The influence of different-sized Ni micro- and nanopowders on the processing and microstructural properties of Sn-Ag-Cu-solder with low Ag content
The development of lead free solder with high thermo-mechanical fatigue resistance is an ongoing task of the electronic industry. One possibility to improve the strength of solder materials is particle reinforcement. This paper reports on the effects of adding different sizes and contents of Ni particles in low silver lead-free solder. For this purpose, nickel (Ni) particles of the sizes 100 nm, 1 (mm and 5 (mm were mixed into a Sn1.0Ag-0.5Cu (SAC105) solder. The wetting and voiding behavior of the solder, as well as the microstructure, shear strength and fracture modes of solder joints were examined. High concentrations and finer powders resulted in higher wetting angles and a decrease in the spreading rate. With increasing particle concentration and smaller particle sizes, the void concentration increased significantly. This is most likely due to an increase in viscosity of the solder melt and a reduction of pore mobility. At low particle additions of 0.1 wt.% a small decrease in thickness of the interfacial intermetallic composites (IMCs) was demonstrated. At high concentrations, however, the IMC thickness increased due to the attachment of IMCs and agglomerates at the interfacial IMC layer. A decrease in shear strength was observed with increasing particle content. This decrease could be attributed to the reduction of the load bearing surface by the pores, as well as the stress concentrations at pore edges. Using a vacuum-overpressure reflow process, the maximum pore content of solder paste reinforced with 1wt.% 1 mm Ni particles was reduced to below 1 vol.%. This resulted in a maximum shear strength improvement of about 5 MPa compared to unreinforced SAC105. With the 100 nm and 1 mm powders, there was a development towards exclusively ductile fracture behavior. With the 5 mm powders, quasi-brittle fracture behavior occurred more frequently, supposedly due to the increasing attachment of IMCs at the interface.