ULTRAMICROTOMY - A UNIQUE METHOD FOR PREPARATION OF COMPOSITE SOLDER FOR TRANSMISSION ELECTRON-MICROSCOPY

Ultramicrotomy was successfully implemented for sectioning solder/Cu and composite solder/Cu samples for TEM. Solder/Cu joints, approximately 10 mm by 2 mm by 3 mm, were carbon coated and dipped in cyanoacrylate ester before being embedded into an epoxy mount. The mounted samples were trimmed in a series of steps to obtain a pyramid-shaped, embedded sample with a flat, trapezoidal face of exposed metal for sectioning. Thin sections were sliced directly from the bulk sample using an ultramicrotome and a diamond knife. Once sectioned, the samples were placed on Formvar and carbon coated copper grids for examination by TEM. Solder/Cu joints made with eutectic (63Sn/37Pb) solder and several composite solders, including Cu, Ni, and Cu6Sn5, were examined. For the first time, it was possible to image simultaneously each phase in the material using a single TEM sample. The various phases present in the solder joints, including the Pb-rich and Sn-rich solder phases, the CU6Sn5 and Cu3Sn intermetallic phases, and Cu were identified using selected area electron diffraction. Artifacts due to sectioning, such as knife marks, intermetallic tearing, and brittle phase extraction, were observed. These artifacts were minimized by controlling the sectioning conditions.