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The formation and growth kinetics of the intermetallic compound (IMC) between lead-free solder and Cu substrate in the surface mount process were studied. Optical and scanning electron microscope (OM and SEM) were used to measure the thickness of intermetallic layers and observe the microstructure evolution of solder joint. The IMC phases were identified by energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The grain size of the intermetallic compound -phase Cu 6 Sn 5 was observed to increase with the increase in peak temperature hold time. The results show that a -phase Cu 6 Sn 5 IMC layer is formed at solder-Cu substrate interface at a very short time. The growth of intermetallics formation was found not to follow the Fick's law that predicts the mean total thickness increases linearly with the square root of the time. It deviates the Fick's law at the early stage of the growth process and then approaches the parabolic law. To explore the growth kinetics, the IMC growth mechanism is suggested and a lagging diffusion model is presented for predicting the intermetallic compound layer growth. Comparison between the model and experimental results demonstrates that the proposed phase-lag model capture the growth history of IMC layers quite well.Index Terms-Intermetallic compound, lead-free solder, phase-lag model, solder joint.

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This study investigates the microstructural evolution and kinetics of intermetallic (IMC) formation in Sn-3.5Ag-0.7Cu lead-free solder joints with different percentages of Sb element, namely, Sn-3.5Ag-0.7Cu-xSb ( =0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0). To investigate the elemental interdiffusion and growth kinetics of IMC formation, isothermal aging test is performed at temperatures of 100 C, 150 C, and 190 C, respectively. Scanning electron microscope (SEM) is used to measure the thickness of intermetallic layer and observe the microstructural evolution of solder joint. The IMC phases are identified by EDX and XRD. Results show that some of the antimony powders are dissolved in the -Sn matrix (Sn-rich phase), some of them participate in the formation of Ag 3 (Sn Sb) and the rest dissolves in the Cu 6 Sn 5 IMC layer. There is a significant drop in IMC thickness when Sb is added to 0.8 wt%. Over this amount the thickness of the IMC increases slightly again. The activation energy and growth rate of the IMC formation are determined. Results reveal that adding antimony in Sn-3.5Ag-0.7Cu solder system can increase the activation energy, and thus reduce the atomic diffusion rate, so as to inhibit the excessive growth of the IMC. The solder joint containing 0.8 wt% antimony has the highest activation energy. SEM images reveal that the number of small particles precipitating in the solder matrix increases with the increase in Sb composition. Based on the observation of the microstructural evolution of the solder joints, a grain boundary pinning mechanism for inhibition of the IMC growth due to Sb addition is proposed.Index Terms-Grain boundary diffusion, intermetallic growth, lead-free solder, Sb addition.

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