The fatigue behaviour of joints between ceramic LTCC modules and FR-4 boards was investigated using thermal cycling tests at temperature intervals of 0 degreesC - 100 degreesC and -40 degreesC - 125 degreesC and analysis of the cycled modules using scanning acoustic microscopy and SEM/EDS. The assemblies were fabricated with Ag-Pd and Ag-Pt metallized pads on LTCC. Fatigue cracks initiated in the solder region of the Ag-Pd metallized LTCC modules, but propagated later in the ceramic module. Propagation was remarkably retarded by Ag-filled vias located under the pads on LTCC. The final cracks that caused an electrical failure in the modules crossed the root region of the vias along the thick Ag(3)Sn+PdSn(4) layer located in this region. The joints of the Ag-Pt metallized modules passed the resistance measurement in both tests. However, they also showed crack growth that initiated on the LTCC/metallization interface, propagated in the ceramic, but was retarded by the vias. These vias were resistant to fracture, since only a thin Ag(3)Sn layer (approximate to 1 mum) existed in their root region.