Characterization of Electroplated BiSn Alloys for Conductive Adhesives

Conductive adhesives are the focus of many investigations in the microelectronic packaging industry as a possible replacement for lead-based solders. These adhesives offer several advantages over traditional solder technology such as finer pitch connections, lower processing temperatures, and environmental friendliness due to their lead-free composition. The present paper discusses one particular conductive adhesive consisting of bismuth tin-coated copper filler particles dispersed within a polymer matrix. Specifically, the bismuth tin alloy was investigated to increase understanding of this metallic system and foster enhancements to the conductive adhesive system. Three aspects of BiSn were characterized: the surface oxidation, the microstructural changes upon heat treatment, and the resistivity of the alloy. Techniques such as x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were utilized to study the surface oxides. The oxide film layer thickness was also determined with the aid of differential scanning calorimetry (DSC). Microstructural changes were observed with scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Lastly, resistivity measurements were made with a 4-point probe. The surface oxide was identified as tin oxide (SnO). While heat treatment did not significantly increase the oxide growth on BiSn surfaces, it did cause oxide crystallites to grow. The resistivity of BiSn alloy was determined to be 16.67 µ(omega) -cm, which is similar to that of 63Sn-37Pb solder, a common solder. These results are promising and indicate that the conductive adhesive investigated is comparable in certain aspects to current lead-based solders.