Abstract: A quarter of WLCSP device was selected for finite element modelling and the stress-strain response of lead-free soldered joints was investigated systematically.The stress of soldered joints is variational with different positions in WLCSP array, the lowest stress of soldered joints appears in the center of array, the largest stress can be found in the outmost solder joints.The relationship between stress and position of soldered joints is as follows:σ(x,y)=1.78x+1.78y+0.33, for creep strain, the correlation is:ε(x,y)=0.006x+0.006y+0.009.Moreover, it is found that the reliability of soldered joints can be influenced by the materials in WLCSP structure, the lower reliability of soldered joints can be found in WLCSP device with polymers than that without it, so it is suitable to select the materials with matched CTE to be utilized in those devices.The reliability of WLCSP device soldered joints decreases when the pin count increases.The higher pin count means larger distance from the outmost soldered joint to the chip center at constant pitch.