Effects of static thermal aging and thermal cycling on the microstructure and shear strength of Sn 95.5 Ag 3.8 Cu 0.7 solder joints

The microstructure and shear strength changes of SnAgCu/Cu and SnAgCu/Ni–P/Cu surface mount solder joints during thermal aging at 150 °C and thermal cycling between ?40 and 150 °C were investigated. The reaction rate between SnAgCu and Cu is higher than that between SnAgCu and Ni–P. After long aging time, the SnAgCu/Cu interface becomes the weakest region in the SnAgCu/Cu solder joint, whereas the shear-force-induced cracks in the SnAgCu/Ni–P solder joint appear at the interface of Ni–P/Cu. During thermal cycling, cracks develop in both solder joints and the shear strength decreases. After extensive thermal cycling, the Ni–P layer separates from the Cu substrate and the shear strength of the SnAgCu/Ni–P solder joint decreases drastically.