3D X-ray microscopy and mechanical analysis of thermal cycling-induced degradation in solder joints

• 3D X-ray imaging revealed internal damage missed by 2D methods. • Solder-to-total volume ratio decreased linearly with thermal cycling. • Incubation period observed before degradation became detectable. • Mechanical testing correlated strength loss with internal degradation. • Void compactness decreased, indicating deformation and shape loss. This study investigates the thermal fatigue behavior of Innolot-soldered chip resistors mounted on printed circuit boards, subjected to accelerated thermal cycling between −40 °C and 125 °C in accordance with the IPC-9701B standard. The degradation of solder joints was analyzed using high-resolution 3D X-ray microscopy at intervals of 500 cycles up to 2500 cycles, complemented by mechanical shear testing and scanning electron microscopy (SEM). Key evaluation metrics included crack length, solder-to-total volume ratio, void diameter, and void compactness. Results showed that crack lengths increased with thermal cycles in some cases across the whole joint, although growth patterns varied across different planes, underscoring the importance of three-dimensional analysis. The solder-to-total volume ratio demonstrated a linear decrease with an initial incubation phase, serving as a robust indicator of joint degradation. Voids became larger and more deformed, with compactness decreasing due to thermal cycling. The peak shear force was significantly reduced in the most degraded component (11.3 N) compared to the other samples (27.7 N and 28.6 N). This can be explained by the highest average crack length, the void growth or with volume loss, however the SEM images revealed another significant factor, which is failure happened in the bulk solder mainly while in the other two cases in the intermetallic layer.