Whisker Formation on SAC305 Soldered Assemblies S. MESCHTER, 1,4 P. SNUGOVSKY, 2 Z. BAGHERI, 2 E. KOSIBA, 2 M. ROMANSKY, 2 J. KENNEDY, 2 L. SNUGOVSKY, 3 and D. PEROVIC 3 1.—BAE Systems, Endicott, NY 13760, USA. 2.—Celestica, Toronto, ON M3C-1V7, Canada 3.—University of Toronto, Toronto, ON M5S-2J7, Canada. 4.—e-mail: stephan.j.meschter@ baesystems.com This article describes the results of a whisker formation study on SAC305 assemblies, evaluating the effects of lead-frame materials and cleanliness in different environments: low-stress simulated power cycling (50–85°C thermal cycling), thermal shock (–55°C to 85°C), and high temperature/high humidity (85°C/85% RH). Cleaned and contaminated small outline transistors, large leaded quad flat packs (QFP), plastic leaded chip carrier packages, and solder balls with and without rare earth elements (REE) were soldered to custom designed test boards with Sn3Ag0.5Cu (SAC305) solder. After assembly, all the boards were cleaned, and half of them were recontaminated (1.56 lg/cm 2 Cl À ). Whisker length, diameter, and density were measured. Detailed metal- lurgical analysis on components before assembly and on solder joints before and after testing was performed. It was found that whiskers grow from solder joint fillets, where the thickness is less than 25 lm, unless REE was present. The influence of lead-frame and solder ball material, microstructure, cleanli- ness, and environment on whisker characteristics is discussed. This article provides detailed metallurgical observations and select whisker length data obtained during this multiyear testing program. INTRODUCTION After the European Union RoHS directive dead- line was enacted in July 2006, the electronics industry began a transition to manufacturing with Sn-Ag-Cu (SAC) near-eutectic alloys to avoid the use of Pb-based solders. This change has had little impact on most commercial consumer products be- cause of the limited life requirements and relatively benign use environments. However, lead-free implementation for the aerospace and defense industry requirements, which includes long service lifetimes, rugged operating environments, and high consequences of failure, requires additional evalu- ation to assure safe systems operation. The new Pb-free and high Sn (about 96%) alloys may be susceptible to spontaneous growths of fila- ment-like tin structures called tin whiskers. Pb is well known as an effective suppressant of whisker formation. Without Pb, the military, aerospace, and other high-reliability industries have experienced increased risks of system failure due to tin whiskers that cause electrical short circuits, impact high- frequency circuits, and create loose debris. 1,2 The majority of recent tin whisker research has con- centrated on coupons or components with an emphasis on the Sn plating composition, thickness, grain size, grain orientations, and Ni underlay- er. 2–12 These studies produced a great deal of knowledge; however, they do not consider the very real situation in which components are assembled on circuit boards using Pb-free solder pastes. Recent observations have demonstrated the ability of Pb-free solder joints to grow whiskers. 13,14 One challenge accompanying real electronic systems is the interplay among the component design and materials, manufacturing process, and service environments that contribute directly or conspire with one another to exacerbate whisker growth. This article is focused on highlights from the Whisker Testing and Modeling project 15 funded by the U.S. Strategic Environmental Research Pro- gram. The objective of this project is to perform systematic tin whisker testing and Monte Carlo risk modeling on lead-free electronic assemblies in order to improve the reliability of military electronic sys- tems. The observations described in this article are based on an extremely large set of data; 129724 JOM, Vol. 66, No. 11, 2014 DOI: 10.1007/s11837-014-1183-9 Ó 2014 The Author(s). This article is published with open access at Springerlink.com 2320 (Published online October 21, 2014)