The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Interfacial Reactions Between Sn-Based Solders and Common Metallisations Used in Electronics
Vesa Vuorinen
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium Edison at Helsinki University of Technology (Espoo, Finland) on the 8th of December, 2006, at 12 noon.
Dissertation in PDF format (ISBN 951-22-8527-4) [11130 KB]
Dissertation is also available in print (ISBN 951-22-8526-6)
Abstract
During their lifetimes the electrical interconnections of portable electronic devices are subjected to high thermal, mechanical, chemical, and electrical loadings, and therefore the metallurgical compatibility between solder alloys and component pad metallisations has become ever more essential from the point of view of interconnection reliability. For this reason, the mechanisms of interfacial reactions are studied in this thesis by making use of thermodynamic calculations together with diffusion kinetic considerations and microstructural analyses. The formation of interfacial intermetallic compounds is investigated experimentally by using solid|liquid and solid|solid diffusion couples of several binary and ternary member systems. These systems include the most important metals used in electronics, tin (Sn), copper (Cu), nickel (Ni), silver (Ag), and gold (Au). The effects of additional elements such as P, Fe, Zn, and Ti on the growth kinetics and defect structures of interfacial intermetallics are also investigated. It is shown that the main factors affecting the thickness and morphology of interfacial intermetallic compounds in soldering are: i) the solubilities and, consequently, the dissolution rates of solid metals in liquid solders; ii) local equilibria and their changes as a function of time and temperature, and iii) the chemical reactions at the supersaturated liquid solder interfaces. The experimental results from isothermal aging at elevated temperatures demonstrate that alloying elements and impurities can change the microstructural evolution of solder interconnections drastically and thus diminish their mechanical integrity. The thermodynamic-kinetic approach, together with detailed microstructural observations, proved to be a very useful tool when studying the interfacial intermetallic reactions, as well as the evolution of solder interconnection microstructures, during long-term aging at elevated temperatures.
Keywords: interfacial reactions, lead-free solder interconnections, microstructural evolution, thermodynamic-kinetic approach, Sn-Cu-Ni system
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© 2006 Helsinki University of Technology
Last update 2011-05-26