Strength and Isothermal Fatigue Resistance of SnBi/SnAgCu Joints Reflowed at Low Temperatures

Soldered micro-electronics assemblies may have to survive a variety of mechanical loads in repeated drops, cyclic bending, or vibration. A very large body of work has addressed the isothermal fatigue performance of SnAgCu solder joints. This work offers a general assessment of the achievable performance of so-called hybrid solder joints formed by soldering with eutectic SnBi or SnBiAg to SnAgCu bumps on area array components. This allows for soldering at much lower temperatures than with SnAgCu alone, but the deformation and damage properties of the resulting structures depend strongly on details of the design and process. A peak reflow temperature of 175 degrees C was shown to be sufficient to ensure that the life of the joints remains limited by fatigue of the unmixed SnAgCu near the component. However, a higher effective stiffness of the mixed region near the substrate means that the life will be lower by 45%.