Formation and growth kinetics of interfacial intermetallics in Pb-free solder joint

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 η-phase Cu₆Sn₅ was observed to increase with the increase in peak temperature hold time. The results show that a η-phase Cu₆Sn₅ 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 captures the growth history of IMC layers quite well.