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Whisker Growth in Tin Alloys on Glass-Epoxy Laminate
August 26, 2014 |Estimated reading time: 1 minute
Abstract
Tin-rich solders are widely applied in the electronic industry in the majority of modern PCBs. Because the use of lead-tin solders has been banned in the European Union since 2006, the problem of the bridging of adjacent conductors due to tin whisker growth (limited before by the addition of Pb) has been reborn. In this study, tin alloys soldered on glass-epoxy laminate (typically used for PCBs) are considered. Scanning ion microscopy with focused ion beam (FIB) system and energy-dispersive X-ray spectroscopy (EDXS) were used to determine correlations between spatial non-uniformities of the glass-epoxy laminate, the distribution of intermetallic compounds, and whisker growth.
Introduction
Tin whiskers are crystals growing from tin or tin-alloy surface that are a threat to the reliability of electronic circuits because of short circuits (due to the bridging of adjacent conductors), increased electromagnetic radiation or device littering. The phenomenon can occur in tin-rich solders, but the addition of lead to the tin alloy inhibits whisker growth. In the twentieth century, the most popular solder was Pb37Sn63 eutectics. However, since July 2006, when the Restriction of Hazardous Substances (RoHS) Directive was adopted by the European Union, the amount of Pb in solders has been limited to 0.1 wt.%. The application of tin-rich lead-free solders in the PCB assembly process has reintroduced the problem of tin whisker growth which had been limited before by the addition of Pb. Whiskers are responsible for many system failures in the military, medical and telecommunication industries.
There is no single, commonly accepted model of whisker growth in the literature. However, most theories involve the role of compressive stress, which may result from chemical, mechanical and thermal factors, with the whisker growth as a phenomenon of stress relief. The growth is affected by such factors as temperature, residual stress, mechanical force, the formation of intermetallic compounds (IMCs), broken oxide layer, electric field, etc. The higher the compressive stress, the greater the volume of Sn contained in whiskers.Read the full article here.Editor's Note: This article originally appeared in the July 2014 issue of SMT Magazine.