Compositional dependence of Sn-Bi solder on the intermetallic compound (IMC) growth kinetics with Cu at thin film scales

• Main Author: Zhou, Yingxiu.
• Other Authors: Gan Chee Lip
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/52092
• Institution: Nanyang Technological University

Many alloys can be used for soldering and tin-lead (Sn-Pb) solder is one of the most widely used. However, the usage of lead leads to undesirable environmental damage. Therefore, there is an increasing demand for lead-free solder, especially low melting temperature solders that can be used in devices that are sensitive to heat. A potential low temperature solder is the Tin-Bismuth (Sn-Bi) alloy. The Sn-Bi alloy binary system has a simple eutectic phase diagram, with a eutectic composition of Sn42Bi58 and melting temperature of 139 °C. This project aims to study the compositional dependence of Sn-Bi solder on the IMC growth kinetics with Cu at thin film scales. From the results obtained, phases that were formed include Cu6Sn5, Cu3Sn IMC and Bi. The approach that we use in this study is to enable real time monitoring of the Cu consumption rate or equivalently, the IMC growth rate. During the experiment, it was observed that all of the annealed samples had their initial colour change from brown copper to shiny white IMC. This shows that Cu was being consumed and the methodology works. It was also discovered that at higher annealing temperatures of 200 °C and 220 °C, the IMC growth rates are independent of Bi at. % composition as it falls fully within the liquid phase. On the other hand, for annealing temperatures at 150 °C and 180 °C, a portion of the Bi at. % composition falls within the Liquid-Bi Solid phase and the IMC growth rate increases and is the fastest when the system approaches the liquid phase eutectic composition of Sn42Bi58. This is due to higher saturation limit and ease of diffusivity of copper atoms into the liquid phase as well as the lowest melting point at the eutectic composition. Therefore, IMC formation rate for lower temperatures of 150 °C and 180 °C are dependent on the composition of Bi, with the fastest rates occurring close to the eutectic composition of Sn42Bi58. The IMC thickness was calculated from the initial Cu thickness with a ratio of 2.74, derived from the chemical reaction between Cu and Sn to form Cu6Sn5. The growth kinetics of Cu6Sn5 was then analyzed using the Power Law to determine if the reaction is reaction limited or diffusion limited from the values of exponent n. A graph of exponent n is then plotted against Bi at. % Composition and results shows that when the composition is lower than the eutectic composition of Sn42Bi58, n ≥ 0.5, suggesting that it is reaction limited. However, when the composition is higher than the eutectic composition, n ≤ 0.5, suggesting that the reaction is diffusion limited.