<p>It is critical to apply wafer-level underfill (WLUF) to establish and improve the stability of microbumps. Although applying WLUF to microbump interconnection layers is common, understanding the behavior of microbump stability without WLUF protection is also critical. Electromigration (EM) and thermal cycling (T/C) tests were performed at a current density of 1.3 × 10⁵ A/cm² at 150℃ and − 45 to 125℃, respectively, to investigate the effect of a WLUF on the reliability for mixed loading of EM and T/C in Cu/Ni/Sn-Ag microbumps. The results of the mixed loading tests showed different failure modes for microbumps with and without the WLUF. Microbump test samples without the WLUF exhibited horizontal cracks. On the other hand, the test samples with WLUF showed cracks propagating obliquely. A series of finite element analyses revealed that the WLUF caused vertical deformation and led to an increase in tensile stress, resulting in oblique crack propagation. These results suggest that the WLUF, due to its relatively high coefficient of thermal expansion, undergoes vertical expansion during thermal cycling, which introduces vertical tensile strain into the microbump structure. As a result, it functions as a stress redistribution layer that modifies the internal stress profile. This redistribution alters the crack propagation path and enhances the mechanical reliability of microbump interconnects under mixed EM and T/C loading.</p> Graphical Abstract <p></p>