Three-dimensional chemical maps by Secondary Ion Mass Spectrometry (3D-SIMS), XPS spectroscopy, and SEM-EDAX microscopy were employed in order to investigate the effects of accelerated fatigue tests on crack formation in 95.5% Pb–2% Sn–2.5% Ag and 95% Pb–5% Sn solder joints. These alloys are used in the die bonding of electronic power device assemblies. The results show that cracks form by Sn-depletion from the inner regions of the soldered joint. Simultaneously, there is a recrystallization of the Pb-rich phase in the same regions of the joint. The crack occurs at a critical number of cycles when a Sn-depleted region is formed, yielding weaker inner layers with lower shear strength. A possible explanation of the Sn-depletion is also discussed.