Investigation of accelerated surface oxidation of Sn-3.5Ag-0.5Cu solder particles by TEM and STEM

The composition and thickness of surface oxide of solder particles has a direct effect on adhesion and electrical resistance of soldering joint and resultant the quality of interconnect and the reliability of packaged system. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to examine the oxide layer on solder powders in the present paper. However, for the surface oxide layer of a lead-free solder particle, the TEM sample for the oxide layer has never been done for studying its thickness or appearance before. And it is the first time in this work to use Focus Ion Beam (FIB) technology to prepare TEM specimen for solder particles and show TEM pictures of their surface oxide layer. High angle annular dark field (HAADF) pattern was applied to distinguish between the oxide layer and the solder matrix by the contrast of average atomic number. The solder powders were exposed in air (70% relative humidity) at 150°C for 0, 120 and 240 h to simulate the accelerated growth of oxide. The surface oxide thickness was 6 nm and 50 nm measured by TEM for 0 h and 120 h samples respectively. Confirming by AES measurement, the thickness of 5 nm and 50 nm were gotten using intersection analysis method for AES depth profiles. It is found that the increase of surface oxide thickness of solder particles is proportional to the rooting of time. The elemental distribution along the oxide was quantified by line scanning using STEM and the atomic ratio of Sn to O in the oxide layer nearer to the outer, the middle, and the inner (adjacent to the solder matrix) were found to be 1:2, 2:3 and 1:1, respectively. The result was validated using XPS which gave Sn to O ratio of 1:2 at 5 nm depth of surface oxide.