Growth of whiskers from Sn surfaces: Driving forces and growth mechanisms
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- 作者：Eric Chason ; eric_chason_phd@brown.edu ; Nitin Jadhav ; Fei Pei ; Eric Buchovecky ; Allan Bower
- 关键词：Sn whiskers or tin whiskers ; Pb-free or lead-free ; Tin ; Tin alloys ; Electronics packaging ; Reliability
- 刊名：Progress in Surface Science
- 出版年：2013
- 出版时间：May, 2013
- 年：2013
- 卷：88
- 期：2
- 页码：103-131
- 全文大小：4014 K
文摘
Sn whiskers are thin filaments that grow spontaneously out of the surface of coatings on Cu and have become a critical reliability problem in Pb-free electronics. In this review, we focus on what creates the driving force for whiskers (or more rounded ¡°hillocks¡±), and what determines where on the surface they will form. Experimental studies are reviewed that quantify the relationship between the Cu-Sn intermetallic (IMC) formation, stress in the layer and whisker/hillock density. Measurements of the mechanical properties show how stress relaxation in the Sn layer is intimately related to how much stress develops due to the IMC formation. Real-time scanning electron microscope (SEM)/focused ion beam (FIB) studies are described that illustrate the whisker/hillock growth process in detail. Whiskers are found to grow out of a single grain on the surface with little lateral growth while hillock growth is accompanied by extensive grain growth and crystallite rotation. Electron-backscattering detection (EBSD) shows the grain structure around where the whiskers/hillocks form, indicating that whiskers can grow out of pre-existing grains and do not require the nucleation of new grains. This has led to a picture in which stress builds up due to IMC growth and causes whiskers/hillocks to form at ¡°weak grains¡±, i.e., grains that have a stress relaxation mechanism that becomes active at a lower stress than its neighbors. FEA (finite element analysis) calculations are used to simulate the evolving stress and whisker growth for several different mechanisms that may lead to ¡°weak¡± grains.