A Study of Pb-Rich Dendrites in a Near-Eutectic 63Sn-37Pb Solder Microstructure via Laboratory-Scale Micro X-ray Computed Tomography (μXCT)

A key challenge in the application of laboratory-scale x-ray computed tomography to the study of metallic alloys is achieving sufficient feature contrast and resolution for the segmentation between solid phases of similar composition and density at spatial length scales suitable for microstructure quantification. In the microelectronic packaging, value exists in the nondestructive evaluation of solder system microstructures resulting from varying compositional and processing factors. A near-eutectic 63Sn-37Pb butt-joint on copper was studied with a custom laboratory-scale microresolved x-ray computed tomography scanner with the goal of quantifying three-dimensional (3D) microstructural constituents resulting from a reproducible reflow process. The 3D character of lead-rich dendrites resulting from non-equilibrium solidification was revealed. The quantification of the dendrite microstructure was made possible through a combination of data acquisition, data processing, and data segmentation techniques. The scanning parameter selection, with respect to the characterization task, is discussed. Data acquisition and processing methods which were determined to be beneficial for 3D microstructure characterization are detailed. A beam-hardening artifact reduction algorithm is provided, without which microstructure quantification would not have been possible. The segmentation of the dendrite features, performed using a semi-automatic 3D region growth method, is described. The segmented solder volume enabled quantitative description of the 3D dendrite microstructure and void content.