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Tin Whisker Growth on the Surface of Tin-Rich Lead-Free Alloys
March 11, 2015 | A. Skwarek, K. Witek, M. Pluska, A. Czerwinski, Institute of Electron TechnologyEstimated reading time: 1 minute
The introduction of lead-free technology into electronics has intensified concern over tin whiskers phenomenon. Tin whiskers are crystals growing from the alloy surface as the result of compressive stress relaxation. This paper presents the dependence of tin whisker growth on copper and oxygen surface content. The observations and measurements were done using scanning electron microscopy with energy- dispersive X-ray spectroscopy. The results show that whisker growth is strongly related to increased copper and oxygen surface content in whisker neighborhood.
Introduction
Soldering is one of the oldest processes known to mankind. The proper solder joint is created by producing the metallic bond between the soldered elements and solder. In the 20th century, the most popular solder was Pb-37Sn63 eutectics. From July 2006, the use of Pb in solders is banned in the European Union as a consequence of RoHS directive. Actually, most of the lead-free alloys are based on Sn, containing in their composition also Ag, Cu, and small additions of Ni, Co, Ge or Sb. The introduction of tin-rich lead free solder has renewed the concern over, known for ages but limited by lead addition, phenomenon: tin whiskers. This phenomenon can significantly influence the reliability of solder connection and as a result influences the work of the entire PCB.
Tin whiskers are the crystals growing from the tin or tin alloy surface which can be dangerous for circuit reliability because of electrical current leakage and shorting due to bridging of adjacent conductors, metal vapor arcing (plasma) at low pressure, increased electromagnetic radiation, and damage from device littering by debris and contamination. The growth of tin whisker is caused by the compressive stress relaxation. The copper intermetallic compounds and surface layer of oxygen can strongly influence whisker growth. This paper presents the results of energy-dispersive X-ray spectroscopy (EDS) studies showing the relationship between tin whisker formation and copper and oxygen surface content in whisker neighborhood.
Editor's Note: This article originally appeared in the February issue of SMT Magazine.
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