Tin Whisker Growth on Electronic Assemblies Soldered with Bi-Containing, Pb-Free Alloys
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https://doi.org/10.1007/s11664-020-08544-6 2020 The Minerals, Metals & Materials Society
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
Tin Whisker Growth on Electronic Assemblies Soldered with Bi-Containing, Pb-Free Alloys ´ M. DELHAISE ,1,3 ZOHREH BAGHERI,1 ANDRE STEPHAN MESCHTER,2 POLINA SNUGOVSKY,1 and JEFF KENNEDY1 1.—Celestica, Inc, Toronto, ON, Canada. 2.—BAE Systems, Endicott, NY, USA. 3.—e-mail: [email protected]
With the introduction of environmental legislation, lead (Pb)-free materials have made their way into the electronics supply chain. As a result, tin whiskers may initiate and grow under specific conditions. Sufficiently long whiskers may cause in-field short circuit failures, which are a serious concern in many high-reliability applications such as aerospace, automotive, and medical. There are efforts in developing third generation Pb-free solder alloys to replace tin-silver-copper (Sn-Ag-Cu, or SAC). Bismuth (Bi) has been considered for inclusion in SAC alloys to reduce melting point and improve reliability. In this study, we examine whisker formation on small outline transistor (SOT) components assembled with SAC-Bi alloys after ambient temperature (25C) or high temperature (85C), high humidity (85% RH) storage. Four alloys were considered—Senju M42 (Sn-2.0Ag-0.7Cu-3.0Bi), Violet (Sn2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and SAC305 (Sn-3.0Ag0.5Cu). The assemblies were finished with immersion tin (ImmSn) or electroless nickel immersion gold (ENIG), and some were intentionally contaminated prior to exposure. It was found that SAC305 tended to grow longer whiskers than SAC-Bi alloys, though more whiskers were found on the latter. Particle Stimulated Nucleation (PSN) of recrystallization of the b-Sn matrix, influenced by Bi precipitation, appears to influence dynamic recrystallization (DRX), the mechanism for whisker growth. Key words: whiskers, SOT, corrosion, contamination, solder, bismuth
INTRODUCTION Tin (Sn) whiskers are one of the most studied reliability concerns existing in electronics manufacturing. They have become increasingly more prevalent due to the introduction of the Restriction of Hazardous Substances (RoHS) legislation the early 2000s, which prohibits the use of lead (Pb) in solder alloys. While Sn whiskers were observed in Sn-Pb solder joints, the removal of lead and usage of Pbfree solders has resulted in the growth of very long
(Received May 7, 2020; accepted October 1, 2020)
(on the order of hundreds to thousands of microns) whiskers. Sn whiskers are typically monocrystalline, highly conductive, and may bridge adjacent solder joints, causing short circuit failures. Failures attributed to tin whiskers have been observed in pacemakers, satellites, and automobiles.1 The growth mechanism for Sn whiskers is believed be dynamic recrystallization (DRX) along with longrange diffusion, promoting the nucleation and growth of whiskers, respectively.2–4 In solder joints and films, stresses may arise due to one of many factors (Fig. 1) th
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