The damage mechanism of lead-free ball grid array (BGA) solder joints is studied whilst the magnitude of the damage was quantitatively evaluated to determine the impact of the thickness of inter-metallic compound (IMC) layer on the reliability of the joints. Five virtual test vehicle assemblies of flip chip (FC) which have 4–12 μm thickness of IMC layer were subjected to the same accelerated high-temperature cycles utilising, in parts, the IEC standard 60749-25. The visco-plastic responses of the assemblies’ solder bump joints to the induced thermal load were simulated using Anand’s plasticity model. The results demonstrate that very thin and thick thickness of IMC layer impact the reliability of the FC solder joints. It was found that strain range at steady state joint damage is approximately 0.07–0.12. This finding indicates that strain controlled studies on bulk solder which are not up to these limits are inadequate to characterise damage behaviour and model damage evolution in the FC BGA solder bump joints. Further results show that the magnitude of the average accumulated visco-plastic energy density per cycle (wacc) depends on the volume percentage of solder and IMC in the joints. This dependency explains why the existing life prediction models based on wacc are not satisfactory in estimating the damage in solder joints. The authors propose the development of new prediction models which will be specific on the volume percentage of solder and IMC in the joints.