Incorporation and corrosion protection mechanism of TiO2 nano-particles in SnAgCu composite alloys: Experimental and density functional theory study

This work studied the incorporation mechanism of TiO2 nano-particles into SnAgCu solders and their effect on the corrosion behavior of the composite alloy. Composite solder alloys have been made from 99Sn0.3Ag0.7Cu solder alloy and TiO2 nano-particles between 0.25 and 1 wt%. The alloys in solder joints were exposed to 4000 h long 85 °C/85RH% thermal-humidity test, and their surface was observed by SEM and FIB techniques. Large localized corrosion spots and numerous Sn whiskers have been found on the samples, except for the samples with 0.25 wt% TiO2 content. The corrosion of the Sn grains resulted in a volumetric increase and mechanical stress, which was relaxed via whisker growth. TiO2 nano-particles in 0.25 wt% incorporated layer-like at the boundaries of the Sn grain. Density functional theory calculations proved that at soldering conditions, the Sn atoms of the solder alloy could bond to the TiO2 NPs through the O atoms of TiO2. The TiO2–Sn oxide layer at the grain boundaries could suppress the corrosion of the composite solder alloys and whisker growth. In the case of higher weight fractions TiO2, the nano-particles were agglomerated and could not perform corrosion protection effect.