Journal of SMT Article

Tin Whisker Formation in Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage, Part I: Whisker Inspection Results

Authors: André M. Delhaise, Stephan Meschter, Polina Snugovsky, Jeff Kennedy, Zohreh Bagheri
Company: Celestica Inc., BAE Systems
Date Published: 4/30/2019   Volume: 32-2

Abstract: With the introduction of the Restriction of Hazardous Substances (RoHS), one issue that has emerged in Pb-free solder alloys is that tin whiskers can initiate and grow 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 and automotive.

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 series of three papers, we discuss whisker formation in 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 as well as discrete chip components, with half of assemblies cleaned and half cleaned and contaminated with low levels of NaCl.

This paper is the first in a series of three in which we share qualitative imaging results from the whisker inspection of the small outline transistor (SOT) components, specifically focusing on whisker growth and corrosion. It was found that contamination was the most influential factor on whiskering and corrosion, with the presence of Ni from the ENIG surface finish and/or Fe42Ni leadframe less significant. Bismuth precipitates were found incorporated in whiskers, appearing to suggest that Bi may influence the mechanism of whisker growth.

Keywords: 

Tin whisker, nucleation, growth, corrosion, ambient temperature, high humidity, lead-free solder, bismuth, SAC



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