SMTA International Conference Proceedings

Evaluation of Tin Whisker Formation on SOT and Discrete Components Assembled with Bismuth-Containing Lead-Free Solder Alloys After Short-Term High Temperature, High Humidity Storage

Authors: André M. Delhaise, Stephan Meschter, Zohreh Bagheri, Polina Snugovsky, Jeff Kennedy
Company: Celestica Inc. and BAE Systems
Date Published: 9/22/2019   Conference: SMTA International

Abstract: With the introduction of environmental legislation such as the Restriction of Hazardous Substances (RoHS), lead (Pb)-free solder alloys have become more prominent. One issue that has emerged is that tin whiskers and initiate and grow under specific conditions, especially when the alloy thickness is less than approximately 50 to 75 microns. Whiskers are thin, highly conductive filaments which have the potential to grow and cause in-field failures in many applications. Most concerning with respect to tin whiskering are high reliability applications such as aerospace, automotive, and medical.

Previously, we discussed whisker formation with bismuth (Bi)-bearing alloys on small outline transistor (SOT) and discrete components after long-term (8000 hours), ambient temperature, high humidity (25°C/85% RH) storage. Three alloys containing Bi, in addition to SAC305, were considered. Boards were assembled with electroless nickel immersion gold (ENIG) and immersion tin (ImmSn) finishes. Whisker density was generally higher on Bi-bearing alloys than SAC. Also, longer whiskers were observed on SAC than Bi-bearing alloys with ImmSn; the opposite was seen for ENIG. These differing results may be caused by variations in the stress state in the solder joint.

In this paper, we evaluate whisker formation on SOT components (with Cu or Alloy 42 leads) and discrete components (with Ni plated terminations) after exposure to a high temperature, high humidity (85°C/85% RH) environment for 3000 hours. It was found that on SOT components, higher whisker densities were observed on ImmSn assemblies for SAC, while on Bi-bearing alloys, higher densities were observed on ENIG. For discrete components, higher densities were observed on ENIG for all alloys, however the difference in density between the two surface finishes increased as Bi in the alloy increased.

Key Words: 

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

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