Sample preparation induced artefacts in soft SAC solders from uncooled and cooled Argon ion milling

Complex combinations of various materials in microelectronic packages pose challenges for sample preparation. Reliable cross-sectional sample preparation is critical for microstructural analysis, especially when utilising low-melting Sn-Ag-Cu (SAC) solders, which are easily deformed or damaged by mechanical grinding. Hence, this study explores the impact of femtosecond laser ablation for bulk material removal and subsequent Ar-ion milling for final cross-sectional polishing, comparing cooled and uncooled milling masks. We reveal that uncooled Ar-ion milling causes artefacts like increased surface roughness and fine Cu-rich structures due to surface heating combined with Cu redeposition on the preferentially milled, intruded SAC-surface. Cryogenic cooling of the milling mask to -100 °C reduces these effects, minimising curtaining and preserving microstructural details. However, we also
show that cooled milling may result in the displacement of recrystallised grains at grain boundaries due to the thermal expansion anisotropy of β-Sn. Thus, milling parameters must be carefully tailored to the desired preservation of microstructure, cracks and strain.