Contributions of Stress and Oxidation on the Formation of Whiskers in Lead-Free Solders
WP-1754
Objective
Over a half century ago it was realized that whiskers spontaneously grow out of solder materials. Preventing whisker growth is particularly important in microelectronic circuits where devices are separated by spacings as small as tens of nanometers. Historically, lead has been added to these solders to prevent whisker growth and improve the manufacturing processes. With the understanding of the harmful environmental effects of lead, this is no longer a valid solution. The introduction of lead-free solders into microelectronics manufacturing requires greater understanding of this failure mechanism. Understanding this growth is complicated in that several driving forces and other factors likely contribute to the nucleation and growth of the whiskers. Two important driving forces are thermal and mechanical stress and oxide formation.
The objective of this project is to increase understanding of the role of stress and oxide formation on whisker growth in lead-free solders. This understanding will support the development of a predictive model, enabling engineers to design solder alloys for whisker growth resistance.
Technical Approach
This project will examine a series of commercial solders. Parallel experiments will be performed over a period of 3 years with Year 1 analysis of single layer solder, Year 2 analysis of dual layer solders, and Year 3 analysis of the effect of applied stress to the solder system. Whisker growth will be measured as a function of time. Experiments will provide a quantitative correlation of stress and oxygen consumption with whisker density. Finite element modeling will be used to determine the stresses within the films. Microstructures of the solders will be examined prior to and at the conclusion of testing to explore additional correlations and elucidate the growth mechanism. A predictive model will be developed based on the experimental results to aid the development of lead-free solders with low susceptibility to whisker formation.
Benefits
Commercially relevant lead-free solders will be evaluated for susceptibility to whisker growth as a function of stress and oxygen consumption. This will be done for both individual solders and solders in combination. On a scientific level, the role of oxide formation and stress on the growth of whiskers will be elucidated. Data will be compiled into a predictive model for whisker growth with respect to key parameters including thermal history, microstructure, stress, and oxygen consumption. (Anticipated Project Completion - 2013)
Points of Contact
Principal Investigator
Dr. Elizabeth Hoffman
Savannah River National Laboratory
Phone: 609-781-0276
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