To describe the phenomenon of the stress generation and stress relaxation occurring simultaneously in the process of whisker growth,a UMAT subroutine in Abaqus is developed to solve the elasto-plastic creep constitutive model by means of the return-mapping algorithm.The finite element simulations show that,compared with the elasto-plastic model in Abaqus,the elasto-plastic creep constitutive model characterizes the stress relaxation of material better.Using this subroutine,we calculate the change of stress in a specimen of Pb-free solder.In terms of the change of stress,the whisker growth rate is evaluated.And the effect of the grain size on the whisker growth rate is discussed.