Tin Whisker Formation on Small Outline Transistors Assembled Using Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage Part 2: Statistical Analysis

  • André M. Delhaise Celestica Inc.
  • Stephan Meschter BAE Systems
  • Polina Snugovsky Celestica Inc. (Retired)
  • Jeff Kennedy
  • Zohreh Bagheri Celestica Inc.
Keywords: Tin whisker, nucleation, growth, corrosion, ambient temperature, high humidity, lead-free solder, bismuth, SAC


With the introduction of environmental legislation such as the Restriction of Hazardous Substances (RoHS), lead (Pb)-free materials have made their way into the electronics manufacturing industry. One issue that has emerged is that Pb-free solder alloys can initiate and grow tin whiskers under specific conditions. These whiskers are thin, highly conductive filaments which have the potential to grow and can cause field failures in many applications. Most concerning with respect to tin whiskering are high reliability applications such as aerospace, automotive, and medical.

Bismuth (Bi) is being considered for inclusion in solder alloys to replace the current industry standard (SAC 305) and provide improved thermomechanical and vibration reliability. In this paper, we discuss whisker formation of several Bi-bearing alloys after long-term (12,000 hours), ambient high humidity (25°C/85% RH) storage. Three alloys containing Bi, in addition to SAC 305 (Sn-3.0Ag-0.5Cu), were considered.  These alloys were Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and Senju (Sn-2.0Ag-0.7Cu-3.0Bi). The boards were fabricated with electroless nickel immersion gold (ENIG) and immersion tin (ImmSn) finishes and populated with parts having Cu and Fe42Ni alloy leads and chip parts, with half of assemblies cleaned and half cleaned and contaminated with low levels of NaCl.

This paper is the second in a series of three in which we share quantitative statistical analysis from the whisker inspection of the small outline transistor (SOT) components. It was found that on ImmSn surface finishes, the longest whiskers were found on SAC, however the longest whiskers were found on Bi-bearing alloys for ENIG. In addition, whiskers were found to generally grow in regions where the tin coverage is thin, and, on ENIG-finished assemblies, near the PCB, likely due to galvanic corrosion between the solder and the finish chemistry.

Author Biographies

André M. Delhaise, Celestica Inc.

André Delhaise is a metallurgist at Celestica, Inc., and is actively involved in several projects in association with the Refined Manufacturing Acceleration Process (ReMAP), which is a Canadian-led consortium of academic and industry partners focused on the commercialization of novel manufacturing technologies and processes. He is also involved in the iNEMI 3rd generation Pb-free alloy characterization effort as well as the Pb-free Risk Management (PERM) group. André is also currently a PhD Candidate in the Department of Materials Science & Engineering at the University of Toronto, specializing in the metallurgy of bismuth-containing lead-free solders, under the supervision of Dr. Doug D. Perovic. He holds a Bachelor of Applied Science (BASc) degree from the same department, obtained in 2013. André was awarded the ‘Best of Proceedings’ award at the SMTA International Conference in 2017, and has published several papers in refereed journals such as the Journal of Electronic Materials (JEM).

Stephan Meschter, BAE Systems

Dr. Stephan Meschter has over 30 years of experience in advanced packaging, failure analysis, and reliability testing of electronic assemblies at BAE Systems Electronic Systems in Endicott, NY. He has designed and evaluated electronic assemblies for power, flight and jet engine control systems used in spacecraft, aircraft and ground vehicles. Starting in 2004, Stephan began evaluating the commercial lead-free materials transition impact to high-reliability, high-performance aerospace and defense electronic systems. He was a member of the 2009 U.S. DoD Lead-free Electronics Manhattan Project team that published a set of best practices to mitigate the risks associated with Lead(Pb)-free electronics usage in high performance DoD systems. Since 2010, Stephan has worked with SERDP on lead-free tin whisker formation research and short circuit risk mitigation using enhanced polymer conformal coatings. He earned a bachelor’s in mechanical engineering from the University of Hartford in Hartford CT in 1984, and he holds both a master’s degree (1987) and a doctoral degree (2001) in Mechanical Engineering from the State University of New York in Binghamton, NY. Stephan currently participates in the IPC Lead(Pb)-free Electronics Risk Management (IPC-PERM) Council and is currently supporting revision of several SAE GEIA Lead-free Aerospace and Defense risk management standards.

Polina Snugovsky, Celestica Inc. (Retired)

Polina Snugovsky recently served as Principal Engineer - Chief Metallurgist at Celestica. Polina was an invited subject matter expert participant in the 2009 Pb-free Manhattan Project tasked with the risk assessment of Pb-free electronics in Aerospace and DoD applications. Polina was an active participant in the Pbfree electronics in aerospace project group (AIA PERM) - American Institute of Aeronautics. She was the principal investigator of two U.S. DoD Strategic Environmental Research and Development Projects (SERDP) that are examining corrosion induced whisker growth and manufacturing mitigation. She has also actively participated in NASA DoD projects. Dr. Snugovsky graduated from the State Metallurgical Academy of Ukraine and received her Ph.D. in Metallurgy, and subsequently in 1985 she earned the higher level Doctoral degree in Metallurgy and Material Science. Before she joined Celestica in 1996, she was a full professor in the department of physical metallurgy of the State Metallurgical Academy of Ukraine. She has published over 160 papers and patented new materials and processes. Polina holds several Outstanding Technical Achievement Awards, including two from Celestica and received Best International Conference Papers awards at SMTA2006, APEX2007, APEX2009, APEX2010, APEX2013 and APEX2014. In 2012 she received SMTA Member of Technical Distinction Award.

Jeff Kennedy

Jeff Kennedy is currently the Strategy and Business Development Manager at ZESTRON Americas. Prior to that he directed Celestica’s advanced technology strategy and development initiatives, including the design and implementation of technology roadmaps for the Industrial and A&D sectors. Jeff has more than 30 years of experience in system integration, process development, PWB fabrication, and packaging in the microelectronics industry. He has engineering and management experience working within mainframe computer industry, microcircuit wire bonding and flip chip, PWB fabrication and laminate packaging substrates, and the last 16 years in contract electronics assembly. Jeff is a past President of SMTA and is an active member of both IPC & IMAPS.

How to Cite
Delhaise, A., Meschter, S., Snugovsky, P., Kennedy, J., & Bagheri, Z. (2020). Tin Whisker Formation on Small Outline Transistors Assembled Using Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage Part 2: Statistical Analysis. Journal of Surface Mount Technology, 33(1), 7-19. https://doi.org/10.37665/smt.v33i1.14