), 2016.
  10. S. Banerjee, I. Dutta and B. S. Majumdar, “A Molecular Dynamics Evaluation of the Effect of Dopant Addition on Grain Boundary Diffusion in Tin: Implication for Whisker Growth,” Materials Science and Engineering A, 666, 2016.
  11. S. Das Mahapatra, H. Yang, L. Meinshausen, S. Bhassyvasantha, S Banerjee, B.S. Majumdar and I. Dutta, “Influence of Indium Addition on Whisker Growth in Electroplated Sn,” Proc. The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9), Japan Inst. Metals and Mater., 2016.
  12. S. Das Mahapatra, B.S. Majumdar, I. Dutta and S. Bhassyvasantha, “Eliminating Whisker Growth by Indium Addition in Electroplated Sn on Copper Substrate,” Journal of Electronic Materials, 46, 2017.
  13. S. D. Mahapatra and I. Dutta, “Role of Surface Oxide in Mitigating Whisker Growth: Finite Element Study,” Materials Science and Engineering A, 706, 2017.
  14. S. Das Mahapatra, B.S. Majumdar and I. Dutta, “Elimination of Tin Whisker Growth by Indium Addition to Electroplated Tin in Electronic Packages,” Proc. 16th IEEE Inter-Society Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), IEEE/ASME, 2017.
  15. S. D. Mahapatra and I. Dutta, “Co-electrodeposition of Sn with 0.2-20% In: Implications on Sn Whisker Growth,” Surface and Coatings Technology, 337, 2018.
  16. B. S. Majumdar, I. Dutta, S. Bhassyvasantha and S. Das Mahapatra, “Recent Advances in Mitigation of Whiskers from Electroplated Tin,” J. Metals (JOM), 72, 2020.
  17. I. Dutta, “Mitigation of Whisker Growth in Tin Coatings by Alloying with Indium,” US Patent #10,879,156, Dec. 29, 2020.
Indranath Dutta, Ph.D., is director of the School of Mechanical and Materials Engineering at Washington State University; idutta@wsu.edu.