Effects of Bi and Pb on oxidation in humidity for low-temperature lead-free solder systems

The oxidation behavior of various Sn-Zn(-Bi) alloys during 60°C/90% and 85°C/85% relative humidity (RH) exposure were investigated by microstructure observation and x-ray diffraction analysis. The mechanical property of the joints of resistor chips (1608R) with two kinds of terminations, Sn and Sn-10Pb, soldered on a printed circuit board with Sn-Zn(-Bi) were evaluated by a shear test. The heat/humidity exposure of Sn-Zn alloys promotes segregation into the grain boundary accompanying oxidation of Zn resulting in the ZnO formation. This segregation induces serious degradation of alloys and Sn whisker growth. Heat/humidity exposure of 85°C/85%RH seriously decreases the shear strength of the surface mounted chip joints, especially Sn-Zn-Bi solder, due to the formation of ZnO at the interface between the solder and the reaction layer. The presence of Bi or Pb in Sn-Zn alloys enhances the diffusion, resulting in severe degradation at 85°C/85%RH exposure. In contrast, the exposure at 60°C/90%RH does not influence the joint strength for up to 1000 h. Under this condition, the oxidation of Zn only reaches a few microns in depth from the free surface.