A unique interferometric system utilizing thermal-conduction loading is developed and implemented to investigate the effect of ramp rates of accelerated test profiles on the thermal deformation of flex package assemblies. The system provides extreme ramp rates to simulate the thermal shock condition with a temperature control much finer than the conventional convection based system can provide. The in-plane and out-of-plane displacements of the flex package are documented through moiré interferometry and Twyman/Green interferometry, respectively. Deformation measured under a thermal shock condition is compared with that under the conventional thermal cycling condition to assess the effect of ramp rates on package deformation. The comparison reveals that a low ramp rate of typical accelerated thermal cycling (ATC) tests causes significant reduction in the maximum level of elastic energy in the package assembly.