Effects of static thermal aging and thermal cycling on the microstructure and shear strength of Sn95.5Ag3.8Cu0.7 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 degreesC and thermal cycling between -40 and 150 degreesC 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.