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Tin Whiskers' Behavior under Stress Load and the Mitigation Method for Immersion Tin Surface Finish
Nor Akmal Fadil1, Siti Zahira Yusof2, Tuty Asma Abu Bakar1
1Department of Materials, Manufacturing, and Industrial Engineering, School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru 81310, Malaysia.
Materials (Basel, Switzerland)
|November 27, 2021
Summary
This study investigated tin whiskers on lead-free electronic materials. A nickel underlayer effectively mitigated whisker formation, while external stress influenced growth patterns and CuO flowers suppressed whisker elongation.
Area of Science:
- Materials Science
- Surface Engineering
- Reliability Engineering
Background:
- Lead-based materials are banned in electronics, driving research into lead-free alternatives like tin.
- Pure tin and its alloys are susceptible to tin whisker formation, posing risks to electronic circuit reliability.
- Tin whiskers can cause short circuits and endanger electronic component functionality.
Purpose of the Study:
- To investigate the impact of stress on tin whiskers' formation and growth on tin-coated copper substrates.
- To evaluate the effectiveness of a nickel underlayer as a mitigation strategy for tin whiskers.
- To analyze the influence of external stress and oxide formation on whisker development.
Main Methods:
- Tin-coated copper substrates were subjected to micro-hardness indentation (2N load) to simulate external stress.
- Samples were stored in a humidity chamber (30 °C/60% RH) for up to 52 weeks.
- Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX) spectroscopy were used for observation and chemical analysis.
- Image analysis measured whisker length according to JEDEC Standard (JESD22-A121A).
Main Results:
- Tin whisker growth was directly proportional to storage time and coating thickness (2.3 μm > 1.5 μm).
- Thicker tin coatings exhibited longer whiskers due to increased internal stress from Cu5Sn6 intermetallic compounds.
- CuO flower formation on thinner coatings suppressed whisker growth.
- External stress from indentation initially halted whisker growth in the indented area but promoted longer whiskers farther away due to tin atom migration.
- Nickel underlayers successfully mitigated tin whisker formation.
Conclusions:
- Tin whisker formation is influenced by storage time, coating thickness, internal stress, and external mechanical factors.
- CuO flower formation can suppress tin whisker growth on thinner tin coatings.
- A nickel underlayer is a highly effective method for mitigating tin whisker formation in tin surface finishes on copper substrates.
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