IMC growth of Sn-3.5Ag/Cu system: Combined chemical reaction and diffusion mechanisms

The growth kinetics of intermetallic (IMC) compound layers formed between Sn-3.5Ag solders and Cu substrate in soldering process are investigated experimentally and analytically. In order to determine the governing mechanisms of IMC formation, an experiment is designed in which samples are fabricated with bonding temperature varied at three levels of 260, 310 and 360 °C while time is varied at seven levels between 1 and 240 min. Microstructural analysis is conducted to analyze the IMC thickness and morphology. A physics-based analytical model (Dybkov), in which net growth of each IMC phase is modeled as result of diffusion and partial chemical reaction, is modified for a case of liquid-solid state for two IMC layers. Dissolution of solid IMCs into liquid solder is included in the model. The model is then used to evaluate governing mechanisms of IMC formation and growth and determine the activation energies for formation and growth of different IMC compounds. The chemical reaction and diffusion rate constants have also been evaluated for different intermetallic compounds of Sn-3.5Ag/-Cu-substrate soldering system. Two intermetallic phases were observed during soldering at the interface: η-phase (Cu ₆Sn ₅) and -phase (Cu ₃Sn) IMC layers. The thickness of the η and IMC phases increase with increasing the soldering time and/or soldering temperature. The increase in the IMC layer thickness during the process is found to obey linear relationship with time during the chemical reaction-controlled stage of formation of Cu ₃Sn IMC layer and it obeys a parabolic relationship with time during the diffusion-controlled growth stage of formation of both IMC layers. It was shown that as temperature increases, the chemical reaction and diffusion rate constants monotonically increase. Modeling results indicate clear distinction of governing mechanisms of growth at different IMC growth stages with chemical reaction-controlled growth at initial stage and diffusion-controlled growth at final stage of IMC formation. The apparent activation energy calculated for the chemical reaction-controlled growth stage is 20.59 (KJ/mol) for IMC phase. For diffusion-controlled growth stage, the activation energy values obtained are 41.98 (KJ/mol) for η IMC phase and 50.10 (KJ/mol) for IMC phase. These values slightly differ from values reported in literature where a simple power-law fit is used to extract those quantities. This is expected because unlike present study, the power-law fit does not explicitly incorporate the interaction between the chemical reaction and diffusion mechanisms in IMC growth. The values obtained in the current study are still within the range of the values found in the literature.