ICSR (Soldering and Reliability) Conference Proceedings

Evaluation of Tin Whisker Formation in Bismuth-Containing Lead-Free Solder Alloys After Long-Term Ambient Temperature, High Humidity Storage

Authors: André M. Delhaise, Stephan Meschter, Polina Snugovsky, Jeff Kennedy, Zohreh Bagheri
Company: Celestica Inc. and BAE Systems
Date Published: 6/5/2018   Conference: ICSR (Soldering and Reliability)

Abstract: 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 focuses on whisker length and density on the small outline transistor (SOT) parts with the Cu and Fe42Ni lead materials for all four alloys.

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