In-situ synchrotron x-ray diffraction during melting and solidification of a lead-free solder paste

The intense flux of high-energy X-rays provided by synchrotron radiation sources allows transmission diffraction experiments to be performed. This can be used to gain non-destructive phase and strain data from the bulk of solid materials. The ability to focus the beam of x-rays into a narrow beam or spot of monochromatic x-rays allows 2d and even 3d mapping to be performed. As with all metals and alloys, the properties of a solder alloy are largely dependent on microstructure. The basis of the microstructure is evolved during a reflow operation from the heating and cooling cycle imposed upon the solder paste. This paper will show results from experiments undertaken at the European Synchrotron Radiation Facility. In these experiments, lead-free solder paste was placed onto a printed circuit board and then reflowed to form a solder joint. Time-resolved x-ray diffraction data were collected both from the bulk solder and interfacial regions during the melting and solidification processes. The paper will discuss the formation of microstructure for different pad finishes and cooling rates