Micromechanical characterization of thermomechanically fatigued lead-free solder joints

Nanoindentation testing (NIT) was used to investigate micromechanical properties of (i) as-fabricated, (ii) thermomechanically fatigued (TMF), and (iii) TMF and crept lead-free solder joints. NIT also served to generate information for a database on lead-free solder joints. Sn–Ag-based solder materials used in this study included a binary eutectic alloy, one ternary alloy, and two quaternary alloys. TMF solder joints were thermally cycled for 0, 250, 500, 1000 cycles between –15 and 150 °C. Using NIT, mechanical properties such as hardness, elastic modulus, strength trends, creep behavior, and stress exponent for power-law creep were obtained on small (nominally, 100 m thick) solder joints. Because the volume of material probed by the indenter during NIT is small and highly localized, the properties observed depended strongly on the particular joint microstructure of the indent location. Scanning electron microscopy (SEM) was used to image the nanoindents and monitor deformation and fracture events that resulted from the indenting.